Our genetic code is the foundation of who we are. The DNA and RNA molecules that carry our genetic information dictate our past (like our innate skills), our present (like the color of our eyes, hair, and skin), and our future (our predisposition toward genetic diseases). But what if we could edit those genes, picking and choosing which ones we want to keep and which ones we want to edit out? Would we even want to?
Scientists using the CRISPR-Cas9 system made headlines last week for being the first to cleanly “fix” a mutation known to cause a heart disorder in regular human embryos. How did they accomplish such a feat, and what does this mean for the future of genetic engineering?
What is CRISPR-Cas9?
Since we’ve discussed CRISPR-Cas9 here at Everyday Einstein before, I’ll only summarize how it works. The CRISPR system works as a type of gene-sized scissors that allows us to snip out unwanted parts of our DNA and replace them with healthy or otherwise preferred copies.
After scientists noticed that the DNA of bacteria and single-celled organisms was structured in a repeating pattern called “clustered regularly interspaced palindromic repeats” (or CRISPR), further research revealed that the nonrepeating spacers between each section of the pattern were copies of the DNA of harmful viruses that had previously posed a threat. The bacteria and simple organisms were keeping a log on the weaknesses of their past foes in case they should encounter them again.
For CRISPR to act effectively as an immune system for the bacteria, it would also need a way of transporting this stored information through the cell. That is where the Cas proteins come in, with Cas-9 being one of the most well understood of the bunch. The Cas proteins hitch a ride with RNA molecules that have been encoded with the intruder DNA looking for a match elsewhere in the cell. Once a match is found, the RNA molecule and Cas protein work together to keep the virus from replicating any further by latching onto the viral DNA and chopping it up.
Can we edit the genetics of human embryos?
In 2012, molecular biologists revolutionized gene editing by demonstrating that they could use the CRISPR-Cas9 system, including a programmable RNA molecule, to find, snip, and replace not just viruses but any sequence of genes. A form of “edit-search-replace” for DNA. Since that discovery, scientists have already been able to use CRISPR to eliminate Hepatitis B and HIV in genomes in human cells.
Last week’s announcement takes the practice of genetic engineering a huge step further by moving beyond human cells to human embryos. A team of scientists at Oregon Health and Science University used CRISPR-Cas9 to correct a genetic mutation linked to hypertrophic cardiomyopathy, a genetic disorder that results in a thickening of the muscles of the heart with no apparent cause and which can lead to heart failure.
CRISPR-Cas9 was applied to sperm from a carrier of the disease as they were injected into eggs from non-carrier donors. The scientists then observed as the CRISPR-Cas9 scissors went to work removing the portion of the DNA with the mutation and replacing it with a healthy copy of the sequence from the maternal genes. Two key aspects of their study was first, the addition of the CRISPR-Cas9 to the egg at the same time as the sperm rather than hours later as had been done in previous studies, and second, the use of a short-lived version of CRISPR that would not hang around and cause potentially unwanted future edits.
In their paper titled ‘Correction of a pathogenic gene mutation in human embryos’, the authors note that the editing worked in 36 of their 54 regular human embryos. (Earlier work by Chinese scientists involved the successful editing of only immature embryos that were not capable of surviving until birth.)
What laws are there concerning the editing of the human genome?
The embryos used in the experiment at OHSU were not implanted in a woman’s uterus, and many more experiments must be run, using different mutations and different donors, for example, before that can be done. But for would-be parents who fear passing on a genetic disease, the possibilities the study suggests are a potential game changer. Those with access t o the gene editing tool could remove any doubt that they had passed on genetic predispositions toward sickle cell anemia, cystic fibrosis, or even breast cancer.
Doctors are already able to create embryos in vitro and then select only the ones that do not carry a specific genetic illness for implantation, a process called “preimplantation genetic diagnosis”. However, those embryos not chosen for implantation are often discarded.
While there is an obvious appeal to be able to control the disease-causing genetic mutations we pass on as parents, that level of control raises a wealth of concerns. Will gene editing eventually be used to create “designer babies” with more desirable traits? Who decides which qualities are desirable?
Currently, in the United States, research in genetic engineering is allowed but regulations prevent the US government from funding any research involved in the genetic engineering of human embryos, including studies like those at OHSU. Opponents of the federal government ban suggest it only moves the research into the realm of private industry while others believe it is an important limitation as the law tries to keep pace with scientific advancement.
While the answers to many of the questions surrounding the ethics of genetic engineering may be murky, one thing is clear. A fair and ethical path forward can only be found if scientists and law makers from diverse cultural backgrounds are included in the conversation.
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.
Image courtesy of shutterstock.
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