Development of small molecule screens for autism using patient-derived iPS cells
Autism Spectrum Disorders (ASDs) are a heritable group of neuro-developmental disorders characterized by language impairments, difficulties in social integrations, and the presence of stereotyped and repetitive behaviors. There are no treatments for ASDs, and very few targets for drug development. Recent evidence suggests that some types of ASDs are caused by defects in calcium signaling during development of the nervous system. We have identified cellular defects in neurons derived from induced pluripotent stem cells (iPSCs) from patients with Timothy Syndrome (TS), caused by a rare mutation in a calcium channel that leads to autism. We propose to use cells carrying this mutant calcium channel to identify drugs that act on calcium signaling pathways that are involved in ASDs.
Our research project has three aims. First, we will determine whether known channel modulators reverse the cellular defects we observe in cells from TS patients. It is possible that we will find that existing drugs already approved for use in humans might be effective for treating this rare but devastating disorder.
Our second aim is to determine whether screens using neuronal cells derived from ASD patients can be used to identify calcium signaling modulators. A bottleneck to therapy development for ASDs has been the lack of appropriate in vitro models for these disorders, and we would like to determine whether our studies could serve as the basis for a new type of screen in human neurons.
Our third aim is to identify signaling molecules that might be affected in patients with ASDs, which could be targets for future drug discovery. There is increasing evidence that several types of ASDs are caused by defects in neuronal activity and calcium signaling. More specifically, the CaV1.2 calcium channel that we are studying has been implicated in syndromic and non-syndromic forms of autism, and also in schizophrenia and bipolar disorder. One of the more exciting aspects of our screen of neurons with a mutation in CaV1.2 is that it gives us a tool to explore calcium-mediated signaling pathways that are defective in ASDs. We will try to modify calcium signaling in neurons from ASD patients by changing the expression of proteins that are known to affect calcium signaling in other contexts. These experiments will identify targets that are active in human neurons and that affect cellular phenotypes that are defective in ASD.
In summary, the work described in this proposal constitutes a critical step to fulfilling the promise that reprogramming of patient-specific cells offers for the treatment of neuropsychiatric disorders such as autism. Our studies will identify lead compounds that could be tested in the clinic for a rare form of autism, and novel molecular targets for therapeutic development in the future. Importantly, these studies will provide a proof of principle that iPSC-derived cells are valuable for drug discovery for neuropsychiatric disorders.
Autism Spectrum Disorders (ASDs) affect approximately 1 in 110 children in California. In addition to the devastating effects that ASDs have on the families of affected individuals, treating and educating people with ASDs imposes a heavy economic burden on the state. In 2007, almost 35,000 individuals with autism were receiving services from the California Regional Centers, and the number was expected to rise to 50,000 by last year. Recent estimates suggest that the lifetime cost of caring for an individual with an ASD can exceed $3 million.
In spite of their impact on our society, there are currently no effective therapies for ASDs. Our lack of cellular and molecular tools to study these disorders means that there are no good targets for drug screening, so there are very limited prospects for developing effective pharmacological treatments in the near future. New drug discovery paradigms are needed to help develop therapies for these neuropsychiatric conditions.
The research described in this proposal could have a dramatic impact on drug discovery methods for ASDs. First, we hope to identify drugs that are effective in treating Timothy Syndrome, a rare form of autism caused by an electrophysiological defect in a calcium channel. Second, we aim to develop new tools to explore calcium-mediated signaling pathways that are defective in ASDs. If successful, our research will identify a family of molecular targets that will be useful for developing therapies for ASDs in the future.
Autism Spectrum Disorders (ASDs) are a heritable group of neuro-developmental disorders characterized by language impairments, difficulties in social integrations, and the presence of stereotyped and repetitive behaviors. There are no treatments for ASDs, and very few targets for drug development. The goal of this CIRM project is to develop a series of in vitro screens for drugs that might affect the underlying cellular defects in ASDs.
Since ASDs are uniquely human, we proposed to design, optimize and conduct high-throughput chemical screens using human neurons derived from induced pluripotent stem cells (iPSCs). Our lab identified cellular defects in neurons derived from patients with Timothy Syndrome (TS), a syndromic disorder often presenting with autism that is caused by a rare mutation in a calcium channel. In our project, we proposed to develop in vitro screening assays for ASDs based on these TS phenotypes, and to screen these assays to identify drugs that might affect behavioral symptoms of autism. In the first year of this award, we conducted preliminary screens and found that certain calcium channel modulators reverse some of the differentiation defects that we observe in these cells. We also extended observations that we had made in mice and showed that TS neurons have defects in the structure and length of their dendrites, measurable features that we can use as the basis for additional drug screens. We have therefore progressed within the aims of the original award.
