Mitochondrial DNA variants in early development and ART

In this thesis, we aimed at unravelling the reasons why children born after assisted reproductive technologies (ART) show an increased risk of being born with a lower birth weight and developing mild cardiometabolic abnormalities later in life.
In the first part of this work, we hypothesized that variants in the mitochondrial DNA (mtDNA) could play a role in the health outcome of ART children. To test this, we compared the mtDNA variants of children born after ART and after spontaneous conception (SC) and found that ART children show higher variant loads of protein- and rRNA-coding mtDNA variants. Logistic regression analysis showed that this mtDNA genotype is associated with a lower birth weight percentile in the SC children but is sex-dependent and influenced by type of culture medium in ART children. Next, we explored the origin of this mtDNA genotype by investigating the maternal inheritance in ART and SC mother-child pairs and the role of ovarian stimulation (OS) on the appearance of de novo mtDNA variants in the next generation. The latter was investigated using oocytes of young fertile donors from natural menstrual cycles and after OS. We found that the mtDNA genotype shown in ART children resulted from both maternal transmission and de novo mutagenesis, which in turn is associated with maternal ageing and not with OS.
To further elucidate the origin and development of these increased risks, we reasoned that mtDNA variant mosaicism on the single-cell level could be the missing link between the mtDNA differences observed in ART children and the higher risk of an adverse health outcome. This led to the second part of the thesis, where we present a study on the timing of appearance of mtDNA mosaicism in human development. We explored mtDNA variant mosaicism in oocytes, preimplantation embryos, adult bulk tissues and adult single cells. We identified several types of variants based on whether they recurred across cells of the same individual or were unique to one cell or bulk sample. These variants differed significantly from each other in mtDNA variant load and location: recurrent variants were more frequently found at higher loads and in less pathogenic regions of the mtDNA while unique variants were found at (very) low loads and are locating in potentially pathogenic mtDNA regions.
The overall conclusion of this thesis is that ART children indeed show a different mtDNA genotype than SC children and this associates with lower birth weight. The exact mechanisms on how these mtDNA findings suggest that mtDNA mosaicism could play an important role in thisvariants affect the pre- and postnatal development are still to be further investigated however, our findings suggest that mtDNA mosaicism could play an important role in this.