Scientists at Northwestern Medicine have identified a potential gene therapy that could reverse involuntary movements in late-stage Parkinson's disease patients, offering a non-surgical alternative to deep brain stimulation. The breakthrough targets a specific type of brain cell malfunction caused by long-term levodopa treatment, the standard medication for Parkinson's symptoms.

What Causes Involuntary Movements in Parkinson's Patients?

Parkinson's disease affects approximately 8.5 million people worldwide and develops when dopamine-producing neurons in the brain gradually die. Dopamine is a chemical messenger that helps coordinate movement, so patients lose the ability to control their muscles smoothly. The first-line treatment, levodopa, converts to dopamine in the brain and helps restore movement control.

However, as the disease progresses and more dopamine neurons are lost, patients need increasingly higher doses of levodopa to manage their symptoms. This is where a serious problem emerges: higher doses trigger a condition called levodopa-induced dyskinesia (LID), characterized by involuntary, jerky movements that patients cannot control. Nearly 80 percent of Parkinson's patients will develop dyskinesia over time, making it one of the most common and distressing complications of long-term treatment.

The involuntary movements occur because levodopa disrupts the strength of connections between different brain regions. "Levodopa disrupts the strength of the connections between the cortex and the striatum," explained D. James Surmeier, senior author of the study and chair of Neuroscience at Northwestern. "These connections between neurons dictate when they become active and how they perform their duties in controlling movement. In late-stage patients, levodopa begins to 'scramble' these connections, which we think leads to uncontrolled movement or dyskinesia".

How Does the New Gene Therapy Work?

Surmeier's team used molecular, cellular, and behavioral studies in mice to understand exactly what goes wrong in the brain during dyskinesia. They discovered that dyskinesia causes an increase in specific receptors called GluN2B-containing NMDA receptors in a subset of striatal neurons responsible for suppressing unwanted movement.

The researchers then tested whether reducing these receptors could help. When they knocked down the expression of GluN2B messenger RNA in these specific neurons, something remarkable happened: not only did dyskinesia fail to develop in the first place, but the therapy also reversed dyskinesia that had already become established in mice.

"What was very exciting was that, in contrast to almost everything else that has been tried in the last 30 years, knocking down this one NMDA receptor subunit in this particular group of cells reversed established dyskinesia," said D. James Surmeier.

D. James Surmeier, Nathan Smith Davis Professor and Chair of Neuroscience at Northwestern Medicine

The approach uses a specially designed viral vector to deliver the therapy systemically, meaning patients would not need brain surgery. This is a significant advantage over current surgical options like deep brain stimulation, which requires implanting electrodes directly into the brain.

Why This Matters for Parkinson's Patients

Current treatment options for dyskinesia are limited. Deep brain stimulation can help some patients, but it requires invasive surgery and carries surgical risks. Many patients have few alternatives once dyskinesia develops, leaving them struggling with involuntary movements that interfere with daily life and quality of life.

A non-invasive gene therapy could transform care for these patients. Surmeier noted that "nearly 80 percent of Parkinson's disease patients will develop dyskinesia. At present, we have a very limited set of tools to help these patients right now. This work points to the possibility of a non-invasive gene therapy that would be transformative".

Surmeier

Steps Toward Human Treatment

• International Consortium Formation: Surmeier is organizing an international research consortium to pursue this therapeutic approach and determine whether it could be safely used in humans.
• Mechanism Validation: The team will continue studying how targeting GluN2B-containing NMDA receptors in specific striatal neurons prevents and reverses dyskinesia without reducing levodopa's beneficial effects on movement.
• Clinical Translation Planning: Researchers will work to develop protocols for testing the gene therapy in human patients, with the goal of eventually offering this as an alternative to surgical interventions.

The research was published in the journal Neuron and was supported by the National Institutes of Health, the Freedom Together Foundation, the Swedish Research Council, the Bumpus Foundation, and the BRAIN Armamentarium. The lead author was Weixing Shen, with co-authors Qiaoling Cui, Zhong Xie, and Tatiana Tkatch from Northwestern's neuroscience department.

While the therapy has only been tested in mice so far, the findings represent a major step forward in understanding dyskinesia and offer hope to the millions of Parkinson's patients who develop this debilitating side effect. If successful in human trials, this gene therapy could provide a non-surgical option that maintains the symptom-relieving benefits of levodopa while eliminating the involuntary movements that make daily life so challenging.