Researchers hope their discovery will show French politicians the need to allow research on human embryonic stem cells. Credit: STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY

There is no cure for the group of hereditary muscle-wasting diseases known as muscular dystrophy. That is particularly alarming because one of its commonest forms — type 1 myotonic dystrophy — becomes more serious as it passes down the generations, manifesting earlier and acquiring pernicious extra symptoms, such as delays to mental development.

A group of French scientists have now unravelled molecular pathways that may be responsible for some symptoms of type 1 myotonic dystrophy. They used a controversial source of material: disease-specific human embryonic stem (hES) cell lines. They hope that their results, published online today in Cell Stem Cell1, will influence a French political debate that threatens to restrict such work. The French Senate will vote on the issue on 5 April, in the first reading of new legislation to update the country's bioethics law (see France mulls embryo research reform).

Type 1 myotonic dystrophy results from a defect in just one gene — dystrophia myotonica-protein kinase (DMPK) — but that damage affects the expression of other healthy genes. The team of researchers, led by Cécile Martinat, a geneticist at the Institute for Stem Cell Therapy in Evry, identified two such genes that are suppressed in the disease. They showed that the suppression prevented neurons from efficiently building connections with muscle cells.

Unlike in most genetic diseases, the damaged DMPK gene is not mutated. Instead, its code is interrupted by a long and unstable string of 'triplet repeats', in which three of the four nucleotides that make up DNA repeat themselves more than fifty times. The string of repeats tends to get longer with each generation.

"This is an excellent example of the value of using hES cells to study a genetic disease and learn something about its biology," says Nissim Benvenisty, a geneticist at the Hebrew University in Jerusalem. Benvenisty has also done research on a disease-specific hES cell line, to learn how the damaged gene that causes fragile-X syndrome, the most common inherited cause of developmental delay, initiates the disease2.

Reused embryos

The cell lines used by the French scientists were derived from two embryos carrying the damaged DMPK gene. These embryos had been identified by pre-implantation genetic diagnosis (PGD), a procedure undertaken during in vitro fertilization by couples at high risk of passing genetic disease to their children. A single cell is removed from an embryo before implantation, and its genes are analysed; only healthy embryos are then implanted into the mother. In countries that allow such procedures, discarded embryos carrying disease genes may be grown into cell lines that model the disease.

Martinet's team developed the hES cells into a class of neuron that would normally connect muscles with nerves. The group looked at gene expression across the whole genome in these cells, comparing it with that in healthy cells, and found 15 genes with altered expression. Two of the those genes belonged to the SLITRK family, which is known to affect how neurons sprout.

The scientists showed that the downregulation of these two genes caused the neural cells to sprout 'neurite' projections in abnormally large amounts — but seemingly to little avail. When they incubated the neural cells with muscle cells, the neurites were not able to form their customary neuromuscular contacts as well as they did in control cells.

When the researchers differentiated the hES cells into classes of neural cells that do not supply muscles with nerves, they didn't find any neurite defects.

Best option

Scientists hope that it will one day be possible to replace hES cells in most experiments and future therapies with reprogrammed adult cells that can differentiate into any cell type, called induced pluripotent stem (iPS) cells.

"But just getting iPS cells with a mutant DMPK gene may not fully mimic the complicated triplet-repeat damage that occurs in type 1 myotonic dystrophy, because of the complex genomic and epigenetic changes associated with reprogramming," says Marc Peschanski, director of the Institute for Stem Cell Therapy and an author of the latest paper. "Also, iPS cells may not in any case help us understand the critical events in very early development that lead to the myotonic dystrophy syndrome," he adds.

Indeed, Benvenisty's team found that, unlike his disease-specific hES cells, iPS cells with the fragile-X mutant gene knocked out did not reproduce the early developmental events leading to fragile-X syndrome3 — which is also caused by a triplet-repeat defect.

Peschanski runs a large iPS-cell research programme in addition to his hES-cell work. "We make iPS cells to model particular diseases when we don't have access to the relevant hES cells — which remain our gold standard," he says.

Politicians who oppose hES-cell research often — wrongly — insist that iPS cells can always substitute for hES cells, says Peschanski. He is frustrated that the lower house of the French parliament invoked this argument when proposing a ban on hES-cell research in France. Peschanski has since been working with other French scientists to persuade the Senate to overturn the proposal next week.