Deficiency of CRTAP in non-lethal recessive osteogenesis imperfecta reduces collagen deposition into matrix.

Osteogenesis imperfecta (OI) is a heterogeneous heritable connective tissue disorder characterized by bone fragility and deformity. The majority of OI cases have dominant inheritance (Sillence types I to IV OI) and result from mutations in the COL1A1 or COL1A2 genes, encoding the proα1(I) and proα2(I) chains of type I collagen, the major structural protein of bone (1, 2). Biochemically, collagen structural defects delay helical folding, exposing the chains to post-translational prolyl 4-hydroxylation and lysyl hydroxylation for a longer time, resulting in ‘over-modification’ and delayed electrophoretic migration of collagen chains. In the last 5 years, a few recessive forms of OI have been shown to be caused by defects in the genes encoding the components of the collagen prolyl 3-hydroxylation complex (3, 4): cartilage-associated protein (CRTAP) (type VII OI, OMIM #610682) (5, 6), LEPRE1 (7–9) (type VIII OI, OMIM #610915), and PPIB (10–12) (type IX OI, OMIM #259440). Recently, additional disease loci responsible for recessive OI have been identified: FKBP10 (13), SERPINH1 (14), SP7/OX (15), and SERPINF1 (16). While both FKBP10 and SERPINH1 code for collagen chaperones resident in the ER, products of the latter two genes instead are not directly involved in collagen production or secretion but are key factors in osteoblasts differentiation and activity. Patients with defects in the components of the ER-resident 3-hydroxylation complex have moderate to severe/lethal OI, with white sclerae, small to normal head circumference and structurally normal collagen. Loss-of-function mutations in CRTAP and LEPRE1 result in rhizomelia, decreased to absent 3-hydroxylation of α1(I)Pro986, and collagen helical overmodification indicative of delayed folding. Of the three components of the 3-hydroxylation complex, CRTAP is known to be secreted into the extracellular matrix (17, 18). Normally, about 10% of CRTAP is secreted, while most is retained in the ER in a complex with prolyl 3-hydroxylase 1 (P3H1). Sixteen CRTAP mutant alleles, occurring in 15 index probands, have been reported (5–7, 18–20). Most null cases are lethal in the perinatal period or within the first year of life. Five non-lethal cases have been described. We present here a 7-year-old Egyptian boy whose severe OI is caused by homozygosity for a frameshift mutation in CRTAP exon 1. His dermal fibroblast type I collagen has typical post-translational modification defects for type VII OI. We report here the novel finding that the collagen content of matrix deposited by patient cells in culture is severely decreased. This data is supported by an in vitro collagen matrix-chase assay. These investigations describe matrix deficiency and disorganization associated with CRTAP deficiency which may reflect the absence of the crucial functions of CRTAP in extracellular matrix.