November 2016 Volume 14, Issue 11, Supplement 11

Authors

Jeffrey Laurence, MD Professor of Medicine Division of Hematology and Medical Oncology New York Presbyterian Hospital and Weill Cornell Medical College Atypical Hemolytic Uremic New York, New York Syndrome (aHUS): Hermann Haller, MD Professor of Medicine Department of Nephrology Essential Aspects of an and Hypertension Hannover Medical School (MHH) Hannover, Germany Accurate Diagnosis

Pier Mannuccio Mannucci, MD Professor of Medicine IRCCS Cà Granda Foundation Maggiore Policlinico Hospital Milan, Italy

Masaomi Nangaku, MD, PhD Professor of Medicine Division of Nephrology and Endocrinology The University of Tokyo Graduate School of Medicine Tokyo, Japan

Manuel Praga, MD Professor of Medicine Division of Nephrology Complutense University Madrid, Spain

Santiago Rodriguez de Cordoba, PhD Professor, Department of Cellular and Molecular Medicine El Centro de Investigaciones Biológicas and El Centro de Investigación Biomédica en Red de Enfermedades Raras Madrid, Spain

ON THE WEB: hematologyandoncology.net

Indexed through the National Library of Medicine (PubMed/MEDLINE), PubMed Central (PMC), and EMBASE REVIEW ARTICLE

Atypical Hemolytic Uremic Syndrome (aHUS): Essential Aspects of an Accurate Diagnosis Jeffrey Laurence, MD, Hermann Haller, MD, Pier Mannuccio Mannucci, MD, Masaomi Nangaku, MD, PhD, Manuel Praga, MD, and Santiago Rodriguez de Cordoba, PhD

Abstract: Atypical hemolytic uremic syndrome (aHUS), a thrombotic microangiopathy (TMA), is a rare, life-threatening, systemic disease. When unrecognized or inappropriately treated, aHUS has a high degree of morbidity and mortality. aHUS results from chronic, uncontrolled activity of the alternative complement pathway, which activates platelets and damages the endothelium. Two-thirds of aHUS cases are associated with an identifiable complement-activating condition. aHUS is clinically very similar to the other major TMAs: Shiga toxin–producing Escherichia coli (STEC)-HUS, thrombotic thrombocytopenic purpura (TTP), and disseminated intravascular coagulation (DIC). The signs and symptoms of all the TMAs overlap, complicating the differential diagnosis. Clinical identification of a TMA requires documentation of micro- angiopathic hemolysis accompanied by thrombocytopenia. DIC must be recognized and treated before it is possible to discriminate among the other 3 major TMAs. STEC-HUS can be excluded through testing for Shiga toxin–producing E. coli. aHUS can be distinguished from TTP on the basis of ADAMTS13 (a disintegrin and metalloproteinase with a thrombos- pondin type 1 motif, member 13) activity, with a severe decrease characteristic of TTP. This test, as both an activity assay and an inhibitor assay, should be ordered before the initiation of plasma therapy in any patient presenting with a TMA. Finally, it is important to recognize that aHUS remains a clinical diagnosis, but in complex scenarios, tissue biopsy may be a useful adjunct in diagnosis.

Introduction angiopathy (TMA) that is clinically fragmented red blood cells or schisto­ very similar to aHUS.2-5 However, a cytes on peripheral blood smear, low In October 2012, this journal pub­ major challenge per­sists: how to make haptoglobin levels, elevated lactate lished a review on “Making the diag­ an efficient and accurate differential dehydrogenase (LDH) and indirect nosis” of atypical hemolytic uremic diagnosis among the TMAs. bilirubin, and a decline in baseline syn­drome (aHUS).1 Recent advances Table 1 lists the 4 major TMAs: hemoglobin—accompanied by throm­ have enabled the implementation aHUS, TTP, Shiga toxin–producing bocytopenia. These laboratory changes of directed therapy, with dramatic Escherichia coli (STEC)-HUS, and must occur in concert with clinical declines in morbidity and mortality disseminated intravascular coagula­ involvement of at least 1 organ system, over the historical interventions used tion (DIC). Clinical recognition of the most common sites being the cen­ in thrombotic thrombocytopenic any TMA requires the documentation tral nervous system, the kidneys, and purpura (TTP), a thrombotic micro­ of microangiopathic hemolysis—with the gastrointestinal tract. DIC, usually

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2 Clinical Advances in Hematology & Oncology Volume 14, Issue 11, Supplement 11 November 2016 ATYPICAL HEMOLYTIC UREMIC SYNDROME (AHUS): AN ACCURATE DIAGNOSIS

secondary to infection or malignancy, Table 1. Distinguishing Among the Major Thrombotic Microangiopathies stands apart from the other TMAs because it has characteristic laboratory Category Defining Characteristic abnormalities indicative of a con­ Disseminated intravascular Coagulation abnormality with elevated INR and sumptive coagulopathy, including an coagulation (DIC) aPTT elevated international normalized ratio Thrombotic thrombocytopenic ADAMTS13 <5%-10%; autoantibody inhibitor of (INR) and activated partial throm­ purpura (TTP) ADAMTS13 (unless one of the rare congenital forms, boplastin time (aPTT). However, the with no inhibitor) signs and symptoms of all the TMAs overlap extensively, complicating the Atypical hemolytic uremic ADAMTS13 >5%-10% (exact cut-off as specified syndrome (aHUS) by the laboratory and assay technique employed); differential diagnosis. associated with a recognized complement-activating condition in two-thirds of cases; congenital mutation Historical Issues in Differentiating in complement system recognized in 70% of cases Among the TMAs E. coli E. coli Since the initial descriptions of 2 Shiga toxin–producing Stool sample or rectal swab positive for – HUS (STEC-HUS) producing Shiga toxin by culture and/or PCR (both major categories of TMA—TTP by should be performed) Moschcowitz in 19246 and HUS by Gasser in 19557—there have been ADAMTS13, a disintegrin and metalloproteinase with a thrombospondin type 1 motif, but a handful of critical diagnostic member 13; aPTT, activated partial thromboplastin time; INR, international normalized ratio; PCR, polymerase chain reaction. and therapeutic breakthroughs. First, there was identification of the utility of plasma exchange (PE) in the treat­ undifferentiated as “TTP/HUS.” Distinguishing Among the ment of TTP. This was followed by Indeed, the definitions of TTP and TMAs: Primary Considerations isolation of Shiga toxin–producing E. HUS had been vague from the outset: coli as the etiologic agent of many cases Gasser included a case of “Mosch­ Development of TTP and aHUS of diarrhea-associated (D+) HUS, now cowitz disease” in his HUS series, appears to require 2 conditions: (1) pre­ known as STEC-HUS. The means to and HUS was originally referred to existing susceptibility factors that are distinguish TTP from aHUS, using as “TTP of children.”8 More recently, either familial (ie, genetic) or acquired, assays for activity of the enzyme von the critical role of congenital defects and are capable of promoting endothe­ Willebrand factor (vWF) cleaving in the regulation of the alternative lial cell activation, platelet aggregation, protease, also known as ADAMTS13 complement pathway in aHUS was or both; and (2) modulating factors, (a disintegrin and metalloproteinase established, solidifying the patho­ encompassing a variety of infectious, with a thrombospondin type 1 motif, physiologic distinction of aHUS from inflammatory, autoimmune, stress, or member 13), was reported 2 decades T T P. 9 The fact that TTP and aHUS drug-related conditions, that are linked ago.5 TTP is accompanied by a severe are rare—occurring in perhaps 2 to 4 epi­demiologically to both TTP and (<5%-10% of normal) deficiency in per million individuals—is a further aHUS, and that can injure the endo­ plasma ADAMTS13 activity, related impediment to an accurate diagnosis. thelium and, often, activate comple­ to an acquired autoantibody against The high morbidity and mortality ment. The latter would account, at least this protease or, in rarer instances, a associated with untreated TTP and in part, for the sporadic development congenital mutation affecting both aHUS, and the different manage­ of overt clinical signs and symptoms alleles of this enzyme. In contrast, ments they require, mandate timely of disorders predicated on congenital plasma ADAMTS13 activity may be recognition of a TMA, followed by the susceptibilities. For example, the first reduced from the normal range of 67% ability to distinguish between the 2 clinical manifestation of familial TTP, to 120% in aHUS or STEC-HUS, but conditions. This review highlights the or Upshaw-Schulman syndrome, re­ should still remain greater than 5% differences in pathophysiology and lated to loss-of-function gene muta­ to 10%, with the exact cut-off value diagnostic findings among the TMAs, tions in ADAMTS13, may not occur dependent upon the assay used. expanding the original 2012 article until late in adolescence, and relapses Until recognition of the path­­o­ with new references to genetic data are infrequent.10 Similarly, overt signs physiologic importance of ADAMTS13 and tissue biopsy. This information of the first episode of acute aHUS may in TTP-type TMAs, aHUS had been should permit a well-structured diag­ occur at any age—it has been recog­ grouped into a heterogeneous­ syn­ nostic approach to patients presenting nized in a 1-day-old newborn and an drome known as non–diarrhea-asso­ with a TMA, facilitating institution of 88-year-old adult11—despite the fact ciated (D–) HUS, or had been simply appropriate therapeutics. that the genetic inability to regulate

