and review article

Sergio S. Maeda ABSTRACT Erika M. Fortes The principal function of the (PTH) is maintenance Ulisses M. Oliveira of calcium plasmatic levels, withdrawing the calcium from bone tissue, Victoria C.Z. Borba reabsorbing it from the glomerular filtrate, and indirectly increasing its intestinal absorption by stimulating active vitamin D (calcitriol) produc- Marise Lazaretti-Castro tion. Additionally, the PTH prompts an increase in urinary excretion of phosphorus and bicarbonate, seeking a larger quantity of free calcium available in circulation. Two mechanisms may alter its function, limiting its control on calcium: insufficient PTH production by the parathyroids (hypoparathyroidism), or a resistance against its action in target tissues (pseudohypoparathyroidism). In both cases, there are significantly reduced levels of plasmatic calcium associated with hyperphos- phatemia. Clinical cases are characterized by nervous hyperexcitability, with paresthesia, cramps, , hyperreflexia, convulsions, and tetanic crisis. Abnormalities such as cataracts and basal ganglia are also typical of these diseases. Treatment consists of oral calcium sup- plementation associated with increased doses of vitamin D derivatives. (Arq Bras Endocrinol Metab 2006;50/4:664-673)

Division of Endocrinology Keywords: Hypoparathyroidism; PTH; ; Pseudohy- (SSM, EMF, UO & ML-C), poparathyroidism; PTH resistance; Albright hereditary osteodistrophy UNIFESP/ EPM, São Paulo, SP; e SEMPR (VCZB), UFPr, RESUMO Curitiba, PR. Hipoparatiroidismo e Pseudohipoparatiroidismo. A principal função do paratormônio (PTH) é a manutenção dos níveis plasmáticos de cálcio, retirando-o do tecido ósseo, reabsorvendo-o do filtrado glomerular e, indiretamente, aumentando sua absorção intesti- nal através do estímulo para a produção de vitamina D ativa (calcitriol). Além disso, o PTH promove um aumento na excreção urinária de fósforo e bicarbonato, objetivando uma maior quantidade de cálcio livre disponível na circulação. Dois mecanismos podem alterar sua função, limitando seu controle sobre o cálcio: produção insuficiente de PTH pelas paratiróides (hipoparatiroidismo), ou uma resistência à sua ação nos órgãos-alvo (pseudohipoparatiroidismo). Em ambos os casos, ocorre uma redução significativa dos níveis plasmáticos de cálcio em associ- ação com hiperfosfatemia. Manifestações clínicas características são: hiperexcitabilidade nervosa, com parestesia, cãimbras, tetania, hiper- reflexia, convulsões e crise tetânica. Catarata e calcificação dos gânglios basais são anormalidades típicas dessas doenças. O tratamen- to consiste da suplementação oral de cálcio, associada com doses ele- vadas de derivados da vitamina D. (Arq Bras Endocrinol Metab 2006;50/4:664-673)

Descritores: Hipoparatiroidismo; PTH; Hipocalcemia; Pseudo- Received in 04/02/06 hipoparatiroidismo; Resistência a PTH; Osteodistrofia hereditária de Accepted in 04/20/06 Albright

