New molecular defects in two RFXANK-defective patients with MHC class II deficiency

MHC deficiency is a genetic disease caused by an absence of MHC class II gene transcription. Patients are classified in 4 complementation groups (A, B, C and D) depending on the transcription factor affected: CIITA for the group A, RFXANK for the group B, RFX5 for the group C and RFXAP for the group D. A patient, JER, presented classical immunological features of MHC class II deficiency. Unexpectedly, HLA-DRA, -DQA and -DMA transcripts were found to be expressed in the JER cell line at nearly wild-type levels, while HLA-DPA and the HLA-D beta chain transcripts were not detected. A defect in RFXANK was first suggested by genetic complementation analyses, then assessed with the demonstration of a homozygous mutation affecting a splice donor site downstream exon 4 of RFXANK. Since the severe deletion of the resulting protein cannot account for the expression of certain HLA-D genes, minor alternative transcripts of the RFXANK gene were analyzed. We thereby showed the existence of a transcript lacking exon 4, encoding a 28 amino-acid-deleted protein (ANKA4) that retains the ankyrin domains and displays a transcriptkmal activity. We propose that the uncoordinated HLA-D expression phenotype is ensured by severely limited amounts of the active ANKA4 protein. The 25 years old SAR patient presents a milder immunodeficiency, eventhough the immunological data are classical for MHC class II deficiency. In this group B cell line, cell surface expression of HLA-DR has been restored by transient expression of RFXANK. Preliminary data, obtained by RT-PCR, suggest that, like in the JER cell line, the intron between exons 4 and 5 is not spliced. The identification of the mutation is currently investigated. A hypothesis concerning the mild immunodeficiency of the SAR patient might be that the ANKA4 protein is expressed at high levels in certain cell types, and might thereby restore locally MHC class II expression. This study will aim to the identification of the molecular mechanisms leading to either severe or mild immunodeficiencies and, thus, to define the requirement for bone marrow transplantation.