Modulation of the W748S mutation in DNA polymerase γ by the E1143G polymorphismin mitochondrial disorders

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Abstract
DNA polymerase gamma (pol γ) is required for replication and repair of mitochondrial DNA. Over 80 mutations in POLG, the gene encoding the catalytic subunit of pol γ, have been linked with disease. The W748S mutation in POLG is the most common mutation in ataxia-neuropathy spectrum disorders and is generally found in cis with the common E1143G polymorphism. It has been unclear whether E1143G participates in the disease process. We investigated the biochemical consequences of pol γ proteins containing W748S or E1143G, or both. W748S pol γ exhibited low DNA polymerase activity, low processivity and a severe DNA-binding defect. However, interactions between the catalytic and accessory subunits were normal. Despite the benefits derived from binding with the accessory subunit, catalytic activities did not reach wild-type (WT) levels. Also, nucleotide selectivity decreased 2.1-fold compared with WT. Surprisingly, pol γ containing only E1143G was 1.4-fold more active than WT, and this increased polymerase activity could be due to higher thermal stability for E1143G pol γ. The E1143G substitution partially rescued the deleterious effects of the W748S mutation, as DNA binding, catalytic activity and fidelity values were intermediate for W748S-E1143G. However, W748S-E1143G had a notably lower change in enthalpy for protein folding than W748S alone. We suggest that when E1143G is in cis with other pathogenic mutations, it can modulate the effects of these mutations. For W748S-E1143G pol γ, the benefits bestowed by E1143G include increased DNA binding and polymerase activity; however, E1143G was somewhat detrimental to protein stability.