Mice lacking the mitochondrial exonuclease MGME1 accumulate mtDNA deletions without developing progeria

https://www.nature.com/articles/s41467-018-03552-x


Stanka Matic, Min Jiang, Thomas J. Nicholls, Jay P. Uhler, Caren Dirksen-Schwanenland, Paola Loguercio Polosa, Marie-Lune Simard, Xinping Li, Ilian Atanassov, Oliver Rackham, Aleksandra Filipovska, James B. Stewart, Maria Falkenberg, Nils-Göran Larsson & Dusanka Milenkovic

• The authors developed a knockout mouse model for the gene MGME1.  Loss of MGME1 expression in siRNA treated cells, or patient fibroblasts, leads to an accumulation of 7S DNA.
• 7S DNA is a single-stranded, ~650 nt long, nascent DNA species that creates a characteristic triple-stranded DNA structure in mtDNA called the D-loop. 7S DNA is formed by premature replication termination of mtDNA. 
• Human patients with loss-of-function MGME1 mutations show depletions/ rearrangements of mtDNA, and a number of devastating phenotypes.
• MGME1 is not essential for embryonic development, but its loss leads to accumulation of multiple deletions and depletion of mtDNA.
• In the heart, these mice possessed ~50% less wild-type mtDNA (Southern blot analysis), and ~x5 more 7SDNA than mtDNA (which is around ~x5 higher ratio than seen in wild-type mice).
• The authors found severe wt-mtDNA depletion in: kidney, liver, brain and heart. Skeletal muscle was least severely affected (if at all). (NB: this is from visual inspection of the Southern blots in Fig 2D).
• Mice with both germline and tissue-specific knockout of Mgme1 are viable and appear healthy, despite the existence of deletions and rearrangements of mtDNA.
• The authors found that the deleted species was a linear mtDNA molecule of ~11kb (~67% mass of normal mtDNA) due to the stalling of mtDNA replication
• Mgme KO mice had similar levels of point mutations as wild-type mice
• There were no clear OXPHOS defects in Mgme KO mice, despite depletion of mtDNA, accumulation of linear subgenomic mtDNA, and replication stalling in young animals. The mice did not show premature ageing.
• In liver tissue, a prominent stalling site is observed, whereas in heart, a range of replication intermediates is observed. This shows the existence of tissue-specific stalling profiles in this system.
• The authors suggest that MGME1 might be part of a regulatory switch acting at the end of the D-loop region that controls mtDNA replication and heavy-strand transcription termination. 

Thoughts
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• Do the authors still see that the mice are susceptible to arrhythmias, as observed here (blog here)? In other words, are there phenotypes in these mice which only become evident when the animals are challenged e.g. through stress?
• How do we resolve the observation that loss of function of MGME1 in humans causes devastating phenotypes, whereas in mice it does not? 
• If these deletions are all linear, can they still transcribe? If not, to what extent are these deletions representative of deletions which occur naturally, for instance the common deletion, which (as I understand it) are circular, see here?