Leukemic HRX fusion proteins inhibit GADD34-induced apoptosis and associate with the GADD34 and hSNF5/INI1 proteins

One of the most common chromosomal abnormalities in acute leukemia is a rec iprocal translocation involving the HRX gene (also called MLL, ALL-1, or HT RX) at chromosomal locus 11q23, resulting in the formation of HRX fusion pr oteins. Using the yeast two-hybrid system and human cell culture coimmunopr ecipitation experiments, we show here that HRX proteins interact directly w ith the GADD34 protein. We have found that transfected cells overexpressing GADD34 display a significant increase in apoptosis after treatment with io nizing radiation, indicating that GADD34 expression not only correlates wit h apoptosis but also can enhance apoptosis. The amino-terminal third of the GADD34 protein was necessary for this observed increase in apoptosis. Furt hermore, coexpression of three different HRX fusion proteins (HRX-ENL, HRX- AF9, and HRX-ELL) had an anti-apoptotic effect, abrogating GADD34-induced a poptosis. In contrast, expression of wild-type HRX gave rise to an increase in apoptosis. The difference observed here between wild-type HRX and the l eukemic HRX fusion proteins suggests that inhibition of GADD34-mediated apo ptosis may be important to leukemogenesis, We also show here that GADD34 bi nds the human SNF5/INI1 protein, a member of the SNF/SWI complex: that can remodel chromatin and activate transcription. These studies demonstrate, fo r the first time, a gain of function for leukemic HRX fusion proteins compa red to wild-type protein. We propose that the role of HRX fusion proteins a s negative regulators of post-DNA-damage-induced apoptosis is important to leukemia progression.