Right Idea, Wrong Target?

Art Courtesy of Jungbin (Jaime) Cha.

Why do treatments fail? Sometimes there is an issue with the treatment’s target. Other times, the translation from an animal model to humans presents too big of a gap. Over the history of research on gene therapy, such failures have brought scientists closer to success. 

The idea behind gene therapy is to treat genetic diseases at their source by altering a missing or faulty gene. For Parkinson’s disease—in which loss of dopamine-producing (dopaminergic) neurons leads to slowness of movement and other severe motor symptoms in late stages—this approach has shown promise, but success has, thus far, been out of reach. 

Past research on gene therapy had targeted a part of the basal ganglia—which is responsible for motor control—called the striatum. In mouse models, this was quite successful, and several papers were published on different genes targeting this area. One of these papers, published in 1994, was covered in the Yale Scientific Magazine by Gautam Mirchandani (Vol. 66 No. 2), who called it a promising study. The proposed therapy was designed to target the gene for tyrosine hydroxylase (TH), an enzyme that is critical for synthesizing dopamine in a Parkinson-like disorder. “Hopes are that such a treatment will soon be available for human benefit,” Mirchandani wrote. Yet, these techniques have all since failed. 

The problem, in many of these cases, was actually the target. While easy to target in a mouse, the basal ganglia in humans has enlarged dramatically over the course of evolution. Since the striatum forms such a large part of the basal ganglia’s structure, it was simply too large of a target. “It is very difficult to cover it by injecting a virus,” said Jim Surmeier, a professor of neuroscience at Northwestern University Feinberg School of Medicine. He is one of the many scientists taking up the task of turning the coal of past failures into the future diamonds that will let gene therapy shine. “We fail more often than we succeed,” Surmeier said. “What distinguishes really good researchers is the ability to learn from your failures.”

A New Target?

Surmeier’s research challenges prevailing theories regarding the function of dopaminergic neurons and their site of action in Parkinson’s disease. Previous researchers had assumed that loss of dopamine in the striatum was sufficient to cause the primary motor symptoms associated with Parkinson’s. However, Surmeier developed a new animal model for Parkinson’s that involved disrupting Complex 1, an important protein for energy generation, in the mitochondria of dopaminergic neurons. The difference in this model was that, unlike most animal models for Parkinson’s which cause rapid onset of the severe motor symptoms, this model more closely mimicked human Parkinson’s with its slow onset. Motor symptoms only became apparent several months after gene editing. This more realistic model led to both a new potential cause of Parkinson’s in humans and a better lens through which to study how brain circuits contribute to the disease.

Using this model, Surmeier discovered something unexpected. “When there was clear loss of striatal dopamine release, the animals were not Parkinsonian, contrary to the prediction of the classical model,” Surmeier said. His new theory takes into account the structure of the basal ganglia. While the striatum is a large complex within this structure, there are other nuclei—clusters of neurons that perform a specific function—modulated by dopaminergic neurons. One of these, which sits between the basal ganglia and the rest of the brain, is the substantia nigra pars reticulata (SNr). While the traditional model of Parkinson’s focuses on almost solely treating the striatum, Surmeier proposed the SNr as a new target for gene therapy. “The basal ganglia are organized like a funnel, with the SNr at the mouth of the funnel. Targeting the mouth of the funnel is the best way to control the output of the basal ganglia,” Surmeier said. 

Now armed with a location, Surmeier needed a gene. In his study published in Nature in 2021, he targeted aromatic acid decarboxylase (AADC), a key enzyme that converts the precursor levodopa into its final form of dopamine. Levodopa is commonly used as a treatment for Parkinson’s. However, its effects wear off with time and more advanced forms of the disease since the dopaminergic neurons that are dying are the primary producers of AADC. Thus, over time, the brain begins to lack sufficient AADC to convert levodopa into dopamine. In this trial in mice, Surmeier was able to use gene therapy to give a new group of neurons the ability to express AADC—those in the SNr—which relieved motor deficits in Parkinsonian mice.

Throughout the process, Surmeier emphasized that one has to be willing to both challenge past assumptions and learn from past failures. When he first received the data from his new Parkinsonian model, Surmeier was skeptical enough to ask others to repeat similar experiments, again and again, when his results did not match preconceived notions. “In science, we never have truth in our hands,” Surmeir said. “It is always an approximation.”

