Animal models — research animals with a form of the disease being studied — are one of the most important tools in ALS research today, says Fernando Vieira, director of in vivo validation at the ALS Therapy Development Institute (ALS TDI) of Cambridge, Mass.
However, animal models must be carefully screened or research results may be misleading and not have applicability to humans, Vieira said in remarks made during an hourlong public webinar on June 29 hosted by ALS TDI, an MDA-supported ALS research facility.
In the webinar (online seminar), Vieira discussed the current state of animal models in ALS research, new ALS mouse models, and the value of the most commonly used ALS research mouse model, the SOD1 mouse.
About animal models
Animal models have long been used in testing new drugs, enabling researchers to evaluate how the body’s cells and tissues interact with a drug, or to determine whether a drug has an effect against one or more processes that cause disease. Researchers also study animal models to better understand how a disease starts and progresses.
Some of the more common animal models, all of which can be useful depending on the application, include invertebrates such as worms and flies; rodents including mice, rats, Guinea pigs and rabbits; large animals such as dogs, horses, pigs and birds; and, finally, nonhuman primates such as monkeys and chimps.
Researchers typically select a model based on cost and its applicability to the problem they’re trying to solve, assessing such trade-offs as size and handling requirements, risk and resemblance to human physiology.
Animal models of ALS
In 1994, researchers associated the human SOD1 gene with some forms of familial (inherited) ALS. When that gene is flawed it can lead to production of mutated SOD1 protein and — following a cascade of events not yet fully understood — motor neurons die.
A number of ALS animal models are based around the SOD1 gene, including some zebrafish, fruit flies and worms. While valuable, Vieira said, typically with these models “you’re modeling parts, but not all, of ALS.”
For example, Vieira explained, to test a potential therapeutic designed to affect axonal growth, a researcher may use a fish model if he only wants to look at axons. “But if you want your drug to impact immune cells and their effect on axons, you can’t use a fish. You’d have to look at something like a mouse.”
Mice have complex systems with many similarities to humans, including an immune system, vascular flow and neurons that are surrounded by immune-specific support cells called astrocytes and microglia.
Studying ALS in the SOD1 mouse
ALS TDI uses a particular variant of the SOD1 mouse model known as SOD1-G93A, which contains 23 to 25 copies of a flawed human SOD1 gene. This mouse exhibits a disease that looks like an aggressive form of ALS, dying at 135 days of age (as opposed to the normal 2 years) following muscle weakening and atrophy (shrinkage), and motor neuron loss.
Like humans with ALS, SOD1 mice experience functional muscle loss, respiratory distress, upper and lower motor neuron involvement, an immune system component, blood-brain barrier disruption, inclusion bodies and protein aggregation (problems found in cells), and neuromuscular junction disruption.
Of mice and men
Although the SOD1 mouse provides a huge advantage over other models, the fact remains that it’s not a human and important differences remain. Still, “what we’re waiting for, and what we want and are looking forward to,” Vieira said, “is a drug that works in the SOD1 mouse model at ALS TDI, that also works in humans.”
Is the SOD1 mouse relevant to sporadic ALS?
The SOD1 form of ALS accounts for approximately 5 to 10 percent of ALS cases in humans. Even though about 90 to 95 percent of human ALS cases are sporadic (noninherited), “it’s important to remember that familial and sporadic ALS look clinically identical,” Vieira said.
A physician can’t tell the difference between familial and sporadic ALS without a family history and a genetic test. Both forms present with the same physical and biological symptoms.
“In the end,” Vieira said, “the SOD1 mouse model is a model in which motor neurons die. And ALS is a disease where motor neurons die. The model is also one where muscles waste away and where there’s central nervous system inflammation — again, all of these are things that we see in both familial and sporadic ALS.”
The hope then is to find a drug that targets these downstream effects in both the mouse model and some, if not all, ALS patients with either familial or sporadic ALS.
In mouse models, details matter
In 2007, ALS TDI research scientists conducted testing called “whole genome expression profiling” in the SOD1 mouse. They looked at all 30,000 genes in the mouse genome and studied how those genes are affected in SOD1 mice compared to healthy mice. They then compared treated SOD1 mice with untreated SOD1 mice.
Starting with approximately 8,000 mice in 2007, the Institute has built up its SOD1 mouse model database to 42,000 mice, and in the process learned that “noise” in the model can confound study results. In other words, details matter.
Failure to control the system carefully — for example, making sure that test and control groups contain equal numbers of male and female mice, as well as equal numbers from the same litter — can lead to false positive or false negative results in testing.
ALS TDI used its optimized study design to test a number of drugs that had previously demonstrated positive results in mouse studies in other labs but which failed to show benefit in subsequent human clinical trials. Its results mirrored those failures seen in human testing.
“We’ve used this type of testing to really understand what’s happening in ALS in these mice, and then over time build a database that helps us home in on the best possible treatment strategies,” Vieira said.
|ALS TDI Director of In Vivo Validation Fernando Vieira briefed webinar viewers June 29, 2010, on the current state of animal models in ALS research.
TDP43: New mouse on the block
Mutant TDP43 protein produced from instructions carried by flawed TDP43 genes has recently been associated with both sporadic and some familial forms of ALS. Now, several models of a new mouse engineered to have a flawed TDP43 gene have been developed by MDA-supported researchers.
ALS TDI currently is working to build a colony of the new TDP43 mice, which TDI researchers plan to fully characterize (describe the qualities and peculiarities of), using gene expression profiling just as they did in the SOD1 mouse, to determine how best to use the TDP43 mouse in studies.
Vieira noted the Institute also is “anxiously awaiting news” from research groups working on mouse models of FUS, another gene that recently was implicated in both familial and sporadic ALS.
“As these models come out, and as we evaluate their relevance to ALS, we’ll be bringing them in-house,” Vieira said. “We want to use every tool we possibly can.”