Pseudohypoparathyroidism, Type Ia

A number sign (#) is used with this entry because pseudohypoparathyroidism type Ia (PHP Ia) is caused by mutation resulting in loss of function of the Gs-alpha isoform of the GNAS gene (139320) on the maternal allele. This results in expression of the Gs-alpha protein only from the paternal allele.

See also pseudopseudohypoparathyroidism (PPHP; 612463), a similar disorder caused by mutations resulting in loss of function of the Gs-alpha isoform of the GNAS gene on the paternal allele and resultant expression of the Gs-alpha isoform only from the maternal allele.

Description

Pseudohypoparathyroidism is a term applied to a heterogeneous group of disorders whose common feature is end-organ resistance to parathyroid hormone (PTH; 168450). In addition to PTH resistance, PHP Ia is characterized by resistance to other hormones, including thyroid-stimulating hormone (TSH; see TSHB, 188540) and gonadotropins. PHP Ia is associated with a constellation of clinical features referred to as Albright hereditary osteodystrophy (AHO), which includes short stature, obesity, round facies, subcutaneous ossifications, brachydactyly, and other skeletal anomalies. Some patients have mental retardation (Mantovani and Spada, 2006).

In contrast, pseudopseudohypoparathyroidism (PPHP; 612463) is characterized by the physical findings of AHO but without hormone resistance (Kinard et al., 1979; Fitch, 1982; Mantovani and Spada, 2006).

PHP1A occurs only after maternal inheritance of the molecular defect, whereas PPHP occurs only after paternal inheritance of the molecular defect (Davies and Hughes, 1993; Wilson et al., 1994). This is an example of imprinting, with differential gene expression depending on the parent of origin of the allele. See INHERITANCE and PATHOGENESIS sections.

Nomenclature

The classification of pseudohypoparathyroidism is based on the phenotype and cellular cyclic AMP (cAMP) response to exogenous PTH administration. Cyclic AMP is produced by an intracellular G protein hormone receptor-adenylyl cyclase (see, e.g., ADCY1, 103072) complex. The GNAS gene encodes the alpha-stimulatory subunit (Gs) of the intracellular G protein, which stimulates the production of cAMP under certain physiologic conditions (Mantovani and Spada, 2006).

Pseudohypoparathyroidism type Ia

Individuals with PHP Ia have AHO features, multiple hormone resistance, decreased cellular cAMP response to PTH infusion, decreased erythrocyte Gs activity, and a GNAS1 mutation in the maternally-derived allele (Mantovani and Spada, 2006).

Pseudopseudohypoparathyroidism

Individuals with PPHP (612463) have AHO without endocrine abnormalities, a normal cellular cAMP response to PTH infusion, decreased erythrocyte Gs activity, and a GNAS1 mutation in the paternally-inherited allele (Mantovani and Spada, 2006).

Pseudohypoparathyroidism type Ib

Individuals with PHP Ib (PHP1B; 603233) have renal PTH resistance, decreased cAMP response to PTH infusion, normal erythrocyte Gs activity, and imprinting/methylation defects at the GNAS locus resulting in lack of expression of the maternal allele in renal tissue. Classically, patients do not have features of AHO. PHP1B is not associated with generalized hormone resistance, although resistance to thyroid-stimulating hormone (TSH; 188540) has been reported (Mantovani and Spada, 2006).

Mariot et al. (2008) noted that features of AHO may rarely be observed in patients with PHP1b who have decreased expression of G-alpha-s due to epimutations at the GNAS locus.

Pseudohypoparathyroidism type Ic

PHP Ic (PHP1C; 612462) is characterized by PTH resistance, generalized hormone resistance, AHO, decreased cAMP response to PTH infusion, and normal erythrocyte Gs activity. It may be a variant form of PHP Ia (Bastepe, 2008).

Pseudohypoparathyroidism type II

Individuals with PHP II (PHP2; 203330) have isolated renal PTH resistance and lack the features of AHO. They have a normal erythrocyte Gs activity and a normal cAMP response to PTH infusion, although the phosphaturic effect of PTH is deficient (Mantovani and Spada, 2006).

