The regulation of mitochondrial DNA (mtDNA) expression is crucial for mitochondrial biogenesis during development and differentiation. We have disrupted the mouse gene for mitochondrial transcription factor A (Tfam; formerly known as m-mtTFA) by gene targetting of loxP-sites followed by cre-mediated excision in vivo. Heterozygous knockout mice exhibit reduced mtDNA copy number and respiratory chain deficiency in heart. Homozygous knockout embryos exhibit a severe mtDNA depletion with abolished oxidative phosphorylation. Mutant embryos proceed through implantation and gastrulation, but die prior to embryonic day (E)10.5. Thus, Tfam is the first mammalian protein demonstrated to regulate mtDNA copy number in vivo and is essential for mitochondrial biogenesis and embryonic development.

Paragangliomas are neuroendocrine tumors frequently associated with mutations in RET, NF1, VHL, and succinate dehydrogenase (SDHx) genes. Methylome analysis of a large paraganglioma cohort identified three stable clusters, associated with distinct clinical features and mutational status. SDHx-related tumors displayed a hypermethylator phenotype, associated with downregulation of key genes involved in neuroendocrine differentiation. Succinate accumulation in SDH-deficient mouse chromaffin cells led to DNA hypermethylation by inhibition of 2-OG-dependent histone and DNA demethylases and established a migratory phenotype reversed by decitabine treatment. Epigenetic silencing was particularly severe in SDHB-mutated tumors, potentially explaining their malignancy. Finally, inactivating FH mutations were identified in the only hypermethylated tumor without SDHx mutations. These findings emphasize the interplay between the Krebs cycle, epigenomic changes, and cancer.

Friedreich ataxia (FRDA), the most common autosomal recessive ataxia, is characterized by degeneration of the large sensory neurons and spinocerebellar tracts, cardiomyopathy and increased incidence in diabetes. FRDA is caused by severely reduced levels of frataxin, a mitochondrial protein of unknown function. Yeast knockout models as well as histological and biochemical data from heart biopsies or autopsies of FRDA patients have shown that frataxin defects cause a specific iron-sulfur protein deficiency and intramitochondrial iron accumulation. We have recently shown that complete absence of frataxin in the mouse leads to early embryonic lethality, demonstrating an important role for frataxin during mouse development. Through a conditional gene-targeting approach, we have generated in parallel a striated muscle frataxin-deficient line and a neuron/cardiac muscle frataxin-deficient line, which together reproduce important progressive pathophysiological and biochemical features of the human disease: cardiac hypertrophy without skeletal muscle involvement, large sensory neuron dysfunction without alteration of the small sensory and motor neurons, and deficient activities of complexes I-III of the respiratory chain and of the aconitases. Our models demonstrate time-dependent intramitochondrial iron accumulation in a frataxin-deficient mammal, which occurs after onset of the pathology and after inactivation of the Fe-S-dependent enzymes. These mutant mice represent the first mammalian models to evaluate treatment strategies for the human disease.