Clinical Advances in Hematology & Oncology Volume 14, Issue 11, Supplement 11 November 2016 3 REVIEW ARTICLE

Table 2. Complement-Activating are genetic defects in complement and aHUS.”19,20 This distinction is impor­ Conditions complement regulatory proteins that tant, as the conditions listed above can permit unregulated amplification of all lead to the signs and symptoms of a • Malignant hypertension the alternative complement pathway. TMA that is simply another manifesta­ • Pregnancy-associated The consequences are massive termi­ tion of that disease process itself, such – Preeclampsia and eclampsia nal complement pathway activation as systemic lupus erythematosus (SLE) – HELLP syndrome with generation of C5a (a potent or malignant hypertension, or associ­ • Autoimmune diseases anaphylatoxin) and C5b-9 (known as ated with pregnancy, drug therapy, or – SLE membrane attack complex [MAC]), other conditions. One must first treat – Scleroderma triggering inflammation, platelet that initial disease process adequately. – APLS activation, platelet aggregation, eryth­ If the TMA does not resolve, then con­ – Vasculitis rocyte lysis, endothelial cell injury, sider that it unmasked aHUS, which • Glomerulonephritis and fibrin microthrombus formation should then be viewed as the primary – C3GN throughout the microvasculature.13 cause of patient morbidity and there­ – IgA nephropathy Any TMA can be associated with fore treated. Making this distinction • Infection activation of the alternative comple­ may require special diagnostic tools, • Malignancy ment system. It is rare, however, for including tissue biopsy, as discussed in • Surgery or trauma the TMA to be sustained, except when section VI. this activation cannot be regulated—as Given the pathophysiologic dif­ • Drug therapy seen in aHUS.14 ferences between aHUS and TTP, one – Immunosuppressive agents – mTOR inhibitors In approximately two-thirds of might think that diagnostic criteria to – Chemotherapy aHUS cases, the TMA is manifested distinguish among the TMAs would be – Antitumor agents by a recognized complement-activat­ relatively simple to apply. Often they – Antimalarial agents ing condition, which causes endo­ are, and this is critical clinically, as it – Antiplatelet therapies thelial cell damage in concert with will guide management decisions. Just – Antiviral agents complement activation, in a person as mortality from TTP declined from – Oral contraceptives with the congenital inability to control more than 90% to less than 10% with – Illicit drugs complement. These conditions are institution of appropriate treatment— • Solid organ/bone marrow transplant listed in Table 2. They include infec­ therapeutic PE21—outcome is highly tions with a wide variety of microor­ unfavorable in inadequately treated APLS, antiphospholipid syndrome; C3GN, C3 glomerulonephritis;­ HELLP, hemolysis, ganisms (prominent ones are H1N1 aHUS. Up to 50% of patients progress elevated liver enzymes, low platelet influenza and those in the vaccines for to end-stage renal disease (ESRD) count; IgA, immunoglobulin A; mTOR, H1N1, adenovirus, cytomegalovirus, within a year, and 25% die during the mammalian target of rapamycin; SLE, Streptococcus pneumoniae, and STEC), acute phase, despite extensive PE.8,22 systemic lupus erythematosus. pregnancy, malignant hypertension, The importance of rapid diagnosis of surgery, organ and tissue transplant, patients with aHUS cannot be stressed the alternative complement pathway is and malignancy.15,16 They can also be more highly.2,23 present at birth. caused by the use of certain immu­ The vast majority of TTP cases nosuppressive drugs (cyclosporine, I. Seven Steps to Consider are idiopathic; disease susceptibility tacrolimus, sirolimus), cancer che­ in Reaching a Specific TMA results from an acquired, autoantibody- motherapeutic agents (gemcitabine, Diagnosis mediated deficiency of ADAMTS13. mitomycin C, cisplatinum, and the This leads to propagation of platelet vascular endothelial growth factor 1. Recognize a TMA. As outlined in aggregates, related to the inability to [VEGF] inhibitor bevacizumab), Table 1 and Figure 1, clinical recogni­ cleave long tethers of platelets bound platelet antagonists (ticlopidine and tion of a TMA involves documentation to ultra–high-molecular-weight multi­ clopidogrel), and the extended-release of the principle laboratory criteria for mers of vWF, which requires an intact form of oxymorphone.15,16 Even classic microangiopathic hemolytic anemia— ADAMTS13. The resultant systemic, TTP may unmask aHUS in a geneti­ schistocytes on peripheral blood smear, uncontrolled microthrombus forma­ cally susceptible individual.17,18 elevated LDH, low haptoglobin, ele­ tion is clinically devastating.12 By As reviewed in Section III.2, aHUS vated indirect bilirubin, and a decline contrast, in the vast majority of aHUS unmasked by a complement-activating in baseline hemoglobin—accompa­ cases, susceptibility factors are familial, condition should not be considered nied by thrombocytopenia. Not all of not acquired. As discussed below, they a “secondary TMA” or “secondary these changes are necessary to make a

4 Clinical Advances in Hematology & Oncology Volume 14, Issue 11, Supplement 11 November 2016 ATYPICAL HEMOLYTIC UREMIC SYNDROME (AHUS): AN ACCURATE DIAGNOSIS

Di erential Diagnosis for TMAs: aHUS, TTP, and STEC-HUS

Thrombocytopenia Microangiopathic hemolysis 3 Platelet count <150,000/mm AND Schistocytes and/or or >25% decrease from baseline Elevated LDH and/or Decreased haptoglobin and/or Decreased hemoglobin

Plus 1 or more of the following:

Neurologic symptoms Renal impairment Gastrointestinal symptoms Confusion and/or Elevated creatinine and/or Diarrhea ± blood and/or Seizures and/or Decreased eGFR and/or Nausea/vomiting and/or Stroke and/or Elevated blood pressure and/or Abdominal pain and/or Other cerebral abnormalities Abnormal urinalysis Gastroenteritis/pancreatitis

Cardiovascular symptoms Pulmonary symptoms Visual symptoms Myocardial infarction and/or Dyspnea and/or Pain and blurred vision Hypertension and/or Pulmonary hemorrhage and/or Retinal vessel occlusion Arterial stenosis and/or Pulmonary edema Ocular hemorrhage Peripheral gangrene

Evaluate ADAMTS13 activity and Shiga toxin/EHEC test

While ADAMTS13 results are awaited, a platelet count >30,000/mm3 and/or serum creatinine >1.7-2.3 mg/dL almost eliminates a diagnosis of severe ADAMTS13 deciency (TTP)

