First, some basic science to lay the groundwork for this blog post. Parkinson’s disease (PD) is all about dopamine, the chemical neurotransmitter that makes our movements smooth. It is produced by cells in the substantia nigra, a structure in the midbrain. The substantia nigra nerves project to the putamen, one of the structures that make up the basal ganglia, somewhere deep in the brain. The substantia nigra nerves are also called the nigrostriatal nerves because the putamen, along with the caudate nucleus and the nucleus accumbens, form a body called the corpus striatum. The work of these so-called nigrostriatal nerves is to produce and deliver dopamine to the putamen. In summary, the putamen is the playpen of dopamine; it is here that it does its work of smoothening our movements.
In Parkinson’s disease, the nogrostriatal system slowly degenerates, therefore becoming unable to supply enough dopamine to the putamen. The obvious solution is to find an alternative supply of dopamine for the putamen. The obvious way again would be to deliver dopamine orally as a tablet, but dopamine unfortunately does not cross the blood brain barrier. However, the similar but more pliant levodopa is able to do so. Once in the brain, levodopa is then converted to the active dopamine by an enzyme called aromatic L‐amino acid decarboxylase (AADC). Because this strategy is reasonably efficient, levodopa has become the foundation of PD treatment. But this strategy is totally dependent on the presence of enough AADC to convert levodopa to dopamine. And this is a vulnerability that PD explores to the full.
Levodopa treatment is usually effective in the early stages of PD. But as the disease progresses, the degenerating nigrostriatal nerves increasingly struggle to produce enough AADC. Remember, AADC is essential for converting levodopa to the active dopamine. Without AADC, in other words, levodopa is useless. The declining ability to produce AADC is therefore the Achille’s heel of levodopa treatment. It is the reason people with advanced PD require increasingly higher doses of levodopa. It is the reason they get unpredictable treatment fluctuations. It is the reason they get abnormal movements called dyskinesias. To remedy this big flaw in the levodopa treatment strategy, and increase AADC levels in the putamen, neuroscientists have investigated the potential role of gene therapy. To unravel this topic, not a ride in the park by any means, I have relied on this excellent 2019 paper titled Magnetic resonance imaging–guided phase 1 trial of putaminal AADC gene therapy for Parkinson’s disease.
If one group of cells becomes unable, or unwilling, to do its job, why not get another group of cells to take over the task? Indeed this simple concept lies at the heart of gene therapy for PD. And neuroscientists have identified the right type of cells to take over the job of producing AADC. These are the medium spiny neurones of the putamen which do not degenerate in PD. The brilliant strategy is to embed the gene for producing AADC into the DNA of the medium spiny neurones. A viral vector is required to carry the gene into the nerves, and the vector of choice here is adenovirus-associated virus (AAV). The vector ‘invades’ the medium spiny neurones and embeds the AADC gene into their DNA. The cells then start producing dopamine from levodopa. It is as simple as that in theory. It is easier said than done in reality.
The intricate steps involved in this strategy are outlined by Chadwick Christine and colleagues who carried out the phase 1 trial of AADC gene therapy. They infused the AAV viral vector directly into the putamen during neurosurgery, and they used magnetic resonance imaging to confirm that the injected material is delivered to the correct target. The detailed protocol refers to technical terms such as bilateral frontal burr holes, intraoperative delivery, neuro‐navigational systems, and the like. The whole affair however appears to be well-tolerated and reasonably successful; the authors reported a dose-dependent increase in AADC enzyme production, and their 15 subjects had more ‘on-time’, less troublesome treatment fluctuations, and required less levodopa. It is interesting that a similar benefit was demonstrated by Karin Kojima and colleagues when they used the same procedure in a genetic disorder called aromatic l-amino acid decarboxylase deficiency. In their paper titled Gene therapy improves motor and mental function of aromatic l-amino acid decarboxylase deficiency, the authors reported ‘remarkable’ motor improvement in all the six subjects they treated.
An alternative approach to PD gene therapy is to use the AAV viral vector to deliver, not the gene for producing AADC this time, but the gene for producing glial cell line‐derived neurotrophic factor (GDNF). The idea behind this is, not to replace, but to flog the dying horse. The theory is that GDNF, a growth factor, should rejuvenate the flagging nigrostriatal nerves, thereby increasing their ability to produce dopamine. This approach was described by John Heiss and colleagues in their paper titled Trial of magnetic resonance–guided putaminal gene therapy for advanced Parkinson’s disease. The authors indeed demonstrated that GDNF-carrying adenovirus vectors can be safely infused into the putamen, and that the process is well-tolerated. They also demonstrated increased dopamine levels in the putamen in 12 of their 13 subjects.
It is clearly early days, but there have been small successes along the way so far. Future trials, already underway, will tell us whether the hope is sustained or dashed. We must wait and see. In the meantime, you can read more about PD gene therapy in this update.