Pseudohypoparathyroidism should not be confused with polyostotic fibrous dysplasia (174800), to which Albright's name is also attached as 'McCune-Albright syndrome.'

Clinical Features

Albright et al. (1942) described 3 patients with short stature, round face, short neck, obesity, subcutaneous calcifications, and shortened metacarpals associated with hypocalcemia and hyperphosphatemia. Although the phenotype resembled parathyroid hormone deficiency, renal function was normal and apparently unresponsive to administration of bovine parathyroid extract. The authors hypothesized that the clinical findings were due to resistance to PTH rather than to PTH deficiency, and termed the disorder 'pseudohypoparathyroidism' (PHP). The specific pattern of observable physical features described by Albright et al. (1942) constitutes 'Albright hereditary osteodystrophy.'

Mann et al. (1962) observed parathyroid hyperplasia in patients with PHP, suggesting increased serum PTH, not deficiency of PTH. Chase and Aurbach (1968) found that PTH circulated in abnormally high concentration in pseudohypoparathyroidism, and that secretion of the hormone responded normally to physiologic control by calcium.

In patients with PHP, Chase et al. (1969) found that urinary cAMP did not increase in response to PTH administration, a finding that was in contrast to controls. The authors suggested that the basic defect in PHP may be a deficient amount or function of PTH-sensitive adenyl cyclase in kidney and bone.

Farfel et al. (1980) found that 5 patients with PHP I and AHO had abnormally low urinary cAMP excretion in response to PTH infusion. There was also a 40 to 50% reduced activity of the G protein, which the authors termed 'N,' that couples the membrane hormone receptor to adenylate cyclase.

Farfel et al. (1981) reported 10 patients with PHP type Ia. Typical features included short stature, brachydactyly, hypocalcemia, elevated serum PTH, and decreased urinary excretion of cAMP following exogenous PTH infusion. Erythrocyte Gs activity was decreased by approximately 50% in 15 patients with PHP type I, but was normal in 19 of their clinically normal first-degree relatives. Inheritance patterns were not consistent with a single mode of transmission. Bourne et al. (1981) reported 5 patients with PHP type I patients who showed a 40% reduction in red cell N protein activity.

Downs et al. (1983) reported a 16-year-old girl with PHP Ia who had resistance to PTH, thyrotropin, and gonadotropins. She had multiple subcutaneous calcifications, uniformly short metacarpals, hypocalcemia, hyperphosphatemia, elevated serum PTH, no response in urinary cAMP to PTH infusion, elevated plasma thyrotropin, low serum thyroxine, no thyroid enlargement, oligomenorrhea, low serum estrogen, high LH and FSH, and normal response to endogenous ACTH during a standard metyrapone test and after insulin-induced hypoglycemia. Red cell membrane Gs activity was 57% of control; renal membranes showed only 30% of control.

Weisman et al. (1985) observed congenital hypothyroidism as the presenting manifestation of PHP. Levine et al. (1985) reported a brother and sister with infantile hypothyroidism whose mother had PHP type Ia. Both children were normocalcemic. Resistance to thyroid-stimulating hormone was reflected in the reduced level of Gs in red cell membranes.

Hewitt and Chambers (1988) reported a patient with PHP type I in whom subcutaneous calcification developed at the age of 3 months. 'Hard lumps' were located on the chest wall and groin.

Izraeli et al. (1992) described a family in which 5 members of 3 successive generations had the clinical features of AHO associated with progressive osseus heteroplasia (POH; 166350), which is caused by activating mutations of GNAS1 inherited on the paternal allele.

Zung et al. (1996) pictured subcutaneous nodules of the left heel in a 7-year-old girl with AHO features. The mother, aged 38 years, had multiple subcutaneous masses of the limbs and bilateral shortening of the fourth metacarpals. A mammogram showed calcified breast nodules which were also palpable.

Aldred et al. (2000) reported a sister and brother with PHP Ia, including hypocalcemia, resistance to PTH, and elevated thyroid-stimulating hormone. Both sibs also had neonatal diarrhea and pancreatic insufficiency that corrected spontaneously after several months.