664 Arq Bras Endocrinol Metab vol 50 nº 4 Agosto 2006 Hypoparathyroidism and Pseudohypoparathyroidism Maeda et al.

HYPOCALCEMIA: CLINICAL MANIFESTATIONS individuals and only 1 to 4% of healthy individuals pre- AND sent a positive sign, which is obtained by pressurizing a sphygmomanometer approximately 20–30 mmHg ERUM CALCIUM CONCENTRATION is kept within a nar- above systolic pressure for 3 minutes. It is character- Srow physiological range due to complex control ized by carpal spasms, with adduction of the thumb, mechanisms involving the parathyroid hormone (PTH), followed by flexion of the metacarpophalangeal joint, active vitamin D (1.25(OH)2D), and calcium sensor extension of the interphalangeal joints, and flexion of receptors, in addition to the concentrations of calcium the wrist, creating the classic “obstetrician’s hand” and phosphate, which act in the renal, intestinal, and pose, in addition to causing paresthesia, muscular ten- bone tissues in order to maintain calcium homeostasis. sion, and local cramps (6). Hypocalcemia occurs when homeostatic mechanisms Severe hypocalcemia may result in bradycardia or fail or when they are not fully compensated. ventricular arrhythmias, cardiovascular collapse, and The normal range of total serum calcium is hypotension that is non-responsive to fluids and vaso- between 8.5 and 10.2 mg/dL (2.12 to 2.55 pressors. A decrease in myocardial contractility occurs, as mmol/L). Symptoms of hypocalcemia occur when the well as a typical electrocardiographic abnormality, which level of ionized calcium is below 2.8 mg/dL (0.7 is the rate-corrected QT interval (QTc) prolongation. mmol/L), equivalent to 7.0 to 7.5 mg/dl (1.75 to 1.87 mmol/l) of total calcium (1,2). The severity of Chronic hypocalcemia symptoms and clinical signs of hypocalcemia correlates Patients with chronic hypocalcemia may or may not with the magnitude and speed at which calcium have symptoms of discreet neuromuscular irritation, declines, influenced by acid-base status and presence even with markedly low calcium levels. Asymptomatic of hypomagnesemia and/or sympathetic hyperactivity. cases may be detected by chance, by the dosage of cal- cium in routine exams, during periods of greater calci- Acute hypocalcemia um demand (i.e.: gestation, lactation, menstrual cycle In acute and/or severe symptomatic hypocalcemia there and states of alkalosis), or during the use of hypocal- is a predominance of neuromuscular, neuropsychiatric, cemic drugs (i.e.: bisphosphonates). Ectodermic and cardiovascular abnormalities. There is an increase in abnormalities from dry skin to dermatitis and even neuromuscular excitability, latent or evident, with senso- alopecia are common. Dental abnormalities suggest ry and motor disruption. Perioral or extremity paresthe- the time of hypocalcemic onset. In infancy, it may lead sia, cramps, myalgia, and muscular weakness are mild to to dental or enamel hypoplasia, the delay or absence of moderate symptoms. Smooth muscle spasms may cause permanent tooth eruption, an increase in cavity occur- biliary and intestinal cramps, dysphagia, bronchospasms, rence, the shortening of molar roots, and, in some laryngeal stridor, premature birth, and detrusor muscle cases, the loss of all teeth (7). dysfunction. Severe hypocalcemia manifests as sponta- Significant cognitive deficits, neuropsychiatric neous tetany, which may appear in the form of carpope- abnormalities, and extrapyramidal symptoms that dal spasms and, more rarely, laryngospasms. Neuropsy- resemble Parkinson’s disease or chorea are associated chiatric manifestations include irritability, anxiety, psy- with the calcification of basal ganglia, which occurs in chosis, hallucinations, dementia, depression, mental con- all forms of chronic hypocalcemia and may be detect- fusion, and extrapyramidal abnormalities. Increased ed with greater sensibility using computerized tomog- intracranial pressure, papilledema, and convulsions can raphy (8). also be present, and must be differentiated from severe Other findings of chronic hypocalcemia include tetany muscular spasms (3-5). sub-capsular cataracts, an increase in bone mineral Typical clinical signs of neuromuscular irritabil- density (BMD), and greater susceptibility to dystonic ity associated with latent tetany include hyperreflexia reactions induced by phenothiazines (4). and Chvostek’s and Trousseau’s signs, respectively. Chvostek’s sign is obtained by the percussion of the Differential diagnosis of hypocalcemia facial nerve approximately 2 cm anterior to the ear Ionized or free calcium dosages are quicker, which will lobe, causing contraction of the ipsilateral facial mus- make them more useful during urgent situations. cles. It has low specificity and sensibility; 25% of Hypocalcemia must be confirmed by the measurement healthy individuals present a positive sign, while 29% of total plasmatic calcium, which should always be cor- of hypocalcemic individuals present a negative sign. rected in hypoalbuminemia cases (plasmatic albumin < Trousseau’s sign is more reliable; 94% of hypocalcemic 4 mg/dL) according to the following formula:

Arq Bras Endocrinol Metab vol 50 nº 4 Agosto 2006 665 Hypoparathyroidism and Pseudohypoparathyroidism Maeda et al.

