Synthesis of mitochondrial DNA in permeabilised human cultured cells

Genetic Counselling for Maternally Inherited Mitochondrial Disorders

The aim of this review was to provide an evidence-based approach to frequently asked questions relating to the risk of transmitting a maternally inherited mitochondrial disorder (MID). We do not address disorders linked with disturbed mitochondrial DNA (mtDNA) maintenance, causing mtDNA depletion or multiple mtDNA deletions, as these are autosomally inherited. The review addresses questions regarding prognosis, recurrence risks and the strategies available to prevent disease transmission. The clinical and genetic complexity of maternally inherited MIDs represent a major challenge for patients, their relatives and health professionals. Since many of the genetic and pathophysiological aspects of MIDs remain unknown, counselling of affected patients and at-risk family members remains difficult. MtDNA mutations are maternally transmitted or, more rarely, they are sporadic, occurring de novo (~25%). Females carrying homoplasmic mtDNA mutations will transmit the mutant species to all of their offspring, who may or may not exhibit a similar phenotype depending on modifying, secondary factors. Females carrying heteroplasmic mtDNA mutations will transmit a variable amount of mutant mtDNA to their offspring, which can result in considerable phenotypic heterogeneity among siblings. The majority of mtDNA rearrangements, such as single large-scale deletions, are sporadic, but there is a small risk of recurrence (~4%) among the offspring of affected women. The range and suitability of reproductive choices for prospective mothers is a complex area of mitochondrial medicine that needs to be managed by experienced healthcare professionals as part of a multidisciplinary team. Genetic counselling is facilitated by the identification of the underlying causative genetic defect. To provide more precise genetic counselling, further research is needed to clarify the secondary factors that account for the variable penetrance and the often marked differential expressivity of pathogenic mtDNA mutations both within and between families.

Mitochondrial DNA disease and developmental implications for reproductive strategies

Mitochondrial diseases are potentially severe, incurable diseases resulting from dysfunctional mitochondria. Several important mitochondrial diseases are caused by mutations in mitochondrial DNA (mtDNA), the genetic material contained within mitochondria, which is maternally inherited. Classical and modern therapeutic approaches exist to address the inheritance of mtDNA disease, but are potentially complicated by the fact that cellular mtDNA populations evolve according to poorly-understood dynamics during development and organismal lifetimes. We review these therapeutic approaches and models of mtDNA dynamics during development, and discuss the implications of recent results from these models for modern mtDNA therapies. We particularly highlight mtDNA segregation—differences in proliferative rates between different mtDNA haplotypes—as a potential and underexplored issue in such therapies. However, straightforward strategies exist to combat this and other potential therapeutic problems. In particular, we describe haplotype matching as an approach with the power to potentially ameliorate any expected issues from mtDNA incompatibility.

Poor correlations in the levels of pathogenic mitochondrial DNA mutations in polar bodies versus oocytes and blastomeres in humans

Because the mtDNA amount remains stable in the early embryo until uterine implantation, early human development is completely dependent on the mtDNA pool of the mature oocyte. Both quantitative and qualitative mtDNA defects therefore may negatively impact oocyte competence or early embryonic development. However, nothing is known about segregation of mutant and wild-type mtDNA molecules during human meiosis. To investigate this point, we compared the mutant levels in 51 first polar bodies (PBs) and their counterpart (oocytes, blastomeres, or whole embryos), at risk of having (1) the "MELAS" m.3243A>G mutation in MT-TL1 (n = 30), (2) the "MERRF" m.8344A>G mutation in MT-TK (n = 15), and (3) the m.9185T>G mutation located in MT-ATP6 (n = 6). Seven out of 51 of the PBs were mutation free and had homoplasmic wild-type counterparts. In the heteroplasmic PBs, measurement of the mutant load was a rough estimate of the counterpart mutation level (R(2) = 0.52), and high mutant-load differentials between the two populations were occasionally observed (ranging from -34% to +34%). The mutant-load differentials between the PB and its counterpart were higher in highly mutated PBs, suggestive of a selection process acting against highly mutated cells during gametogenesis or early embryonic development. Finally, individual discrepancies in mutant loads between PBs and their counterparts make PB-based preconception diagnosis unreliable for the prevention of mtDNA disorder transmission. Such differences were not observed in animal models, and they emphasize the need to conduct thorough studies on mtDNA segregation in humans.

