T lymphocyte-derived malignancies are pediatric cancers often carrying poor prognoses. The proto-oncogenes underlying these malignancies frequently are also fundamental to normal lymphocyte development and function. Therefore, the discovery of heretofore unrecognized lymphocyte oncogenes and tumor suppressors is of potentially profound significance to both clinical medicine and scientific understanding.

To address this, we pioneered a phenotype-driven forward-genetic screen in zebrafish (Danio rerio). Using transgenic animals with T lymphocyte-specific enhanced green fluorescent protein (EGFP), we performed chemical mutagenesis, screened fish for GFP⁺ tumors, and identified several lines developing heritable T cell malignancies. One of these lines, oscar the grouch (otg), is characterized by recessive inheritance. Validation of otg as a true leukemia predisposition model was accomplished using histology, immunohistochemistry, gene expression studies, confirmation of clonality, and allogeneic transplantation.

In search of the genetic mutation underlying otg, we compared in vivo responses to glucocorticoids and γ-irradiation (therapies known to affect human T-ALL). Both dexamethasone (DXM) and irradiation treatment (XRT) were of limited or no utility in otg. In addition to diminished sensitivity, otg larvae were resistant to radiation-induced apoptosis and showed decreased activation of caspase 3. We determined this resistance is due to a block in the intrinsic mitochondrial apoptosis pathway.

To discover genetic changes that may cause T-ALL, and those contributing to relapse, we used array comparative genomic hybridization (aCGH) in both humans and zebrafish. We identified copy number aberrations (CNAs) in 17 zebrafish T-ALLs, and compared all D. rerio genes found in any CNA to a cohort of 75 human T-ALL samples. We found significant overlap (62%) with genes in CNAs from the human T-ALLs. Additionally, 15 genes recurrently altered (≥3 samples) in zebrafish T-ALL were also in CNAs from 5 or more human T-ALLs. Finally, we used aCGH to study CNA genes acquired during iterative allo-transplantations of 3 zebrafish malignancies. We compared these genes with human aCGH results from 23 patients who either failed induction, or had already relapsed. Again, we observed a large overlap (53%) as well as 9 genes found in 2 fish and ≥5 humans. These genes are candidates for causing the increased severity of these tumors.