Mitochondrial abnormalities in fibroblasts carrying DNMT1 mutations

ABSTRACT: Autosomal dominant cerebellar ataxia, deafness and narcolepsy (ADCA-DN) and hereditary sensory neuropathy with dementia and hearing loss (HSE-IN) are rare neurodegenerative syndromes recently linked to dominant pathogenic mutations in the DNA methyltransferase 1 (DNMT1) gene. ADCA-DN and HSN-IE phenotypes share some features such as optic atrophy, deafness, cerebellar ataxia, peripheral neuropathy and narcolepsy. Moreover, both ADCA-DN and HSN-IE phenotypes present clinical features typical of mitochondrial diseases and some biochemical evidence of mitochondrial dysfunction in muscle of ADCA-DN patients was previously reported. Interestingly, DNMT1 has been recently recognized as responsible for mitochondrial DNA (mtDNA) methylation, in addition to nuclear DNA methylation, suggesting that DNMT1 mutations may directly affect mtDNA, supporting also the hypothesis of mitochondrial dysfunction in ADCA-DN and HSN-IE. To investigate the pathogenic mechanisms of DNMT1 mutations, and how these genetic defects may affect mitochondrial function, we studied fibroblasts from ADCA-DN and HSE-IN families carrying four different mutations in DNMT1 (p.A570V, p.G605A, p.V606F, p.P507R). All mutants showed a significant increase of about 1,5 fold in DNMT1 mRNA expression and a significant reduction of about 40% in the global DNA methyltransferase activity, as evaluated in total cellular protein extracts, compared to controls. Analysis of mitochondrial transcription revealed a significant increase of mitochondrial gene expression in the mutants compared to controls. We observed the mitochondrial network after 48 hours of growth in galactose medium, finding a marked decrease in the percentage of cells with filamentous mitochondria and, concomitantly, an increased number of cells with fragmented mitochondria. These preliminary data demonstrate that mutations in DNMT1 responsible for ADCA-DN and HSE-IN may produce a defect in the methyltransferase activity, possibly causing a reduction in mtDNA methylation, which in turn leads to the increased mtDNA transcription. Moreover, the network fragmentation induced by galactose suggests a mitochondrial defect impinging on the mito-network dynamics.