Epigenetic Influences in RA
Synovial Fibroblasts
The RA fibroblast-like synoviocytes (RASFs) are central mediators of tissue destruction via the production of a range of disease-related molecules, including chemokines, adhesion molecules and proteases. In addition, RASFs have a semi-transformed phenotype in vitro, with loss of contact inhibition, high proliferative activity and resistance to apoptosis. Engraftment of normal human cartilage and RASFs into the severe combined immunodeficiency mouse revealed this aggressive phenotype to be maintained for up to 60 days and be independent of adaptive immune cells. The mechanism(s) responsible for this stable, aggressive phenotype is unknown, however, there is increasing evidence implicating the epigenome. A methylation array study reported lower levels in RASFs compared with OA synovial fibroblasts (OASFs), particularly in genes regulating cell adhesion, transendothelial migration and extracellular matrix interactions. Furthermore, treating OASFs with the DNA demethylating agent 5-azadeoxycytidine (AZA) resulted in conversion to an RASF-like phenotype. Decreased expression of miR-34a*, as a result of increased promoter methylation, results in up-regulation of the X-linked inhibitor of apoptosis protein, potentially contributing to the resistance of RASFs to apoptosis.
The acetylation of histone proteins is regulated by the relative activities of two enzyme families: histone acetyltransferases and histone deacetylases (HDACs). The HDAC superfamily is important in the regulation of a wide range of developmental and physiological processes. As HDACs lack DNA-binding activity, they are recruited to target genes via interactions with transcription factors. A shift towards histone hyperacetylation has been reported in RASFs compared with OASFs, with overexpression of HDAC1 in the former. Targeted knockdown of HDAC1 in RASFs, using small interfering RNA, resulted in decreased proliferation and increased apoptosis.
Peripheral Blood
Global DNA methylation has been reported to be lower in T cells and leucocytes of RA patients compared with controls, although both involved small numbers. A single CpG motif in the IL-6 promoter, ~1 kb upstream of the transcriptional start site, was significantly less methylated in peripheral blood mononuclear cells from RA cases compared with controls, and correlated with higher lipopolysaccharide-induced IL-6 mRNA levels by monocyte-derived macrophages. Higher expression of CD40L and lower promoter methylation is found in RA CD4 T cells.