Three letters that could
save your life
New insights into
our genetic code may combat illnesses such as Aids and hepatitis,
says Roger Highfield
Scientists have found a new and highly-precise
way to toss a spanner into the works of a cell. Much remains to
be understood about this powerful way to shut down, or "silence",
genes but it seems to play a central role in the development of
plants and animals, as well as in fending off illnesses.
Now they are beginning to exploit this knowledge.
Some are using it to investigate what genes do, while others hope
to create new treatments for diseases such as cancer. Recently,
the technique passed its first test to show it could be used to
combat Aids, hepatitis and other illnesses. Silence is indeed
golden.
The advance rests on new insights into genetic
code called ribonucleic acid (RNA), long thought to be the dowdy
ancestor of its glamorous cousin deoxyribonucleic acid (DNA).
While most are unsure what RNA is, DNA is a household word. Indeed,
events across the planet this month will mark the 50th anniversary
of the discovery, in Cambridge, that DNA is formed from two molecular
strands wound into a double helix.
Like DNA, RNA consists of strings of chemicals
- "letters" - that spell out the genetic code. The letters
are represented by A, C, G and either T in DNA or U in RNA. C
always binds to G. A binds only to T or U. A single-strand of
DNA or RNA can bind to another strand consisting of complementary
letters.
Until recently, it was thought that RNA carried
out DNA's commands, in effect deciding what genetic information
to turn into action.
Then, the DNA's double helix unwinds and its
genetic code is copied on to a single-stranded "messenger"
RNA. In turn, the messenger RNA shuttles the code from the heart
of cells to ribosomes, the "factories" that make the
proteins that build and operate cells.
So why bother, then, to study the RNA middleman
when DNA holds the recipe of life? The reason is that RNA has
found a new starring role in the cell.
The first hint of this came six years ago, when
Dr Andrew Fire of the Carnegie Institution, Washington, Dr Craig
Mello of the University of Massachusetts and colleagues found
that double-stranded RNA can be used to shut down specific genes.
Their discovery, called RNA interference, or
RNAi, was published in the journal Nature and is now widely used
as a research tool.
Another clue was from Prof David Baulcombe and
Andrew Hamilton in the Sainsbury Laboratory, Norwich. They discovered
RNA fragments called short interfering RNAs, or siRNAs, are used
by cells to fight viruses.
Although they were working with plants, their
work complemented the animal studies of Fire and Mello because
siRNAs are fragments of double-stranded RNA.
RNAi seems to occur when double-stranded RNA
is chopped into siRNA. The siRNA then guides an enzyme that degrades
the messenger RNA that turns genes into action.
Many scientists are excited by this because
gene silencing is highly specific - the messenger RNA will only
be degraded if it has the same sequence of A,C,G and U letters
as the double-stranded RNA.
This means a messenger RNA, for example from
a disease gene or a virus, can be targeted without the risk of
side effects that would be caused if the other messenger RNAs
needed by a healthy cell were silenced.
Recently, scientists have reported the first
evidence RNAi can be used to prevent liver injury and death in
mice that suffer hepatitis.
"This is the paper everyone has been waiting
for," commented Dr Phillip Zamore of the University of Massachusetts
Medical School.
"It is really the first paper that says
clinicians should be thinking about siRNAs and hoping that they
will prove to be suitable for use in humans in some form.''
"As far as I know this is the first successful
test in animals," said Dr Judy Lieberman of Harvard Medical
School, Boston, head of the team that reported the work in the
journal Nature Medicine.
Her team at Harvard's Centre for Blood Research,
working with two Chinese medical institutes, successfully used
RNAi technology to treat or prevent liver failure and fibrosis
in two mouse models of hepatitis.
In the study, Dr Lieberman's team used RNAi
to silence Fas, a gene implicated in liver disease, including
hepatitis. The siRNA worked like a drug, so that Fas was only
temporarily silenced. Scientists wanted to avoid long-term suppression,
linked to cancer and other problems.
The results were startling. "All the mice
died in the control groups, and 80 per cent of the treated mice
lived," said Dr Lieberman, adding it will take at least three
years before there are human studies.
"I was very skeptical when we started,"
said her colleague, Prof Premlata Shankar. "This opens up
so many avenues if it really works, and it seems to be working.
It takes the whole siRNA field one step further.''
Dr Lieberman's team believes RNAi may also be
an effective weapon against viral infection, especially HIV, the
Aids virus. Working with colleagues at MIT, she published a study
last summer in which RNAi blocked the virus in laboratory cell
studies.
Then Prof Shankar and her colleagues used RNAi
to remove the docking point used by the virus to invade human
immune cells. The virus enters using a protein called CCR5 on
blood cells called macrophages. Because earlier work showed that
the one per cent of the Caucasian population who lack the CCR5
gene still have relatively normal immune functions, the team reasoned
RNAi could be used to shut this protein "door" without
side effects.
The researchers were surprised by how well small
RNAs prevented infection of macrophages and also prevented viral
replication in infected macrophages. But how to deliver the RNA
safely?
Paradoxically, Aids may provide the answer.
Dr David Baltimore and colleagues at the California Institute
of Technology have created a genetically modified Aids virus that
can use RNAi to turn off the CCR5 gene in immune cells: when exposed
to the Aids virus, the cells were more resistant: between three
and seven-fold fewer were infected than usual.
The field is expanding rapidly. At Cold Spring
Harbor Laboratory on America's east coast, researchers have also
tried using RNAs folded over like hairpins to quash specific genes.
These seem to work as efficiently as siRNAs, heralding what they
playfully call short hairpin activated gene silencing or "SHAGging".
Dr Julie Ahringer and colleagues at the Wellcome
Trust/Cancer Research UK Institute, University of Cambridge, have
created a library of more than 16,000 bacteria, each of which
can use RNAi to turn off one of the nematode worm's 19,000 genes
to reveal what they do.
"This is the first time we can systematically
investigate how a genome works," she said. "Since more
than half of worm genes have a human counterpart, there is enormous
scope for learning about human gene function.''
Companies such as Ribopharma in Germany, Cyclacel
in Britain and Alnylam Pharmaceuticals in the US are also using
RNA to find new treatments. No wonder then, that the journal Science
hailed RNAi as the top scientific advance of last year.
10 April
2003

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