Causes of ALS
Amyotrophic lateral sclerosis (ALS), also referred to as Lou Gehrig’s disease, is a neurodegenerative disease that has a wide range of symptoms and potential causes. In more than nine out of every 10 cases diagnosed, there is no clear identifying cause – most people living with ALS don’t have large numbers of affected family members that signal an obvious genetic history.
Nor have scientists been able to find clues about what causes ALS in anything about the way patients live their lives. So far, nothing in any individual’s diets, where they’ve lived, how they’ve lived, or what they’ve done with their lives seems to explain why they’ve developed a progressive neurodegenerative disease that slowly causes motor neurons to malfunction and die.
In five to ten percent of all ALS cases, a clear genetic history exists. The disease is classed as autosomal dominant in these individuals. If a parent carries a genetic mutation that causes ALS, each child has a fifty percent chance of inheriting that mutation and developing ALS.
Studies in the early 1990s on genetic forms of ALS revealed that a single gene defect could account for a portion of these familial cases. Roughly fifteen to twenty percent of familial ALS stems from mutations in the gene for the enzymes superoxide dismutase 1 (SOD1) or copper zinc superoxide dismutase. That means approximately one to two percent of all cases of ALS stem from SOD mutations. An additional three percent of ALS results from mutations in other genes.
In October 2011, an international team of scientists, including Packard researchers, identified the most common genetic cause of ALS in a gene called C9ORF72. In this gene, found on chromosome 9, six letters of the genetic code are normally repeated only a handful of times. In up to forty-five percent of those with familial ALS, these six letters are repeated hundreds, even thousands of times. Since this discovery, scientists at Packard and at institutions around the world have been working to figure out how this mutation causes ALS.
Uncovering the Cause of ALS
Still, for the majority of ALS cases–ALS seems to occur in the absence of a family history of the disease. Known as sporadic ALS, this means that the disease appears to occur at random with no clearly associated risk factors or family history.
ALS is primarily an illness of motor neurons. People with ALS experience progressive weakness in muscles responsible for arm and leg movement, speaking, swallowing, and breathing. As lower motor neurons are lost, the muscles that they supply atrophy. Upper motor neuron loss causes stiffness, or “spasticity,” which results in slow and poorly coordinated movements.
However, ALS is also an illness of astrocytes, the common nervous system cells that support motor neurons by providing energy and helping to repair damaged neurons. Throughout the disease, a number of cell processes go awry, contributing to the progress of ALS. So far, none of them appear more important than the other in contributing to disease progression.
Packard Center researchers are targeting these pathogenic processes as the best way to understand the causes of ALS and to develop treatments for the disease:
The SOD1 enzyme, encoded by the SOD1 gene helps to reduce damage from highly reactive and toxic chemicals called free radicals. Scientists assumed that losing this ability via a mutation caused ALS in these patients, but Packard Center scientists and others have shown that mutant SOD1 brings about motor neuron death by other means.
Patients and ALS model animals can have good SOD1 enzyme function and still suffer progressive motor neuron loss. Packard scientists are actively exploring how mutant SOD1 damages cells, trying to show the important missing links between the damaged gene and all of the other pathological processes linked to ALS.
Because we know for certain that mutations in the SOD1 gene are toxic to motor neurons and are responsible for a subset of ALS cases, there’s likely great value in understanding how mutated SOD1 causes motor neuron death. Investigators refer to the resulting line
An abnormal clumping of proteins in neurons is a common feature of Alzheimer’s, Parkinsons, and other neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Researchers now have multiple lines of evidence suggesting that these protein clumps are toxic to cells.
Packard researchers found that aggregates of SOD1 protein appear early on accumulate as the disease progresses, suggesting that aggregation may be an early event in ALS’s pathology. SOD1 protein clumps are also found in the motor neurons of model ALS mice, just before or at the same time that ALS symptoms begin.
Recently, aggregates of two other proteins, TDP-43 and FUS, were isolated in ALS patients—mostly those with the familial disease, though some with sporadic. The fact that they appear in both types of patients is important. More studies will show if the proteins are part of what triggers ALS onset or progression. The appearance of these aggregates provides, in principle, the possibility of a prime drug target—one that lets us stop or slow the disease early on.
Mitochondria supply energy crucial to the survival of each and every cell. Abnormal, poorly functioning, or dying mitochondria are common in neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Unusual-appearing mitochondria appear early in animal models of ALS—even before disease symptoms surface.
Past Packard Center research has shown that normal mitochondrial movement throughout motor neurons becomes erratic, although how this affects the availability of energy at key places in motor neurons, such as synapses, isn’t well understood. It’s a current topic of our Center’s grantees’ research.
Packard Center scientists have been actively pursuing possible biochemical causes of ailing, abnormal mitochondria in mouse and yeast models of ALS, in the hopes that understanding what’s happening to the tiny cell bodies will shed light on what starts the disease, both in familial and sporadic ALS.
Growth factors stimulate cell growth and proper cell development, as well as regulating a variety of cell processes.
Several years ago, scientists discovered that mice carrying a human mutated VEGF
(vascular endothelial growth factor) gene developed ALS symptoms, which led to the idea that VEGF might play a part in starting or maintaining ALS. The mutant mice failed to
produce enough VEGF, also developing progressive loss of muscle function and motor neuron death.
In humans, a large study of European patients identified three different mutations in the VEGF gene as significant risk factors for ALS.
VEGF isn’t the only growth factor implicated. Like VEGF, other growth factors such as IGF-1 (insulin-like growth factor) and GCLN (glial cell line-derived neurotrophic factor) also appear to have neuro-protective properties. In principle, loss or deficiency of these factors could make motor neurons more vulnerable to damage.
Ongoing clinical trials of VEGF should tell if it’s a useful therapy.
