Therapeutic potential of somatic cell nuclear transfer for degenerative disease caused by mitochondrial DNA mutations

Mitochondrial Dysfunction and Therapeutic Perspectives in Cardiovascular Diseases

High mortality rates due to cardiovascular diseases (CVDs) have attracted worldwide attention. It has been reported that mitochondrial dysfunction is one of the most important mechanisms affecting the pathogenesis of CVDs. Mitochondrial DNA (mtDNA) mutations may result in impaired oxidative phosphorylation (OXPHOS), abnormal respiratory chains, and ATP production. In dysfunctional mitochondria, the electron transport chain (ETC) is uncoupled and the energy supply is reduced, while reactive oxygen species (ROS) production is increased. Here, we discussed and analyzed the relationship between mtDNA mutations, impaired mitophagy, decreased OXPHOS, elevated ROS, and CVDs from the perspective of mitochondrial dysfunction. Furthermore, we explored current potential therapeutic strategies for CVDs by eliminating mtDNA mutations (e.g., mtDNA editing and mitochondrial replacement), enhancing mitophagy, improving OXPHOS capacity (e.g., supplement with NAD+, nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), and nano-drug delivery), and reducing ROS (e.g., supplement with Coenzyme Q10 and other antioxidants), and dissected their respective advantages and limitations. In fact, some therapeutic strategies are still a long way from achieving safe and effective clinical treatment. Although establishing effective and safe therapeutic strategies for CVDs remains challenging, starting from a mitochondrial perspective holds bright prospects.

Double sperm cloning (DSC) is a promising strategy in mammalian genetic engineering and stem cell research

Embryonic stem cells (ESCs) derived from somatic cell nuclear transfer (SCNT) and induced pluripotent stem cells (iPSCs) are promising tools for meeting the personalized requirements of regenerative medicine. However, some obstacles need to be overcome before clinical trials can be undertaken. First, donor cells vary, and the reprogramming procedures are diverse, so standardization is a great obstacle regarding SCNT and iPSCs. Second, somatic cells derived from a patient may carry mitochondrial DNA mutations and exhibit telomere instability with aging or disease, and SCNT-ESCs and iPSCs retain the epigenetic memory or epigenetic modification errors. Third, reprogramming efficiency has remained low. Therefore, in addition to improving their success rate, other alternatives for producing ESCs should be explored. Producing androgenetic diploid embryos could be an outstanding strategy; androgenic diploid embryos are produced through double sperm cloning (DSC), in which two capacitated sperms (XY or XX, sorted by flow cytometer) are injected into a denucleated oocyte by intracytoplasmic sperm injection (ICSI) to reconstruct embryo and derive DSC-ESCs. This process could avoid some potential issues, such as mitochondrial interference, telomere shortening, and somatic epigenetic memory, all of which accompany somatic donor cells. Oocytes are naturally activated by sperm, which is unlike the artificial activation that occurs in SCNT. The procedure is simple and practical and can be easily standardized. In addition, DSC-ESCs can overcome ethical concerns and resolve immunological response matching with sperm providers. Certainly, some challenges must be faced regarding imprinted genes, epigenetics, X chromosome inactivation, and dosage compensation. In mice, DSC-ESCs have been produced and have shown excellent differentiation ability. Therefore, the many advantages of DSC make the study of this process worthwhile for regenerative medicine and animal breeding.

Lessons Learned from Somatic Cell Nuclear Transfer

Somatic cell nuclear transfer (SCNT) has been an area of interest in the field of stem cell research and regenerative medicine for the past 20 years. The main biological goal of SCNT is to reverse the differentiated state of a somatic cell, for the purpose of creating blastocysts from which embryonic stem cells (ESCs) can be derived for therapeutic cloning, or for the purpose of reproductive cloning. However, the consensus is that the low efficiency in creating normal viable offspring in animals by SCNT (1–5%) and the high number of abnormalities seen in these cloned animals is due to epigenetic reprogramming failure. In this review we provide an overview of the current literature on SCNT, focusing on protocol development, which includes early SCNT protocol deficiencies and optimizations along with donor cell type and cell cycle synchrony; epigenetic reprogramming in SCNT; current protocol optimizations such as nuclear reprogramming strategies that can be applied to improve epigenetic reprogramming by SCNT; applications of SCNT; the ethical and legal implications of SCNT in humans; and specific lessons learned for establishing an optimized SCNT protocol using a mouse model.

Patient-specific neural progenitor cells derived from induced pluripotent stem cells offer a promise of good models for mitochondrial disease

Mitochondria are the primary generators of ATP in eukaryotic cells through the process of oxidative phosphorylation. Mitochondria are also involved in several other important cellular functions including regulation of intracellular Ca²⁺, cell signaling and apoptosis. Mitochondrial dysfunction causes disease and since it is not possible to perform repeated studies in humans, models are essential to enable us to investigate the mechanisms involved. Recently, the discovery of induced pluripotent stem cells (iPSCs), made by reprogramming adult somatic cells (Takahashi and Yamanaka 2006; Yamanaka and Blau 2010), has provided a unique opportunity for studying aspects of disease mechanisms in patient-specific cells and tissues. Reprogramming cells to neuronal lineage such as neural progenitor cells (NPCs) generated from the neural induction of reprogrammed iPSCs can thus provide a useful model for investigating neurological disease mechanisms including those caused by mitochondrial dysfunction. In addition, NPCs display a huge clinical potential in drug screening and therapeutics.