Corrected Serum Ca= Total Serum Ca mg/dL (0.8 x nent hypoparathyroidism varies and depends on the [(4 – albumin mg/dL)]. root disease, extent of surgery, and surgeon’s experi- ence. Symptoms generally begin 1 or 2 days after the Differential diagnosis of hypocalcemia will procedure, and in approximately 50% of the cases this depend largely upon PTH and phosphorus levels, eval- abnormality is transient and caused by edema or glan- uated along with other clinical and laboratory data dular hemorrhage. After the removal of 1 or more (table 1). Cases presenting should parathyroids for the treatment of primary hyper- include differential diagnosis of vitamin D, while cases parathyroidism, hypocalcemia indicate 2 etiologies: associated with are determined Hungry Bone Syndrome or hypoparathyroidism. Dif- according to PTH levels. ferentiation is made via measurements of phosphorus, which will be low in Hungry Bone Syndrome and high in hypoparathyroidism. Hypoparathyroidism may be a HYPOPARATHYROIDISM rare complication after radioactive iodine treatment or external radiotherapy on the cervical region (9,10). Hypoparathyroidism is an abnormality caused by a Autoimmunity may also be another cause of parathyroid hormone (PTH) secretion deficiency, and hypoparathyroidism, which may be isolated or associ- encompasses heterogeneous conditions (table 1), ated with other autoimmune diseases (11). Antibodies which makes etiological differentiation crucial to the against parathyroids were detected in more than 30% detection of abnormalities associated with some of of patients with isolated hypoparathyroidism and more these diseases beforehand, thereby preventing compli- than 40% of patients with hypoparathyroidism associ- cations (4). are caused by hypocal- ated with other endocrine deficiencies. Antibodies cemia. Laboratory measurements present hypocal- against the calcium sensor-receptor (CASR) have been cemia, hyperphosphatemia, and inappropriately low or found in more than 50% of patients with polyglandu- undetectable PTH. Generally, levels of 1.25(OH)2D lar autoimmune syndrome type I or hypoparathy- are low and the alkaline phosphatase is normal. In the roidism associated with autoimmune hypothyroidism. majority of cases, hypoparathyroidism is sporadic, but The polyglandular autoimmune syndrome type I is there are familial cases in which transmission may be characterized by the triad: hypoparathyroidism (which autosomic recessive, dominant, or X-linked. is the most frequent endocrinopathy), mucocutaneous The most common cause of hypoparathy- candidiasis, and adrenal insufficiency. It is caused by a roidism is after thyroid surgery. Incidence of perma- mutation in the autoimmune regulator gene (AIRE),

Table 1. Causes of hypocalcemia.

Low PTH levels (hypoparathyroidism) High PTH levels (secondary hyperparathyroidism)

Parathyroid Destruction PTH resistance Surgery Pseudohypoparathyroidism Auto-immune (isolated or polyglandular) Hypomagnesemia Cervical irradiation Infiltration by metastasis or systemic diseases Nutritional (Sarcoidosis, amyloidosis, hemochromatosis, Lack of sunlight Wilson’s disease, thalassemia) Malabsorption Reduced parathyroid function Vitamin D dependent rickets Hypomagnesemia type I (lack of activity of 1a-hydroxylase) PTH gene defects type II (Vitamin D receptor resistance) Calcium sensing receptor mutations Chronic renal disease Parathyroid agenesis Drugs DiGeorge Syndrome Bisphosphonates, cisplatin, ketoconazole, gallium Isolated x-linked hypoparathyroidism nitrate, anticonvulsants Kenny-Caffey syndrome Hyperphosphatemia Mitochondrial neuropathies Renal insufficiency Massive tumor lysis Acute rhabdomyolysis Acute pancreatitis Hungry bone Toxic shock syndrome Acute severe illness Calcium chelators (citrated blood transfusions, phosphate)

