Altered relation of two collagen types in osteogenesis imperfecta.

To ascertain if the features of osteogenesis imperfecta could be due to an abnormality in two genetically distinct collagens, Type III and Type I, we measured, in pepsin digests of skin, the ratio of the alpha 1, (III) to alpha 1 (I) chains derived from the two types, using a method of interrupted polyacrylamide-gel electrophoresis. In 40 control subjects this ratio decreased from about 0.45 in early fetal life to a mean (+/-2 S.D.) of 0.14 +/- 0.06 in 14 adults. Seven of nine adult patients with mild osteogenesis imperfecta had ratios of more than 4 S.D., above this mean, as did two of five patients with severe osteogenesis imperfecta. The increased ratio was probably due to a reduction of Type I collagen. If this reduction were generalized it could contribute to the bony fragility of osteogenesis imperfecta, since mineralized bone contains Type I collagen only.