Mitochondrial (mt) diseases are neurodegenerative conditions due to mutations either in nuclear or mtDNA genes. More than 50% of
mtDNA mutations reside in transfer RNA (tRNA) genes (http://www.mitomap.org) and are responsible for a wide range of syndromes,
such as the Mitochondrial Encephalopathy with Lactic Acidosis and Stroke like episodes (MELAS) and the myoclonic epilepsy with
Ragged-Red-Fibers (MERRF) for which non effective treatment is available at present. Development of therapeutic strategies has been
hampered by the lack of animal models of disease. Thus, there is a need to develop cellular models able to recapitulate the tissue
related physiopathology determined by mt-DNA mutations. Post-mitotic cells, differentiated from induced pluripotent stem cells (iPSCs)
with pathogenic mutations in mt-tRNA genes appear as a promising tool to study the mechanisms underlying tissue-specific
manifestations and pathogenesis of mtDNA disorders and to test tissue-specific rescuing mechanisms of novel therapeutic
approaches. The main objectives of this project are: to obtain neurons and myotubes from iPSCs derived from patients' fibroblasts
bearing either the m.3243A>G mutation in mt-tRNALeu(UUR) gene causing MELAS or the m.8344A>G mutation in the mt-tRNALys
gene causing MERRF; to investigate the mechanisms related to tissue-specific pathologic phenotype.
Specifically, we will:
i) select MELAS- and MERRF-iPSCs with high and low levels of heteroplasmy and characterize their phenotype, by evaluation of cell
viability, oxygen consumption, bioenergetic proficiency;
ii) generate neurons and myotubes from the selected iPSCs, and verify their phenotype, as above.
Given that brain and skeletal muscle are the most typically and severely affected tissues in mt-tRNA related disease, we believe that
this innovative approach will provide information on the pathogenic tissue-specific mechanisms. This is an essential step towards the
use of new molecules in the clinical context.
One of the major challenges in mt-tRNAs related disease is to understand the molecular link between disease phenotypes and
genotypes. For example, the heteroplasmic m.3243A>G mutation in MTTL1, which is the most common among MTTs mutations can
occur in association with a number of clinical syndromes such as encephalomyopathy, lactic acidosis and stroke-like episodes
(MELAS) syndrome; chronic progressive external ophthalmoplegia (CPEO); maternally inherited diabetes and deafness (MIDD) and
cardiomyopathy. The mechanisms underlying the heterogeneity of clinical phenotypes are not fully understood.
We expect that iPSC-derived neurons and myocytes will be reliable cellular model useful to better define the molecular consequences of mt tRNAs mutations and to test the effects of putative therapeutic molecules. This will have a significant impact on the development of therapeutic strategies for mt-tRNA related disease. The knowledge of pathogenic mechanisms related to mt-tRNA mutations will represent a key achievement towards the development of a pharmacological therapy with significant technological, economic and social impact.