Mantovani et al. (2003) investigated the presence of pituitary resistance to hypothalamic hormones acting via G-coupled receptors in patients with PHP Ia. Six of 9 patients showed an impaired growth hormone (GH; 139250) responsiveness to GHRH (139190) plus arginine, consistent with a complete GH deficiency; partial and normal responses were found in 2 and 1 patient, respectively. Accordingly, IGF1 (147440) levels were below and in the low-normal range in 7 and 2 patients, respectively. The authors concluded that in addition to PTH and TSH resistance, patients with PHP Ia display variable degrees of GHRH resistance, consistent with Gs-alpha imprinting in the human pituitary. The findings were consistent with abnormalities of secretion of thyroid hormone (de Wijn and Steendijk, 1982) and gonadotropins (Shapiro et al., 1980) that had been described in patients with PHP.

Brachydactyly, classically described as shortening of III, IV, and V metacarpals and I distal phalanx, is the typical and most specific feature of AHO. This phenotype has been reported in 70% of PHP subjects from routine radiologic examinations. De Sanctis et al. (2004) evaluated the metacarpophalangeal pattern profile in 14 genetically characterized patients with PHP Ia and determined the prevalence and patterns of left hand bone shortening. Shortening below -2 SD score was present in at least 1 bone in each subject, with a prevalence of 100%; however, great variability existed between subjects and between hand bone segments. Brachymetacarpia and brachytelephalangy characterized the hands of the subjects.

Clinical Variability

De Nanclares et al. (2007) reported 4 unrelated patients who were thought to have PHP1A because of PTH and TSH resistance and mild features of Albright hereditary osteodystrophy. Two patients showed decreased G-alpha activity in erythrocytes. However, genetic analysis did not reveal germline point mutations in the GNAS gene in any of the patients; instead, all were found to have GNAS methylation defects, which are usually associated with PHP1B. Furthermore, 1 of the patients with normal G-alpha activity was found to have a 3.0-kb STX16 deletion (603666.0001), which is usually associated with PHP1B. The findings suggested that there may be an overlap between the molecular and clinical features of PHP1A and PHP1B, and that methylation defects may manifest as mild PHP1A.

Other Features

Allen et al. (1988) observed that hypomagnesemia can prevent the elevation of PTH hormone concentrations in familial PHP; this finding indicated that the parathyroid gland retains its physiologic response to hypomagnesemia in this disorder.

Stirling et al. (1991) reported a woman with PPHP; both she and her son had deficiency in production of growth hormone-releasing factor (GHRH; 139190). The son responded well to growth hormone treatment.

Studies in frog neuroepithelium showed that the sense of smell is mediated by a Gs-adenylate cyclase system. Weinstock et al. (1986) found that all Gs-deficient patients with type Ia PHP had impaired olfaction, whereas all Gs-normal patients with PHP type Ib had normal olfaction. This suggested that Gs-deficient patients may be resistant or impaired in other cAMP-mediated actions in other nonendocrine systems.

Doty et al. (1997) noted that variably decreased olfactory ability had been described in some patients with PHP Ia, suggesting that Gs-alpha may play a role in human olfactory transduction. Tests of odor detection, identification, and memory among patients with PHP Ia, PHP Ib, and PPHP showed that patients with PHP Ia and PHP Ib had olfactory dysfunction, and that patients with PPHP had normal olfactory function. The authors concluded that the olfactory dysfunction associated with PHP is not the result of generalized Gs-alpha protein deficiency.

Mental deficiency occurs in 47 to 75% of patients with PHP type I. Because mutations in the adenylate cyclase-cAMP system may affect the learning ability of Drosophila, Farfel and Friedman (1986) assessed mental deficiency in 25 patients whose Gs protein activity had been determined. Nine of 14 patients with type Ia and none of 11 patients with type Ib pseudohypoparathyroidism had mental deficiency. Farfel and Friedman (1986) concluded that Gs-protein deficiency, reduced cAMP levels, or both are involved in the mental deficiency of these patients.

