Protein, Neuron Abnormalities Implicated in Juvenile Batten Disease
Results were derived from lab-grown neurons from a patient with most common mutation
Alterations in producing, transporting, and degrading proteins, as well as in neuronal activity, are likely involved in juvenile Batten disease, according to data from lab-grown neurons from a patient with the most common disease-causing mutation.
These findings point to previously unknown mechanisms that may lead to neurodegeneration among patients and that may serve as new therapeutic targets.
The study, “Lysosomal alterations and decreased electrophysiological activity in CLN3 disease patient-derived cortical neurons,” was published in Disease Models & Mechanisms.
Juvenile Batten, also called CLN3 disease, is the most common form of Batten. It’s caused by mutations in both copies of the CLN3 gene. This gene provides instructions to make battenin, a protein in lysosomes — cell compartments that act as “recycling centers.”
CLN3 mutations result in battenin levels reducing, which leads to “waste” building up to toxic levels that particularly affect neurons.
More than 90 CLN3 mutations have been reported to cause juvenile Batten, the most common being a large deletion — called the “1kb” deletion — that’s in nearly all patients. It can show up either in both CLN3 copies or combined with another rare mutation.
Analyzing one patient’s mutation to ID juvenile Batten processes
Researchers in Australia analyzed neurons derived from a CLN3 disease patient with the “1kb” deletion in one CLN3 copy to identify abnormal processes in juvenile Batten.
These cells were generated from patient-derived induced pluripotent stem cells (iPSCs) and grown in the lab. iPSCs are generated from fully matured cells (often blood and skin cells) that are reprogrammed back to a stem cell-like state, where they can develop into almost every type of human cell given certain biochemical cues.
When derived directly from patients, these cells can be used as cellular models that mimic a disease’s genetic and clinical diversity. The researchers also used a genetic tool called CRISPR/Cas9 to correct the “1kb” deletion in the patient’s iPSCs to generate “corrected” neurons with the exact same genetic background.
Neurons also were derived from iPSCs of an unrelated healthy donor.
The researchers analyzed how the mutation affected different processes, such as lysosomal function and neuron development.
Imaging results in the patient-derived neurons showed cell storage materials that were not apparent in the “corrected” and healthy neurons, indicating the possibility of a deficit in degrading types of cell waste, including proteins and lipids.
The patient-derived neurons also showed increased levels of some lysosomal proteins and impaired endocytosis, wherein cells take in and transport molecules from the outside.
The researchers saw the “1kb” deletion was associated with changes in the levels of more than 1,500 proteins when they analyzed these changes associated with CLN3 disease.
The proteins that were significantly reduced in patient-derived neurons versus “corrected” cells were mostly involved in endocytosis and axon growth and guidance, while those present at higher levels were mainly involved in ribosomal and lysosomal pathways.
Axon growth and guidance is when neurons send out their nerve fibers (axons) to reach their correct targets. Ribosomes are the molecular machines responsible for protein production and have been previously implicated in CLN3 disease.
The data also provided more support for changes in neuron development and function in juvenile Batten disease. “Corrected” neurons were found to fire more often, for longer, and in a more coordinated way than patient-derived neurons.
These findings “suggest that CLN3 deficiency alters protein homeostasis [balance] by affecting [production], trafficking and degradation, and that disruption of these pathways may negatively affect [electrical signal] transmission or axonal growth,” the researchers wrote, noting the study identified “disease-related changes relating to protein synthesis, trafficking and degradation, and in neuronal activity that weren’t apparent in CLN3-corrected or healthy control neurons.”
“These data implicate inter-related pathways in protein homeostasis and [neuron development] as contributing to CLN3 disease, and which could be potential targets for therapy,” they said, adding more research in other disease models, including those with two “1kb” deletions, would help confirm these findings.
“More broadly, our data demonstrate the utility of iPSCs for modeling CLN3 disease, and imply that such models will be a powerful tool for identifying and testing new therapies for this disease,” they said.
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