Modulation of mtDNA copy number ameliorates the pathological consequences of a heteroplasmic mtDNA mutation in the mouse

Heteroplasmic mtDNA mutations typically act in a recessive way and cause mitochondrial disease only if present above a certain threshold level. We have experimentally investigated to what extent the absolute levels of wild-type (WT) mtDNA influence disease manifestations by manipulating TFAM levels in mice with a heteroplasmic mtDNA mutation in the tRNA^Ala gene. Increase of total mtDNA levels ameliorated pathology in multiple tissues, although the levels of heteroplasmy remained the same. A reduction in mtDNA levels worsened the phenotype in postmitotic tissues, such as heart, whereas there was an unexpected beneficial effect in rapidly proliferating tissues, such as colon, because of enhanced clonal expansion and selective elimination of mutated mtDNA. The absolute levels of WT mtDNA are thus an important determinant of the pathological manifestations, suggesting that pharmacological or gene therapy approaches to selectively increase mtDNA copy number provide a potential treatment strategy for human mtDNA mutation disease.

Complex genetic counselling and prenatal analysis in a woman with external ophthalmoplegia and deleted mtDNA

Single large mitochondrial DNA deletions (DeltamtDNA) are usually spontaneously occurring and cause a wide variety of symptoms, ranging from severe infantile multisystem disorders to adult onset progressive external ophthalmoplegia (PEO). There is always heteroplasmy with a mixture of normal and mutant mtDNA and the levels usually vary widely between tissues. There is at present insufficient scientific basis for accurate genetic counselling of women with DeltamtDNA, but it is reasonable to assume that DeltamtDNA can be transmitted if it is present in the female germ cells. Here, we present the results of prenatal analysis in a woman with DeltamtDNA and PEO. No DeltamtDNA was detected by Southern blot and PCR analyses of chorionic villi from the first trimester of pregnancy, in cord blood obtained at birth or in peripheral blood from the child at six months of age. This makes it unlikely that the child will develop a severe infantile mitochondrial disorder due to transmission of high levels of DeltamtDNA. However, the complex mitochondrial genetics and the limited access to human tissues makes it impossible to exclude transmission of low levels of DeltamtDNA that possibly could cause disease later in life.

Effects of high resistance training in patients with myotonic dystrophy

Nine ambulatory subjects with myotonic dystrophy participated in a supervised 12-week progressive high-resistance training program. Knee extensor muscles were trained 3 times a week with free weights, 3 x 10 repetitions at 80% of 1RM. One leg was randomly chosen for training and the other served as control. Six patients completed the training program. In the trained leg, 1RM increased from 16.4 +/- 3.4 kg to 21.8 +/- 2.6 kg (p = 0.0002). There was no difference between pre- and post-training concentric or eccentric isokinetic values at 30 degrees/second in either leg. Muscle biopsy from m. vastus lateralis in the trained leg revealed no systematic difference in the degree of histopathological abnormalities before and after training. After training, there was a tendency toward increase in cross-sectional area of type I muscle fibres. However, the number of subjects was too small to draw conclusions regarding the effects of training on the histopathological changes. Magnetic resonance imaging revealed no difference in the m. quadriceps area after training. In conclusion, patients with myotonic dystrophy improved their muscle strength without any observed negative side effects after a 12-week high-resistance training program.