Because obesity is frequently associated with Gs deficiency and Gs is part of the pathway transducing the lipolytic signal through beta-adrenergic receptors, Carel et al. (1999) tested whether epinephrine resistance was part of the spectrum of hormonal resistance in patients with Gs deficiency. Using a nonradioactive tracer dilution approach to measure glycerol flux, they analyzed the lipolytic response to epinephrine in 6 patients with Gs deficiency and PHP Ia and compared it in 6 age-matched normal controls and 9 massively obese children. Basal glycerol production was reduced by 50%, and lipolytic response to epinephrine was reduced by 67% in Gs-deficient children, as compared with controls. The degree of impairment of lipolysis was similar in Gs-deficient children who were only moderately overweight and in morbidly obese children.

Inheritance

Pseudohypoparathyroidism with AHO was originally thought to be transmitted as an X-linked dominant disorder, most likely based on initial observations that females were affected twice as often as males. However, Mann et al. (1962) found that whereas only 4 of 36 female cases were of the incomplete form, 6 of 14 male cases failed to show full expression. This finding, contrary to that in other X-linked traits, made it possible that the disorders were in fact sex-influenced autosomal dominant.

Chase et al. (1969) reported a family in which 2 patients with PHP were the progeny of a mother with PPHP (612463). Another family had 3 brothers with PHP; the mother had PPHP. Weinberg and Stone (1971) described a family in which a brother and sister had PHP1, and the son and daughter of the brother had PPHP. All had clinical features of AHO, which were more prominent in the patients with PHP1. The patients were of normal intelligence but showed ectopic calcification and ossification, rounded facies, 'absent 4th knuckles,' and short feet and hands with particularly short 4th metacarpals.

Williams et al. (1977) described 4 females and 1 male in a family pedigree who showed wide clinical variability encompassing both PHP and PPHP. Farfel et al. (1981) reported a woman with PPHP whose daughter had classic PHP type Ia. The mother had AHO with no history of hypocalcemia or other endocrine abnormalities and normal urinary cAMP response to PTH, but reduced erythrocyte N protein activity.

Kinard et al. (1979) and Fitch (1982) reported kindreds in which some individuals had AHO without hormone resistance (PPHP) while others had hormone resistance as well (PHP1A), suggesting that PHP and PPHP are genetically related.

Cederbaum and Lippe (1973) reported 2 sibs with PHP1A transmitted in an apparently autosomal recessive pattern. Farfel et al. (1981) restudied this family and found that the 2 affected sibs had decreased N-protein while the parents were clinically and biochemically normal. They suggested that this may have been an instance of gonadal mosaicism in one of the parents or that one of the parents was in fact carrying the gene and was only very mildly affected.

In a review of the literature, Fitch (1982) favored autosomal dominant inheritance with sex modification.

Van Dop et al. (1984) found renal resistance to endogenous and exogenous PTH as well as reduced activity of the Gs protein in a man with PHP Ia. Both of his children also had reduced Gs activity levels and short stature. The 11-year-old daughter had evidence of partial resistance to PTH, whereas the 7-year-old son had no apparent abnormalities in calcium metabolism and a normal cAMP response to administered PTH, consistent with PPHP. The occurrence of reduced erythrocyte Gs activity in the father and son was consistent with autosomal dominant inheritance for the primary biochemical defect (see Spiegel et al., 1985).

In 8 kindreds, Levine et al. (1988) found that the AHO phenotype and erythrocyte alpha-Gs deficiency were transmitted together in a dominant inheritance pattern.

Lecumberri et al. (2010) reported 2 unrelated families in which PHP1A and PHP1B occurred coincidentally within different branches of each of the families. In the first family, 2 sibs with PHP1A inherited a GNAS mutation from their affected mother. A man from another branch of the family with PHP1B was found to have paternal uniparental isodisomy of chromosome 20q, with presumed lack of expression of the maternal allele. The diagnosis was confusing in this patient before molecular analysis because he had mild features of AHO and cognitive impairment, suggestive of PHP1A. Genetic analysis confirmed that he did not have a germline GNAS mutation. In the second family, 1 individual with PHP1A had a GNAS mutation (139320.0011) inherited from his mother, and a second cousin had PHP1B with an epigenetic defect at the GNAS locus. Lecumberri et al. (2010) emphasized the importance of molecular diagnosis for proper genetic counseling.

