Evasion of DNA damage-induced cell death, via mutation of the p53 tumor suppressor or overexpression of prosurvival Bcl-2 family proteins, is a key step toward malignant transformation and therapeutic resistance. We report that depletion or acute inhibition of checkpoint kinase 1 (Chk1) is sufficient to restore gamma-radiation-induced apoptosis in p53 mutant zebrafish embryos. Surprisingly, caspase-3 is not activated prior to DNA fragmentation, in contrast to classical intrinsic or extrinsic apoptosis. Rather, an alternative apoptotic program is engaged that cell autonomously requires atm (ataxia telangiectasia mutated), atr (ATM and Rad3-related) and caspase-2, and is not affected by p53 loss or overexpression of bcl-2/xl. Similarly, Chk1 inhibitor-treated human tumor cells hyperactivate ATM, ATR, and caspase-2 after gamma-radiation and trigger a caspase-2-dependent apoptotic program that bypasses p53 deficiency and excess Bcl-2. The evolutionarily conserved "Chk1-suppressed" pathway defines a novel apoptotic process, whose responsiveness to Chk1 inhibitors and insensitivity to p53 and BCL2 alterations have important implications for cancer therapy.

Summary The molecular events underlying the progression of T-lymphoblastic lymphoma (T-LBL) to acute T-lymphoblastic leukemia (T-ALL) remain elusive. In our zebrafish model, concomitant overexpression of bcl-2 with Myc accelerated T-LBL onset while inhibiting progression to T-ALL. The T-LBL cells failed to invade the vasculature and showed evidence of increased homotypic cell-cell adhesion and autophagy. Further analysis using clinical biopsy specimens revealed autophagy and increased levels of BCL2, S1P1 and ICAM1 in human T-LBL compared to T-ALL. Inhibition of S1P1 signaling in T-LBL cells led to decreased homotypic adhesion in vitro and increased tumor cell intravasation in vivo. Thus, blockade of intravasation and hematologic dissemination in T-LBL is due to elevated S1P1 signaling, increased expression of ICAM1 and augmented homotypic cell-cell adhesion.

Activating mutations in the NOTCH1 gene have been found in about 60% of patients with T-cell acute lymphoblastic leukemia (T-ALL). In order to study the molecular mechanisms by which altered Notch signaling induces leukemia, a zebrafish model of human NOTCH1-induced T-cell leukemia was generated. Seven of sixteen mosaic fish developed a T-cell lymphoproliferative disease at about 5 months. These neoplastic cells extensively invaded tissues throughout the fish and caused an aggressive and lethal leukemia when transplanted into irradiated recipient fish. However, stable transgenic fish exhibited a longer latency for leukemia onset. When the stable transgenic line was crossed with another line overexpressing the zebrafish bcl2 gene, the leukemia onset was dramatically accelerated, indicating synergy between the Notch pathway and the bcl2-mediated antiapoptotic pathway. Reverse transcription-polymerase chain reaction analysis showed that Notch target genes such as her6 and her9 were highly expressed in NOTCH1-induced leukemias. The ability of this model to detect a strong interaction between NOTCH1 and bcl2 suggests that genetic modifier screens have a high likelihood of revealing other genes that can cooperate with NOTCH1 to induce T-ALL.