Nuclease S1 mapping of a homozygous mutation in the carboxyl-propeptide-coding region of the pro alpha 2(I) collagen gene in a patient with osteogenesis imperfecta.

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Abstract
The molecular defect in a patient with a moderately severe form of osteogenesis imperfecta was characterized by nuclease S1 mapping. Single-stranded 5'' and 3'' endlabeled DNA probes coding for 80% of the carboxylpropeptide of the pro.alpha.2(I) collagen gene were hybridized to mRNA isolated from cultured fibroblasts of the patient and his parents. Nuclease S1 digestion revealed a homozygous mutation in the patient and a heterozygous pattern in the consanguineous parents. As a result of the defect in the gene, none of the pro.alpha.2(I) chains synthesized by the patient''s fibroblasts were incorporated into a type I procollagen heterotrimer consisting of 2 pro.alpha.1(I) chains and 1 pro.alpha.2(I) chain. Cultured skin fibroblasts from the patient were previously shown to secrete only pro.alpha.1(I) trimers. Fibroblasts from both parents, who do not have osteogenesis imperfecta, secrete both pro.alpha.1(I) trimers and normal type I procollagen. Synthesis of pro.alpha.2(I) chains was decreased in fibroblasts from the patient and his parents. The decrease in the synthesis of pro.alpha.2(I) chains is not caused by decreased transcription of the pro.alpha.2(I) collagen alleles, since the pro.alpha.1(I)/pro.alpha.2(I) mRNA ratios were normal in the patient and his parents.