A gene therapy for chronic pain

For Rajesh Khanna, conducting basic research on voltage-gated ion channels as a graduate student at the University of Toronto unexpectedly led to tackling a pressing societal problem. The channels’ vital role in pain signaling drove him to develop treatments for chronic pain, a condition with limited and addiction-prone treatment options. “This is really laying on the minds in the psyche of the nation,” said Khanna, now a neurobiologist at the University of Florida. “There's a need to develop nonopioids for chronic pain.”

In a recent study published in Proceedings of the National Academy of Sciences, Khanna and his collaborators at New York University identified a small peptide that blocks a pain-associated interaction between the sodium channel Na_v1.7 and its regulatory protein collapsin response mediator protein 2 (CRMP-2) (1). They used an adeno-associated virus to deliver code for the peptide into sensory neurons in vivo, demonstrating a potential gene therapy for long-term reduction or even prevention of chronic pain. “Wouldn't it be nice to have a chronic solution to a chronic problem?” said Khanna.

Rajesh Khanna studies ion channel complexes as targets for treating chronic pain.

credit: Rajesh Khanna

Some people with mutations in Na_v1.7 experience frequent bouts of burning pain, while other mutations leave people incapable of feeling any pain at all (2). Researchers have attempted to design chronic pain drugs that directly block the channel, but this is challenging because Na_v1.7 is one member of a family of channels that appears throughout the body (3). None of the blockers so far have blocked this channel without also blocking other related channels. “Our whole philosophy and approach are different in that we are not trying to block the channel. We’re trying to indirectly regulate its activity,” said Khanna.

Khanna has been exploring this approach for over a decade. As an early-career professor, he and his team successfully reduced pain in rodents by mutating CRMP-2 to inhibit its interaction with Na_v1.7 (4). Yet, it remained unclear how CRMP-2 preferentially bound Na_v1.7 over other Na_v1.x channels. “That was really bothering us,” said Khanna.

To investigate this matter in the current study, the researchers set out to locate CRMP-2 binding sites within Na_v1.7. They split Na_v1.7 into an array of peptide fragments and pinpointed one fragment that exclusively bound CRMP-2. The team then swapped this CRMP-2 regulatory sequence peptide in Na_v1.7 with equivalent peptide regions from other Na_v1.x proteins, which greatly reduced the flow of sodium ions through the channel. This confirmed that the interaction between CRMP-2 and Na_v1.7 is selective and crucial to the channel’s function. “It was a long journey that got us here, and we're pretty happy that we're able to figure out exactly why the coupling happens,” said Khanna.

Armed with this mechanistic insight, Khanna’s team injected the CRMP-2 regulatory sequence peptide into sensory neurons in vitro and observed the same reduction in Na_v1.7’s current. Neurons injected with the peptide were also less excitable, requiring significantly higher currents to fire and releasing fewer sensory neurotransmitters in the process.

The team then injected the peptide into rats with spared nerve injury, a model of neuropathic pain, and tested their hypersensitivity to weight, a hallmark symptom of chronic pain. When the researchers applied increasingly heavy weights to the rats’ paws, the pain from the nerve injury caused untreated rats to withdraw their paws almost immediately, whereas injected rats tolerated much heavier stimuli before withdrawing their paws. Importantly, the treated rats maintained motor function as well as sensitivity to heat and acute pain, all of which can be lost as unwanted side effects of chronic pain medications.

It was a long journey that got us here, and we're pretty happy that we're able to figure out exactly why the coupling happens.
- Rajesh Khanna, University of Florida

Finally, the team designed a plasmid that coded for the CRMP-2 regulatory sequence peptide and packaged it in an adeno-associated virus that delivers genes to neurons without evoking an immune response. Injecting the gene product into the spinal cords of mice with spared nerve injury recovered their tolerance for the applied weights, while giving mice the treatment before the injury prevented loss of tolerance altogether. The treatment also reversed hypersensitivity to weight in mice administered paclitaxel, a common chemotherapy drug that causes acute and chronic pain (5). In each of these in vivo experiments, the effects of the gene therapy lasted several weeks, demonstrating the potential for long-lasting relief from chronic pain.

The study marks one of the first successful demonstrations of gene therapy for chronic pain. “I will caution [against] any kind of false hope that this is going to be the next best thing in chronic pain,” Khanna said. “Nevertheless, this is a major milestone.”

Vladimir Yarov-Yarovoy, a neuroscientist at the University of California, Davis who was not involved in the study, shared this cautious optimism. While impressed by the specificity and durability of the treatment, he pointed out potential roadblocks, including safe delivery, the ethics of gene therapy, and accessibility and cost. Still, he is interested to see if the approach can be applied to treat various types of pain. “As we learn more about potential differences in specific pain conditions and the role of sodium channels in each, [researchers] might be able to develop more precise and safe therapies,” said Yarov-Yarovoy.

Having validated their results in monkey neurons, Khanna’s team now plans to conduct long-term efficacy and toxicity studies in more clinically relevant models. They also hope that the therapy will show promise for treating a broad range of pain types, including migraine, dental, and orofacial pain.