Functional analysis of a novel POLγA mutation associated with a severe perinatal mitochondrial encephalomyopathy

The disease-causing mutation p.F907I reveals a novel pathogenic mechanism for POLγ-related diseases

Mutations in the catalytic domain of mitochondrial DNA polymerase γ (POLγ) cause a broad spectrum of clinical conditions. POLγ mutations impair mitochondrial DNA replication, thereby causing deletions and/or depletion of mitochondrial DNA, which in turn impair biogenesis of the oxidative phosphorylation system. We here identify a patient with a homozygous p.F907I mutation in POLγ, manifesting a severe clinical phenotype with developmental arrest and rapid loss of skills from 18 months of age. Magnetic resonance imaging of the brain revealed extensive white matter abnormalities, Southern blot of muscle mtDNA demonstrated depletion of mtDNA and the patient deceased at 23 months of age. Interestingly, the p.F907I mutation does not affect POLγ activity on single-stranded DNA or its proofreading activity. Instead, the mutation affects unwinding of parental double-stranded DNA at the replication fork, impairing the ability of the POLγ to support leading-strand DNA synthesis with the TWINKLE helicase. Our results thus reveal a novel pathogenic mechanism for POLγ-related diseases.

Polymerase Gamma Mitochondrial DNA Depletion Syndrome Initially Presenting as Disproportionate Respiratory Distress in a Moderately Premature Neonate: A Case Report

A 32-week premature infant presented with respiratory failure, later progressing to pulmonary hypertension (PH), liver failure, lactic acidosis, and encephalopathy. Using exome sequencing, this patient was diagnosed with a rare Polymerase Gamma (POLG)-related mitochondrial DNA (mtDNA) depletion syndrome. This case demonstrates that expanding the differential to uncommon diagnoses is important for complex infants, even in premature neonates whose condition may be explained partially by their gestational age (GA). It also shows that patients with complex neonatal diseases with significant family history may benefit from exome sequencing for diagnosis.