ABSTRACT
The objective of this study was to investigate clinical, biochemical, and genetic features in 7 probands (a total of 11 patients) with nicotine-amide adenine dinucleotide (NADH) dehydrogenase (complex I) deficiency. We screened the mitochondrial DNA for mutations and found pathogenic mutations in complex I genes (mitochondrial NADH dehydrogenase subunit (MTND) genes) in three probands. The 10191T>C mutation in MTND3 and the 14487T>C mutation in MTND6 were present in two probands with Leigh's-like and Leigh's syndrome, respectively. Four siblings with a syndrome consisting of encephalomyopathy with hearing impairment, optic nerve atrophy, and cardiac involvement had the 11778G>A mutation in MTND4, previously associated with Leber hereditary optic neuropathy. These findings demonstrate that mutations in MTND genes are relatively frequent in patients with complex I deficiency. Biochemical measurements of respiratory chain function in muscle mitochondria showed that all patients had a moderate decrease of the mitochondrial adenosine triphosphate production rate. Interestingly, the complex I deficiency caused secondary metabolic alterations with decreased oxaloacetate-induced inhibition of succinate dehydrogenase (complex II) and excretion of Krebs cycle intermediates in the urine. Our results thus suggest that altered regulation of metabolism may play an important role in the pathogenesis of complex I deficiency.
Subject(s)
Electron Transport Complex I/deficiency , Electron Transport Complex I/genetics , Metabolism, Inborn Errors , Mutation , NADH Dehydrogenase/genetics , Adenosine Triphosphate/metabolism , Adolescent , Adult , Blotting, Western/methods , Child , Child, Preschool , DNA Mutational Analysis , DNA, Mitochondrial/genetics , DNA, Mitochondrial/metabolism , Female , Humans , Infant , Male , Metabolism, Inborn Errors/genetics , Metabolism, Inborn Errors/metabolism , Metabolism, Inborn Errors/physiopathology , Models, Biological , NADH Dehydrogenase/metabolism , Threonine/geneticsABSTRACT
Mitochondrial DNA (mtDNA) polymerase gamma (Polg) is a heterodimeric enzyme containing a Pol I-like catalytic core (PolgA) and an accessory subunit. Mutations in POLGA, affecting the stability of mtDNA, have been identified in several human pathologies such as progressive external ophthalmoplegia and Alpers' syndrome. Extensive literature shows mitochondrial toxicity effects nucleoside analogue reverse transcriptase inhibitors used in the treatment of HIV and chronic hepatitis B as a consequence of an inhibitory effect on Polg. We have previously shown that mice with an error-prone version of PolgA accumulate higher levels of somatic mtDNA mutations resulting in a premature aging phenotype. In the present paper, we demonstrate PolgA deficiency in mouse embryos causes an early developmental arrest between embryonic days 7.5 and 8.5 associated with severe mtDNA depletion. Heterozygous knockout mice have half the wild-type levels of PolgA transcripts and a slight reduction in mtDNA levels but develop normally. Surprisingly, amounts of PolgA transcripts in heterozygous knockout mice are increased in response to artificially elevated mtDNA copy number, revealing a possible regulatory link between mtDNA maintenance and PolgA expression. Our results show that Polg indeed is the only DNA polymerase capable of maintaining mtDNA in mammalian mitochondria. In addition, presence of Polg is absolutely essential for the organogenesis during mammalian embryonic development.
Subject(s)
DNA, Mitochondrial/genetics , DNA-Directed DNA Polymerase/genetics , Gene Deletion , Mitochondria/genetics , Organogenesis/genetics , Animals , DNA Polymerase gamma , Diffuse Cerebral Sclerosis of Schilder/genetics , Heterozygote , Humans , Mice , Mice, Knockout , Ophthalmoplegia/geneticsABSTRACT
We performed global gene expression analyses in mouse hearts with progressive respiratory chain deficiency and found a metabolic switch at an early disease stage. The tissue-specific mitochondrial transcription factor A (Tfam) knockout mice of this study displayed a progressive heart phenotype with depletion of mtDNA and an accompanying severe decline of respiratory chain enzyme activities along with a decreased mitochondrial ATP production rate. These characteristics were observed after 2 weeks of age and became gradually more severe until the terminal stage occurred at 10-12 weeks of age. Global gene expression analyses with microarrays showed that a metabolic switch occurred early in the progression of cardiac mitochondrial dysfunction. A large number of genes encoding critical enzymes in fatty acid oxidation showed decreased expression whereas several genes encoding glycolytic enzymes showed increased expression. These alterations are consistent with activation of a fetal gene expression program, a well-documented phenomenon in cardiac disease. An increase in mitochondrial mass was not observed until the disease had reached an advanced stage. In contrast to what we have earlier observed in respiratory chain-deficient skeletal muscle, the increased mitochondrial biogenesis in respiratory chain-deficient heart muscle did not increase the overall mitochondrial ATP production rate. The observed switch in metabolism is unlikely to benefit energy homeostasis in the respiratory chain-deficient hearts and therefore likely aggravates the disease. It can thus be concluded that at least some of the secondary gene expression alterations in mitochondrial cardiomyopathy do not compensate but rather directly contribute to heart failure progression.
