Glutamine a molecular indicator for embryo assessment and aneuploidy testing
Considerable efforts have been made to seek noninvasive methods for the assessment of embryo development and aneuploidy, but accurate, rapid, and efficient methods are still lacking. One of the most important reasons for this is the lack of a robust marker for embryo evaluation. The amino acid glutamine is of central importance in the metabolic and biosynthetic pathways in cells. Although classified as nonessential, abundant evidence suggests that glutamine may become a conditionally essential amino acid during periods of metabolic stress. Its intracellular levels are regulated both by the uptake of extracellular glutamine via specific transport systems and by its intracellular synthesis by glutamine synthetase. Glutamine is closely related to cell growth because it is one of the raw materials for protein synthesis and the provision of carbon and nitrogen for de novo purine and pyrimidine synthesis.
There have been many studies investigating the role of amino acids in embryo development. The addition of glutamine to embryo culture medium has beneficial effects on embryo development in mice, pigs, and humans. Glutamine is crucial to cell growth and development, and differences in glutamine consumption in embryos may reflect differences in embryo viability.
Aneuploidy is the principal genetic factor influencing embryo development, characterized by an abnormal number or structure of chromosomes, which is responsible for most embryo developmental arrest, failure to implant, and spontaneous abortions. Abnormal chromosomes in aneuploid cells can result in proteome and secretome alterations in human preimplantation embryos. Amino acid turnover was also affected by aneuploidy.
The aim of the study by Sui‑Bing Miao et al was to investigate the association of glutamine consumption in embryos with embryo quality and aneuploidy using gas chromatography–mass spectrometry (GC–MS) and preimplantation genetic testing (PGT) approaches. The results demonstrated that glutamine was a primary contributor to the classification of the good-quality and poor-quality embryos based on the orthogonal partial least-squares discriminant analysis model.
To determine whether glutamine consumption is associated with embryo quality and aneuploidy, a retrospective study was conducted in an in vitro fertilization center. Spent embryo culture media from patients undergoing assisted reproduction treatment and preimplantation genetic testing (PGT) were obtained on day 3 of in vitro culture. Embryo quality was assessed for cell number and fragmentation rate. PGT for aneuploidy was performed using whole genome amplification and DNA sequencing.
Glutamine levels in spent embryo culture media were analyzed by gas chromatography–mass spectrometry. The results demonstrated that glutamine was a primary contributor to the classification of the good-quality and poor-quality embryos based on the orthogonal partial least-squares discriminant analysis model.
Glutamine consumption in the poor-quality embryos was significantly higher than that in the good-quality embryos (P<0.05). A significant increase in glutamine consumption was observed from aneuploid embryos compared with that from euploid embryos (P<0.01).
The findings of the present study show that there is an increase in glutamine consumption in poor-quality and aneuploidy embryos, and the increased glutamine consumption is a compensatory mechanism to mitigate oxidative stress in embryos.
Aneuploidy, associated with chromosome instability, can lead to transcriptome and proteome alterations, which would induce cellular stress leading to proteotoxicity and dysregulated metabolism, replication, and mitosis. Under these conditions of stress, the demand for glutamine may substantially increase.
First, as unbalanced gene expression due to alterations in chromosome stoichiometry, aneuploid cells may experience proteotoxic stress, manifested as enhanced protein turnover in which glutamine plays a key role. In addition, proteotoxic stress is accompanied by protein synthesis, folding, and degradation, all of which are energy consumption processes and place extra demands on mitochondrial output. Hence, increased glutamine consumption may reflect the requirements for energy production and protein synthesis in aneuploid cells.
Second, to accommodate the energy burden in aneuploid cells, the number and activity of mitochondria are upregulated accordingly, accompanied by elevated reactive oxygen species, which are free radicals derived from molecular oxygen, usually as a byproduct of aerobic cellular metabolism in mitochondria. To maintain the redox balance, more glutamine may be required to be converted to GSH.
Finally, the accumulative reactive oxygen species and replication stress induced by aneuploidy are causes of DNA damage in the form of double-strand breaks or increased mutational load. DNA damage can elicit cellular signaling response initiating DNA repair, particularly in poor-quality human embryos. As glutamine physiologically functions to generate nucleotides, aneuploid cells need to enhance glutamine metabolism to facilitate DNA repair.
PGT using trophectoderm biopsy is currently the most widely used genetic test for identification of de novo aneuploidy in embryos in clinical in vitro fertilization (IVF). However, potential safety concerns regarding biopsy and restrictions to only those embryos suitable for biopsy pose limitations. The removal of TE cells is inherently traumatic and may lead to a decrease in implantation potential. In addition, there are so many procedures in PGT that it usually takes a few days to obtain testing results and the cost is high. Compared with invasive PGT, noninvasive methods are characterized by harmlessness, time efficiency, and low cost. Combining glutamine analysis of embryo culture media with morphological assessment may help better assess the reproductive potential of individual embryos in assisted reproductive technology. This would lead to an increase in the implantation rate and pregnancy rate per embryo transfer and facilitate single embryo transfer. Additional studies are required to provide further insight into the clinical value of glutamine and large-scale prospective randomized trials are needed to substantiate the results.
Source: Sui‑Bing Miao,Yan‑Ru Feng;Xiao‑Dan Wang; Reproductive Sciences (2022) 29:1721–1729
https://doi.org/10.1007/s43032-021-00812-y