POLG mutations cause decreased mitochondrial DNA repopulation rates following induced depletion in human fibroblasts

Disorders of mitochondrial DNA (mtDNA) maintenance have emerged as an important cause of human genetic disease, but demonstrating the functional consequences of de novo mutations remains a major challenge. We studied the rate of depletion and repopulation of mtDNA in human fibroblasts exposed to ethidium bromide in patients with heterozygous POLG mutations, POLG2 and TK2 mutations. Ethidium bromide induced mtDNA depletion occurred at the same rate in human fibroblasts from patients and healthy controls. By contrast, the restoration of mtDNA levels was markedly delayed in fibroblasts from patients with compound heterozygous POLG mutations. Specific POLG2 and TK2 mutations did not delay mtDNA repopulation rates. These observations are consistent with the hypothesis that mutations in POLG impair mtDNA repopulation within intact cells, and provide a potential method of demonstrating the functional consequences of putative pathogenic alleles causing a defect of mtDNA synthesis.

Transmission of mitochondrial DNA diseases and ways to prevent them

Recent reports of strong selection of mitochondrial DNA (mtDNA) during transmission in animal models of mtDNA disease, and of nuclear transfer in both animal models and humans, have important scientific implications. These are directly applicable to the genetic management of mtDNA disease. The risk that a mitochondrial disorder will be transmitted is difficult to estimate due to heteroplasmy—the existence of normal and mutant mtDNA in the same individual, tissue, or cell. In addition, the mtDNA bottleneck during oogenesis frequently results in dramatic and unpredictable inter-generational fluctuations in the proportions of mutant and wild-type mtDNA. Pre-implantation genetic diagnosis (PGD) for mtDNA disease enables embryos produced by in vitro fertilization (IVF) to be screened for mtDNA mutations. Embryos determined to be at low risk (i.e., those having low mutant mtDNA load) can be preferentially transferred to the uterus with the aim of initiating unaffected pregnancies. New evidence that some types of deleterious mtDNA mutations are eliminated within a few generations suggests that women undergoing PGD have a reasonable chance of generating embryos with a lower mutant load than their own. While nuclear transfer may become an alternative approach in future, there might be more difficulties, ethical as well as technical. This Review outlines the implications of recent advances for genetic management of these potentially devastating disorders.

Anticancer DNA intercalators cause p53-dependent mitochondrial DNA nucleoid re-modelling

Many anticancer drugs, such as doxorubicin (DXR), intercalate into nuclear DNA of cancer cells, thereby inhibiting their growth. However, it is not well understood how such drugs interact with mitochondrial DNA (mtDNA). Using cell and molecular studies of cultured cells, we show that DXR and other DNA intercalators, such as ethidium bromide, can rapidly intercalate into mtDNA within living cells, causing aggregation of mtDNA nucleoids and altering the distribution of nucleoid proteins. Remodelled nucleoids excluded DXR and maintained mtDNA synthesis, whereas non-remodelled nucleoids became heavily intercalated with DXR, which inhibited their replication, thus leading to mtDNA depletion. Remodelling was accompanied by extensive mitochondrial elongation or interconnection, and was suppressed in cells lacking mitofusin 1 and optic atrophy 1 (OPA1), the key proteins for mitochondrial fusion. In contrast, remodelling was significantly increased by p53 or ataxia telangiectasia mutated inhibition (ATM), indicating a link between nucleoid dynamics and the genomic DNA damage response. Collectively, our results show that DNA intercalators can trigger a common mitochondrial response, which likely contributes to the marked clinical toxicity associated with these drugs.

