New technology keeps some of these insects, which spread
malaria, from reproducing
BY TINA HESMAN SAEY 7:19AM, DECEMBER 18, 2015
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| Anopheles gambiae, one of the mosquito species known to transmit the germ that causes malaria. |
A new genetic tool may help wipe out Africa’s main
malaria-carrying mosquitoes. It’s known
as a “gene drive.” In tests, it sterilized Anopheles gambiae mosquitoes. In
other words, it kept them from reproducing.
A gene is the basic package of genetic instructions that
tells cells how to build some protein. Proteins are the chemicals that do much
of the work of cells. Gene drives are pieces of DNA that have been tweaked by
scientists. These snippets of DNA have been designed to find a target gene,
slice into it and then insert themselves. This cut-and-paste system alters the
function of the original gene. And it is self-propagating, which means that it
makes more of itself.
Other human-tweaked (or engineered) genes don’t reproduce
themselves. A gene drive’s ability to make more of itself helps it get into
more mosquitoes than would a regular inserted gene.
This is the second gene drive aimed at knocking out malaria.
The first, announced two weeks earlier, works quite differently. It stops
mosquitoes from transmitting the parasite that causes malaria. The newer gene
drive instead eliminates the mosquitoes themselves by making it impossible for
females to make new baby mosquitoes.
Scientists reported their achievement December 7 in the
journal Nature Biotechnology.
Technique appears potent
Most genes come in two copies. One copy comes from the
mother. The other comes from the father. So each of a parent’s genes has only a
50-50 chance of getting passed on to a kid.
But like Star Trek’s Borg, gene drives become part of every
unaltered target gene they encounter. These ambitious bits of DNA break
standard inheritance rules. With the new technique, 76.1 percent to 99.9
percent of the offspring inherited the drive. As a result, the edited genes
“drive” themselves quickly through populations.
Austin Burt first conceived of the idea for gene drives in
2003. This evolutionary geneticist works at Imperial College London, in
England. For more than a decade, gene drives remained mostly just an idea. But
this year, thanks to precision molecular “scissors” known as CRISPR/Cas9, four
gene drives were put to use in several labs. These include the two in
mosquitoes.
“They all work terrifically,” says George Church. He’s a
geneticist at Harvard University in Boston, Mass.
Cas9 is a DNA-cutting enzyme borrowed from bacteria.
Researchers can design genetic cousins of DNA — molecules known as RNA — to
guide the enzyme to desired genes.
Not ready for prime time
Church is pleased to see that this latest mosquito gene
drive works. Still, he says, further tweaks may be needed before it is ready
for release into mosquitoes in the wild.
Researchers also may want to combine approaches. They might
first release a gene drive that would prevent mosquitoes from carrying malaria.
Later, they might release another to control mosquito reproduction, he
suggests.
In the new study, Burt and colleagues first used CRISPR/Cas9
and another type of gene editor known as TALENs. These disrupted each of three
genes that are very active in the egg-making organ of mosquitoes. Females
carrying two copies of any one of the three disrupted genes were sterile. Once
the researchers had confirmed that messing with these genes kept the mosquitoes
from reproducing, the team then built gene drives to insert them into the
genes.
Gene drives that interfere with reproduction make some
people nervous. Such drives could make species go extinct. Many people would
not miss mosquitoes. But no one really knows what getting rid of the insects
might do to their ecosystem — the animals, plants and other living organisms in
their neighborhood. For instance, some of those neighbors might rely on
mosquitoes as lunch.
This gene drive also has some technical glitches. That means
it won’t be the final version used to control wild mosquitoes. But the
scientists are hopeful that future gene drives could cut populations of malaria
mosquitoes dramatically, says study coauthor Tony Nolan. He’s a molecular
biologist, also at Imperial College London. The public-health community needs new
approaches for mosquito control, he says — “and this is a promising one.”
Cas9 An enzyme
that geneticists are now using to help edit genes. It can cut through DNA,
allowing it to fix broken genes, splice in new ones or disable certain genes.
Cas9 is shepherded to the place it is supposed to make cuts by CRISPRs, a type
of genetic guides. The Cas9 enzyme came from bacteria. When viruses invade a
bacterium, this enzyme can chop up the germ's DNA, making it harmless.
cell The smallest
structural and functional unit of an organism. Typically too small to see with
the naked eye, it consists of watery fluid surrounded by a membrane or wall.
