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Rhabdomyosarcoma (RMS) is the most common pediatric soft-tissue cancer, which primarily develops in muscle tissue during childhood. Despite advances in treatment, outcomes for many patients remain poor. This thesis aims to investigate RMS progression and regression in different genetic contexts, using zebrafish RMS models to evaluate a novel molecule as a potential RMS treatment.

Pax3 and Pax7 transcription factors play important roles in fusion-positive RMS. However, their role in fusion-negative RMS remains less clear. Therefore, we aimed to examine how the lack of Pax3 and Pax7 influences fusion-negative RMS formation and progression.

In the first study, we investigated the role of Pax3 in RAS-driven, fusion-negative RMS. Using pax3a^-/-;pax3b^-/- double mutant zebrafish, we found that the RAS/MAPK signalling pathway was downregulated in the mutants compared to controls, correlating with a delay in tumour progression. These findings suggest that, in fusion-negative RMS, Pax3 also contributes to tumour development by modulating oncogenic signalling. 

In the second study, we aimed to explore the role of Pax7 in fusion-negative RMS, using a pax7-deficient zebrafish model. Our RNA-seq and lipidomics analyses showed downregulation of mitochondrial-related genes, decreased cardiolipin levels, and altered microtubule dynamics in pax7a^-/-;pax7b^-/- mutants compared to healthy wild-type fish. We also showed lower mitochondria labelling in pax7a^-/-;pax7b^-/- tumours compared to wild-type tumours. These findings indicate that Pax7 plays an important role in maintaining mitochondrial membrane integrity and function in healthy and tumourigenic conditions.

In the last part of the thesis, we aimed to evaluate the feasibility of photodynamic therapy using a novel photosensitizer for cancer treatment.

In the third study we designed and synthesized a heavy-atom-free light-inducible compound called dibenzothioxanthene imide (DBI), which binds to G-quadruplex (G4) DNA. Upon targeted photoactivation of DBI, it produced reactive oxygen species and caused DNA damage in human cancer cells and healthy zebrafish embryos. 

Complementing this, the last study explored the potential of photodynamic therapy using the photosensitizer DBI for treating RMS in zebrafish models. Treatment with light-activated DBI resulted in targeted apoptosis and tumour regression, compared to control treatments. In addition, RMS tissue was more affected by photoactivated DBI than healthy tissue, likely due to the higher accumulation of G4 structures in tumours compared to healthy regions.

Together, these findings uncover new roles of Pax3 and Pax7 in RMS biology using zebrafish models and highlight the therapeutic potential of G4-targeted photodynamic therapy for RMS.