A 19-base pair deletion in the pro-alpha 2(I) gene of type I procollagen that causes in-frame RNA splicing from exon 10 to exon 12 in a proband with atypical osteogenesis imperfecta and in his asymptomatic mother.

Previous observations established that fibroblasts from a proband with atypical osteogenesis imperfecta synthesized about equal amounts of normal pro-alpha 2(I) chains and shortened pro-alpha 2(I) chains of type I procollagen. The pro-alpha 2(I) chains were shortened because of an in-frame deletion of most or all of the 18 amino acids encoded by exon 11 of the pro-alpha 2(I) gene. Here it was demonstrated that one of the proband's alleles for the pro-alpha 2(I) gene contained a 19-base pair deletion at the junction of intervening sequence 10 and exon 11 that produced an RNA splicing defect. Probe protection experiments did not reveal any evidence for use of cryptic splice sites, and they suggested that the major species of abnormally spliced pro-alpha 2(I) mRNA in the proband's fibroblasts was completely spliced from exon 10 to 12. The defect in RNA splicing is unusual among RNA-splicing mutations in producing an abnormal polypeptide chain that is used for protomer assembly. Since the probe protection experiments showed the same defect in the mRNA from the fibroblasts of the asymptomatic mother, the mutation was inherited in an autosomal dominant manner but showed variable phenotypic expression in the proband's family.