June 7, 2022
1: Can you describe the central function of oligodendrocytes and myelin generation?
Oligodendrocytes and neurons are types of brain cells. Neurons have a cell body that holds the nucleus (where the cell’s genes are located), dendrites that branch off from the cell body, and an axon that extends away from the cell body like a cable. Neurons communicate with one another by receiving electrical signals (called “action potential”) via dendrites and sending these signals along their axons. An action potential travels from the cell body, down the length of an axon, ultimately triggering the release of chemical messengers (called “neurotransmitters”) from the end of the axon onto the dendrites and cell bodies of other neurons, which can cause a new action potential to form in the next neuron in the network. The regions of the brain in which the neurons are located is commonly referred to as “grey matter.”
Oligodendrocytes have many extensions, known as processes, each of which contacts and wraps tightly around a piece of an axon. A useful metaphor is an octopus with multiple tentacles, with each tentacle (a process) wrapping around a cable (an axon). In wrapping around the axon, the oligodendrocyte process forms numerous layers of compacted cell membrane in a spiral around the axon segment. The compacted membrane is called myelin, and the process of wrapping is called myelination. A single oligodendrocyte can myelinate segments of up to 50 axons, and conversely adjacent myelin segments on the same axon could belong to different oligodendrocytes. The regions of the brain in which the oligodendrocytes and myelinated axons are located is commonly referred to as “white matter.”
Myelination of axon segments provides insulation for the axon, allows for high-speed conduction of neural impulses (i.e., the electrical signals) over long distances, and thereby increases the speed of neuronal communication. Myelinated axon fibers can carry impulses up to 100 times faster than non-myelinated fibers. Oligodendrocytes also provide growth factors and metabolic support to the brain and spinal cord on the whole to maintain the overall health of the central nervous system. When oligodendrocyte function is impaired and myelination is disrupted, such as in the leukodystrophies, severe, adverse consequences on brain function can ensue.
2: How does Myrtelle’s novel class of nonpathogenic recombinant adeno-associated virus (rAAV) vectors enable gene therapy for disorders involving myelin?
Gene therapies generally treat disease by delivering a healthy, functional gene to a patient in whom that gene is defective due to a genetic mutation, or by silencing a disease-causing gene that is not functioning properly. It isn’t easy to put a gene into a cell. A “vector” is required that acts as a vehicle to carry therapeutic genetic material to a target cell. This is something that a special type of nonpathogenic virus can safely do. A useful metaphor is a letter that comes in the mail. A virus consists of a protein coat (which is like the envelope that has an address), inside of which are genetic instructions (like the letter with directions inside the envelope).
While many types of viruses have been explored as potential nonpathogenic gene therapy vectors, the most widely used are recombinant adeno-associated viruses (rAAV) due to their long track record of safety in hundreds of clinical trials worldwide. Many types of rAAVs have been discovered. Each type has a slightly different protein coat (called a capsid), resulting in differing abilities to carry genetic information into various cell types (much like different addresses on the envelopes result in differences in which mailboxes get a letter). Different rAAVs have different abilities to deliver genetic instructions to the various kinds of cells in the body, such as cells of the retina, lungs, liver, pancreas, kidneys, heart, muscles, and the neurons of the brain. Historically, none of these available rAAV types are effective in targeting oligodendrocytes. However, a new class of rAAVs that has been shown to efficiently target oligodendrocytes has recently been developed. Myrtelle is exploring this class of novel rAAVs as a potential treatment for Canavan disease, which belongs to a group of rare, genetic, neurological disorders called leukodystrophies that result from disruption of myelin function in the white matter of the brain. Our lead vector in this class is rAAV-Olig001. rAAV-Olig001 was shown to efficiently deliver genetic information to oligodendrocytes in several species, including a mouse model of Canavan disease. Since the genes that contribute to proper myelin formation and maintenance are found in oligodendrocytes, rAAVs that efficiently target oligodendrocytes, like rAAV-Olig001, offer new opportunities to treat disorders involving myelin, such as the leukodystrophies and other white matter diseases.