≤5% ADAMTS13 activity >5% ADAMTS13 activity Shiga toxin/EHEC positive

TTP aHUS STEC-HUS

Figure 1. Diagnosis of a thrombotic microangiopathy requires specific laboratory findings coupled with evidence of involvement of at least 1 organ system. Six such organ systems are illustrated here, but any tissue may be injured, with development of clinical signs related to microthrombosis and ischemia. Two possible exceptions are the lung, which is frequently involved in aHUS but rarely, if ever, structurally involved in TTP, and the skin, which is frequently involved at a histopathologic level in the TMAs but, apart from petechiae related to thrombocytopenia, presents gross clinical signs of ischemia in less than 5% of cases. In terms of hematologic parameters, median platelet counts in TTP are less than 20,000/mm3, whereas in aHUS, median counts are greater than 40,000/mm3, with many patients having values that appear within the normal range (>150,000/mm3), but that actually represent a greater than 25% decrease from their usual baseline (ie, the counts may normally be in the 250,000/mm3-350,000/mm3 range). ADAMTS13, a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13; aHUS, atypical hemolytic uremic syndrome; eGFR, estimated glomerular filtration rate; EHEC, E. coli; LDH, lactate dehydrogenase; STEC-HUS, Shiga toxin–producing E. coli hemolytic uremic syndrome; TMA, thrombotic microangiopathy; TTP, thrombotic thrombocytopenic purpura.

TMA diagnosis. (The rationale for this TTP and aHUS in the setting of active normal range in DIC, the bilirubin statement is discussed below.) DIC. All TMAs may be associated may not be elevated, at least initially, with thrombocytopenia, red cell frag­ in other TMAs as well, even in the • DIC must be ruled out by establish­ mentation, and release of D-dimers context of a markedly elevated LDH ing that the TMA is occurring in the and fibrin degradation products. If (see below). context of an INR and aPTT within DIC is present, the underlying condi­ the normal ranges. It is difficult, if not tion must first be corrected. Although • Schistocytes are the sine qua non of impossible, to distinguish between indirect bilirubin is often within the a TMA. However, they may be infre­

Clinical Advances in Hematology & Oncology Volume 14, Issue 11, Supplement 11 November 2016 5 REVIEW ARTICLE

quent on initial presentation, overlap­ gross blood is present in the stool. tually never directly involved in TTP, ping with levels in healthy controls of Given the variability in competence but frequently involved in untreated 1 or less per high-power microscopic among laboratories, both polymerase aHUS.26,27 field. Intact reticuloendothelial and chain reaction (PCR) and culture- splenic function is capable of clearing based assays for Shiga toxin–produc­ 4. Consider the absolute platelet red cells with damaged membranes— ing E. coli must be employed, using count and serum creatinine values. eg, schistocytes—from the periphery stool or a rectal swab. It is important The patient’s presenting platelet count for several days, despite other labora­ to determine if the laboratory tests and serum creatinine can serve as a tory and clinical manifestations of a for more than the common O157:H7 guide to whether TTP or aHUS is the TMA. This argues for daily review of STEC variant, as recent outbreaks in more likely diagnosis.28 In an “uncom­ the peripheral blood smear to evalu­ Europe have been linked to a much plicated” case, a platelet count greater ate changes in schistocyte frequency. less common isolate, O104:H4.14 than 30,000/mm3 at presentation is Biopsy may be useful in the absence Gastrointestinal signs and sym­p­ highly unusual in TTP, but classic for of peripheral schistocytes, as charac­ toms cannot be relied upon to distin­ aHUS.20,29 (An “uncomplicated” case teristic features of a TMA in tissue guish among the TMAs. Bloody diar­ is one that is not linked to an ongo­ can still be present, as discussed in rhea is a classic sign of STEC-HUS, ing complement-activating condition, section VI. but approximately 30% of aHUS and such as autoimmune disease, infection, An elevated LDH, usually at least TTP cases involve diarrhea, which can can­cer, and use of certain medications, 2 times the upper limit of normal be bloody, perhaps as a consequence all of which can unmask aHUS, but range,24 is also characteristic of a TMA. of colonic microinfarcts. The infec­ themselves may alter platelet counts At disease onset, its rise is a conse­ tious pathogens responsible for diar­ and renal function.) TTP is a platelet- quence of both hemolysis and tissue rhea are among the most potent acti­ consumptive disorder, and median ischemia. Once PE has been initiated, vators of the alternative complement platelet counts are less than 20,000/ LDH levels may decline dramatically pathway. They may unmask aHUS, mm3. In aHUS, however, typical fibrin in both TTP and aHUS, yet usually do and prolong its course, as breaching microthrombi, rather than platelet- not normalize in aHUS with PE alone. of intestinal epithelial barriers leads to rich clots, predominate, with less plate­ LDH isoenzyme analysis has shown microbial translocation and a positive let consumption. The median platelet that a substantial portion of LDH ele­ feedback loop for complement activa­ count in aHUS is 30,000/mm3 to vation in a TMA may be attributable tion. Indeed, STEC-HUS that does 40,000/mm3. Indeed, platelet counts to its release from tissues damaged as not respond to usual supportive care within the normal range occur in up a result of microthrombosis-associated may unmask aHUS in a susceptible to 20% of aHUS cases at presentation, ischemia, which is not corrected by PE patient.14 reflecting the fact that those “normal” in aHUS.25 This can also account for numbers may still indicate a greater the fact that in an acute TMA, LDH 3. Document clinical involvement of than 25% change from the patient’s elevations may be far out of proportion at least 1 organ system. Laboratory usual baseline.30 to the degree of red cell destruction abnormalities consistent with a TMA In terms of serum creatinine, suggested by minimal initial changes must be accompanied by clinical signs values greater than 2.3 mg/dL (200 in indirect bilirubin or hemoglobin. linked to at least 1 organ system. Fig­ μmol/L) are unusual in TTP. The ure 1 lists the 6 most common sites, adjusted odds ratio for aHUS vs TTP • Decreased or undetectable serum but both aHUS and TTP can affect is 9.1 for serum creatinine values haptoglobin levels are classic for any any tissue. In approximately 20% of exceeding 1.7 mg/dL to 2.3 mg/dL hemolytic anemia. However, hapto­ initial aHUS presentations, there is (150-200 μmol/L).20,29 globin may fall within the normal little to no involvement in terms of range despite an active TMA. This serum creatinine, despite the word 5. Evaluate ADAMTS13 activity. relates to the role of haptoglobin as an uremic in the disease’s name, although TTP can be distinguished from aHUS acute-phase reactant, and underlies the microscopic hematuria and microal­ on the basis of ADAMTS13 testing. recommendation to not utilize hapto­ buminuria are usually present. In con­ Reductions to less than 5% to 10% of globin as a diagnostic criterion for a trast, classic TTP involves the kidneys normal activity levels are characteris­ TMA.24 in more than 50% of cases, although tic of TTP, but not seen in aHUS or with much less severity than aHUS STEC-HUS. An increasing number 2. Rule out STEC-HUS. In the pres­ or HUS. The one possible exception of university hospitals are conduct­ ence of a TMA and diarrhea, STEC to universal tissue involvement in the ing these assays in-house, permitting needs to be evaluated, whether or not TMAs relates to the lung, which is vir­ results to be available within hours. It is

