Pten Inactivating Mutations Promote Loss of MYC “Oncogene Addiction” in a Conditional Zebrafish Model of T-ALL.

Abstract 3977

Poster Board III-913

The MYC oncogene is overexpressed in most human cases of T-cell acute lymphoblastic leukemia (T-ALL), and our laboratory previously developed a zebrafish model of MYC-induced T-ALL that closely recapitulates the most common subtype of human T-ALL, which demonstrates overexpression of TAL1 and LMO2. We have now developed a conditional zebrafish model of MYC-induced T-ALL, based on the rag2-driven expression of a MYC-estrogen receptor (MYC-ER) fusion transgene, in which T-ALL is induced by 4-hydroxytamoxifen treatment. Most cases of T-ALL in this model exhibit 'oncogene addiction' to MYC, as evidenced by the regression of established tumors when 4HT treatment is discontinued. However, the genetic determinants of 'addiction' to the MYC oncogene are unknown. Deletion of the PTEN tumor suppressor has been found to be a recurrent abnormality in human T-ALL, and we have now identified abnormalities in PTEN, PI3K or AKT in 48% of human T-ALL. Due to a partial genome duplication that occurred during evolution, zebrafish have two pten genes, pten-A and pten-B, which are both ubiquitously expressed and are largely redundant. In order to examine the consequences of pten inactivation in MYC-induced T-ALL, we mated zebrafish loss-of-function mutations in pten-A and pten-B into our rag2-MYC-ER line that also carries the rag2-GFP transgene, allowing the in vivo monitoring of tumor onset and regression. Zebrafish were raised in 4-hydroxytamoxifen beginning at 5 days post-fertilization, and tumor onset was monitored via fluorescence microscopy. At the time of tumor onset, zebrafish were removed from 4-hydroxytamoxifen, and tumor regression was monitored weekly. Although pten loss-of-function mutations did not accelerate the onset of MYC-induced T-ALL, these did affect tumor regression. We found that T-ALL underwent regression in 68% (21/31) of zebrafish whose germline pten status was wild-type, while regression occurred in only 21% (7/32) of zebrafish harboring loss of any of their germline pten alleles (p=0.0003). The effect of pten inactivation on tumor regression was phenocopied by the expression of a constitutively active murine Akt2 transgene in the MYC-ER line, indicating that the effect of pten inactivation is mediated through the activation of AKT. Our studies provide new evidence that the PTEN-PI3K-AKT pathway is an important modulator of MYC ‘oncogene addiction’ in zebrafish T-ALL.