{
  "abstract": "Background The use of immunologic agents to treat bladder cancer has been beneficial in both muscle-invasive and non-muscle invasive cases. While some patients with advanced or metastatic bladder cancer have experienced durable responses to immune checkpoint inhibitors (ICI), the majority of patients do not respond to this therapeutic option. Compared to other cancers, bladder cancer exhibits some of the highest tumor mutation burdens, suggesting a potential benefit from ICI therapy. Approximately 70% of single nucleotide variant mutations seen in bladder cancer can be attributed to APOBEC3 enzymes, which catalyze the conversion of cytosine to uracil in both single-stranded DNA and RNA substrates. The APOBEC3 mutational signatures are primarily driven by the APOBEC3A (A3A) and APOBEC3B (A3B) family members.Methods To investigate the potential roles of A3A and A3B in modulating immunotherapy response, we performed in-silico analysis of bladder tumors from The Cancer Genome Atlas (TCGA). Additionally, we utilized a tetracycline-inducible A3A and A3B system to overexpress each enzyme individually in a mouse syngeneic bladder cancer cell line.Results Analysis of TCGA bladder tumors revealed that APOBEC3 mutational load was significantly associated with previously observed positive (interferon-gamma response) and negative (stromal fraction and intratumoral heterogeneity) indicators of response to ICI therapy. When examining RNA expression of A3A and A3B in these tumors, A3A was significantly correlated with features of an inflamed tumor microenvironment, while A3B was significantly correlated with features of genomic instability. These distinct correlations suggest isoform-specific functions for the APOBEC3 enzymes. To further interrogate these findings in-vivo, we generated mouse BBN963 tumors to express A3A or A3B. The efficacy of anti-PD-1 therapy in this model was attenuated in BBN963 tumors expressing A3A. In contrast, A3B expression sensitized BBN963 tumors to anti-PD-1 therapy. Furthermore, we have validated in multiple human datasets that A3A—and no other APOBEC3 family member—is significantly correlated with squamous differentiation in bladder cancer.Conclusions These findings may help explain the differential responsiveness to ICI therapy in bladder cancer, given the widespread activity of APOBEC3 mutagenesis. Future work is aimed at further delineating why APOBEC3 isoforms differentially modulate ICI response. These associations may provide key insights into why A3 isoforms attenuate and/or sensitize tumors to therapeutic interventions.Ethics Approval All animal studies were reviewed and approved by The University of North Carolina at Chapel Hill Institutional Animal Care and Use Committee (IACUC) #24-184.",
  "authors": [
    {
      "affiliations": [
        "University of North Carolina at Chapel Hill, Chapel Hill, NC, USA"
      ],
      "name": "Michael S Sturdivant"
    },
    {
      "affiliations": [
        "University of North Carolina at Chapel Hill, Chapel Hill, NC, USA"
      ],
      "name": "Andrew Truong"
    },
    {
      "affiliations": [
        "University of North Carolina at Chapel Hill, Chapel Hill, NC, USA"
      ],
      "name": "Skyla Marchesi"
    },
    {
      "affiliations": [
        "University of North Carolina at Chapel Hill, Chapel Hill, NC, USA"
      ],
      "name": "Elliott Drew Toomer"
    },
    {
      "affiliations": [
        "University of North Carolina at Chapel Hill, Chapel Hill, NC, USA"
      ],
      "name": "Mi Zhou"
    },
    {
      "affiliations": [
        "University of North Carolina at Chapel Hill, Chapel Hill, NC, USA"
      ],
      "name": "Milargo Ortiz"
    },
    {
      "affiliations": [
        "University of North Carolina at Chapel Hill, Chapel Hill, NC, USA"
      ],
      "name": "Jeffrey Damrauer"
    },
    {
      "affiliations": [
        "University of North Carolina at Chapel Hill, Chapel Hill, NC, USA"
      ],
      "name": "William Kim"
    }
  ],
  "title": "906 APOBEC3 isoforms differentially modulate response to immunotherapy in bladder cancer",
  "uid": "776542b4-2d65-5029-98d0-b1ae4a714f56"
}