Copy number variations of the mitochondrial DNA as potential cause of mitochondrial diseases

Abstract

Aim: The aim of this thesis was the analysis of copy number variations of the mitochondrial DNA (mtDNA) in several tissues and cell types with regard to different mitochondrial associated disorders.<br /> Background: The mtDNA copy number can be reduced due to mutations in the nuclear encoded DNA polymerase gamma (POLG) or damages caused by deleterious reactive oxygen species (ROS), which are created by the respiratory chain. This leads to the insufficient expression of mitochondrial encoded subunits of complexes of the oxidative phosphorylation system (OXPHOS). Consequently an impairment of the biochemical activity and integrity of the cells occurs. <br /> Methods: The quantification of the mtDNA was performed by quantitative PCR (qPCR). Biochemical activities were determined by enzymatic assays such as direct measurement of the citrate synthase (CS) activity or comprehensive measurement of the respiratory activity. Results: Mutations in the nuclear inherited gene POLG result in mtDNA depletion in mitochondrial disorders including a mild phenotype of progressive external ophthalmoplegia (PEO) with epilepsy/ataxia. A mtDNA depletion was detected in different tissues and cell types of Alpers-Huttenlocher patients with pathogenic nuclear mutations. The mtDNA copy number was reduced in specific hippocampal regions of temporal lobe epilepsy (TLE) patients with Ammons’ horn sclerosis (AHS) accompanied by a decreased CS activity. An in vitro reduction of the mtDNA in fibroblasts results in an impaired respiratory activity.<br /> Conclusions: The mtDNA content is proportional to the mitochondria content and the energy demand of the respective tissue or cell type under normal conditions. A cell type- and tissue-specific depletion of the mtDNA can be present in several inherited and somatic mitochondrial disorders in vivo or can be generated by an in vitro system. The mtDNA depletion diminishes the biochemical activity and integrity of the cells and can contribute to the disease phenotype.