bovine SCNT clones, the donor mtDNA is preferentially replicated over the recipient (e.g. enucleated egg) mtDNA (Do et al. 2002, Takeda et al. 2003). This finding is important as it’s possible that in heteroplasmic embryos there is a kind of competition between the donor and recipient mtDNA, so that the net effect could be reduced numbers of mitochondria which could result in phenotypes that look like mtDNA-depletion syndromes. Other studies have also pointed out that mutations in mitochondrial genes as well as in the nuclear genes involved in mitochondrial translation can give rise to a number of diseases (often called mtDNA-depletion syndrome) in humans, some of which are very severe (Hiendleder et al. 2005, Jacobs and Turnbull 2005, St John et al 2004). Since mitochondria are the powerhouses of the cells, those cell types most in need of energy, e.g. muscles, nerve cells, liver, etc. have the largest number of mitochondria, while cells that are less energy dependent have much lower numbers of mitochondria. For example, in humans there are roughly 6,800 mtDNA copies in each cardiac muscle cell (Miller et al. 2003), compared to about 409 mtDNA copies in each peripheral blood mononuclear cell (Gahan et al. 2001) and only 2 copies of nuclear genes. Thus, cell types most in need of energy (and therefore lots of mitochondria) often are adversely affected in mtDNA-depletion syndrome leading to myopathies and neuropathies; examples include cardiac myopathy, mitochondrial myopathy, Leber’s hereditary optic neuropathy, diabetes mellitus and deafness (DAD). However, since energy production is so important to the cell, if mitochondrial disease, depending on the cell type affected, can cause muscle wasting, nerve damage, seizure, strokes, blindness, deafness and more, with recent studies also implicating mitochondria in diseases such as Alzheimer’s and Parkinson’s (Lemonick, 2006). Indeed, there is now a field of study on “mitochondrial diseases,” as “defects of mitochondrial metabolism cause a wide range of human diseases that include examples from all medical subspecialties” (Schapira 2006: 70).
A number of researchers have noticed similarities between some of the mitochondrial depletion diseases in humans and some of the abnormalities seen in SCNT and hypothesize that aberrant nuclear-mitochondrial interactions in SCNTs could be responsible for some of these abnormalities. A research team in Germany found that “A survey of perinatal clinical data from human subjects with deficient mitochondrial respiratory chain activity has revealed a plethora of phenotypes that have striking similarities with abnormalities commonly encountered in SCNT fetuses and offspring. We discuss the limited experimental data on nuclear-mitochondrial interaction effects in SCNT and explore the potential effects in the context of new findings about the biology of mitochondria” (Hiendleder et al. 2005: 69). Researchers in the United Kingdom have suggested that potential overpopulation of mitochondria could lead to the large offspring syndrome often seen in SCNT cow clones and call for more work in this area: “a cytoplasm over-populated with mitochondria would lead to cellular expansion that might be indicative of the reported large-offspring syndromes. This under-researched area of investigation could provide clear answers to some of the developmental abnormalities witnessed in NT offspring and aborted feotuses, whether mediated through failure of somatic cell reprogramming or independently” (St John et al. 2004: 638-639).