Heterozygous Polg mutation causes motor dysfunction due to mtDNA deletions

Abstract

ObjectiveMutations in nuclear-encoded mitochondrial DNA (mtDNA) polymerase (POLG) are known to cause autosomal dominant chronic progressive external ophthalmoplegia (adCPEO) with accumulation of multiple mtDNA deletions in muscles. However, no animal model with a heterozygous Polg mutation representing mtDNA impairment and symptoms of CPEO has been established. To understand the pathogenic mechanism of CPEO, it is important to determine the age dependency and tissue specificity of mtDNA impairment resulting from a heterozygous mutation in the Polg gene in an animal model.MethodsWe assessed behavioral phenotypes, tissue-specific accumulation of mtDNA deletions, and its age dependency in heterozygous PolgD257A knock-in mice carrying a proofreading-deficient mutation in the Polg.ResultsHeterozygous PolgD257A knock-in mice exhibited motor dysfunction in a rotarod test. Polg+/D257A mice had significant accumulation of multiple mtDNA deletions, but did not show significant accumulation of point mutations or mtDNA depletion in the brain. While mtDNA deletions increased in an age-dependent manner regardless of the tissue even in Polg+/+ mice, the age-dependent accumulation of mtDNA deletions was enhanced in muscles and in the brain of Polg+/D257A mice.InterpretationHeterozygous PolgD257A knock-in mice showed tissue-specific, age-dependent accumulation of multiple mtDNA deletions in muscles and the brain which was likely to result in neuromuscular symptoms. Polg+/D257A mice may be used as an animal model of adCPEO associated with impaired mtDNA maintenance.

Depression-like episodes in mice harboring mtDNA deletions in paraventricular thalamus

“…17 The forebrain neuron-specific mutant mice did not exhibit either somatic symptoms as we expected 18 or sensorimotor dysfunction including ataxia and Parkinsonism, working memory deficits or cognitive impairment during the episodes and non-episode periods (Supplementary Figure 4a–c and Supplementary Movies 1–3). Among the psychiatric diagnoses of mitochondrial disease patients, the most common diagnosis is major depression (54%) followed by bipolar disorder (17%) and panic disorder (11%).…”

What Can Mitochondrial DNA Analysis Tell Us About Mood Disorders?

“…Small truncated mtDNAs with large deletions (for example, 30% of the total length) can be maintained in mitochondria if the region involved in replication (mtDNA control region or the D-loop region) is intact. Replication of deleted mtDNAs can be completed in a shorter time and they may increase in abundancy preferentially (27,28). Probably by chance, human mtDNA has two identical 13-bp sequences separated by 4977 bp (Figure 1), and the approximately 5-kb DNA stretch between the two sites is likely to be lost (called "common deletion").…”

“…This suggests that a repair system working on mtDNA is similar to microhomology-mediated end joining, one of the nuclear DNA repair systems (29), and a mtDNA replication-dependent repair pathway (30), although mtDNA was previously thought to lack efficient DNA repair mechanisms. Our sensitive quantitative polymerase chain reaction method for assaying deleted mtDNA in mutant mtDNA polymerase (Polg) knock-in mice showed that multiple deleted mtDNAs were more abundantly accumulated in the neural tissues (28). Although neurons are in a postmitotic state, mtDNAs in the cells are actively metabolized (i.e., replicated and degraded).…”

“…Although neurons are in a postmitotic state, mtDNAs in the cells are actively metabolized (i.e., replicated and degraded). Pulse labeling of mtDNA using 5-bromo-2-deoxyuridie incorporation revealed that the time to reach the half maximum levels of 5-bromo-2-deoxyuridie incorporation was shortened in the brain (approximately 4 hours) compared to the liver (28). The small truncated mtDNAs are able to be replicated and transcribed, and thus produce mutant mitochondrial proteins, which accelerate the vicious cycle of ROS in mitochondria (16).…”

Roles of the mitochondrial replisome in mitochondrial DNA deletion formation

“…In contrast to primary mtDNA diseases, defects of the mitochondrial replisome may lead to different, frequently coexisting, mtDNA aberrances: point mutations are often found along with mtDNA deletions in the same patient/tissue. A mouse line expressing a proofreading-deficient Pol g-a variant, the so-called mutator mouse, may serve as an important model for the understanding of this phenomenon, as the accumulation of multiple large-scale deletions in its mtDNA has been observed (Trifunovic et al, 2004;Fuke et al, 2014), in addition to the expected accumulation of point mutations (Vanderstraeten et al, 1998;Spelbrink et al, 2000). Despite a previously considered possibility that the accumulation of point mutations may predispose mtDNA molecules to deletions, no correlation has been found between the distribution of point mutations and the deletion breakpoints (Wanrooij et al, 2004;Hudson and Chinnery, 2006).…”