This study used a proteomic approach to screen the proteins with decreased expression in the synovial tissues of rheumatoid arthritis (RA) patients by comparing their expression profiles to that of osteoarthritis (OA) and ankylosing spondylitis (AS) patients. The result was complemented by a SNP analysis.
Proteins extracted from the synovial membranes (n=10 for each disease) were separated by 2-D electrophoresis. The proteins with significantly decreased expression in the RA samples were subjected to MALDI-TOF/TOF MS. The result was verified using western blotting. Tag SNPs located in the targeted gene were assessed using the Taqman assay in a cohort of 267 Chinese patients with RA, 51 patients with AS and 160 healthy controls. The genotyping result was confirmed in a large cohort of 389 patients with RA, 200 patients with AS and 371 healthy controls.
The proteomic approach detected significantly decreased expression of vitamin D-binding protein (VDBP) in the synovial membranes from patients with RA, which was confirmed with western blot analysis. rs2282679 was significantly associated with RA and AS (p=0.026794 and 0.007566, respectively). The result was confirmed in a large cohort of RA (OR=0.678639, 95%CI=[0.541113~0.851118], p=0.000776) and AS (OR=0.564053, 95%CI=[0.433716~0.733558], p=1.79e-005).
1,25-dihydroxyvitamin D3 inhibits cell proliferation, immunoglobulin production and the release of cytokines through binding to VDBP. VDBP also mediates bone resorption by activating osteoclasts. The decreased expression and the genetic effect of VDBP in RA suggest a novel pathogenic pathway that vitamin D contributes to the arthritic process of the disease.