Homozygosity for autosomal dominant Marfan syndrome.

Marfan syndrome is an autosomal dominant condition with varying phenotypic manifestations. Affected persons are usually heterozygotes. A family is presented in which the gene for this syndrome is segregating in a large number of members. Two sibs suffered from unusually severe, identical, and fatal manifestations from birth, their parents having mild cardiovascular and somatic symptoms common in Marfan syndrome. Investigation of collagen biosynthesis in fibroblasts revealed no abnormalities in fibronectin and procollagen I and III synthesis and secretion or in the procollagen to collagen conversion. We suggest that these two sibs are examples of homozygosity for the Marfan syndrome gene, based on the large number of affected members, the absence of additional consanguinity, manifestation of the syndrome in both parents, and the severity of the disease in the two sibs.