Molecular basis for the functional roles of the multimorphic T95R mutation of IRF4 causing human autosomal dominant combined immunodeficiency

Interferon regulatory factor 4 (IRF4) is a transcription factor that regulates the development and function of immune cells. Recently, a new multimorphic mutation T95R was identified in the IRF4 DNA-binding domain (DBD) in patients with autosomal dominant combined immune deficiency. Here, we characterized the interactions of the wild-type IRF4-DBD (IRF4-DBD^WT) and T95R mutant (IRF4-DBD^T95R) with a canonical DNA sequence and several noncanonical DNA sequences. We found that compared to IRF4-DBD^WT, IRF4-DBD^T95R exhibits higher binding affinities for both canonical and noncanonical DNAs, with the highest preference for the noncanonical GATA sequence. The crystal structures of IRF4-DBD^WT in complex with the GATA sequence and IRF4-DBD^T95R in complexes with both canonical and noncanonical DNAs were determined, showing that the T95R mutation enhances the interactions of IRF4-DBD^T95R with the canonical and noncanonical DNAs to achieve higher affinity and specificity. Collectively, our data provide the molecular basis for the gain-of-function and new function of IRF4^T95R.