Normocalcemic pseudohypoparathyroidism. Association with normal vitamin D3 metabolism

The pseudohypoparathyroidism type lb locus is linked to a region including GNAS1 at 20q13.3

Pseudohypoparathyroidism (PHP) is characterized by biochemical hypoparathyroidism with elevated parathyroid hormone levels owing to reduced target tissue responsiveness to parathyroid hormone. Patients with PHP la have somatic defects termed Albright's hereditary osteodystrophy (AHO) and exhibit resistance to additional hormones because of heterozygous mutations in the GNAS1 gene that lead to a generalized deficiency of the a subunit of Gs, the heterotrimeric G protein that couples receptors to adenylyl cyclase. By contrast, patients with PHP 1b lack AHO and have selective parathyroid hormone (PTH) resistance, presumably because of an imprinting defect that impairs expression of G(s)alpha in the proximal renal tubule. Although an epigenetic defect in GNAS1 has been identified in subjects with PHP1b, the genetic defect is unknown. To define the genetic defect in PHP 1b, we performed a genome-wide linkage analysis in five multi-generational PHP lb families. Of the 408 polymorphic microsatellite markers examined, markers located on chromosome 20q13.3, the region containing GNAS1, demonstrated linkage to PHP lb. Fine-mapping and multipoint linkage analysis of this region demonstrated linkage to a 5.7-cM region between 907rep2 and the telomere. Haplotype analysis established that affected individuals shared a 5-cM region including part of the GNAS1 gene to the telomere. Our data confirm that PHP1b is linked to a region that includes GNAS1, and further refine the locus, although the primary genetic mutation(s) that causes defective imprinting of GNAS1 remains undefined.

Chronic hypocalcaemia due to selective skeletal resistance to parathyroid hormone

A 71-year-old man was referred for evaluation of asymptomatic hypocalcaemia dating back at least 20 years. There were no somatic abnormalities and Chvostek and Trousseau signs were negative. Serum total calcium varied from 1.88 to 2.03 mmol/l, albumin 37-44 g/l, phosphate 0.54-1.12 mmol/l and ionized calcium 1-1.13 mmol/l. Serum intact PTH levels were 69 and 55 ng/l (10-65), 25-OHD was 40 nmol/l (2.25-107.5) and 1,25-(OH)2D was 54.6 nmol/l (39-156). Serum and urine magnesium and creatinine clearance were normal. Twenty-four-hour urine calcium was 2.15 mmol and calcium/creatinine ratio 0.07. TM phosphate (maximal rate of tubular reabsorption of phosphate in mmol/l glomerular filtrate (GF)) was 0.84 mmol/l GF (0.80-1.34). Bone formation and resorption markers were normal. Bone mineral densities measured by dual-energy X-ray absorptiometry (DEXA) were within normal limits at the hip, forearm and lumbar spine. Infusion of 200 units of synthetic 1-34 PTH was associated with a rise in urinary cyclic AMP from 43 mmol/l GF to 344 mmol/l GF and TM phosphate fell from 0.93 to 0.76 mmol/l GF; 1-34 PTH infusions of 300 units twice daily for 5 days were associated with an increase in serum 1,25-(OH)2D from 80.6 to 114.4 pmol/l but no increase in serum calcium. This is a most unusual case of chronic hypocalcaemia similar to that reported by Frame et al. resulting from isolated skeletal resistance to PTH that is not related to renal insufficiency, osteomalacia or a magnesium-deficient state. These two cases appear to represent a new variant of pseudohypoparathyroidism ?type III.

Endocrine manifestations of stimulatory G protein alpha-subunit mutations and the role of genomic imprinting

