ARTICLE IN BRIEF
A new transgenic mouse model with mutant TDP-43 appears to be similar to human sporadic amyotrophic lateral sclerosis, opening up new opportunities for research on targeted therapies.
Investigators at Washington University in St. Louis have developed a new animal model for familial amyotrophic lateral sclerosis (ALS) based on a mutation in the TAR DNA binding protein-43 (TDP-43) gene identified in some ALS patients. The new model may be closer to the more common sporadic form of the illness.
The TDP-43 transgenic mice initially develop normally, but by three months of age they have problems walking, lose muscle mass, and eventually become paralyzed. By five months, they are dead. Under the microscope, the investigators strained for ubiquitin and identified clumps of protein tagged for degradation within the motor neurons, but they were not sure what they were.
The new model is phenotypically different from the first transgenic model for familial ALS based on a mutation in the superoxide dismutase 1 gene developed in the 1990s. The SOD1 mice appear to have paralysis of their back legs caused by predominantly lower motor neuron degeneration. By comparison, the TDP-43 transgenic mice have mainly upper motor neuron development.
But both lines have upper and lower motor neuron damage to some degree, similar to ALS patients who may have more or less of one than the other.
“We hope that the new model helps us understand ALS and can be used to test experimental medicines,” said Robert H. Baloh, MD, PhD, an assistant professor of neurology and lead author of the finding, published online in the Proceedings of the National Academy of Sciences in October.
THE ROLE OF TDP-43
In 2006, Virginia Lee, PhD, John Trojanowski, MD, PhD, and colleagues at the University of Pennsylvania discovered accumulations of a protein called TDP-43 in autopsy material from patients with both ALS and frontotemporal dementia (FTD). This was the first clue that TDP-43 may be involved in ALS, but direct evidence was lacking.
Across the world, scientists turned to their patients with familial ALS to ask whether TDP-43 mutations might play a role in selectively damaging the motor neurons. By 2008, Nigel Cairns, PhD, and Alison Goate, PhD, at Washington University, identified a point mutation in one of their ALS patients. On closer inspection, the team learned that three people in two generations — this patient was the fourth in the lineage — had similar symptoms. The patient's aunt was in a nursing home with increasing paralysis, and her two brothers died with identical symptoms. Almost simultaneously, similar reports of TDP-43 mutations in patients with familial ALS appeared from other groups worldwide, solidifying the importance of the finding.
“The discovery of the TDP-43 mutations in ALS led to a paradigm shift,” said Dr. Baloh, who led the work in developing a TDP-43 animal model of ALS. “It was a door that opened up for the field that we didn't know was there.”
No one is sure how TDP-43 mutations lead to ALS. They don't even know much about the role of the normal functioning of the gene. What they know is that the TDP-43 protein is involved in regulating alternative mRNA splicing and RNA metabolism.
Dr. Baloh said that studies are under way to answer these questions. The hope, of course, is to have a model to develop and screen new medicines for the treatment of ALS.
Dr. Baloh and his colleagues used the TDP-43 mutation from their ALS patient to develop the transgenic mouse model. They made a mutant transgene and attempted to express it at high levels in the brain and spinal cord. They targeted neurons in every region of the brain and spinal cord, including motor neurons. Normally, such an experiment would lead to the production of many different transgenic mouse lines. But only one mouse line survived. They learned quickly that the gene is fatally toxic if over-expressed too much. “You probably need to express it just right to get the phenotype that we observed,” said Dr. Baloh. “We were lucky to get one line. We still need more.”
They learned other lessons about TDP-43 along the way. “If you express it equally in all neurons you predominantly get degeneration of layer 5 cortical and spinal motor neurons,” Dr. Baloh said.
Intriguingly, human postmortem studies of FTD, performed by William Seeley, MD, and colleagues at the University of California-San Francisco, have challenged the long-held view that FTD begins in superficial layers, suggesting instead early injury to a subset of large, neurofilament-rich Layer 5 projection neurons called von Economo neurons. Therefore the new TDP-43 mouse model and human anatomical findings may converge to suggest that ALS and FTD, in addition to their strong molecular linkage, share early damage to cortical layer 5.
“The contribution of the SOD1 model to familial ALS is tremendous,” said Dr. Baloh. “That said, I think that two models are better than one, as they likely reflect different underlying pathways of disease. If a drug works on both, it is more likely to work on sporadic ALS, as this likely has many underlying pathways which lead to it.”
The animal line has been shipped to Jackson Laboratories where they are being expanded for open distribution to facilitate more research.
Commenting on the new mouse model, Dr. Trojanowski said “the mutation seems to give a consistent pathology. All of the phenotype is there but we need multiple models to sort out the biology of ALS.”
He said that the major pathological features observed in postmortem tissue from patients with ALS are motor neuron loss, cortical spinal tract degeneration, gliosis and cytoplasmic neuronal inclusions formed by TDP-43.
The Washington University mouse model doesn't appear to have an accumulation of TDP-43 protein in the motor neurons. There are accumulations of ubiquitinated protein in the motor neurons in the TDP-43 mice, but the investigators don't know what the protein or proteins are, other than they do not contain TDP-43. Dr. Baloh said that it remains a puzzle that they are trying to understand.
Since 2006, there have been 300 papers on TDP-43. “The progress is amazing,” said Dr. Trojanowski. There have been 29 different missense mutations in 51 unrelated sporadic or familial ALS cases. “This mutation will certainly accelerate the research.”