Wiszniewski W, etal., Immunogenetics. 2003 Feb;54(11):747-55. Epub 2003 Jan 16.
MHC class II deficiency is a combined immunodeficiency caused by defects in the four regulatory factors, CIITA, RFXANK, RFX5 and RFXAP, that control MHC II expression at the transcriptional level. The RFXANK gene encodes one
subunit of the heterotrimeric RFX complex that is involved in the assembly of several transcription factors on MHC II promoters. Seven different RFXANK mutations have previously been reported in 26 unrelated patients. The most frequent mutation, a 26-bp deletion (752delG-25), has been identified in 21 patients. The other mutations are all nonsense or splice-site mutations, leading to proteins lacking all or part of the RFXANK ankyrin repeat region. We report two novel missense mutations, D121V and R212X, resulting in loss of function of the gene. We investigated the in vivo effects of these mutations and of three other point mutations on the expression of the RFXANK RNA and protein. The number of RFXANK transcripts was severely reduced in all patients except one. The RFXANK protein was barely detected in two cases. In addition, guided by a structural model of RFXANK, we investigated experimental mutants of the C-terminal tyrosine 224. Substitution Y224A, but not Y224F, led to the loss of function of RFXANK. Two null mutants, D121V and Y224A, were tested in protein interaction and DNA binding assays. The D121V mutant was unable to form the RFX complex, indicating that D121 is required for RFXAP binding. The Y224A mutant formed an RFX complex that bound normally to the MHC II promoter, but did not lead to MHC class II expression, whereas Y224F RFXANK retained the wild-type function. This indicates that an aromatic ring, but not the phenyl chain of tyrosine, is necessary at position 224 for normal RFXANK function. Studies on the Y224A mutant suggest that, in addition to the RFX subunits and CIITA, another protein is essential for MHC class II expression. This protein appears to interact with the fourth ankyrin repeat of RFXANK.
Lennon-Duménil AM, etal., J Immunol. 2001 May 1;166(9):5681-7. doi: 10.4049/jimmunol.166.9.5681.
We describe the analysis of a patient, JER, presenting classical immunological features of MHC class II deficiency. Unexpectedly, some HLA transcripts (HLA-DRA, HLA-DQA, and HLA-DMA) were found to be expressed in the JER cell line at nearly wild-type levels, while HLA-DPA and the HLA-D beta-chain tr
anscripts were not detected. Gene reporter experiments confirmed the differential transcriptional activities driven by the HLA-D promoters in the JER cells. 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. Because 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-aa-deleted protein that retains a transcriptional activity. Altogether, we characterize a new type of mutation in the RFXANK gene in a MHC class II-defective patient leading to an uncoordinated expression of the HLA-D genes, and propose that this phenotype is ensured by severely limited amounts of an active, although truncated RFXANK protein.
PURPOSE: Major histocompatibility complex class II (MHC-II) deficiency is a rare inborn error of immunity (IEI). Impaired antigen presentation to CD4 + T cells results in combined immunodeficiency (CID). Patients typically present with severe respiratory and gastrointestinal tract infecti
ons at early ages. Hematopoietic stem cell transplantation (HSCT) is the only curative therapy. METHODS: We describe the clinical, immunologic, and genetic features of eighteen unrelated Iranian patients with MHC-II deficiency. RESULTS: Consanguinity was present in all affected families. The median age at the initial presentation was 5.5 months (range 7 days to 18 years). The main symptoms included failure to thrive, persistent diarrhea, and pneumonia. Autoimmune and neurologic features were also documented in about one-third of the patients, respectively. Thirteen patients carried RFXANK gene mutations, two carried RFX5 gene mutations, and three carried a RFXAP gene mutation. Six patients shared the same RFXANK founder mutation (c.162delG); limited to the Iranian population and dated to approximately 1296 years ago. Four of the patients underwent HSCT; three of them are alive. On the other hand, nine of the fourteen patients who did not undergo HSCT had a poor prognosis and died. CONCLUSION: MHC-II deficiency is not rare in Iran, with a high rate of consanguinity. It should be considered in the differential diagnosis of CID at any age. With the limited access to HSCT and its variable results in MHC-II deficiency, implementing genetic counseling and family planning for the affected families are mandatory. We are better determined to study the c.162delG RFXANK heterozygous mutation frequency in the Iranian population.
