| Summary: | Osteoarthritis (OA) is a common, multifactorial disease that is characterized by focal degeneration of the smooth articular cartilage in any of the synovial joints. Although the underlying molecular mechanisms for OA are still not fully understood, epidemiological studies have evidenced a significant genetic component to OA susceptibility. Genome-wide linkage scans and large-scale association studies have had success in unraveling the genetic architecture underlying OA with the identification of a number of susceptibility genes. In this work, functional analyses are reported of OA associated polymorphisms within two susceptibility genes: <em>BMP5</em> and <em>GDF5</em>, both members of the TGF-β superfamily of secreted proteins. The extent of differential allelic expression (DAE) of <em>BMP5</em> in human mesenchymal tissues was first examined with significant differences in <em>BMP5</em> allelic output observed (allelic ratios exceeding 4:1 in the tissues of some donors). Significant variability in allelic expression within the different tissues of donors was also observed, suggesting that polymorphism in <em>cis</em>-regulation of <em>BMP5</em> expression is common and that there is a considerable effect of tissue specific elements on <em>BMP5</em> expression. DAE was then used as a phenotype to map tissue-specific <em>cis</em>-regulatory polymorphisms with the identification of a single nucleotide polymorphism (SNP) located downstream of <em>BMP5</em> that was significantly associated with DAE as well as OA, suggesting that variability in <em>cis</em>-regulation of <em>BMP5</em> is important in OA susceptibility. Moreover, the functional effect of a previously identified OA associated microsatellite within intron 1 of <em>BMP5</em> was investigated using luciferase reporter assays and electrophoretic mobility shift assays (EMSAs) with significant differences observed both in the ability of various microsatellite alleles to modulate <em>BMP5</em> promoter activity and to bind GATA-3 nuclear proteins, further suggesting a role for variability in <em>BMP5</em> expression in OA susceptibility that may in part be due to altered GATA-3 binding. Finally, functional characterization of a previously reported OA associated SNP in the 5′ UTR of GDF5 is presented in which EMSAs show differential binding of nuclear factors between the two SNP alleles, strengthening the possible functional contribution of this SNP to OA susceptibility. Overall, this work demonstrates that polymorphism in <em>cis</em>-regulation is likely to play a role in OA susceptibility.
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