Genetic Variants Associated with Ankylosing Spondylitis
HLA-B27 is the strongest genetic risk factor for ankylosing spondylitis, present in approximately 92% of patients, but at least 113 additional genetic loci contribute to disease susceptibility, with the most significant non-MHC variants located in genes involved in immune regulation and peptide processing.
Primary Genetic Association: HLA-B27
- HLA-B27 positivity increases lifetime risk of axial spondyloarthritis to approximately 25% in first-degree relatives of affected patients, compared to only 3% risk in HLA-B27 negative individuals 1, 2.
- In Swedish cohorts, 92.3% of ankylosing spondylitis cases are HLA-B27 positive, confirming the dominant role of this allele across European populations 3.
- HLA-B27 carriage does not increase mortality risk in either affected patients or the general population, providing reassurance for asymptomatic carriers 1.
Non-MHC Genetic Variants
Major Immune-Related Loci
High-density genotyping has identified 13 novel risk loci and confirmed associations at 11 additional loci beyond the original discoveries 4. The most significant non-MHC associations include:
Aminopeptidase Gene Cluster
- Four aminopeptidase-encoding genes (ERAP1, ERAP2, LNPEP, NPEPPS) show strong associations with ankylosing spondylitis 4.
- These enzymes are involved in peptide processing before MHC class I presentation, directly linking to HLA-B27 pathogenic mechanisms 4.
- Protective variants at two aminopeptidase loci reduce both enzyme function and MHC class I cell surface expression, suggesting a mechanistic pathway for disease protection 4.
IL23R-IL12RB2 Intergenic Region
- The rs11209032 SNP functions as an enhancer variant that modulates transcription factor binding 5.
- The risk 'A' allele shows reduced luciferase activity and decreased H3K4me1 methylation in CD4+ T-cells 5.
- Homozygosity for the risk allele is associated with increased IFN-γ-secreting Th1 cells (p=0.004), suggesting a shift toward Th1-mediated inflammation 5.
- This locus has been replicated in Swedish populations (p < 1.47 × 10⁻³) 3.
RUNX3 Upstream Region
- The lead SNP rs6600247 (p = 2.6 × 10⁻¹⁵) disrupts c-MYC transcription factor binding approximately 13kb upstream of the RUNX3 promoter 6.
- The AS-risk 'C' allele shows reduced binding to c-MYC protein in electrophoretic mobility shift assays (p < 0.05) 6.
- A second SNP, rs4648889, demonstrates clear chromatin looping with the RUNX3 promoter, indicating complex regulatory mechanisms at this locus 6.
Shared Inflammatory Bowel Disease Loci
- CDKAL1 achieved genome-wide significance in exome-wide association studies 7.
- Suggestive associations were found at FAM118A, C7orf72, and FAM114A1 (common variants) and PNPLA1 (low-frequency variant) 7.
- CDKAL1 and C7orf72 are also associated with inflammatory bowel disease, reinforcing the genetic overlap between these conditions 7.
Additional Confirmed Loci
MICA (MHC class I chain-related gene A) was successfully replicated in Swedish populations (p < 1.47 × 10⁻³), confirming its role across European ancestry groups 3.
Genetic Architecture Summary
- Total of >113 genetic loci are now associated with ankylosing spondylitis when combining HLA-B27, the 12 previously identified loci, and the 13 novel loci plus 11 additional haplotypes identified through Immunochip genotyping 4.
- Substantial heritability remains unexplained despite these discoveries, indicating additional rare variants or gene-gene interactions contribute to disease risk 7.
Clinical Implications of Genetic Testing
- HLA-B27 testing is diagnostically useful but not sufficient for diagnosis, as only 25% of HLA-B27 positive first-degree relatives develop disease 1, 2.
- Acute anterior uveitis in HLA-B27 positive first-degree relatives significantly predicts axSpA development (odds ratio 4.7; 95% CI 2.2-10.5) 1.
- Genetic variants at IL23R and RUNX3 may eventually guide targeted therapy selection, though this is not yet standard practice 5, 6.