A novel gene therapy approach shows promise in selectively rescuing the function of the TDP-43 protein in diseased nerve cells affected by amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases, while leaving healthy cells untouched. This precision is achieved by targeting problematic sequences called cryptic exons, which are present only in cells where TDP-43 is not functioning correctly. When these sequences are not properly removed from genetic messages, the therapy activates and produces a therapeutic protein that helps restore lost functions.
Avoiding Off-Target Effects
"Many potential therapies also alter important cellular processes and this may cause toxicity," said Pietro Fratta, MD, PhD, head of the Molecular Neurodegeneration Laboratory at the Francis Crick Institute and a University College London (UCL) professor. "Therefore, limiting their action to diseased cells, while leaving healthy cells untouched, will increase the safety of gene therapies and allow researchers to pursue many more treatment options."
The Role of TDP-43 in ALS
ALS, also known as motor neuron disease, is characterized by the progressive damage and degeneration of motor neurons. In approximately 97% of ALS cases, toxic clumps of the TDP-43 protein are evident and contribute to nerve cell damage. These clumps are also observed in many individuals with frontotemporal dementia (FTD) and Alzheimer’s disease.
TDP-43, normally located in the nucleus, helps control how genes are read. When TDP-43 forms clumps, it fails to reach the nucleus, leading to the erroneous incorporation of small extra sequences, known as cryptic exons, into protein-making templates. This ultimately affects protein production and leads to disease.
"TDP-43 controls many aspects of cellular health, and its dysfunction is a key driver of disease," said Claire Le Pichon, PhD, an investigator at the Eunice Kennedy Shriver National Institute of Child Health and Human Development. "Gene therapy holds promise for treating neurodegenerative diseases like ALS and FTD, which are relatively common but for which there are few treatments."
TDP-REG: A Targeted Therapeutic Approach
To address the challenge of off-target effects, the research team at Francis Crick and UCL designed a gene therapy approach, named TDP-REG, that is only active in cells containing cryptic exons. The system is delivered to cells and mice using a harmless adeno-associated virus (AAV).
In stem cell-derived neurons, TDP-REG successfully removed cryptic exons from the UNC13A gene, which produces a protein essential for regulating neurotransmitter release at synapses. Dysfunction of UNC13A, due to mutations or cryptic exons, has been linked with an increased risk of ALS and FTD.
The therapeutic approach also effectively removed cryptic exons from other genes known to be processed by TDP-43, with no production of TDP-REG observed in healthy cells.
Implications for Precision Medicine
"There’s huge potential in precision medicine approaches that only target the cells that need treatment," said Oscar Wilkins, the study’s first author and a research fellow at UCL. "Whether it’s cancer, heart disease, or motor neuron disease, the key challenge is finding something unique about the diseased cells that we can use for our own purposes — in this case, we chose the TDP-43 protein, as it becomes dysfunctional in so many different neurodegenerative disorders."
Max Chien, a PhD student at UCL, added, "As well as potentially delivering gene therapies, our approach can also be used to detect diseased cells. This could help scientists test whether their treatments perform correctly, before entering into a clinical trial."
