Research Update: APC C-terminal Derived Peptide APC11 Offers a New Therapy for Immunotherapy-Resistant Colorectal Cancer
Today, we share research published in Cell Research, reporting a novel immune evasion mechanism in colorectal cancer (CRC) that is independent of the canonical Wnt/β-catenin signaling. The study found that loss-of-function mutations in the APCgene, present in 80-90% of CRC cases, lead to the activation of the protein tyrosine phosphatase PTPN13. This, in turn, dephosphorylates and inhibits the key immune transcription factor STAT1. Consequently, downstream IRF1 expression is downregulated, the MHC class I antigen presentation pathway is impaired, ultimately weakening CD8+ T cell infiltration and function, and promoting tumor immune evasion. Based on this mechanism, the research team developed a peptide derived from the 11 C-terminal amino acids of the APC protein (APC11). This peptide effectively blocks the interaction between PTPN13 and STAT1, restoring anti-tumor immune responses. In various preclinical models, the APC11 peptide, used either alone or in combination with anti-PD-1 antibodies, demonstrated significant tumor-suppressive effects, providing a new target and strategy for immunotherapy in APC-mutant CRC.
01 Research Background
Colorectal cancer generally has a low response rate to immune checkpoint inhibitors (such as anti-PD-1/PD-L1 antibodies). Except for a minority of microsatellite instability-high (MSI-H) subtypes, the vast majority of patients with microsatellite stable (MSS) tumors derive little benefit. APCgene mutation is the most common driver event in CRC (80-90%). Traditionally, it is believed to primarily promote tumor proliferation by causing abnormal β-catenin accumulation and excessive activation of the Wnt signaling pathway. However, whether and how APCmutations directly mediate immune evasion remains unclear. Meanwhile, the role of PTPN13 in cancer is controversial; it can act as an oncogene by inhibiting Fas-mediated apoptosis or suppress tumor invasion by stabilizing cell junctions. Therefore, elucidating the specific function of APC loss in remodeling the tumor immune microenvironment is crucial for developing new immunotherapies.
02 Innovative Highlights
Discovery of a Novel APC/PTPN13/STAT1 Immunosuppressive Axis:
Using high-throughput functional genomics via in vivoCRISPR screening, the study found that knocking down Ptpn13specifically inhibited the growth of Apc-deficient CT26 cells in immunocompetent mice (but not in nude mice), initially identifying PTPN13 as a key effector molecule mediating immune escape downstream of APC loss. This reveals a novel pathway independent of β-catenin.
Elucidation of the Key Role of the APC C-terminus in Immune Regulation:
The study confirmed that the full-length APC protein can directly bind to the PDZ2a domain of PTPN13 via its C-terminus, thereby "occupying" its binding site and preventing PTPN13 from dephosphorylating STAT1. Upon APC loss, PTPN13 is "released," subsequently inhibiting the STAT1 signaling pathway. Crucially, point mutation experiments (e.g., APCV2860A) demonstrated that the valine residue in the APC C-terminus is essential for its binding to PTPN13, and this function is independent of APC's role in regulating β-catenin.
Development of a Therapeutically Promising APC11-Targeting Peptide:
Based on the above mechanism, the team rationally designed and synthesized an 11-amino-acid peptide (APC11) mimicking the function of the APC C-terminus. Surface plasmon resonance (SPR) and X-ray crystallography analyses precisely resolved the high-affinity binding mode of APC11 to the PTPN13 PDZ2a domain (KD~6 µM) and confirmed that its C-terminal valine is the key binding site. This peptide effectively mimics the function of full-length APC, restoring the IFNγ-STAT1-IRF1-MHC I pathway in both cellular and animal models.
Confirmation of the Synergistic Effect Between APC11 Peptide and Immune Checkpoint Inhibitors:
The study not only proved the efficacy of the APC11 peptide as a monotherapy in various mouse models (subcutaneous and orthotopic transplantation models, ApcMin/+spontaneous tumor model) but, more importantly, found that the combination of APC11 with anti-PD-1 antibodies produced a powerful synergistic anti-tumor effect. This combination successfully reversed immunotherapy resistance in APC-mutant CRC, showing high clinical translational potential.
