Originally published as Genetics Published Articles Ahead of Print on October 8, 2006.

Genetics, Vol. 174, 1963-1972, December 2006, Copyright © 2006
doi:10.1534/genetics.106.064477

Contribution of Growth and Cell Cycle Checkpoints to Radiation Survival in Drosophila

Burnley Jaklevic*,1, Lyle Uyetake*, Willy Lemstra{dagger}, Julia Chang*, William Leary*, Anthony Edwards*, Smruti Vidwans*, Ody Sibon{dagger} and Tin Tin Su*,2

* Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, Colorado 80309-0347 and {dagger} Department of Cell Biology/Section of Radiation and Stress Cell Biology, University Medical Center Groningen, University of Groningen, 9713 AV Groningen, The Netherlands

2 Corresponding author: University of Colorado, Campus Box 0347, Boulder, CO 80309-0347.
E-mail: tin.su{at}colorado.edu

Cell cycle checkpoints contribute to survival after exposure to ionizing radiation (IR) by arresting the cell cycle and permitting repair. As such, yeast and mammalian cells lacking checkpoints are more sensitive to killing by IR. We reported previously that Drosophila larvae mutant for grp (encoding a homolog of Chk1) survive IR as well as wild type despite being deficient in cell cycle checkpoints. This discrepancy could be due to differences either among species or between unicellular and multicellular systems. Here, we provide evidence that Grapes is needed for survival of Drosophila S2 cells after exposure to similar doses of IR, suggesting that multicellular organisms may utilize checkpoint-independent mechanisms to survive irradiation. The dispensability of checkpoints in multicellular organisms could be due to replacement of damaged cells by regeneration through increased nutritional uptake and compensatory proliferation. In support of this idea, we find that inhibition of nutritional uptake (by starvation or onset of pupariation) or inhibition of growth factor signaling and downstream targets (by mutations in cdk4, chico, or dmyc) reduced the radiation survival of larvae. Further, some of these treatments are more detrimental for grp mutants, suggesting that the need for compensatory proliferation is greater for checkpoint mutants. The difference in survival of grp and wild-type larvae allowed us to screen for small molecules that act as genotype-specific radiation sensitizers in a multicellular context. A pilot screen of a small molecule library from the National Cancer Institute yielded known and approved radio-sensitizing anticancer drugs. Since radiation is a common treatment option for human cancers, we propose that Drosophila may be used as an in vivo screening tool for genotype-specific drugs that enhance the effect of radiation therapy.