The heterotrimeric G protein G(s) couples hormone receptors (as well as other receptors) to the effector enzyme adenylyl cyclase and is therefore required for hormone-stimulated intracellular cAMP generation. Receptors activate G(s) by promoting exchange of GTP for GDP on the G(s) alpha-subunit (G(s)alpha) while an intrinsic GTPase activity of G(s)alpha that hydrolyzes bound GTP to GDP leads to deactivation. Mutations of specific G(s)alpha residues (Arg(201) or Gln(227)) that are critical for the GTPase reaction lead to constitutive activation of G(s)-coupled signaling pathways, and such somatic mutations are found in endocrine tumors, fibrous dysplasia of bone, and the McCune-Albright syndrome. Conversely, heterozygous loss-of-function mutations may lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, brachydactyly, sc ossifications, and mental deficits. Similar mutations are also associated with progressive osseous heteroplasia. Interestingly, paternal transmission of GNAS1 mutations leads to the AHO phenotype alone (pseudopseudohypoparathyroidism), while maternal transmission leads to AHO plus resistance to several hormones (e.g., PTH, TSH) that activate G(s) in their target tissues (pseudohypoparathyroidism type IA). Studies in G(s)alpha knockout mice demonstrate that G(s)alpha is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues (e.g., renal proximal tubule, the major site of renal PTH action), while being biallelically expressed in most other tissues. Disrupting mutations in the maternal allele lead to loss of G(s)alpha expression in proximal tubules and therefore loss of PTH action in the kidney, while mutations in the paternal allele have little effect on G(s)alpha expression or PTH action. G(s)alpha has recently been shown to be also imprinted in human pituitary glands. The G(s)alpha gene GNAS1 (as well as its murine ortholog Gnas) has at least four alternative promoters and first exons, leading to the production of alternative gene products including G(s)alpha, XLalphas (a novel G(s)alpha isoform that is expressed only from the paternal allele), and NESP55 (a chromogranin-like protein that is expressed only from the maternal allele). A fourth alternative promoter and first exon (exon 1A) located approximately 2.5 kb upstream of the G(s)alpha promoter is normally methylated on the maternal allele and transcriptionally active on the paternal allele. In patients with isolated renal resistance to PTH (pseudohypoparathyroidism type IB), the exon 1A promoter region has a paternal-specific imprinting pattern on both alleles (unmethylated, transcriptionally active), suggesting that this region is critical for the tissue-specific imprinting of G(s)alpha. The GNAS1 imprinting defect in pseudohypoparathyroidism type IB is predicted to decrease G(s)alpha expression in renal proximal tubules. Studies in G(s)alpha knockout mice also demonstrate that this gene is critical in the regulation of lipid and glucose metabolism.

Evolution of pseudohypoparathyroidism: an informative family study

An adult woman with pseudopseudohypoparathyroidism had a child with normal calcium and parathyroid hormone concentrations and cyclic AMP response to injected parathyroid hormone in infancy. By 2.5 years he had features of pseudohypoparathyroidism with raised parathyroid hormone and 'flat' cyclic AMP response. This is the first documented case of a change in parathyroid hormone responsiveness. The abnormal cyclic AMP response to parathyroid hormone in pseudohypoparathyroidism can evolve during childhood.

Imprinting in Albright's hereditary osteodystrophy

Review of published reports of Albright's hereditary osteodystrophy (AHO) involving two or more generations shows a marked excess of maternal transmission. Full expression of the gene (AHO + hormone resistance, pseudohypoparathyroidism) occurs in maternally transmitted cases and partial expression (AHO alone) when the gene is inherited from the father, suggesting the involvement of genomic imprinting in the expression of this disorder.

Plasma cyclic AMP response to intravenous parathyroid hormone in pseudohypoparathyroidism

Highly purified bovine parathyroid hormone (PTH) was given by intravenous bolus injection to patients being investigated for disorders of mineral metabolism, and to adult volunteer controls. Plasma cyclic AMP measured basally and at 10 min gave reliable discrimination between the normal response and cases of pseudohypoparathyroidism. Infants under 3 months of age tended to have higher basal levels of cAMP and a flatter pattern of response to the dose of PTH used. This simplified test procedure in children offers considerable advantages over previous tests of PTH responsiveness which involve urine collections and multiple blood sampling. It is suitable for selective screening of individuals suspected of pseudohypoparathyroidism on the basis of their family history or physical abnormalities.

Heterogeneity of pseudohypoparathyroidism type I from the aspect of urinary excretion of calcium and serum levels of parathyroid hormone

Urinary excretion of calcium (Ca) was measured in 9 patients with pseudohypoparathyroidism (PHP) type I--3 with Albright's hereditary osteodystrophy (AHO): AHO(+) and 6 without AHO: AHO(-)--and in 13 with idiopathic hypoparathyroidism (IHP), treated with active vitamin D3 (1,25(OH)2D3 or 1 alpha OHD3) to maintain serum Ca levels at 8.4-9.5 mg/dl. Fasting urinary excretion of Ca in PHP was significantly lower than that in IHP. Moreover, fasting urinary excretion of Ca in PHP AHO(+) was lower than that in PHP AHO(-). This difference was also seen in the urine after oral loading of Ca. Urinary excretion of sodium (Na) was not different between PHP AHO(+) and PHP AHO(-). Serum levels of immunoreactive PTH in PHP AHO(+) were higher than those in PHP AHO(-). The difference in urinary excretion of Ca between PHP AHO(+) and PHP AHO(-) may come from the difference in the circulating levels of PTH.

