in mitochondrial translation are encoded by nuclear genes, meaning that proper reproduction of the mitochondria requires interaction of the mtDNA and nuclear DNA (nDNA) (Jacobs and Turnbull, 2005); if this interaction is aberrant, many things could go wrong, such as overproduction or underproduction of mitochondria. In addition, many of the proteins involved in the ETC are coded for by nDNA, so that any aberrant interaction between mtDNA and nDNA could also adversely impact energy production in cells.
Mitochondria are located outside the nucleus in the cytoplasm of the cell. Egg cells contain a large amount of mitochondria, while sperm contain many fewer mitochondria (sperm cells consist almost solely of genetic material). During normal mating, sperm mitochondria get labeled and are eliminated from the embryo before the 8- cell stage in cattle and rhesus monkeys, so that the developing embryo only has maternal mtDNA (Sutovsky et al., 1999). However, the process of SCNT often results in the mixture of two different mtDNAs—one from the donor cell (the somatic cell), the other from the recipient cell (the enucleated egg)—because the donor cell is often fused with the enucleated egg cell. The presence of only one type of mtDNA in a cell is called homoplasy, while the presence of two types of mtDNA in a cell is called heteroplasmy. Thus, SCNT clones may be heteroplasmic (e.g. with mtDNA from both “parents”—the nuclear donor [e.g. somatic cell] and the [enucleated] egg recipient), while animals produced via normal mating or other ARTs (artificial reproductive technologies, e.g in- vitro fertilization, embryo culture and transfer) are homoplasmic.
In SCNT clones produced via the fusion of a somatic cell and an enucleated egg, in addition to the possibility of heteroplasmy, the cytoplasm from the somatic cell also contains compounds involved in the replication (e.g. polymerase γ [PolG]) and transcription (e.g. mitochondrial transcription factor A [TFAM]) of mitochondria that are encoded by nuclear genes. In embryos produced via IVF (in vitro fertilization), PolG and TFAM levels are dramatically reduced between the 2-cell and 4-cell stage, while in SCNT embryos, the PolG and TFAM levels are not reduced at the 4-cell stage. The persistence of PolG and TFAM in SCNT embryos but not in IVF embryos suggests that nuclear-mitochondrial interaction following SCNT is out of sequence as the onset of mitochondrial replication is a post-implantation even (Lloyd et al. 2006). This could lead to disruption in the number of mitochondria that are passed on to different cells and lead to a number of problems.
The FDA does discuss the issue of mtDNA but only refers to one article: “Although there has been speculation that mitochondrial dimorphism may affect development of SCNT embryos, only one study was identified that looked specifically at mitochondrial effects on embryo development (Takeda et al. 2005)” (FDA, 2006: 185). FDA ignores the fact that numerous studies have found that SCNT clones do exhibit heteroplasmy. Indeed, a review article on mtDNA and SCNT—published in 2004—has a table which lists 6 different studies, published between 1998 and 2003, that demonstrate varying levels of heteroplasmy in cattle and sheep clones; some of the studies have shown that up to 59% of the mtDNA in a clone may come from donor mtDNA (St. John et al. 2004). Other studies suggest that in some heteroplasmic embryos, particularly