Loss of matrilin 1 does not exacerbate the skeletal phenotype in a mouse model of multiple epiphyseal dysplasia caused by a Matn3 V194D mutation

Objective Mutations in matrilin 3 can result in multiple epiphyseal dysplasia (MED), a disease characterized by delayed and irregular bone growth and early‐onset osteoarthritis. Although intracellular retention of the majority of mutant matrilin 3 was previously observed in a murine model of MED caused by a Matn3 V194D mutation, some mutant protein was secreted into the extracellular matrix. Thus, it was proposed that secretion of mutant matrilin 3 may be dependent on the formation of hetero‐oligomers with matrilin 1. The aim of this study was to investigate the hypothesis that deletion of matrilin 1 would abolish the formation of matrilin 1/matrilin 3 hetero‐oligomers, eliminate the secretion of mutant matrilin 3, and influence disease severity. Methods Mice with a Matn3 V194D mutation were crossed with Matn1 ‐null mice, generating mice that were homozygous for V194D and null for matrilin 1. This novel mouse was used for in‐depth phenotyping, while cartilage and chondrocytes were studied both histochemically and biochemically. Results Endochondral ossification was not disrupted any further in mice with a double V194D mutation compared with mice with a single mutation. A similar proportion of mutant matrilin 3 was present in the extracellular matrix, and the amount of retained mutant matrilin 3 was not noticeably increased. Retained mutant matrilin 3 formed disulfide‐bonded aggregates and caused the co‐retention of matrilin 1. Conclusion We showed that secretion of matrilin 3 V194D mutant protein is not dependent on hetero‐oligomerization with matrilin 1, and that the total ablation of matrilin 1 expression has no impact on disease severity in mice with MED. Mutant matrilin 3 oligomers form non‐native disulfide‐bonded aggregates through the misfolded A domain.