Deficiency of Adenosine Deaminase 2 Masquerading as Behçet's Disease: Phenotypic Mimicry with HLA-B*51 Positivity

Deficiency of adenosine deaminase 2 (DADA2) is a rare monogenic autoinflammatory disease resulting from biallelic loss-of-function mutations in ADA2 gene. It has variable clinical manifestations, some of which can mimic Behçet's disease (BD). Herein, we present a family of three siblings diagnosed with DADA2, two of whom were initially misdiagnosed as BD based on clinical phenotype including positive human leukocyte antigen B51 (HLA-B*51).

Gene mutational analysis was performed by whole exome (WES) and Sanger sequencing.

We reported two siblings presented with recurrent oral ulcers, fever, arthritis, and skin lesions, alongside elevated inflammatory markers and HLA-B*51 positivity, leading to an initial misdiagnosis of BD. Genetic testing later revealed a homozygous ADA2 variant (c.139G > A p.Gly47Arg) in both siblings and their asymptomatic younger sister, confirming DADA2 diagnosis. Thereafter, we reviewed the literature to identify other patients misdiagnosed with BD but later found to have DADA2. This resulted in a cohort of 10 DADA2 patients, including our two reported siblings. The median time from symptoms onset to the final diagnosis of DADA2 was 7 years. All patients exhibited BD-like phenotype, except for uveitis, and 8 were HLA-B*51 positive, which likely contributed to the diagnostic confusion.

These findings highlight the broad clinical spectrum of DADA2, which can resemble BD, and suggest that HLA-B*51 positivity in DADA2 may further complicate diagnosis. Clinicians should maintain a high index of suspicion for DADA2 in early-onset BD-like cases, particularly without uveitis, or a family history of similar symptoms. Further studies are warranted to explore HLA-B*51 role in DADA2 phenotype.