Sporadic Pseudohypoparathyroidism 1B in Monozygotic Twins: Insights Into the Pathogenesis of Methylation Defects

Background: Pseudohypoparathyroidism (PHP) 1B is an imprinting disorder characterized by renal resistance to parathyroid hormone (PTH) without Albright Hereditary Osteodystrophy (AHO). PHP1B is associated with methylation defects at the GNAS differentially methylated regions (DMRs). In sporadic cases with PHP1B, the mechanistic basis of methylation defects remains to be solved, except in rare cases with uniparental disomy of chromosome 20. In addition, to date, monozygotic twin cases with sporadic PHP1B have not been reported. Clinical Case: The patients were 26-year-old Japanese female monozygotic twins. They had been born to nonconsanguineous parents after an uneventful pregnancy. Both twins had common biochemical features, including hypocalcemia, hyperphosphatemia, elevated PTH levels, and impaired urinary excretion of phosphorus and cAMP in response to teriparatide. They showed no signs of AHO. The serum calcium levels of their parents and brother were within the normal range, and family history was unremarkable. Based on these findings, the twins were diagnosed with PHP1B. Targeted bisulfite sequencing of the GNAS DMRs in all family members revealed almost complete gain-of-methylation at the NESP55 DMR, and loss-of-methylation at the AS, XL, and A/B DMRs in the twins, but not in other family members. Except for the GNAS locus, we did not find clear methylation defects in other imprinted genome loci in the twins. Methylation defects at the GNAS locus were further confirmed by methylation-sensitive restriction enzyme-qPCR. Whole-genome sequencing of the twins showed no pathogenic variants in the GNAS exons encoding the Gs alpha subunit. No large deletions or insertions were found at the STX16 locus or in the region from AS exon 5 to XL. Based on the SNP genotyping results, large paternal isodisomies in the GNAS DMRs were unlikely. Collectively, these results suggested that the twins had concordant methylation defects that are seen in the sporadic form of PHP1b. We speculate that an early developmental event before the twin splitting is responsible for the abnormal methylation of the GNAS DMRs. Conclusion: We report, for the first time, monozygotic twins with sporadic PHP1B who were phenotypically and epigenetically concordant. Our comprehensive molecular genetic analyses have thus far ruled out the previously described genetic defects underlying PHP1B. The current findings provide new insights into the mechanistic basis of the GNAS methylation defects in sporadic PHP1B.