COL5A1 haploinsufficiency is a common molecular mechanism underlying the classical form of EDS

We have identified haploinsufficiency of the COL5A1 gene that encodes the p ro alpha 1(V) chain of type V collagen in the classical form of the Ehlers- Danlos syndrome (EDS), a heritable connective-tissue disorder that severely alters the collagen-fibrillar structure of the dermis, joints, eyes, and b lood vessels. Eight of 28 probands with classical EDS who were heterozygous for expressed polymorphisms in COL5A1 showed complete or nearly complete l oss of expression of one COL5A1 allele. Reduced levels of pro alpha 1(V) mR NA relative to the levels of another type V collagen mRNA, pro alpha 2(V), were also observed in the cultured fibroblasts from EDS probands. Products of the two COL5A1 alleles were approximately equal after the addition of cy cloheximide to the fibroblast cultures. After harvesting of mRNAs from cycl oheximide-treated cultured fibroblasts, heteroduplex analysis of overlappin g reverse transcriptase-PCR segments spanning the complete pro alpha 1(V) c DNA showed anomalies in four of the eight probands that led to identificati on of causative mutations, and, in the remaining four probands, targeting o f CGA-->TGA mutations in genomic DNA revealed a premature stop at codon in one of them. We estimate that approximately one-third of individuals with c lassical EDS have mutations of COL5A1 that result in haploinsufficiency. Th ese findings indicate that the normal formation of the heterotypic collagen fibrils that contain types I, III, and V collagen requires the expression of both COL5A1 alleles.