Achieving Automated Organelle Biopsy on Small Single Cells Using a Cell Surgery Robotic System

Single cell surgery such as manipulation or removal of subcellular components or/and organelles from single cells is increasingly used for the study of diseases and their causes in precision medicine. This paper presents a robotic surgery system to achieve automated organelle biopsy of single cells with dimensions of less than 20 μm in diameter. The complexity of spatial detection of the organelle position is reduced by patterning the cells using a microfluidic chip device. A sliding mode nonlinear controller is developed to enable extraction of organelles, such as the mitochondria and the nucleus, from single cells with high precision. An image processing algorithm is also developed to automatically detect the position of the desired organelle. The effectiveness of the proposed robotic surgery system is demonstrated experimentally with automated extraction of mitochondria and nucleus from human acute promyelocytic leukemia cells and human fibroblast cells. Extraction is followed by biological tests to indicate the functionality of biopsied mitochondria as well as the cell viability after removal of mitochondria. The results presented here have revealed that the proposed approach of automated organelle biopsy on single small cells is feasible.

Novel reproductive technologies to prevent mitochondrial disease

BACKGROUND

The use of nuclear transfer (NT) has been proposed as a novel reproductive treatment to overcome the transmission of maternally-inherited mitochondrial DNA (mtDNA) mutations. Pathogenic mutations in mtDNA can cause a wide-spectrum of life-limiting disorders, collectively known as mtDNA disease, for which there are currently few effective treatments and no known cures. The many unique features of mtDNA make genetic counselling challenging for women harbouring pathogenic mtDNA mutations but reproductive options that involve medical intervention are available that will minimize the risk of mtDNA disease in their offspring. This includes PGD, which is currently offered as a clinical treatment but will not be suitable for all. The potential for NT to reduce transmission of mtDNA mutations has been demonstrated in both animal and human models, and has recently been clinically applied not only to prevent mtDNA disease but also for some infertility cases. In this review, we will interrogate the different NT techniques, including a discussion on the available safety and efficacy data of these technologies for mtDNA disease prevention. In addition, we appraise the evidence for the translational use of NT technologies in infertility.

OBJECTIVE AND RATIONALE

We propose to review the current scientific evidence regarding the clinical use of NT to prevent mitochondrial disease.

SEARCH METHODS

The scientific literature was investigated by searching PubMed database until Jan 2017. Relevant documents from Human Fertilisation and Embryology Authority as well as reports from both the scientific and popular media were also implemented. The above searches were based on the following key words: 'mitochondria', 'mitochondrial DNA'; 'mitochondrial DNA disease', 'fertility'; 'preimplantation genetic diagnosis', 'nuclear transfer', 'mitochondrial replacement' and 'mitochondrial donation'.

OUTCOMES

While NT techniques have been shown to effectively reduce the transmission of heteroplasmic mtDNA variants in animal models, and increasing evidence supports their use to prevent the transmission of human mtDNA disease, the need for robust, long-term evaluation is still warranted. Moreover, prenatal screening would still be strongly advocated in combination with the use of these IVF-based technologies. Scientific evidence to support the use of NT and other novel reproductive techniques for infertility is currently lacking.

WIDER IMPLICATIONS

It is mandatory that any new ART treatments are first adequately assessed in both animal and human models before the cautious implementation of these new therapeutic approaches is clinically undertaken. There is growing evidence to suggest that the translation of these innovative technologies into clinical practice should be cautiously adopted only in highly selected patients. Indeed, given the limited safety and efficacy data, close monitoring of any offspring remains paramount.

Transdifferentiation and reprogramming: Overview of the processes, their similarities and differences

Reprogramming, or generation of induced pluripotent stem (iPS) cells (functionally similar to embryonic stem cells or ES cells) by the use of transcription factors (typically: Oct3/4, Sox2, c-Myc, Klf4) called "Yamanaka factors" (OSKM), has revolutionized regenerative medicine. However, factors used to induce stemness are also overexpressed in cancer. Both, ES cells and iPS cells cause teratoma formation when injected to tissues. This raises a safety concern for therapies based on iPS derivates. Transdifferentiation (lineage reprogramming, or -conversion), is a process in which one mature, specialized cell type changes into another without entering a pluripotent state. This process involves an ectopic expression of transcription factors and/or other stimuli. Unlike in the case of reprogramming, tissues obtained by this method do not carry the risk of subsequent teratomagenesis.

