Impact of a novel homozygous mutation in nicotinamide nucleotide transhydrogenase on mitochondrial DNA integrity in a case of familial glucocorticoid deficiency

• Mutations in NNT were reported for a subset of FGD patients.
• A gene–dose effect is reported for a novel NNT pathogenic mutation (c.644T > C).
• Homozygous carrier (FGD patient) has negligible NNT activity.
• Patient exhibited OXPHOS deficits with oxidative damage to mtDNA and proteins.
• Heterozygous carriers (parents) were asymptomatic with ~ 60% NNT activity.
• All carriers showed deficits in mitochondrial biogenesis and mtDNA replication.

BackgroundFamilial glucocorticoid deficiency (FGD) is a rare autosomal recessive disorder that is characterized by isolated glucocorticoid deficiency. Recently, mutations in the gene encoding for the mitochondrial nicotinamide nucleotide transhydrogenase (NNT) have been identified as a causative gene for FGD; however, no NNT activities have been reported in FGD patients carrying NNT mutations.MethodsClinical, biochemical and molecular analyses of lymphocytes from FDG homozygous and heterozygous carriers for the F215S NNT mutation were performed.ResultsIn this study, we described an FGD-affected Japanese patient carrying a novel NNT homozygous mutation (c.644T>C; F215S) with a significant loss-of-function (NNT activity = 31% of healthy controls) in peripheral blood cells' mitochondria. The NNT activities of the parents, heterozygous for the mutation, were 61% of the controls.ConclusionsOur results indicated that (i) mitochondrial biogenesis (citrate synthase activity) and/or mtDNA replication (mtDNA copy number) were affected at ≤ 60% NNT activity because these parameters were affected in individuals carrying either one or both mutated alleles; and (ii) other outcomes (mtDNA deletions, protein tyrosine nitration, OXPHOS capacity) were affected at ≤ 30% NNT activity as also observed in murine cerebellar mitochondria from C57BL/6J (NNT−/−) vs. C57BL/6JN (NNT+/+) substrains.General significanceBy studying a family affected with a novel point mutation in the NNT gene, a gene–dose response was found for various mitochondrial outcomes providing for novel insights into the role of NNT in the maintenance of mtDNA integrity beyond that described for preventing oxidative stress.