A GENE ENCODING A NOVEL RFX-ASSOCIATED TRANSACTIVATOR IS MUTATED IN THE MAJORITY OF MHC CLASS-II DEFICIENCY PATIENTS

Major histocompatibility class II (MHC-II) molecules are transmembrane proteins that have a central role in development and control of the i mmune system. They are encoded by a multigene family and their express ion is tightly regulated. MHC-II deficiency (OMIM 209920) is an autoso mal recessive immunodeficiency syndrome resulting from defects in tran s-acting factors essential for transcription of MHC-II genes(1,2). The re are four genetic complementation groups(3-5) (A, B, C and D), refle cting the existence of four MHC-II regulators(2). The factors defectiv e in groups A (CIITA), C (RFX5) and D (RFXAP) have been identified(6-8 ). CIITA is a non-DNA-binding co-activator that controls the cell-type specificity and inducibility of MHC-II expression(6,9) RFX5 and RFXAP are two subunits of RFX, a multi-protein complex that binds the X box motif of MHC-II promoters(10,11). Mutations in the genes encoding RFX 5 (RFX5) or RFXAP (RFXAP) abolish binding of RFX (refs 7,8,12), Simila r to groups C and D, group B is characterized by a defect in RFX bindi ng(2,10,11), and although it accounts for the majority of patients, th e factor defective in group B has remained unknown. We report here the isolation of RFX by a novel single-step DNA-affinity purification app roach and the identification of RFXANK, the gene encoding a third subu nit of RFX. RFXANK restores MHC-II expression in cell lines from patie nts in group B and is mutated in these patients. RFXANK contains a pro tein-protein interaction region consisting of three ankyrin repeats. I ts interaction with RFX5 and RFXAP is essential for binding of the RFX complex to MHC-II promoters.