The mitochondrial DNA polymerase gamma degrades linear DNA fragments precluding the formation of deletions.

The mitochondrial DNA polymerase gamma degrades linear DNA fragments precluding the formation of deletions.

I posted recently how two labs have published data showing that the mitochondrial DNA  (mtDNA) polymerase gamma (POLGA) would degrade linear mtDNA fragments (Peeva et al. 2018) or mtDNA under starvation (Medeiros et al. 2018). Now a third group published similar results and they come from the lab of Carlos Moraes so you know are going to enjoy it (Nissanka et al. 2018).

In this study they used both mouse embryonic fibroblasts (MEFs) and actual mice. First, they expressed mitochondrially targeted restriction enzymes to produce linear mtDNA fragments and followed how quickly these are degraded in WT MEFs and mtDNA mutator MEFs. Clearly, MEFs expressing the exonuclease-deficient POLGA were not efficient in removing the linear mtDNA fragment which is a similar result to the Peeva et al. study.

Previously, Medeiros et al. and Peeva et al studied the degradation of mtDNA in yeast and cell culture, respectively, but it was unclear whether this would also take place in vivo. Moraes lab is rather experienced in introducing enzymes (restriction enzymes and mitoTALENs) into mitochondria in mice and as could be expected they introduced these mitochondrially targeted restriction enzymes into mice using adenovirus to study mtDNA degradation. As a result, it seems that POLGA is participating to the degradation of linear mtDNA fragments also in vivo.

Nissanka et al. also assessed whether the presence of linear mtDNA leads to mtDNA rearrangements such as circular mtDNA molecules with deletions. This seems to indeed be the case and it would be interesting to know whether these rearrangements would also take place in patients carrying pathogenic mutations in the replication machinery proteins. These results also suggest that mitochondrial zinc fingers (mtZFN) and mitoTALENs (Gammage et al. 2017) might have some unintended consequences. Both of these approaches are based on cutting the mtDNA molecules carrying a pathogenic mutation leading to the degradation of the molecule. In the ideal case, the loss of these pathogenic molecules would be replaced by the replication of the WT mtDNA molecules. Based on the results of Nissanka et al. in the non-ideal case the presence of these linear molecules could increase the amount of mtDNA rearrangements.

References:

Medeiros TC, Thomas RL, Ghillebert R, Graef M. Autophagy balances mtDNA synthesis and degradation by DNA polymerase POLG during starvation. J Cell Biol. 2018. PMID: 29519802

Nissanka N, Bacman SR, Plastini MJ, Moraes CT. The mitochondrial DNA polymerase gamma degrades linear DNA fragments precluding the formation of deletions. Nat Commun. 2018. PMID: 29950568

Gammage PA, Moraes CT, Minczuk M. Mitochondrial Genome Engineering: The Revolution May Not Be CRISPR-Ized. Trends Genet. 2017. PMID: 29179920

Peeva V, Blei D, Trombly G, Corsi S, Szukszto MJ, Rebelo-Guiomar P, Gammage PA, Kudin AP, Becker C, Altmüller J, Minczuk M, Zsurka G, Kunz WS. Linear mitochondrial DNA is rapidly degraded by components of the replication machinery. Nat Commun. 2018. PMID: 29712893