CRTAP and LEPRE1 mutations in recessive osteogenesis imperfecta

Abstract

Autosomal dominant osteogenesis imperfecta (OI) is caused by mutations in the genes (COL1A1 or COL1A2) encoding the chains of type I collagen. Recently, dysregulation of hydroxylation of a single proline residue at position 986 of both the triple-helical domains of type I collagen alpha1(I) and type II collagen alpha1(II) chains has been implicated in the pathogenesis of recessive forms of OI. Two proteins, cartilage-associated protein (CRTAP) and prolyl-3-hydroxylase-1 (P3H1, encoded by the LEPRE1 gene) form a complex that performs the hydroxylation and brings the prolyl cis-trans isomerase cyclophilin-B (CYPB) to the unfolded collagen. In our screen of 78 subjects diagnosed with OI type II or III, we identified three probands with mutations in CRTAP and 16 with mutations in LEPRE1. The latter group includes a mutation in patients from the Irish Traveller population, a genetically isolated community with increased incidence of OI. The clinical features resulting from CRTAP or LEPRE1 loss of function mutations were difficult to distinguish at birth. Infants in both groups had multiple fractures, decreased bone modeling (affecting especially the femurs), and extremely low bone mineral density. Interestingly, "popcorn" epiphyses may reflect underlying cartilaginous and bone dysplasia in this form of OI. These results expand the range of CRTAP/LEPRE1 mutations that result in recessive OI and emphasize the importance of distinguishing recurrence of severe OI of recessive inheritance from those that result from parental germline mosaicism for COL1A1 or COL1A2 mutations.

Figure 1

Early radiographs of Proband 2 with CRTAP mutation. A, B, C, and D, radiographs at age one day, consistent with severe OI. This infant was a compound heterozygote for mutations in CRTAP (c.278_293dup; c.822_826delAATACinsT).

Figure 2

Early radiographs of Proband 7 with LEPRE1 nonsense mutation. A, B, C, D, radiographs of Proband 7 at birth, consistent with severe OI, found to have a homozygous nonsense mutation in LEPRE1, c.392C>A, p.Ser131X.

Figure 3

Late radiographs of Proband 3 with CRTAP missense mutation. A–H, radiographs at age 8 years, consistent with OI, found to have a homozygous missense mutation in CRTAP, c.200T>C; p.Leu67Pro.

Figure 4

Late radiographs of Proband 10 with LEPRE1 nonsense mutation. A, B, C, radiographs of Proband 10 at age 11 years, previously described at age 6 years by Cabral et al.(Cabral, et al., 2007b) and confirmed to have a homozygous nonsense mutation in LEPRE1, c.1656C>A; p.Tyr552X.

Figure 5

Bulbous “popcorn epiphyses.” A, radiograph of femur of Proband 3 with a homozygous missense mutation in CRTAP, c.200C>T; p.Leu67Pro. B, radiograph of leg of Proband 10 with a homozygous nonsense mutation in LEPRE1, c.1656C>A; p.Tyr552X.

Figure 6

Schematic diagram of the exons and the locations of mutations found in the CRTAP and LEPRE1 genes. The mutations described in this report are shown above each gene and those previously described are shown below the respective gene. The number of alleles identified are in brackets [] and the references are indicated by superscript. Amino acids are represented by the single letter code: I, isoleucine; M, methionine; Q, glutamine; R, arginine; Y, tyrosine; and X, termination codon.

Figure 7

Tandem mass spectrometric analyses of α1(I) collagen from patient tissues and cultured cells with CRTAP (Proband 1) or LEPRE1 (Proband 11) mutations shows decreased 3-hydroxyproline relative to controls. A, B, and C, analyses from patient and control fibroblast-conditioned media. D, E, and F, analyses from patient and control bone and cultured osteoblast collagen.