RFXANK

RFXANK (RFX-B) is the ankyrin repeat-containing subunit of the trimeric RFX transcription factor complex that drives MHC class II gene transcription by simultaneously engaging RFXAP and CIITA on opposite faces of its ankyrin repeats, thereby nucleating enhanceosome assembly on MHC II promoters (PMID:10072068, PMID:11463838). Class IIa histone deacetylases HDAC4 and HDAC5 bind the same ankyrin repeat domain to repress MHC II expression; CaMK-mediated phosphorylation of these HDACs triggers CRM1-dependent nuclear export of HDAC/RFXANK complexes, relieving repression and enabling signal-responsive control of antigen presentation (PMID:16236793, PMID:15964851). Loss-of-function mutations in RFXANK, including a 26-bp founder deletion prevalent in North African populations, abolish MHC class II transcription and cause bare lymphocyte syndrome complementation group B, a combined immunodeficiency correctable by hematopoietic stem cell transplantation (PMID:10072068, PMID:10803838, PMID:21908431).

1. 1999 High

   Identification of RFXANK as the smallest RFX complex subunit and the gene mutated in BLS group B established that a single ankyrin repeat-containing protein is required for MHC class II transcription.

   Evidence Biochemical purification of the RFX complex, subunit sequencing, and functional complementation of BLS cell lines

   PMID:10072068

   Open questions at the time

   • How RFXANK ankyrin repeats engage individual partners was unknown
   • No structural model of the ankyrin repeat fold existed
   • Whether RFXANK has regulatory inputs beyond scaffolding was untested
2. 2000 Medium

   Discovery of a 26-bp founder deletion in North African BLS patients confirmed that RFXANK loss of function is the predominant molecular cause of group B immunodeficiency and linked a specific allele to abolition of MHC class II expression.

   Evidence Mutation screening and haplotype analysis in 20 group B BLS patients

   PMID:10803838

   Open questions at the time

   • Clinical study without biochemical reconstitution of the mutant protein
   • Long-term immunological consequences and transplant outcomes were not yet characterized
   • Whether the deletion produces a truncated protein or triggers nonsense-mediated decay was not resolved
3. 2001 High

   Mutagenesis of RFXANK ankyrin repeats revealed that RFXAP and CIITA bind simultaneously on opposite faces, explaining how RFXANK nucleates enhanceosome assembly and why a single patient point mutation in the ankyrin groove causes disease by selectively disrupting RFXAP binding.

   Evidence Computational modeling of ankyrin repeats, alanine scanning mutagenesis, in vitro and in vivo protein interaction assays

   PMID:11463838

   Open questions at the time

   • No crystal or cryo-EM structure of the ternary complex
   • Whether other transcription factors or coactivators contact the RFXANK scaffold was unexplored
   • Quantitative binding affinities for each face were not determined
4. 2005 High

   Discovery that class IIa HDACs (HDAC4/5) bind RFXANK ankyrin repeats to repress MHC II transcription, and that CaMK phosphorylation triggers CRM1-dependent nuclear export of the HDAC/RFXANK complex, established RFXANK as a signal-responsive switch controlling antigen presentation.

   Evidence Reciprocal co-immunoprecipitation, yeast two-hybrid domain mapping, reporter assays, CaMK phosphorylation and nuclear export assays in two independent studies

   PMID:15964851 PMID:16236793

   Open questions at the time

   • Whether HDAC4/5 binding and CIITA binding to RFXANK are mutually exclusive was not tested
   • Physiological signals upstream of CaMK that trigger this switch in antigen-presenting cells were not identified
   • In vivo validation in primary dendritic cells or macrophages was lacking
5. 2011 Medium

   Demonstration that HSCT restores MHC class II expression and immune function in RFXANK-deficient patients confirmed the cell-autonomous hematopoietic origin of the defect and validated the pathway model in vivo.

   Evidence Clinical and immunological follow-up of 35 BLS patients after HSCT

   PMID:21908431

   Open questions at the time

   • Clinical cohort study without molecular reconstitution experiments
   • Whether residual RFXANK-independent MHC II expression occurs in non-hematopoietic tissues was not addressed
   • Genotype–phenotype correlations for different RFXANK mutations remain limited

• No high-resolution structure of the RFX trimer or the RFXANK–HDAC complex exists, leaving unresolved how the competing interactions on the ankyrin repeat scaffold are coordinated and which physiological signals in antigen-presenting cells control the HDAC export switch.
• No crystal or cryo-EM structure of full RFX complex or RFXANK–HDAC interface
• Upstream signaling pathways activating CaMK at MHC II loci in dendritic cells not defined
• Whether RFXANK participates in transcription of non-MHC II target genes is unknown