DNA methylation of DKK‐1 may correlate with pathological bone formation in ankylosing spondylitis

OBJECTIVE: To investigate DNA methylation (DNAm) status of dickkopf‐associated protein 1 (DKK‐1) in ossified hip capsule synovium and serum among patients with ankylosing spondylitis (AS). METHODS: Western blot was applied to detect the level of DKK‐1 protein expression in hip joint capsule tissues from four patients with AS as well as four patients with femoral neck fracture (FNF) caused by trauma as control. DKK‐1 gene promoter methylation (GPM) was examined by methylation‐specific polymerase chain reaction. Reverse transcription‐polymerase chain reaction was performed to examine the messenger RNA (mRNA) levels of DKK‐1, β‐catenin, and Wnt3a in both tissue and serum. The DNAm status of serum DKK‐1 was measured among 36 patients with AS and syndesmophytes (AS + syndesmophytes group), 40 patients with AS but no syndesmophyte (AS group), and 42 healthy individuals (control group). Also, the serum levels of DKK‐1 were measured by enzyme‐linked immunosorbent assay. The modified New York criteria (mNYC) together with the modified Stoke Ankylosing Spondylitis Spinal Score (mSASSS) were adopted to examine the radiographic progression of AS. The receiver operating characteristic (ROC) curve was applied to investigate the diagnostic value of the methylation rate of DKK‐1 with regard to radiographic progression. RESULTS: The expressions of DKK‐1 protein and mRNA in hip joint capsule tissues of AS patients were significantly lower, while DKK‐1 GPM rate, β‐catenin mRNA, and Wnt3a mRNA were markedly higher when compared with FNF group. For serum samples, the DKK‐1 methylation rate was significantly higher in AS+ syndesmophytes group in contrast to AS group and healthy controls. Serum levels of DKK‐1 protein and mRNA in AS with syndesmophytes group were markedly decreased, while β‐catenin mRNA and Wnt3a mRNA expressions were significantly increased than AS with no syndesmophyte group and the healthy control group. AS patients in Grade 4 showed a significantly higher serum DKK‐1 GPM rate than those in Grade 3 based on mNYC. Serum DKK‐1 GPM level was markedly and positively correlated with mSASSS. Serum levels of DKK‐1 in AS+ syndesmophytes group were markedly lower compared with AS but no syndesmophyte group and healthy controls. ROC curve analysis indicated that serum DKK‐1 methylation rate serves as a decent indicator for AS radiographic progression. CONCLUSION: DNAm of DKK‐1 may correlate with pathological bone formation in AS, which may provide new strategies for the treatment of AS abnormal bone formation.