6 Clinical Advances in Hematology & Oncology Volume 14, Issue 11, Supplement 11 November 2016 ATYPICAL HEMOLYTIC UREMIC SYNDROME (AHUS): AN ACCURATE DIAGNOSIS

critical to draw blood for ADAMTS13 TMA.21 If an apheresis station is not a typical case of TTP with high titer analysis prior to instituting PE, as immediately available, and renal func­ anti-ADAMTS13 IgG autoantibod­ interpretation of values obtained after tion permits, fresh frozen plasma (FFP) ies, it is highly unlikely that 1 or 2 initiation of plasma therapy is difficult. infusions may be initiated instead, cycles of PE can raise levels of enzyme (This concern is expanded upon in awaiting eventual PE. Plasma therapy activity from a TTP diagnostic level Section II.) One should also recognize is continued pending ADAMTS13 of less than 5% to greater than 20% that any disease process that injures activity results. Based on those results, (Han-Mou Tsai, MD, personal com­ endothelium thereby releases vWF there are 3 possibilities. The first 2 are munication). However, this possibility multimers into the circulation, which straightforward: is still an important consideration. The can bind to plasma ADAMTS13 and major effect of plasma infusions or PE reduce levels to below the normal 1. TMA in the setting of an in TTP is restoration of a functional range for a healthy individual. How­ ADAMTS13 activity less than 5%. ADAMTS13 enzyme. After several ever, absent TTP, these conditions will If the ADAMTS13 activity is less than cycles of plasma, particularly in the not reduce ADAMTS13 activity to the 5% of normal control levels (or <10% setting of a low-titer ADAMTS13 TTP diagnostic range of less than 5% in some assays), the diagnosis is TTP, inhibitor, the patient will have some to 10%, nor would they be linked to and therapeutic PE should be contin­ exogenous enzymes derived from an ADAMTS13 inhibitor. ued. The presence of acquired inhibi­ donor plasma. tors of ADAMTS13, most commonly Alternatively, what if the treating 6. Cobalamin C deficiency–associ- immunoglobulin G (IgG) autoanti­ physician believes that the diagnosis of ated TMA, diagnosed by high plasma bodies, should also be evaluated. They TTP is firm despite ADAMTS13 levels levels of homocysteine and methyl- are detectable in 80% to 90% of TTP greater than 5% to 10%, based on his or malonic acid, should also be consid- patients with acquired ADAMTS13 her interpretation of certain published ered once STEC-HUS and TTP have deficiency,31 but are not present in studies? In one series, ADAMTS13 been ruled out by the above tests. patients with congenital ADAMTS13 activity of less than 5% was seen in This is particularly pertinent if the deficiency. Additional treatments for only 33% of patients with “idiopathic reticulocyte count is low, although the recalcitrant TTP, including various T T P.” 32 In a parallel study, 29% of proinflammatory state characteristic of immunosuppressive regimens, should patients diagnosed with idiopathic aHUS may also be associated with a be considered if only minor responses or secondary TTP “responsive” to PE low reticulocyte count. in laboratory and clinical abnormali­ did not have a severe ADAMTS13 ties defining the TMA are seen after deficiency.33 Furthermore, some hema­ 7. Review the kidney biopsy, if 3 to 9 PEs, each representing replace­ tologists reject the term aHUS as it obtained for reasons other than ment of 1.0 to 1.5 plasma volumes or “lacks both specificity and a suggestion defining a TMA, or consider obtain- approximately 20 to 40 L of FFP. (The of cause,” with “nonspecific diagnostic ing a biopsy of the kidney or other meaning of “response” is discussed criteria,” including microangiopathic organs/tissues in a complicated case. below.) hemolytic anemia, thrombocytopenia, aHUS and TTP are clinical diagnoses. and ADAMTS13 activity at or greater However, histopathology and immu­ 2. TMA in the setting of an than 5%, which “may also occur in all nohistochemistry (IHC) may be useful ADAMTS13 activity greater than other primary TMA syndromes.”34 If in helping to identify a TMA in dif­ 5% to 10%. If the ADAMTS13 there is any doubt about the diagnosis ficult cases, as outlined in sections III.2 activity is greater than 5% to 10%; of aHUS, despite ADAMTS13 levels and VI. there are no complicating, untreated greater than 5% to 10%, additional complement-activating conditions methods to distinguish among the II. Distinguishing Between present (in that scenario, see section TMAs should be rapidly pursued. aHUS and TTP: Additional III.2); and cobalamin C deficiency has Four strategies to further solidify the Considerations been excluded then the diagnosis is diagnosis of aHUS are outlined below. aHUS. Plasma-based treatment should A new patient presenting with labo­ be stopped. III. Additional Methods to ratory and clinical signs of a TMA, Distinguish Between TTP recognized by the criteria in the first 2 3. TMA in the setting of an and aHUS rows of Figure 1, usually begins treat­ unknown/unreliable ADAMTS13 ment with plasma therapy. PE, rather result.­­ But what if an ADAMTS13 1. Response to PE. A clinically signifi­ than plasma infusion, is the initial assay was not obtained, and the patient cant response to PE involves improve­ standard of care for an undifferentiated has undergone several cycles of PE? In ment or complete correction in the

Clinical Advances in Hematology & Oncology Volume 14, Issue 11, Supplement 11 November 2016 7 REVIEW ARTICLE

ADAMTS13 activity >5% (n=22) ADAMTS13 activity ≤5% (n=22)

100 95% 95%

75 71% 71% 62%

50

Patients (%) Patients 25% 23% 25

n=15/21 n=21/22 n=13/21 n=21/22 n=3/12 n=5/7 n=5/22 0% 0 Platelet Count Recovery LDH Normalization Renal Function Recovery Deaths (platelet count >150,000/µL (normal LDH (normal creatinine level by day 21) by day 21) by day 21)

Figure 2. Outcomes in a series of 44 TMA patients, diagnosed and treated pre-2011. Twenty-two patients were diagnosed with TTP on the basis of ADAMTS13 activities at or less than 5%, and 22 had ADAMTS13 values greater than 5%. The high mortality in the patients without ADAMTS13 deficiency, with 5 patients dying over the 21-day follow-up period, is in stark contrast to no deaths in the TTP group. (The causes of death included NSTEMI/aspiration pneumonia, NSTEMI/abdominal abscess, multiorgan failure, respiratory failure, and sepsis.) ADAMTS13, a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13; LDH, lactate dehydrogenase; NSTEMI, non-SDT wave elevation myocardial infarction; TMA, thrombotic microangiopathy; TTP, thrombotic thrombocytopenic purpura. Adapted from Pishko AM, Arepally GM. Predicting the temporal course of laboratory abnormality resolution in patients with thrombotic microangiopathy [ASH abstract 4192]. Blood. 2014;124(suppl 21).42 parameters of Figure 1. A complete In an additional 31% of patients, 1 or H (CFH) and complement factor I response is defined by the following: 2 further cycles of PE were required (CFI), the 2 most commonly mutated normalization of the hemoglobin, to effect a complete response.21 This genes in aHUS, to effect those tran­ LDH, and platelet count, and a is similar to many later trials of PE, sient changes.8,39 But, unlike the use decrease of at least 25% from baseline where remissions were obtained with a of PE in TTP, serum creatinine rarely in serum creatinine after plasma ther­ mean number of PEs of 19 ± 17 in one normalizes, and the risk of ESRD and apy has been completed. Guidelines as study,36 and a median of 9 PEs (mean death is not altered. to the median times for response to PE cumulative infused FFP of 43 ± 77 L) One can document this differen­ for the various signs and symptoms of in another.37 tial response to PE in TTP vs aHUS TTP have been published.35 However, approximately 80% of in the laboratory. Normalization of the Response may also be defined patients with aHUS may also have platelet count following PE in a TTP in terms of the amount of plasma dramatic responses to PE, based patient results in normally functioning required. The first randomized study of upon normalization in platelet count, platelets, with little expression of acti­ PE vs plasma infusion in TTP (defined hemoglobin, and haptoglobin, and a vation markers, such as P-selectin, as clinically and by serum creatinine <1.6 decline, but often not a normalization, assessed by flow cytometry. In contrast, mg/dL, but without the benefit of in LDH.2,13 Yet tissue damage per­ PE also often leads to normalization of ADAMTS13 testing), demonstrated sists, and maintenance of even those platelet counts in an aHUS patient, the superiority of PE over plasma infu­ responses usually requires continued but platelet activation persists, with sion.21 Forty-seven percent of those PE.13 The variability in initial response high expression of P-selectin40 reflected receiving PE had a complete response rates is dependent upon the nature clinically in continued organ system after the first treatment cycle, which of the complement-related mutation involvement.41 involved an average of 21.5 ± 7.8 L involved,13,38 as FFP contains suffi­ In summary, if a patient diag­ of FFP exchanged throughout 9 days. cient quantities of complement factor nosed as having TTP has a limited