Animals are made of anywhere from thousands to trillions of cells, depending on
their size.
bacterium (plural bacteria)
A single-celled organism. These dwell nearly everywhere on Earth, from
the bottom of the sea to inside animals.
CRISPR An
abbreviation — pronounced crisper — for the term “clustered regularly
interspaced short palindromic repeats.” These are pieces of RNA, an
information-carrying molecule. They are copied from the genetic material of
viruses that infect bacteria. When a bacterium encounters a virus that it was
previously exposed to, it produces an RNA copy of the CRISPR that contains that
virus’ genetic information. The RNA then guides an enzyme, called Cas9, to cut
up the virus and make it harmless. Scientists are now building their own
versions of CRISPR RNAs. These lab-made RNAs guide the enzyme to cut specific
genes in other organisms. Scientists use them, like a genetic scissors, to edit
— or alter — specific genes so that they can then study how the gene works,
repair damage to broken genes, insert new genes or disable harmful ones.
disrupt (n.
disruption) To break apart something; interrupt the normal operation of
something; or to throw the normal organization (or order) of something into
disorder.
ecosystem A group of
interacting living organisms — including microorganisms, plants and animals —
and their physical environment within a particular climate. Examples include
tropical reefs, rainforests, alpine meadows and polar tundra.
enzymes Molecules
made by living things to speed up chemical reactions.
evolutionary genetics
A field of biology that focuses on how genes — and the traits they lead
to — change over long periods of time (potentially over millennia or more).
People who work in this field are known as evolutionary geneticists.
gene (adj. genetic)
A segment of DNA that codes, or holds instructions, for producing a protein.
Offspring inherit genes from their parents. Genes influence how an organism
looks and behaves.
gene editing The
deliberate introduction of changes to genes by researchers.
gene drive A
technique for introducing new bits of DNA into genes to change their function.
Unlike other such genetic engineering techniques, gene drives are
self-propagating. That means they make more of themselves, becoming part of
every unaltered target gene they encounter. As a result, they get passed on to
more than 50 percent of an altered animal’s offspring, “driving” themselves
quickly into populations.
genetic Having to do
with chromosomes, DNA and the genes contained within DNA. The field of science
dealing with these biological instructions is known as genetics. People who
work in this field are geneticists.
genetic engineering
The direct manipulation of an organism’s genome. In this process, genes
can be removed, disabled so that they no longer function, or added after being
taken from other organisms. Genetic engineering can be used to create organisms
that produce medicines, or crops that grow better under challenging conditions
such as dry weather, hot temperatures or salty soils.
malaria A disease
caused by a parasite that invades the red blood cells. The parasite is
transmitted by mosquitoes, largely in tropical and subtropical regions.
molecule (adj. molecular) An electrically neutral group of
atoms that represents the smallest possible amount of a chemical compound.
Molecules can be made of single types of atoms or of different types. For
example, the oxygen in the air is made of two oxygen atoms (O2), but water is
made of two hydrogen atoms and one oxygen atom (H2O).
organ (in biology)
Various parts of an organism that perform one or more particular functions. For
instance, an ovary is an organ that makes eggs, the brain is an organ that
interprets nerve signals and a plant’s roots are organs that take in nutrients
and moisture.
parasite An organism
that gets benefits from another species, called a host, but doesn’t provide it
any benefits. Classic examples of parasites include ticks, fleas and tapeworms.
population (in
biology) A group of individuals from the same species that lives in the same
area.
proteins Compounds
made from one or more long chains of amino acids. Proteins are an essential
part of all living organisms. They form the basis of living cells, muscle and
tissues; they also do the work inside of cells. The hemoglobin in blood and the
antibodies that attempt to fight infections are among the better-known,
stand-alone proteins.Medicines frequently work by latching onto proteins.
RNA A molecule that
helps “read” the genetic information contained in DNA. A cell’s molecular
machinery reads DNA to create RNA, and then reads RNA to create proteins.
sterile An
adjective that means devoid of life — or at least of germs. (in biology) An
organism that is physically unable to reproduce.
TALEN An acronym for
transcription activator-like effector nucleases. These are a class of enzymes
that may be used as molecular “scissors” by some gene-editing processes.
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