The announcement this spring that yet another promising drug had failed to help people with ALS in a clinical trial has led some in the field to re-examine some assumptions.
Minocycline, an antibiotic with anti-inflammatory and anti-cell death properties, had shown positive effects in ALS-affected mice, but results in humans weren’t the same.
It joins a list of medications — creatine, celecoxib (Celebrex) and gabapentin (Neurontin) are examples — for which the same story can be told.
The mice used in all research on these medications had a form of ALS caused by a mutation in the gene for the SOD1 protein. The SOD1 mouse develops an ALS-like disease that researchers hope overlaps enough with human ALS that mouse responses to treatment might predict human responses.
SOD1 gene mutations, however, cause a familial (inherited) form of human ALS that’s present in only 1 percent to 3 percent of ALS cases. For some 90 percent of patients, the disease isn’t familial. Its onset is unpredictable (sporadic) and the cause is unknown. (The remaining 7 percent or so have non-SOD1 familial forms of ALS.)
A different mechanism?
It may be that SOD1-caused familial ALS and sporadic ALS don’t have as much in common as had been hoped.
The absence of deposits of the TDP-43 protein in tissue samples from humans with SOD1-caused ALS “implies that motor neuron [nerve cell] degeneration in these cases may result from a different mechanism, and that cases with SOD1 mutations may not be the familial counterpart of sporadic [nonfamilial] ALS,” warn the authors of a recent report.
It then becomes important, if distressing, to ask: How relevant are research results in SOD1 mouse models to the vast majority of ALS patients?
Story not fully told
Jeffrey Rothstein, who directs the MDA/ALS Center at Johns Hopkins University in Baltimore, says he isn’t ready to give up on SOD1 rodents yet.
“I don’t buy that we’re barking up the wrong tree with the SOD1 mouse,” he says, speculating that TDP-43 deposits may be a feature of the end stage of slowly progressive ALS and that many of the SOD1 patients in the TDP-43 study had a mutation that causes a very rapidly progressive disease. Had they lived longer, they might have acquired the deposits, he says.
However, he adds, “We have to come up with new mouse models and additional means of evaluating drug candidates beyond just the mice. And that’s now being done.”
Teepu Siddique, who sees patients at the MDA/ALS Center at Northwestern Memorial Hospital in Chicago, isn’t ready to give up on SOD1 mice either.
“Right now, TDP-43 is just a marker,” says Siddique, who was an author on the TDP-43 study. “We don’t know what it does. Maybe we are looking at two different kinds of disease, but the story is not yet fully told.”
Lost in Translation?
In his article “Lost in Translation,” published in the April issue of Neurobiology of Disease, Michael Benatar raises questions about the methodology of SOD1 mouse studies.
Benatar, who co-directs the MDA clinic at Emory University in Atlanta, notes that the SOD1 mouse may or may not be a good model of human sporadic ALS, but he says there could be other reasons for its failure to predict treatment responses in patients.
An important consideration, Benatar says, is that most mouse trials treat the animals before ALS symptoms appear, while people are always treated after disease onset.
All about SOD1?
Relatively few researchers propose that abnormal SOD1 protein causes most or all ALS cases. Most think abnormal SOD1 protein molecules are only involved in SOD1-related familial ALS. They look to SOD1 rodent models for insight into the sporadic ALS disease process “downstream of its unknown cause.”
However, there’s one scientist whose interpretation of ALS causation suggests a major role for SOD1 in sporadic, as well as familial, ALS.
Neurodegenerative disease specialist Neil Cashman believes ALS may have a lot in common with prion diseases, which are degenerative disorders resulting from misfolded, toxic proteins. Cashman directs the ALS Centre at the University of British Columbia and is chief scientific officer at Amorfix Life Sciences, a Toronto biotechnology firm.
A prion is a substance that’s able to induce abnormal folding of normal cellular proteins. So far, all the known prion diseases, which include bovine spongiform encephalopathy (mad cow disease) and others, are caused by a misfolded PrP protein that acts as a folding template and causes otherwise normal PrP protein molecules to assume an abnormal, toxic shape.
Cashman wants to find out whether ALS is a disease of misfolded SOD1 protein molecules, arising in some cases from a mutated SOD1 gene and in others from a random misfolding of a normal SOD1 protein molecule.
His hypothesis, for which he has some evidence and is gathering more, is appealing, because if it’s true, it implies that SOD1 rodent models are a good replication of human disease after all.
“I think the SOD1 models are great models for protein aggregation [clumping],” Cashman says, “but they don’t capture a critical feature of [human] ALS, which is propagation of the disease.” To do that, he says, they’ll need some tweaking.
The SOD1 animals, he notes, are “transgenic,” meaning they’re born with mutated human SOD1 genes in every cell (as are people with mutated SOD1 genes). There’s no spread of the disease in SOD1 transgenic mice, he notes. “It presents simultaneously in the most affected regions, the hindlimbs,” he says.
“In sporadic ALS, there’s this creeping paralysis, which may reflect a propagation of the disease throughout the nervous system. I think it’s crucial to our understanding of ALS.”
In Cashman’s ALS hypothesis, the regional spread of weakness is caused by the gradual spread of misfolded SOD1 protein molecules.
It could be argued that this hypothesis doesn’t hold up, because humans with mutated SOD1 genes, present from birth, also show gradual spread of paralysis.
But Cashman believes that human SOD1 mutations aren’t by themselves enough to cause ALS. This is supported by the long delay between being born with an SOD1 mutation and disease onset, usually some 50 years later, as well as the lack of 100 percent correlation between having an SOD1 mutation and developing ALS.
“You can have a mutant protein that functions normally for 50 or 60 years, and you can bear mutant SOD1 for an entire lifetime without developing disease,” Cashman says. In his model of ALS, “there’s an event that must occur. The mutation itself is not enough to produce the disease.”
The event, he says, could be a viral infection or another type of stress to a cell. But “once the template is established, the process propagates and runs its course.”
“I don’t think we’ve wasted our time with the SOD1 mouse models,” says Jonathan Glass, director of the MDA/ALS Center at Emory University in Atlanta. “I think we’ve learned an enormous amount. But we can’t just keep banging on the same nail with the same hammer.
“We need to understand the mechanisms that underlie the disease and develop hypotheses based on these mechanisms and not necessarily on mouse tests. Using mice as gatekeepers for determining which drugs go on to clinical trials is probably not going to be the case anymore.”
Glass says he doesn’t think we should “jump to human trials quickly,” but he’s also “not sure if taking the extra step of going to the mouse is the best way to do it.” It might be better to identify mechanisms in humans and then find a way to test drugs based on those data, perhaps through a test tube screening process.
“The human is the best model we’ve got,” he says.
Siddique says he thinks SOD1 mice will continue to be used until we have better models. SOD1 models, he says, are “low-hanging fruit.”
The higher fruit may be out of reach for now, but eventually we’ll get there, he says. He noted that a key aspect of Parkinson’s disease pathology was found as a result of a gene mutation identified in only three families.
“In science,” he says, “you get clarity and then confusion and then clarity.”