Functional mtDNA replication defect in a fibroblast line from a patient with mtDNA depletion.

Abstract

Mitochondrial DNA (mtDNA) depletion causes infantile cytochrome oxidase deficiency in a number of cases (Moraes et al 1991 ; Trischler et al 1992 ; Poulton et al 1994). Diagnosis is based on a quantitative reduction in the level of qualitatively normal mtDNA relative to nuclear DNA. Where there is a family history, inheritance appears to be autosomal recessive or dominant, suggesting that depletion of mtDNA is secondary to a nuclear defect rather than a mitochondrial gene defect which would be maternally inherited. mtDNA depletion could be caused by a defect in mitochondrial biogenesis such as mtDNA replication, decreased mtDNA stability or increased DNA damage. Unfortunately, few potential candidate genes for causing mtDNA depletion have been identified. One candidate is mitochondrial transcription factor A (mtTFA) which may be an important regulator of both replication and transcription of mtDNA (Parisi and Clayton 1991). Earlier work (Poulton et al 1994) indicated that reduced mtTFA levels correlated with mtDNA depletion and in some cases reduced mtTFA expression. Studies of mtDNA depletion in vitro are difficult owing to the tissue-specific nature of mtDNA depletion. Fibroblasts from mtDNA depletion patients are not always depleted of mtDNA (Poulton et al 1994). In this study we used an artificial depletion method to determine whether cell lines from mtDNA depletion patients have a functional defect in mtDNA replication. The method allows gene expression to be studied during depletion and during the recovery phase where mtDNA levels increase. mtDNA-depleted fibroblasts from one patient were unable to recover mtDNA levels following dideoxycytidine (ddC) treatment.