Homozygous Cys(542)-> Arg substitution in GPIIIa in a Swiss patient with type I Glanzmann's thrombasthenia

Glanzmann's thrombasthenia (GT) arises from a qualitative or quantitative d efect in the GPIIb-IIIa complex (integrin alpha(IIb)beta(3)), the mediator of platelet aggregation. We describe a patient in whom clinical and laborat ory findings typical of type I GT were found together with a second patholo gy involving neurological and other complications symptomatic of tuberous s clerosis, Analysis of platelet proteins by Western blotting revealed trace amounts of normally migrating GPIIb and equally small amounts of GPIIIa of slightly slower than normal migration. Flow cytometry confirmed a much decr eased binding to platelets of monoclonal antibodies to GPIIb, GPIIIa or GPI Ib-IIIa, and an antibody to the alpha(v) subunit also showed decreased bind ing. Nonradioactive PCR single-strand conformation polymorphism analysis fo llowed by direct sequencing of PCR-amplified DNA fragments showed a homozyg ous point mutation (T to C) at nucleotide 1722 of GPIIIa cDNA and which led to a Cys(542)-->Arg substitution in the GPIIIa protein. The mutation gave rise to a HinP1 I restriction site in exon II of the GPIIIa gene and allele -specific restriction enzyme analysis of family members confirmed that a si ngle mutated allele was inherited from each parent. This amino acid substit ution presumably changes the capacity for disulphide bond formation within the cysteine-rich core region of GPIIIa and its study will provide new info rmation on GPIIb-IIIa and alpha(v)beta(3) structure and biosynthesis.