Yet Another Target?

Surmeier’s work is only the tip of the iceberg when it comes to developing gene therapies for previously intractable diseases. His successful process of choosing just the right nucleus for targeting, and just the right enzyme to target for modification, is the result of not just personal failures, but also the failures, and successes, of many of his colleagues. 

One such colleague is Krzysztof Bankiewicz at the Ohio State College of Medicine. Bankiewicz has been interested in dopamine and Parkinson’s disease since the 1980s, when he joined one of the first clinical trials to restore normal dopamine levels in the brain to reverse Parkinsonian symptoms. The clinical trial intended to transplant cells that could produce dopamine into the striatum. But despite trying many stem and fetal cell lines, many dopaminergic cells did not survive the transplantation process. It did not seem like cell transplantation was going to work. 

The advent of gene therapy, where a clinician can change just one gene at a time as opposed to transplanting entire new cell lines, was exciting to Bankiewicz. He began studying specific gene-delivery systems and experimenting with different targets in the brain for the delivery of enzymes that could convert levodopa into dopamine. He also tried many variations of the viral vector, which is used to deliver a properly functioning copy of a gene in gene therapy. After twenty years of repeated failure and learning, he has managed to launch clinical trials intended to deliver genes of interest to treat many neurodegenerative diseases, such as pediatric Parkinson’s disease. He looks at commonalities between diseases to figure out the best way to approach new ones. “In this way, the disease itself becomes more of an application for the operating system,” Bankiewicz said. The operating system is the common pipeline that enables him to optimize a gene therapy’s chance of clinical success.

Bankiewicz is currently trying to develop gene therapies for neurotrophic factors, which are proteins in the brain that support the development and maintenance of neurons (since neuronal damage and death characterizes various dementias). He has worked on increasing the production of the protective brain-derived neurotrophic factor to the entorhinal cortex, a key memory center in the brain, to slow and possibly even reverse memory loss in Alzheimer’s disease. He is currently working on a trial to increase the production of glial-derived neurotrophic factor that promotes dopaminergic neuron survival, which may ameliorate Parkinson’s disease symptoms. He says that there are other diseases to address through the neurotrophic factor pathway, including Huntington’s disease and childhood dementias. 

Bankiewicz’s theory also supports the development of various gene therapies to reduce and replace proteins that misfold and aggregate in various neurodegenerative diseases. For example, he is currently working on a trial to reduce levels of alpha-synuclein, which accumulates in Parkinson’s disease and multiple system atrophy, and is interested in developing a trial to replace the protein progranulin which leads to amyloid protein accumulation in patients with fronto-temporal dementia. 

Bankiewicz’s research is especially important because gene therapy can be a highly effective treatment that works for most patients, regardless of their disease etiology. This means that it does not matter whether a patient has a genetic cause for their disease or if their disease is idiopathic (unknown cause). For example, when patients are given levodopa, the drug addresses their symptoms regardless of the cause of their Parkinson’s disease. Similarly, gene therapy addresses common disease pathology, such as loss of dopaminergic neurons or levodopa resistance in Parkinson’s disease, regardless of whether it is driven by idiopathic or familial factors. This means it has a higher chance of working across patient populations. 

Moreover, gene therapy is a one-time surgical treatment, which is clinically preferable for patients, whose alternatives are either to live with a deep-brain stimulation electrode within their head, spend the rest of their life taking a medication that progressively loses its effectiveness, or let their disease continually advance. Bankiewicz stated that his patients feel safe undergoing the treatment, knowing that its effects are very localized. “They love the idea of just getting one therapeutic for life,” Bankiewicz said. 

This field has made significant leaps since YSM last reported on the 1994 study on gene therapy. Bankiewicz’s and Surmeier’s clinical trials may bring treatment to hundreds of thousands of people who would otherwise live without the hope of recovering from or surviving their disease. Yet all this would not have been possible without countless mistakes. If Bankiewicz has learned one thing along the way, it is that scientists are hardly right every single time. “One lesson that I tell my students and scientists: don’t be afraid to try. You learn from your failures. So, don’t be afraid to fail,” he said.