8 Clinical Advances in Hematology & Oncology Volume 14, Issue 11, Supplement 11 November 2016 ATYPICAL HEMOLYTIC UREMIC SYNDROME (AHUS): AN ACCURATE DIAGNOSIS

Complement-Activating Conditions Can Unmask aHUS

Uncontrolled Complement Ampli cation

• Malignant hypertension • Pregnancy-associated Endothelial damage • Autoimmune Complement Defective Complement Platelet activation TMA • Infection activation Regulatory Proteins Thrombin generation • Surgery/trauma Gastrointestinal symptoms • Medicationa • Transplant (organ/ bone marrow) Feedback Ampli cation

Figure 3. aHUS is a TMA linked to the inability to regulate the alternative complement pathway. This defect sets up a positive feedback loop among generation of the terminal complement components C5a (an anaphylatoxin) and C5b-9 (MAC), endothelial cell activation/ injury, and thrombin generation. In approximately two-thirds of cases, a complement-activating condition can be temporally linked to an unmasking of aHUS in a susceptible individual. However, some of those disorders—such as those listed in the box at the left of the diagram—can themselves cause a TMA-like disorder. It is imperative to treat those conditions, but if the signs and symptoms of the TMA do not resolve following such therapy, one should consider a diagnosis of unmasked aHUS. In some situations, renal, cutaneous, or other tissue biopsy may be required. aEspecially calcineurin, mTOR inhibitors, ticlopidine, clopidogrel, and certain cancer chemotherapies. aHUS, atypical hemolytic uremic syndrome; MAC, membrane attack complex; mTOR, mammalian target of rapamycin; TMA, thrombotic microangiopathy. Adapted from Saab K et al. Thrombotic microangiopathy in the setting of HIV infection: a case report and review of the differential diagnosis and therapy. AIDS Patient Care STDs. 2016;30(8):359-364.44

response to PE, or is requiring quan­ greater than 5%—what we would ADAMTS13 deficiency.42 These data tities of plasma exceeding those out­ now recognize as aHUS. Cases linked were recently corroborated by a review lined above for TTP, it is prudent to to chemotherapeutic agents or bone of 186 adult patients entered into the reevaluate the diagnosis and consider marrow transplant were excluded. All Harvard TMA Research Collaborative aHUS. It is critical to recognize that patients were treated with PE. There registry with a TMA thought to be the majority of aHUS patients treated were no deaths reported among the clinically “suggestive of TTP,” but with with PE alone may have a complete or 22 patients in the “ADAMTS13- ADAMTS13 activities exceeding near-complete hematologic remission deficiency” group, but 5 deaths 10%.43 The authors concluded that yet go on to develop ESRD or die. among the 22 patients in the “with­ outcomes were not improved by the That is, they may lose an organ, or die, out–ADAMTS13-deficiency” cohort use of PE in this setting. but do so with normal lab numbers. throughout an observation period This was illustrated by a retrospective of 21 days (Figure 2). In addition, 2. Diagnosing aHUS in the setting of review of TMA patients treated with 71% of patients with ADAMTS13 a complement-activating condition. PE at Duke University from 2007 deficiency normalized their serum An additional source of misdiagnosis to mid-2013.42 The “ADAMTS13- creatinine by day 21, compared with or delay in diagnosis may be failure to deficiency” group included patients only 25% of the patients without consider that a variety of conditions with a TMA occurring in the context severe ADAMTS13 deficiency (Figure that activate complement, both physi­ of ADAMTS13 less than 5%—those 2). Yet while 95% of patients with ologic and pathologic, can present with TTP; whereas the patients in the ADAMTS13 deficiency normalized with signs and symptoms character­ “without–ADAMTS13-deficiency” their platelet counts and LDH, so did istic of a TMA. As outlined in Table group had a TMA and ADAMTS13 approximately 70% of patients without 2, these conditions include pregnancy;

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the hemolysis, elevated liver enzymes, • Finally, discontinue or change medi­ acteristic of malignant hypertension, and low platelets (HELLP) syndrome; cations linked to aHUS. In terms of is a not uncommon feature of aHUS infection; autoimmune diseases, such the calcineurin or mammalian target (Manuel Praga, MD, personal obser­ as SLE and scleroderma; malignant of rapamycin (mTOR) inhibitors, this vation). hypertension; organ and tissue trans­ is typically for at least one half-life (7-8 plant; and the use of certain medica­ days and 3-4 days, respectively). In • Bone marrow transplant: TMAs per­ tions. If those signs and symptoms of addition, treat the underlying comple­ sisting despite discontinuation of graft- TMA do not resolve once the comple­ ment-activating condition for as long vs-host disease (GVHD) prophylaxis ment-activating condition has been as your experience suggests you should involving an mTOR or calcineurin treated and medications associated continue that intervention. If your inhibitor, and resolution of an under­ with aHUS have been discontinued or treatments are working and the TMA lying infection or GVHD, if present, changed to other drugs, recognize that has resolved, then no other therapy are often aHUS.48 it may have unmasked aHUS or TTP would be required. However, bear in and undertake appropriate diagnostic mind the initial differential diagnosis • Drugs: TMAs temporally related procedures to investigate that pos­ that led to consideration of a TMA to certain chemotherapeutic agents, sibility. unmasked by the complement-activat­ VEGF inhibitors, anti-GVHD med­ Figure 3 illustrates a general ing condition, in case the TMA should ications, and extended-release opiates,­ scheme by which one can consider recur. Alternatively, if those treatments which do not resolve following with­ the possibility of aHUS arising in the do not mitigate the TMA, and TTP drawal of those agents, are usually setting of what appears to be an acute has been ruled out by ADAMTS13 aHUS, not TTP. TMAs temporally presentation of a new complement- testing, then conclude that aHUS is related to the antiplatelet agent ticlo­ activating condition, or exacerbation involved. pidine, and possibly also to clop­ of an existing one.44 Major examples of a complement- idogrel, may unmask aHUS in a activating condition are: genetically susceptible individual even • First, establish the existence of a if ADAMTS13 activity and inhibi­ microangiopathic hemolytic anemia. • Pregnancy: TMAs occurring during tor levels are classic for a TTP-type This is usually based on interpreta­ pregnancy, which resolve following TMA.18 Indeed, whenever thrombin tion of the peripheral blood smear, pregnancy termination, are virtually is generated in an acute coagulation with documentation of schistocytes always TTP (with approximately disorder, there is a positive feedback and thrombocytopenia, as defined in one-quarter previously undiagnosed loop between thrombin generation; Figure 1. Renal biopsy is usually not con­genital TTP cases).46 TMAs occur­ C5 cleavage, as thrombin functions performed, even if there appears to ring late in the third trimester, or post­ as a C5 convertase49; and formation be minimal risk of thrombosis linked partum, are usually aHUS.46 TMAs of of C5a and C5b-9. In an individual to the use of platelet transfusions to HELLP syndrome that occur late in genetically susceptible to aHUS, TTP perform the biopsy.45 However, a TMA preg­nancy and do not resolve once the could thereby unmask aHUS, and it can be diagnosed on renal biopsy in preg­nancy was terminated are usually is the latter disease process that may the absence of peripheral blood schis­ aHUS. be responsible for the clinical se­ tocytes, based on the clinical setting quelae.17,18 and findings of subendothelial edema • SLE: What appeared to be exacer­ in arterioles and glomerular capil­ bation of lupus nephritis in a patient IV. Complement Genetics lary loops, which may or may not be with known SLE was instead aHUS- associated with luminal fibrin micro­ linked renal failure, with evidence of The complement system is an essen­ thrombi. This subject, and the utility microthrombi on renal biopsy. tial part of innate immunity, with of staining for C5b-9, is discussed in critical roles in response to pathogens detail in section VI. • Malignant hypertension: Renal func­ and the removal of cell debris and tion should improve in the vast major­ immune complexes. Complement is • Second, eliminate TTP if the ity of patients with associated acute activated through the classic, lectin, ADAMTS13 is greater than 5% to kidney injury once blood pressure is and alternative pathways, resulting in 10%. brought under control.47 Persistence the formation of unstable bimolecular of anemia, thrombocytopenia, and complexes called C3 convertases. C3 • Third, as outlined below, recognize renal injury should raise the suspicion can hydrolyze spontaneously into that certain complement-activating of aHUS unmasked by the malignant C3(H2O), a molecule that mimics the conditions are much more likely to be hypertension. In addition, grade III C3 cleavage product C3b and confers linked to one type of TMA. to IV hypertensive retinopathy, char­ upon the alternative pathway the

