Background/Purpose: Osteoarthritis (OA) is the leading cause of chronic disability in the U.S., affecting 40% of individuals over the age of 70 and costing $128 billion annually in the US alone.  Late-stage OA chondrocytes exhibit a host of gene transcription changes leading to upregulation of enzymes that contribute to cartilage breakdown. Herein, we characterize epigenome-wide DNA methylation changes in osteoarthritic compared to healthy cartilage from the same joints.

Methods: Twelve femoral heads were obtained at the time of hip arthroplasty for primary OA.  Articular cartilage tissue was dissected from grossly affected and grossly normal areas, flash frozen in liquid nitrogen, and DNA was extracted.  Following sodium bisulfite-treatment, DNA methylation was quantified at >485,000 CpG sites across the genome using Illumina HumanMethylation450 arrays.  CpG sites with an absolute methylation difference between OA and normal cartilage (Db) ≥ 15%, and P <0.01 after correction for multiple testing using a false discovery rate (FDR) of 5%, were considered statistically significant and used for subsequent analysis. 

Results: We found 442 differentially methylated CpG sites in OA compared to normal cartilage from the same joints: 260 hypo- and 182 hypermethylated.  Overrepresented gene sets included “Connective tissue disorder” (n=53, p=4.73E-6 to 7.36E-3), “Developmental disorder” (n=44, p=4.73E-6 to 7.36E-3), and “Skeletal & muscular disorder” (n=59, p=4.73E-6 to 7.36E-3).  Particularly interesting methylation changes in OA include hypermethylation of COL11A2, which functions to maintain spacing and diameter of type II collagen and is mutated in patients with Stickler syndrome and OSMED.  Additional DNA methylation changes detected include hypermethylation of COL6A2, hypomethylation of the fibrillar collagen gene COL1A1, and hypermethylation of COL18A1. Hypermethylation was also noted in multiple CpG sites within the WNT pathway co-receptor LRP5, which is associated with OA in mice and is a genetic susceptibility gene for OA in humans.  Hypomethylation was found at multiple CpG sites in the transcription factor RUNX1.  Upstream regulator analysis identified significant association of TGFβ1 with 44 differentially methylated genes.  Finally, canonical pathway analysis identified enrichment of several pathways, most prominently the ERK signaling pathway among differentially methylated genes (p=1.51E-4).

Conclusion: We detected significant methylation changes in multiple collagen genes in OA. Our data suggest an epigenetic basis for defective collagen production in OA.  Furthermore, we found evidence for epigenetic dysregulation of WNT signaling, and enrichment of genes associated with TGFβ- and ERK-pathway signaling, both of which are enticing targets for OA therapeutics.  This work reinforces a role for genetic/epigenetic interaction in the pathogenesis of OA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/epigenome-wide-dna-methylation-study-reveals-hypermethylated-collagen-genes-and-suggests-a-role-for-tgf%ce%b2-in-osteoarthritis/