Hematopoietic stem cells Genetic defects in hematopoietic stem cells. Focus on myeloid cell differentiation and function C. Klein eutrophil granulocytes were first iden- cue mechanisms cannot prevent undue ER University Children’s Hospital tified when Paul Ehrlich, aged 25 years, stress, cells undergo apoptosis. Neutrophils Ludwig-Maximilians-University Ndeveloped staining techniques. He from patients with mutations in ELANE Munich, Germany described three different types of granulocytes: show increased signs of ER stress and apop- basophilic, neutrophilic, and eosinophilic poly- tosis8,12,13 suggesting that ER stress is critically morphonuclear cells.1 Independently, Elie involved in the pathophysiology of severe Acknowledgements: I wish Metchnikoff’s studies in starfish larvae congenital neutropenia associated with to acknowledge the contributions revealed that certain cells are capable of mutations in ELANE. of a large international net of phagocytosis.2 Both investigators shared the Using a genome-wide linkage study and clinical partners and basic scientists. Nobel Prize in 1908 for their discoveries, candidate gene sequencing in consanguineous Furthermore, I am grateful to our patients and their families, to my laying the foundation of cellular immunity. pedigrees with severe congenital neutropenia, research team, to the members Diseases related to dysfunction of neu- loss-of-function mutations in HAX1 (HCLS1 trophil granulocytes were discovered around associated protein X-1) have been identified.14 of the international registries, th th and to Karin Leitz for administrative the turn of the 19 and 20 centuries, when HAX1 is also the gene mutated in the patients support. This work was funded Philip King Brown first described a patient originally identified by Rolf Kostmann. by the European E-RARE program with lethal pharyngitis due to agranulocyto- Originally, HAX1 was cloned as an interact- Neutro-NET. sis.3 Similar cases were reported thereafter,4 ing partner of HCLS1, a kinase involved in B- many associated with the medical use of cell receptor signal transduction. HAX1 is pre- aminopyrines becoming available as a drug at dominantly localized at the mitochondria Hematology Education: that time. In 1950, the Swedish pediatrician membrane and stabilizes the mitochondrial the education program for the Rolf Kostmann published the first report of membrane potential. In the absence of HAX1, annual congress of the European inherited neutrophil deficiency.5 cells are prone to undergo apoptosis. Hematology Association Kostmann recognized severe congenital Interestingly, two isoforms of HAX1 (A and neutropenia (“agranulocytosis infantilis heredi- B) with tissue-specific expression patterns 2011;5:146-150 taria”) as an autosomal recessive trait charac- have been described. Biallelic mutations terized by severe neutropenia and “matura- affecting both isoforms (A+B) lead to severe tion arrest” in the bone marrow.5,6 Children congenital neutropenia and associated neuro- with severe congenital neutropenia (SCN), logical problems, whereas mutations affect- usually defined as an absolute neutrophil ing only isoform A lead to a phenotype of iso- count of less than 500 /mL, present with lated congenital neutropenia.15 invasive bacterial infections, such as Patients with mutations in the zinc finger omphalitis, skin abscesses, pneumonia, or molecule GFI1 (growth factor independent-1) septicemia. Characteristically, sites of infec- also present with congenital neutropenia.16 tion lack formation of pus. Today, we recog- GFI1 is a master-regulator of hematopoiesis nize a great variety of genetic defects that can and coordinates transcription17 and splicing18 in lead to congenital neutropenia.7 The gene hematopoietic, as well as in non-hematopoiet- encoding neutrophil elastase (ELANE) was ic cells. Similar to mutations in ELANE, GFI1- the first found to be mutated in patients with mutations are inherited in an autosomal dom- cyclic neutropenia8 and severe congenital inant pattern. Mutations affect the DNA-bind- neutropenia.9 ELANE-mutated patients repre- ing domain, generate a dominant-negative sent the largest group among Caucasian variant of GFI1, and thus interfere with tran- patients with SCN.10 Neutrophil elastase scriptional networks on multiple target genes belongs to the class of serine proteases and is and regulatory RNAs. As a consequence, expressed exclusively in mature myelomono- patients show not only a severe maturation cytic cells and their committed immature arrest of myeloid cells but also aberrations in precursor cells. Cells expressing a mutated monocytoid and lymphoid cells. ELANE allele induce the so-called unfolded Mutations in the X-chromosome encoded protein response, a physiological rescue Wiskott–Aldrich