Modeling POLG-related disorders in the zebrafish organism: towards a pharmacological treatment

The mitochondrial DNA polymerase γ (POLγ) is an enzyme involved in mitochondrial DNA (mtDNA) replication and maintenance. The human holoenzyme is composed of a catalytic component (POLG) and two accessory subunits (POLG2) acting as processivity factors. Mutations in genes involved in mtDNA replication (POLG, POLG2, TWNK and SSBP1) lead to a subset of autosomal-inherited mitochondrial diseases, collectively known as POLG-related disorders, presenting with broad clinical expression and often leading to premature death at a young age. Despite advancements, therapies for POLG-related disorders remain complex and non-curative, although recent investigations have identified clofilium tosylate as a potential therapeutic compound. Zebrafish emerged as a suitable model in genetic disease research to study POLG-related disorders, due to the high conservation between human and zebrafish mitochondrial polymerase subunits, an ideal situation to identify potential targets for a drug treatment. To better understand the role of the subunits of POLγ and the mechanisms of disease progression in vivo, we characterized the morphofunctional traits associated to polg and polg2 mutations in two zebrafish mutant lines (polgia302 and polg2ia304), identifying severe mtDNA depletion, slower development and altered mitochondrial network and dynamics in homozygous mutants. Moreover, we conducted preliminary analyses to assess a panel of candidate molecules that have already been shown to rescue disease phenotypes in yeast Polg models. The library includes antiarrhythmic drugs, as well as anti-rheumatism, anti-inflammatory and nonsteroidal oestrogen molecules. We evaluated the efficacy of these compounds in rescuing the lower content of mtDNA and the motor behaviour in the aforementioned zebrafish mutants, detecting positive responses after drug treatment. Altogether, our results point at zebrafish as an effective model to study the etiopathology of human POLG-related disorders during early developmental phases, besides being an excellent platform for the screening of therapeutic drugs able to rescue Polg and Polg2-related defects.