GNAS defects identified by stimulatory G protein alpha-subunit signalling studies in platelets.

Context. GNAS is an imprinted region that gives rise to several transcripts, antisense transcripts and noncoding RNAs, including transcription of RNA encoding the alpha subunit of the stimulatory G protein (Gsalpha). The complexity of the GNAS cluster results in ubiquitous genomic imprints, tissue-specific Gsalpha expression and multiple genotype-phenotype relationships. Phenotypes resulting from genetic and epigenetic abnormalities of the GNAS region include Albright's Hereditary Osteodystrophy (AHO), pseudohypoparathyroidism types Ia (PHPIa) and Ib (PHPIb), and pseudopseudohypoparathyroidism (PPHP). Objective. To study the complex GNAS pathology by a functional test, as an alternative to the generally used but labour-intensive erythrocyte complementation assay. Design and patients. We report the first platelet-based diagnostic test for Gsalpha hypofunction, supported by clinical, biochemical, and molecular data for 6 patients with PHPIa or PPHP and 9 patients with PHPIb. The platelet test is based on the inhibition of platelet aggregation by cAMP, produced after Gsalpha stimulation. Results. Platelets are easily accessible, and platelet aggregation responses were found to reflect Gsalpha signaling defects in patients, in concordance with the patient's phenotype and genotype. Gsalpha hypofunction in PHPIa and PPHP patients with GNAS mutations was clearly detected by this method. Mildly decreased or normal Gsalpha function was detected in patients with PHPIb with either an overall or exon1A-only epigenetic defect, respectively. Platelet Gsalpha expression was reduced in both PHPIb patient groups while XLalphas was upregulated only in PHPIb patients with the broad epigenetic defect. Conclusion. The platelet-based test is a novel tool for establishing the diagnosis of Gsalpha defects, which may be otherwise quite challenging.