Genetic and Epigenetic Characteristics of Autosomal Dominant Pseudohypoparathyroidism Type 1B: Case Reports and Literature Review

Hypocalcemic Tetany Transiently "Cured" by Pregnancy: A Case Report

Hypocalcemia is an uncommon electrolyte abnormality. We evaluated a young woman with episodes of recurrent symptomatic hypocalcemia, which had started in her late teens. She was hypertensive and laboratory evaluation revealed elevated parathyroid hormone, elevated phosphate, and decreased renal phosphate excretion along with low ionized calcium levels. Her renin and aldosterone levels were elevated. While the response to treatment with calcium and 1,25(OH)2 dihydroxyvitamin D was modest, her laboratory values and symptoms improved significantly during pregnancy and then recurred post-delivery. Based on clinical and laboratory features, we made a diagnosis of pseudohypoparathyroidism type Ib (PHP1B). Her genomic DNA revealed broad methylation changes at the GNAS locus, which encodes the α-subunit of the stimulatory G protein (Gαs), without evidence for a deletion or duplication, consistent with PHP1B. Analyses of several microsatellite markers on chromosome 20 performed on our patient and her parents provided no evidence for paternal uniparental isodisomy/heterodisomy of chromosome 20q13.3 (patUPD20q). The approach to hypocalcemia; classification, clinical features, and genetic/epigenetic basis for pseudohypoparathyroidism; alterations of mineral metabolism in PHP1B and pregnancy; and implications for the high renin and aldosterone levels and hypokalemia are discussed.

STX16 exon 5-7 deletion in a patient with pseudohypoparathyroidism type 1B

OBJECTIVES

Pseudohypoparathyroidism (PHP) comprises a cluster of heterogeneous diseases characterized by hypocalcemia and hyperphosphatemia due to parathyroid hormone (PTH) resistance. PHP type 1B (PHP1B) is caused by heterozygous maternal deletions within GNAS or STX16. STX16 exon 2-6 deletion is commonly observed in autosomal dominant (AD)-PHP1B, while sporadic PHP1B commonly results from methylation abnormalities of maternal differentially methylated regions and remains unclear at the molecular level.

CASE PRESENTATION

A 39-year-old male patient with PHP1B, who had his first seizure at 15 years of age, presented to our hospital. The methylation-specific multiplex ligation-dependent probe amplification results showed a half-reduced copy number of STX16 exon 5-7 and loss of methylation at GNAS exon A/B. His mother also had a half-reduced copy number of STX16 exon 5-7 but with normal methylation of GNAS. His father has a normal copy number of STX16 and normal methylation of GNAS.

CONCLUSIONS

For the recognition and early diagnosis of this kind of disease, here we report the clinical symptoms, auxiliary examinations, genetic testing characteristics, and treatment of the patient.

GNAS locus: bone related diseases and mouse models

GNASis a complex locus characterized by multiple transcripts and an imprinting effect. It orchestrates a variety of physiological processes via numerous signaling pathways. Human diseases associated with the GNAS gene encompass fibrous dysplasia (FD), Albright's Hereditary Osteodystrophy (AHO), parathyroid hormone(PTH) resistance, and Progressive Osseous Heteroplasia (POH), among others. To facilitate the study of the GNAS locus and its associated diseases, researchers have developed a range of mouse models. In this review, we will systematically explore the GNAS locus, its related signaling pathways, the bone diseases associated with it, and the mouse models pertinent to these bone diseases.

A Case of Sporadic Pseudohypoparathyroidism Type 1B Presented with Hypokalemia

Luo et al. 1 reported two cases of autosomal dominant pseudohypoparathyroidism type 1B (AD-PHP1B) and reviewed literature about the genetic and epigenetic characteristics of AD-PHP1B. Pseudohypoparathyroidism (PHP) is a cluster of heterogeneous diseases characterized by hypocalcemia and hyperphosphatemia due to resistance to parathyroid hormone (PTH). PHP1B almost results from methylation abnormalities of the maternal differentially methylated regions (DMRs) and can be divided into sporadic PHP1B and AD-PHP1B 1. As mentioned in this article 1, AD-PHP1B is caused by heterozygous maternal deletions within GNAS or STX16, which are associated with loss of methylation at the A/B DMR alone or at all maternally methylated GNAS exons. While sporadic PHP1B remains unclear at the molecular level, except for approximately 10% of the patients caused by paternal uniparental isodisomy or heterodisomy involving chromosome 20q (patUPD20q) 2. Here, we would like to present a rare case of sporadic PHP1B occurring in association with hypokalemia.