Thesis title: Characterization of conserved radioresistance mechanisms in Drosophila melanogaster and human Cell Lines
Radiotherapy is a cornerstone of cancer treatment, but its long-term efficacy is often compromised by tumor recurrence due to intrinsic or acquired radioresistance. This resistance stems from altered cellular mechanisms that enable tumor cells to withstand genotoxic stress more effectively than healthy tissues. A fundamental understanding of these mechanisms is therefore critical for improving clinical outcomes. Model organisms like Drosophila melanogaster, which exhibit natural radioresistance, provide valuable systems for dissecting the cellular response to ionizing radiation.
In this thesis work, I investigated the role of loquacious (loqs) in the radiation response of Drosophila. I demonstrated that three copies of loqs, that encodes a double-stranded RNA-binding protein known to participate in the biogenesis of small non-coding RNAs, confer radiosensitivity in Drosophila mitotic cells. This finding, which is the opposite of the radioresistance previously observed in heterozygous loqs mutants by our lab, is not restricted to a specific tissue or cell type. Furthermore, the opposing phenotypic effects of loqs haploinsufficiency and triplosensitivity suggest that its function in the DNA damage response is inherently dosage-sensitive.
I have also demonstrated that the double-stranded RNA-binding protein PACT, the human ortholog of loqs, is also involved in a radiation response in human cells, including cancer cells, which is similar with that described in flies. Indeed, my cytological and molecular analyses revealed that depletion of PACT, which is known to participate in the biogenesis of small non-coding RNAs, in the antiviral response, and to act as an activator of the protein kinase PKR, confers radioresistance in both tumor and non-tumor human cell lines. Conversely, PACT overexpression in radioresistant Embryonal Rhabdomyosarcoma cells increased their sensitivity to X-ray exposure, further supporting the involvement of PACT in the radiation response. Finally, my analyses of histone H3 post-translational modifications suggest that modulation of both PACT and Loqs affects chromatin state in human and Drosophila cells, respectively. These chromatin changes may represent one of the key mechanisms underlying the radioresistant/radiosensitive phenotype.
Taken together, my findings identify PACT as a novel regulator of the radiation response and highlight it as a promising therapeutic target to enhance radiotherapy efficacy.