Genetic contribution to intervertebral disc maintenance in mice

Abstract

Intervertebral disc (IVD) degeneration is a major cause of back pain, affecting the quality of life in an aging society. Currently, there are no effective treatments as the etiology of disc degeneration is not well understood. It is influenced by environmental and genetic factors. Over 90% of the population will develop disc degeneration by the age of 50. Intriguingly, approximately 10% of the population is “protected”, indicating possible genetic factors influencing disc homeostasis.

The LG/J (large) mice possess remarkable healing capacity for damage to elastic and articular cartilages, in contrast to the poor healer SM/J (small) mice. We hypothesize that the genetic makeup of these mice will influence intervertebral disc homeostasis as the IVD share similarity with cartilage. Using these parental strains of mice and recombinant inbreeds, we assessed the IVD changes in these mice by histology and studied the potential genetic contributions to disc maintenance in mice.

Analyses of whole spine of mice showed SM/J mice displayed “degenerative” changes in tail discs as early as 4-weeks of age, with an undefined nucleus pulposus (NP) structure that appear fibrotic, and the boundary between NP and annulus fibrosus (AF) become indistinguishable compared to that of LG/J. Assessment of IVD cell markers and extracellular matrix (ECM) components showed distinct changes consistent with degenerative process in SM/J tail discs while discs in LG/J maintains notochordal-like cells and ECM proteins consistent with a “healthy” disc, supported by semi quantitative comparison of tail discs using a histological scoring system.

In addition to the parental strains, I studied recombinant inbreds LGXSM-6 and LGXSM-33. Interesting, both inbred strains displayed severe disc changes in the tail discs at 4 weeks. Comparing the genetic makeup that are similar between SM/J and the recombinant inbreds and absence in LG/J, 178 SNPs on 119 nearby genes were identified of which seven genes contained missense variants for Tacr3, Mttp, Xpa, Cd19, Qprt, Cep112 and Abca6. Of the genes that feature highly in the Panther gene ontology pathways, Vdr was identified and is a known genetic risk factor for IVD degeneration in human studies.

Additional filtering the candidate genes with structural variations identified 89 variants in 50 genes. All variants are in the non-coding regions. Of these, the genes Cacng5, Cacng4 and Shank3 were clustered together for the pathway regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate selective glutamate receptor activity (GO: 2000311), p value = 0.00630. Significant, using these as candidate genes, a test association in our human cohort with GWAS data showed positive correlations with IDD traits such as high intensity zone and Schmorl’s nodes that indicate degenerative changes in the annulus fibrosus and cartilage endplates of the disc.

This study provided novel insight into the genetic contribution to disc integrity in mice. Addition analyses are needed to provide finer mapping of the genetic loci and the identification of functional changes. The information will provide important clues for genetic analysis of the human cohort to enhance the genetic power as we begin to unravel the loci for IVD maintenance and possible repair mechanisms.