NeuroVoices: Robert Bowser, PhD, on TDP-43’s Role in ALS

TDP-43 has emerged as one of the most consequential proteins in ALS research, present in the neuropathology of approximately 97% of all ALS patients and increasingly at the center of drug development programs targeting the disease's underlying biology. First identified as the predominant protein in ALS neuropathologic inclusions roughly 20 years ago, TDP-43 has since been established as a major regulator of RNA metabolism, and its loss of function in the nucleus is now understood to drive a cascade of downstream consequences that contribute to motor neuron death. At ALS Nexus, the annual conference of The ALS Association, a dedicated session brought together leading experts to break down what TDP-43 is and why it may be key to making ALS livable.

Robert Bowser, PhD, is Chief Scientific Officer, Professor, and Chair of the Department of Translational Neuroscience at Barrow Neurological Institute in Phoenix, Arizona, where his research is focused on biomarker discovery and disease mechanisms in ALS and other neurodegenerative diseases. He was among the speakers at ALS Nexus presenting on TDP-43 and the basics behind its biology.

In this iteration of NeuroVoices, Bowser walked through the mechanistic rationale for targeting TDP-43 in ALS, surveyed the current therapeutic landscape including small molecules, nanobodies, and gene therapy approaches, and discussed the biggest unanswered questions that remain. In addition, he shared his perspective on how biomarkers tied to TDP-43 loss of function could reshape ALS clinical trial design in the years ahead.

NeurologyLive: For our clinical audience, provide some background on the mechanism and rationale of TDP-43 as a pathway to potentially treat ALS.

Robert Bowser, PhD: The background of TDP-43, its mechanisms, and why the interest really started about 20 years ago. Neuropathologically, when we look at tissue from ALS patients, we can see pathology in the tissue that occurs in about 97% of all ALS patients. The question is, what protein is in that pathology? About 20 years ago, almost to the date, two different groups published papers saying they had purified out what was in the neuropathology in ALS patients, and it was predominantly a protein called TDP-43. Everyone was scratching their heads. What is TDP-43? What does it do? Why is it in these aggregates and inclusions in ALS patients?

What we knew at the time was that TDP-43 was an RNA/DNA-binding protein. It was actually identified from those working in the HIV field because it was relevant in how viruses, when they enter a cell, hijack some of the RNA processing events in order to replicate themselves. TDP-43 was part of that process, and the cell was trying to protect itself. From there, we learned that TDP-43 was a major player involved in how mRNAs are spliced, assembled into mature messages, and involved in the location of these messages within the cytoplasm of cells. So, we know it was a major player in RNA metabolism within the cell.

During this time, a number of other RNA-binding proteins were identified that played a role in ALS, and a major theme in ALS became that it was a disease around RNA metabolism. Back to TDP-43, we learned about thousands of messenger RNAs that it binds to and regulates their splicing, and we started to piece together this puzzle of how TDP-43 is involved predominantly in the nucleus. It normally shuttles between the nucleus and cytoplasm, but in ALS patients and in other neurodegenerative diseases, one sees these aggregates and inclusions of TDP. Envisioning then that you have a loss of function of what it should be doing in the nucleus, it became a very interesting protein for targeting and thinking about its major role in ALS.

What does the therapeutic landscape look like for TDP-43-targeted agents?

The landscape of TDP-43-targeted approaches is quite exciting right now. A number of companies are developing small molecules with the hope of breaking up the aggregates of TDP within inclusions in the cytoplasm, and hopefully eliminating those aggregated species such that when new TDP-43 gets translated within that cell, it goes back into the nucleus to enable its normal function. So, the idea is to eliminate these aggregates so that TDP can return to the nucleus and function normally.

