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Making cells ignore mutations could treat genetic diseases

Friday, November 25, 2016 15:15
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If the instructions for what you’re building are wrong, what can you do? That’s the problem posed when DNA mutations in people with genetic disease lead to the production of faulty proteins. But a new technique could help cells get around that problem and potentially treat conditions like some genetic types of cystic fibrosis.

Most of our genes are recipes for making proteins. Each successive three-letter DNA sequence – known as a codon – specifies which amino acid should be added next to a growing chain of amino acids to create a protein. This goes on until the protein-making machinery reaches a codon that says stop.

But sometimes, DNA mutations create a stop codon in the wrong place. A single mutation can truncate a protein that should be 100 amino acids long to one that is just 15 long, rendering it completely useless. These are known as nonsense mutations, and they cause about 10 per cent of all genetic diseases.

Ignoring the stop sign

There might be ways to get around these premature stop codons. One approach was first proposed in the 1980s, but only now have people managed to get it to work in human cells. It focuses on tRNAs, the molecules that recognise codons during the production of a protein, and match the right amino acid to them.

It’s possible to make artificial tRNAs that recognise a premature stop codon, and instead of terminating the protein-making process, add the amino acid required to make a useful protein.

In 2014, Carla Oliveira at the University of Porto in Portugal and her team restored the production of a healthy protein in cells carrying a mutation that leads to hereditary stomach and breast cancer. The only option currently available for people who have this mutation is to have their stomach or breasts removed.

Now Christopher Ahern at the University of Iowa in Iowa City and his team have used artificial tRNAs to restore some production of the protein that is usually missing or broken in people who have cystic fibrosis.

Smuggling into cells

Like Oliveira’s team, they did this with cells in a dish, but after further experiments, it may offer an alternative to drugs and gene therapy for those looking to treat cystic fibrosis. “If we could recapitulate it in the lung, that would do it,” says Ahern.

It should one day become possible to cure cystic fibrosis by fixing or replacing the mutant gene, Ahern says. But getting the long gene sequences needed for gene therapy or gene editing into lung cells is a huge challenge.

Artificial tRNAs are smaller, so  it might be possible to develop treatments more quickly.

Once inside a cell, the artificial tRNAs compete with the proteins that normally bind to stop codons and halt protein production. This means that artificial tRNAs will not fix every protein made by the faulty gene, but they may be able to fix just enough to make a difference – for many genetic disorders, even low levels of protein can make a huge difference.

“I think it’s really exciting,” says Malcolm Brodlie of Newcastle University, UK, who studies cystic fibrosis and also treats people with the condition at the Great North Children’s Hospital in Newcastle. But he points out that while Ahern’s team got cells to produce proteins with the right amino acid sequence, they have not yet shown these proteins are fully functional.

Messing with the code

Is it safe to muck about with the genetic code like this? One danger is that the artificial tRNAs may interfere with correct stop codons, messing up other proteins. This might happen occasionally, but cells do have other ways of telling when they have reach the end of the instructions.

Evidence from other kinds of research suggest that artificial tRNAs should be safe. “It is known that the introduction of tRNAs that are targeted to stop codons is tolerated in animals,” says Jason Chin of the University of Cambridge, whose team is using artificial tRNAs to expand the genetic code in animals such as worms and fruit flies.

There have already been trials of drug compounds that interfere with the protein-making machinery, making it ignore premature stop codons. The most advanced of these read-through drugs, called ataluren, has been shown to be safe. Unfortunately, it isn’t very effective. Earlier this year the US decided not to approve its use for Duchenne’s muscular dystrophy.

Artificial tRNAs should be more effective, not least because the resulting proteins are completely normal. With a read-through drug, by contrast, there is either a missing amino acid or an incorrect substitute in the final protein.

Any treatment based on this approach is still a long way off, warns Brodlie, not least because artificial tRNAs are harder to deliver to cells than conventional drugs. The field is advancing rapidly, though, Ahern says. “The delivery systems are becoming available.”

Both teams are hoping industrial partners will come forward to help develop this early work into potential treatments.

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