A frameshift mutation results in a truncated nonfunctional carboxyl-terminal Proα1(I) propeptide of type I collagen in osteogenesis imperfecta

Abstract

A codon frameshift mutation caused by a single base (U) insertion after base pair 4088 of preproα1(I) mRNA of type I procollagen was identified in a baby with lethal perinatal osteogenesis imperfecta. The mutation was identified in fibroblast RNA by a new method that allows the direct detection of mismatched bases by chemical modification and cleavage in heteroduplexes formed between mRNA and control cDNA probes. The region of mismatches was specifically amplified by the polymerase chain reaction and sequenced. The heterozygous mutation in the amplified cDNA most likely resulted from a T insertion in exon 49 of COL1A1. The frameshift resulted in a truncated proα1(I) carboxyl-terminal propeptide in which the amino acid sequence was abnormal from Val1146 to the carboxyl terminus. The propeptide lacked Asn118a7, which normally carries an N-linked oligosaccharide unit, and was more basic than the normal propeptide. The distribution of cysteines was altered and the mutant propeptide was unable to form normal interchain disulfide bonds. Some of the mutant proα1(I)' chains were incorporated into type I procollagen molecules but resulted in abnormal helix formation with overhydroxylation of lysine residues, increased degradation, and poor secretion. Only normal type I collagen was incorporated into the extracellular matrix in vivo resulting in a tissue type I collagen content approximately 20% of that of control (Bateman, J. F., Chan, D., Mascara, T., Rogers, J. G., and Cole, W. G. (1986) Biochem. J. 240, 699-708).