DNA Scalpel Fixes Mutations; Leaves No Scars

A new technique to repair errors in DNA while leaving no trace...
16 October 2011

Interview with 

Professor David Lomas, Cambridge Institute for Medical Research




Ben -   A new technique to repair errors in DNA while leaving no trace has been reported in the journal Nature.  The researchers have corrected an error that leads to an untreatable liver disease, and this technique could eventually lead to treatments for an extremely wide range of genetic illnesses.  Joining us to explain more is Professor David Lomas from the Cambridge Institute for Medical Research.  David, thank you so much for joining us.

David -   My pleasure.  Thank you for the invitation.

Ben -   First of all, what is this liver disease?  What causes it and what are the symptoms?

David -   Alpha 1-antitrypsin is a protein that's produced from the liver, and it bathes all the tissues in the body.  The role of the protein is to protect the tissues against enzyme damage.  There is a genetic mutation in this protein that's found in 1 in 25 of the population.  So 1 in 2,000 are homozygous and it affects 30,000 people as homozygotes in the U.K.  So it's common.  This mutation causes the protein to misfold and accumulate within hepatocytes.  That gives rise to liver disease for which the only treatment is transplantation.  Now you imagine the lack of an important circulating protein means there's a lack of protection for the tissues and the lung is most susceptible, and these people will develop volume onset emphysema particularly if they smoke.

Ben -   So, it is a liver disease but it also affects other systems?

David -   Correct.  The primary abnormality is the protein misfolding in the liver, accumulating in the liver and then the secondary effect is on the lung.

Ben -   And as you said it's quite common so it's obviously a good target for us to look at.  So what have you done?  What have you reported in this particular paper?

David -   So, what we're trying to do is to think of a different way of producing liver cells that one day may take over from the damaged cells within patients.  So we started with my patients and we took a skin biopsy, and we isolated the fibroblasts.  We then reprogrammed these fibroblast so they became stem cells.  Now, these stem cells still have the genetic mutation, so we corrected the genetic mutation and to give you some idea of the magnitude of the problem, we changed one or two base pairs in 6 billion.  We left the rest of them unchanged and then, having made that correction, we have a normal healthy stem cell.  We differentiated those to liver cells, which now work beautifully in the test tube, and then we put them in mice.  We showed that they were viable in mice and they produced the relevant liver proteins.

Ben -   So your technique is sort of a genetic scalpel;  you're going in there with extreme precision to cut open the DNA, take out the errors, repair it, and not leave a genetic scar.

David -   Absolutely right.  Molecular scalpel is a good description.

Ben -   What at the moment is stopping us from using this in humans?  It seems to be very well received in mice, and I understand you put them into mice with no immune system so you don't get rejection.  But quite often, we've seen with stem cell research, like the cells don't really integrate properly, they might form tumours, there are all sorts of problems.  So, what's standing in the way now?

David -   So there's two issues to highlight.  The first is that the cells that we get from reprogramming look like liver cells and behave like liver cells but are an immature version of the liver cell  You can consider them as a foetal cell, that's the good way of looking at them.  Now, they seem to function well in the mouse.  They seem to produce the relevant proteins and they seem to integrate.  But we don't know whether long term, they can take over the function of the liver and repopulate and replace damaged liver cells.  The second issue is that when you reprogram skin cells into stem cells and then differentiate them into hepatocytes, you collect point mutations in the genome.  And from our sequencing, we found about two dozen point mutations.  Now, we think looking at the bioformatics that they're okay so they're probably not going to have any effect, but you don't know for sure.  The safety signal that we saw in these experiments is that when we put them in the mouse six weeks later, they function normally.  There didn't appear to be any malignant potential.  But those two issues need to be addressed, particularly the last one, before we can move this through to clinical trials.

Ben -   Two dozen mutations doesn't actually sound like that many.  I imagine there are far more just going from parent to child?

David -   Exactly, but the problem is that we need to understand what they mean and what they do, and it's the context of the mutation that's important.  So they may be fine.  This may not be a problem in real life.  However, we need to think about it very carefully, and that's one thing that we need to address as we develop our clinical studies.

Ben -   I guess the reprogramming method is a very artificial situation in which these mutations are forming.  So they could form in regions that would otherwise be very well protected.

David -   Correct, and it's just important to stress the safety side of these things.  I mean it's a nice story and we can go from skin, correct the genetic defect, and come up with liver cells that appear to function beautifully.  But if we're going to put them into patients, we need to develop strategies whereby we first of all do no harm, and the cells that we put it are not only viable and healthy but, they won't have that malignant side effect.

Ben -   What do you see the future for this sort of technique?  If you can pick out one or two bases in an entire genome then surely you can do this for pretty much any genetic disorder.

David -   That's correct, you can use this molecular scalpel for any genetic disease and you can correct the genome and leave the rest of genome pristine and that's the central part of this paper.  So you can apply it to any genetic technique.  It's then getting cells that are relevant and clean, and viable, and healthy that you can use in patients.  So we're grappling with that.  So having done this, we're trying to think, "how can we put this through in patients in a safe way?"  And our next step probably, because the grappling is still ongoing, is to encapsulate the cells in some way in a polymer mix, an alginate, which means that we can put it in the human body in a test environment whereby it's safe.  So, if the cells do develop a malignant potential, they won't escape beyond the capsule and then we can retrieve the cells at a later date and show that they function normally and there's no malignant potential; and I think that stepping stone is quite important before we get through to clinical trials.

Ben -   And that, of course, would show you how they behave in context with all of the other chemical factors around that would be there in a real situation.

David -   Absolutely, and remember of course these have come from the individual that you are about put them back into, so they have the same genome and theoretically, there should be no requirement for immunosuppressive treatments, and there should be no rejection.

Ben -   Excellent.  Well, that's extremely promising.  Well, thank you ever so much for joining us.

David -   Thank you.


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