3: Myrtelle has just expanded its pipeline with Pelizaeus-Merzbacher Disease (PMD). Can you explain why and what has been learned from the Canavan program that might apply to PMD?
Like Canavan disease, PMD is a rare leukodystrophy in which dysfunction of myelination produces loss of coordination, motor abilities, and intellectual function, often in the first year of life. In more severe forms, the prognosis is poor, with progressive deterioration resulting in death. The most common form of PMD is caused by a duplication mutation of the gene encoding the myelin protein “proteolipid protein 1” (PLP1). This means that the oligodendrocytes of a PMD patient make too much PLP1. Normally, PLP1 is thought to help the layers of compacted myelin to adhere properly to each other. Unfortunately, too much PLP1 in oligodendrocytes triggers dysfunction and disrupts proper myelin formation. Reducing PLP1 expression to normal levels in patients with gene duplications would be expected to restore oligodendrocyte function and improve outcome. By coupling rAAVs that target oligodendrocytes (the addressed envelope) with “gene-silencing” genetic instructions (the therapeutic letter), Myrtelle is working toward developing new, safe, and effective gene therapies for PMD.
Like Myrtelle’s gene therapy for Canavan disease, we believe that the optimal approach to delivering a gene therapy for PMD will be to administer it directly into the fluid that surrounds the brain and spinal cord (cerebrospinal fluid) to best access the cells in need of the therapeutic genetic material. Because of the Canavan program, Myrtelle has built a team with deep and unique experience in manufacturing oligodendrocyte-selective rAAVs and other novel gene therapy product candidates, testing their safety and efficacy, running clinical development programs for rare pediatric leukodystrophies, and keeping the patient community informed. Much of what we have learned as a company with the Canavan program should apply to the PMD program.
4: There has been some recent excitement about the role and importance of Oligodendrocytes in relation to the CNS. Can you provide some details of that?
Myelin was first described over 100 years ago by the great German pathologist Rudolf Virchow. He called it “Nervenkitt” (German for “nerve-glue”). Since that time, myelin has come to be appreciated as the fatty substance that wraps around and insulates neuronal axons and speeds up neuronal conduction. It is often compared to the rubber insulation on a copper wire. However, new research indicates that myelin might be more than nerve glue or rubber insulation. White matter consists mostly of myelinated axons and takes up about half of the human brain. Collections of myelinated axons called “tracts” connect the various parts of the brain and spinal cord and enable their communication. Neurons communicating in a network can be close by or far away in the human body. For example, wiggling a toe requires microscopic neurons in the brain (near the top of the head) to send the signals along their axons all the way to the lower spinal cord (midway down the back). This creates a problem: if two neurons need to communicate with a third neuron simultaneously, but their axons are of different lengths, how can the network compensate? One way to do this is to alter the degree of myelination to adjust the conduction velocity (more myelination means faster conduction) in order to synchronize the arrival of the two action potentials. In short, it is becoming clear that oligodendrocytes play a much more dynamic role in the capacity of the brain to process information than previously thought.
Further, new MRI techniques have revealed that human white matter can change during learning. Changes in white matter structure or abnormalities in myelin genes are also associated with psychiatric disorders like schizophrenia, depression, and obsessive-compulsive disorder, neurodevelopmental disorders like autism, and neurodegenerative diseases like Alzheimer’s. These studies indicate that white matter structure can change after learning, and conversely, that sub-optimal adaptation of white matter may contribute to a host of neuropsychiatric and neurological disorders.
In addition to their role in forming myelin, oligodendrocytes perform other important functions. They make proteins, enzymes, and nutrients that provide energy, promote growth, and support overall brain health. The growing list of the capabilities of oligodendrocytes sheds new light on their wide-ranging and diverse functions, which historically have been overlooked and underappreciated. This new understanding is providing novel opportunities to intervene in disorders that involve oligodendrocytes, myelin, and white matter as a whole. Myrtelle’s research into oligodendrocyte-selective rAAVs may open new avenues to developing innovative gene therapies for these diseases.