Interplay between Genetic Risk Factors and Protective Mechanisms for Intervertebral Disc Degeneration in Mice

Abstract

Introduction Intervertebral disc degeneration is a major cause of back pain. It is clear that there is a significant genetic contribution to the susceptibility of an individual-to-disc degeneration. Study in Hong Kong population cohort for disc degeneration cohort showed that while approximately 90% of the people will develop some form of disc degeneration by the age of 50 years, there are individuals who showed no indication of disc degeneration by MRI, suggesting the possibility of “protective factors,” either genetic and/or environmental. We have previously shown that Aspirin, an extracellular matrix protein, is a genetic risk factor of disc degeneration. Furthermore, our unpublished data have revealed that transgenic mice (ASPN-tg) overexpressing human ASPN gene in cartilage and disc tissues results in an accelerated or abnormal differentiation of cells in the mouse nucleus pulposus that could enhance initiation of disc degeneration. It is possible to study protective genes in humans using case–control association studies, but this would require the collection of a very large cohort of “protected” individuals. Another approach is to study in animal models and map the identified loci to the human genome. In mice, there are “super healer” strains (e.g., LG/J) with superior healing potentials of damaged tissues including ear elastic cartilage and hyaline cartilage of synovial joints, and potential genetic loci for improved joint cartilage repair have been identified in these mice. Materials and Methods Using the LG/J mice as a source of “protective” genes, we aim to perform sequential backcross of the ASPN-tg mice in C57 background to an LG/J background and compare the degenerative outcome in each generation. Our hypothesis is that if there is a protective or modulating effect, this will be reflected in histological analysis of the tail discs, coupled with the use of specific molecular analysis of differentiation markers for the different population of nucleus pulposus (NP) cells. Results Our data showed that ASPN-tg in successive backcross generations to LG/J background showed progressive improvement in the repair potential to ear punch wound compared with the parental strain, ASPN-tg/C57, for the repair of a 2 mm through-and-through ear punch wound. This is our positive indicator for the presence of “healing” genes in corresponding F1, F2, and F3 generations for the repair of damages to ear cartilage. In the NP for the backcrossed mice at F3, histological and immunostaining of cells in the NP showed fewer or absence of clusters of small round-shaped cells and Brachyury/Sox-9 positive chondrocyte-like cells were observed in ASPN-tg/C57 mice, suggesting there are genes in the LG/J background that reduces the risk or susceptibility to disc degeneration. Conclusion Our study reveals that genetic components in mice with a genetic background for better wound healing can provide protection against the morphological changes to chondrocyte-like cells in the NP that we use as an indicator of disc degeneration in intervertebral discs, induced by enhanced expression of ASPN in NP cells. Further investigation is needed to map the protective genes and study their functional impact on genetic risk factors to provide mechanistic insight for this protection or modulation of disc degeneration.