Genetic testing of Behçet's disease using next-generation sequencing to identify monogenic mimics and HLA-B*51

Objective Several monogenic autoinflammatory disorders and primary immunodeficiencies can present early in life with features that may be mistaken for Behcet's disease (BD). We aimed to develop a genetic analysis workflow to identify rare monogenic BD-like diseases and establish the contribution of HLA haplotype in a cohort of patients from the UK.Methods Patients with clinically suspected BD were recruited from four BD specialist care centres in the UK. All participants underwent whole-exome sequencing (WES), and genetic analysis thereafter by (i) examining genes known to cause monogenic immunodeficiency, autoinflammation or vasculitis by virtual panel application; (ii) scrutiny of variants prioritized by Exomiser using Human Phenotype Ontology (HPO); (iii) identification of copy number variants using ExomeDepth; and (iv) HLA-typing using OptiType.Results Thirty-one patients were recruited: median age 15 (4-52), and median disease onset age 5 (0-20). Nine/31 (29%) patients had monogenic disease mimicking BD: five cases of Haploinsufficiency of A20 with novel TNFAIP3 variants (p.T76I, p. M112Tfs*8, p. S548Dfs*128, p. C657Vfs*14, p. E661Nfs*36); one case of ISG15 deficiency with a novel nonsense variant (ISG15: p.Q16X) and 1p36.33 microdeletion; one case of common variable immune deficiency (TNFRSF13B: p.A181E); and two cases of TNF receptor-associated periodic syndrome (TNFRSF1A: p.R92Q). Of the remaining 22 patients, eight (36%) were HLA-B*51 positive.Conclusion We describe a novel genetic workflow for BD, which can efficiently detect known and potentially novel monogenic forms of BD, whilst additionally providing HLA-typing. Our results highlight the importance of genetic testing before BD diagnosis, as this has an impact on choice of therapy, prognosis and genetic counselling.