Researchers demonstrate efficacy of antisense therapy for spinal muscular atrophy

In December 2009, Isis Pharmaceuticals announced it had added a SMA drug to its development pipeline. This drug is called ISIS-SMNRx and is specifically designed to potentially treat SMA through the correction of SMN2 RNA splicing, the low-functioning back-up gene found in all SMA patients.
The first animal model results were published on July 12th in the journal Genes and Development: scientists at Cold Spring Harbor Laboratory (CSHL) and Isis have succeeded in reversing symptoms of Type III SMA, a relatively mild form of the disease, in mice by introducing ISIS-SMNRx, an antisense oligonucleotide (ASO), into their spinal cords. The ASO fixes the molecular mistake underlying SMA by redirecting the cellular editing process called alternative splicing.
“Validating ASO efficacy in animal models is a crucial pre-clinical step before this strategy can be applied in SMA patients”, says CSHL Professor Adrian Krainer, Ph.D. “We have now successfully demonstrated this therapeutic efficacy in the mouse nervous system. Although the mice only have the mild symptoms of Type III SMA, our treatment can effectively correct them”.
Dr. C. Frank Bennett, Ph.D., Senior Vice President, Research, ISIS Pharmaceuticals, states: “ISIS-SMNRx is currently in pre-clinical development. Studies evaluating the safety and distribution of the drug in animals will begin later this year. These studies are necessary to support studies in SMA patients and will take about 1 year to complete”.

RNA is the molecule that translates information stored in our genes into proteins. By targeting RNA, the amount of disease causing proteins, such as the SMN protein that is chronically depleted in SMA, can be modulated. The primary technology used to target disease RNA is called antisense technology, where antisense oligonucleotides (ASOs) are used as the drug substance. Antisense oligonucleotides are chemically synthesized pieces of nucleic acid designed to bind to RNA in a very selective manner and thereby regulate the function of the RNA. This results in either increased or decreased amounts of the protein produced from the targeted RNA. In the case of SMA, the antisense drug has been shown to increase the production of the SMN protein, by targeting the RNA of the SMN2 gene. The SMN2 gene is the closely related back-up gene to the SMN1 gene that is typically deleted in SMA patients. It normally produces a truncated and low-functioning form of SMN protein. The ISIS drug is designed to bind the SMN2 RNA, driving the production of full-length, functional SMN protein, by altering the RNA splicing of the SMN2 gene. This type of drug is called a splicing modulator.
“We first showed that this ASO corrected SMN2 splicing in the test tube, and in patients’ cells grown in the lab”, says CSHL Research Investigator Yimin Hua, Ph.D., who spearheaded this work in Krainer’s lab. In 2008, the team injected these ASO molecules into the bloodstream of mice engineered to carry a human SMN2 gene that display symptoms of type III SMA. This regimen corrected SMN2 splicing in the animals’ liver and kidneys but not in spinal cord neurons-where they are most needed because the ASOs failed to breach the blood-brain barrier and enter the spinal cord. The collaborators have now overcome this limitation by directly delivering ASOs into the animals’ central nervous system, a common route of administration for other drugs such as chemotherapy agents. Infusing ASOs into the fluid that surrounds the brain and spinal cord resulted in a robust increase in the levels of SMN protein in individual motor neurons throughout the spinal cord of type III SMA mice.
“This effect persisted for half a year after the treatment ended, indicating that the ASO is extremely stable”, explains Krainer. “And equally importantly, none of the ASO doses tested triggered toxicity or inflammation in any of the mice tested”.
Mice with type III SMA typically develop necrosis-the death of cells and destruction of tissue-in adulthood. Although they do not display muscle weakness, these animals begin to lose their tail and ears 3-4 weeks after they are born, with complete loss occurring within a few more weeks. To test the ability of the ASOs to prevent these symptoms, which resemble clinical features observed in some infants with the more severe type I SMA, the team treated neonatal mice or 15-day-old mouse embryos with a single ASO injection. This treatment prevented both tail and ear necrosis in the neonates and embryos, which developed into adults with normal tails and ears. The researchers suggest that supplying the therapeutic ASO to the animals’ central nervous system, which restored cellular SMN protein levels, might in turn prevent neuronal deterioration, muscle wasting, and vascular problems in the tail and ears.
“Antisense correction of SMN2 splicing in the CNS rescues necrosis in a Type III SMA mouse model” has appeared online, ahead of print in Genes and Development on July 12th. The full citation is: Yimin Hua, Kentaro Sahashi, Gene Hung, Frank Rigo, Marco A. Passini, C. Frank Bennett and Adrian R. Krainer.
The paper is available online here.

(source: CSHL website)