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.