Abstract
Mitochondrial transcription factor A (TFAM) is compacting mitochondrial DNA (dmtDNA) into nucleoids and directly controls mtDNA copy number. Here, we show that the TFAM-to-mtDNA ratio is critical for maintaining normal mtDNA expression in different mouse tissues. Moderately increased TFAM protein levels increase mtDNA copy number but a normal TFAM-to-mtDNA ratio is maintained resulting in unaltered mtDNA expression and normal whole animal metabolism. Mice ubiquitously expressing very high TFAM levels develop pathology leading to deficient oxidative phosphorylation (OXPHOS) and early postnatal lethality. The TFAM-to-mtDNA ratio varies widely between tissues in these mice and is very high in skeletal muscle leading to strong repression of mtDNA expression and OXPHOS deficiency. In the heart, increased mtDNA copy number results in a near normal TFAM-to-mtDNA ratio and maintained OXPHOS capacity. In liver, induction of LONP1 protease and mitochondrial RNA polymerase expression counteracts the silencing effect of high TFAM levels. TFAM thus acts as a general repressor of mtDNA expression and this effect can be counterbalanced by tissue-specific expression of regulatory factors.
Highlights
Treatment of mitochondrial diseases caused by dysfunctional oxidative phosphorylation (OXPHOS) remains a challenge
To study the consequences of moderately increased TFAM levels in vivo, we generated bacterial artificial chromosome (BAC) transgenic mice harbouring an introduced 203 kb mouse genomic fragment containing Tfam expressed from its endogenous promoter under the control of adjacent regulatory elements (Fig 1A)
BAC constructs are typically randomly inserted into the mouse genome and analysis of the three different founder lines, BAC TG 137 (Figs 1 and S1), BAC TG 188 (Fig S1), and BAC TG 91 (Fig S1) allowed us to rule out effects caused by insertional mutagenesis
Summary
Treatment of mitochondrial diseases caused by dysfunctional oxidative phosphorylation (OXPHOS) remains a challenge. Mutations in two different genomes, that is, nuclear DNA and mitochondrial DNA (mtDNA), can cause OXPHOS defects and mitochondrial disease (Rahman, 2020; Russell et al, 2020). The majority of mitochondrial proteins (~99%), including all that regulate mtDNA maintenance and expression, are encoded in the nucleus and imported into mitochondria. To add to the complexity, mutations in about 300 nuclear genes have been shown to cause mitochondrial disorders by affecting different aspects of mitochondrial function, such as biogenesis of the OXPHOS system, nucleotide transport/synthesis, membrane dynamics, mtDNA maintenance, and mtDNA expression at different levels, including transcription, RNA maturation, and translation (Vafai & Mootha, 2012; Thompson et al, 2020)
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