Shared Epitope and Anti-Cyclic Citrullinated Peptide Antibodies: Relationship with Age at Onset and Duration of Disease in Rheumatoid Arthritis

To the Editor:

Rheumatoid arthritis (RA) is a complex disease with a strong immune inflammatory component. Recent progress in defining the role of genetic factors in RA has been remarkable. It has been proposed that the shared epitope (SE) is a primary genetic element of susceptibility. Citrullination is a post-translational protein modification that is accomplished by deimination of arginine by peptidyl-arginine deiminases. Interestingly, polymorphisms in one member of the family, PADI4, have been associated with RA in Asian populations¹, although this association could not be reproduced in Caucasian populations^2,3. An association has been described between the presence of the SE and anti-cyclic citrullinated peptide antibody (anti-CCP) levels, and of each variable with disease severity^4,5. Also, anti-CCP-positive has been reported to be immunogenetically distinct from anti-CCP-negative disease, with the former subgroup being primarily responsible for association and linkage to HLA-DRB1 SE⁶.

In this cross-sectional study we aimed at analyzing whether the presence of the SE and anti-CCP caused an acceleration in disease development that could be reflected in an earlier age at onset. This acceleration might be considered a sign of a worse prognosis.

We analyzed 411 Spanish patients with RA (76% women) consecutively recruited from a single center, all meeting American College of Rheumatology criteria for RA. Patients’ mean age at disease onset was 51 ± 15 years, and median disease duration was 8 years (range 3–15); 58% of patients carried the SE (Lifecodes HLA-SSO kit; Terpenel-Diagnostics Ltd., Abingdon, UK). The following alleles were identified: DRB1*0101, *0102, *0401, *0404, *0405, *0408, *1001 or *1402. Fifty-four percent of patients presented anti-CCP antibodies (determined by ELISA with Immunoscan Euro-Diagnostica assay; Euro-Diagnostica, Malmoe, Sweden; single testing) and 69% were rheumatoid factor (RF)-positive (all patients who had been RF-positive in just one analysis irrespective of when the analysis had been performed). Statistical analysis was performed with standard statistical software (Stata v 9.0). Normality was tested by skewness and kurtosis tests.

There was a high association between anti-CCP and SE: 68% of 222 anti-CCP-positive individuals were also positive for SE, and only 47% of 189 anti-CCP antibody-negative patients were SE-positive (p = 0.00002; odds ratio 2.39, 95% CI 1.60–3.56; Table 1). In both groups of patients (anti-CCP-positive and anti-CCP-negative), SE positivity was statistically higher than in healthy Spanish controls (32% of 595 individuals; OR 4.53, 95% CI 3.22–6.40 for anti-CCP-positive and OR 1.90, 95% CI 1.34–2.69 for anti-CCP-negative patients). We analyzed whether age at onset is dependent on presence of SE or anti-CCP antibodies; in both cases an earlier age at onset was found in patients with positive SE or anti-CCP, although no statistical significance was reached (Table 1; p = 0.12 and p = 0.07, respectively). Positivity for both factors simultaneously was associated with an even earlier age at onset when compared with all other patients (48 ± 14 yrs vs 53 ± 18 yrs; p = 0.05), although this comparison did not withstand Bonferroni correction.

It seemed reasonable that patients with early age at onset will probably have a longer disease duration. We found that a clear and highly significant link existed between age at onset and disease duration (p < 10⁻⁷; r = −0.43).

We analyzed whether disease duration was dependent on SE or presence of anti-CCP (positivity); only the presence of anti-CCP antibodies was associated with disease duration (p = 0.005). The presence of SE showed no statistically significant association with disease duration (p = 0.62; Table 2).

In order to ascertain the relationship between SE, anti-CCP antibodies, age at onset, and disease duration, we performed a univariate analysis of variance test considering all these variables. As Table 1 shows, the trend for association (p = 0.07) observed between anti-CCP antibodies and age at onset disappeared when disease duration was taken into account (p = 0.52). Positivity for both factors (SE and anti-CCP antibodies) simultaneously was not associated with earlier age at onset, when adjusted by disease duration, even when compared with double-negative patients (p = 0.15). Therefore, our data suggest that association of anti-CCP with age at onset is secondary to disease duration.

It can been argued that, as a result of an inflammatory process, anti-CCP antibodies may appear not only before the first symptoms, as has been occasionally described⁷, but also during the course of the disease. According to this idea, we have shown in our study that it is more probable to find anti-CCP antibodies after a long disease history. Indeed, our data suggest that an association exists between anti-CCP antibodies and disease duration (Table 2), but not primarily with age at onset (Table 1). These results are supported by reports that have described the acquisition of anti-CCP antibodies, resulting from an immune response⁸. There are no studies analyzing the prevalence of anti-CCP antibodies during the whole course of the disease (anti-CCP antibodies have been a recent addition to the RA diagnostic arsenal), but Rönnelid, et al⁹ showed that during the first 5 years for anti-CCP-positive individuals, antibody levels remained fairly constant. Nonetheless, it is remarkable that 3 out of 119 anti-CCP-negative patients (2.5%; analyzed with the same anti-CCP assay) became positive after only 1 year of followup.

The fact that anti-CCP-negative patients have higher SE positivity than that found in our healthy control population (49% vs 32%) suggests that the SE contributes independently to RA development, even in absence of anti-CCP antibodies. We are aware that our hypothesis contrasts with current thinking on the role of the SE in RA pathogenesis¹⁰, and further studies will be necessary to fully address this issue.