The Mitochondrial Genome in Embryo Technologies

Abstract

The mammalian mitochondrial genome encodes for 37 genes which are involved in a broad range of cellular functions. The mitochondrial DNA (mtDNA) molecule is commonly assumed to be inherited through oocyte cytoplasm in a clonal manner, and apparently species-specific mechanisms have evolved to eliminate the contribution of sperm mitochondria after natural fertilization. However, recent evidence for paternal mtDNA inheritance in embryos and offspring questions the general validity of this model, particularly in the context of assisted reproduction and embryo biotechnology. In addition to normal mt DNA haplotype variation, oocytes and spermatozoa show remarkable differences in mtDNA content and may be affected by inherited or acquired mtDNA aberrations. All these parameters have been correlated with gamete quality and reproductive success rates. Nuclear transfer (NT) technology provides experimental models for studying interactions between nuclear and mitochondrial genomes. Recent studies demonstrated (i) a significant effect of mtDNA haplotype or other maternal cytoplasmic factors on the efficiency of NT; (ii) phenotypic differences between transmitochondrial clones pointing to functionally relevant nuclear-cytoplasmic interactions; and (iii) neutral or non-neutral selection of mtDNA haplotypes in heteroplasmic conditions. Mitochondria form a dynamic reticulum, enabling complementation of mitochondrial components and possibly mixing of different mtDNA populations in heteroplasmic individuals. Future directions of research on mtDNA in the context of reproductive biotechnology range from the elimination of adverse effects of artificial heteroplasmy, e.g. created by ooplasm transfer, to engineering of optimized constellations of nuclear and cytoplasmic genes for the production of superior livestock.

Nuclear-Cytoplasmic Interactions Affect In Utero Developmental Capacity, Phenotype, and Cellular Metabolism of Bovine Nuclear Transfer Fetuses1

“…It has been speculated that induced artificial heteroplasmy could be responsible for low SCNT efficiency [37], but two fetuses in resorption that yielded DNA and could be assayed for heteroplasmy in our study turned out to be homoplasmic for oocyte mtDNA. Mitochondrial functions are nevertheless extremely diverse and complex [38], and natural variation in the mitochondrial genome affects a variety of parameters in natural and assisted reproduction [39]. Smith et al [40] reported mtDNA effects on blastocyst rate in reconstructed mouse embryos; epigenetic effects seemed negligible in this model.…”

“…The observed differences are relevant for and could improve SCNT cloning efficiency and, if present in human, might have consequences for technologies such as ooplasmic transfer in assisted reproduction [39,45]. Our data point to complex oocyte cytoplasm-dependent epigenetic modifications and/or nuclear DNA-mtDNA interactions that could be functionally dissected by SCNT after controlling the nuclear genome of oocyte donors by repeated backcrossings [38], or by using groups of transmitochondrial individuals [46] as oocyte donors for the production of embryos, fetuses, and offspring in further studies.…”

A large-scale screening of the normalized mammalian mitochondrial gene expression profiles

“…A non-coding mitochondrial control region contains the main regulatory sequences for transcription and replication initiation, including a triplestranded D-loop. Though the great majority of mitochondrial proteins are encoded by the nuclear genome, mtDNA encodes 13 proteins involved in oxidative phosphorylation (for review see Hiendleder &Wolf, 2003 andFernandez-Silva et al, 2003). Individual mammalian mitochondrial-genome-encoded genes have demonstrated a number of other important biological functions, such as serving as histocompatibility antigens for the NADH dehydrogenase 1 (mt-Nd1) and cytochrome c oxidase subunit I (mt-Co1)-derived peptides (Loveland et al, 1990 ;Morse et al, 1996) or playing a role in protein folding for 12S and 16S ribosomal RNA (mt-Rnr1 and -Rnr2 ; Sulijoadikusumo et al, 2001).…”

The Influence of Nuclear Content on Developmental Competence of Gaur × Cattle Hybrid In Vitro Fertilized and Somatic Cell Nuclear Transfer Embryos1

“…In SCNT embryos, nuclear-mitochondrial interaction is complicated by the fact that the donor nucleus has to function properly in either a heteroplasmic environment consisting of two mitochondrial lineages or a homoplasmic environment consisting of a mitochondrial lineage unrelated to the nuclear genome. Heteroplasmy has been associated with poor developmental outcome in SCNT embryos and possibly with the phenotypic variation observed in cloned offspring [14]. Examination of heteroplasmic SCNT embryos produced from Bos taurus oocytes and donor cells of different mtDNA haplotypes indicates that the type of recipient cytoplasm affected the developmental ability of the embryo and the fetus, with changes in fetal phenotype and fetal cell metabolism being observed [15].…”