Mitoepigenetics investigations in neurodegenerative diseases

Neurodegenerative diseases (NDs) represent a group of disorders characterized by the progressive neuronal loss in specific areas of the central nervous system. NDs are incurable and often fatal shortly after diagnosis. The global prevalence of these disorders is dramatically increasing worldwide as populations age and life expectancies increase. The identification of valuable biomarkers for an early diagnosis is of outmost importance as it would promote early interventions able to prevent or delay as much as possible the onset of the disease. Despite their high heterogeneity and the differences in their primary etiologies, NDs share many common aspects in relation to their clinical, biochemical, and pathological features and multiple lines of evidence suggest that mitochondrial dysfunction is involved in the pathogenesis of many NDs, especially Alzheimer´s disease (AD), Parkinson´s disease (PD) and amyotrophic lateral sclerosis (ALS). To date, several papers showed that aberrant epigenetic mechanisms in the nuclear DNA may be involved in the onset and development of NDs, and several studies have found altered gene methylation levels in both post-mortem brain specimens and peripheral tissues from patients with these diseases. In recent years growing evidence for a potential role of altered mitochondrial DNA (mtDNA) methylation and hydroxymethylation in several diseases has emerged. Although mitochondrial impairment is a classical feature of neurodegeneration, little attention has been given until now to the role of the mitochondrial epigenome itself in NDs. Particularly, studies performed so far have investigated mtDNA methylation in animal models of ALS, and in brain tissue of patients with AD, PD and ALS. However, potential mtDNA methylation alterations in peripheral tissues of NDs patients have not been investigated in those studies. The main aim of the work presented in the current thesis was to investigate the presence of mitoepigenetic signatures in peripheral blood of patients with AD (Study 1), ALS (Study 2) and PD (Study 3). DNA methylation analysis of the mitochondrial D-loop region, which regulates mitochondrial transcription and replication, was performed by means of Methylation Sensitive-High Resolution Melting and Pyrosequencing techniques. In study 1 D-loop methylation levels were analyzed in people affected by AD with different clinical dementia impairment degrees and results suggest that mtDNA methylation could vary with the stage of the disease. In study 2 D-loop methylation levels were analyzed in ALS patients with mutations in SOD1, TARDBP, FUS or C9ORF72 genes, and in their relatives, and results showed that mtDNA methylation levels were decreased in ALS tissues, partcularly in carriers of SOD1 mutations. In study 3 higher Dloop methylation levels, although not statistically significant, were detected in peripheral blood from PD patients. Results presented in the current thesis indicate a potential involvement for impaired mtDNA methylation in NDs, which is detectable in peripheral blood suggesting that this field of research deserves to be further studied. Moreover, current results suggest that mtDNA methylation could be sensitive to different disease stages and dementia levels, thus adding a new layer of interest in the search for peripheral mitoepigenetic biomarkers for neurodegeneration. Given the pivotal role of mitochondrial dysfunction and of epigenetic mechanisms in neurodegeneration, the field of mitoepigenetics in neurodegenerative diseases is a timely and attractive recent area of investigation, where preliminary results really seem encouraging, but more research is warranted to clarify the connections between epigenetic changes occurring in the mitochondrial genome, mitochondrial DNA dynamics, and the neurodegenerative process.