635. Mechanisms of Caspase-1 Mediated Schwannoma Regression

Abstract

Schwannomas are tumors composed of Schwann-lineage cells that form along peripheral nerves. These tumors can cause pain, sensory/motor dysfunction, and death through compression of peripheral nerves, the spinal cord, and/or the brain stem. Management of these tumors-essentially restricted to operative resection and gamma-knife debulking- is limited in scope and efficacy and has significant associated morbidity. Chemotherapeutic treatment strategies have shown incomplete and transient tumor regression. Treatment of Schwannomas remains a major unmet clinical need.Schwannomas are appealing targets for gene therapy as they: 1) grow slowly, 2) have cellular and genetic homogeneity, and 3) can be readily localized using magnetic resonance and/or ultrasound imaging for direct intratumoral vector injection. Gene-therapy is potentially advantageous compared to resection as gene therapy is minimally invasive, kills tumor cells without damaging tumor-associated nerve, and may allow treatment of lesions not amenable to resection. We have previously published that schwannomas can be effectively treated with a gene therapy strategy that uses an adeno-associated virus (AAV) vector to deliver the apoptotic and inflammatory enzyme ICE (IL1-converting enzyme, aka caspase-1) under the Schwann cell specific promoter, myelin Protein 0 (P0). It efficiently debulkes schwannomas and resolves schwannoma associated pain, without any neuronal toxicity.However, the mechanisms by which AAV-P0-ICE selectively kills schwannomas without causing schwann-cell damage and associated neural demyelination are not clear. Understanding these mechanisms would provide the basis for broader utilization of schwannoma gene therapy and insight into how we might increase the efficacy of our current therapy.Through this project we have explored how the role of the P0 promoter and the inflammasome in regulating selective killing of schwannoma cells. Our results indicate that P0 is a highly stringent promoter as it is not expressed in neurons or even in schwannoma cells grown in vitro. P0 is active only in schwann and schannoma cells in vivo, and lack of P0 activity prevents any caspase-1 induced death in other cell types. Second, our data indicates that caspase-1 killing requires concomitant activation of the inflammasome. This pathway was active in 2 different schwannoma cell lines and may be the differentiating factor that prevents caspase-1 to cause axonal demyelination following AAV-P0-ICE injection into peripheral nerve. Finally, our data indicate that adjuvant treatment with factors that stimulate the inflammasome activity increases the efficacy of caspase-1 mediated schwannoma killing. Taken together, these data strongly support the safety of using the P0 promoter for the development of gene therapies for schwannomas and provide new, more efficient, venues for schwannoma capsase-1 mediated gene therapy.