Pseudo hypoparathyroidism type Ib with disturbed imprinting in the GNAS1 cluster and Gsα deficiency in platelets

Pseudohypoparathyroidism lb (PHPlb), characterized by parathyroid hormone-resistant hypocalcemia and hyperphosphatemia, is caused by a deregulation in the imprinting status of the GNAS1 cluster, comprising exons XL, NESP55 and 1A and the coding exons of Gsalpha. Differences in methylation of exon 1A and sporadically also of exons XL and NESP55 were found and thought to result in long-range effects on Gsa expression, limited to the proximal renal tubules. The exact imprinting defect Is not precisely localized, and the expected differences in Gsa protein level and function are mainly hypothetical. We describe a PHPIb patient with lack of methylation of the exon XL and 1A promoters, and biallelic methylation of the NESP55 promoter. Platelets of this patient show a :functional Gs defect, decreased AMP formation upon Gs-receptor stimulation, normal Gsa sequence but reduced Gsa protein levels. Transcriptional deregulation between the now biallelically active promoters of both exon 1A and exon 1 of Gsa could explain the decreased Gsa expression in platelets and presumably in the proximal renal tubules. We found decreased NESP55 and increased XLalphas protein levels in platelets, in agreement with the methylation status of their corresponding first exons. In a megakaryocytic cell line MEG-01, exon 1A is methylated on both alleles, in contrast to the normally maternally methylated exon 1A in leukocytes. Experimental demethylation of exon 1A in MEG-01 cells led to reduced Gsa expression, in agreement with the observations in the patient. Platelet studies may therefore allow easy evaluation of disturbances of the GNAS1 cluster in PHPIb patients.