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ability to bypass a particular activator. of aHUS patients, with anti-CFH Mukherjee, in his monumental work It is always “on,” at a low level. Under autoantibodies leading to decreased The Gene, stated that a mutation “. . . physiologic conditions, this low-level CFH function.22,52 More than 90% can provide no real information about activation is controlled by comple­ of these patients are homozygous for a disease or disorder. The definition ment regulatory proteins, both soluble a common polymorphism that deletes of disease rests, rather, on the specific and present on the surface of most CFHR3 and CFHR1 genes. An aHUS disabilities caused by an incongru- eukaryotic cells. Pathogens do not usu­ patient with such mutations should be ity between an individual’s genetic ally have complement regulators on evaluated for an anti-CFH autoanti­ endowment and his or her current their surfaces to inhibit this spontane­ body.52 environment. . . .”56 ous activation. Incorporation of C3b Mutations cannot be identified In making a decision to order molecules to surface-bound C3 con­ in cases of aHUS 30% of the time.51 genetic testing, one must recognize that vertases then generates C5 convertases, Expansion of genetic analyses among it may take months. As noted above, cleaving C5 and leading to formation patients with aHUS, particularly 30% of the time a negative result will of C5a (an anaphylatoxin) and C5b-9 those enrolled in international aHUS be obtained, often with the puzzling (MAC), setting up an inflammatory registries, and pursuit of laboratory comment that a genetic variant in a response and destroying the pathogen. studies to functionally characterize complement gene “of unknown sig­ Regulation of such alternative comple­ genetic variants not yet recognized as nificance” is present.57 But it can be ment pathway activation is a complex pathogenic, will be critical to improv­ of value in family genetic counseling, process involving 2 soluble proteins, ing diagnostic criteria for aHUS. as carriers might be closely monitored CFH and CFI, previously mentioned For example, genotyping for risk during conditions triggering marked in the context of FFP infusions, and hap­lotypes CFH-H3 and MCPggaac complement activation, such as sur­ several membrane-bound proteins: should be pursued, along with evalua­ gery, trauma, infection, malignancy, mem­brane cofactor protein (MCP; tion of copy number variation, hybrid and pregnancy. Concerning long-term CD46), thrombomodulin (THBD), genes, and other complex genomic prognosis, certain mutations, particu­ complement receptor 1 (CR1; CD35), rearrangements.53-55 larly in MCP, may be associated with and decay-accelerating factor (DAF; It is understandable that failure milder disease and fewer relapses.22 CD55). The activity of these molecules to identify a recognized mutation in preserves complement homeostasis and a patient with aHUS might generate V. Circulating Complement prevents endothelial cell activation and uncertainties regarding diagnosis and/ Levels injury, platelet activation and aggrega­ or treatment duration, but it should tion, and inflammation. Many of these not. Not finding a mutation function­ Measurements of complement and proteins are encoded by genes within ally characterized in the literature as soluble complement regulatory pro­ a cluster known as RCA (Regulator of pathogenic does not exclude dysregu­ tein levels in plasma or serum are Complement Activation) on human lation of the alternative pathway, nor unreliable markers for aHUS. A low chromosome 1q32.50 prove that a genetic component is not C3 level accompanied by a normal or Over the past 15 years, the involved. elevated C4 level would be classic for analy­sis of hundreds of aHUS Incomplete penetrance must also activation of the alternative comple­ patients through international col­ be considered. A key paradigm in using ment pathway, which underlies aHUS, laborative studies has established that genotyping to diagnose a clinical dis­ but this pattern is only occasionally approximately 70% carry identifiable ease (the phenotype) is the realization observed. Serum C3 is normal in up genetic abnormalities that alter the that gene mutations or genetic vari­ to 80% of aHUS patients, and C3 and regulation of the alternative pathway. ants are not the sole determinants of C4 levels are too variable for diagnostic These mutations are heterozygous phenotype. The environment can be purposes.30,58 Complement can also be in approximately 90% of cases, and critical to the incomplete penetrance activated in TTP, leading to elevated include pathogenic variants in CFH, of aHUS. For example, this may levels of C5a and C5b-9, as in aHUS.59 CD46, CFI, C3, complement factor involve the type and degree of daily Measurement of C5b-9 in urine or B (CFB), THBD, CFHR1, CFHR3, pathogen exposure in different cities properly processed plasma—recogniz­ diacylglycerol kinase-ε (DGKE), and or countries of residence; physiologic ing the need for rapid freezing of a plasminogen.51 Most lead to loss of triggers of complement activation, plasma sample, given the short half-life protein function, with the exception such as pregnancy; and chance, linked of C5b-9—may help identify a TMA, of those in C3 and CFB, identified in to divergent epigenetic modifications but it cannot distinguish a primary 15% of aHUS cases, which are gain-of- or random fluctuations in unrecog­ TMA from a related complement- function mutations.22,38 There may be nized complement or thrombosis activating condition, nor TTP from an acquired component in 7% to 10% regulatory molecules. Dr Siddhartha aHUS.59

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A B C

D E

Figure 4. (A) Immunohistochemistry for terminal complement component complex C5b-9 deposition in a cutaneous punch biopsy obtained from a 33-year-old man with aHUS linked to an allogeneic hematopoietic stem cell transplant. Reprinted from Chapin J et al. Clin Adv Hematol Oncol. 2014;12(9):565-573.48 (B) Histology of a renal biopsy in a 30-year-old woman with aHUS and a C3 and thrombomodulin mutation. Courtesy of Dr Hermann Haller. (C) Immunohistochemistry showing C5b-9 deposition in the renal biopsy of the same patient as in panel A. Courtesy of Dr Hermann Haller. (D) Classic findings of a TMA, defined histologically, on a renal biopsy of a 43-year-old man with a TMA associated with malignant hypertension. Findings include subendothelial edema with luminal fibrin thrombi (arrows). Courtesy of Dr Helen Liapis, Arkana Laboratories. (E) A more subtle change characteristic of a TMA on light microscopy of a renal biopsy in a 52-year-old woman with a TMA associated with antiphospholipid syndrome. Findings include glomerular mesangiolysis (arrow) and membranoproliferative glomerulonephritis, which may be recognized in the absence of fibrin microthrombi. Courtesy of Dr Helen Liapis, Arkana Laboratories.

