Biochemical screening of type I collagen in osteogenesis imperfecta: detection of glycine substitutions in the amino end of the alpha chains requires supplementation by molecular analysis.

BACKGROUND: The biochemical test for osteogenesis imperfecta (OI) detects structural abnormalities in the helical region of type I collagen as delayed electrophoretic migration of alpha chains on SDS-urea-PAGE. Sensitivity of this test is based on overmodification of alpha chains in helices with a glycine substitution or other structural defect. The limits of detectability have not been reported.
METHODS: We compared the collagen electrophoretic migration of 30 probands (types III or IV OI) with known mutations in the amino half of the alpha1(I) and alpha2(I) chains. Differences in sensitivity were examined by 5% and 6% SDS-urea-PAGE, and with respect to alpha chain, location along the chain, and substituting amino acid.
RESULTS: Sensitivity was enhanced on 5% gels, and by examination of intracellular and secreted collagen. In alpha1(I), substitutions in the first 100 residues were not detectable; 7% of cases in the current Mutation Consortium database are in this region. alpha1(I) substitutions between residues 100 and 230 were variably detectable, while those after residue 232 were all detected. In alpha2(I), variability of electrophoretic detection extended through residue 436. About a third of cases in the Consortium database are located in the combined variable detection region. Biochemical sensitivity did not correlate with substituting residue.
CONCLUSIONS: Complete testing of probands with normal type I collagen biochemical results requires supplementation by molecular analysis of cDNA or gDNA in the amino third of alpha1(I) and amino half of alpha2(I). Mutation detection in OI is important for counselling, reproductive decisions, exclusion of child abuse, and genotype-phenotype correlations.