The suppression of apoptosis is thought to be a requirement for the successful generation of most, if not all, cancers. The goal of my research is to discover therapeutic targets whose inhibition restores normal cell death pathways to cancer cells that are resistant to radiation and cytotoxic drugs. Bcl-2 overexpression in cancer cells is believed to confer resistance to radiation therapy and most chemotherapies. Virtually all anti-apoptotic members of the Bcl-2 family, as well as the pro-apoptotic Bax-like members, have been implicated in carcinogenesis as either oncogenes or tumor suppressors, respectively. In mouse models, overexpression of the anti-apoptotic Bcl-2 family members, or loss of both Bax and Bak, has been shown to decrease the sensitivity of tumor cells to radiation and chemotherapies.
In my laboratory, we use the zebrafish as a model system to study apoptotic mechanisms in cancer. The zebrafish has many attributes making it an attractive vertebrate system for study of apoptosis, such as optical clarity, rapid development, and conservation of the complex mitochondrial apoptosis pathway. Using a forward genetic approach, we have identified a number of recessive mutations that give rise to radiosensitization phenotypes during zebrafish development. These mutations resensitize Bcl-2-overexpressing cells to pro-apoptotic stimuli. We are currently placing these genes in apoptotic and cell cycle checkpoint pathways through genetic epistasis analysis.
To investigate the mechanisms controlling Bcl-2 mediated oncogenic pathways in human cancer cells, we also study the effects of zBcl-2 in zebrafish lymphoid cells in a transgenic line in which the zebrafish rag2 promoter directs the expression of egfp-bcl-2 in T- and B-lymphoid cells. Fluorescence microscopic analysis of living rag2-egfp-bcl-2 transgenic fish reveals that their thymocytes are resistant to radiation- and dexamethasone-induced apoptosis. T-cell leukemias induced in zebrafish by the rag2-egfp-mMYC transgene are eradicated by total body irradiation, whereas in fish expressing both MYC and egfp-bcl-2, the leukemic cells demonstrate resistance to radiation. These transgenic zebrafish lines serve as models for human tumors that are resistant to mitochondrial apoptosis induced by radiotherapy and chemotherapy. Thus, we analyze our zebrafish radiosensitizing mutations in this cancer model system to determine if they resensitize cancer cells to apoptosis-inducing therapies in vivo. Ultimately, we employ loss-of-function approaches in mouse and human cells to determine if the mechanisms underlying the radiosensitization are conserved.