Heterogeneity of axial spondyloarthritis: genetics, sex and structural damage matter

Objective Axial spondyloarthritis (axSpA) comprises both radiographic and non-radiographic disease. However, the paucity of specific objective measures for the disease and current classification criteria showing suboptimal specificity contribute to disease heterogeneity observed in clinical practice and research. We used a historical cohort of patients with axSpA to assess sources of heterogeneity. Methods The study involved 363 axSpA probands recruited from membership of the Swiss Ankylosing Spondylitis Patient Society. Participants underwent examination by a rheumatologist, completed questionnaires and provided blood samples for HLA typing. Patients underwent radiography of sacroiliac joints and were categorised according to the New York (NY) criteria (ankylosing spondylitis (AS) or non-radiographic axSpA (nr-axSpA)) and HLA-B27 status. Genetic characterisation by single nucleotide polymorphism microarray was performed and AS polygenic risk scores (PRS) were calculated. Results Considerable heterogeneity was observed. The male to female ratio for AS (NY+) was 3:1, but 1:1 for nr-axSpA. For HLA-27(+) AS, the ratio was 2.5:1, but nearly 1:1 for HLA-B27(-) disease. Women with nr-axSpA had strikingly lower mean PRS and lower HLA-B27 prevalence than men with nr-axSpA or NY(+) male and female patients with AS. PRS was able to distinguish male but not female patients with nr-axSpA from related healthy first-degree relatives. Radiographic sacroiliitis was strongly associated with HLA-B27, especially in men. Conclusion Women clinically diagnosed with axSpA but without radiographic sacroiliitis as a group have a disease that is distinct from AS by the modified New York criteria overall and from nr-axSpA in men. Given the high degree of heterogeneity, stratified or adjusted analysis of effectiveness studies is indicated, taking genetics, sex and radiographic damage (sacroiliitis) into account.