Roles of DNA topoisomerase II isozymes in chemotherapy and secondary malignancies

Drugs that target DNA topoisomerase II (Top2), including etoposide (VP-16), doxorubicin, and mitoxantrone, are among the most effective anticancer drugs in clinical use. However, Top2-based chemotherapy has been associated with higher incidences of secondary malignancies, notably the development of acute myeloid leukemia in VP-16-treated patients. This association is suggestive of a link between carcinogenesis and Top2-mediated DNA damage. We show here that VP-16-induced carcinogenesis involves mainly the β rather than the α isozyme of Top2. In a mouse skin carcinogenesis model, the incidence of VP-16-induced melanomas in the skin of 7,12-dimethylbenz[a]anthracene-treated mice is found to be significantly higher in TOP2β+ than in skin-specific top2β-knockout mice. Furthermore, VP-16-induced DNA sequence rearrangements and double-strand breaks (DSBs) are found to be Top2β-dependent and preventable by cotreatment with a proteasome inhibitor, suggesting the importance of proteasomal degradation of the Top2β-DNA cleavage complexes in VP-16-induced DNA sequence rearrangements. VP-16 cytotoxicity in transformed cells expressing both Top2 isozymes is, however, found to be primarily Top2α-dependent. These results point to the importance of developing Top2α-specific anticancer drugs for effective chemotherapy without the development of treatment-related secondary malignancies

Potential role of oxidative stress-induced apoptosis in mediating chromosomal rearrangements in nasopharyngeal carcinoma

BACKGROUND: Genetic aberrations have been identified in nasopharyngeal carcinoma (NPC), however, the underlying mechanism remains elusive. There are increasing evidences that the apoptotic nuclease caspase-activated deoxyribonuclease (CAD) is one of the players leading to translocation in leukemia. Oxidative stress, which has been strongly implicated in carcinogenesis, is a potent apoptotic inducer. Most of the NPC etiological factors are known to induce oxidative stress. Although apoptosis is a cell death process, cells possess the potential to survive apoptosis upon DNA repair. Eventually, the surviving cells may carry rearranged chromosomes. We hypothesized that oxidative stress-induced apoptosis may cause chromosomal breaks mediated by CAD. Upon erroneous DNA repair, cells that survive apoptosis may harbor chromosomal rearrangements contributing to NPC pathogenesis. This study focused on the AF9 gene at 9p22, a common deletion region in NPC. We aimed to propose a possible model for molecular mechanism underlying the chromosomal rearrangements in NPC. RESULTS: In the present study, we showed that hydrogen peroxide (H(2)O(2)) induced apoptosis in NPC (HK1) and normal nasopharyngeal epithelial (NP69) cells, as evaluated by flow cytometric analyses. Activity of caspases 3/7 was detected in H(2)O(2)-treated cells. This activity was inhibited by caspase inhibitor (CI). By nested inverse polymerase chain reaction (IPCR), we demonstrated that oxidative stress-induced apoptosis in HK1 and NP69 cells resulted in cleavages within the breakpoint cluster region (BCR) of the AF9 gene. The gene cleavage frequency detected in the H(2)O(2)-treated cells was found to be significantly higher than untreated control. We further found that treatment with CI, which indirectly inhibits CAD, significantly reduced the chromosomal breaks in H(2)O(2)-cotreated cells. Intriguingly, a few breakpoints were mapped within the AF9 region that was previously reported to translocate with the mixed lineage leukemia (MLL) gene in acute lymphoblastic leukemia (ALL) patient. CONCLUSIONS: In conclusion, our findings suggested that oxidative stress-induced apoptosis could be one of the mechanisms underlying the chromosomal rearrangements in NPC. CAD may play an important role in chromosomal cleavages mediated by oxidative stress-induced apoptosis. A potential model for oxidative stress-induced apoptosis mediating chromosomal rearrangements in NPC is proposed