Functional analysis of the osteoarthritis susceptibility–associated GDF5 regulatory polymorphism

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Abstract
Objective Single‐nucleotide polymorphism (SNP) rs143383 (T to C) in the 5′‐untranslated region (5′‐UTR) of GDF5 has recently been reported to be associated with osteoarthritis (OA) susceptibility, with lower expression of the risk‐associated T allele observed in vitro and in vivo. The in vivo studies were performed on cartilage tissue from OA patients. The present study was undertaken to expand the analysis of the effect of this SNP on GDF5 allelic expression to more joint tissue types, to investigate for cis and trans factors that interact with the SNP, and to examine novel cis‐acting GDF5 regulatory polymorphisms. Methods Tissue samples were collected from OA patients undergoing joint replacement of the hip or knee. Nucleic acid was extracted, and, using rs143383 and an assay that discriminates and quantifies allelic expression, the relative amount of GDF5 expression from the T and C alleles was measured. Additional common variants in the GDF5 transcript sequence were interrogated as potential regulatory elements using allelic expression and luciferase reporter assays, and electrophoretic mobility shift assays were used to search for trans factors binding to rs143383. Results We observed a consistent allelic expression imbalance of GDF5 in all tissues tested, implying that the functional effect mediated by rs143383 on GDF5 expression is joint‐wide. We identified a second polymorphism, located in the 3′‐UTR of GDF5, that influenced allelic expression of the gene independent of rs143383. Finally, we observed differential binding of deformed epidermal autoregulatory factor 1 (DEAF‐1) to the 2 alleles of rs143383. Conclusion These findings show that the OA susceptibility mediated by polymorphism in GDF5 is not restricted to cartilage, emphasizing the need to consider the disease as involving the whole joint. The existence of an additional cis‐acting regulatory polymorphism highlights the complexity of the regulation of expression of this important OA susceptibility locus. DEAF‐1 is a trans‐acting factor that merits further investigation as a potential tool for modulating GDF5 expression.