ARTICLE IN BRIEF
Investigators reported that TAR DNA-binding protein 43, or TDP-43, pathology in amyotrophic lateral sclerosis possibly disseminates in a sequential pattern that permits recognition of four neuropathological stages consistent with the hypothesis that TDP-43 pathology is propagated along axonal pathways.
Proteinaceous inclusions in amyotrophic lateral sclerosis (ALS) accumulate in a sequential pattern, beginning in motor neurons and later appearing in other neurons to which the motor neurons connect, according to a study published online on May 20 in advance of the print edition of Annals of Neurology. The results suggest that ALS pathology may spread along axonal pathways, but that interpretation is controversial. Much hangs in the balance, since targeting misfolded protein as it moves between cells may offer a new therapeutic strategy, and may be easier than preventing it from misfolding in the first place. But how strong is the evidence?
The study is the latest neuropathological tour-de-force from the laboratory of Heiko Braak, MD, professor of neurology at the Center for Biomedical Research at the University of Ulm in Germany. Dr. Braak has previously defined neuropathological stages in Parkinson's disease and Alzheimer's disease consistent with the spread of pathology between connected neurons.
His co-investigator in the current study, John Q. Trojanowski, MD, PhD, told Neurology Today that over the past decade, evidence has accumulated that misfolded tau, amyloid-beta, and alpha-synuclein are all capable of transmission between cells. “It is clear now from the experimental literature that there is a cell-to-cell spread of these pathological proteins, and this informs the staging studies that Dr. Braak has done,” said Dr. Trojanowski, co-director of the Center for Neurodegenerative Research and the William Maul Measey-Truman G. Schnabel, Jr. MD, professor of geriatric medicine and gerontology at the University of Pennsylvania in Philadelphia.
Mutations in TAR DNA-binding protein 43 (TDP-43) are a rare cause of ALS, but misfolded wild-type TDP-43 accumulates to form aggregates in affected neurons in almost all ALS cases, regardless of mutation status. How, or indeed whether, TDP-43 aggregates contribute to ALS pathogenesis is unknown, but the protein has a prion-like domain, and some in vitro evidence has suggested that misfolded TDP-43 may propagate in a prion-like manner. That led Drs. Trojanowski and Braak to ask whether accumulation of TDP-43 pathology in ALS might also display a sequential pattern.
The authors examined central nervous system (CNS) tissue of 76 patients at autopsy with confirmed clinical and neuropathological diagnoses of ALS. Subjects included 30 men and 46 women, with a disease duration ranging from one to 13 years. Twenty-two regions in the CNS were examined, including cortical, subcortical, brainstem, and spinal cord areas. Tissue was stained with antibodies for phosphorylated TDP-43, and the severity of TDP-43 pathology was rated from 0 to 3.
In every case, they found TDP-43 aggregates in the frontal agranular motor cortex, the motor neurons of the spinal cord, and bulbar motor neurons, except for oculomotor neurons. They defined this distribution as Stage 1. Of the 76 patients, 6 patients showed Stage 1 pathology only, including the longest-lived patient (156 months) and one of the shortest-lived (16 months).
Fifteen patients were classified as being in Stage 2, defined as having all the pathology of Stage 1, plus some involvement of the prefrontal cortex, the reticular formation, and precerebellar nuclei. There was no involvement of cerebellar cortex neurons.
Twenty-seven patients were in Stage 3, with the pathology of Stages 1 and 2, plus more extensive involvement of the prefrontal cortex, postcentral neocortex, and basal ganglia. Several patients exhibited mixed Stage3/4 pathology, and the remainder were classified as being in Stage 4, with aggregates in all areas from Stages 1 through 3, plus dissemination into anteromedial areas of the temporal lobe, including the hippocampus.
In 11 patients with the mutation in the chromosome 9 open reading frame 72 (C9ORF72) gene, the sequence of distribution of pathology was the same, but the amount of TDP-43 aggregation was higher. None of the study patients had the superoxide dismutase 1 mutation. Dr. Trojanowski pointed out that the question of whether these patients have any TDP-43 aggregates is controversial, but it is not extensive, in any event.
The results offer a framework in which to understand ALS progression, and imply that there may be under-recognized clinical correlates to pathology in some affected areas, such as post-central sensory areas affected in Stage 3, and the hippocampal involvement in Stage 4. “This will take us some time to sort out,” Dr. Trojanowski said, but he expects there will be more attention paid to the potential clinical consequences as a result of this study.
Potentially the most important implication from the study is the support it provides for the hypothesis that ALS propagates from cell to cell. There is no proof, Dr. Trojanowski cautioned, that the pathology in any individual patient spreads according to this model. “You don't know if early stages will evolve into late stages, which is a caveat with any cross-sectional study,” he said. Neither does the study provide insight into what the initiating event is for ALS, or why it begins where it does.
But the study does suggest that a disease that begins in the motor neurons propagates to other regions of the brain in a fashion that is predictable based on proximity and connection. “One might speculate that, based on limited data on TDP-43 transmission in cell culture, a few cells will develop these inclusions, they will either die or release in some other way the abnormally shaped TDP-43, which then enter another cell, and corrupt the normal protein in the unaffected cell, so they also adopt the pathological conformation. This is how it can spread from cell to cell.”
In this model, the appearance of aggregates in the post-central neocortex after the motor cortex, for example, “is clearly trans-synaptic — a chain, if you will, of passaging of disease proteins from one neuron to another,” Dr. Trojanowski said. If that model is correct, then interrupting that cell-to-cell passage, perhaps with anti-TDP-43 antibodies, could be therapeutic. Dr. Trojanowski noted that he and his colleagues are currently working on mouse models to determine if propagation can be demonstrated in vivo. If so, he said, “This offers us a chance to talk about disease-modifying therapy in ALS.”
Ian Mackenzie, MD, professor of pathology and laboratory medicine at the University of British Columbia in Vancouver, Canada, commended the study authors, but noted some important reservations, as well. “The best thing this paper does is to provide an anatomical template which further studies can investigate,” if it is shown in the lab that propagation does indeed occur in vivo.
However, he said, the idea of propagation is far from proven. “You could just as easily argue that it shows there are different neuronal populations in different areas of the CNS that have differing levels of vulnerability,” and the pattern of progression reflects those differences. “I don't think this is something that can be solved by looking at human post-mortem tissue.”
“Having looked at my own cases, I agree that, from case to case, there are these variations in the amount of severity in different anatomic areas,” he said, “and there probably is this hierarchical pattern.” However, he said, “the validation comes when you can prove all new cases fit somewhere into that staging system, and that hasn't been done yet.” And if the experience with the staging systems in Alzheimer's disease and Parkinson's disease is a guide, “there will be lots of cases that don't fit.”
Finally, he noted, ALS and frontotemporal dementia (FTD) are increasingly being seen as two ends of a spectrum, but this study was restricted only to ALS cases. The next step needs to be to determine if FTD cases also show a complementary hierarchical pattern consistent with spread from frontal areas to motor areas. That work, Dr. Trojanowski said, is under way.