Measurement of absolute copy number variation of Glutathione S-Transferase M1 gene by digital droplet PCR and association analysis in Tunisian Rheumatoid Arthritis population.

Background: The investigation of copy number variations (CNVs) analysis of candidate genes is currently an important research area in modulating human diseases. We aimed to quantify CNVs in glutathione S-transferase M1 (GSTM1) gene and determine its genetic contribution in Tunisian rheumatoid arthritis (RA) and its subsets through an innovative technique for quantification.

Methods: A total of 165 RA cases and 102 healthy controls were included in the study. Using a recently powerful approach of digital droplet PCR (ddPCR), we quantified GSTM1 gene to determine the presence of no, one, or multiple copy number (CN) at high levels of sensitivity and specificity. Odds ratio and Fisher exact test were performed to estimate the association risk for GSTM1CNVs in RA.

Results: Copy number identified by ddPCR was 0, 1, and 2 copies per diploid genome. A high frequency of '0' copy was revealed with 54% in RA patients. The deletion ('0' copy) of GSTM1 was found to be a significant risk factor for anti-cyclic citrullinated peptide (anti-CCP) positive RA (OR=4.16, CI =[1.17-14.7]). In addition, a lack of association was found when comparing between the CNVs of RA patients and those of controls.

Conclusion: This study highlights the powerful accuracy of ddPCR for the quantification of CNVs and suggests that the variation in the CN of GSTM1 is associated with anti-CCP positivity in RA. However, it does not indicate a specific role in the susceptibility to the disease in our Tunisian sample.