Pseudohypoparathyroidism: current concepts

Following a brief discussion of the diagnosis and classification of hypoparathyroidism, this review will focus on current concepts of pseudohypoparathyroidism. Topics to be covered will include differing resistance of kidney and bone to parathyroid hormone, relationship of estrogen and pregnancy to Ca homeostasis, normocalcemic pseudohypoparathyroidism, and current understanding of pathogenesis based on various defects in the hormone receptor-adenylate cyclase system. Evidence for physiologic derangements beyond the impaired generation of cyclic AMP will be reviewed, as well as involvement of nonendocrine systems by the deficiency of the stimulatory guanine nucleotide connecting protein. Finally, semantic confusion resulting from the faulty term "pseudopseudohypoparathyroidism" will be addressed.

Pseudohypoparathyroidism, parkinsonism syndrome, with no basal ganglia calcification

A 20 year old woman with pseudohypoparathyroidism, Parkinsonism and no basal ganglia calcifications shown by computed tomography is reported. She has typical features of pseudohypoparathyroidism and biochemical evidence of end-organ resistance to parathyroid hormone. She is mentally retarded and has tremor, rigidity, bradykinesia, and stooped posture. The cause of Parkinsonism in pseudohypoparathyroidism is thought to be basal ganglia calcification. This patient must have another pathophysiology, perhaps directly related to a G protein defect, causing impaired neurotransmission.

Studies on the attainment of normocalcemia in patients with pseudohypoparathyroidism

Basal serum calcium and parathyroid hormone (PTH) levels were measured, and urinary excretion of cyclic adenosine monophosphate (AMP) and phosphate was determined before and after the infusion of 250 U of PTH in four patients with pseudohypoparathyroidism when they were hypocalcemic and again when they spontaneously became normocalcemic. These data were compared to those observed in a group of patients with pseudohypoparathyroidism before and after they became normocalcemic after treatment with vitamin D and calcium. Serum PTH levels were very high in patients with untreated pseudohypoparathyroidism and decreased, although not to normal, when normocalcemia occurred either spontaneously or through treatment. Of the four patients who became normocalcemic spontaneously, basal and PTH-stimulated urinary excretion of cyclic AMP, and clearance of phosphate increased. These changes were all significantly different from the changes which occurred when patients became normocalcemic as a result of treatment with vitamin D anc calcium. The factors which govern the apparent increased renal sensitivity to endogenous and exogenous PTH when normocalcemia develops spontaneously in patients with pseudohypoparathyroidism remain to be explained. However, these changes are dissimilar from those which occur from treatment with vitamin D anc calcium.

Hypoparathyroidism

Recent advances in our understanding of the physiologic actions of PTH and vitamin D have clarified certain aspects of the pathogenesis, classification, and management of hypoparathyroidism. Central to pathogenesis and categorization is the recognition that hypoparathyroidism may result from PTH deficiency, ineffectiveness, or resistance, with a resultant inability to stimulate adenylate cyclase in target tissues. This aberration in adenylate cyclase activity impairs certain physiologic responses such as renal phosphate excretion and renal calcium reabsorption that are required for proper calcium homeostasis. Also critical is the subnormal production of 1 alpha,25-dihydroxycholecalciferol (1,25-DHCC). Although the precise mechanism for the deficiency of 1,25-DHCC remains unclear, one may hypothesize that in hormone-deficient or hormone-ineffective hypoparathyroidism, decreased synthesis results from the absence of the two recognized stimuli for 1 alpha-hydroxylase--bioactive PTH and hypophosphatemia. Provision of either one of these stimuli would then be expected to restore 1,25-DHCC to normal levels, which could explain the calcemic response to PTH in these patients. There is some evidence that the synthesis of 1,25-DHCC may be "primarily" affected in PTH-resistant hypoparathyroidism, and thus may be unresponsive to any of the known stimuli. It remains conceivable, however, that during normocalcemic phases, such patients may improve their renal cyclic AMP and phosphaturic responses to PTH, with associated improvement in 1,25-DHCC synthesis. Certain acquired forms of PTH resistance such as hypomagnesemia and end-stage renal disease may also be associated with defective 1-hydroxylation. Whether occurring primarily or as a secondary process, the subnormal production of 1,25-DHCC may influence calcium and skeletal metabolism directly or by modifying response to PTH. The availability of 1,25-DHCC provides an effective and physiologically meaningful mode of therapy for most cases of hypoparathyroidism.