666 Arq Bras Endocrinol Metab vol 50 nº 4 Agosto 2006 Hypoparathyroidism and Pseudohypoparathyroidism Maeda et al.

located in locus 21q22.3, which produces a protein tion in the chromosome region 1q43-44); Kenny-Caf- that functions as a transcription regulator. This syn- fey Syndrome (stunted growth, facial-cranial abnor- drome may be sporadic or familial, with autosomic malities, long bone cortical thickening, with medullary recessive transmission. Clinical signs begin to appear in stenosis and basal ganglia calcification); mitochondrial patients approximately 5 years after birth with the myopathies, such as Kearns-Sayre Syndrome emergence of candidiasis, followed by hypoparathy- (encephalomyopathy, oftalmoplegia, retinal pigment roidism before the age of 10, and later adrenal insuffi- dystrophy and cardiomyopathy) and Pearson’s mar- ciency, found in 100%, 79%, and 72% of the cases, row-pancreas syndrome (neutropenia, sideroblastic respectively (11,12). Hypogonadism, hypothyroidism, anemia, vacuolization of the bone marrow cells, and insulin-dependent mellitus diabetes, alopecia, vitiligo, exocrine pancreas dysfunction) (10). keratoconjunctivitis, intestinal malabsorption syn- Patients classified as having idiopathic drome, chronic hepatitis, and pernicious anemia may hypoparathyroidism must be carefully evaluated in also be present. order to exclude genetic syndromes. Genetic studies Maternal hyperparathyroidism, which inhibits have linked these cases to mutations in the CASR and PTH secretion by newborns’ parathyroids, is another PTH genes. The CASR gene, bound to the G protein cause of hypoparathyroidism. Hypocalcemia general- and comprising 1078 amino acid residues, is located in ly develops in the third week of life and is self-con- chromosome 3q and codifies a cell surface receptor tained (10). Variations in magnesium levels may also belonging to family C of the seven transmembrane cause hypoparathyroidism. stimu- receptor (7TM) superfamily (16,17). Predominantly lates the CASR and inhibits PTH secretion. In such expressed in the parathyroids and kidneys, this recep- circumstances, the clinical case of neuromuscular tor regulates PTH secretion and the tubular excretion hyperexcitability is usually more discreet. Hypomag- of calcium in response to fluctuations in calcium serum nesemia, commonly seen in chronic alcoholism and levels. When extra-cellular calcium concentrations burn victims, may also cause hypocalcemia because it increase, the CASR activates the G protein pathway, promotes the diminution of PTH secretion levels in stimulating the activity of phospholipases C, leading to addition to causing renal and bone resistance to PTH the accumulation of inositol 1,4,5 triphosphate (IP3) action (13). and the liberation of calcium from intra-cellular stor- DiGeorge Syndrome is characterized by thymus age. The increase of intracellular calcium results in the and parathyroid dysgenesis, cardiac malformation, and activation of protein kinase C, which in turn activates facial dysmorphogenesis. There is a defect in the for- the MAPK’s pathway, reducing the PTH gene tran- mation of the third and fourth bronchial arches, owed scription (15-17). The CASR is found in tissues unre- to the deletions of locus 22q11.2, but the gene lated to calcium homeostasis, such as the mammal responsible for the syndrome has yet to be identified. gland, keratinocytes, central nervous system, pancreas, Some patients with phenotype similar to DiGeorge and epithelial cells, although its function has yet to be Syndrome present deletions in other chromosomes defined (16). Mutations that lead to function loss in (10p13 and 10p14) (14). The agenesis or dysgenesis this gene are related to familial benign hypocalciuric of isolated parathyroids have X-linked or autosomic hypercalcemia and severe neonatal primary hyper- recessive transmission. Cases with X-linked transmis- parathyroidism (15,17). Activating mutations, on the sion are related to a mutation in locus Xq26-27, which other hand, cause a decrease of the calcium set point, likely plays a role in the embryological development of promote the suppression of PTH, and induce hyper- parathyroids (10). calciuria, which may aggravate the hypocalcemia. Another genetic cause is HDR Syndrome, char- Mutations are generally located in the aminoterminal acterized by hypoparathyroidism, neuro-sensorial deaf- domain of the extra-cellular and transmembrane por- ness, and renal dysplasia. HDR is related to haploinsuf- tions of the CASR. Exons 3, 4 and 7 are considered ficiency or function loss of gene GATA3 (GATA bind- hotspots for activating mutations in this gene. Exons 3 ing protein 3), located in chromosome 10p14-10, and 4 involve the extra-cellular domain, an essential which belongs to the family of transcription factors portion of the receptor for glycosylation, cell surface involved in embryological development (15). expression, dimerization, and binding of the ligand. Other complex syndromes associated with Mutations in exon 7 involve domains that participate hypoparathyroidism are: Sanjat-Sakati syndrome in the activation of G proteins associated with the (hypoparathyroidism, mental retardation, stunted receptor (18). Lienhardt and colleagues showed a high growth, dysmorphism, and seizures, related to a muta- frequency of CASR mutations (42%) in patients with