For the remainder of the grant, however, we are proposing to broaden the scope of this project to include iPSC-based screens using neurons from patients with more prevalent forms of ASDs. In other research in our lab, we have characterized phenotypes in neurons derived from patients with two other diseases that are more prevalent than TS: DiGeorge Syndrome (DGS) and Phelan-McDermid Syndrome (PMDS), two neurodevelopmental disorders resulting from deletions within chromosome 22 and patients present symptoms that often include autism. We have shown that these cells have defects in the length of their dendrites, in the structure and function of their synapses, and in their ability to transmit electrical impulses. We propose to broaden the scope of our work to develop screens for TS, DGS, and PMDS. These screens will serve as a basis for identifying drugs that lessen or reverse cellular defects in these disorders, and thus may lead to more generalized treatments for ASDs.
We believe that this research not only fulfills critical steps in the development of a novel test for potential ASD treatments, but demonstrates the power of iPSC technology for understanding the underlying mechanisms of neurological disorders. Expanding the scope of our original project will help us increase the impact of our studies on therapeutic development and on the understanding of the neurobiology of ASDs.
Autism Spectrum Disorders (ASDs) are a heritable group of neurodevelopmental disorders that affect the verbal, social, and behavioral abilities of affected individuals. There are no pharmacological treatments for ASDs, in part because of a lack of validated cellular and animal models for use in drug screens. The goal of this project is to develop and validate a cell-based high throughput screening method that we will use to identify therapies for ASDs.
Our laboratory has established methods for collecting skin samples from patients and reprogramming these cells into induced pluripotent stem (iPS) cells, which we then differentiate into neurons. We have characterized neurons from patients with ASDs, and identified cellular phenotypes that are amenable to high-throughput methods to identify drug targets. Our efforts in Year 2 of our CIRM funding have focused on Phelan-McDermid Syndrome (PMDS), an inherited progressive neurodevelopmental disorder characterized by developmental delay, absent or severely impaired speech, and an increased risk of autism. We have discovered that neurons from PMDS patients who have autism have defects in excitatory synaptic transmission caused by the loss of one copy of the gene Shank3. Shank3 lies in the region of Chromosome 22 that is deleted in PMDS, and is important for the development of synapses. Based on our studies, PMDS neurons can be distinguished from their wildtype counterparts by low expression levels of Shank3 measured by quantitative PCR, decreased number of excitatory synapses labeled by immunocytochemistry and imaged with a microscope, and reduced excitatory cellular currents measured electrophysiologically. Each of these phenotypes is amenable to high throughput screening of therapeutic compounds. We tested several candidate therapeutics and found that prolonged treatment with the growth factor IGF-1 partially reverses the defects we have discovered in PMDS neurons. While IGF-1 is highly bioactive and therefore not an ideal drug candidate, it can be used to validate our screening method.
We are currently running trials to select the best phenotype and assay for larger-scale screening. In parallel, we have developed protocols to culture large numbers of iPSC-derived neurons for high throughput screens, and we are growing and banking working stocks of PMDS and control neurons. These experiments will help us identify drug candidates for PMDS, and will represent a significant advance in HTS approaches for the testing of ASD therapies using iPSC-based systems.
Autism Spectrum Disorders (ASDs) are a heritable group of neurodevelopmental disorders that affect the verbal, social, and behavioral ability of affected in individual. There are no treatments for ASD, in part because the biological basis for the disorders are not know. In addition, there are no methods for screening drugs that may be therapeutic. The goal of this project was to develop screening assays based on stem cells that were derived from individuals with autism.
Using skin samples from affected individuals, our laboratory was able to generate induced pluripotent stem cells (iPSC) and use these stem cells to generate neurons. With CIRM support, we have now generated iPSC from many individuals, some of whom carry genetic alterations that cause autism. Work under this award focused on two genetic disorders, Timothy Syndrome (TS) and Phelan-McDermid Syndrome (PMDS). Both are inherited syndromes that affect several body systems and also greatly increase the risk of autism. In each case, we found that neurons from affected individuals displayed changes in the way neurons connect and communicate. The effects were pronounced in PMDS neurons, in part due to the loss of the Shank3 gene that is involved in the function of the excitatory synapse. Work in year 3 has focused on identifying a robust alteration in neuron function that can be used for drug screening.
One such phenotype was discovered and involves a change in the way calcium is utilized when neurons communicate by generating an electrical current. Using chemicals that detect calcium, fluorescent assays were developed that show a robust difference in calcium response in PMDS neurons relative to neurons from unaffected individuals. Adapting the fluorescent calcium reporter assay to a high-throughput format also required the invention of new stem cell culture methods for generating neurons that were more efficient and less costly. Ultimately, a novel strategy was developed that now permits the production of very large numbers of neurons that can be assayed in high throughput screens. A limited screen using candidate drugs has confirmed the utility of the assay and future work will utilize these assays in large scale screens for drugs that normalize or augment the synaptic defects.
- Nature (2013) SHANK3 and IGF1 restore synaptic deficits in neurons from 22q13 deletion syndrome patients. (PubMed: 24132240)
- Nature (2011) MicroRNA-mediated conversion of human fibroblasts to neurons. (PubMed: 21753754)
- Nat Med (2011) Using iPSC-derived neurons to uncover cellular phenotypes associated with Timothy syndrome. (PubMed: 22120178)