Replication pauses of the wild-type and mutant mitochondrial DNA polymerase gamma: a simulation study

Roles of the mitochondrial replisome in mitochondrial DNA deletion formation

Mitochondrial DNA (mtDNA) deletions are a common cause of human mitochondrial diseases. Mutations in the genes encoding components of the mitochondrial replisome, such as DNA polymerase gamma (Pol γ) and the mtDNA helicase Twinkle, have been associated with the accumulation of such deletions and the development of pathological conditions in humans. Recently, we demonstrated that changes in the level of wild-type Twinkle promote mtDNA deletions, which implies that not only mutations in, but also dysregulation of the stoichiometry between the replisome components is potentially pathogenic. The mechanism(s) by which alterations to the replisome function generate mtDNA deletions is(are) currently under debate. It is commonly accepted that stalling of the replication fork at sites likely to form secondary structures precedes the deletion formation. The secondary structural elements can be bypassed by the replication-slippage mechanism. Otherwise, stalling of the replication fork can generate single- and double-strand breaks, which can be repaired through recombination leading to the elimination of segments between the recombination sites. Here, we discuss aberrances of the replisome in the context of the two debated outcomes, and suggest new mechanistic explanations based on replication restart and template switching that could account for all the deletion types reported for patients.

Successful Treatment of Intracranial Glioblastoma Xenografts With a Monoamine Oxidase B-Activated Pro-Drug

The last major advance in the treatment of glioblastoma multiforme (GBM) was the introduction of temozolomide in 1999. Treatment with temozolomide following surgical debulking extends survival rate compared to radiotherapy and debulking alone. However, virtually all glioblastoma patients experience disease progression within 7 to 10 months. Although many salvage treatments, including bevacizumab, rechallenge with temozolomide, and other alkylating agents, have been evaluated, none of these clearly improves survival. Monoamine oxidase B (MAOB) is highly expressed in glioblastoma cell mitochondria, and mitochondrial function is intimately tied to treatment-resistant glioblastoma progression. These glioblastoma properties provide a strong rationale for pursuing a MAOB-selective pro-drug treatment approach that, upon drug activation, targets glioblastoma mitochondria, especially mitochondrial DNA. MP-MUS is the lead compound in a family of pro-drugs designed to treat GBM that is converted into the mature, mitochondria-targeting drug, P⁺-MUS, by MAOB. We show that MP-MUS can successfully kill primary gliomas in vitro and in vivo mouse xenograft models.

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Monoamine oxidase B (MAOB) is upregulated in glioblastoma.

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MAOB is used to convert a nitrogen mustard pro-drug into a mtDNA targeting drug.

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In vitro data show MAOB dependent, mitochondrial-based, toxicity.

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In vivo primary glioblastoma mouse models show efficacy.

Glioblastoma multiforme (GBM) is an inevitably fatal disease with inadequate treatment. MP-MUS, the lead compound in a family of pro-drugs specifically designed to treat GBM, is converted to the mature drug P⁺-MUS by the enzyme MAOB. This lipophilic cation accumulates in mitochondria, attacking mitochondrial DNA and promoting oxidative stress. GBM has markedly increased levels of MAOB compared to that in other cells; thus, mitochondrial and cell toxicity is restricted to GBM cells. MP-MUS significantly reduced tumor size in flank and intracranial xenograft models of GBM and induced permanent changes in the phenotype of surviving cells without causing systemic or organ-specific toxicities.

Targeted enrichment and high-resolution digital profiling of mitochondrial DNA deletions in human brain

Due largely to the inability to accurately quantify and characterize de novo deletion events, the mechanisms underpinning the pathogenic expansion of mtDNA deletions in aging and neuromuscular disorders remain poorly understood. Here, we outline and validate a new tool termed 'Digital Deletion Detection' (3D) that allows for high-resolution analysis of rare deletions occurring at frequencies as low as 1 × 10(-8) . 3D is a three-step process that includes targeted enrichment for deletion-bearing molecules, single-molecule partitioning of genomes into thousands of droplets for direct quantification via droplet digital PCR, and breakpoint characterization using massively parallel sequencing. Using 3D, we interrogated over 8 billion mitochondrial genomes to analyze the age-related dynamics of mtDNA deletions in human brain tissue. We demonstrate that the total deletion load increases with age, while the total number and diversity of unique deletions remain constant. Our data provide support for the hypothesis that expansion of pre-existing mutations is the primary factor contributing to age-related accumulation of mtDNA deletions.