Miriam Meisler, Ph. D.
Department of Human Genetics
University of Michigan
October 18, 2016
Recent reports of exciting progress in gene therapy for the disorders Spinal Motor Atrophy (SMA) and Duchene Muscular Dystrophy have raised the question in all of our minds – when will Dravet syndrome patients benefit from this breakthrough? The potential for gene therapy is different for every genetic disorder, and many unique factors come into play for each gene target. Here we look at some recent successes and how they compare with Dravet syndrome.
Spinal motor atrophy is a deficiency disorder, like Dravet syndrome, that is caused by mutation of the gene SMN1. In this unusual case, there is a closely related gene, SMN2, that is normally inactive because a sequence in intron 7 prevents the synthesis of a complete messenger RNA. Dr. Adrian Krainer at Cold Spring Harbor Laboratory discovered that a small DNA fragment (an “ASO”) that binds the intron 7 sequence can correct the splicing defect. In this special situation, administration of this ASO, by injection into the cerebral spinal fluid, results in activation of SMN2 and clinical improvement, based on early results from a Phase 3 clinical trial. Repeated injection of the ASO is required to maintain expression of SMN2. Unfortunately, in the case of Dravet syndrome there is no related gene that could be activated in this way.
Another area in which treatment with short DNA fragments (ASOs) shows promise is for Huntington’s Disease and related triplet expansion disorders. These mutant genes contain a long stretch of nucleotide repeats that is not present in the normal gene. Targeting of ASOs to reduce gene expression has resulted in clinical improvement in mouse models of Huntington’s Disease, and Phase 1 trials for patients are in progress. The same approach is being developed for related diseases such as Spinal Cerebellar Ataxia. This approach, turning off a dominant mutant gene with ASOs, is sometimes referred to as Gene Suppression. This is not applicable to Dravet syndrome, which is a gene deficiency disorder.
SCN8A encephalopathy, which is clinically similar to Dravet syndrome, is a dominant disorder resulting from mutations in the SCN8A gene that usually increase the activity of the channel. Gene Suppression by ASO targeting might become feasible for this disorder, although a minimal level of SCN8A expression must be maintained for normal neurological function.
Another type of gene therapy for deficiency disorders like Dravet syndrome is Gene Replacement. The messenger RNA encoding the deficient gene is cloned into an RNA virus that can be injected into the cerebrospinal fluid and then enter neurons in the brain. Gene Replacement is under development for lysosomal storage diseases and other disorders. The currently available viruses can accomodate RNA of length up to 4,500 nucleotides. However, the messenger RNA for SCN1A is 6,000 nucleotides long, and the entire protein seems to be required for channel activity. Therefore gene replacement with viral vectors is not yet feasible. Even if a delivery vehicle for the SCN1A gene were developed, it would be challenging to deliver the gene to the appropriate cells in the brain. It is not yet clear which brain regions are most important to target.
As these examples demonstrate, each genetic disorder has unique features, and therapy for each disease must be tailored to the specific gene. In the case of Dravet syndrome, there is deficiency due to mutation of one copy of the SCN1A gene that causes loss of gene function, and patients retain one good copy of the gene. It is very attractive to consider increasing the expression level of the good copy of SCN1A, but we don’t yet know how to do that. Stabilization of the messenger RNA or the protein are being considered. Unfortunately, the SCN1A messenger RNA is too large for current viral vectors used in Gene Replacement. We do not know of an appropriate target to ‘knock-down’ with an ASO, as for Huntington’s Disease, or a target to ‘activate’ with an ASO, as for Spinal Motor Atrophy. So we are challenged to come up with something new, through basic research, that will enable us to compensate for SCN1A deficiency in Dravet syndrome. Currently, delivery methods for gene therapy are being improved in many laboratories. When a good target is identified for increasing SCN1A expression, we can expect to see rapid progression into animal models and to the clinic.
What this means: Gene therapy has shown early success in treating other diseases, but every gene is unique. Increasing SCN1A production is difficult because the gene is bigger than current delivery systems, and the target neurons for gene replacement are not well defined.