Incident rheumatoid arthritis in patients living in Turkey and in Denmark: a comparative clinical, genetic, and serological study.

Objective: The north-south gradient hypothesis proposes that individuals with rheumatoid arthritis (RA) residing in southern regions manifest a younger age of onset and milder disease compared to their northern counterparts. This study aimed to compare treatment-naïve, new-onset RA patients in Denmark and Turkey, examining demographic, clinical, laboratory, and genetic parameters.

Method: Prospective data collection was conducted, with all patients meeting the 2010 American College of Rheumatology/European League Against Rheumatism criteria. Shared epitope (SE) allele carrier frequencies were examined for genetic comparisons between patients and normal controls.

Results: Out of 223 RA patients, 109 were Danish and 114 Turkish. Danish patients exhibited a median age at onset of 60 years, whereas Turkish patients were younger at 51 years (p = 0.0007). The Danish cohort displayed significantly more swollen and tender joints, resulting in higher Disease Activity Score based on 28-joint count-C-reactive protein (DAS28-CRP). Danish RA patients and controls possessed more RA risk-enhancing alleles (S2 + S3P) and fewer risk-protective (S1 + S3D) alleles than Turkish patients and controls.

Conclusion: This study substantiates the north-south gradient hypothesis, highlighting that new-onset RA patients in Denmark tend to experience an older age of onset and more severe disease activity than their Turkish counterparts. Variations in risk-enhancing alleles and fewer risk-protective alleles in Danish patients and controls are associated with these distinctions. Future research should investigate the genetic and environmental factors underlying these regional disparities, exploring their persistence in the long-term course of the disease through follow-up studies.