Coexistence of two different pseudohypoparathyroidism subtypes (Ia and Ib) in the same kindred with independent Gsα coding mutations and GNAS imprinting defects

Background: Pseudohypoparathyroidism (PHP) defines a rare group of disorders whose common feature is resistance to the parathyroid hormone. Patients with PHP-Ia display additional hormone resistance, Albright hereditary osteodystrophy (AHO) and reduced Gsα activity in easily accessible cells. This form of PHP is associated with heterozygous inactivating mutations in Gsα-coding exons of GNAS, an imprinted gene locus on chromosome 20q13.3. Patients with PHP-Ib typically have isolated parathyroid hormone resistance, lack AHO features and demonstrate normal erythrocyte Gsα activity. Instead of coding Gsα mutations, patients with PHP-Ib display imprinting defects of GNAS, caused, at least in some cases, by genetic mutations within or nearby this gene. Patients: Two unrelated PHP families, each of which includes at least one patient with a Gsα coding mutation and another with GNAS loss of imprinting, are reported here. Results: One of the patients with GNAS imprinting defects has paternal uniparental isodisomy of chromosome 20q, explaining the observed imprinting abnormalities. The identified Gsa coding mutations include a tetranucleotide deletion in exon 7, which is frequently found in PHP-Ia, and a novel single nucleotide change at the acceptor splice junction of intron 11. Conclusions: These molecular data reveal an interesting mixture, in the same family, of both genetic and epigenetic mutations of the same gene.