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