VI. Examining Tissue TTP. In contrast, biopsy of similar sites antemortem. The gingiva, rectum, or in an aHUS patient typically reveals skin are the suggested sites to sample, Biopsies can be useful in difficult “red clots” in which fibrin predomi­ whether or not there is a visible lesion. diagnostic situations. Sampling of any nates, and an inflammatory infiltrate Several groups have utilized cutane­ accessible, highly vascular site may help may be seen31,60 together with deposits ous punch biopsies in confirming an inform the differential diagnosis of a of C5b-9.39,61,62 This may account for aHUS diagnosis,46,61,62 even though TMA, based on pathologic distinctions the higher median platelet counts skin is rarely involved clinically in between TTP and aHUS. The micro­ characteristic of aHUS, as well as the aHUS.63 Figure 4A illustrates the clas­ thrombi of TTP are typically “white profound fatigue often reported by sic immunohistopathology in aHUS, clots,” composed of platelets and vWF, patients, given the generalized inflam­ based on a random cutaneous punch with only small amounts of fibrin; vas­ matory state induced by aHUS, with biopsy. cular or perivascular inflammatory cell unregulated cleavage of C5 releasing Renal tissue can also be sampled,64 infiltrations are minimal or absent.60 the anaphylatoxin C5a. and may be diagnostic in the setting As noted earlier, the consumption of These differences have been rec­ of many complement-activating con­ platelets in these clots helps explain ognized postmortem for decades,60 ditions. This point was emphasized the marked thrombocytopenia seen in but until recently were rarely explored in section III.2. However, it may be

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technically difficult to perform such Summary complement pathway, which activates a procedure in the presence of throm­ platelets and damages endothelium. bocytopenia. It should also be noted • aHUS is a rare, life-threatening, sys­ This defect is related to a genetic defi­ that aHUS cannot be distinguished temic disease. When unrecognized or ciency in one or more soluble and/or from STEC-HUS on the basis of renal inappropriately treated, it has a high membrane-bound complement regula­ pathology alone.13 Biopsy results using degree of morbidity and mortality. tory proteins. CFH is most frequently light microscopy are typically avail­ implicated. Acquired autoantibodies able in 24 to 48 hours, with further • The clinical presentation of the 4 against CFH have been identified in information derived from electron major TMAs—aHUS, STEC-HUS, some cases, with greater than 90% of microscopy and immunofluorescence TTP, and DIC—can be very similar. such patients homozygous for a poly­ assay (IFA) or IHC. Classic findings DIC must be recognized and treated morphism deleting CFHR3 and/or of a TMA consist of endothelial cell before one is able to reliably dis­ CFHR1 genes. swelling and denudation in arterioles criminate among the other 3 types of and glomerular capillary loops, often TMAs. STEC-HUS can be ruled out • The lung is often involved in aHUS, associated with luminal fibrin thrombi using both culture and PCR-based but is rarely, if ever, structurally (Figures 4B and 4D). As with cutane­ assays of stool samples or rectal swabs involved in TTP. ous biopsies, C5b-9 deposition on for Shiga toxin–producing E. coli. microvessels may be seen in renal biop­ • In an uncomplicated case—ie, one sies of an aHUS patient (Figure 4C). • A TMA may be documented in not associated with an ongoing com­ Glomerular mesangiolysis (Figure 4E) 2 ways: plement-activating disease state that and membranoproliferative glomeru­ has “unmasked” aHUS—a platelet lonephritis are more subtle findings of – Clinically, by signs of a microangio­ count greater than 30,000/mm3 and/ an acute or chronic TMA, respectively. pathic hemolytic anemia (schistocyto­ or a creatinine greater than 2.3 mg/ Those findings should raise the suspi­ sis on peripheral blood smear, elevated dL is highly indicative of aHUS rather cion of a TMA even without evidence LDH, elevated indirect bilirubin, and than TTP. of thrombi in a biopsy sample. Occa­ low haptoglobin) along with thrombo­ sionally, diffuse or segmental cortical cytopenia and clinical involvement of • Two-thirds of aHUS cases are associ­ necrosis may also be seen, particularly at least 1 organ system. ated with an identifiable complement- in catastrophic antiphospholipid syn­ activating condition. These condi­ drome. Electron microscopy offers – Pathologically, with endothelial cell tions include autoimmune diseases, further clues, with recognition of swelling, subendothelial accumulation infection, malignant hypertension, endothelial cell swelling, necrosis, and of electron-lucent material and, in the pregnancy, organ or tissue transplant, distinction between platelet and fibrin case of renal biopsy, mesangiolysis and and treatment with chemotherapeutic microthrombi. double contours of the glomerular or anti-GVHD medications. In order It must be emphasized that clini­ basement membrane. If microthrombi to distinguish aHUS unmasked by cal trials have not authenticated the are present, the predominance of fibrin those conditions from the labora­ sensitivity and specificity of C5b-9 vs platelet-rich clots in cases of aHUS tory and clinical signs of a TMA that staining of any tissue in distinguish­ vs TTP, respectively, should be consid­ simply reflects the pathology of those ing TTP from aHUS, or in classifying ered. IFA or IHC may demonstrate conditions, one must treat the comple­ a TMA arising in the setting of any C5b-9 deposition in aHUS. ment-activating condition, and assess complement-activating disorder. On whether the TMA has resolved. If not, occasion, the use of light and electron • aHUS can be distinguished from and ADAMTS13 is greater than 5% microscopy together with IFA or TTP on the basis of ADAMTS13 to 10% and cobalamin C deficiency IHC for C5b-9 may be necessary to activity, with a severe decrease (<5%- has been excluded, consider that the fully evaluate a biopsy. For example, a 10% of control) characteristic of TTP. primary cause of pathology is aHUS recent report documented deposition It is important to order this test in any and treat accordingly. of C5b-9 in the microvasculature of patient presenting with laboratory and renal biopsies from 2 infants with con­ clinical signs of a TMA before plasma • Assessment of circulating comple­ genital TTP. However, vWF tethered therapy is initiated. ment levels, including C3, C4, C5a, “white” platelet clots were also present, and C5b-9, is of low diagnostic value. which are classic for TTP but not part • aHUS is distinct pathophysiologi­ of the histology, or pathophysiology, of cally from TTP. It results from chronic, • Genetic testing may be useful in aHUS.65 uncontrolled activity of the alternative patients with suspected aHUS. The

