Leukemic HRX Fusion Proteins Inhibit GADD34-Induced Apoptosis and Associate with the GADD34 and hSNF5/INI1 Proteins

Abstract

One of the most common chromosomal abnormalities in acute leukemia is a reciprocal translocation involving the HRX gene (also called MLL, ALL-1, or HTRX) at chromosomal locus 11q23, resulting in the formation of HRX fusion proteins. Using the yeast two-hybrid system and human cell culture coimmunoprecipitation experiments, we show here that HRX proteins interact directly with the GADD34 protein. We have found that transfected cells overexpressing GADD34 display a significant increase in apoptosis after treatment with ionizing radiation, indicating that GADD34 expression not only correlates with apoptosis but also can enhance apoptosis. The amino-terminal third of the GADD34 protein was necessary for this observed increase in apoptosis. Furthermore, coexpression of three different HRX fusion proteins (HRX-ENL, HRX-AF9, and HRX-ELL) had an anti-apoptotic effect, abrogating GADD34-induced apoptosis. In contrast, expression of wild-type HRX gave rise to an increase in apoptosis. The difference observed here between wild-type HRX and the leukemic HRX fusion proteins suggests that inhibition of GADD34-mediated apoptosis may be important to leukemogenesis. We also show here that GADD34 binds the human SNF5/INI1 protein, a member of the SNF/SWI complex that can remodel chromatin and activate transcription. These studies demonstrate, for the first time, a gain of function for leukemic HRX fusion proteins compared to wild-type protein. We propose that the role of HRX fusion proteins as negative regulators of post-DNA-damage-induced apoptosis is important to leukemia progression.

ACKNOWLEDGMENTS

We thank Hye Son Yi and Thanh Nyugen for providing valuable technical support, Boguslaw Kwiatkowski for providing the pCSmycTEL DNA, Nancy Zeleznik-Le for providing an HRX-AF9 cDNA, Mike Thirman for providing an HRX-ELL cDNA, and Michael Cleary for a full-length HRH cDNA.

Funding from the T. J. Martell Foundation and a National Cancer Institute Cancer Center Support Grant (CA 68485) to R.C. supported this work. A National Institutes of Health grant (CA73969) and a Department of Veterans Affairs Merit Review grant to D.C.T. also supported this work.