A Tool for Editing the
Genome: Supplemental
information
Background
CRISPR refers to a pattern found in some species’ DNA. For
example, bacteria exposed to viruses add strands of DNA that act
like a “recording of exposure” to the particular viruses. These
CRISPR ‘recordings’ have proven helpful to cheese and yogurt
producers; most major U.S. producers compare various strains of
the bacteria to select those with the most preferred immunities to
make their cheese or yogurt1.
Scientists have leveraged this insight to create a tool that may be
able to not just identify but also ‘edit’ genes, in organisms well
beyond bacteria. CRISPR DNA strands are synthesized into
matching RNA; the RNA is combined with a ‘guide’ that helps it
find DNA that matches it in the target cell. This CRISPR tool ‘snips’
or removes the DNA segment. Scientists can then replace the
segment with alternate DNA. For example, a segment of DNA
associated with an inherited disease might be removed and
replaced with a segment that is not associated with that disease.
This could improve the health of animals as well as plants,
suggesting potential applications in the food supply.
1
http://www.ncbi.nlm.nih.gov/pubmed/22224556
With this technique, scientists can replace multiple genes at one
time, speeding up a process which used to entail only one ‘edit’
per generation of an organism. Changes made with this technique
may be permanent and can be passed down to future
generations.
Safety
This technology allows for more targeted (though not perfect)
genome editing at a lower cost than many other technologies.
Scientists also appreciate the relative ease of the technique –
many more science professionals can use this technique than
many of the other techniques for genome editing.
This technique also offers more specificity than many of the other
techniques, which may mean fewer inadvertent changes to the
genome. However, the technology is still quite new and
occasionally cuts DNA in unintended places when the target gene
is too similar to other sequences in the genome.
Unlike other techniques, the CRISPR leaves none of the specific
markers that other forms of genetic modification do; this makes
monitoring of the use of the technology more difficult. This may
also complicate governance and regulation of the tool’s use.
Mosquitos
The Aedes aegypti mosquito is the carrier for dengue fever, which
the World Health Organization calls the “world’s fastest-growing
vector-borne viral disease”. It also carries other viruses such as
chikungunya and yellow fever. These diseases infect hundreds of
millions of people and kill an estimated 50,000 people each year.
This mosquito is expected to be present in the United States as
climates change.
These mosquitos thrive in urbanized areas and are very difficult to
control because their eggs can survive without water for months.
If a population of mosquitos is eradicated, a rainfall months later
can bring it roaring back as eggs hatch. There is also evidence that
some strains of this mosquito are developing pesticide resistance.
In March 2015, scientists from Rockefeller University published a
study showing that they were able to use the CRISPR tool to
introduce precise, specific (if not practical; the changes were all
made to demonstrate the technology) mutations in the Ae.
aegypti in the laboratory. It is hoped that use of CRISPR in the
mosquitos will eventually reduce the spread of these diseases.
Engineered mosquitos have been released in the Cayman Islands
and Brazil.
 Potential problems: Despite being one of the most wellstudied mosquitos, little is known about its ecology. It is
unclear how large-scale changes that might eliminate this
species in a local area would affect the food chain and the
birds, frogs, and fish that eat this mosquito. If a human were
to be bitten by a modified mosquito, it is highly unlikely that
any modified DNA would be transferred, although the FDA is
confirming this claim.
 Potential benefits: Human health would greatly improve if
these diseases were no longer spread by this mosquito. Of
the 3,500 species of mosquito, only a few hundred bite or
bother humans, so some scientists argue that eliminating
one species would have minimal impact on the environment
or the food chain. We would also no longer be harming
other species by spraying pesticides.
Poison Ivy/Oak
Poison ivy and poison oak are irritating, noxious plants that cause
a painful rash to most people who make skin contact. Poison ivy
and oak are irritating to people because of a chemical urushiol
(ur-ROO-she-all) the plants secrete in their sap.
The exact reason for the chemical is unknown, but it might help
the plants retain water or help seal wounds to the leaves when
eaten. These plants are an important food source for many
animals, including deer, raccoons, and many birds. These animals
are not allergic to the chemical.
These plants and their relatives affect 50 million Americans every
year. Hundreds of firefighters in California who fight summer
forest fires breathe in the smoke, which contains the irritant, and
their lungs are so affected they are unable to work.
Some scientists are concerned that climate change is making
these plants stronger. Increased carbon dioxide levels tell the
plant make a more potent version of urushiol and to grow bigger
leaves.
 Potential problems: The exact function for urushiol in these
plants is unknown. If it helps the plants retain water or seal
wounds after being eaten, the plants may die off faster than
anticipated. This would help achieve the goal of reducing
plant numbers, but this might remove an important food
source for many animals.
 Potential benefits: Removing the irritant from the plants
could drastically reduce the number of Americans affected
by poison ivy or oak every year. Beyond the direct benefits,
this may even have economic benefits: poison oak and
poison ivy account for an estimated ten percent of lost work
time in the U. S. Forest Service. In fact, hundreds of fire
fighters who battle forest fires in California can develop
serious inflammation of respiratory mucous membranes
from breathing in smoke containing urushiol. Because of the
serious economic impact due to lost employment time,
poison oak "injuries" cost approximately one percent of the
California’s workers' compensation budget.
Sources and Read More (listed by date published)
On the CRISPR technique:
Stan J. J. Brouns. “A Swiss Army Knife of Immunity”. Science 17 August 2012: 337 (6096), 808-809.
[DOI:10.1126/science.1227253]Pennisi, E.
“The CRISPR Craze”. Science 23 August 2013: 341 (6148), 833-836. [DOI:10.1126/science.341.6148.833]
Pollack, Andrew. “A Powerful New Way to Edit DNA”. (3 March 2014). New York Times. <
http://www.nytimes.com/2014/03/04/health/a-powerful-new-way-to-edit-dna.html> [12 June 2015].
“CRISPR reverses disease symptoms in living animals for first time.” (31 March 2014). Genetic Engineering
and Biotechnology News. <http://www.genengnews.com/gen-news-highlights/crispr-reverses-diseasesymptoms-in-living-animals-for-first-time/81249682/> [12 June 2015].
On safety and ethical considerations:
Trafton, Anne. “Genome editing becomes more accurate.” (22 July 2013). MIT News.
<http://mcgovern.mit.edu/news/news/genome-editing-becomes-more-accurate/> [12 June
2015].Gallagher, James.
“‘Designer babies’ debate should start, scientists say”. (19 January 2015). BBC News.
<http://www.bbc.com/news/health-30742774> [12 June 2015].
Vogel, Gretchen. “Embryo engineering alarm.” Science 20 March 2015: 347 (6228), 1301.
[DOI:10.1126/science.347.6228.1301]
Stockton, Nick. “America needs to figure out the ethics of gene editing now.” (23 April 2015). Wired.
<http://www.wired.com/2015/04/america-needs-figure-ethics-gene-editing-now/> [12 June 2015].
Engineered mosquitos have been released in the Cayman Islands and Brazil.
http://www.cbsnews.com/news/fda-debates-releasing-genetically-modified-mosquitoes-into-floridakeys/
Credits and rights
Copyright 2015, AAAS, Washington, D.C. Published under a Creative Commons Attribution--‐Noncommercial--‐
ShareAlike license: http://creativecommons.org/licenses/by--‐nc--‐sa/3.0
This activity was created as a collaboration of the Multi-Site Public Engagement with Science Synthetic Biology project. This project was supported by the National Science Foundation under Award
Number: 1421179; Any opinions, findings, and conclusions or recommendations expressed in this
program are those of the author and do not necessarily reflect the views of the Foundation.