Genomic Instability and Enhanced Radiosensitivity in Hsp70.1- and Hsp70.3-Deficient Mice

Abstract

Heat shock proteins (HSPs) are highly conserved among all organisms from prokaryotes to eukaryotes. In mice, the HSP genes Hsp70.1 and Hsp70.3 are induced by both endogenous and exogenous stressors, such as heat and toxicants. In order to determine whether such proteins specifically influence genomic instability, mice deficient for Hsp70.1 and Hsp70.3 (Hsp70.1/3^−/− mice) were generated by gene targeting. Mouse embryonic fibroblasts (MEFs) prepared from Hsp70.1/3^−/− mice did not synthesize Hsp70.1 or Hsp70.3 after heat-induced stress. While the Hsp70.1/3^−/− mutant mice were fertile, their cells displayed genomic instability that was enhanced by heat treatment. Cells from Hsp70.1/3^−/− mice also display a higher frequency of chromosome end-to-end associations than do control Hsp70.1/3^+/+ cells. To determine whether observed genomic instability was related to defective chromosome repair, Hsp70.1/3^−/− and Hsp70.1/3^+/+ fibroblasts were treated with ionizing radiation (IR) alone or heat and IR. Exposure to IR led to more residual chromosome aberrations, radioresistant DNA synthesis (a hallmark of genomic instability), increased cell killing, and enhanced IR-induced oncogenic transformation in Hsp70.1/3^−/− cells. Heat treatment prior to IR exposure enhanced cell killing, S-phase-specific chromosome damage, and the frequency of transformants in Hsp70.1/3^−/− cells in comparison to Hsp70.1/3^+/+ cells. Both in vivo and in vitro studies demonstrate for the first time that Hsp70.1 and Hsp70.3 have an essential role in maintaining genomic stability under stress conditions.

This investigation was supported by grant NS34746 from NIH, by a grant from the Department of Army, by the A-T Children's Society, and by funds from Radiation Oncology, Washington University School of Medicine, St. Louis, Mo., to T.K.P. and from the U.S. Environmental Protection Agency to D.J.D.

This study has been subjected to review by the National Health and Environmental Effects Research Laboratory and approved for publication. Approval does not signify that the contents reflect the views of the Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.