Mutations in the genes encoding cartilage associated
protein (CRTAP) and
prolyl 3-hydroxylase 1 (P3H1 encoded by LEPRE1) were the first identified causes of recessive
Osteogenesis Imperfecta (OI). These
proteins, together with
cyclophilin B (encoded by PPIB), form a complex that 3-hydroxylates a single
proline residue on the α1(I) chain (Pro986) and has
cis/trans isomerase (
PPIase) activity essential for proper
collagen folding. Recent data suggest that prolyl 3-hydroxylation of Pro986 is not required for the structural stability of
collagen; however, the absence of this post-translational modification may disrupt
protein-
protein interactions integral for proper
collagen folding and lead to
collagen over-modification. P3H1 and CRTAP stabilize each other and absence of one results in degradation of the other. Hence, hypomorphic or loss of function mutations of either gene cause loss of the whole complex and its associated functions. The relative contribution of losing this complex's 3-hydroxylation versus
PPIase and
collagen chaperone activities to the phenotype of recessive OI is unknown. To distinguish between these functions, we generated knock-in mice carrying a single amino acid substitution in the catalytic site of P3h1 (Lepre1(H662A) ). This substitution abolished P3h1 activity but retained ability to form a complex with Crtap and thus the
collagen chaperone function. Knock-in mice showed absence of prolyl 3-hydroxylation at Pro986 of the α1(I) and α1(II)
collagen chains but no significant over-modification at other
collagen residues. They were normal in appearance, had no growth defects and normal cartilage growth plate histology but showed decreased trabecular bone mass. This new mouse model recapitulates elements of the bone phenotype of OI but not the cartilage and growth phenotypes caused by loss of the prolyl 3-hydroxylation complex. Our observations suggest differential tissue consequences due to selective inactivation of P3H1
hydroxylase activity versus complete ablation of the prolyl 3-hydroxylation complex.