By taking advantage of a natural process of gene silencing, a new gene therapy approach appears to prevent the toxicity todorsal root ganglion(DRG) a specific cluster of sensory neurons seen in non-human primates during gene therapy studies for neurological disorders, researchers report.

The approach successfully protected the primates DRG from excessive activity of the introduced gene known as a transgene and subsequent toxicity, without affecting the transgenes activity elsewhere in the nervous system.

DRG inflammation and toxicity was observed in non-human primates after spinal canal injection of AVXS-101 approved asZolgensma when given as an intravenous infusion prompting a partial hold of a Phase 1/2 trial (NCT03381729) to allow for further investigation.

That trial, calledSTRONG, is testing intrathecal (spinal canal) injection of this gene therapy in children with spinal muscular atrophy (SMA) between 6 months and 5 years old.

The study, MicroRNA-mediated inhibition of transgene expression reduces dorsal root ganglion toxicity by AAV vectors in primates, was published in the journal Science Translational Medicine.

We believe that this new approach could improve safety in genetherapyuniversally, as well as in SMA, Juliette Hordeaux, PhD, the studys first author at University of Pennsylvanias Perelman School of Medicine (Penn Medicine) said in a press release.

This approach could be used to design other gene therapy [carriers] to repress transgene [activity] in the cell types that are affected by the toxicity and not others, which is critical, because you need [such activity] everywhere else to effectively treat the disorder, added Hordeaux, who is also senior director of translational research for Penn Medicines gene therapy program.

Gene therapy works to deliver a functional version of a gene to correct or replace a faulty gene within specific cells in the body. Most therapy approaches today use a modified and harmlessadeno-associated virus(AAV) as a carrier for the working gene, a vehicle to transport it to a target cell.

But previous studies, including primate work by Penn researchers, showed that AAV-based gene therapies targeting the central nervous system (CNS; brain and spinal cord) can damage the DRG, a cluster of neurons of the spinal nerve that bring sensory information from the periphery to the spinal cord.

Notably, DRG toxicity was observed in studies regardless of the therapys route of administration (directly into the bloodstream or into the spinal canal). No reports of such toxicity in humans, including children treated in STRONG, are known.

Conventional immunosuppressive regimens were ineffective in preventing this toxicity, strongly indicating that an excessive immune response was not its cause. This led Hordeaux and her Penn colleagues to evaluate whether the damage was related to excessive levels of the transgenes product, which could cause cellular stress.

To test their idea, they took advantage of RNAi, a natural process of gene silencing, in which microRNA (miR) molecules bind to a specific messenger RNA (mRNA), targeting it for destruction and preventing the production of that protein. (mRNA is the molecule derived from DNA and used as a template for protein production.)

Since the miR183 complex is specifically produced in sensory tissues and organs such as dorsal root ganglion, the researchers introduced miR183s sequence targets at the endof the transgene sequence in an AVV. With this, any mRNA produced from the transgene would be destroyed in DRG neurons by the naturally present miR183, preventing protein production in these cells.

Researchers then compared the effects of administering AAV with a transgene containing or not containing miR183s sequence targets into the cerebrospinal fluid (the fluid that bathes the brain and spinal cord) of non-human primates.

Introducing miR183s sequence targets in the transgene were found to significantly reduce its mRNA levels and subsequent toxicity in DRG neurons, without affecting the transgenesmRNA levels elsewhere in the primates brain.

Steroids given to primates treated with unmodified AAVs, in contrast, failed to alleviate DRG damage, despite their known anti-inflammatory and immunosuppressive effects. This ineffectiveness was consistent with the proposal that immune system activity does not mediate this neuronal toxicity, the researchers wrote.

We were concerned about the DRG [toxicity] that was observed in most of our [non-human primate] studies, said James M. Wilson, MD, PhD, the studys senior author, and gene therapy program director and a professor of medicine and pediatrics at Penn Medicine.

This modified [viral] vectorshows great promise to reduce DRGtoxicityand should facilitate the development of safer AAV-based gene therapies for many CNS diseases, Wilson added.

Novartis gene therapy Zolgensma, when given directly into the bloodstream, is currently available for use in newborns and toddlers up to age 2 with any type of SMA in the U.S. and Japan, and to those with almost all types who weigh up to 21 kilograms (about 46 pounds) inEurope.

To meet aU.S. Food and Drug Administration(FDA) request for a pivotal confirmatory study of the gene therapys use with older SMA patients, who would be treated via intrathecal (IT) injection, Novartisplans to launch a new AVXS-101 IT trial.

This administration route is favored for those beyond toddler age, as it is thought to better target themotor neuronsdamaged by the disease.

According to the company, this IT trial cannot include U.S. sites until the hold on STRONG is lifted.

Marta Figueiredo holds a BSc in Biology and a MSc in Evolutionary and Developmental Biology from the University of Lisbon, Portugal. She is currently finishing her PhD in Biomedical Sciences at the University of Lisbon, where she focused her research on the role of several signalling pathways in thymus and parathyroid glands embryonic development.

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Ana holds a PhD in Immunology from the University of Lisbon and worked as a postdoctoral researcher at Instituto de Medicina Molecular (iMM) in Lisbon, Portugal. She graduated with a BSc in Genetics from the University of Newcastle and received a Masters in Biomolecular Archaeology from the University of Manchester, England. After leaving the lab to pursue a career in Science Communication, she served as the Director of Science Communication at iMM.

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Intrathecal Gene Therapy Use Might Be Safer With 'Silencing' Step - SMA News Today