Collagen (I) homotrimer potentiates the osteogenesis imperfecta (oim) mutant allele and reduces survival in male mice

Abstract

Type I collagen is the major structural component of bone where it exists as an (α1)2(α2)1 heterotrimer in all vertebrates. The oim mouse model comprising solely homotrimeric (α1)3 collagen-1, due a dysfunctional α2 chain, has a brittle bone phenotype implying that the heterotrimeric form is required for physiological bone function. However, humans with null alleles preventing synthesis of the α2 chain have connective tissue and cardiovascular abnormalities (cardiac valvular Ehlers Danlos Syndrome), without evident bone fragility. Col1a2 null and osteogenesis imperfecta (oim) mouse lines were used in this study and bones analysed by microCT and 3-point bending. RNA was also extracted from heterozygote tissues and allelic discrimination analyses performed using qRT-PCR. Here we show that mice lacking the α2(I) chain do not have impaired biomechanical or bone structural properties, unlike oim homozygous mice. However Mendelian inheritance was affected in male mice of both lines and male mice null for the α2 chain exhibited age-related loss of condition. The brittle bone phenotype of oim homozygotes could result from detrimental effects of the oim mutant allele, however the phenotype of oim heterozygotes is known to be less severe. We used allelic discrimination to show that the oim mutant allele is not downregulated in heterozygotes. We then tested whether gene dosage was responsible for the less severe phenotype of oim heterozygotes by generating compound heterozygotes. Data showed that compound heterozygotes had impaired bone structural properties as compared to oim heterozygotes, albeit to a lesser extent than oim homozygotes. Hence, we concluded that the presence of heterotrimeric collagen (I) in oim heterozygotes alleviates the effect of the oim mutant allele but a genetic interaction between homotrimeric collagen (I) and the oim mutant allele leads to bone fragility.