Effects of Imprinting on Inheritance Pattern

Davies and Hughes (1993) pointed out that published reports of AHO involving 2 or more generations showed a marked excess of maternal transmission. Furthermore, full expression of the disorder (AHO plus hormone resistance in the form of PHP) occurred in maternally transmitted cases, whereas only partial expression (isolated AHO, or PPHP) occurred in paternally transmitted cases. Davies and Hughes (1993) suggested that genomic imprinting is involved in the expression of the disorder.

Schuster et al. (1994) reported 1 family (Schuster et al., 1993) in which the pattern of inheritance did not support the imprinting hypothesis. One member of the family with 'normocalcemic PHP' who had an impaired cAMP response to PTH was born from a father with PPHP. Schuster et al. (1994) hypothesized that mechanisms other than genomic imprinting were responsible for the full expression of hormone resistance, at least within this family. Additional components of signal transduction (e.g., calmodulin, cAMP phosphodiesterase, or protein kinase A) may be responsible for the differences between PHP Ia and PPHP.

Pathogenesis

PHP1A is caused by decreased Gs signaling in the hormone receptor-adenylate cylase complex involved in intracellular signal transduction. GNAS is a heavily imprinted locus, with different expression of its isoforms in different tissues dependent on the parental origin of the gene. Individuals with PHP1A and PPHP (612463) show about a 50% decrease in Gs expression in erythrocytes, which normally express both parental alleles. In PHP1A, physiologic responses are impaired in tissues in which 50% residual Gs activity from the paternal allele is not sufficient to maintain normal signaling, thus resulting in haploinsufficiency. Moreover, renal tubule cells are unique in that they only express the maternal allele of Gs; the paternal allele is not expressed. Thus, renal cells of PHP1A patients have complete lack of Gs expression resulting from an inactive maternal allele. Hormone resistance in patients with maternal Gs-alpha mutations affects those tissues in which expression of Gs-alpha is predominantly from the maternal allele (i.e., renal tubule cells). The features of AHO are believed to result from defective signaling in other cells due to Gs haploinsufficiency (Bastepe and Juppner, 2005; Mantovani and Spada, 2006).

Chase and Aurbach (1968) demonstrated that PTH stimulated adenyl cyclase in the renal cortex. Maguire et al. (1977) showed that the PTH receptor-adenylate cyclase complex involved in response to PTH consists of a hormone receptor, adenylate cyclase, and G proteins. Gs, also termed 'Ns,' is the stimulatory G subunit that mediates the activation of adenylate cyclase to form cAMP.

Drezner and Burch (1978) observed altered kinetic properties of adenylate cyclase in renal cell membrane preparations from a patient with PHP. The abnormalities were corrected by addition of guanosine-5-prime-triphosphate to the reaction mixtures. These results suggested an abnormal nucleotide receptor site on the G protein, resulting in partial uncoupling of the PTH hormone receptor and adenylate cyclase. This was predicted to render the organ refractory to hormonal stimulation.

Decreased activity of the G protein is found in patients with PHP type I who usually also have AHO (PHP type Ia), whereas normal G protein activity is found in patients with PHP type I who do not have AHO (PHP type Ib). Heinsimer et al. (1984) found that beta-adrenergic agonist-specific binding properties of red cell membranes were 45% of controls in 5 patients with PHP Ia and 97% of controls in 5 patients with PHP Ib. Further studies were consistent with a single defect causing deficient hormone receptor-nucleotide complex formation and adenylate cyclase activity in PHP Ia, whereas the biochemical lesion(s) appeared not to affect the complex formation in PHP Ib.

Levine et al. (1983) demonstrated that a Gs defect was responsible for resistance to multiple hormones in some cases of PHP type I. End-organ resistance to a variety of hormones included dysfunction of thyroid, gonadotropin, prolactin, and glucagon action. A blunted plasma cAMP response to the infusion of the beta-adrenergic agonist isoproterenol was described by Carlson and Brickman (1983).

Levine et al. (1986) found that the activity of Gs was reduced in red cells from patients with both PHP Ia and PPHP in 6 kindreds. However, the authors noted that patients with PPHP usually do not have obvious endocrine dysfunction, suggesting that factors other than Gs activity must be involved in determining hormone resistance.

In a review of G protein diseases, Farfel et al. (1999) stated that the different phenotypes between PHP1A and PPHP probably result from tissue-specific combinations of haploinsufficiency and paternal imprinting.

