Therapeutic Targeting of Disrupted Nucleocytoplasmic Transport in C9orf72-ALS Motor Neurons
Abstract
Mutations in over a dozen genes have now been identified as the cause of both familial and a small proportion of apparently sporadic forms of amyotrophic lateral sclerosis (ALS). Collectively, these mutations account for ~15% of all ALS cases. These genetic discoveries have taught us that ALS, despite the shared phenotype of progressive degeneration of motor neurons, is in fact etiologically diverse. This observation raises the possibility that a single treatment may not be effective for patients with ALS due to different causes. Rather, specific treatments may be needed for patients with the various forms of the disease. Moreover, knowledge of the genetic cause(s) of ALS offers an unprecedented opportunity to develop potential therapeutic agents that target the specific underlying genetic abnormality. In this proposal, we outline a strategy for developing treatments for patients with ALS (and/or frontotemporal dementia [FTD]) that is caused by the "hexanucleotide repeat expansion" mutation (whereby six "letters" of the genetic code -- GGGGCC -- are repeated over and over) in the C9ORF72 gene. The C9ORF72 repeat expansion leads to the generation of two types of toxic substances (RNA species that contain the expanded transcript and abnormal proteins that are generated by translation of these RNA species). It has recently emerged that these toxic RNAs and proteins lead to dysfunction of transport (i.e., communication) between the nucleus and the cytoplasm, a process that is critically important to health of motor neurons and supporting cells. Since this "nucleocytoplasmic transport" dysfunction seems to lie at the heart of the biology of C9ORF72-ALS/FTD, we believe it to be an ideal tool to use for drug screening -- i.e., if we can identify drugs that improve or restore nucleocytoplasmic transport to normal, there s an excellent chance that these drugs may be worthwhile to test in patients with ALS through a clinical trial. Having determined that we will screen potential drugs in an assay that quantifies nucleocytoplasmic transport, we then need to decide what types of drugs to screen. To understand our choice, we need to first explain the complicated concept of "epigenetics" and how this differs from DNA and RNA. DNA is the substance that carries our genetic code, which we inherit from our parents. For DNA to exert its effect, however, it must first produce RNA, and RNA in turn must make proteins. Proteins are the "workers" responsible for all cellular activities. Epigenetics, on the other hand, describes heritable and modifiable processes that affect gene expression (i.e., RNA production) but are not caused by changes (i.e., mutations) in the DNA sequence itself. Since changing the epigenetic environment of the C9ORF72 gene may lead to decreased production of the toxic RNA and protein products described above, we anticipate great value in screening epigenetically active compounds in our assay of nucleocytoplasmic transport. In this application, we outline an approach for developing a tool (Aim 1) that we can then use in a semi-high-throughput fashion to screen hundreds (if not thousands) of drugs, asking which of these drugs impact nucleocytoplasmic transport in a favorable manner (Aim 2). By performing this drug screen in motor neurons derived from induced pluripotent stem cells from patients with C9ORF72-ALS/FTD, we are able to preserve the epigenetic environment that is native to C9ORF72 repeat expansion in patients with ALS. This approach maximizes the likelihood that compounds we identify can be translated into benefit for patients in the clinic.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 07, 2017
- Source ID
- W81XWH1710132
Entities
People
- Michael Benatar
Organizations
- United States Army
- University of Miami