Elaine Remmers and colleagues report a genome-wide association study for Behçet's Disease in a Turkish population. They identify associations in the Class I region of the MHC, IL10 and IL23R-IL12RB2. Behçet's disease is a genetically complex disease of unknown etiology characterized by recurrent inflammatory attacks affecting the orogenital mucosa, eyes and skin. We performed a genome-wide association study with 311,459 SNPs in 1,215 individuals with Behçet's disease (cases) and 1,278 healthy controls from Turkey. We confirmed the known association of Behçet's disease with HLA-B*51 and identified a second, independent association within the MHC Class I region. We also identified an association at IL10 (rs1518111, P = 1.88 × 10−8). Using a meta-analysis with an additional five cohorts from Turkey, the Middle East, Europe and Asia, comprising a total of 2,430 cases and 2,660 controls, we identified associations at IL10 (rs1518111, P = 3.54 × 10−18, odds ratio = 1.45, 95% CI 1.34–1.58) and the IL23R-IL12RB2 locus (rs924080, P = 6.69 × 10−9, OR = 1.28, 95% CI 1.18–1.39). The disease-associated IL10 variant (the rs1518111 A allele) was associated with diminished mRNA expression and low protein production.

Daniel Kastner and colleagues report genome-wide association analyses for Behçet's disease, a condition characterized by episodic inflammation of the skin and eyes and an important cause of blindness. They identify four loci newly associated with Behçet's disease, an epistatic interaction between HLA-B*51 and ERAP1 and overlap with loci previously associated to related inflammatory disorders. Individuals with Behçet's disease suffer from episodic inflammation often affecting the orogenital mucosa, skin and eyes. To discover new susceptibility loci for Behçet's disease, we performed a genome-wide association study (GWAS) of 779,465 SNPs with imputed genotypes in 1,209 Turkish individuals with Behçet's disease and 1,278 controls. We identified new associations at CCR1, STAT4 and KLRC4. Additionally, two SNPs in ERAP1, encoding ERAP1 p.Asp575Asn and p.Arg725Gln alterations, recessively conferred disease risk. These findings were replicated in 1,468 independent Turkish and/or 1,352 Japanese samples (combined meta-analysis P < 2 × 10−9). We also found evidence for interaction between HLA-B*51 and ERAP1 (P = 9 × 10−4). The CCR1 and STAT4 variants were associated with gene expression differences. Three risk loci shared with ankylosing spondylitis and psoriasis (the MHC class I region, ERAP1 and IL23R and the MHC class I–ERAP1 interaction), as well as two loci shared with inflammatory bowel disease (IL23R and IL10) implicate shared pathogenic pathways in the spondyloarthritides and Behçet's disease.

Genome-wide association studies (GWAS) are a powerful means of identifying genes with disease-associated common variants, but they are not well-suited to detecting genes with disease-associated rare and low-frequency variants. In the current study of Behçet disease (BD), nonsynonymous variants (NSVs) identified by deep exonic resequencing of 10 genes found by GWAS ( IL10, IL23R , CCR1 , STAT4 , KLRK1 , KLRC1, KLRC2, KLRC3, KLRC4 , and ERAP1 ) and 11 genes selected for their role in innate immunity (IL1B, IL1R1, IL1RN, NLRP3, MEFV, TNFRSF1A, PSTPIP1, CASP1, PYCARD, NOD2, and TLR4 ) were evaluated for BD association. A differential distribution of the rare and low-frequency NSVs of a gene in 2,461 BD cases compared with 2,458 controls indicated their collective association with disease. By stringent criteria requiring at least a single burden test with study-wide significance and a corroborating test with at least nominal significance, rare and low-frequency NSVs in one GWAS-identified gene, IL23R ( P = 6.9 × 10 ⁻⁵ ), and one gene involved in innate immunity, TLR4 ( P = 8.0 × 10 ⁻⁴ ), were associated with BD. In addition, damaging or rare damaging NOD2 variants were nominally significant across all three burden tests applied ( P = 0.0063–0.045). Furthermore, carriage of the familial Mediterranean fever gene ( MEFV ) mutation Met694Val, which is known to cause recessively inherited familial Mediterranean fever, conferred BD risk in the Turkish population (OR, 2.65; P = 1.8 × 10 ⁻¹² ). The disease-associated NSVs in MEFV and TLR4 implicate innate immune and bacterial sensing mechanisms in BD pathogenesis.

