The Molecular Genetics and Epigenetics of COLGALT2, a Risk Locus for Osteoarthritis

Abstract

Osteoarthritis (OA) is a debilitating joint disorder affecting millions globally. No disease-modifying osteoarthritis drugs exist to date, highlighting the need for in-depth research into the molecular underpinnings of OA. The aetiology of OA is multifactorial, with genetics playing a crucial role in its development. Genome-wide association studies (GWAS) have to date linked over 125 genetic variants to OA. Functional follow-up studies on these variants are needed in order to gain profound insights into the biology of this complex trait and diseases and create therapeutic opportunities. In this thesis I investigated two independent OA GWAS signals, rs11583641 and rs1046934, associated with a collagen galactosyltransferase gene COLGALT2. Cartilage and synovium tissues from arthroplasty patients were used to detect quantitative trait loci (QTLs) and link the genetic variants with gene expression and DNA methylation (DNAm) levels at enhancer regions. These effects were also investigated in developmental cartilage samples in order to gain insight into the developmental origins of OA. CRISPR/Cas9 tools were used to identify causal relationships between DNAm at the enhancers and gene expression in vitro cell models of cartilage and synovium. The role of the COLGALT2 enzyme in chondrocyte biology was studied using a proteomic analysis of mesenchymal stem cells differentiated into chondrocytes The OA associated variants were found to act as both expression and methylation -QTLs in arthroplasty and in developmental cartilage samples. The risk alleles of these two independent variants were found to correlate with DNAm levels at enhancer regions and with the expression of COLGALT2. Targeted epigenetic modulation of the enhancers revealed causal links between DNA methylation and COLGALT2 expression. The rs11583641 locus was active in synovial tissue but the mechanism driving the effect was distinct from that observed in cartilage. Depletion of the COLGALT2 enzyme revealed it to be crucial for the development of chondrocytes and for the secretion of healthy cartilage ECM. The work presented in this thesis highlights the complex genetic landscape of OA, underscoring the importance of genetic and epigenetic factors in its pathogenesis. The identification of novel genetic targets opens avenues for the development of DMOADs, offering hope for improved management and treatment of OA.