Investigators have identified a common pathogenic mechanism for multiple forms of amyotrophic lateral sclerosis.
DR. TEEPU SIDDIQUE: “This is a novel pathology that opens up a very real possibility for treatment. This is the first time that a reliable functional relationship to neurodegeneration has been identified that can explain all forms of ALS. All points converge on this pathway.”
Using old-fashioned genetic sleuthing, a team of neurologists at Northwestern University specializing in amyotrophic lateral sclerosis (ALS) has identified mutations in a gene called UBQLN2 that cause dominant X-linked juvenile and adult-onset ALS and ALS/dementia. The gene codes for a protein called ubiquilin 2 that has a major role in regulating the protein degradation pathways.
The scientists — led by Teepu Siddique, MD, Les Turner ALS Foundation/Herbert C. Wenske Foundation Professor and director of the Neuromuscular/Neurogenetics Division at Northwest University — first identified the genetic mutation in one large ALS pedigree and went on to assess other familial cases, as well as individuals with the sporadic form of the disease. They reported in the Aug. 21 online edition of Nature that there was ubiquilin 2 pathology, which they believe leads to a build-up of excess protein that in turn damages motor neurons.
While it is still not clear whether ubiquilin 2 pathology is the cause of ALS, Dr. Siddique said that it is “the first known mechanism that can unite the field.”
“These protein aggregates have not been seen before,” Dr. Siddique said. “This is a novel pathology that opens up a very real possibility for treatment. This is the first time that a reliable functional relationship to neurodegeneration has been identified that can explain all forms of ALS,” he said. “All points converge on this pathway.”
The scientists added that it is a “common pathogenic mechanism that can be exploited for therapeutic intervention.”
The story that led to this latest finding began in 1984 when Dr. Siddique and his colleagues showed that scientists could apply genetic linkage techniques to ALS. They had been studying autopsy tissue samples from a World War II Navy veteran who had ALS. The linkage studies pointed to an intriguing gene — superoxide dismutase 1 (SOD1). But the SOD1 mutation and two subsequent genetic mutations explain 30 percent of familial cases, which only represent less than 5 percent of all ALS cases.
As it turned out, the SOD1 mutation was not the culprit in the veteran's family. They went back to the genetic linkage method with DNA from 18 other individuals with ALS in his family. They identified a region on the X-chromosome and identified the mutation in the UBQLN2 gene. It was a dominantly inherited disease with reduced penetrance in the female patients in this family. The mutation results in a single substitution of proline with histidine. They also conducted sequencing studies and the mutation was not in the single nucleotide polymorphism database.
They set out to look for this mutation in other families. They analyzed DNA from 188 ALS patients with a familial form of the disease. These families also included patients with ALS dementia. They excluded patients with known mutations, including SOD1, TAR DNA-binding protein (TDP43), and fused in sarcoma (FUS, also known as translocated in liposarcoma).
The investigators identified four other UBQLN2 mutations in four unrelated families. When they looked at their medical records, they observed that 35 of 40 individuals identified had ALS and the remaining five had the mutation but no evidence of disease. They said that there was a 90 percent penetrance of the disease by the age of 70. In these families, the age of onset spanned from 16 to 71 years old. Males had an earlier age of onset — at an average of 33 years old — compared to females whose average age of onset was 47 years. Eight of the 35 patients also had ALS dementia.
Virtually all neurodegenerative diseases are marked by abnormal protein aggregation. In ALS, protein aggregates are primarily in the spinal motor neurons, and are characterized by skein-like morphologies, linear finger-like structures in the cytoplasm.
The investigators conducted a series of immunochemistry studies to see whether ubiquilin 2 is present in the skein-like inclusions in post-mortem spinal cord sections from ALS patients as well as inclusion formations in the X-linked ALS. All tests were positive for ubiquilin 2 protein aggregates. They found the same inclusions in other types of ALS, including sporadic cases. They even looked for this pathology in the hippocampus in post-mortem samples from patients with ALS dementia and found inclusions in the hippocampal regions that have not been seen before, said Dr. Siddique. They went on to look at hippocampal regions from ALS dementia patients without the ubiquilin 2 mutation and also found accumulations of the protein.
DR. HIROSHI MITSUMOTO said the study suggests “if you study more cases that you may resolve the puzzle of sporadic ALS,” but added that no one knows why the abnormality occurred in the first place. “There are probably many roads to ALS and at least we know the final pathway.”
Dr. Siddique said that the exact function of ubiquilin 2 is not clear but it does have housekeeping functions that clean away misfolded or damaged proteins. Proteins need to be recycled for the synapse and other regions of the neurons to function normally, he said.
DR. STANLEY APPEL said the new findings establish “a new pathway that is important,” adding: “I am worried that no single pathway will be the panacea for ALS.”
“These data provide robust evidence for an impairment of protein turnover in the pathogenesis of ALS and ALS/dementia,” the authors wrote. “Elucidation of this process may be central to the understanding of pathogenic pathways. These pathways should provide novel molecular targets for the design of rational therapies for these disorders.”
Dr. Siddique said that this convergence means that there is a pathway they can target. “We are very excited. It is now a molecular biology question. We need small molecule testing to find a drug that acts on this pathway.”
“This is a major step forward,” said Jeremy Shefner, MD, PhD, professor and chair of neurology at Upstate Medical University in Syracuse. He is also co-chair of the Northeast ALS Clinical Trials Consortium. “The study is well done and it points us in a new direction. The meaning for patients with sporadic ALS has yet to be worked out.”
Stanley Appel, MD, director of the Methodist Neurological Institute in Houston, added that this study “establishes a new pathway that is important.” That said, he added: “I am worried that no single pathway will be the panacea for ALS.”
Still, he said that this finding “gives us more insight into the basic processes and it is a seminal discovery. This is direct evidence that this pathway is involved in many types of ALS. We just don't know whether normalizing ubiquilin 2 will protect the motor neurons from damage.”
Hiroshi Mitsumoto, MD, the Wesley J. Howe Professor of Neurology at Columbia University and head of the Neuromuscular Division, sees this study “as one of the most important studies in ALS and is a key missing piece of the jigsaw puzzle. It is present in many different kinds of ALS.”
“This tells me that if you study more cases that you may resolve the puzzle of sporadic ALS,” he said, adding that no one knows why the abnormality occurred in the first place. “There are probably many roads to ALS and at least we know the final pathway.”
The ALS clinician sees the potential for molecular therapies. “But we still need to do a lot of work to identify therapies and test them, “ Dr. Mitsumoto said.
ALS clinicians and researchers are converging in Tarrytown, NY at the beginning of September for an international conference hosted by the Columbia University neurologist. He suspects that this study will be a focus of the talk. “We have to pay attention to pathogenesis. And this study does just that.”