Coexistence of two different pseudohypoparathyroidism subtypes (Ia and Ib) in the same kindred with independent Gs{alpha} 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 Gsalpha activity in easily accessible cells. This form of PHP is associated with heterozygous inactivating mutations in Gsalpha-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 Gsalpha activity. Instead of coding Gsalpha 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 Gsalpha 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 Gsalpha 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.