Other companies are using antibodies or nanobodies to target these pathologic species of TDP without targeting normal functioning TDP-43. People are trying to deliver those into cells by a number of different routes. Personally, one of the more exciting approaches is using gene therapy to really deliver these nanobodies directly into the cell and try to eliminate the toxic species of TDP-43. Others are looking at whether CRISPR approaches can be used to gene edit and fix mutations in individuals who harbor genetic alterations of the gene itself, which is another exciting avenue that is ongoing and hopefully will become a reality in clinical trials in the not-too-distant future.

What are some of the biggest unanswered questions that we still have about TDP-43, whether it comes to its involvement in ALS or in finding a successful therapeutic agent?

There remain several unanswered questions and lots of exciting areas in development. One exciting area is around biomarkers related to the loss of function of TDP-43. In the last few years, several groups have identified what are basically cryptic peptides. When TDP-43 loss of function occurs, you get missplicing of mRNAs, and for some of those mRNAs, they get translated into a novel peptide sequence within the body. If you can identify that, you have a biomarker of loss of TDP-43 function. Several labs are pursuing that.

We can see this in brain tissue, and it's hard to identify these novel cryptic peptides in biofluids, but work is ongoing to develop improved assays. That would be quite encouraging because if you have biomarkers of loss of function, then when you target therapies to TDP, you should see a reduction of those cryptic peptides within a biofluid if the therapy is effective. So, they could be really good pharmacodynamic biomarkers in upcoming clinical trials targeting TDP.

Another area with much less knowledge is around what are the pathologic species of TDP-43. We know that C-terminal fragments develop and accumulate in the aggregates and inclusions. We also know that you can transfer some of this from one cell to another and seed TDP pathology through intracellular connections.

What are the species being transported from one cell to the other? Are those pathologic species the same across sporadic or familial ALS patients, or are there different strains of TDP pathology? A lot of unknown questions at the moment, but a lot of work trying to figure this out, because if you can identify those toxic species, it provides another opportunity for developing drugs that target them.

How does the outlook look for using TDP-43-targeted therapies in combination or sequenced with other targets for ALS?

In ALS, I'm a proponent of precision medicine approaches and using biomarkers to identify what might be the best drug or combinations of drugs within any one individual. If we can develop therapies that really target TDP, which, again, 97% of patients exhibit in tissue postmortem, that's going to be a major component of combination approaches moving forward. There are multiple other mechanisms that contribute to both the onset and progression of ALS, and I do see combination therapies as the main route eventually for treating ALS more effectively.

The other exciting avenue around this is that TDP-43 pathology exists in several other neurodegenerative diseases. As you mentioned, there's Alzheimer's disease, Parkinson's disease, and frontotemporal dementia, all of which have pathology and inclusions and aggregates of TDP. So, a drug developed for TDP in ALS patients might obviously be quite beneficial in these other neurodegenerative diseases.

How do you envision ALS clinical trials continuing to evolve over the next couple of years, particularly with biomarkers?

At the moment, neurofilament light chain is clearly the top biomarker in the ALS field, especially following the accelerated approval of the ASO for SOD1 mutation carriers. Everyone is including it in their clinical trial design, and it's a very important biomarker around axonal injury in neurodegeneration. But it is only one biomarker, and so we're going to need a combination of different biomarkers that likely target different mechanisms of action that are contributing to ALS. Beyond axonal injury and death, TDP is definitely right there. As biomarker development around cryptic peptides related to TDP-43 loss of function matures, that will add another layer of input into the clinical trials arena. And really looking at whether, in any early-phase clinical trial, you can see an impact on these biomarkers, whether it's neurofilament, TDP, immune-related biomarkers, or other mechanisms.

The goal will be to very quickly identify go/no-go decisions on clinical trials in order to say, yes, this drug should expand and move forward, versus no, let's pivot, stop this trial, and move patients into other trials. Allowing patients more at-bats, so to speak, with ALS and participation in more than one trial, I think, will both benefit the patients and benefit drug development in ALS.

Transcript edited for clarity. Click here for previous iterations of NeuroVoices.