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Genetically Modified Humans?
How Genome Editing Works
by Tanya Lewis, Staff Writer | April 24, 2015 11:06am ET
Chinese scientists have edited the genomes of human embryos for the first time,
confirming a storm of rumors and igniting an ethical debate.
Researchers at Sun Yat-sen University in Guangzhou, China, used an experimental
gene-editing technique to modify a gene in human embryos that causes a
fatal blood disorder. The procedure, which was done in nonviable embryos, was only
partially successful, Nature News reported.
The study, which was published online Monday (April 18) in the journalProtein & Cell,
has raised questions in the scientific community over the risks of the pro cedure and the
ethics of its use in humans. [Unraveling the Human Genome: 6 Molecular Milestones]
"Their study should be a stern warning to any practitioner who thinks the technology is
ready for testing to eradicate disease genes," George Daley, a stem-cell biologist at
Harvard Medical School in Boston, told Nature News.
The technique involves an enzyme complex known as CRISPR/Cas9, found in many
bacteria. CRISPR (short for "clustered regularly interspaced short palindromic repeats"),
is a short, repeated sequence of RNA that matches the genetic sequence the
researcher wants to modify. It works in concert with Cas9, an enzyme that cuts DNA like
a pair of molecular scissors.
First, the CRISPR/Cas9 complex searches through the cell's DNA until it finds and binds
to a sequence that matches the CRISPR, said John Reidhaar-Olson, a biochemist at
Albert Einstein College of Medicine in New York, who was not involved in the study.
Then, the Cas9 cuts the DNA. Lastly, the cell repairs the cut, in this case by inserting a
piece of DNA supplied by the experimenter, Reidhaar-Olson told Live Science.
In bacteria, the complex provides resistance against foreign DNA, such as plasmids
(small, circular pieces of DNA) and phages (viruses that infect bacteria). But since 2013,
scientists have used the system to edit genesin the cells of other species, including
adult human cells andanimal embryos. But this is the first time it has been used to
modify human embryos.
In the study, Junjiu Huang, a genetics researcher at Sun Yat-sen University, injected the
CRISPR/Cas9 complex into human embryos in order to repair a gene for Beta
thalassaemia, a potentially fatal blood disorder that reduces the production of
hemoglobin. The embryos, which were obtained from local fertility clinics, could not
result in live births because they had been fertilized by two sperm, which prevents the
embryos from developing properly.
The researchers performed the procedure on 86 embryos, and waited four days to allow
the gene editing to take place. Seventy-one of the embryos survived, and the
researchers genetically tested 54 of them.
Only 28 embryos were spliced successfully, meaning the faulty gene was removed, and
just a few of those incorporated the healthy gene in its place. The success rate would
need to be closer to 100 percent before the technique could be used in viable human
embryos, the researchers said.
The procedure also caused worrisome mutations in other parts of the genome — and at
a much higher rate than in mouse embryos or adult human cells undergoing the same
procedure. These mutations could have detrimental effects on cells, which is one of the
big concerns about gene editing.
Because of these safety issues, the use of this technique in humans poses serious
ethical questions, some scientists say. The editors of the journals Nature and Science
refused to publish the study for such ethical reasons, Huang told Nature News.
Discussion:
1. Read only the title of the article first. Then write at least a couple of sentences
explaining what you think the article will be about.
2. Read the article and then write a 5 sentence summary.
3. Did their experiment work? What needs to happen before it could ever be tested
in viable human embryos?
4. Write at least 5 sentences about your feelings on the topic of using such
techniques to genetically modify human embryos to remove disorders.