Abstract
Mutations in POLG disrupt mtDNA replication and cause devastating diseases often with neurological phenotypes. Defining disease mechanisms has been hampered by limited access to human tissues, particularly neurons. Using patient cells carrying POLG mutations, we generated iPSCs and then neural stem cells. These neural precursors manifested a phenotype that faithfully replicated the molecular and biochemical changes found in patient post‐mortem brain tissue. We confirmed the same loss of mtDNA and complex I in dopaminergic neurons generated from the same stem cells. POLG‐driven mitochondrial dysfunction led to neuronal ROS overproduction and increased cellular senescence. Loss of complex I was associated with disturbed NAD + metabolism with increased UCP2 expression and reduced phosphorylated SirT1. In cells with compound heterozygous POLG mutations, we also found activated mitophagy via the BNIP3 pathway. Our studies are the first that show it is possible to recapitulate the neuronal molecular and biochemical defects associated with POLG mutation in a human stem cell model. Further, our data provide insight into how mitochondrial dysfunction and mtDNA alterations influence cellular fate determining processes.
Highlights
Mitochondria are membrane enclosed, intracellular organelles involved in multiple cellular functions, but best known for generating adenosine triphosphate (ATP)
We found that patient-specific neural stem cells (NSCs) replicated the findings of mtDNA depletion and complex I deficiency identified in post-mortem tissues
While others have generated induced pluripotent stem cells (iPSCs) from patients with POLG mutations (Zurita et al, 2016; Chumarina et al, 2019) or investigated valproate toxicity in iPSC-derived POLG patient hepatocytes (Li et al, 2015), we believe our study is the first to confirm that the pathological changes seen in post-mortem studies are faithfully replicated in iPSC-derived NSCs
Summary
Mitochondria are membrane enclosed, intracellular organelles involved in multiple cellular functions, but best known for generating adenosine triphosphate (ATP). Mitochondria are the only organelles besides the nucleus that possess their own DNA (mitochondrial DNA; mtDNA) and their own machinery for synthesizing RNA and proteins. Mutations in POLG cause a wide variety of diseases that vary in age of onset and severity. More than 200 disease-causing mutations are known, and these cause diverse phenotypes including devastating early onset encephalopathy syndromes such as Alpers’ syndrome (Naviaux & Nguyen, 2004; Ferrari et al, 2005) or severe adult onset disorders with progressive spinocerebellar ataxia and epilepsy (Van Goethem et al, 2004; Hakonen et al, 2005; Winterthun et al, 2005).
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