A Novel GNAS Duplication Associated With Loss-of-Methylation Restricted to Exon A/B Causes Pseudohypoparathyroidism Type Ib (PHP1B)

Abstract

Pseudohypoparathyroidism type Ib (PHP1B) is characterized by resistance to parathyroid hormone (PTH) leading to hypocalcemia and hyperphosphatemia, and in some cases resistance toward additional hormones. Patients affected by this disorder all share a loss‐of‐methylation (LOM) at the differentially methylated GNAS exon A/B, which reduces expression of the stimulatory G protein α‐subunit (Gsα) from the maternal allele. This leads in the proximal renal tubules, where the paternal GNAS allele does not contribute much to expression of this signaling protein, to little or no Gsα expression thereby causing PTH resistance. We now describe a PHP1B patient with a de novo genomic GNAS duplication of approximately 88 kb, which is associated with LOM restricted to exon A/B alone. Multiplex ligation‐dependent probe amplification (MLPA), comparative genomic hybridization (CGH), and whole‐genome sequencing (WGS) established that the duplicated DNA fragment extends from GNAS exon AS1 (telomeric breakpoint) to a small region between two imperfect repeats just upstream of LOC105372695 (centromeric breakpoint). Our novel duplication is considerably shorter than previously described duplications/triplications in that portion of chromosome 20q13 and it does not affect methylation at exons AS and XL. Based on these and previous findings, it appears plausible that the identified genomic abnormality disrupts in cis the actions of a transcript that is required for establishing or maintaining exon A/B methylation. Our findings extend the molecular causes of PHP1B and provide additional insights into structural GNAS features that are required for maintaining maternal Gsα expression and for preventing PTH‐resistance. © 2020 American Society for Bone and Mineral Research (ASBMR).

GNAS AS2 methylation status enables mechanism-based categorization of Pseudohypoparathyroidism Type 1B

“…STX16 deletions, leading to the loss of an enhancer for NESP55 transcription, are the most common cause in this category ( 9 ). Other less frequent causes include duplications spanning the NESP55 promoter ( 13 ), in which the telomeric (downstream) copy of the duplicated NESP55 promoters that normally dictates AS2 methylation is unlikely to be sufficiently active (as explained above). Deletions restricted to NESP55 exon and/or its promoter, which reportedly show a similar pattern of GNAS methylation defect as STX16 deletions ( 15 , 22 ), may also belong to this category; however, such samples were not available for our current study.…”

“…Patient characteristics are summarized in Supplemental Table 1 . Thirty-one patients with PHP1B, including previously described cases ( 9 , 11 , 13 , 14 , 20 , 34 – 36 ), and 21 unaffected controls were included. Patients were clinically diagnosed as PHP1B based on elevated PTH levels with or without hypocalcemia, hyperphosphatemia, and normal renal function.…”