Identification of Y591 mutation of Flt3 in patients sharing a profile of p53 phosphorylation and Bcl-2 overexpression. (A) Phosphorylation of p53 at serine 15 and Bcl-2 expression were measured in PBMCs from healthy donors or in AML blast cells. Four samples with Flt3 mutations had duplication of Y591 (DupY591). Seven samples had Flt3 mutations that did not duplicate Y591 (SPY591; 6 representatives shown). The remaining samples displayed other Flt3 mutations or were considered wild type (6 representatives shown). (B) Bcl-2 expression, relative to the median in AML blast cells, is shown for normal PBMCs or AML patient samples with different Flt3 mutational status. Significantly greater Bcl-2 expression was observed in samples from AML patients with Flt3 LM that duplicated Y591 than in cells from patients with Flt3 LM that did not (DupY591 versus SPY591; P < .001). Wild-type Flt3 AML patient samples displayed a spread of Bcl-2 values centered at the median observed for all 30 AML samples (a value of 0 on the log₁₀ scale). The median Bcl-2 expression for each group is marked with a black line. (C) A percentage of STAT and MAPK signaling we previously analyzed in this cohort of AML patients³  was plotted for each patient against the Bcl-2 expression in patients without (left plot) or with (right plot) a Flt3 LM. Among patients with a Flt3 LM, potentiated STAT and MAPK signaling was associated with Bcl-2 expression (r² = 0.51). In contrast, significantly higher levels of aggressive signaling were observed in patients whose Flt3 mutation duplicated Y591, as compared to patients with other Flt3 LMs (P < .01).