Artificial Chromosomes for Muscular Dystrophy

Duchenne muscular dystrophy, or DMD, is a genetic condition in which sufferers lack the ability to produce dystrophin – an essential structural protein in muscles. The size of the...
21 August 2011

Interview with 

Giulio Cossu, University of Milan


Chris -   Duchene Muscular Dystrophy or DMD is a genetic condition in which sufferers lack the ability to produce dystrophin which is an essential structural protein in muscles.  This leads to muscle wasting and ultimately, difficulty with walking and breathing, and symptoms usually appear by the age of about 5.  Duchene Muscular Dystrophy could be treated by giving patients a healthy copy of that dystrophin gene, but the problem is that the gene is too big to fit into the viruses that we might normally use to add genes to cells.  Now researchers in Italy have got around the problem by building an artificial chromosome to carry the gene into some cells and then adding this to stem cells which can then be used to repair damaged muscles.  And to tell us more, we're joined by Professor Giulio Cossu from the University of Milan who's the scientist behind this work.  Hello, Giulio...

Giulio -   Hi.

Chris -   First of all, how common is DMD, Duchene Muscular Dystrophy?

Giulio -   It is a quite common disease affecting approximately 1 in 3500 males, because the disease is linked to the X chromosome, so that only male children are affected while females are carriers.  They can carry the disease, but not be affected themselves.

Chris -   And the gene is monumentally big.

Giulio -   Yes.

Chris -   How big is big?

Giulio -   It is actually the largest gene known on this planet.  It is larger than many bacterial entire genomes.  It's more than 2 million bases which is the main problem for classic gene therapy where the healthy copy of the gene is normally vehiculated through viral vectors, but this gene is too big.

Chris -   So, it's not simple to just add a new copy of the gene into the affected muscle cells on account of its size, so what approach have you been taking instead?

Giulio -   Well, the approach we took was to use a human artificial chromosome, created in Japan by our collaborator, Mitsu Shimora, who has progressively taken away pieces of chromosome 21 and replace the missing part with the whole dystrophin gene, with all these intervening sequences, regulatory sequences.  So it's sort of a mini chromosome that is able to replicate every time the cell replicate its own chromosomes.  So it goes around every time that the cell divides and can carry the whole dystrophin gene inside the cells.

Chris -   So, you could put that chromosome into a diseased cell that lacks a healthy copy of the dystrophin gene.  It would replace the unhealthy gene, make healthy dystrophin protein, and that should make the muscle function improve.

Giulio -   That what we found in mice, but as you all know, everything is much easier in mice than in patients!  And one reason is that the transfer of the artificial chromosome is inefficient.  So that you need to select for the cells that took the chromosome versus all the other many, many more which did not, and human cells do not have, at variance with rodent cells, this infinite ability to proliferate in vitro.  Therefore, immediate transfer of this technology to human cells is not possible and will require further technical steps to make it possible.  That's what we are working on and we hope to solve this problem in the next years.

Chris -   So, given that you can't just add the artificial chromosome directly to the muscle, what approach have you taken instead?

Giulio -   So we took this cell that we've been working on for a number of years.  They have a terrible name.  I apologise this - mesoangioblasts.  We weren't able to come out with anything better.  Essentially, these are progenitor cells that are associated to the blood vessels, but we've discovered that they're also able to make muscle if taken up from the biopsy of skeletal muscle.  The cells can be grown in culture, and in the past we've shown that normal cells can be delivered through the arterial circulation and reach the downstream muscle.  That leads to an even distribution of the cells through the downstream muscles and in the past, we were able to show that this transplantation of these donor cell ameliorates the symptoms of muscular dystrophy in dystrophic mice and dogs.

And at the moment, we have a clinical trial running, but the problem is that the strategy with donor cells requires two things - first, the patient need to have an initially identical donor, normally a brother, pretty much like bone marrow transplantation and second, this requires immune suppression which is not a simple thing to do.  

Chris -   So a better way of doing it would be to take the person's own stem cells, their own mesoangioblasts, and your chromosome to them and then put them back into the patient, and then they'd only be getting their own cells, and that would get around that problem, wouldn't it?

Giulio -   Absolutely.  The problem now is that we don't have enough cells to do all these things.  All these steps can be done with mouse cells but not yet with human cells.  So in order for this to be tested on patients, this strategy still requires a few years of laboratory work.

Chris -   And when you try to do this on mice, so if you take this mesoangioblast cells from the mouse and you put your artificial chromosome into them, can they improve the function of a mouse with muscular dystrophy and remedy its problem?

Giulio -   Yes, that's what we observed.  After transplantation of these cells into dystrophic mice, the morphology of the muscles ameliorated.  The motility of these animals increased even though it didn't reach the level of a normal mouse. In no case, should we talk about a cure, but just an experimental treatment that in preclinical setting, is producing an amelioration of the symptoms of the disease.

Chris -   Thank you very much, Giulio.  That's Giulio Cossu from University of Milan.  Still work to do, but a wonderful discovery which you can find them out this week in the journal Science Translational Medicine.


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