Stable transformation of CHO Cells and human NARP cybrids confers oligomycin resistance (oli(r)) following transfer of a mitochondrial DNA-encoded oli(r) ATPase6 gene to the nuclear genome: a model system for mtDNA gene therapy.

Point and deletion mutations and a general depletion of mammalian mitochondrial DNA (mtDNA) give rise to a wide variety of medical syndromes that are refractory to treatment, possibly including aging itself. While gene therapy directed at correcting such deficits in the mitochondrial genome may offer some therapeutic benefits, there are inherent problems associated with a direct approach. These problems are primarily due to the high mitochondrial genome copy number in each cell and the mitochondrial genome being "protected" inside the double-membrane mitochondrial organelle. In an alternative approach there is evidence that genes normally present in the mitochondrial genome can be incorporated into the nuclear genome. To extend such studies, we modified the Chinese Hamster Ovary (CHO) mtDNA-located ATPase6 gene (possessing a mutation which confers oligomycin resistance- oli(r)) by altering the mtDNA code to the universal code (U-code) to permit the correct translation of its mRNA in the cytoplasm. The U-code construct was inserted into the nuclear genome (nucDNA) of a wild type CHO cell. The expressed transgene products enabled the transformed CHO cell lines to grow in up to 1000 ng mL(-1) oligomycin, while untransformed sensitive CHO cells were eliminated in 1 ng mL(-1) oligomycin. This approach, termed allotopic expression, provides a model that may make possible the transfer of all 13 mtDNA mammalian protein-encoding genes to the nucDNA, for treatments of mtDNA disorders. The CHO mtATPase6 protein is 85% identical to both the mouse and human mtATPase6 protein; these proteins are highly conserved in the region of the oligomycin resistance mutation. They are also well conserved in the regions of the oligomycin resistance mutation of the mouse, and in the region of a mutation found in Leigh's syndrome (T8993G), also called NARP (neurogenic weakness, ataxia, retinitis pigmentosum). It is likely that the CHO oli(r) mtATPase6 Ucode construct could impart oligomycin-resistance in human and mouse cells, as well as function in place of the mutant ATPase subunit in a NARP cell line. Preliminary experiments on human cybrids homoplasmic for the NARP mutation (kindly supplied by D.C. Wallace), transformed with our construct, display an increased oligomycin resistance that supports these suppositions.

High incidence of propionic acidemia in greenland is due to a prevalent mutation, 1540insCCC, in the gene for the beta-subunit of propionyl CoA carboxylase.

Propionyl CoA carboxylase (PCC) is a mitochondrial, biotin-dependent enzyme involved in the catabolism of amino acids, odd-chain fatty acids, and other metabolites. PCC consists of two subunits, alpha and beta, encoded by the PCCA and PCCB genes, respectively. Inherited PCC deficiency due to mutations in either gene results in propionic acidemia (PA), an autosomal recessive disease. Surprisingly, PA is highly prevalent among Inuits in Greenland. We have analyzed reverse transcriptase-PCR products of the beta-subunit mRNA, to characterize the responsible mutation(s). A 3-bp insertion, 1540insCCC, was found in homozygous form in three patients and in compound heterozygous form in one patient. The resulting PCC has no measurable activity, and the mutant beta-subunit appears to be very unstable. To test the hypothesis that a common mutation is responsible for PA in the Greenlandic Inuit population, 310 anonymous DNA samples of Inuit origin were screened for 1540insCCC. We found a carrier frequency of 5%, which is very high compared with those of most other autosomal recessive diseases. Analysis of alleles of a very closely linked marker, D3S2453, revealed a high degree of linkage disequilibrium between one specific allele and 1540insCCC, suggesting that this mutation may be a founder mutation.

Changes in the carboxyl terminus of the beta subunit of human propionyl-CoA carboxylase affect the oligomer assembly and catalysis: expression and characterization of seven patient-derived mutant forms of PCC in Escherichia coli.

Propionyl-CoA carboxylase (PCC) catalyzes the biotin-dependent carboxylation of propionyl-CoA to d-methylmalonyl-CoA in the mitochondrial matrix. Human PCC is a dodecamer composed of pairs of nonidentical alpha and beta subunits encoded by PCCA and PCCB genes, respectively. Deficiency of PCC results in propionic acidemia (PA), a metabolic disorder characterized by severe metabolic ketoacidosis, vomiting, lethargy, and hypotonia. To date, almost 60 mutations have been reported in both genes. Exon 15 of the beta subunit is one of the two sites where a number of mutations have been identified in PA patients. In the primary betaPCC sequence, these mutations lead to three substitutions (R512C, L519P, and N536D), three truncations (R499X, R514X, and W531X), and one insertion (A51_R514insP). We expressed these mutant proteins in Escherichia coli in which the GroESL complex was overexpressed. The only mutation that does not impact the stability of mutant betaPCC in bacteria is W531X. The remaining mutations lead to either complete (L519P, N536D) or partial (R499X, R512C, A513_R514insP, and R514X) degradation of the mutant subunits. Size-exclusion chromatography revealed that R512C and W531X do not affect the assembly of alphaPCC and betaPCC to active oligomers. Specific activities for these mutant proteins, however, were only 3.9 and 10% of the wild type, respectively. Taken together, the carboxyl-terminal portion of 40 amino acid residues of the beta subunit affects the stability and the assembly of the alpha and beta subunits as well as the carboxylation of propionyl-CoA.

Enhanced mitochondrial biogenesis is associated with increased expression of the mitochondrial ATP-dependent Lon protease.

Rats bearing the Zajdela hepatoma tumor and T3-treated hypothyroid rats were used to study the role of protein degradation in the process of mitochondrial biogenesis. It was shown that the activity, protein and mRNA levels of the ATP-dependent Lon protease increased in rapidly growing Zajdela hepatoma cells. The increase in the rate of mitochondrial biogenesis by thyroid hormone was similarly accompanied by enhanced expression of the Lon protease. The results imply that mitochondrial biogenesis in mammalian cells is, at least partially, regulated by the matrix Lon protease.

In vitro association of mitochondrial ATP- dependent protease with mitochondrial heat-shock proteins.

Specific antibodies against the mitochondrial ATP- dependent protease and heat-shock proteins were used to study the association of these proteins with an abnormal bacterial protein, CRAG. It was shown that the mitochondrial ATP-dependent protease from rat liver and Zajdela hepatoma bind to the CRAG protein and that this binding was mediated through the heat-shock proteins. It was also demonstrated that the protease associated with heat-shock proteins is capable of degrading large proteins as well as small peptides in an ATP-dependent fashion. Zajdela hepatoma mitochondria, with enhanced mitochondrial proteolysis, were shown to contain more ATP-dependent protease associated with heat-shock proteins.