University of California Berkeley researchers have used brain organoids to identify a key pathogenic mechanism behind tuberous sclerosis complex (TSC), a genetic neurodevelopmental disorder that is a leading cause of childhood epilepsy.

The study, published in Nature, highlighted hyperreactive astrocytes as one of the primary drivers of the inflammation that can cause seizures and highlights possible therapies that could reduce the inflammation.

Helen Bateup, PhD, a Berkeley professor of neuroscience and of molecular and cell biology, and co-authors explain that TSC is characterized by potato-shaped lesions in the brain called cortical tubers. These cause severe seizures that are often resistant to common antiseizure medications. The disease is also accompanied by debilitating symptoms in other parts of the body, including the heart, skin, and kidneys.

The cortical tubers are thought to result from “second-hit” mutations in the TSC1 or TSC2 genes, whereby patients with an inherited heterozygous mutation then acquire a second, random mutation that inactivates the remaining functional TSC1 or TSC2 allele in a subset of neural progenitor cells.

It is unclear, however, how mutations in these two genes, which regulate the mTOR developmental and metabolic pathway, cause tuberous lesions in the brain. The assumption was that they were making neurons overreact, leading to seizures.

To investigate, Bateup and colleagues used single-cell transcriptomics and cyclic immunostaining on human brain organoid models and resected tuber tissue from patients to track the development of mutated progenitor cells in the absence of seizures.

They found that loss of the TSC2 protein causes the neural progenitor cells to differentiate into enlarged, pro-inflammatory reactive astrocytes that show distinct molecular and morphological alterations. These include increased inflammatory cytokine secretion and elevated expression of neurodegenerative disease risk genes such as APOE and CLU.

The findings indicate that abnormal astrocytes and other glial cells are not just a consequence of seizures but actually cause the lesions themselves.

“As soon as these astrocytes are born, they are reactive and look like they’ve been triggered into a disease state. This is arising as a primary result of the mutation,” said Bateup. “So now we can rethink the disease pathophysiology. It’s not necessarily the case that the neurons are the only cause of seizure activity and the glia become involved later. It could be the other way around, or it could be that both cell types contribute to seizures and epilepsy.”

If the mechanism is confirmed, it may be possible that existing immunosuppressant drugs targeting reactive cells could reverse the inflammation and alleviate the intractable seizures.

“If it’s really glia-driven and there’s all these angry cells causing problems, how much can we fix by just suppressing that?” Bateup remarked. “Can you calm down the glia and ideally bring them back to a homeostatic state, or if that’s not possible, just shut off their ability to cause damage to the surrounding cells? I think that’s feasible.”