Novel RUNX1 isoforms determine the fate of acute myeloid leukemia cells by controlling CD56 expression.

CD56(high) acute myeloid leukemias (AMLs) have a poor prognosis, but it has been unclear how CD56 expression is controlled and how it relates to clinical aggressiveness. We show that CD56 expression on AML cells correlates with an abnormal expression pattern of runt-related transcription factor 1 (RUNX1) isoforms. Whereas full-length p48 RUNX1 (p48) up-regulated CD56 in AML cells, 3 previously unknown shorter RUNX1 isoforms, p38a, p30, and p24, suppressed CD56 expression. Both p48 and CD56 induced nuclear translocation of nuclear factor (NF)-kappaB and increased bcl2L12 expression, and inhibition of this pathway by small inhibitory RNA-mediated p48 knock down or NF-kappaB blockade substantially increased apoptosis in CD56(+) AML cell lines. These findings indicate the potential for new therapy of CD56(high) AML by suppression of the "overactive" RUNX1/CD56/NF-kappaB signaling pathway(s).