Genetics and ART: Selection versus correction
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Following the completion of the Human Genome Project in 2003 and major technologic advancements in the subsequent years, the field of human genetics became the focal point of convergence for several distinct but interrelated disciplines: bioinformatics, computational biology, and sequencing technologies. As the result, individual human genomes can now be sequenced at a single base pair level, and with higher fidelity, at a fraction of the original cost and at a much faster speed.
This molecular progress, however, has not been accompanied by an equivalent clinical progress, because in a significant number of cases a defined and predictable clinical phenotype cannot be attributed to a detected molecular genotype. This has resulted in an overabundance of variants of uncertain significance. Variable expressivity, incomplete penetrance, epigenetics, mosaicism, and the polygenic nature of many human traits further complicate reliable interpretation and prognostication of the colossal amount of molecular genetic data that are being generated by the above-mentioned technologic advances.
Considering these limitations, at this juncture it is crucial to acknowledge that any attempts to prematurely commercialize these preclinical and research studies (such as polygenic risk scores for embryos) are perilous and have the potential to cause significant harm in terms of unnecessary stress and anxiety for intended parents as well as the potential for yet-unmapped societal and legal implications.
However, it is just a matter of time until more accurate clinical phenotyping catches up with molecular genotyping. As we get closer to this next historic milestone, precision medicine in the postnatal life (with regard to both diagnostics and therapeutics) and preimplantation genetic testing (PGT) at the prenatal stage for a much wider spectrum of conditions—including both monogenic and polygenic traits—may indeed become a reality.
The potential of germline editing
Specifically regarding PGT (which requires IVF), it is important to recognize that due to the limited and nonrenewable endowment of human oocytes (ovarian reserve), combined with the detrimental impact of advancing age on the quality of the remaining cohort as manifested by a higher risk of aneuploidy, the current clinical practice of trying to “select” a nonaffected embryo can be very inefficient. As a result, the intended parents pursuing such treatments may need to undergo multiple cycles of ovarian stimulation and oocyte retrieval.
A potential solution for genes associated with known diseases is the prospect of remedial germline editing by CRISPR–Cas9 technology or its future descendants. This would take advantage of the existing embryos to try to “correct” the defective gene instead of trying to “select” a normal embryo. These technologies are still in the early stages of development and are remotely distant from clinical applications. On the other hand, although germline gene editing, if actualized, would be a monumental breakthrough in the history of genetics and medicine, we must be cognizant of its serious legal, societal, and ethical ramifications, which are currently unknown. Furthermore, even at the biologic and technical level, the technology still is not advanced enough to reliably rule out off-target modifications, and the unintended clinical consequences of the on-target corrections have not been studied either.
Regulation of genetic modifications
Due to these myriad concerns and the lack of an existing appropriate regulatory framework and oversight for such interventions, current US law (since December 2015, through provisions in annual federal appropriations laws passed by Congress and renewed annually thereafter) bars the US Food and Drug Administration from considering any clinical trial application “in which a human embryo is intentionally created or modified to include a heritable genetic modification.” Notably, this moratorium also prohibits mitochondrial replacement technology (MRT), which is a less controversial and relatively better-studied innovation.
Mitochondrial genetic disorders caused by the mutations in mitochondrial DNA (versus nuclear DNA) are amenable to a specific treatment strategy aimed at substituting the defective maternal mitochondrial genome with the mitochondrial genome of an unaffected donor oocyte. This can be achieved via either pronuclear transfer, which involves isolation and transfer of the male and female pronuclei from an affected embryo to an enucleated normal donor embryo, or maternal spindle transfer, which involves isolation and transfer of the metaphase II spindle complex of an affected oocyte to an enucleated disease-free donor egg. It is noteworthy that in 2015 in the United Kingdom, Parliament expanded the definition of “permitted eggs and embryos” to include those “where unhealthy mitochondrial DNA is replaced by healthy mitochondrial DNA from a donor.” This thereby allows the UK Human Fertilisation and Embryology Authority to formally direct and oversee clinical trials in MRT.
Summing up
Although the future of assisted human reproduction cannot be clearly outlined at this time, it is likely to be radically different from the current state given these emerging applications at the intersection of ART and diagnostic and therapeutic genetics. To ensure that exploring this uncharted territory will ultimately be in the interest of humankind and civilization, proper regulatory oversight—after careful consideration of all ethical, societal, and legal implications—needs to be developed for all preclinical and clinical research in this field. Participatory public engagement must be an integrated part of this process. ●
WHAT THIS EVIDENCE MEANS FOR PRACTICE
The field of human genetics has already transformed medicine. However, the convergence of the interrelated disciplines of bioinformatics, computational biology, sequencing technologies, and CRISPR–Cas9 technology is creating incredible new advances that will bring great benefits but also major societal challenges.
The field of human genetics has already transformed medicine. However, the convergence of the interrelated disciplines of bioinformatics, computational biology, sequencing technologies, and CRISPR–Cas9 technology is creating incredible new advances that will bring great benefits but also major societal challenges.