IQCE

IQCE is a ciliary scaffold protein that, together with EFCAB7, anchors the EVC-EVC2 signaling complex at the EvC zone at the base of primary cilia, thereby positively regulating Hedgehog pathway activation through GLI2 and GLI1 transcription factors (PMID:24582806, PMID:37576597). Loss of IQCE disrupts EVC-EVC2 ciliary localization and impairs Hedgehog target gene induction, and biallelic loss-of-function variants in IQCE cause postaxial polydactyly in humans and ciliopathy phenotypes—including kidney cysts, body curvature, and left-right asymmetry defects—in zebrafish (PMID:28488682, PMID:31549751). SUMO3 modification of EVC-EVC2 cytosolic tails enhances their accumulation at the EvC zone, likely by promoting binding to the EFCAB7-IQCE module, and EVC2 engages EFCAB7 through two distinct binding motifs (PMID:37576597).

1. 2014 High

   The fundamental question of how EVC-EVC2 is retained at a discrete ciliary signaling microdomain was answered by demonstrating that IQCE forms a complex with EFCAB7 at the ciliary base, and this module anchors EVC-EVC2 to positively regulate Hedgehog-dependent GLI2 activation.

   Evidence Reciprocal co-immunoprecipitation, ciliary localization imaging, siRNA depletion with GLI2 activation readout in mammalian cells

   PMID:24582806

   Open questions at the time

   • Direct structural basis of IQCE-EFCAB7 interaction not determined
   • Whether IQCE has functions independent of the EFCAB7-EVC-EVC2 module is unknown
   • In vivo mammalian loss-of-function model for IQCE not yet reported
2. 2017 Medium

   The clinical relevance of IQCE was established when a homozygous splice-site variant causing a frameshift and premature stop was identified as the cause of postaxial polydactyly type A, linking IQCE loss of function to a human limb patterning defect consistent with disrupted Hedgehog signaling.

   Evidence Exome sequencing in a consanguineous family, Sanger validation, mini-gene splicing assay confirming aberrant splicing

   PMID:28488682

   Open questions at the time

   • Single family reported; additional kindreds needed to confirm genotype-phenotype spectrum
   • Patient-derived cell Hedgehog signaling not directly measured in this study
   • Whether partial IQCE loss produces milder phenotypes is unknown
3. 2019 High

   Confirmation that IQCE loss disrupts Hedgehog signaling at the transcriptional level came from patient fibroblast RNA analysis, and zebrafish iqce knockdown expanded the phenotypic spectrum to include kidney cysts, body curvature, cilia misdirection, and left-right asymmetry defects, establishing IQCE as broadly required for ciliary function.

   Evidence RNA expression profiling in patient fibroblasts with biallelic IQCE variants; morpholino knockdown in zebrafish with multiple ciliary phenotype readouts

   PMID:31549751

   Open questions at the time

   • Zebrafish data relied on morpholino knockdown rather than a stable genetic mutant
   • Which specific Hedgehog target genes are most sensitive to IQCE loss is not fully catalogued
   • Whether IQCE functions outside Hedgehog signaling (e.g., in Wnt or other ciliary pathways) is unresolved
4. 2023 High

   Proteomic and biochemical dissection revealed that IQCE and EFCAB7 are the principal endogenous interactors of EVC, that SUMO3 modification of EVC-EVC2 enhances EvC zone accumulation likely through increased EFCAB7-IQCE binding, and that two separable EFCAB7-binding motifs in EVC2 mediate this targeting; concurrent genetic evidence from EFCAB7-mutant families confirmed heterotetramer integrity is required for Hedgehog-dependent GLI1 induction.

   Evidence Endogenous co-IP/mass spectrometry, SUMO modification assays, mutational mapping of EFCAB7-binding motifs, and exome sequencing in EFCAB7-mutant families with GLI1 induction assay

   PMID:37576597 PMID:37684519

   Open questions at the time

   • Structural resolution of the IQCE-EFCAB7-EVC-EVC2 heterotetramer is lacking
   • Whether SUMO3-dependent regulation of EvC zone targeting is dynamic during Hedgehog signaling is unknown
   • A mammalian IQCE knockout model has not been reported

• Key unresolved questions include the atomic structure of the IQCE-EFCAB7-EVC-EVC2 complex, whether IQCE participates in signaling pathways beyond Hedgehog, and whether mouse Iqce knockout recapitulates human polydactyly and broader ciliopathy phenotypes.
• No high-resolution structural data for IQCE or the EvC complex
• No mammalian genetic knockout model published
• Potential IQCE roles in non-Hedgehog ciliary signaling are unexplored