Gene therapy with apoptosis-associated speck-like protein, a newly described schwannoma tumor suppressor, inhibits schwannoma growth in vivo

BACKGROUND: We evaluated apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) as a schwannoma tumor suppressor and explored its utilization in a schwannoma gene therapy strategy that may be translated to clinical use. METHODS: ASC protein expression and mRNA level were assessed in human schwannoma by immunohistochemistry and quantitative PCR, respectively. Methylation- specific PCR was used to assess ASC promoter methylation. The effect of ASC overexpression in schwannoma cells was evaluated through ATP-based viability, lactate dehydrogenase release, and apoptosis staining. Western blotting and colorimetric assay were used to test the effect of ASC overexpression on endogenous pro-apoptotic pathways. Bioluminescence imaging, behavioral testing, and immunohistochemistry in human xenograft and murine allograft schwannoma models were used to examine the efficacy and toxicity of intratumoral injection of adeno-associated virus (AAV) vector encoding ASC. RESULTS: ASC expression was suppressed via promoter methylation in over 80% of the human schwannomas tested. ASC overexpression in schwannoma cells results in cell death and is associated with activation of endogenous caspase-9, caspase-3, and upregulation of BH3 interacting-domain death agonist. In a human xenograft schwannoma model, AAV1-mediated ASC delivery reduced tumor growth and resolved tumor-associated pain without detectable toxicity, and tumor control was associated with reduced Ki67 mitotic index and increased tumor-cell apoptosis. Efficacy of this schwannoma gene therapy strategy was confirmed in a murine schwannoma model. CONCLUSION: We have identified ASC as a putative schwannoma tumor suppressor with high potential clinical utility for schwannoma gene therapy and generated a vector that treats schwannomas via a novel mechanism that does not overlap with current treatments.