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How Did Scientists Edit the Genes of Human Embryos?

Scientists have successfully edited the genes of human embryos. What does this mean for the future of genetic engineering?

By
Sabrina Stierwalt, PhD,
August 8, 2017
Episode #249

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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.

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