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isolated hypoparathyroidism, placing added impor- did not promote an increase in serum calcium or phos- tance on the screening of this gene, especially in phaturia (21). These individuals presented develop- hypocalcemic patients showing normal urinary calcium ment abnormalities: rounded face, short stature, obe- excretion (relative hypercalciuria). Since there is an sity, brachydactyly, short and low-set nasal bridge, excellent correlation between the presence of muta- strabismus, and ectopic ; which were des- tion in the CASR and hypocalcemia, the measurement ignated Albright’s Hereditary Osteodystrophy of the serum calcium may be used as a sufficiently sen- (AHO). Due to biochemical similarities with hypo- sitive screening in the affected families with this abnor- parathyroidism, but associated with high levels of PTH mality (18). Patients are generally asymptomatic, the was called pseudo-hypoparathyroidism (PHP). In serum PTH is normal or low, and they receive the 1966, Tashjian et al. (22) demonstrated an increase in hypoparathyroidism diagnosis (15). It is important to serum PTH concentrations in these patients and, in distinguish these patients from those with 1969, Chase et al. (23) presented that there was no hypoparathyroidism, because their treatment with cal- urinary AMPc generation in patients with PHP and cium and vitamin D may lead to hypercalciuria, the pathogenesis of this abnormality should be related nephrocalcinosis, and loss of renal function (10). to a renal defect in the generation of urinary AMPc by Mutations of the PTH gene, located in locus the PTH receptor. 11p15, are uncommon causes of hypoparathyroidism, and they can present autosomic recessive or dominant PTH receptor type I modes of inheritance. They result in defective PTH PTH receptor type I is expressed in the kidney and bone molecule synthesis and undetectable PTH levels. as a glycoprotein of approximately 75 kDa that connects Another gene related to hypoparathyroidism is GCMB to both PTH and PTH-related protein (PTHrP). This (a human gene homologous to the Drosophila glial cell receptor, attached to regulatory heterotrimeric protein missing gene b), located in chromosome 6p24.2, G (α, β, γ), presents a single polypeptide chain with expressed exclusively in the parathyroids, and which seven transmembrane domains, 3 extracellular loops, codifies an important transcription factor for the devel- and 3 or 4 intracellular loops (20). opment of these glands (10). The Gcm2 (glial cells Hormone binding leads to the activation of missing 2) is the murine gene homologous for GCMB. protein Gs, with disassociation of subunit α. This, Homozygote knockout rats (-/-) for the Gcm2 do not when activated, disconnects itself from the receptor present parathyroids and develop hypocalcemia and and functions as a primary modulator of the adenyl hyperphosphatemia. PTH levels are normal or low, but cyclase, promoting the intracellular formation of cyclic insufficient to control hypocalcemia. In these rats there AMP (AMPc), which rapidly activates the protein may also be PTH production by parathyroid cells locat- kinase A (PKA) (21-25). The intrinsic GTPase activity ed in the thymus capsule, explaining the diminished, of subunit α permits the linking of βγ dimer for a new but detectable, PTH levels. In humans it is unknown if activation cycle. there is intrathymic PTH production or if this small Patients with AHO present inactivating muta- PTH production is due to the gene’s residual activity tions in the GNAS1 gene, which codifies the subunit (15,19). α of protein Gs, mapped in chromosome 20q13.2- 20q13.3. The GNAS1 gene is codified by 13 exons, in addition to 3 initial exons that correspond to different PSEUDOHYPOPARATHYROIDISM promoters, allowing, by alternative splicing, the for- mation of 4 different isoforms. Exons 2-13 are com- The term pseudo-hypoparathyroidism describes a group mon to all transcripts. Mutations were described in all of abnormalities characterized by clinical and laborator- exons, except in 3, which despite suffering splicing out ial hypoparathyroidism findings (hypocalcemia, hyper- still produced a functional protein (26). phosphatemia), but with high plasmatic levels of PTH Recent studies confirm that the GNAS1 gene due to a target tissue resistance, if chronic renal failure presents the imprinting phenomenon, which explains or were excluded (20). the variations in phenotype depending upon the maternal or paternal mutation’s origin (24,27,28). Historical Three promoters present opposing models of allele- In 1942, Fuller Albright and cols. described 3 patients specific methylation and monoallelic transcription. with clinical and laboratorial hypocalcemia findings in The promoter closest to the 5’ extremity codifies which the administration of parathyroid tissue extracts region NESP55 (neuroendocrine secretory protein

