Mitochondria, organelles found in cells, play a vital role during embryo development—working as specialized “powerhouses” that supply individual cells with energy and perform other critical cellular functions. Over the past decade, understanding of mitochondrial function has advanced, yet little is known about how variations in mitochondrial DNA (mtDNA), or the genetic information found within each mitochondrion, impacts pregnancy outcomes.
With up to one-third of in vitro fertilization (IVF) transfers of “chromosomally normal” embryos failing, this writer and fellow researchers specializing in preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD) investigated the role of mtDNA in early fetal development. We used a new clinical diagnostic test to establish a critical threshold of mtDNA quantity within embryos. Our study, published in the journal PLOS Genetics and also presented at the American Society for Reproductive Medicine (ASRM2015) annual Scientific Sessions, demonstrates that the level of mtDNA has a strong relationship with the ability of a human embryo to implant in the uterus following IVF.
A novel biomarker
Preimplantation development is an energy-demanding process, as cells rapidly divide in early pregnancy. Because mitochondrial functions are critical during the first few days of embryo development, the study examined the possible correlation between mtDNA content and female age, embryo chromosome status, viability, and implantation potential among 379 embryos. It also analyzed at what stage an embryo initiates replication of its own mtDNA and carried out a detailed assessment of the mitochondrial genome sequence, searching for mutations, deletions, and polymorphisms. A combination of microarray comparative genomic hybridization (aCGH), quantitative PCR, and next-generation sequencing (NGS) were used during the course of this study.
The study showed that embryos produced by reproductively older women contain higher levels of mtDNA (P=0.003) than those from their younger counterparts, implicating mitochondria in reproductive aging. Additionally, mtDNA levels were elevated in embryos with an abnormal number of chromosomes (aneuploid embryos), independent of age (P=0.025). Aneuploidy is responsible for the majority of miscarriages and serious genetic disorders. Healthy embryos that successfully implanted in the uterus and resulted in a live birth were associated with lower levels of mtDNA than those that failed to produce a viable pregnancy (P=0.007).
Importantly, an mtDNA quantity threshold was established, above which implantation was never observed. For embryos with quantities of mtDNA below the threshold, there was a better than average chance of producing a pregnancy. Embryos that had high levels of mtDNA—above the previously established threshold—did not implant successfully, thus leading to a 100 percent negative predictive value for these failed cases. The overall pregnancy rate in the group was 38 percent when mtDNA was not considered prior to transfer.
The results of this study suggest that increased mtDNA may be related to elevated metabolism and embryo viability; mitochondria may have a role in female reproductive aging as well as the genesis of aneuploidy. Of clinical significance, mtDNA represents a novel biomarker with potential value for IVF treatment, revealing chromosomally normal embryos incapable of producing a viable pregnancy.
There is an urgent need for new methods to improve the efficiency and success rates of IVF. These important findings show that mtDNA can help to highlight the embryos most likely to produce a pregnancy, allowing them to be given top priority for transfer to the woman’s uterus. The discovery of a new biomarker of embryo viability, independent of standard assessments such as morphology, is a rare event and of great clinical potential.
Subsequently, the predictive value of this threshold was confirmed in an independent blinded prospective study, indicating that abnormal mtDNA levels are present in 30 percent of non-implanting euploid embryos, but are not seen in embryos forming a viable pregnancy.
A new laboratory tool
A first validated clinical test is now available that can measure the quantity of mtDNA in trophectoderm biopsies, and be applied to any biopsy specimens sent for PGS or PGD analysis. It can only be offered for treatment cycles involving vitrification of all embryos after biopsy. The turnaround time for results is two weeks or less. By helping to identify the embryos with the greatest probability of forming a successful pregnancy, this new clinical test is predicted to provide a further improvement in implantation rates, above and beyond what is currently done in the laboratory setting using PGS on its own.
Established to work on the same trophectoderm samples used for PGS, this clinical test does not require any additional work in the embryology laboratory and, as a result, embryos do not need to be subjected to any interventions beyond those associated with routine chromosome screening. Since IVF can be a costly endeavor for those undergoing this medical approach to pregnancy, this new clinical test brings a simple and inexpensive approach that could help improve chances of successful fertility outcomes.
The success rate of IVF dramatically decreases with increasing female age. The higher levels of mtDNA observed with advancing age raise the question of whether mitochondria and their genome might play a direct role in the decline of female fertility with age. Increased levels of mtDNA may indicate compromised mitochondria that are unable to generate the expected amount of energy to support embryo development.
My colleagues’ and my study demonstrates a clear association between mtDNA quantity and the ability of a human embryo to implant in the uterus. A large (100 IVF cycles) blinded non-selection study is also being conducted in collaboration with the IVF clinic at the NYU Langone Medical Center to further assess the rate of outcome improvement if mtDNA quantification is combined with PGS.
