The role of mitochondrial genome (mtDNA) in somatic and embryo cloning of mammals. A review

Abstract

Mitochondria are semiautonomous organelles that contain double-stranded (α-helix) circular DNA molecules (mtDNAs) of about 16300 to 16500 base pairs (bp). The mtDNA encodes only 13 proteins, 22 tRNAs and 2 rRNAs. Up to 95% of proteins involved in biogenesis and functions of mitochondria are encoded by the cell nucleus. The copy number of mitochondrial genome in a typical mammalian somatic cell ranges from approximately 2 × 103 to about 5 × 103, whereas the number of mtDNA molecules in a single mature (Metaphase II) oocyte is about 1.6 × 105 in mice, 2.5 × 105 in cattle and 3-8 × 105 in humans. In the procedure of somatic or embryo cell cloning (nuclear transfer), mitochondria of nuclear donor cells, together with the nucleus, are transplanted to the enucleated recipient oocyte (ooplast). Thus, the cloned embryo should harbour the mtDNAs from both the donor cell and the recipient oocyte. In cloned animals, mitochondria are inherited primarily from recipient oocytes, and mitochondria from donor cells appear to be rapidly eliminated during the first few cleavage divisions and are almost undetectable by the blastocyst stage. This selective segregation of the donor mitochondrial genome, which takes place in the preimplantation development of nuclear transfer-derived embryos, leads gradually to cellular mtDNA homoplasmy. Only in some cases does the mitochondria originating from both donor cells and recipient oocytes coexist (the so-called mtDNA heteroplasmy). When cloning both embryos and adult individuals, what is often forgotten is the presence in the cytoplasm of donor- and recipient-cells of mtDNA. This contains small (approximately 0.01%) amounts of total cell genetic information, but it is different from information recorded in the nuclear DNA (99.99% of cellular genome). Thus, introduction of donor cell nucleus into the oocyte derived from another individual leads to generation of hybridic (in terms of genetic mitochondrial material) reconstructed oocytes, which can have a certain influence on the genotype and phenotype identity of offspring produced as a result of cloning. The "ideal" clone can be obtained only with donor cell nucleus transplanted into the oocyte originating from the same individual. Therefore "ideal mammalian clones" can only be the clones of females, whose overall mitochondrial genome has a completely homogenous pattern of regulatory and coding nucleotide sequences of all cellular mtDNA copies in all the somatic and germ cell lines.