668 Arq Bras Endocrinol Metab vol 50 nº 4 Agosto 2006 Hypoparathyroidism and Pseudohypoparathyroidism Maeda et al.

55), which is expressed exclusively by the maternal ness to Gs protein deficiency, which may also be allele, while exons XLαs (extra large as like protein) explained by the differences in the quantity of AMPc and 1A (1 alternative) are expressed by the paternal necessary in order to activate the kinase proteins and allele (figure 1). Despite simultaneous imprinting generate physiological responses in each tissue. The (paternal and maternal), expression of Gsα appears to absence of imprinting in other target tissues would be be biallelic in the majority of examined human tissues, another explication for patients with PHP1a who do but expressed maternally in only some tissues such as not develop resistance to other hormones that stimu- the proximal renal tubules (25). late the Gsα protein and paths linked to the AMPc (for example: ACTH and vasopressin) (25,26). Pathogenesis and classification The characterization of the molecular bases of pseudo- Pseudo-hypoparathyroidism type 1b (PHP1b) hypoparathyroidism began with the observation that Presents normal appearance, normal Gs activity, and the AMPc is the mediator of various PTH actions in isolated resistance to PTH. Molecular studies do not the kidney and bone fiber. show an intrinsic defect in the PTH receptor, and pre- PTH infusion remains the most consistent test sent normal Gs function in the erythrocytes (25). The for the distinction of the diverse variants of this syn- majority of cases appear to be sporadic, but some drome. Based in the abnormalities of the various com- familial cases were described as having autosomic ponents of the transduction of the cell membrane sig- dominant transmission (29,33). As in PHP1a, a defec- nal, the pseudo-hypoparathyroidism may be divided in tive nephrogenic answer to AMPc occurs in relation to distinct forms. PTH. However, these individuals present elevated PTH concentrations and frequently manifest skeletal Pseudo-hypoparathyroidism type 1a (PHP1a) abnormalities similar to those that occur in patients Presents AHO phenotype. There is a reduction in with hyperparathyroidism (30). Fibroblast studies activity of the stimulatory protein G (Gs), limiting show small accumulation of AMPc in response to PTH AMPc synthesis. In this situation, patients are not only in some cases associated with lower quantities of the resistant to PTH, but to other peptide hormones like PTH type I Receptor RNAm. A molecular defect was TSH, gonadotrophins, and glucagon. Transmission is mapped in a small region of chromosome 20q13.3 autosomic dominant (29). Hypocalcaemia and hyper- (exon 1A) next to the GNAS1 gene. Exon 1A is phosphatemia are not typically present until five years important to maintain the tissue-specific imprinting of of age, but PTH elevations may be documented much Gsα and is normally methylated in the maternal allele earlier and sometimes are associated with light hyper- and not methylated in the paternal allele (25). One of calcemia. Primary hypoparathyroidism frequently pre- the proposed mechanisms to explain PHP1b cases was cedes the development of functional hypoparathy- suggested by Weinstein and cols. (figure 2). Exon 1A roidism and may be detected during neonatal screen- contains a cis-acting, negative regulatory element ing. Gonadal dysfunction (hypogonadism or delayed (silencer) that is both methylation-sensitive and tissue- puberty) is very frequent, especially in women (30). specific (25,26). A tissue-specific trans-acting repres- There are accounts of resistance to GHRH and pro- sor (R) binds to the silencer and suppresses Gs expres- lactin deficiency in PHP1a cases (25,31,32). There is sion on the paternal allele, but is unable to bind the considerable variability in relation to tissue responsive- maternal allele due to methylation, allowing Gsα to be expressed from the maternal allele. In the majority of tissues, exon 1A is methylated but the repressor is not

αs expressed, and as a result the Gsα is expressed bialleli- cally. In PHP1b, the methylation of this exon is absent in the maternal allele, allowing the repressor to link to both alleles and suppress the Gsα expression in the renal proximal tubules, raising the Gsα deficiency and αs PTH resistance (26,29,34), which normally only express the maternal allele. These individuals do not Figure 1. General organization and imprinting patterns of present AHO, because the defective imprinting does the GNAS1 gene. Promoter NESP55 is only expressed by the not have an effect in the G α expression of the major- maternal allele, while promoters XLαs and 1A are expressed s by the paternal allele. ity of the tissues where it is expressed biallelically.

Arq Bras Endocrinol Metab vol 50 nº 4 Agosto 2006 669 Hypoparathyroidism and Pseudohypoparathyroidism Maeda et al.

