CAAP1

CAAP1 is a conserved RNA-processing factor whose best-defined role is as a structural bridge within the SOS splicing system, a spliceosome-independent RNA defense that excises DNA transposons from host mRNAs (PMID:41372403). CAAP1 physically links the RNA ligase RTCB and the mRNA-binding splicing factor AKAP17A, an arrangement required for RTCB to ligate the mRNA fragments generated after transposon excision, and this function is conserved between C. elegans and human cells (PMID:41372403). The interaction with AKAP17A was first detected by IP-MS in ovarian cancer cells, where CAAP1 overexpression increased cisplatin sensitivity via the mRNA splicing pathway (PMID:36870674). Independently of this splicing role, CAAP1 has been characterized as an anti-apoptotic factor: knockdown induces caspase-10-dependent activation of a mitochondrial caspase-3/9 amplification loop involving tBid cleavage (PMID:21980415), and in hepatoma cells CAAP1 suppresses intrinsic apoptosis while promoting proliferation and invasion (PMID:34018363). CAAP1 was also identified in a haploid knockout screen as a factor whose loss augments DNA double-strand break repair downstream of topoisomerase IIα, sensitizing cells to topoisomerase poisons (PMID:26260527); however, a CRISPR knockout mouse model showed no defect in DSB repair (γH2AX kinetics) or apoptotic sensitivity in primary pre-B cells and MEFs, arguing against a direct role in these processes in primary cell contexts (PMID:30629682).

1. 2011 Medium

   Established the first functional assignment for CAAP1 as an anti-apoptotic protein, defining how its loss triggers caspase activation rather than leaving the gene a cipher.

   Evidence RNAi knockdown in A-549 and MCF7/casp3-10b cells with caspase activity assays and genetic rescue via caspase-3/-10 inactivation

   PMID:21980415

   Open questions at the time

   • No molecular partner or biochemical activity identified for CAAP1 itself
   • Mechanism of caspase-10 upregulation upon knockdown unresolved
   • Tested in transformed cell lines only
2. 2015 Medium

   Placed CAAP1 in a DNA-damage context by showing its deletion enhances DSB repair downstream of topoisomerase IIα, linking it to chemoresistance.

   Evidence Genome-wide haploid knockout screen with doxorubicin and etoposide sensitivity assays

   PMID:26260527

   Open questions at the time

   • No direct biochemical mechanism connecting CAAP1 to DSB repair machinery
   • Screen-derived phenotype not validated across cell types
3. 2019 Medium

   An in vivo knockout model directly tested and failed to confirm the proposed DSB-repair and apoptosis roles, sharpening the question of CAAP1's true physiological function.

   Evidence CRISPR/Cas9 knockout mouse with γH2AX kinetics and drug sensitivity assays in primary pre-B cells and MEFs

   PMID:30629682

   Open questions at the time

   • Negative result leaves the cell-line phenotypes unexplained (context dependence vs artifact)
   • Did not assay RNA processing or splicing functions later found to be central
4. 2023 Medium

   Identified the first direct physical partner of CAAP1, AKAP17A, and linked CAAP1 to the mRNA splicing pathway, redirecting the field from apoptosis toward RNA metabolism.

   Evidence IP-MS in platinum-resistant A2780/DDP ovarian cancer cells with overexpression and cisplatin sensitivity assay

   PMID:36870674

   Open questions at the time

   • Interaction not reciprocally validated or reconstituted at the time
   • Mechanism connecting CAAP1-AKAP17A to splicing unresolved in this study
5. 2025 High

   Resolved the core molecular mechanism: CAAP1 bridges RTCB and AKAP17A in the SOS splicing complex, enabling spliceosome-independent excision of DNA transposons from host mRNAs, with conservation across two organisms.

   Evidence Genetic screens, co-IP/pulldown, functional rescue, and in vivo/in vitro splicing assays in C. elegans and human cells (Nature; corroborating bioRxiv preprint)

   PMID:40027818 PMID:41372403

   Open questions at the time

   • Structural basis of the RTCB-CAAP1-AKAP17A bridge not defined
   • Relationship between this RNA-ligation role and earlier apoptosis phenotypes unclear
6. 2026 Low

   Computational phosphoproteomics nominated regulatory phosphosites on CAAP1 co-enriched with splicing machinery, hinting at post-translational control of its RNA role.

   Evidence Meta-analysis of 885 human phosphoproteomic datasets with co-regulation enrichment

   PMID:41701571

   Open questions at the time

   • Phosphosite function not experimentally validated
   • No kinase assigned
   • Functional consequence on splicing or apoptosis untested

• How CAAP1's defined role as an RNA-ligation bridge in the SOS splicing system mechanistically connects to its reported anti-apoptotic activity remains unresolved.
• No experiment links transposon-excision splicing to caspase regulation
• Structure of the SOS complex unknown
• Physiological relevance in mammalian tissues beyond cell lines undefined