Is a variant of uncertain significance always ‘insignificant’? A systematic review on PRF1 A91V in Hemophagocytic Lymphohistocytosis and comparative analysis with Still’s disease

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory disorder that may arise secondary to rheumatic diseases such as Still’s disease, where macrophage activation syndrome (MAS) represents its clinical counterpart. The pathogenic significance of the PRF1 A91V variant remains uncertain, although functional data suggest partial perforin dysfunction and a possible contribution to late-onset or atypical HLH. This study aimed to clarify the clinical implications of the PRF1 A91V variant through a systematic review of published HLH and MAS cases and a comparative analysis with a single-center Still’s disease cohort. A total of 20 studies, including 38 individual HLH or MAS cases carrying the PRF1 A91V variant, were identified. The median age at diagnosis was 22 years, and 18.4% of patients were homozygous. Fever (82.6%), splenomegaly (57.9%), and hepatomegaly (36.8%) were the most frequent clinical findings. Anemia (57.1%) and thrombocytopenia (85.7%) were the predominant laboratory abnormalities, accompanied by marked hyperferritinemia (median 9319 ng/mL). Compared with 43 active Still’s disease cases, PRF1-mutated HLH patients showed significantly higher rates of cytopenias, hepatomegaly, and central nervous system involvement, together with substantially elevated ferritin levels (9,193 vs 800 ng/mL, p = 0.0023), whereas C-reactive protein levels were comparable. Receiver-operating characteristic analysis identified a ferritin cutoff of 7000 ng/mL (sensitivity 63.2%, specificity 84.6%) as the optimal discriminator for PRF1 A91V positivity. In multivariate regression, ferritin ≥ 7,000 ng/mL remained the only independent predictor (OR 17.3, 95% CI 2.0–146.3, p = 0.009). Patients carrying the PRF1 A91V variant represent a distinct subgroup within the spectrum of hyperinflammatory syndromes. Extreme hyperferritinemia combined with cytopenias should raise suspicion for perforin-related HLH rather than cytokine-driven MAS or classic Still’s disease. Recognition of this variant as a risk-modifying allele may guide early genetic testing and therapeutic decisions, including consideration of advanced interventions in selected cases.