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RNA interference:
2006 Nobel Prize in Physiology or Medicine to
for their discovery of
In 1998, the American scientists Andrew Fire (MIT)
and Craig Mello (Harvard) published their discovery
of a mechanism that can degrade mRNA made by a specific gene. This
mechanism, RNA interference, is activated when RNA molecules occur
in double-stranded form in the cell. Double-stranded RNA activates a
biochemical machinery which degrades those single stranded mRNA
molecules carrying a genetic code identical to that of the
double-stranded RNA. When such mRNA molecules disappear, the
corresponding gene is silenced and no protein of the encoded type is
made. RNA interference occurs in plants, animals, and humans. It is
of great importance for the regulation of gene expression,
participates in defense against viral infections, and keeps jumping
genes under control.
Fire and Mello were investigating gene expression in the nematode
worm
Caenorhabditis
elegans
(Nobel Committee Fig. 1).
Injecting the normal mRNA molecules (sense
RNA) that encodes a muscle protein in the nematodes
led to the normal muscle protein and no changes in the behavior of
the worms (Nobel Committee Fig. 2).
Injecting 'antisense' RNA,
the compliment of the normal mRNA, but which can pair with the
normal mRNA, also had no effect. But when Fire and Mello injected
the paired, double stranded, sense and antisense RNA together, they
observed that the worms displayed peculiar, twitching movements.
Similar twitching movements were seen in worms that completely
lacked the normal functioning gene for this muscle protein. What had
happened?
The sense and antisense RNA molecules met, bound to each other and
formed a double-stranded RNA and that double-stranded RNA molecule
silenced the gene’s mRNA Fire and Mello tested this hypothesis by
injecting double-stranded RNA molecules containing the genetic codes
for several other worm proteins. In every experiment, injection of
double-stranded RNA carrying a specific mRNA’s genetic code led to
silencing of the gene containing that particular code. The protein
encoded by that gene was no longer formed.
After a series of simple but elegant experiments, Fire and Mello
deduced that double-stranded RNA can silence genes, that this RNA
interference is specific for the gene whose code matches that of the
injected RNA molecule, and that RNA interference can spread between
cells and even be inherited. It was enough to inject tiny amounts of
double-stranded RNA to achieve an effect, and Fire and Mello
therefore proposed that RNA interference (now commonly abbreviated
to RNAi) is a catalytic process.
Fire and Mello published their findings in the journal
Nature on February 19, 1998. Their discovery
revealed a natural mechanism for controlling the flow of genetic
information, which heralded the start of a new research field.
The mechanism of how RNAi work naturally in cells preventing the
translation of normal mRNA was worked out over the next several
years (Nobel Committee Fig. 3).
Double-stranded RNA binds to a protein complex,
Dicer, which cleaves the dsRNA
into fragments. Another protein complex,
RISC, binds these fragments.
One of the RNA strands is eliminated, but the other remains bound to
the RISC complex and serves as a probe to detect mRNA molecules.
When an mRNA molecule can pair with the RNA fragment on RISC, it is
bound to the RISC complex, cleaved, and degraded. The gene served by
this particular mRNA has therefore been silenced.
RNA interference – a defense against viruses and jumping genes:
RNA interference is important natural defense against viruses,
particularly in lower organisms. Many viruses have a genetic code
that contains double-stranded RNA. When such a virus infects a cell,
it injects its RNA molecule, which immediately binds to
Dicer (Nobel
Committee Fig. 4). The RISC
complex is activated, viral RNA is degraded, and the cell
survives the infection. Jumping genes, also known as transposons,
are DNA sequences that can move around in the genome. They are
present in all organisms and can cause damage if they end up in the
wrong place. Many transposons operate by copying their DNA to RNA,
which is then reverse-transcribed back to DNA and inserted at
another site in the genome. Part of this RNA molecule is often
double-stranded and can be targeted by RNA interference. In this
way, RNA interference protects the genome against transposons.
New opportunities in biomedical research, gene technology and health
care:
RNA interference opens up exciting possibilities for use in gene
technology. Double-stranded RNA molecules have been designed to
silence specific genes in humans, animals or plants (Nobel
Committee Fig. 4c). Such artificially-made silencing RNA
molecules are introduced into the cell and activate the RNA
interference machinery to break down mRNA with an identical code.
This method has already become an important research tool in biology
and biomedicine. Several successful gene silencing experiments in
human cells and animals have been accomplished. For instance, a gene
causing high blood cholesterol levels was recently shown to be
silenced by treating animals with a silencing RNA. Plans are
underway to develop silencing RNA as a treatment for virus
infections, cardiovascular diseases, cancer, endocrine disorders and
several other conditions. |