Every cell in the body has a built-in suicide program that’s normally kept under control. Programmed cell death, known as apoptosis, can occur at various stages during development, or when a cell becomes too damaged by infection, cancer, or old age.
Studies by Packard scientists and others show that in ALS, however, this process occurs prematurely. In the SOD1 mouse model of the disease, scientists have verified that motor neurons ultimately die via apoptosis. More recent work by our researchers shows that the continued, early activation of apoptosis pathways occurs before ALS symptoms appear in the model mice. The end stages of the process are turned on shortly before motor neuron death.
Drugs that inhibit apoptosis have shown some success in ALS mice by delaying disease onsent and progress. These agents are not yet available for human use, although many companies are beginning to develop them. Understanding cell death pathways is an active area for future ALS research at both the basic science and the therapeutic level.
Motor neurons are often extremely long cells with their axons leading from the spinal cord to muscles. The sheer distance that nutrients and other materials must travel back and forth from the cell body to the synapse creates logistical issues. Just the added energy requirement for transport makes these cells vulnerable in a way others aren’t.
Much of Packard Center research has focused on impaired axon transport as a potential cause of ALS, with much work concentrated on neurofilaments, the long protein tubules in cells that play a key role in stimulating axon growth. One of the hallmarks of ALS is the over-accumulation of neurofilaments in parts of motor neurons. mice with mutations in neurofilament genes also have poorly functioning motor neurons.
If SOD1 mice are made to overproduce neurofilaments, the mice have longer lifespans. Since the excess neurofilaments collect in the cell body rather than the axons, Packard scientists believe that, with fewer neurofilaments in the way, axon transport may be improved in these animals.
Axon transport involves more than neurofilaments. Microscopic molecular “motors” can move materials across entire axons. Mutations in the motors’ genes can paralyze mice. One family study showed that a specific mutation in the gene for the motor protein dynactin resulted in members with paralyzed vocal cords.
Past Packard Center research has focused on what happens to axon transport of mitochondria – the cell’s powerhouses – in ALS. Early studies show distinct differences in people and animals with ALS.
A growing number of studies suggest that the processes that damage motor neurons in ALS don’t exclusively affect those cells. Neighboring astroglial cells, Packard research has shown, also contribute to ALS’s progression.
Astrocytes help to clear away toxic glutamate from motor neuron synapses. In the mid-1990s, our scientists discovered that astrocytes in many ALS patients and the SOD1 rodent models of the disease are unable to do this properly due to abnormal transport molecules in the astrocytes.
These astrocytes also contain toxic protein aggregates that increase in number as the idsease progresses. Packard studies in animal models have shown that damaged astrocytes increases the vulnerability of motor neurons to damage.
Exactly how astrocytes single out motor neurons for destruction isn’t well understood; it’s likely that these cells may produce something toxic, as well as fail to clear glutamate away properly. This is an area of intense study.
One of the most-studied mechanisms for motor neuron death involves their abnormal stimulation by the amino acid glutamate.
Normally, glutamate acts as a neurotransmitter, carrying messages across synapses throughout the brain and spinal cord. The presence of too much glutamate, however, kills neurons. This buildup can occur if neurons are overstimulated (known as excitotoxicity) or if cell chemistry goes awry. Normally, it’s prevented by glutamate transporters, molecules embedded in cell membranes that act like “sponges” to remove the neurotransmitter. Glutamate transporters are most plentiful at synapses. Even more exist on the cell membranes of astrocytes, neurons’ central nervous system companion cells.
In the early 1990s, a large study of nearly 400 patients with sporadic ALS showed some 40 percent had increased glutamate levels in cerebrospinal fluid – a byproduct of excitotoxicity. The higher the glutamate levels, scientists found, the more severe the disease.
Packard scientists and others proved glutamate-based excitotoxicity is part of a process leading to motor neuron death in ALS. Glutamate transporters are either inefficient or don’t exist in sufficient supply to prevent glutamate buildup. Up to 80 percent of ALS patients have some abnormality in glutamate transporters, our studies show.
ALS model mice have less than half of the normal glutamate transporters in their spinal cords, a reduction that begins well before symptoms start. Remove glutamate transporters from ALS mice entirely and neurons swiftly die.
Earlier Packard research showed that overexpression of glutamate transporters in mouse models delays onset of symptoms and extends life. More recent Center-assisted studies showed that beta-lactam antibiotics boost transporter numbers, delay disease onset, slow its progression, and prolong life in animal studies.
Excitotoxicity also makes a good therapeutic target and clinical studies are ongoing.
More than a decade ago, scientists identified mutations in TDP-43 as a relatively common genetic cause of ALS.
TDP-43 directs the production of proteins that bind RNA. RNA directs the assembly of proteins from amino acids and helps to shape the final product. Scientists suspect that mutations in TDP-43 somehow upsets this process.
Packard investigators are especially eager to discover exactly how mutant TDP-43 damages cells, to see if it does, indeed, upset RNA metabolism. That’s because their earlier discovery of ALS excitotoxicity showed that faulty RNA metabolism plays a major part in that greatly destructive process. Being able to link a mutant gene to one of the most cell-damaging happenings in ALS would be the most important find in ALS pathology research to date.
Mutations in the gene coding for TDP-43 also causes clumps or aggregates in motor neurons. Alzheimer’s and Parkinson’s disease also involve toxic protein aggregates in nervous system cells.
Forming aggregates in an important sign. But what most excites Packard and other scientists is that TDP-43 mutations have been identified both in familial and in sporadic ALS patients.
As it is, TDP-43 has the potential to lead to an entirely different set of scientific observations, not to mention finding biomarkers for ALS, improved imaging of the disease and new animal models. All of that would set us more firmly on a road to therapy.
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