Early embryo ontogeny and parental genetic legacy: From developmental mechanisms to diagnosis and treatment

To sustain protein synthesis during early mammalian development, at least two conditions can justify the use of maternal mRNAs accumulated throughout oogenesis: i) the condensation and silencing of the maternal chromatin shortly before meiotic resumption at ovulation; II) the absence of major transcription of the embryonic genome during the first cleavage cycles. Therefore, during early embryogenesis, synthesis of new proteins largely depends on maternal factors (mRNA, microRNAs, and small interfering RNAs), which are then progressively replaced by embryonic transcripts. This exposes the embryo to the risk that defects in quantity and quality of RNAs produced during oogenesis affects the early cleavage stages, with potentially fatal consequences for embryo development.

Likewise, maternal proteins can also be stored during oogenesis and used from meiotic resumption until the early cleavage stages, while the maternal or embryonic genome remain silent. One of the first described genes of maternal origin showing an embryonic effect (referred to as maternal effect genes, MEGs) is MATER. In the human, biallelic mutations in MATER that cause decreased protein synthesis in oocytes are found in infertile women whose embryos arrest at the first cleavage stages. Together with other MEGs products, MATER shares a common and well-defined cell localization. These factors are organized in the subcortical maternal complex (SCMC), a protein aggregate distributed below the oocyte cortex and inherited by early embryos. The SCMC is involved in the regulation of multiple processes including meiotic spindle formation and positioning, regulation of translation, organelle redistribution, embryonic cleavage and epigenetic reprogramming. Notably, the unique localization of the SCMC suggests that not only is the correct expression of MEG-encoded proteins essential for early development, but also that the spatial arrangement of such factors may be finely regulated.

Another example of MEG, not associated to the SCMC, is TUBB8. The product of this gene is a specific β‐tubulin isotype present only in primates and exclusively found in oocytes, where it operates as the major structural component of the meiotic spindle. As such, TUBB8 is essential for ensuring cytoskeletal functions required for the completion of meiosis during fertilization and the correct accomplishment of the first mitotic divisions. Defective variants of TUBB8 detected in Journal Pre-proof 3 infertile women produce disruptive spindle‐assembly deficiencies during oocyte maturation and fertilization, as well as cleavage aberrations and arrest in early embryos.

Possible paternal effects on embryo development and newborn health are less defined and mainly associated with the sperm chromosome and centriolar constitution, due to the relative cellular contribution of the oocyte and sperm to the formation of the zygote. However, recent studies suggest that paternal factors in the form of RNAs may support early embryo development. In the mouse, several microRNAs are delivered to the sperm during post-testicular maturation and transit from the caput to the cauda of the epididymis. ICSI embryos generated with sperm collected from caput epididymis show diverse overexpressed regulatory factors, implant poorly and undergo postimplantation arrest. However, molecular and developmental defects of such embryos are completely rescued by microinjection of purified cauda-specific small RNAs. Further evidence on the role of sperm-derived small RNAs in shaping embryo development and offspring health is growing and will probably be soon extended to human infertility.

Overall, investigations on possible genetic causes of early developmental failure are progressively revealing the key role of maternally – and, to a less extent, paternally - inherited factors. Mutations affecting such factors can impair crucial regulatory networks and cause developmental arrest at any stage between fertilization an implantation. This influence likely extends beyond implantation, with a significant phenotypic spectrum ranging from preimplantation arrest to imprinting disorders in newborns. This highlights the clinical utility of such investigations, not only in preventing prolonged infertility but also in informing carrier couples about potential obstetrical and neonatal risks.

So far, classical single-gene association studies have been instrumental to generate these notions, offering a diagnostic explanation to selected cases of reproductive failure and initial insights in the genetics governing early human development. This has made assessment of genetic risk in reproduction already possible, although mainly in relation to monogenic traits. But, similar to other medical disciplines, novel more accessible genome‐wide sequencing and analysis techniques are bringing about a revolution in the genetics of infertility. The ability to screen thousands of genes with a predicted role in gamete competence and embryo development at the preconception stage is now technically feasible and available at sustainable costs. Carrier screening for recessive genetic disorders has become commonplace in many IVF settings, representing the most established and validated application of preconception genomics. The field is rapidly transitioning from gene-panel to exome sequencing-based approaches, enabling a more comprehensive genomic assessment of key genes involved in reproductive risk, as well as infertility and embryonic lethality.

To fully realize the potential of preconception genomic medicine, further efforts should focus on the development of large biobanks combining genomic data with well-curated electronic medical records (EMR) in infertility. Genome-wide association studies will then have the potential to generate several outcomes: to shed light on the relationship between biological mechanisms and phenotype, to enhance the precision of infertility diagnosis, to assess the genetic risk of prospective parents toward prenatal and postnatal complications, and to better inform clinical strategies and the development of personalized treatments. Soon, genetic diagnoses of infertility will be much more accurate and reliable, especially for cases previously classified as “idiopathic”. Couples will gain increased awareness of their reproductive risk, even before attempting to conceive. Choice on possible alternative treatments will be more informed. Women at risk of a premature decline of their fertility due to genetic factors will learn about their condition and have the opportunity to resort to fertility preservation solutions.

To fully unlock the potential of preconception genomics, several critical steps must be taken. First, robust statistical methodologies are needed to identify outlier cases in IVF for association studies. To date, most studies have selected cases in an arbitrary and subjective manner, often neglecting to account for the role of chance in poor IVF outcomes. Second, large genomic datasets are essential for conducting agnostic association studies and gene discovery, which will optimize the diagnostic yield for this field. Finally, comprehensive biobanks, enriched with ancestry diversity, are crucial for validating findings across different populations, ensuring the development of equitable and generalizable testing programs.

Increased knowledge of developmental regulatory networks has already the potential to re-tune the emphasis of reproductive studies from observational to truly experimental, offering an opportunity for a paradigm shift. Once in the spotlight of association studies, maternal genes products suspected to govern pivotal oocyte and early embryo processes can be specifically targeted to directly test their function. Indeed, harnessing recently developed technologies, these products can be rapidly degraded within the oocyte environment to assess the effects of their depletion.

In the longer term, novel avenues for treatment are envisaged. In preclinical studies, injection of functional TRIP13 cRNA in MI arrested oocytes obtained from a woman carrier of biallelic TRIP13 pathogenic variants resulted in phenotypic rescue, with treated oocytes showing normal subsequent maturation, fertilization and development to blastocyst stage. Ideally, in the future, exogenous delivery of key maternal factors to oocytes could be instrumental for groundbreaking achievements, such as to free infertility treatment from the yoke of maternal age. The detrimental effect of maternal age on embryo competence is largely caused by aneuploidies originating during female meiosis. A major source of such meiotic errors is the progressive depletion with age of specific proteins (e.g. shogushin 2) that normally assure sister chromatid cohesion. Supplying fully grown prophase-arrested oocytes with these proteins or their transcript could replenish the stock of cohesion proteins and prevent or mitigate the age-depended increase in meiotic errors occurring during the prophase-metaphase II transition.

Ultimately, revealing the genetics of early human development will revolutionize the treatment of infertility, opening to the full potential of precision medicine.

Source: Coticchio G, Cimadomo D, Capalbo A, Rienzi L, Early embryo ontogeny and parental genetic legacy: from developmental mechanisms to diagnosis and treatment, Fertility and Sterility (2024), doi: https://doi.org/10.1016/j.fertnstert.2024.10.043.