Abstract

Effective cancer therapy, using surgery, radiotherapy, or chemotherapy results in the absence of macroscopic disease either at the site of the primary tumor or common distal sites of disease dissemination. Despite this initial tumor clearance, many patients will eventually relapse. Thus, small cohorts of tumor cells can survive in cryptic anatomical loci following therapy. These surviving cancer cells represent minimal residual disease (MRD). While cell-intrinsic processes are known to play decisive roles in chemotherapeutic response and drug resistance, relatively little is known about the impact of the tumor microenvironment on the persistence of MRD. Using a preclinical model of B cell lymphoma, we have been able to show that paracrine release of prosurvival cytokines protects subsets of lymphoma cells from cell death induced by genotoxic chemotherapy. Thus, while genotoxic damage can eradicate systemic disease, it can also, paradoxically, elicit prosurvival signaling in select anatomical sites. To further examine the role of the tumor microenvironment on therapeutic response, we have adapted large-scale loss of function genetic screen approaches for use in models of B cell malignancy in vivo. Specifically, we have made use of comprehensive shRNA libraries to target the genome in an unbiased manner. By introducing large shRNA pools into leukemia cells and transplanting modified cells into syngeneic recipients, we could identify up to 9,000 unique hairpins in individual mice, suggesting that library diversity can be maintained in this setting, and that genome-scale shRNA libraries can be screened in vivo. Genes identified using this approach represent central mediators of leukemia cell survival and potential cancer drug targets.