Functional Analysis Of A Novel Pol Gamma A Mutation Associated With A Severe Perinatal Mitochondrial Encephalomyopathy

Mutations in the mitochondrial DNA polymerase gamma catalytic subunit (POL. A) compromise the stability of mitochondrial DNA (mtDNA) by leading to mutations, deletions and depletions in mtDNA. Patients with mutations in POL gamma A often differ remarkably in disease severity and age of onset. In this work we have studied the functional consequence of POL gamma A mutations in a patient with an uncommon and a very severe disease phenotype characterized by prenatal onset with intrauterine growth restriction, lactic acidosis from birth, encephalopathy, hepatopathy, myopathy, and early death. Muscle biopsy identified scattered COX-deficient muscle fibers, respiratory chain dysfunction and mtDNA depletion. We identified a novel POL.A mutation (p.His1134Tyr) in trans with the previously identified p.Thr251Ile/Pro587Leu double mutant. Biochemical characterization of the purified recombinant POL gamma A variants showed that the p.His1134Tyr mutation caused severe polymerase dysfunction. The p.Thr251Ile/Pro587Leu mutation caused reduced polymerase function in conditions of low dNTP concentration that mimic postmitotic tissues. Critically, when p.His1134Tyr and p.Thr251Ile/Pro587Leu were combined under these conditions, mtDNA replication was severely diminished and featured prominent stalling. Our data provide a molecular explanation for the patients mtDNA depletion and clinical features, particularly in tissues such as brain and muscle that have low dNTP concentration. (c) 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)