Mitophagy-driven selection against heteroplasmic mitochondrial DNA mutations

Beth Israel Deaconess Medical Center, Boston MA

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Abstract

DESCRIPTION (provided by applicant): Disorders caused by maternally inherited pathogenic mitochondrial DNA (mtDNA) mutations can lead to a wide array of neurological, cardiac, and other disorders. Unfortunately, clearly effective clinical treatments for these often devastating disorders are lacking. An ideal strategy would eliminate the mutant mtDNA and replace it with wild type (WT) DNA. However, classic gene therapy approaches are difficult to apply to mtDNA mutations. On the other hand, mitochondria undergo frequent turnover (every few days), even in postmitotic cells, with only a subset of copies of the mitochondrial genome being replicated during this process, providing an opportunity to influence which mtDNA molecules are replicated. We now propose to test a novel strategy to promote the selective elimination of deleterious mtDNA mutations that can be applied to heteroplasmic mtDNA mutations. Heteroplasmy is a common feature of pathogenic mtDNA mutations, and refers to a mix of WT and mutant mtDNA within the same cells or tissue. Our hypothesis takes advantage of a natural cellular process known as mitophagy (mitochondrial degradation by autophagy), which is a mechanism for selectively eliminating dysfunctional mitochondria. We hypothesize that some mitochondria within a cell will harbor greater levels of a heteroplasmic mtDNA mutation than others. Those with greater levels of a deleterious mutation will tend to have relatively greater impairment of mitochondrial function. Therefore, we propose to test the novel hypothesize that stimulating mitophagy by inhibiting mTOR kinase activity in cells harboring a heteroplasmic pathogenic mtDNA mutation will drive selection against the mutant mtDNA, over time leading to a substantial reduction in the mutational burden and hence an improvement in mitochondrial function. We have a unique resource available for testing this hypothesis: multiple SH-SY5Y cybrid cell lines harboring different levels of a heteroplasmic G11778A complex I (CI) gene mutation associated with Leber's Heredity Optic Neuropathy (LHON), all prepared at the same time from members of a single family. Our preliminary data with these cell lines support our hypothesis. A second important resource in our laboratory is the mutator mouse that expresses a proofreading deficient mtDNA polymerase 3 (Polg) leading to accumulation with age of heteroplasmic somatic mtDNA mutations in association with a premature aging phenotype. Our preliminary data demonstrate substantial metabolic, behavioral, and neurochemical deficits in these mice. We now hypothesize that enhancing mitophagy in the Polg mutator mice will attenuate the accumulation of somatic mtDNA mutations and ameliorate the deficits in these mice. Ultimately, clinical applications of this strategy have the potential to be of benefit to patients with classic mitochondrial disorders associated with heteroplasmic mtDNA mutations, to families harboring Polg mutations associated with familial parkinsonism and other disorders, and potentially for age-related neurodegenerative disorders.