Why do we need alternative markers of rheumatoid arthritis (RA)?
A: Diagnosis of RA can be difficult because other disorders cause similar symptoms, especially early in the course of disease. Traditional tests for RA detect the autoantibodies rheumatoid factor (RF) and anticyclic citrullinated peptide (CCP), but both of these may be negative in a small but significant percentage of patients with RA. We need better markers that can close this gap and help predict which patients need early intervention, target treatment, and improve our ability to monitor therapy.
What other autoantibodies have been proposed as markers of RA?
The most common methods for measuring RF are turbidimetry and nephelometry, but these primarily detect IgM RF. The use of enzyme immunoassay allows differentiation of RF into IgM, IgA, and IgG classes. Detecting IgA and IgG RF in addition to IgM RF does not increase diagnostic sensitivity, but it may add specificity when combined with IgM RF and anti-CCP.
Modern anti-CCP assays detect many different (and proprietary) peptides. The general term anticitrullinated protein antibodies, or ACPA, has been proposed as a name for these markers. Citrullination refers to the modification of an arginine residue to citrulline via enzymatic deimination. The enzymes that create citrulline (peptidyl arginine deiminases or PADs) are active in synovial inflammation and the production of antibodies against these altered proteins is characteristic of RA. With regard to alternatives to the anti-CCP assay, most of the focus has been on developing tests that detect antibodies to individual proteins that have been citrullinated or that have undergone a different posttranslational modification.
Assays are commercially available for antibodies specific for citrullinated alpha-enolase (a glycolytic enzyme) and citrullinated vimentin (a cytoskeletal protein). There are two different antibodies to citrullinated vimentin. Inflammatory cells in RA not only citrullinate vimentin, but the protein also undergoes somatic mutation, increasing the number of arginine residues and therefore the degree of citrullination. The original antibody, called anti-Sa, targets wild-type citrullinated vimentin. Antibody against the mutated version of citrullinated vimentin, called anti-MCV, has been shown to be much more sensitive and may be the best alternative marker for identifying patients who may be anti-CCP negative.
The other major type of post-translational modification in RA is carbamylation, in which non-enzymatic conversion of a lysine residue produces homocitrulline. Antibodies to carbamylated protein are not as sensitive as anti-MCV, but they may be a useful alternative marker to help confirm the diagnosis.
What other types of alternative markers have been proposed?
Two markers of joint inflammation and bone erosion have been investigated. Matrix metalloproteinases (MMPs) contribute to joint destruction in RA, primarily by degrading cartilage. Serum levels of MMP-3, the major MMP present in RA synovia, and 14-3-3 eta, a chaperone protein present in the inflamed joint and also detectable in serum, correlate with disease activity in early RA and also with disease progression. These markers may be helpful additions to conventional monitoring with C-reactive protein. 14-3-3 eta also shows high sensitivity and specificity for RA and can help identify RA patients who are negative for RF and anti-CCP.
What new markers of RA are on the horizon?
The search for other antibodies to specific citrullinated proteins continues. Antibodies to PAD4, the enzyme that creates the citrulline residue, may also have prognostic value. Analysis of RNA transcript expression may help determine whether patients are likely to respond to biologic disease-modifying antirheumatic drugs. Finally, a study presented at the recent American College of Rheumatology meeting reported that epigenetic features of almost 1,000 genes showed discrimination between patients with and without RA. The ultimate alternative RA marker may wind up being a molecular genetic one.
James D. Faix, MD, is the medical director of immunology for Quest Diagnostics, based in San Juan Capistrano, California. +Email: [email protected]