X X

X X

Figure 2. Mechanisms that explain the molecular pathogenesis of PTH resistance (res PTH) and Albright’s Hereditary Osteodystrophy (AHO) by inactivation and imprinting of the GNAS1 gene. Exon 1A is only methylated in the maternal allele and is transcriptionally active in the paternal allele in all tissues. In the majority of tissues, exon 1A does not affect Gsα expression, which is biallelically expressed. In the proximal renal tubules, the non-methylated exon possibly suppresses Gsα expression in the paternal allele via a repressor (R), leading to a maternal-specific expression of Gsα. In PPHP and PHP1a, the inactive mutations in the paternal and maternal alleles, respectively, reduce the Gsα expression by 50%, and this haploinsufficiency produces the Albright phenotype. In the proximal tubules, mutations in the inactive paternal allele do not affect Gsα expression, while mutations in the active maternal allele produce PTH resistance. In PHP1b, the methylation defect in the maternal allele leads to a paternal-specific imprinting model of exon 1A in both alleles. When this happens, Gsα loses expression in the proximal tubules and there is a PTH resistance, but not affecting the expression in other tissues, which explains why these individuals do not possess Albright’s phenotype (25).

Pseudo-hypoparathyroidism type 1c (PHP1c) torial findings (35). Some clinical syndromes, howev- Presents Albright’s phenotype and resistance to multi- er, may present the same skeletal abnormalities as ple hormones (PTH, TSH, gonadotrophins, and AHO without evidence of PTH resistance. PPHP car- glucagon). Gs activity is normal, and so is Gi. Studies riers are often times relatives of PHP patients, and may show reduced activity of the membrane’s adenyl present periods of hypocalcemia and normocalcemia. cyclase catalytic subunit. It has yet to be clarified AHO patients who inherit Gsα mutations from their which molecular defect is responsible (29). mothers also develop resistance to TSH, PTH, and gonadotrophins (PHP1a), while those who inherit it Pseudo-hypoparathyroidism type 2 (PHP2) from their fathers develop only OHA (PPHP). This is Some patients present normal phenotype, normal owed to the fact that the Gsα gene is expressed pri- AMPc urinary excretion but absent phosphaturic marily from the maternal allele in target-tissues for the response to PTH. It is probably associated with defects respective hormones (renal proximal tubules, thyroid, in stages posterior to AMPc formation, because Gs and ovaries). Mutations in the active maternal allele activity is normal. Its defect has yet to be identified lead to Gsα expression deficiency and hormonal resis- and suggests that there is not any evidence of familial tance, while mutations of the paternal allele have little transmission and that it may be an acquired defect. or no effect on genetic expression or hormonal signal- ing (26). The presence of a Gsα allele, however, is not Pseudo-pseudohypoparathyroidism (PPHP) sufficient in all tissues. Different phenotypes probably The term was used by Albright to describe patients result from tissue-specific combinations of haploinsuf- with the Albright phenotype, but without the labora- ficiency to paternal imprinting (maternal allele expres-

