We studied six unrelated children with depletion of
mitochondrial DNA (
mtDNA). They presented with
Leigh syndrome, infantile hepatocerebral
mtDNA depletion syndrome, or
Alpers-Huttenlocher syndrome. Several genes have been implicated in
mtDNA depletion. Screening of candidate genes indicated that all six patients were compound heterozygous for missense mutations in the gene for the catalytic subunit of
DNA polymerase gamma (POLG). Three of the identified mutations, c.3328C>T (p.H1110Y), c.3401A>G (p.H1134R), and c.3406G>A (p.E1136K), have not been reported earlier. To investigate the functional consequences of the mutations, we carried out a series of biochemical assays in cultured fibroblasts. These studies revealed that fibroblast cultures from the patients with infantile hepatocerebral
mtDNA depletion syndrome progressively lost their
mtDNA during culturing, whereas fibroblast cultures from patients presenting with
Leigh syndrome or
Alpers-Huttenlocher syndrome had reduced but stable levels of
mtDNA.
DNA polymerase gamma activity was below the normal range in all patient cultures, except for one; however, this culture showed low levels of the heterodimeric
enzyme and poor
DNA polymerase gamma processivity. Parental fibroblast cultures had normal catalytic efficiency of
DNA polymerase gamma, consistent with the observation that all carriers are asymptomatic. Thus, we report the first patient with
Leigh syndrome caused by POLG mutations. The cell culture experiments established the pathogenicity of the identified POLG mutations and helped to define the molecular mechanisms responsible for
mtDNA depletion in the patients' tissues. The assays may facilitate the identification of those patients in whom screening for POLG mutations would be most appropriate.