E2F1 promotes, JNK and DIAP1 inhibit, and chromosomal position has little effect on radiation-induced Loss of Heterozygosity in Drosophila

Abstract

Loss of Heterozygosity (LOH) can occur when a heterozygous mutant cell loses the remaining wild type allele to become a homozygous mutant. LOH can have physiological consequences if, for example, the affected gene encodes a tumor suppressor. We used two fluorescent reporters to study mechanisms of LOH induction by X-rays, a type of ionizing radiation (IR), in Drosophila larval wing discs. IR is used to treat more than half of cancer patients, so understanding its effects is of biomedical relevance. Quantitative analysis of IR-induced LOH at seven different positions between the telomere and the centromere, the first such study that we know of, showed that while a homologous chromosome is required to produce LOH after irradiation, position along the chromosome makes little difference. This suggests that IR-induced LOH occurs via non-recombination-based mechanisms, unlike what was previously reported for spontaneously occurring LOH. Using a focused screen, we identified E2F1 as a key promotor of LOH and further testing suggests a mechanism involving its role in cell cycle regulation rather than apoptosis. We leveraged the loss of a transcriptional repressor through LOH to express transgenes specifically in cells that have already acquired LOH. This approach identified JNK signaling and apoptosis as key determinants of LOH maintenance. These studies reveal previously unknown mechanisms for generation and elimination of cells with chromosome aberrations after exposure to IR. SummaryLoss of Heterozygosity (LOH) can lead to homozygous mutant cells, with potential physiological consequences if it affects tumor suppressor genes. Using fluorescent reporters of LOH in Drosophila wing discs, we found that X-ray-induced LOH requires a homologous chromosome but was unaffected by position along the chromosome. This suggests non-recombination mechanisms for X-ray-induced LOH, unlike spontaneous LOH. We found that E2F1 promotes LOH while JNK signaling and apoptosis eliminates cells with LOH, uncovering new insights into chromosomal aberrations post-exposure to radiation.