By David Altman
Translational neuroscience research has seen a disappointing streak of failed clinical drug trials. While the need for therapeutics that target the nervous system is growing, recent results in diseases like Alzheimer’s and autism have disappointed, and many companies have begun to downsize their R&D investments. Prospects are glum for patients who need new therapies to help manage their disorders.
The frustration is that drug candidates that have shown promise in animal models have not demonstrated efficacy in humans. Mouse models are not proving to be sufficient surrogates for human neurologic disease. Human brains and brain cells are built and function differently, and many neurodevelopmental disorders – hard enough to diagnose in human children – don’t have identifiable behavioral counterparts in mice. As I hear over and over from scientists, there is no such thing as a mouse with autism.
A study, published in Cell Stem Cell this June and conducted by Clifford Woolf, MD, PhD, et al, is among the first to demonstrate the power of an alternative technique: modeling disease in neurons derived from induced pluripotent stem cells (iPS cells). Applying this technique in patients with the neurodegenerative disease amyotrophic lateral sclerosis (ALS), the researchers created motor neurons and used them to screen multiple compounds in a dish. One compound, kenpaullone, outperformed two drugs that had failed in previous ALS clinical trials. It increased the life of motor neurons by weeks; maintained normal neuronal processes, synapses and electrophysiologic characteristics and lowered levels of mutant SOD1 protein and ubiquitin.
The last finding is important, because some familial forms of ALS are caused by SOD1 mutations. Also notable is that the two failed Phase III compounds, which performed well in mouse models, had limited effects in the neuron-in-a-dish model – proof of principle that in vitro motor neuron screening could increase the chances of discovering treatments effective in humans.
Stem cell advances
The process of creating patient-specific neurons has matured over the past few years. Researchers are better able to revert a patient’s skin cells back to pluripotency, capable of forming any tissue, and then push these iPS cells into a neuronal lineage. Like replaying a videotape, researchers can then recapitulate the developmental stages of the neuron and get clues about how disease develops.
Gerald Berry, MD, and George Daley, MD, PhD, at Boston Children’s Hospital, recently did this for galactosemia, a life-threatening disorder in newborns caused by the body’s inability to process a simple sugar. Children with galacatosemia accumulate metabolites of galactose, in particular galactitol. Neurons cultured from these children’s iPS cells were shown to accumulate these same metabolites – reproducing their disease in a dish.
Genome, transcriptome and environment
Phenotype and transcriptome analyses are a crucial part of neuron-in-a-dish drug screening. Researchers now have a powerful way to correlate a patient’s disease phenotype with his or her genome and transcriptome – the full group of RNA molecules that reflect gene expression, directing production of the proteins that drive cellular function. The hope is that the abnormal gene expression profile seen in patients is also abnormal in a dish.
"The idea is that this will act like a clinical trial," says Woolf, who directs the F.M. Kirby Neurobiology Center at Boston Children’s and is working with the Harvard Stem Cell Institute to put together a collaborative consortium to conduct clinical trials in a dish. "Unlike human trials, you are able to test your compound in multiple doses and test multiple drugs. The ‘patients’ never go away."
But here’s a question. Human disease is often a combination of inherited genetics and the environment, which manifests through epigenetics. Can this too be replicated in a dish?
"It will depend on the disease," says Daley, director of the Stem Cell Transplantation Program at Boston Children’s. "For some conditions, the cells need an environment that is supportive to their functioning. When skin cells are reprogrammed into a pluripotent state, epigenetic modifications to certain genes caused by the environment are erased. However, by manipulating the culture conditions, it appears possible to restore some of these epigenetic changes to more faithfully model the disease."
Will mouse models go away?
If disease-in-a-dish lives up to its potential, mouse models will still be used to complement the in vitro strategy. "What is harder to recapitulate in a dish is the complex whole-animal physiology," notes Daley. "When we invoke mouse models, we are really asking for the whole physiologic context."
"In a dish, we will never get a mechanistic insight into how the nervous system is behaving in the presence of a genetic mutation," adds Woolf. "We will always need to do systems biology, asking why 100 genes in a neurological disease result in some change in social interaction."
But the disease-in-a-dish method does have a place in tackling neurodevelopmental and neurodegenerative disease, both believe.
"There has been excitement in some groups and skepticism from others as to whether insights into very complex neurodevelopmental disorders can be gleaned by this disease-in-a-dish approach," Daley says. "Yet, for complex diseases like Alzheimer’s and Parkinson’s, we have observed real in vitro signals. There is real pathology in the dish, which I do think is going to be relevant to drug discovery."