Weinstein and Yu (1999) reviewed the clinical and molecular evidence that supports the role of genomic imprinting of Gs-alpha in the pathogenesis of AHO features.

Gs-alpha transcripts are imprinted in pituitary somatotrophs that secrete growth hormone (Hayward et al., 2001), leading Germain-Lee et al. (2003) to hypothesize that maternally inherited GNAS1 mutations would impair GH secretion through disruption of mediation of the pituitary response to GHRH by G-alpha-s. They studied GH status in 13 subjects with PHP type Ia. GH responses to arginine/L-DOPA and arginine/GHRH were deficient in 9 subjects, all of whom were obese and had low serum concentrations of IGF1. By contrast, none of the 4 GH-sufficient subjects were obese, and all had normal IGF1 levels. The authors concluded that GH deficiency is common in PHP type Ia (69%) and may contribute to the obesity and short stature typical of AHO, and proposed that GH status be evaluated in all patients with PHP type Ia.

Using hot-stop PCR analysis on total RNA from 6 normal human thyroid specimens, Liu et al. (2003) showed that the majority of the Gs-alpha mRNA (72 +/- 3%) was derived from the maternal allele. This was considered consistent with the presence of TSH resistance in patients with maternal Gs-alpha-null mutations (PHP Ia). The authors concluded that there is tissue-specific imprinting of Gs-alpha in the thyroid, which may explain mild to moderate TSH resistance in PHP Ia.

Hsu et al. (2007) demonstrated that mutation of 1 GNAS gene caused reduced Gs-alpha signaling in patients with AHO as evidenced by decreased maximal cAMP production. The reduced Gs-alpha signaling in AHO patients was associated with decreased activation of the downstream target cystic fibrosis transmembrane conductance regulator (CFTR; 602421) in vivo when compared with normal controls.

Clinical Management

Drezner et al. (1976) presented evidence to suggest that the skeletal resistance to PTH in PHP may result from a vitamin D deficiency. In 3 of 4 patients with PHP, they found low levels of the active form of vitamin D, 1,25-dihydroxycholecalciferol. Treatment with vitamin D restored the serum calcium and the calcemic response to PTH, without affecting the impaired renal response. The authors suggested that hypocalcemia and bone disease in PHP may be due to active vitamin D deficiency caused by inability of the kidney to convert 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, rather than to primary bone resistance to PTH.

Metz et al. (1977) showed normal 25-hydroxyvitamin D but extremely low 1,25-dihydroxycholecalciferol in a patient with a unique variant of PHP. He had elevated serum PTH and normal renal response to PTH, which could be consistent with PHP type II. Treatment with 1,25-dihydroxycholecalciferol restored bone responsiveness to PTH. The authors suggested that homeostatic release of calcium from bone in classic PHP was not responsive to PTH because of a renal deficiency in the production of 1,25-vitamin D.

Mapping

Hall (1990) noted that the GNAS1 gene maps to a region of chromosome 20 that is homologous to an area of mouse chromosome 2 involved in both maternal and paternal imprinting. She suggested that AHO may show imprinting by virtue of location in this area.

Molecular Genetics

Levine et al. (1988) studied 8 kindreds with 1 or more members affected with AHO or PHP and decreased activity of the Gs protein. RNA blot analysis showed about 50% reduced GNAS1 mRNA levels in fibroblasts of affected members from 6 of the pedigrees, but normal levels in affected members of the other 2 pedigrees. No abnormalities of restriction fragments or gene dosage were identified using a cDNA hybridization probe, leading the authors to conclude that major rearrangements or deletions of GNAS1 were not a common cause for the disorder.

In a proband and his mother with PHP Ia, Patten et al. (1989, 1990) identified a heterozygous mutation in the GNAS gene (139320.0001).

In a mother with PPHP (612463) and her 4 daughters with PHP Ia (Kinard et al., 1979), Gejman et al. (1990) and Weinstein et al. (1990) identified a heterozygous mutation in the GNAS gene (139320.0002). A son of 1 of the affected daughters also had PHP Ia and carried the mutation. A second heterozygous mutation (139320.0003) was identified in a second family in which the mother had PPHP and her daughter had PHP Ia.