03 Results and Discussion
3.1 APC Loss Drives Immune Evasion and Leads to Anti-PD-1 Resistance
In genetically engineered mouse models and syngeneic tumor transplantation models, Apcloss (but not p53loss or Krasmutation) was confirmed as the key factor driving immune evasion, manifested as a significant reduction in intratumoral CD8+ T cell infiltration. Analysis of clinical data showed that APCmutation is significantly associated with low expression of T cell markers (CD3E, CD8A, etc.), and patients with APCmutations had shorter overall survival after receiving anti-PD-1/PD-L1 therapy.
Fig. 1 APC loss leads to CRC immune evasion and invalidation of anti-PD1 in tumor therapy.
3.2 PTPN13 is a Key Immune Regulator Downstream of APC Loss
In vivocompetitive assays showed that Ptpn13-knockout tumor cells were selectively eliminated in immunocompetent mice, an effect dependent on CD8+ T cells. Clinical sample analysis revealed that high PTPN13 expression correlates with poor prognosis in CRC patients, reduced intratumoral CD8+ T cells, and downregulated HLA-ABC expression. In the context of Apcloss, knocking out Ptpn13restored STAT1 phosphorylation, upregulated MHC-I-related gene expression, promoted CD8+ T cell infiltration and function, and inhibited tumor growth.
Fig. 2 APC loss drives immune evasion in CRC via PTPN13.
3.3 The APC/PTPN13 Axis Affects Antigen Presentation via STAT1 and is Independent of β-catenin
RNA-seq and GSEA analyses indicated that APC loss specifically suppresses type II interferon response and antigen processing and presentation pathways. Functional experiments demonstrated that APC loss impairs IFNγ-induced STAT1 phosphorylation and IRF1 expression, subsequently downregulating MHC-I molecules and antigen presentation-related genes. Crucially, neither knocking down β-catenin nor overexpressing an activated form of β-catenin (ΔN-β-catenin) could reverse the immunosuppressive phenotype caused by APC loss, proving that this immune evasion mechanism is independent of the Wnt/β-catenin pathway.
Fig. 3 APC loss inactivates IFNγ-STAT1-IRF1-MHC-I antigen presentation signaling.
3.4 Therapeutic Efficacy Validation of the APC11 Peptide
The APC11 peptide effectively blocked the PTPN13-STAT1 interaction and restored the STAT1 signaling pathway. The researchers further developed PEGylated TAT-APC11 and nanoparticle-encapsulated NP-APC11 to improve stability and targeting. In various tumor models, APC11 treatment significantly inhibited tumor growth and increased CD8+ T cell infiltration and activation. When combined with anti-PD-1, the effect was significantly superior to monotherapy.
Fig. 4 Ptpn13 knockout suppresses APC-loss-induced activation of IFNγ-STAT1-IRF1 signaling.
04 Conclusion and Future Perspectives
This study overturns the previous understanding of APC function, revealing that it is not only a "gatekeeper" of the Wnt pathway but also a "regulator" of tumor immunity. By discovering the novel APC/PTPN13/STAT1 immunosuppressive axis and successfully developing the therapeutically promising APC11 targeting peptide, it provides a new therapeutic concept and combination strategy for the vast majority of CRC patients with APCmutations and MSS subtypes.
The profound significance of this research lies in directly linking a common oncogenic mutation (APC loss) to a targetable immunosuppressive mechanism. APC11, as a "mechanism-driven" therapy, has the advantage of specifically targeting the immune deficiency directly caused by APC loss, rather than non-specifically enhancing immune responses. Future clinical translation of APC11 could focus on: 1) Optimizing the pharmacokinetic properties of the peptide, such as developing more stable small-molecule mimetics; 2) Exploring its application in other cancers with a high incidence of APCmutations (e.g., certain gastric cancers); 3) Conducting clinical trials to verify its safety and efficacy in human CRC patients, particularly in combination regimens with existing immunotherapies. In summary, this work achieves a perfect transition from basic mechanism discovery to potential therapeutic strategy, representing a significant breakthrough in the field of tumor immunology research.
Orginal Article：
Ma WH, Li WY, Chen T, Jing L, Chen YH, Li K, Xu ZL, Shen RF, He Y, Mou T, Luo TY, Sun X, Wu ZK, Wang LJ, Liu HJ, Qiu X, Gao Y, Bai X, Wang W, Wu D, Li G, Zhou WJ. Targeting PTPN13 with 11-amino-acid peptides of C-terminal APC prevents immune evasion of colorectal cancer. Cell Res. 2026 Jan;36(1):72-93.