Inter-Species Therapeutic Cloning: The Looming Problem of Mitochondrial DNA and Two Possible Solutions

A Novel Possible Approach to The Creation of Genetically Personalized Human Embryonic Stem-Like Cell Lines

A novel possible approach to the derivation of human embryonic stem (ES) cell's analogues, named tetraploid meiosis (TM) cells, derived from a prospective patient's or a senior's (PS) somatic cells is described. In this procedure, most of the resulting cells' mitochondrial DNA, if not all, is also from the PS's cells. The procedure involves several rounds of the following three steps, starting from an allogeneic human ES cell line: the creation of tetraploid ES cells by fusion of somatic cells from the prospective PS with the ES cells, the differentiation of these cells into tetraploid oogonia that give rise to diploid oocytes following meiosis, and the isolation of ES cells from parthenogenetic blastocysts, morula, 8-cell embryos, or 4-cell embryos derived from these oocytes. After sufficient repetition of this process using the prospective PS's somatic cells, the genetic contribution of the original allogeneic ES cells will be low enough not to trigger an immune response. Because this procedure avoids most challenging aspects of standard somatic cell nuclear transfer, it may be a promising way to generate immunocompatible ES cells in the numbers needed to meet likely future demand.

The plasma membrane redox system: a candidate source of aging-related oxidative stress

The plasma membrane redox system (PMRS) is an electron transport chain in the plasma membrane that transfers electrons from either intra- or extracellular donors to extracellular acceptors. Unlike the superoxide-generating NADPH oxidase of phagocytes and the homologous (but much less active) enzymes found in some other cells, the PMRS is still incompletely characterised at the molecular level. Much is known, however, concerning its function and affinity for both physiological and non-physiological substrates. A role for it in aging, the 'reductive hotspot hypothesis' (RHH), was proposed in 1998 as part of an explanation for the apparently indefinite survival in vivo of cells that have entirely lost mitochondrial respiratory capacity as a result of the accumulation of mitochondrial mutations. Stimulation of the PMRS might allow the cell to maintain redox homeostasis even while continuing to operate the Krebs cycle, which may be advantageous in many ways. However, the PMRS may, like the mitochondrial respiratory chain, be prone to generate superoxide when thus dysregulated - and in this case superoxide would be generated outside the cell, where antioxidant defences are more limited than inside the cell and where much highly oxidisable material is present. Cascades of peroxidation chain reactions initiated by this process may greatly amplify the oxidative stress on the organism that is caused by rare mitochondrially mutant cells. Since such cells increase in abundance with aging (though remaining rare), this is an economical hypothesis to explain the rise in oxidative stress seen in (and generally believed to contribute substantially to) mammalian aging. In an extension of previously published accounts of RHH, I propose here that the lysosomal toxicity of oxidised cholesterol derivatives (oxysterols) may contribute to the toxicity of mitochondrial mutations by affecting lysosomal function in many cell types in the same way as they have been proposed to do in arterial macrophages.

Reactive Oxygen Species Production in the Mitochondrial Matrix: Implications for the Mechanism of Mitochondrial Mutation Accumulation

The vicious cycle theory postulates that typical mitochondrial DNA (mtDNA) mutations cause their host mitochondria to generate more superoxide and other reactive oxygen species (ROS) than do normal mitochondria, thereby promoting the occurrence of additional mtDNA mutations at an ever-accelerating rate. However, nearly all the loss-of-function mtDNA mutations seen in vivo are large deletions, which (as the original statement of the theory indeed noted, though this has been widely overlooked) should not trigger a vicious cycle because they will prevent the assembly of the potentially superoxide-generating enzyme complexes. Consistent with this is the observation that each cell exhibiting loss of mtDNA-encoded function in vivo contains copies of a single, evidently clonally expanded, mutant mtDNA species, whereas the vicious cycle theory predicts a spectrum of mutant forms in each cell. Two recent papers, however, unveil a way in which mtDNA mutations could indeed promote ROS production of their host mitochondria. MtDNA mutations probably shift the intramitochondrial NAD(+)/NADH redox couple towards NADH, and this is now shown in vitro to cause ROS production by alpha-ketoglutarate dehydrogenase, an essential enzyme of the TCA cycle. This does not revive the vicious cycle theory, but it has complex implications for the two most plausible more recent theories, known as "survival of the slowest" and "crippled mitochondria." It may also prove to explain other recent observations in mitochondrially mutant cells in vivo.

Forces maintaining organellar genomes: is any as strong as genetic code disparity or hydrophobicity?

It remains controversial why mitochondria and chloroplasts retain the genes encoding a small subset of their constituent proteins, despite the transfer of so many other genes to the nucleus. Two candidate obstacles to gene transfer, suggested long ago, are that the genetic code of some mitochondrial genomes differs from the standard nuclear code, such that a transferred gene would encode an incorrect amino acid sequence, and that the proteins most frequently encoded in mitochondria are generally very hydrophobic, which may impede their import after synthesis in the cytosol. More recently it has been suggested that both these interpretations suffer from serious "false positives" and "false negatives": genes that they predict should be readily transferred but which have never (or seldom) been, and genes whose transfer has occurred often or early, even though this is predicted to be very difficult. Here I consider the full known range of ostensibly problematic such genes, with particular reference to the sequences of events that could have led to their present location. I show that this detailed analysis of these cases reveals that they are in fact wholly consistent with the hypothesis that code disparity and hydrophobicity are much more powerful barriers to functional gene transfer than any other. The popularity of the contrary view has led to the search for other barriers that might retain genes in organelles even more powerfully than code disparity or hydrophobicity; one proposal, concerning the role of proteins in redox processes, has received widespread support. I conclude that this abandonment of the original explanations for the retention of organellar genomes has been premature. Several other, relatively minor, obstacles to gene transfer certainly exist, contributing to the retention of relatively many organellar genes in most lineages compared to animal mtDNA, but there is no evidence for obstacles as severe as code disparity or hydrophobicity. One corollary of this conclusion is that there is currently no reason to suppose that engineering nuclear versions of the remaining mammalian mitochondrial genes, a feat that may have widespread biomedical relevance, should require anything other than sequence alterations obviating code disparity and causing modest reductions in hydrophobicity without loss of enzymatic function.