Syndrome (WAS) gene typical- mechanism to prevent toxic effects by ly cause loss-of-function of the Wiskott– improperly folded proteins. The unfolded Aldrich Syndrome protein (WASP). As a con- protein response signal cascade is initiated by sequence, affected boys suffer from a com- three ER-localized protein sensors: IRE1alpha bined immunodeficiency syndrome associ- (inositol-requiring 1alpha), PERK (double- ating thrombocytopenia, autoimmunity, and stranded RNA-dependent protein kinase immunodeficiency. WASP is a cytosolic (PKR)-like ER kinase, and activating tran- adaptor protein expressed in all nucleated scription factor 6 (ATF6).11 In case these res- blood cells and is required for actin polymer- | 146 | Hematology Education: the education programme for the annual congress of the European Hematology Association | 2011; 5(1) London, United Kingdom, June 9-12, 2011 ization. In WASP deficiency, lymphoid and myeloid ty of neutrophil granulocytes. For example, Hermansky– cells are functionally perturbed in multiple aspects. Rare Pudlak syndrome type II, caused by defects in AP3B1, is human patients were found expressing a constitutively associated with congenital neutropenia and hypopig- active variant of WASP, leading to increased actin poly- mentation.23 The gene encodes for a subunit of the adap- merization and congenital neutropenia.19 Increased tor protein 2 complex controlling vesicular trafficking. WASP activity also causes defective cytokinesis, increas- Mutations in P14/MAPBPIP, encoding an endosomal pro- ing the risk of myelodysplasia.20 tein, have been discovered in a single pedigree with con- Neutrophil granulocytes critically depend on glucose genital neutropenia, combined immunodeficiency, and metabolism. This is highlighted by mutations in hypopigmentation. In contrast to patients with severe SLC37A421 and G6PC3,22 respectively. SLC37A4 congenital neutropenia, patients with defects in AP3 and encodes the glucose-6-phosphate transporter (G6PT) MAPBPIP do not have a myeloid maturation arrest. The mediating transition of G6P from cytosol to the endo- mechanism of neutropenia remains to be resolved. plasmic reticulum, whereas G6PC3 encodes a ubiqui- In American Indians, a rare variant of congenital neu- tously expressed homologue of glucose-6-phosphatase. tropenia has been identified that is associated with poikilo- Whereas in G6PT, deficiency congenital neutropenia is derma, hyperkeratosis, and nail dystrophy.24 Using a target- associated with metabolic complications (glycogen stor- ed next generation sequencing approach, Volpi et al. iden- age, hypoglycemia), G6PC3 deficiency causes congeni- tified mutations in C16Orf57 as the cause for Clericuzio- tal neutropenia in conjunction with variable structural type neutropenia.25 Recently, Clericuzio et al. confirmed defects or the cardiovascular and urogenital system. that C16Orf57 is also the mutated gene in Athabascan This disease is phenotypically quite variable and may patients with poikiloderma and neutropenia.26 The func- also present with inner ear deafness or growth failure. tion of the mutated protein is still unknown. Interestingly, in contrast to glycogen storage disease Some other monogenetic diseases, such as warts- type Ia and type Ib, G3PC3 deficiency does not result in hypogammaglobulinemia, immunodeficiency-myelokathex- metabolic complications, presumably because glucose is (WHIM) syndrome, Cohen syndrome, Shwachman homeostasis is regulated mainly be G6PC1 and G6PT. Diamond syndrome, cartilage-hair-hypoplasia, or Barth A number of other rare genetic defects have been syndrome may also be associated with congenital neu- reported to control differentiation, migration, and viabili- tropenia (Table 1). Table 1. Synopsis of disease with congenital neutropenia and related animal models. Hematology Education: the education programme for the annual congress of the European Hematology Association | 2011; 5(1) | 147 | 16th Congress of the European Hematology Association Table 1. Synopsis of disease with congenital neutropenia and related animal models. | 148 | Hematology Education: the education programme for the annual congress of the European Hematology Association | 2011; 5(1) London, United Kingdom, June 9-12, 2011 Currently the epistatic relationships between these 23. Dell’Angelica EC, Shotelersuk V, Aguilar RC, Gahl WA, Bonifacino JS. Altered trafficking of lysosomal proteins in variant proteins and their role in governing differentia- Hermansky-Pudlak syndrome due to mutations in the beta 3A tion and homeostasis of neutrophil granulocytes remain subunit of the AP-3 adaptor. Mol Cell. 1999;3(1):11-21. obscure. Cross-species perspectives may open novel 24. Clericuzio C, Hoyme HE, Asse JM. Immune deficient poikilo- horizons in our understanding