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identification of a pathogenic genetic Oechslin R. Hämolytisch-urämische syndrome: bilat­ March 19, 2016.] J Nephrol. doi:10.1007/s40620-016- variant in complement or comple­ erale nierenrindennekrosen bei akuten erworbenen 0288-3. hämolytischen anämien. Schweiz Med Wochenschr. 24. Ho VT, Cutler C, Carter S, et al. Blood and mar­ ment-regulatory protein, or in anti- 1955;85(38-39):905-909. row transplant clinical trials network toxicity commit­ CFH autoantibodies, reinforces the 8. Rossi EC. Plasma exchange in the thrombotic micro­ tee consensus summary: thrombotic microangiopathy diagnosis and may provide prognostic angiopathies. In Rossi EC, Simon TL, Moss GS, Gould after hematopoietic stem cell transplantation. Biol Blood SA, eds. Principles of Transfusion Medicine. 2nd ed. Marrow Transplant. 2005;11(8):571-575. information and/or help guide treat­ Baltimore, MD: Williams and Wilkins; 1996. 25. Cohen JA, Brecher ME, Bandarenko N. Cellular ment duration. 9. Caprioli J, Noris M, Brioschi S, et al; International source of serum lactate dehydrogenase elevation in Registry of Recurrent and Familial HUS/TTP. Genetics patients with thrombotic thrombocytopenic purpura. J of HUS: the impact of MCP, CFH, and IF mutations Clin Apher. 1998;13(1):16-19. • Biopsy of any highly vascular site, on clinical presentation, response to treatment, and 26. Mitra D, Jaffe EA, Weksler B, Hajjar KA, Soderland including the skin, gingiva, rectum, outcome. Blood. 2006;108(4):1267-1279. C, Laurence J. Thrombotic thrombocytopenic purpura or kidney, with IHC or IFA for C5b-9 10. Levy GG, Nichols WC, Lian EC, et al. Muta­ and sporadic hemolytic-uremic syndrome plasmas tions in a member of the ADAMTS gene family induce apoptosis in restricted lineages of human micro­ deposition on microvessels can be a cause thrombotic thrombocytopenic purpura. Nature. vascular endothelial cells. Blood. 1997;89(4):1224- useful adjunct to diagnosis in difficult 2001;413(6855):488-494. 1234. cases. 11. Sullivan M, Rybicki LA, Winter A, et al. Age-related 27. Hofer J, Rosales A, Fischer C, Giner T. Extra-renal penetrance of hereditary atypical hemolytic uremic syn­ manifestations of complement-mediated thrombotic drome. Ann Hum Genet. 2011;75(6):639-647. microangiopathies. Front Pediatr. 2014;2:97. • Unlike TTP, plasma therapy has no 12. Vesely SK, George JN, Lämmle B, et al. 28. Mannucci PM, Cugno M. The complex differential role in the long-term management of ADAMTS13 activity in thrombotic thrombocytope­ diagnosis between thrombotic thrombocytopenic pur­ nic purpura-hemolytic uremic syndrome: relation to pura and the atypical hemolytic uremic syndrome: lab­ aHUS. If a putative TTP patient is not presenting features and clinical outcomes in a pro­ oratory weapons and their impact on treatment choice responding to plasma therapy by all spective cohort of 142 patients. Blood. 2003;102(1): and monitoring. Thromb Res. 2015;136(5):851-854. clinical criteria, or requires pro­longed 60-68. 29. Coppo P, Schwarzinger M, Buffet M, et al; French 13. Noris M, Remuzzi G. Atypical hemolytic-uremic Reference Center for Thrombotic Microangiopathies. PE to effect or maintain a remission, syndrome. N Engl J Med. 2009;361(17):1676-1687. Predictive features of severe acquired ADAMTS13 defi­ reevaluate the diagnosis and consider 14. Noris M, Mescia F, Remuzzi G. STEC-HUS, atypi­ ciency in idiopathic thrombotic microangiopathies: the aHUS. cal HUS and TTP are all diseases of complement activa­ French TMA reference center experience. PLoS One. tion. Nat Rev Nephrol. 2012;8(11):622-633. 2010;5(4):e10208. 15. Lapeyraque A-L, Malina M, Fremeaux-Bacchi V, et 30. Noris M, Caprioli J, Bresin E, et al. Relative role al. Eculizumab in severe Shiga-toxin-associated HUS. of genetic complement abnormalities in sporadic and Acknowledgment and Disclosures N Engl J Med. 2011;364(26):2561-2563. familial aHUS and their impact on clinical phenotype. 16. Waters AM, Licht C. aHUS caused by complement Clin J Am Soc Nephrol. 2010;5(10):1844-1859. This article was supported by funding dysregulation: new therapies on the horizon. Pediatr 31. Tsai H-M. Pathophysiology of thrombotic throm­ from Alexion Pharmaceuticals, Inc. This Nephrol. 2011;26(1):41-57. bocytopenic purpura. Int J Hematol. 2010;91(1):1-19. article reflects the opinions and views 17. Tsai E, Chapin J, Laurence JC, Tsai HM. Use of 32. Vesely SK, George JN, Lämmle B, et al. eculizumab in the treatment of a case of refractory, ADAMTS13 activity in thrombotic thrombocytopenic of the authors and was developed with ADAMTS13-deficient thrombotic thrombocytopenic purpura-hemolytic uremic syndrome: relation to pre­ minimal editorial support from Alexion. purpura: additional data and clinical follow-up. Br J senting features and clinical outcomes in a prospective The authors have all received honoraria Haematol. 2013;162(4):558-559. cohort of 142 patients. Blood. 2003;102(1):60-68. 18. Chapin J, Eyler S, Smith R, Tsai HM, Laurence 33. Veyradier A, Obert B, Houllier A, Meyer D, Girma from Alexion. J. Complement factor H mutations are present in J-P. Specific von Willebrand factor-cleaving protease in ADAMTS13-deficient, ticlopidine-associated throm­ thrombotic microangiopathies: a study of 111 cases. References botic microangiopathies. Blood. 2013;121(19):4012- Blood. 2001;98(6):1765-1772. 4013. 34. George JN, Nester CM. Syndromes of thrombotic 19. Scully M, Goodship T. How I treat thrombotic microangiopathy. N Engl J Med. 2014;371(7):654-666. 1. Laurence J. Atypical hemolytic uremic syndrome thrombocytopenic purpura and atypical haemolytic 35. Wun T. Thrombotic thrombocytopenic purpura. (aHUS): making the diagnosis. Clin Adv Hematol uraemic syndrome. Br J Haematol. 2014;164(6):759- Medscape. Emedicine.medscape.com/article/206598. Oncol. 2012;10(suppl 17):2-8. 766. Updated June 20, 2016. Accessed July 12, 2016. 2. Laurence J. Atypical hemolytic uremic syndrome 20. Kato H, Nangaku M, Hataya H, et al; Joint Com­ 36. Marn Pernat A, Buturović-Ponikvar J, Kovac J, et (aHUS): treating the patient. Clin Adv Hematol Oncol. mittee for the Revision of Clinical Guides of Atypical al. Membrane plasma exchange for the treatment of 2013;11(suppl 15):4-15. Hemolytic Uremic Syndrome in Japan. Clinical guide­ thrombotic thrombocytopenic purpura. Ther Apher 3. Cataland SR, Wu HM. How I treat: the clinical lines for atypical hemolytic uremic syndrome in Japan. Dial. 2009;13(4):318-321. differentiation and initial treatment of adult patients Pediatr Int. 2016;58(7):549-555. 37. Lara PN Jr, Coe TL, Zhou H, Fernando L, Hol­ with atypical hemolytic uremic syndrome. Blood. 21. Rock GA, Shumak KH, Buskard NA, et al; Cana­ land PV, Wun T. Improved survival with plasma 2014;123(16):2478-2484. dian Apheresis Study Group. Comparison of plasma exchange in patients with thrombotic thrombocytope­ 4. Legendre CM, Licht C, Muus P, et al. Terminal com­ exchange with plasma infusion in the treatment of nic purpura-hemolytic uremic syndrome. Am J Med. plement inhibitor eculizumab in atypical hemolytic- thrombotic thrombocytopenic purpura. N Engl J Med. 1999;107(6):573-579. uremic syndrome. N Engl J Med. 2013;368(23):2169- 1991;325(6):393-397. 38. Bresin E, Rurali E, Caprioli J, et al; European 2181. 22. Fremeaux-Bacchi V, Fakhouri F, Garnier A, et al. Working Party on Complement Genetics in Renal 5. Campistol JM, Arias M, Ariceta G, et al. An update Genetics and outcome of atypical hemolytic uremic Diseases. Combined complement gene mutations in for atypical haemolytic uraemic syndrome: diagnosis syndrome: a nationwide French series comparing chil­ atypical hemolytic uremic syndrome influence clinical and treatment. A consensus document. Nefrologia. dren and adults. Clin J Am Soc Nephrol. 2013;8(4):554- phenotype. J Am Soc Nephrol. 2013;24(3):475-486. 2015;35(S):421-447. 562. 39. Licht C, Weyersberg A, Heinen S, et al. Successful 6. Moschcowitz E. Hyaline thrombosis of the terminal 23. Walle JV, Delmas Y, Ardissino G, Wang J, Kincaid plasma therapy for atypical hemolytic uremic syndrome arterioles and capillaries: a hitherto undescribed disease. JF, Haller H. Improved renal recovery in patients with caused by factor H deficiency owing to a novel muta­ Proc NY Pathol Soc. 1924;24:21-24. atypical hemolytic uremic syndrome following rapid tion in the complement cofactor protein domain 15. 7. Gasser C, Gautier E, Steck A, Siebenmann RE, initiation of eculizumab treatment [published online Am J Kidney Dis. 2005;45(2):415-421.

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