DNA‐editing enzymes as potential treatments for heteroplasmic mtDNA diseases

Abstract

Mutations in the mitochondrial genome are the cause of many debilitating neuromuscular disorders. Currently, there is no cure or treatment for these diseases, and symptom management is the only relief doctors can provide. Although supplements and vitamins are commonly used in treatment, they provide little benefit to the patient and are only palliative. This is why gene therapy is a promising research topic to potentially treat and, in theory, even cure diseases caused by mutations in the mitochondrial DNA (mtDNA). Mammalian cells contain approximately a thousand copies of mtDNA, which can lead to a phenomenon called heteroplasmy, where both wild‐type and mutant mtDNA molecules co‐exist within the cell. Disease only manifests once the per cent of mutant mtDNA reaches a high threshold (usually >80%), which causes mitochondrial dysfunction and reduced ATP production. This is a useful feature to take advantage of for gene therapy applications, as not every mutant copy of mtDNA needs to be eliminated, but only enough to shift the heteroplasmic ratio below the disease threshold. Several DNA‐editing enzymes have been used to shift heteroplasmy in cell culture and mice. This review provides an overview of these enzymes and discusses roadblocks of applying these to gene therapy in humans.

Therapeutic Options in Hereditary Optic Neuropathies

“…After an initial pioneering proof of principle based on the use of restriction enzymes engineered for mitochondrial import [ 205 , 206 ], several DNA-editing enzymes have now been tested, targeted to specific mtDNA mutations, to lower mutant heteroplasmy levels in both cell cultures and mouse models [ 207 , 208 ].…”

Mitochondrial Heteroplasmy Shifting as a Potential Biomarker of Cancer Progression

“…Heteroplasmy is a common phenomenon, and low levels of heteroplasmy are commonly found in healthy tissues; the presence of potentially pathogenic mutations is not always translated into changes in the cell function or in a specific phenotype [ 72 , 73 , 74 ]. The maintenance of normal mitochondrial function is explained by the existence of a “biochemical threshold”, a state where normal mtDNA buffers the damaging effect of mutated mtDNA molecules through the modulation of mitochondrial DNA replication, mitochondrial fission and fusion processes [ 73 , 75 ]. The threshold is tissue-specific, and the level of the mutated allele (heteroplasmy level) required to produce a pathological phenotype is dependent on the nature of the mutation [ 57 , 74 , 76 ].…”

Potential of Mitochondrial Genome Editing for Human Fertility Health

“…Nanoparticles for chemical nonviral nucleic acid delivery usually come in the form of lipoplexes (DNA/catatonic lipids), polyplexes (DNA/ catatonic polymers), and lipopolyplexes (DNA/catatonic polymers/catatonic lipids) for transferring large plasmid DNA molecules and small DNA molecules in the form of oligodeoxynucleotides and RNAs (Zekonyte et al, 2020). Inorganic nanoparticles can be made of silica or gold, and organic particles can be made of lipid emulsions, lipid nanoparticles, and a wide variety of organic polymers (Zekonyte et al, 2020). Delivery of these molecules includes physical methods, such as direct injection, electroporation, sonoporation, and magnetofection, to induce gene delivery (Ramamoorth and Narvekar, 2015;Hardee et al, 2017).…”