Topoisomerase II inhibitors induce an illegitimate genome rearrangement common in infant leukemia.

摘要

Infant acute leukemias account for ~30% of all malignancy seen in childhood across the Western world. They are aggressive and characterized by rapid onset shortly after birth. The majority of these have rearrangements involving the MLL (mixed lineage leukemia) gene. Although MLL fusion to more than 75 genes have been identified, AF9 is one of its most common translocation partners. Since MLL breakpoint sequences associated with infant acute leukemia are similar to those in secondary AML following exposure to the topoisomerase II (topo II) poison etoposide, it has been hypothesized that exposure during pregnancy to biochemically similar compounds may promote infant acute leukemia. Some studies have shown an epidemiological link between bioflavonoid intake and increased incidence of MLL-rearranged infant leukemias. Hundreds of unregulated nutritional supplements are widely available and perceived to prevent cardiovascular disease, inflammation and cancer. However, their potential to promote leukemic translocations should be determined. My goal was to test the hypotheses that 1) MLL and AF9 breakpoint cluster regions (bcrs) undergo double strand breaks (DSBs), and are highly recombinogenic independent of chromosomal context, and in the absence of a leukemic fusion protein; and 2) a number of topoisomerase II inhibitors have the potential to promote these translocations analogous to those observed in infant leukemia. To test these hypotheses, I developed transgenic reporter cell lines such that a translocation between MLL and AF9 bcrs results in a full-length GFP transcript and quantifiable green fluorescence. I show here that topo II inhibitors such as the bioflavonoids genistein, quercetin, luteolin and myricetin lead to MLL-AF9 bcr translocations, genistein and quercetin being the most potent promoters. Notably, benzoquinone which is associated with non-MLL-rearranged leukemias do not promote MLL-AF9 translocations even at a high dose. I demonstrate here the sensitivity of the MLL and AF9 bcrs to topo II poisons and bioflavonoids independent of their normal chromatin context and independent of the formation of a leukemic fusion protein. In addition, this reporter gene system allows for rapid and reproducible screening of hundreds of compounds that may have the potential to promote leukemogenic translocations.