CHIC2

CHIC2 functions as an adapter protein that pairs with the E3 ubiquitin ligase STUB1 to negatively regulate cytokine receptor surface levels by promoting their ubiquitination and degradation (PMID:36108743, PMID:40796662). The STUB1–CHIC2 complex binds CSF2RB, the common β chain of the IL-3, IL-5, and GM-CSF receptors, driving its ubiquitination and lysosomal degradation; loss of either STUB1 or CHIC2 stabilizes CSF2RB, with the strongest effect at low cytokine concentrations (PMID:36108743). In CD8+ T cells the same complex restrains expression of IL-27 receptor α, and knockout of CHIC2 (or STUB1) enhances anti-tumor function across murine tumor models in an IL-27Rα-dependent manner (PMID:40796662). CHIC2 also has a recurrent role in hematologic malignancy: it lies at chromosome 4q12 between FIP1L1 and PDGFRA, such that the interstitial deletion fusing those kinase genes removes CHIC2, making CHIC2 loss a surrogate marker for the FIP1L1-PDGFRA fusion (PMID:12842979), and CHIC2 itself can fuse to ETV6 in AML with t(4;12), generating a chimeric transcript that retains the ETV6 HLH and ETS DNA-binding domains under the CHIC2 promoter (PMID:10477709).

1. 1999 Medium

   Established CHIC2 as a recurrent partner in leukemia-associated chromosomal rearrangement, defining its first disease relevance before any molecular function was known.

   Evidence RT-PCR detection and molecular cloning of the BTL(CHIC2)-ETV6 fusion in t(4;12) AML patient samples

   PMID:10477709

   Open questions at the time

   • Functional consequence of the chimeric protein not tested
   • Native CHIC2 protein function not addressed
   • Single lab, limited patient numbers
2. 2003 Medium

   Positioned CHIC2 within the 4q12 genomic neighborhood, showing its deletion marks the FIP1L1-PDGFRA fusion and clarifying that CHIC2 loss is a bystander of an oncogenic kinase rearrangement.

   Evidence FISH for CHIC2 deletion and RT-PCR for FIP1L1-PDGFRA in fractionated hematopoietic cell populations

   PMID:12842979

   Open questions at the time

   • Does not establish a causal role for CHIC2 loss itself
   • Molecular function of CHIC2 still undefined
   • Limited patient cohort
3. 2022 High

   Defined the first biochemical function of CHIC2 as a STUB1-associated adapter that targets the cytokine receptor common β chain CSF2RB for ubiquitination and lysosomal degradation.

   Evidence Genetic screens, co-IP, ubiquitination assays, and protein stability measurements after STUB1/CHIC2 knockout in multiple cellular contexts

   PMID:36108743

   Open questions at the time

   • Structural basis of CHIC2–STUB1–CSF2RB assembly not resolved
   • Whether CHIC2 confers substrate specificity vs. acting as scaffold unclear
   • Range of receptor substrates beyond CSF2RB not fully mapped
4. 2025 High

   Extended the STUB1–CHIC2 mechanism to immune regulation, showing the complex restrains IL-27Rα on CD8+ T cells and that its loss enhances anti-tumor immunity.

   Evidence In vivo CRISPR screens, knockout in murine tumor models, co-IP, and receptor profiling in mouse and human CD8+ T cells

   PMID:40796662

   Open questions at the time

   • Direct ubiquitination of IL-27Rα by the complex not biochemically demonstrated here
   • Mechanism of receptor selectivity across cell types unknown
   • Therapeutic targetability not established

• How CHIC2 selects its receptor substrates and is recruited to STUB1, and whether its leukemia-associated rearrangements perturb this adapter function, remain unresolved.
• No structural model of the CHIC2–STUB1 complex
• No defined CHIC2 substrate-recognition determinant
• Link between CHIC2 fusion/deletion events and its adapter function untested