Mutations in collagen I are more commonly related with Osteogenesis Imperfecta (OI, brittle bone disease) and Ehlers Danlos Syndrome (EDS, hypermobility), along with a spectrum of phenotypes within these classifications. Potter et al. reported that mice heterozygous for a Col1a2 mutation – resulting in alternate splice site and inclusion of an extra amino at the start of exon 22 – developed a late onset osteoarthritis (OA) around 12 months of age. The purpose of this study was to investigate the presence of OI, ESD and OA phenotypes through mechanical testing in wild type and mutant mice.
Female C3H mice heterozygous to the Col1a2 mutation (n=5) and their wild-type controls (n=5) were used. Tissues were isolated at around 3 months of age. All mechanical testing was conducted using a multiaxial mechanical tester (Mach-1 v500css, Biomomentum, Canada). The left and right humeri were extracted for 3-point bending performed at a loading velocity of 0.05 mm/s. The abdominal skin was collected for tensile rupture tests performed at 2 mm/s. The tibial plateaus and femoral condyles from left knee joints were harvested and used to perform automated indentation mapping (≈15 positions/surfaces). At each position, a spherical indentation (D=0.3mm, Amplitude=30um) is performed by simultaneously moving the 3 orthogonal stages along an axis (perpendicular to the surface) while recording the resulting load components.
3-point bending revealed that heterozygous mice have more fragile humeri indicated by a lower maximum flexural load and work to fracture compared to wild-type mice. Similar humeri outer diameter and cortical wall thickness were measured for both groups. Moreover, heterozygous mice have reduced tensile properties denoted with an increased extensibility which is evident in the early phase of displacement. Indentation mappings showed a significantly higher structural stiffness in the medial femoral condyles of heterozygous mice.
Our results confirm the OI (brittle bones) and EDS (hyperelasticity of skin) phenotypes in mutant mice. Also, indentation mapping of articular surfaces reveals significant changes in mechanical properties of cartilage in mutant mice compared to wild-type. Overall, this study provides an essential insight into how the mutant collagen protein could result in OA in this model, and potentially in OI and EDS patients through mechanical testing of various tissues.