Wnt signaling modulates the response to DNA damage in the Drosophila wing imaginal disc by regulating the EGFR pathway

Despite the deep conservation of the DNA damage response pathway (DDR), cells in different contexts vary widely in their susceptibility to DNA damage and their propensity to undergo apoptosis as a result of genomic lesions. One of the cell signaling pathways implicated in modulating the DDR is the highly conserved Wnt pathway, which is known to promote resistance to DNA damage caused by ionizing radiation in a variety of human cancers. However, the mechanisms linking Wnt signal transduction to the DDR remain unclear. Here, we use a genetically encoded system in Drosophila to reliably induce consistent levels of DNA damage in vivo, and demonstrate that canonical Wnt signaling in the wing imaginal disc buffers cells against apoptosis in the face of DNA double-strand breaks. We show that Wg, the primary Wnt ligand in Drosophila, activates Epidermal Growth Factor Receptor (EGFR) signaling via the ligand-processing protease Rhomboid, which in turn modulates the DDR in a Chk2, p53, and E2F1-dependent manner. These studies provide mechanistic insight into the modulation of the DDR by the Wnt and EGFR pathways in vivo in a highly proliferative tissue. Furthermore, they reveal how the growth and patterning functions of Wnt signaling are coupled with pro-survival, anti-apoptotic activities, thereby facilitating developmental robustness in the face of genomic damage. Author SummaryEctopic activation of the highly conserved Wnt signaling pathway has been previously demonstrated to promote resistance to radiation and chemoradiation therapy in a variety of human cancers, yet the mechanisms by which Wnt modulates the DDR pathway are not clearly established. Furthermore, putative interactions between Wnt signaling and the DDR outside the context of pathological Wnt over-expressing tumors have not been clearly elucidated. Here, we show that, in Drosophila, loss of canonical Wnt signaling during development of the highly proliferative wing imaginal disc sensitizes cells to DNA damage, biasing them towards apoptosis and ultimately disrupting normal wing development. In contrast, ectopic Wnt signaling reduces the level of apoptosis for a given level of DNA damage. Mechanistically, we demonstrate that Wnt signaling acts via Epidermal Growth Factor Receptor (EGFR) signaling, a well characterized pro-survival pathway, by activating the ligand-processing protease Rhomboid, and that this effect requires core DDR components Chk2, p53, and E2F1. Altogether, we show that Wnt signaling can promote developmental robustness by opposing apoptosis in the face of DNA damage, and reveal a mechanism by which Wnt signaling modulates the DDR via EGFR signaling.