Daniel Kastner, Elaine Remmers and colleagues perform an association study of Behçet's disease based on dense genotyping of immune-related loci. They identify new association signals near genes involved in host response to microbial exposure and extend evidence for shared susceptibility loci with Crohn's disease and leprosy. We analyzed 1,900 Turkish Behçet's disease cases and 1,779 controls genotyped with the Immunochip. The most significantly associated SNP was rs1050502, a tag SNP for HLA-B*51. In the Turkish discovery set, we identified three new risk loci, IL1A–IL1B, IRF8, and CEBPB–PTPN1, with genome-wide significance (P < 5 × 10−8) by direct genotyping and ADO–EGR2 by imputation. We replicated the ADO–EGR2, IRF8, and CEBPB–PTPN1 loci by genotyping 969 Iranian cases and 826 controls. Imputed data in 608 Japanese cases and 737 controls further replicated ADO–EGR2 and IRF8, and meta-analysis additionally identified RIPK2 and LACC1. The disease-associated allele of rs4402765, the lead marker at IL1A–IL1B, was associated with both decreased IL-1α and increased IL-1β production. ABO non-secretor genotypes for two ancestry-specific FUT2 SNPs showed strong disease association (P = 5.89 × 10−15). Our findings extend the list of susceptibility genes shared with Crohn's disease and leprosy and implicate mucosal factors and the innate immune response to microbial exposure in Behçet's disease susceptibility.

The HLA protein, HLA-B*51, encoded by HLA-B in MHC, is the strongest known genetic risk factor for Behçet disease (BD). Associations between BD and other factors within the MHC have been reported also, although strong regional linkage disequilibrium complicates their confident disentanglement from HLA-B*51. In the current study, we examined a combination of directly obtained and imputed MHC-region SNPs, directly obtained HLA-B locus types, and imputed classical HLA types with their corresponding polymorphic amino acid residues for association with BD in 1,190 cases and 1,257 controls. SNP mapping with logistic regression of the MHC identified the HLA-B/MICA region and the region between HLA-F and HLA-A as independently associated with BD (P < 1.7 × 10⁻⁸). HLA-B*51, -A*03, -B*15, -B*27, -B*49, -B*57, and -A*26 each contributed independently to BD risk. We directly examined rs116799036, a noncoding SNP upstream of HLA-B that was recently suggested to underlie the association of HLA-B*51 with BD, but we were unable to replicate that finding in our collection. Instead, we mapped the BD association to seven MHC class I (MHC-I) amino acid residues, including anchor residues that critically define the selection and binding of peptides to MHC-I molecules, residues known to influence MHC-I–killer immunoglobulin-like receptor interactions, and a residue located in the signal peptide of HLA-B. The locations of these variants collectively implicate MHC-I peptide binding in the pathophysiology of BD. Furthermore, several lines of evidence suggest a role for altered regulation of cellular cytotoxicity in BD pathogenesis. Significance To elucidate their role in Behçet disease (BD), we used imputation to analyze MHC class I (MHC-I) alleles and amino acids in a large case-control collection. Several MHC-I alleles influenced BD susceptibility, some conferring risk and others affording protection. The association of MHC-I with BD was mapped to six positions around the peptide-binding groove and to one position in the signal peptide. These residues are critical in defining the peptide-binding specificity of MHC-I molecules, and they also affect the engagement of MHC-I molecules by killer immunoglobulin-like receptors on natural killer and T cells. These data implicate peptide binding by MHC-I in BD pathogenesis and suggest that altered regulation of cytotoxic cells by MHC-I may be pathogenic in BD.

Objectives To determine clinical and genetic features of individuals with relapsing polychondritis (RP) likely caused by pathogenic somatic variants in ubiquitin-like modifier activating enzyme 1 (UBA1). Methods Fourteen patients with RP who met the Damiani and Levine criteria were recruited (12 men, 2 women; median onset age (IQR) 72.1 years (67.1–78.0)). Sanger sequencing of UBA1 was performed using genomic DNA from peripheral blood leukocytes or bone marrow tissue. Droplet digital PCR (ddPCR) and peptide nucleic acid (PNA)-clamping PCR were used to detect low-prevalence somatic variants. Clinical features of the patients were investigated retrospectively. Results UBA1 was examined in 13 of the 14 patients; 73% (8/11) of the male patients had somatic UBA1 variants (c.121A>C, c.121A>G or c.122T>C resulting in p.Met41Leu, p.Met41Val or p.Met41Thr, respectively). All the variant-positive patients had systemic symptoms, including a significantly high prevalence of skin lesions. ddPCR detected low prevalence (0.14%) of somatic variant (c.121A>C) in one female patient, which was subsequently confirmed by PNA-clamping PCR. Conclusions Genetic screening for pathogenic UBA1 variants should be considered in patients with RP, especially male patients with skin lesions. The somatic variant in UBA1 in the female patient is the first to be reported.