Mitochondrial transfer: Implications for assisted reproductive technologies

The use of mitochondrial transfer as a clinic procedure is drawing closer to reality. Here we provide a detailed overview of mitochondrial transfer techniques – both established and recent – including pronuclear, spindle, ooplasmic and blastomere transfer. Reasons as to why some techniques are more suitable for the prevention of mitochondrial DNA disease than others, as well as the advantages and disadvantages of each methodology, are discussed. The possible clinical introduction of these techniques has raised concerns about the adverse effects they may have on resultant embryos and offspring. Success rates of each technique, embryo viability and developmental consequences post mitochondrial transfer are addressed through analysis of evidence obtained from both animal and human studies. Counterarguments against potential mitochondrial-nuclear genome incompatibility are also provided. Additional clinical applications of mitochondrial transfer techniques are discussed. These include the rescue or enhancement of fertility in women of advanced maternal age or those suffering from diabetes. An alternative to using mitochondrial DNA transfer for germ line therapies is the therapeutic use of somatic cell nuclear transfer for the generation of personalised stem cells. Although ethically challenging, this method could offer patients already suffering from mitochondrial DNA diseases a novel treatment option.

Concise reviews: Assisted reproductive technologies to prevent transmission of mitochondrial DNA disease

While the fertilized egg inherits its nuclear DNA from both parents, the mitochondrial DNA is strictly maternally inherited. Cells contain multiple copies of mtDNA, each of which encodes 37 genes, which are essential for energy production by oxidative phosphorylation. Mutations can be present in all, or only in some copies of mtDNA. If present above a certain threshold, pathogenic mtDNA mutations can cause a range of debilitating and fatal diseases. Here, we provide an update of currently available options and new techniques under development to reduce the risk of transmitting mtDNA disease from mother to child. Preimplantation genetic diagnosis (PGD), a commonly used technique to detect mutations in nuclear DNA, is currently being offered to determine the mutation load of embryos produced by women who carry mtDNA mutations. The available evidence indicates that cells removed from an eight-cell embryo are predictive of the mutation load in the entire embryo, indicating that PGD provides an effective risk reduction strategy for women who produce embryos with low mutation loads. For those who do not, research is now focused on meiotic nuclear transplantation techniques to uncouple the inheritance of nuclear and mtDNA. These approaches include transplantation of any one of the products or female meiosis (meiosis II spindle, or either of the polar bodies) between oocytes, or the transplantation of pronuclei between fertilized eggs. In all cases, the transferred genetic material arises from a normal meiosis and should therefore, not be confused with cloning. The scientific progress and associated regulatory issues are discussed. Stem Cells2015;33:639–645

Extreme-Depth Re-sequencing of Mitochondrial DNA Finds No Evidence of Paternal Transmission in Humans

Recent reports have questioned the accepted dogma that mammalian mitochondrial DNA (mtDNA) is strictly maternally inherited. In humans, the argument hinges on detecting a signature of inter-molecular recombination in mtDNA sequences sampled at the population level, inferring a paternal source for the mixed haplotypes. However, interpreting these data is fraught with difficulty, and direct experimental evidence is lacking. Using extreme-high depth mtDNA re-sequencing up to ~1.2 million-fold coverage, we find no evidence that paternal mtDNA haplotypes are transmitted to offspring in humans, thus excluding a simple dilution mechanism for uniparental transmission of mtDNA present in all healthy individuals. Our findings indicate that an active mechanism eliminates paternal mtDNA which likely acts at the molecular level.

Emerging evidence raises the possibility that human mitochondrial DNA (mtDNA) is not strictly maternally inherited, but it has not been technically possible to test this hypothesis directly. We identified trios with discordant mtDNA haplotypes, parent-offspring trios were validated using polymorphic microsatellites, and then used extreme-high depth mtDNA re-sequencing to look for paternally transmitted mtDNA. Despite having up to ~1.2 million-fold coverage of mtDNA, we find no evidence that paternal mtDNA haplotypes are transmitted to offspring in humans. Our findings exclude a simple dilution mechanism for uniparental transmission of mtDNA present in all healthy individuals.

Modified Adherence Method (MAM) for Electrofusion of Anchorage-Dependent Cells

The artificially induced cell fusion is a useful experimental tool in biology, biotechnology and medicine. The electrofusion is a physical method for cell fusion that applies high-voltage electric pulses. The use of electric pulses causes cell membrane structural changes which bring the cell membrane in the so-called fusogenic state. When such fusogenic membranes are in close contact cell fusion takes place. Physical contact between fusion partners can be achieved by various methods and one of them is modified adherence method (MAM) described in detail here on B16-F1 cell line. The method is based on the fact that living cells form contacts in confluent culture. However, instead of using confluent cell culture, in modified adherence method cells are plated in suitable concentration and allowed to form contacts for only short predetermined period of time. During that time the cells are only slightly attached to the dish surface maintaining the spherical shape. Observed high fusion yields up to 50 % obtained by MAM in situ by dual-color fluorescence microscopy are among the highest in field of electrofusion. The method can be readily adapted to other anchorage-dependent cell lines.