Stem Cell Model Offers a New Target in ALS


by Kayt Sukel

November 21, 2013

Amyotrophic lateral sclerosis (ALS), better known as Lou Gehrig's disease, is a devastating neurodegenerative disorder that attacks the brain's motor neurons, leading to muscle weakness, paralysis and, eventually, to death. To date, there is only one Food and Drug Administration (FDA) approved drug available to treat the disease, but while this treatment can marginally slow the disease's destructive effects, it cannot undo the damage already done to the nervous system.

Researchers at Johns Hopkins University are using an innovative induced pluripotent stem cell (iPS) technique to track not only how a specific gene mutation disrupts normal cell function in ALS but also to show how a new treatment can stop those disruptions in their tracks.

The trouble with heterogeneous diseases

One of the biggest challenges of studying ALS is its heterogeneity. The disease never manifests itself the same way twice and may have various genetic contributors. Huda Zoghbi, a neurologist at the Baylor College of Medicine and member of the Dana Alliance for Brain Initiatives (DABI), argues that many common neurodegenerative disorders are not singular diseases but rather a collection of rare disorders with some shared genetic contributors (as well as similar symptoms) that just happen to be categorized under the same name.

"In Parkinson's disease, we've found an accumulation of a protein called synuclein in the brain post-mortem. But there are many ways that you might get to that accumulation," she says. "Maybe one patient inherits an extra copy of the gene and too much protein is made. Maybe in another patient, there's a mutation on a different gene that makes it more difficult to clear the synuclein. In still maybe another patient, there's a gene for a third protein that interacts with synuclein and because that interaction is altered, the synuclein is hanging out in the cell longer. So you have this common pathology for Parkinson's, but it could be due to one or more of many different factors, each of which just happen to converge on an important pathway."

ALS, Zoghbi argues, is a similar story. Multiple genes have been implicated in the disorder's debilitating symptoms, but they are likely working in different ways to result in death of motor neurons, the hallmark pathology of the disorder. The trick to finding more effective treatments, she says, is not to ignore this heterogeneity, but to embrace it.

A gene of interest

Yet, in one case a single gene may provide clues to more effective treatments for some people. In 2011, Rosa Rademakers and colleagues at the Mayo Clinic discovered that a mutation in one gene in particular, C9ORF72, was found in more than 40 percent of patients with inherited ALS and at least 10 percent of patients with the sporadic form of the disease. Stanley Appel, Director of the Houston Methodist Neurological Institute, says that this discovery raised the question of where, why and how this mutation was leading to the disorder's pathology.

"One theory was that this gene was resulting in a toxic gain of function. That the mutation turns out at an RNA level," he says. "And because of that mutation, the related RNA is now toxic. That is, when this RNA binds to another protein, something goes wrong and then the cell can't make other important things, like a chloride channel or an insulin receptor or something like that and that causes a lot of other problems down the line."

To better understand this single gene's role in ALS pathology, Jeffrey Rothstein and Rita Sattler, researchers at Johns Hopkins University used iPS technique to create neurons from fibroblasts, or skin cells, of ALS patients with the C9ORF72 mutation.  The researchers used viruses to deliver genetic stem cell factors to the cell and then, once back in a stem cell-like state, "reprogrammed" the cell into a motor neuron, the type of cell which is most affected by ALS pathology (http://www.dana.org/news/features/detail.aspx?id=21636).

The Hopkins group discovered that the mutation resulted in the abnormal production of RNAs, which interfered with RNA-binding proteins, ultimately inhibiting normal cell function. To counter those effects, the group then administered an antisense therapy, or strand of synthesized nucleic acid, developed by Isis Pharmaceuticals, which, in effect, ramped down the protein production of mutated C9ORF72 gene, allowing the RNA-binding proteins to do their jobs. The results were published in the Oct. 16, 2013, issue of Neuron.

Sattler says that the results are extremely promising but very preliminary. "When you work with human cells, you can be much closer to what's really happening in patients and you can be much more comfortable about those results translating to treatment," she says. "But we're not sure yet whether knocking down this gene is bad for you. We don't think it's critical but we need to test it very thoroughly so we can ensure safety."

The future of personalized medicine

Rothstein and Sattler are making preparations to eventually take the antisense therapy to clinical trials in ALS patients who have the C9ORF72 mutation, focusing on safety and efficacy. But even they argue C9ORF72 is probably not the only contributing factor in ALS-and they have a way to go before antisense treatments are available to patients. Appel agrees.

"There are many factors involved with ALS. Other studies have shown that neuronal injury can't explain the whole disease process," he says. "That doesn't mean if you decrease the injury in motor neurons with antisense therapy that you wouldn't help treat the disease. But it still may not give you all the underlying mechanisms of the disease. It may not treat all the underlying mechanisms of the disease."

Still, Zoghbi  says that using iPS is a valuable way to come up with novel, personalized treatments for heterogeneous neurodegenerative disorders.

"This kind of study opens up the way for scientists to go back and think about how we might do these kinds of studies in patients, how we can design experiments to explore the different pathologies in animal models, where we can look for new therapeutic targets" she says. "iPS is an excellent model to dissect disease pathogenesis. And while we should not rely on this model exclusively, because like all models it does have some pitfalls, it does help us dig deeper and really guide us to the next steps in researching and developing effective treatments."