The Journal of clinical endocrinology and metabolism | 2021
A novel familial PHP1B variant with incomplete loss-of-methylation at GNAS-A/B and enhanced methylation at GNAS-AS2.
Abstract
CONTEXT\nPseudohypoparathyroidism type 1B (PHP1B), also referred to as inactivating PTH/PTHrP Signaling Disorder (iPPSD), is characterized by proximal renal tubular resistance to parathyroid hormone (PTH) leading to hypocalcemia, hyperphosphatemia and elevated PTH values. Autosomal dominant PHP1B (AD-PHP1B) with loss-of-methylation at the maternal GNAS A/B:TSS-DMR (transcription start site-differentially methylated region) alone can be caused by maternal deletions involving STX16.\n\n\nOBJECTIVES\nCharacterize a previously not reported AD-PHP1B family with loss-of-methylation at GNAS A/B:TSS-DMR, but without evidence for a STX16 deletion on the maternal allele and assess GNAS-AS2:TSS-DMR methylation.\n\n\nPATIENTS AND METHODS\nDNAs from 24 patients and 10 controls were investigated. AD-PHP1B patients without STX16 deletion from a single family (n=3), AD-PHP1B patients with STX16 deletion (n=9), sporPHP1B (n=10), unaffected controls (n=10), patUPD20 (n=1), and matUPD20 (n=1). Methylation and copy number analyses were performed by pyrosequencing, MS-MPLA, and MLPA, respectively.\n\n\nRESULTS\nMolecular cloning of PCR-amplified, bisulfite-treated genomic DNA from healthy controls revealed evidence for two distinct GNAS-AS2:TSS-DMR subdomains, named AS2-1 and AS2-2, which showed 16.0±2.3% and 31.0±2.2% methylation, respectively. DNA from affected members of a previously not reported AD-PHP1B family without the known genetic defects revealed incomplete LOM (loss-of-methylation) at GNAS A/B:TSS-DMR, normal methylation at the three well-established maternal and paternal DMRs, and, surprisingly, increased methylation at AS2-1 (32.9±3.5%), but not at AS2-2 (30.5±2.9%).\n\n\nCONCLUSION\nThe distinct methylation changes at the novel GNAS-AS2:TSS-DMR will help characterize further different PHP1B/iPPSD3 variants and will guide the search for underlying genetic defects, which may provide novel insights into the mechanisms underlying GNAS methylation.