Drosophila melanogaster as a model to study mitochondrial diseases

Human mitochondrial diseases known as mitochondrial encephalomyopathies affecting the oxidative phosphorylation (OXPHOS) system can result from a large number of different mutations, both in the nuclear genome or in the maternally inherited mitochondrial DNA (mtDNA). The manifestations of mitochondrial diseases are extremely diverse, including numerous symptoms of variable severity, affecting tissues highly dependent on mitochondrial energy, such as brain, muscle, heart, and the sensorineural epithelia. Moreover these disorders can appear early in life, in children or in young adults (Wallace et al. 1999). We used Drosophila melanogaster as a model organism to study a number of different mitochondrial disorders. The main reasons why the fruit fly is considered a good model are several. It is small, easy to grow, it has a short life cycle and generation time, it has a large number of progeny and finally it has a small completed sequenced genome, (Adams et al. 2000), with a large number of orthologs to human genes. In particular we employed Drosophila to characterize the molecular function of three different genes whose dysfunctions result in encephalomyopathies affecting the functionality of the mitochondrial respiratory chain at different levels. However, in Drosophila, mutations in these genes cause a common larval lethal phenotype. Furthermore another gene characterized in the present work is involved in the maintenance of mitochondrial DNA (mtDNA). Structural alterations of the mitochondrial genome or decrease of its copy number have profound consequences on mitochondrial function and cause severe human diseases which ultimately, also lead to death (Spinazzola and Zeviani 2005). These genes, and the pathologies deriving from their impairments, were studied in Drosophila with different approaches which follow a common general scheme. Firstly we emloyed an approach which would allow ubiquitous or tissue-specific gene knock down, produced by exploiting the yeast UAS/GAL4 binary system. Specific dsRNA allows the targeted silencing of the interesting gene which can be spatially and temporally modulated (Brand and Perrimon 1993). When possible, we adopted procedures to obtain a gene knock out model. Generation of knock out lines is now a well established method which involves the recombination between transgenic lines bearing insertional elements present within the fruit fly genome. It is rather straightforward to obtain genomic deletions through the combination of FRT bearing transgenes flanking the region of interest, in the presence of FLP recombinase (Parks et al. 2004). Furthermore, when possible, confirmation of the genetic specificity of the observed defects in the Drosophila models was obtained by implementing appropriate gene rescue strategies.