Previous studies in both animals and humans suggest that qualitative and quantitative defects in mitochondrial DNA (mtDNA) are associated with abnormal oocyte maturation, fertilization, and implantation failure. Synthesis of adenosine triphosphate (ATP) by maternally inherited mitochondria provides the enormous amounts of energy needed to support oocyte maturation, fertilization, and embryo development. When supplies of ATP provided by mitochondria are inadequate, the chromosomal segregation process in meiosis is adversely affected, susceptibility to aneuploidy is increased, and there is increased risk of a number of mitochondrial diseases. The severity of many mitochondrial diseases is dependent on the ratio of mutant to wild-type mtDNA molecules; this ratio is called heteroplasmy. Recent studies have shown that mitochondrial haplogroup variants may mediate the chromosomal segregation process and oocyte cell growth through a differential level of ATP production.
To investigate the association between aneuploidy and patterns of mtDNA segregation in human oocytes, the level of heteroplasmy in the 3 resulting products of female meiosis, polar body 1 and 2, and corresponding oocytes was assessed by examining the segregation pattern of mutant and wild-type mtDNA molecules in the hypervariable region I (HVRI) of the D-loop region. The possible association between aneuploidy and specific mitochondrial haplogroups was also investigated. Whole genome amplification was used to amplify the DNA from 122 polar bodies (PBs) and 51 oocytes of 16 infertile patients undergoing intracytoplasmic sperm injection cycles in combination with preimplantation genetic screening. An aliquot of the amplified product was used to assess aneuploidy, and another aliquot was processed for mtDNA sequencing.
The efficiency of amplification and sequencing of the HVRI was 75.4% and 63%, respectively, in PBs and 100% in oocytes. Comparison of the mtDNA sequences from blood of the individual donors with the matching oocytes showed full correspondence of polymorphisms, whereas the degree of concordance in PBs dropped to 89.6%. Haplogroups for each of the 16 patients were inferred. Five different haplogroups (H, J, T, U, and K) were observed in macrohaplogroup R. Of the 89 PBs diagnosed from the 13 patients belonging to macrohaplogroup R, 23 were euploid and 66 aneuploid. The incidence of total anomalies in all analyzed PBs was significantly lower in haplogroup H (6.5%) than in haplogroups J and T (17.6% and 13.4%, respectively; P < 0.001). Hypoaneuploidy occurred more frequently than hyperaneuploidy in haplogroup J (10.5% vs 7.0%, P < 0.05). Eighty-one percent of PBs in the 3 patients belonging to haplogroup N were aneuploid, and rates of chromosome hypoaneuploidy and hyperaneuploidy were similar (4.7% and 4.3%, respectively).
These findings suggest that the presence in PBs of mtDNA base changes not found in the corresponding oocytes may reflect a selection mechanism against severe mtDNA mutations, while permitting a high mtDNA evolution rate that could result in bioenergetic diversity. The differential susceptibility to aneuploidy among some haplogroups strongly supports this hypothesis.