Mystery Protein May Be Key in ALS

by Jim Schnabel

July 31, 2008

During the past two years, a growing number of researchers investigating the cause of amyotrophic lateral sclerosis (ALS) have begun to focus on a protein known as TDP-43. Abnormal clumps of the protein have been found in the brains of people with ALS, in a pattern reminiscent of amyloid in Alzheimer’s; in recent months, seemingly disease-causing TDP-43 mutations have been found in some people with rare, familial forms of ALS. Many researchers are now optimistic that the study of TDP-43 will lead to an understanding of ALS’s major causes as well as better diagnostic tests and treatments.

“If we believe that the protein inclusions in Alzheimer’s and Parkinson’s tell us something about the causes of those diseases, then by analogy I think that TDP-43 is something that we should explore in great depth,” says Virginia Lee, a neuropathologist at the University of Pennsylvania whose laboratory first linked TDP-43 to ALS.

“I’m very excited by it,” says Jeff Rothstein, who heads the Robert Packard Center for ALS Research at Johns Hopkins University. “I believe we now should be spending a lot of effort on TDP-43.”

Lou Gehrig’s disease

Made famous in the United States seven decades ago when it struck down baseball star Lou Gehrig in his prime, ALS is characterized by the destruction of muscle-controlling neurons in the motor cortex and spinal cord. It is estimated to affect about 1 in 100,000 people worldwide, with a median age at diagnosis of about 55 but a wide age distribution that reaches even into early adulthood.

In clinical trials, no drug has yet seemed truly effective in slowing the disorder, which in rare patients (such as the physicist Stephen Hawking) stops progressing on its own, but in most patients causes respiratory failure within two or three years. The only FDA-approved drug for the condition, riluzole, prolongs the lives of people with ALS by only a few months on average.

As has been the case for other neurodegenerative diseases, such as Alzheimer’s and Parkinson’s, researchers have found a number of specific gene mutations that can cause rare, familial forms of ALS but the cause of the “sporadic” majority of cases has been elusive.

Two years ago, however, a team of researchers led by Virginia Lee and her husband and laboratory co-director, John Trojanowski, reported in Science that they had found a major clue to sporadic ALS. Examining the autopsied brains of people with ALS, or with clinically related disorders known as the frontotemporal dementias (FTDs), Lee and Trojanowski and their colleagues found many clumps of a misfolded protein, TDP-43, inside neurons and other cells in disease-affected brain areas.

TDP-43 vs. SOD1

TDP-43, a DNA-binding protein, is usually found in the cell’s nucleus and is presumed to have a role in managing the translation of DNA into RNA and thence into proteins. Its precise function remains unknown. But in the study by Lee and Trojanowski’s lab, the TDP-43 clumps were found outside the nucleus, in the cytoplasm, and the ways in which the proteins had been processed hinted that cellular mechanisms meant to remove damaged or harmful proteins had tried but failed to dispose of them.

That finding on its own was far from being proof that abnormal forms or processing of TDP-43 could cause ALS. But last year, a group of researchers including Lee and Trojanowski looked at the brains of 111 people who had died of ALS, including a rare but well-studied form of familial ALS caused by mutations to the gene superoxide dismutase 1 (SOD1). The researchers detected the clumps of TDP-43 in the brains of all cases except the SOD1 cases, suggesting that most forms of ALS were driven by TDP-43 abnormalities, while SOD1 was relevant only in rare cases.

If so, this finding would have major implications for ALS research, since the standard tool for drug testing in ALS since the mid 1990s has been a transgenic mouse with mutant SOD1—a “model” assumed to share most of its disease pathways with ordinary ALS [see story “Mouse Models: Handle with Care,” May 19, 2008].

In recent months, genetic evidence has persuaded even more researchers that TDP-43 is the lead suspect in ALS. In several publications, beginning with a paper in Science in March, competing teams of researchers have reported finding a total of 15 separate mutations to the TDP-43 gene in ALS patients—including a few patients previously thought to have non-familial, “sporadic” forms of ALS. In these patients with TDP-43 mutations, ALS appeared to be transmitted in an autosomal dominant manner—just like SOD1-driven ALS—so that even a single bad copy could cause disease.

Before these findings, Rothstein had found the neuropathological evidence for TDP-43’s central role unpersuasive, calling it “patho-babel” in an editorial. “No one had genetic information that allowed them to make a cogent scientific statement,” he remembers. “But that evidence exists now, and so now is the time to really spend the effort.”

In fact, Rothstein now wonders whether he narrowly missed finding the TDP-43 connection a decade ago.  “We had a paper in Neuron in which we showed that RNA metabolism was abnormal in ALS,” he says.  Rothstein and his colleagues didn’t know what to make of it at the time, but he recognizes now that the specific RNA-processing abnormalities seen then “are the kinds of things that TDP-43 would normally control.”

The evidence that TDP-43 plays a central, causative role in ALS is not yet conclusive, note Rothstein and other researchers. It is not even clear that TDP-43 aggregations are totally absent from SOD1-driven ALS. “Several labs in Canada have shown at meetings that TDP-43 aggregates in SOD1 tissues as well,” Rothstein says. “So I’m not sure there’s not an overlap, [but if there is] it may not be a dramatic overlap.”

TDP-43 might be merely a marker of the disease, produced somewhere in the course of illness, and perhaps in greater amounts for the more common forms of ALS. The finding of TDP-43 mutations in some people with ALS hints that TDP-43 abnormalities in some cases are a cause of the disease, but they may be causative only rarely. Researchers in two other recent studies have scanned a total of several hundred ordinary ALS patients and found no TDP-43 mutations, suggesting that such mutations are uncommon. It may be that factors other than TDP-43 mutations damage TDP-43 proteins or somehow disrupt their normal RNA-handling activities in the nucleus.

In any case, one of the obvious next steps for ALS researchers is to develop a transgenic mouse model carrying mutant TDP-43 genes, to see if it develops pathology like that seen in ALS in humans.

“Until somebody does that in an animal model or modulates TDP-43 in a human and affects disease, you can’t hang your hat on it,” says Sean Scott, president of the ALS-Therapy Development Institute, a nonprofit biotech company in Cambridge, Mass., that works extensively with SOD1 mice.

But if such a transgenic model were developed, and if a drug were then found that could slow disease both in the TDP-43 mice and in people with sporadic, common ALS, it would confirm TDP-43’s central role, and the protein would be seen as an important target not only for therapies but also for ALS diagnostic tests, which might enable doctors to treat patients much earlier.

Expectations currently are high that a successful TDP-43 animal model will be developed. “I’m extremely excited about the TDP-43 findings and having those translated into mouse models or other kinds of models to help develop therapeutics for the sporadic [ALS] patients,” says Greg Cox, a mouse-model expert at Jackson Laboratories, which creates and sells transgenic mice.

Rothstein says that the Packard Center, which he heads, is funding two separate groups that are trying to develop TDP-43 mice. Lee and Trojanowski and their colleagues are in the race too, and Lee suspects that at least one group could cross the finish line and publish a TDP-43 mouse paper as early as next year. “Stay tuned,” she says.