Targeted Deletion of the S-Phase-Specific Myc Antagonist Mad3 Sensitizes Neuronal and Lymphoid Cells to Radiation-Induced Apoptosis

Abstract

The Mad family comprises four basic-helix-loop-helix/leucine zipper proteins, Mad1, Mxi1, Mad3, and Mad4, which heterodimerize with Max and function as transcriptional repressors. The balance between Myc-Max and Mad-Max complexes has been postulated to influence cell proliferation and differentiation. The expression patterns of Mad family genes are complex, but in general, the induction of most family members is linked to cell cycle exit and differentiation. The expression pattern ofmad3 is unusual in that mad3 mRNA and protein were found to be restricted to proliferating cells prior to differentiation. We show here that during murine developmentmad3 is specifically expressed in the S phase of the cell cycle in neuronal progenitor cells that are committed to differentiation. To investigate mad3 function, we disrupted the mad3 gene by homologous recombination in mice. No defect in cell cycle exit and differentiation could be detected in mad3 homozygous mutant mice. However, upon gamma irradiation, increased cell death of thymocytes and neural progenitor cells was observed, implicating mad3 in the regulation of the cellular response to DNA damage.

ACKNOWLEDGMENTS

We are indebted to Leni Sue Carlos and Pei Feng Cheng for technical assistance and to Keesook Lee for expert work in gene targeting. We are grateful to Philippe Soriano for advice and reagents. We are also thankful to to George Sale and the pathology service and to Barbara Johnston and the technicians of the animal facility.

This work was supported by grants CA57138 and HL54881 to R.N.E., a fellowship from the Damon Runyon-Walter Winchell Foundation (DRG-076) and a Special Fellowship of the Leukemia Society of America to G.A.M., and an NIH Mentored Clinical Investigator Award (K08 AJ01445) to B.M.I. R.N.E. is a Research Professor of the American Cancer Society.