Deficiency of the human mitochondrial transcription factor h-mtTFA in infantile mitochondrial myopathy is associated with mtDNA depletion

Abstract

Recent studies show that patients presenting with cytochrome oxidase (COX) deficiency in infancy may have reduced mitochondrial DNA (mtDNA) in muscle. The human mitochondrial transcription factor A (h-mtTFA) may be an important regulator of both transcription and replication of mtDNA. h-mtTFA levels were investigated in cell lines which were either free of mtDNA (rho 0) or temporarily depleted by treatment with dideoxycytidine (ddC), and in tissue from three patients with mtDNA depletion and cytochrome oxidase deficiency. h-mtTFA was compared with other mitochondrial proteins such as pyruvate dehydrogenase and porin by Western blotting. The ratio of mtDNA and h-mtTFA mRNA to reference nuclear probes was measured by dual labelling of dot blots. The ratio of mtDNA to nuclear DNA in skeletal muscle was low in muscle in the three patients and in other tissues in one. h-mtTFA was low in cells depleted either permanently or transiently of mtDNA, and this reduction in h-mtTFA roughly paralleled mtDNA levels. Similarly, treatment of rho 0 cell lines with ddC induced a reduction in mtDNA as well as h-mtTFA protein. The relationship between h-mtTFA and mtDNA levels suggests that they may be causally linked. MtDNA depletion was accompanied by an increase in the level of h-mtTFA RNA in the cell lines but low levels in the patient. This suggests that either h-mtTFA regulates mtDNA levels, or that h-mtTFA expression may be regulated by a feedback mechanism initiated by MtDNA Depletion.

Composition and Dynamics of Human Mitochondrial Nucleoids

“…TFAM levels, in contrast with mtSSB or POLG and POLG2 levels, were severely reduced. This agrees with previous studies showing a strong correlation between TFAM protein levels and mtDNA levels, using rho-zero cell lines, cell lines partially depleted by dideoxycytidine treatment, material from patients suffering from mtDNA depletion and heterozygous TFAM knockout mice (Larsson et al, 1994;Poulton et al, 1994;Larsson et al, 1998). This strongly suggested that TFAM protein stability and turnover depends on its interaction with mtDNA and is supported by our observation of an apparent dependence of the occurrence of TFAM degradation products on a lack of DNA binding.…”

MicroRNA-149 Inhibits PARP-2 and Promotes Mitochondrial Biogenesis via SIRT-1/PGC-1α Network in Skeletal Muscle

“…For example, overexpression of let-7 inhibits Lin28, induces insulin resistance, and impaired glucose tolerance in skeletal muscle (31). As occurs in myogenic C2C12 cells, endurance exercise significantly downregulates miR-494 and upregulates its target genes mtTFA and forkhead box j3 (30) that have been shown to play important roles in mitochondrial biogenesis (34,35). Transgenic mice overexpressing miR-23a displayed by immunoblot 48 h after transfection.…”

Inherited Mitochondrial DNA Depletion

“…While decreased mtDNA content was invariably detected in affected tissues, nonaffected tissues exhibited a wide range of mtDNA abundance levels. For example, in a patient who presented with neonatal encephalopathy and expired in infancy muscle mtDNA content was 23%, liver 7%, kidney 24%, and brain 37% of the control (14). It is a matter of conjecture whether organs such as kidney or muscle would become involved had this patient lived longer, given the known variability of clinical threshold among tissues (15).…”

“…A search for mutations in the Tfam gene was initiated by the finding of decreased abundance of the Tfam protein in the cells of several patients with mtDNA depletion but none were found. Thus, in the study of candidate genes, proteins that require mtDNA molecules for their own stability and are secondarily decreased, should be distinguished from those that are the primary cause of the disease (14,52).…”