Characterization of the human homozygous R182W POLG2 mutation in mitochondrial DNA depletion syndrome

Mutations in mitochondrial DNA (mtDNA) have been linked to a variety of metabolic, neurological and muscular diseases which can present at any time throughout life. MtDNA is replicated by DNA polymerase gamma (Pol γ), twinkle helicase and mitochondrial single-stranded binding protein (mtSSB). The Pol γ holoenzyme is a heterotrimer consisting of the p140 catalytic subunit and a p55 homodimeric accessory subunit encoded by the nuclear genes POLG and POLG2, respectively. The accessory subunits enhance DNA binding and promote processive DNA synthesis of the holoenzyme. Mutations in either POLG or POLG2 are linked to disease and adversely affect maintenance of the mitochondrial genome, resulting in depletion, deletions and/or point mutations in mtDNA. A homozygous mutation located at Chr17: 62492543G>A in POLG2, resulting in R182W substitution in p55, was previously identified to cause mtDNA depletion and fatal hepatic liver failure. Here we characterize this homozygous R182W p55 mutation using in vivo cultured cell models and in vitro biochemical assessments. Compared to control fibroblasts, homozygous R182W p55 primary dermal fibroblasts exhibit a two-fold slower doubling time, reduced mtDNA copy number and reduced levels of POLG and POLG2 transcripts correlating with the reported disease state. Expression of R182W p55 in HEK293 cells impairs oxidative-phosphorylation. Biochemically, R182W p55 displays DNA binding and association with p140 similar to WT p55. R182W p55 mimics the ability of WT p55 to stimulate primer extension, support steady-state nucleotide incorporation, and suppress the exonuclease function of Pol γ in vitro. However, R182W p55 has severe defects in protein stability as determined by differential scanning fluorimetry and in stimulating function as determined by thermal inactivation. These data demonstrate that the Chr17: 62492543G>A mutation in POLG2, R182W p55, severely impairs stability of the accessory subunit and is the likely cause of the disease phenotype.