Preimplantation embryo metabolism as a biomarker of embryonic viability and health

Abstract

Worldwide, 12% of couples suffer from infertility and therefore rely on assisted reproductive technologies to conceive. The current success rate of an initial IVF cycle is ~20%, leaving ~80% of couples unsuccessful after their first cycle. Although success rates increase with subsequent cycles, represented by an increase in cumulative pregnancy rates, IVF treatment is expensive and can have a significant impact on the psychological wellbeing of couples. It is therefore imperative that research is focused on increasing the success rate of initial IVF cycles to reduce the time to pregnancy.

The success of an IVF cycle significantly relies on the ability to select the most competent embryo from a patient’s cohort that has the greatest chance of establishing a viable pregnancy. Current embryo selection methods focus on the morphological and/or the morphokinetic development of the preimplantation embryo and preimplantation genetic testing can be utilized to ensure the embryo transferred is genetically normal. However, despite an embryo being regarded as high quality based on these selection methods, its success post-transfer is not guaranteed.

Blastocyst metabolism is a key regulator of embryo development and through metaboloepigenetic interactions, embryonic health, and its assessment represents an additional biomarker that may improve the accuracy of embryo selection. A comparison between blastocyst metabolism, morphology, time-lapse annotations, artificial intelligence, chromosomal status and transfer success was conducted. High glucose uptake and high amino acid consumption were found to be associated with human blastocysts of high viability according to current methods of selection. Further, glucose uptake was significantly higher in human blastocysts that established a viable pregnancy.

Genetically abnormal, or aneuploid, human embryos developed slower and were assigned lower viability scores. Additionally, blastocyst amino acid utilization appeared to be perturbed due to aneuploid associated stress. An analysis of vitrified mouse and human blastocyst pyruvate and/or glucose uptake post-warm was unable to provide a measure of viability. However, a morphological assessment of human blastocyst reexpansion post-warm revealed blastocysts with a greater degree of re-expansion were associated with higher live birth rates. Finally, using a mouse model to demonstrate how changes in embryo culture media can impact blastocyst metabolism and health, the addition of antioxidants (acetyl-L-carnitine, N-acetyl-L-cysteine and alpha-lipoic acid) to embryo culture media, individually and in combination, was investigated. A reduction in oxidative stress, regulation of blastocyst carbohydrate metabolism, lower NADH levels and improved blastocyst development were identified.

Together the data presented in this thesis provides a comprehensive analysis of human blastocyst physiology and lay the foundation for the development of an algorithm incorporating morphological, morphokinetic and metabolic biomarkers which could assist in the identification of the most viable and healthy blastocyst for transfer. Such an algorithm may also be used to validate future advances in embryo culture conditions. Therefore, these data provide an opportunity to significantly improve human IVF success rates and reduce the time to pregnancy.