Mitochondria transfection by oligonucleotides containing a signal peptide and vectorized by cationic liposomes

The progress of research in gene therapy allows hope for treatment of mitochondrial genetic disorders provided that efficient methods for gene transfer into mitochondria can be found. In this work, we have used an oligonucleotide coupled covalently to a mitochondria-targeted peptide at one end and a cationic liposome prepared from trimethyl aminoethane carbamoyl cholesterol iodide (TMAEC-Chol) to carry it in living cells. With a fluorescent probe to label the oligonucleotide at the other end and by means of confocal microscopy, we show that such modified oligonucleotides complexed to liposomes enter into the cytoplasm of human fibroblasts in primary culture, and then, after dissociation from the complexes, they penetrate into the mitochondria. The fluorescence was still observed after 8 days, suggesting the continued presence of oligonucleotides. At the concentrations used for this study, the cationic liposomes have practically no effect on cell growth, as revealed by the MTT assay.

For debate: defective mitochondria, free radicals, cell death, aging-reality or myth-ochondria?

As both experimental evidence and theoretical considerations may suggest that free radicals and mitochondria might be associated as key factors in aging, these organelles have been implicated in various versions of the free radical theory of aging. However, except for a few cases, no evidence for a death process specifically activated in respiratory defective cells could be found in patients with a mitochondrial disorder, including those harboring high levels of mutant mtDNA associated with profound respiratory chain deficiencies. This and more recent evidence suggest that damages produced by free-radicals endogenously generated in the mitochondria result in a distinctive biochemical profile, only occur under exceptional conditions and that a dysfunction of the respiratory chain does not cause opening of the permeability transition pore and is not sufficient per se to trigger massive entrance of cells into death processes, neither apoptosis nor necrosis. Therefore, defective mitochondria and their particular genome, should not be considered as a major and primary source of free radicals either leading cells into a death cascade or resulting in an accelerated aging process.

Renal failure from mitochondrial cytopathies

Mitochondrial cytopathies are metabolic diseases, expressing mutations in nuclear DNA, punctiform mutations or depletions in mitochondrial DNA. These genetic lesions alter mitochondrial oxidative phosphorylation, with a reduction in energy produced for cell activity. Renal disease may be the first sign of mitochondrial cytopathy, or it may appear together with neurological and neuromuscular signs. Fanconi's syndrome, a benign sign of renal tubulopathy, is particularly frequent in newborns with mitochondrial cytopathy, whereas tubulointerstitial nephropathy, which affects infants and adults, is more serious because it develops into terminal uremia. Findings of hyperlactatemia and reduced enzymatic activity on the respiratory chain in tissue biopsies are of diagnostic significance in mitochondrial cytopathy. A breakthrough is being made in our understanding of genetic alterations in mitochondrial DNA, and with future therapy, the kidney, a target organ, may be safeguarded.

Perinatal maturation of rat kidney mitochondria

In the rat kidney, NaK-ATPase activity increased between days 19 and 20 of gestation (+50%) and between 1 and 24 h after birth (+20%), requiring an increased energy supply. In order to determine whether mitochondrial changes were involved, renal mitochondrial development was investigated from day 19 of gestation to 1 day after birth. Slot-blot analyses of mitochondrial-DNA/nuclear-DNA ratio and determination of citrate synthase activity showed a doubling in the mitochondrial pool between days 19 and 20 of gestation. In isolated mitochondria, oxygen consumption remained unchanged between days 19 and 20 of gestation, and then it was enhanced between days 20 and 21 of gestation (+70%) and between 1 and 24 h after birth (+50%). We also focused on one of the respiratory-chain complexes, ATP synthase, and measured its activity and content during the perinatal period. We demonstrated increases in both activity and content of ATP synthase between days 20 and 21 of gestation and between 1 and 24 h after birth, thus suggesting that changes in ATP synthase activity are ascribed to an increase in the mitochondrial density of ATP synthase complexes. Moreover, the mitochondrial ATP/ADP ratio only increased between 1 and 24 h (+90%), indicating a critical step in the renal respiratory-chain maturation at that time. We therefore conclude that the postnatal enhancement of renal mitochondrial oxidative capacity might depend on protein synthesis de novo and on changes in the adenine nucleotide concentrations.

3-Methylglutaconic aciduria associated with Pearson syndrome and respiratory chain defects

3-Methylglutaconic aciduria was detected in four patients with Pearson syndrome, a multitissue disorder with hematologic abnormalities, lactic acidosis resulting from defective oxidative phosphorylation, and deletions in the mitochondrial genome. 3-Methylglutaconic acid may be an additional useful marker for Pearson syndrome and may be a more specific marker than other organic acids identified in this disorder.

Autosomal dominant deletions of the mitochondrial genome in a case of progressive encephalomyopathy

Multiple deletions of the mitochondrial genome were found in a family in which the proband had ataxia and ketoacidotic comas. A progressive multiorgan involvement appeared in the course of the disease, and histopathological investigation demonstrated mitochondrial myopathy features with ragged red fibers. A defect of oxidative phosphorylation was found in both skeletal muscle and lymphocytes. It is surprising that various mtDNA deletions were detected both in the proband and in his healthy mother and maternal aunt but not in the rest of the maternal progeny. All the deletions were located between Cox II and cytochrome b genes, and short (4-5 bp) repeated sequences were consistently present at the boundaries of the rearrangements in different tissues. Therefore, the deletions appear not to be transmitted per se but to be inherited in a Mendelian manner, being possibly dominant. Both the Mendelian inheritance of the trait and the variety of the deletions in carriers suggest that a nuclearly encoded factor(s) might be involved in the triggering of the deletions. However, the presence of the rearrangements in healthy individuals raises the question of whether mtDNA deletions actually cause the clinical expression of the disease.

Carrier detection in a partially dominant X-linked disease: ornithine transcarbamylase deficiency

Ornithine transcarbamylase (OTC) deficiency is an X-linked disease responsible for lethal neonatal hyperammonemia in males. Partial OTC deficiency also occurs in females and can be responsible for life-threatening hyperammonemic comas in heterozygotes (15%). Increased orotic acid excretion occurs in both symptomatic and asymptomatic carriers, especially under protein loading tests. The disease is therefore partially dominant with neonatal lethality in the hemizygous male; the fraction of new mutations has previously been estimated to be low in males (point estimation = 0, upper bound of the confidence interval = 0.16) and 57% in females. Genetic counseling in this disease is difficult because it is not clear whether a negative protein loading test rules out carrier status. In an attempt to determine how reliable the test is for carrier detection, we investigated ten obligate carriers for orotic acid excretion; considering all data available, we concluded that the test is rarely negative in obligate carriers (8%). Consequently, a negative test in a mother decreases the minimum risk of being a carrier from 84% a priori to 30% if she had an affected son and from 43% a priori to 5% if she had a heterozygous daughter. Finally, the diagnosis of a new mutation in the germ cells of the maternal grandfather in one particular family could be ascertained by extensive DNA analysis.

[Respiratory chain diseases in infancy. Clinical presentation and diagnosis]

Nineteen children with defects of the mitochondrial respiratory chain are described. First symptoms appeared during the first two years of their lives. Four types of clinical pictures were identified: 1/neonatal hypotonia. 2/cardiomyopathy. 3/progressive neurological deterioration. 4/multisystem disease. A study of pyruvate and fatty acids metabolism and a skeletal muscle biopsy were performed in all cases. Elevation of beta-hydroxybutyrate/acetoacetate and lactate were the most frequent biochemical abnormalities. Muscular biopsy in light microscopy showed in most cases abnormal lipid storage.