In an Italian patient with PHP Ia and features of AHO, Mantovani et al. (2000) detected a heterozygous 1-bp deletion (139320.0025) in the GNAS gene. This mutation was also found in the patient's mother, who had PPHP.

In a brother and sister with PHP type Ia, who also had neonatal diarrhea and pancreatic insufficiency, Aldred et al. (2000) identified heterozygosity for a 12-bp insertion in the GNAS1 gene (139320.0035). The mutation was not found in 2 unaffected sibs and was also not detected in the lymphocyte DNA of either of the clinically unaffected parents. Haplotype analysis confirmed germline mosaicism and indicated that the mutation was maternal in origin. Makita et al. (2007) performed biochemical and intact cell studies of the 12-bp insertion (AVDT) and suggested that the PHP-Ia phenotype results from instability of the Gs-alpha-AVDT mutant and that the accompanying neonatal diarrhea may result from its enhanced constitutive activity in the intestine.

Aldred et al. (2002) reported 2 patients with Albright hereditary osteodystrophy and deletions of chromosome 20q, including complete deletion of the GNAS1 gene. One boy had a paternally inherited deletion of chromosome 20q13.13-q13.32 and a normal biochemical evaluation consistent with PPHP. The other patient had a maternally derived deletion of chromosome 20q13.31-q13.33 and PHP type Ia.

History

Pseudohypoparathyroidism was the first endocrine syndrome in humans to be recognized as a failure of end-organ response to a hormone. Albright et al. (1942) was right about AHO being a hormone resistance state but wrong about the Sebright bantam male, the name of which he misspelled in his 1942 article and the nature of which has been shown to be excessive conversion of testosterone to estrogen, not resistance to androgen.

Johnson (1980) suggested a form of simple inheritance that was neither dominant nor recessive to explain the transmission of AHO, i.e., that resulting from metabolic interference. In this situation both homozygotes would be normal. Only the heterozygous condition would produce an abnormal phenotype because the 2 alleles interact to have a deleterious effect. Metabolic interference may occur when 2 allelic genes code for different subunits of a multisubunit enzyme or structural protein or 2 nonallelic genes may interact in other ways. Albright hereditary osteodystrophy, manic depressive psychosis (309200), and 'X-linked' pterygium syndrome (312150) were cited as examples of diseases compatible with X-linked dominance but not worse in males.

The possibility of an anomalous PTH in PHP type I was suggested by observations of Loveridge et al. (1982). When exogenous parathormone was added to plasma of normal subjects and those with hyperparathyroidism or primary hypoparathyroidism, response was commensurate with the amount added; 50 to 90% of the exogenous hormone was 'recovered.' When this was done with the plasma of 10 PHP type I patients, recovery ranged from less than 1% up to 35%. This seemed to indicate an inhibitor in PHP plasma.

Hedeland et al. (1992) described a mother and daughter with classic features of pseudohypoparathyroidism type Ia in association with proximal deletion of 15q, del(15)(q11q13), similar to that seen in Prader-Willi syndrome (176270) and Angelman syndrome (105830). Using a series of DNA probes that often show deletion or uniparental disomy in the latter 2 conditions, Hedeland et al. (1992) found no evidence for either in the mother or the daughter.

Animal Model

Yu et al. (1998) disrupted the Gnas gene by targeted mutagenesis in the mouse and demonstrated that Gnas is imprinted in a tissue-specific manner. Consistent with Gnas being an imprinted gene, heterozygous mice with paternal (+/p-) and maternal (m-/+) transmission of the Gnas null allele (GsKO) had distinct phenotypes. M-/+ but not +/p- mice were resistant to parathyroid hormone, whereas both m-/+ and +/p- mice had a normal maximal physiologic response to vasopressin. Studies of the expression of the alpha subunit demonstrated that Gnas is imprinted, with the paternal allele relatively inactive in renal cortex (the site of PTH (168450) action) and brown and white adipose tissue. In contrast, Gnas was not imprinted in the renal inner medulla (the site of vasopressin action). Tissue-specific imprinting of Gnas was thought to be the mechanism for variable and tissue-specific hormone resistance in these mice and in Albright hereditary osteodystrophy.