IL36RN mutations impact on protein expression and function: a basis for genotype-phenotype correlation in pustular diseases.

Homozygous or compound heterozygous IL36RN gene mutations underlie the pathogenesis of psoriasis-related pustular eruptions including generalized pustular psoriasis (GPP), palmoplantar pustular psoriasis (PPP), acrodermatitis continua of Hallopeau (ACH), and acute generalized exanthematous pustular eruption (AGEP). We identified two unreported IL36RN homozygous mutations (c.41C>A/p.Ser14X and c.420_426del/p.Gly141MetfsX29) in familial GPP cases. We analyzed the impact of a spectrum of IL36RN mutations on IL-36Ra protein by using site-directed mutagenesis and expression in HEK293T cells. This enabled us to differentiate null mutations with complete absence of IL-36Ra (the 2 previously unreported mutants, c.80T>C/p.Leu27Pro, c.28C>T/p.Arg10X, c.280G>T/p.Glu94X, c.368C>G/p.Thr123Arg, c.368C>T/p.Thr123Met, and c.227C>T/p.Pro76Leu) from mutations with decreased (c.95A>G/p.His32Arg, c.142C>T/p.Arg48Trp, c.308C>T/p.Ser113Leu) or unchanged (c.304C>T/p.Arg102Trp and c.104A>G/p.Lys35Arg) protein expression. Functional assays measuring the impact of mutations on the capacity to repress IL-36-dependent activation of the NFκB pathway revealed complete functional impairment for null mutants, whereas partial or no impairment was observed for other mutations considered as hypomorphic. Finally, null mutants were associated with severe clinical phenotypes (GPP, AGEP), while hypomorphic ones were identified in both localized (PPP, ACH) and generalized variants. These results provide a preliminary basis for genotype-phenotype correlation in patients with deficiency of the IL-36Ra (DITRA), and suggest the involvement of other factors in the modulation of clinical expression.