Abstract
Classical osteogenesis imperfecta (OI) is a dominant genetic disorder of connective tissue caused by mutations in either of the two genes encoding type I collagen, COL1A1 and COL1A2. Recent investigations, however, have generated a new paradigm for OI incorporating many of the prototypical features that distinguish dominant and recessive conditions, within a type I collagen framework. We and others have shown that the long-sought cause of the recessive form of OI, first postulated in the Sillence classification, lies in defects in the genes encoding cartilage-associated protein (CRTAP) or prolyl 3-hydroxylase 1 (P3H1/LEPRE1). Together with cyclophilin B (PPIB), CRTAP and P3H1 comprise the collagen prolyl 3-hydroxylation complex, which catalyzes a specific posttranslational modification of types I, II, and V collagen, and may act as a general chaperone. Patients with mutations in CRTAP or LEPRE1 have a lethal to severe osteochondrodystrophy that overlaps with Sillence types II and III OI but has distinctive features. Infants with recessive OI have white sclerae, undertubulation of the long bones, gracile ribs without beading, and a small to normal head circumference. Those who survive to childhood or the teen years have severe growth deficiency and extreme bone fragility. Most causative mutations result in null alleles, with the absence or severe reduction of gene transcripts and proteins. As expected, 3-hydroxylation of the Pro986 residue is absent or severly reduced, but bone severity and survival length do not correlate with the extent of residual hydroxylation. Surprisingly, the collagen produced by cells with an absence of Pro986 hydroxylation has helical overmodification by lysyl hydroxylase and prolyl 4-hydroxylase, indicating that the folding of the collagen helix has been substantially delayed.