The Flt3 ITD Accelerates An Already Established Myeloid Leukemia and Alters Chemotherapy Response In Vitro and In Vivo in a p53 Dependent Manner.

Abstract 1719

Poster Board I-745

Acute myeloid leukemia (AML) is an aggressive disease with heterogeneous genetics and variable prognosis. The presence of an internal tandem duplication within the FLT3 gene (Flt3 ITD) is a marker for poor prognosis and has been linked to anthracycline resistance in cell lines and primary patient samples in vitro. The effect of this mutation on response to chemotherapy in vivo has not been examined and its effect on response to cytarabine is not known. In this study we use a genetically defined mouse model of AML to examine the effects of the Flt3 ITD on response to cytarabine and the anthracycline doxorubicin in vitro and in vivo. In vitro the Flt3 ITD conferred resistance to doxorubicin and the combination of doxorubicin and cytarabine but sensitivity to cytarabine alone in comparison to the identical leukemia without the Flt3 ITD. In vivo the presence of the Flt3 ITD provided an advantage in leukemic engraftment and accelerated disease onset. This advantage could be partially reversed by treatment of the animals with cytarabine but not by treatment with doxorubicin. Surprisingly, in vivo the Flt3 ITD conferred a marked increase in sensitivity to cytarabine when compared to the parental leukemia without this mutation. In contrast to the parental leukemia, the addition of doxorubicin to cytarabine provided no advantage over cytarabine alone. When the DNA damage response was assessed the presence of the Flt3 ITD resulted in an increase in the levels of p53 following treatment with either doxorubicin or cytarabine. Induction of the p53 target genes p21 and MDM2 was also increased. Surprisingly, the Flt3 ITD had no effect on disease onset or chemotherapy response in vitro or in vivo in the setting of p53 null AML. These data when taken together demonstrate that the Flt3 ITD confers a mixed sensitivity and resistance to standard chemotherapy and provides an engraftment advantage in a manner that depends on an intact p53 allele. This may at least in part explain the rarity of dual p53 null and Flt3 ITD positive AML. Furthermore, these data suggest that patients with Fl3 ITD positive AML may benefit more from treatment with high dose Ara-C then with combinations containing an anthracycline.