Osteogenesis imperfecta type IV. Detection of a point mutation in one α1(I) collagen allele (COL1A1) by RNA/RNA hybrid analysis

We have identified a point mutation in one α1(I) collagen allele (COL1A1) of a child with the type IV osteogenesis imperfecta phenotype. When compared to parental and control samples, skin fibroblasts of the proband synthesized two populations of type I collagen molecules. One population was normal; the other was delayed in secretion and electrophoretic migration due to post-translational overmodification. Two-dimensional gel electrophoresis of the CNBr peptides demonstrated a gradient of overmodification beginning near the carboxyl-terminal CB peptides. This predicts that the mutation delaying helix formation is near the carboxyl-terminal end of one of the component chains of type I collagen. The mRNA of the patient was probed with overlapping antisense riboprobes to type I collagen cDNA. Cleavage of a mismatch in RNA/RNA hybrids to RNase A allowed the location of the mutation to a 225-base pair region of α1(I) cDNA. The mismatch was not present in RNA/RNA hybrids from either parent. This region of both α1(I) alleles of the patient was isolated by screening a λZAP cDNA library. Sequence determination of both alleles demonstrated a single nucleotide change, G→A, resulting in the substitution of a serine for a glycine at amino acid residue 832. This point mutation occurs in the coding region for α1(I) CB6 and is concordant with the protein data. The finding of a glycine substitution in a α1(I) chain of a patient with the milder type IV osteogenesis imperfecta phenotype requires modification of current molecular models for type II and IV osteogenesis imperfecta.