670 Arq Bras Endocrinol Metab vol 50 nº 4 Agosto 2006 Hypoparathyroidism and Pseudohypoparathyroidism Maeda et al.

sion). The AHO phenotype results from deficient sig- cium per day in the various forms of salts available naling in cells in which the Gsα gene is haploinsuffi- (table 2). Although calcium carbonate is the most cient but does not suffer imprinting (both alleles are used salt, it needs gastric acid so that it may be solu- expressed) (24). The diminution of signaling Gsα bilized and absorbed, and causes more collateral gas- appears to promote osteoblastic differentiation, which trointestinal effects than other salts. (39). All patients may explain ectopic calcifications and premature clos- with hypoparathyroidism or pseudohypoparathy- ing of growth plates (25). roidism who become hypocalcemic must use vitamin D or analogous in addition to calcium (table 3). The Defects outside the endocrine system vast majority of patients obtain control with calcitriol Although it is characterized by endocrine dysfunction, in dosages of 0.25 µg, taken twice daily, up to 0.5 µg it may have abnormalities in organs whose functions four times daily. High doses of oral Cholecalciferol are mediated by the AMPc path. Weinstock et al. (36) can also be used (50.000 to 150.000 UI/day), but showed that patients with PHPIA have altered olfac- the risk of intoxication after years of treatment is ele- tory function if compared with PHPIB patients. The vated due to their long half-life. Treatment objectives mental deficiency that occurs 50–75% of PHP-carrying are to maintain free ionized calcium levels within the individuals is also associated with Gs deficiency. Stud- normal interval, to avoid hypercalciuria, and to sup- ies also show evidence of paternal imprinting in the press PTH levels in patients with pseudohypoparathy- brown and white adipose tissue, suggesting that obe- roidism. Hypoparathyroidism causes increased excre- sity may be resultant of decreased Gsα expression in tion of urinary calcium in relation to serum calcium the adipose tissues, with diminution of the AMPc and and predisposes hypercalciuria, nephrolithiasis, and lipolysis formation in response to β-adrenergic stimu- nephrocalcinosis. On the other hand, patients with lus in the adipocyte (25,29). pseudohypoparathyroidism that have low urinary cal- cium in relation to serum calcium may tolerate serum calcium levels within the normal interval without TREATMENT OF HYPOCALCEMIA developing hypercalcemia. Vitamin D intoxication must be remembered in patients with hypercalcemic Management of acute or severe symptomatic hypocal- symptoms. Thiazide diuretics are used in cases of cemia must be made with intravenous calcium, with hypercalciuria and nonabsorbable antacid can be the goal of interrupting symptoms, preventing laryn- added to reduce hyperphosphatemia and prevent geal spasm, and maintain total calcium levels above . The product of calcium x 7.0–7.5 mg/dL (ionized calcium greater than 0.7 phosphate must be kept below 55. Those patients must mmol/L). Hyperphosphatemia, alkalosis, and hypo- have their kidneys radiologically evaluated regularly in magnesemia, when present, must be corrected. At that order to rule out nephrocalcinosis (4). moment, the 10 to 30 mL of 10% calcium gluconate PTH1-34 treatment has the advantage of nor- should be made slowly (10 minutes – maximum of 30 malizing calcemia without increasing calciuria, reduc- mg/min) in bolus IV (93 to 279 mg of elementary ing the risk of nephrocalcinosis and renal insufficiency, calcium), and repeated as many times as necessary. and, theoretically, the antagonists of the calcium sen- After the grave acute symptoms have been suspended, sor receptor may be utilized in the treatment to pro- a maintenance of calcium levels via continuous mote the inactivation of the receptor and, conse- endovenous infusion of 0.5 to 1.5 mg/Kg/h (maxi- quently, increase PTH secretion, but there still have mum of 100 mg/h) of elementary calcium for 4 to 6 not been sufficient studies (40). hours should be made, using a solution with SG5% 900 ml + 100 ml of 10% gluconate (930 mg elemen- tary calcium/liter). Serum calcium levels must be mea- REFERENCES sured frequently in this period, and electrocardio- graphic monitoring must be done, especially in those 1. Broadus AE. Mineral balance and homeostasis. In: Favus MJ, editor. Primer on the metabolic bone diseases and patients using digitalics, due to growing levels or cal- disorders of mineral metabolism, 5th ed. American Soci- cium predisposed to digitalis intoxication. Endove- ety for Bone and Mineral Research; 2003. p. 105-11. nous transition to the oral must be made as soon as 2. Bushinsky DA, Monk RD. Electrolyte quintet: calcium. possible (2,3,38). Lancet 1998;352:306-11. Long-term treatment of patients with chronic 3. Tohme JF, Bilezikian JP. Hypocalcemic emergencies. hypocalcemia is done with 1 to 3 g of elementary cal- Endocrinol Metab Clin North Am 1993;22:363-75.

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Table 2. Most used calcium salt forms.

Salt Element Milligrams of salt needed Parenteral content in order to obtain 1 g preparation elementary calcium

Calcium carbonate 40% 2500 Calcium phosphate 38% 2631 Calcium chloride 27% 3700 10% Solution= 273 mg/10 ml Calcium citrate 21% 4762 Calcium lactate 13% 7700 Calcium gluconate 9% 11100 10% Solution= 93 mg/10 ml

Table 3. Vitamin D analogues.

Calciferol D3 / D2 Calcidiol Calcitriol Alfacalcidol 25(OH)D 1,25(OH)2D1αOHD

25-hydroxylation necessary + – – – 1α-hydroxylation necessary + + – – Physiological dose/dosage 2.5–10 µg 1–5 µg 0.25–0.5 µg 1–3 µg (1 µg= 40 UI) (1 µg= 40 UI) Pharmacological dose/dosage 0.625–5 mg 20–250 µg 0.5–3 µg 1–3 µg Time needed to normalize calcium 4–8 weeks 2–4 weeks 3–7 days 7–14 days Duration of effect 6–12 weeks 2–6 weeks 3–7 days 7–14 days

Table 4. Classification and clinical features of Pseudohypoparathyroidism subtypes.

PHP1a PHP1b PHP1c PHP2 PPHP

Hypocalcemia Yes Yes Yes Yes No OHA Yes No Yes No Yes Hormonal resistance Multiple PTH Multiple PTH None Response to IV PTH No No No Yes Yes Gs Mutation Yes No No No Yes Cause Gs mutation Non-methylation Unknown Acquired Genomic of promoter 1A defect? imprinting

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