Defects in maintenance of mitochondrial DNA are associated with intramitochondrial nucleotide imbalances

Defects in mtDNA maintenance range from fatal multisystem childhood diseases, such as Alpers syndrome, to milder diseases in adults, including mtDNA depletion syndromes (MDS) and familial progressive external ophthalmoplegia (AdPEO). Most are associated with defects in genes involved in mitochondrial deoxynucleotide metabolism or utilization, such as mutations in thymidine kinase 2 (TK2) as well as the mtDNA replicative helicase, Twinkle and gamma polymerase (POLG). We have developed an in vitro system to measure incorporation of radiolabelled dNTPs into mitochondria of saponin permeabilized cells. We used this to compare the rates of mtDNA synthesis in cells from 12 patients with diseases of mtDNA maintenance. We observed reduced incorporation of exogenous alpha (32)P-dTTP in fibroblasts from a patient with Alpers syndrome associated with the A467T substitution in POLG, a patient with dGK mutations, and a patient with mtDNA depletion of unknown origin compared to controls. However, incorporation of alpha (32)P-dTTP relative to either cell doubling time or alpha (32)P-dCTP incorporation was increased in patients with thymidine kinase deficiency or PEO as the result of TWINKLE mutations compared with controls. The specific activity of newly synthesized mtDNA depends on the size of the endogenous pool diluting the exogenous labelled nucleotide. Our result is consistent with a deficiency in the intramitochondrial pool of dTTP relative to dCTP in cells from patients with TK2 deficiency and TWINKLE mutations. Such DNA precursor asymmetry could cause pausing of the replication complex and hence exacerbate the propensity for age-related mtDNA mutations. Because deviations from the normal concentrations of dNTPs are known to be mutagenic, we suggest that intramitochondrial nucleotide imbalance could underlie the multiple mtDNA mutations observed in these patients.

Human mitochondrial DNA with large deletions repopulates organelles faster than full-length genomes under relaxed copy number control

Partially-deleted mitochondrial DNA (DeltamtDNA) accumulates during aging of postmitotic tissues. This accumulation has been linked to decreased metabolic activity, increased reactive oxygen species formation and the aging process. Taking advantage of cell lines with heteroplasmic mtDNA mutations, we showed that, after severe mtDNA depletion, organelles are quickly and predominantly repopulated with DeltamtDNA, whereas repopulation with the wild-type counterpart is slower. This behavior was not observed for full-length genomes with pathogenic point mutations. The faster repopulation of smaller molecules was supported by metabolic labeling of mtDNA with [3H]thymidine during relaxed copy number control conditions. We also showed that hybrid cells containing two defective mtDNA haplotypes tend to retain the smaller one as they adjust their normal mtDNA copy number. Taken together, our results indicate that, under relaxed copy number control, DeltamtDNAs repopulate mitochondria more efficiently than full-length genomes.

N-terminal protein acylation confers localization to cholesterol, sphingolipid-enriched membranes but not to lipid rafts/caveolae

When variably fatty acylated N-terminal amino acid sequences were appended to a green fluorescent reporter protein (GFP), chimeric GFPs were localized to different membranes in a fatty acylation-dependent manner. To explore the mechanism of localization, the properties of acceptor membranes and their interaction with acylated chimeric GFPs were analyzed in COS-7 cells. Myristoylated GFPs containing a palmitoylated or polybasic region colocalized with cholesterol and ganglioside GM(1), but not with caveolin, at the plasma membrane and endosomes. A dipalmitoylated GFP chimera colocalized with cholesterol and GM(1) at the plasma membrane and with caveolin in the Golgi region. Acylated GFP chimeras did not cofractionate with low-density caveolin-rich lipid rafts prepared with Triton X-100 or detergent-free methods. All GFP chimeras, but not full-length p62(c-yes) and caveolin, were readily solubilized from membranes with various detergents. These data suggest that, although N-terminal acylation can bring GFP to cholesterol and sphingolipid-enriched membranes, protein-protein interactions are required to localize a given protein to detergent-resistant membranes or caveolin-rich membranes. In addition to restricting acceptor membrane localization, N-terminal fatty acylation could represent an efficient means to enrich the concentration of signaling proteins in the vicinity of detergent-resistant membranes and facilitate protein-protein interactions mediating transfer to a detergent-resistant lipid raft core.