Molecular overlap hyperlinks tuberous sclerosis, fragile X | Spectrum
Low protein: Shank2 protein levels (red) are decreased in cerebellar cells from mice lacking TSC1.
Brain cells from the cerebellum of mice that model the tuberous sclerosis complex (TSC) show subdued protein levels, according to a new study. Levels of the affected proteins are controlled by FMRP, the protein that is lacking in people with fragile X syndrome – a finding that suggests that the two disorders have molecular roots.
“We thought even before this study that there might be some overlap at the cellular level between FMRP and TSC,” says lead researcher Mustafa Sahin, professor of neurology at Harvard University. He and his team previously showed that cerebellar cells derived from people with TSC reduced levels of FMRP and its targets.
“That was another way to show that it can also happen in the cerebellum,” says Sahin about the new work.
Like Fragile X Syndrome, TSC is characterized by intellectual disability and, often, autism. The condition arises from mutations in the TSC1 or TSC2 genes, which supply proteins needed to control the mTOR signaling pathway. Without regulation, this path works in overdrive and increases protein production and cell proliferation.
The new study suggests that the mTOR pathway itself regulates FMRP, which is known to bind to RNA transcripts and block their translation into proteins. However, the mechanism that links mTOR and FMRP remains unknown, says Angelique Bordey, a professor of neurosurgery and cell and molecular physiology at Yale University who was not involved in the study.
“It’s a dataset and a study that actually opens new doors and requires additional research,” she says.
Sahin and colleagues constructed mice that lacked the TSC1 gene in a class of cerebellar cells called Purkinje cells and measured the levels of RNA in those cells to measure gene expression.
The researchers found that mice lacking TSC1 showed decreased gene expression compared to controls. In fact, 72 percent of the genes that differed between the groups were downregulated in the model mice. The results appeared in eLife on July 14th.
The RNA fragments from the TSC1 mice were also more prone to decay than those from the controls, which could explain the downgraded gene expression in the animals. And the excessive breakdown of RNA specifically affected a core group of genes that are regulated by FMRP and help support synapses, the team found. Many are also linked to autism.
Despite lower overall RNA levels, the Purkinje cells lacking TSC1 showed no change in the amount of RNA bound to ribosomes – the cellular machines that convert RNA into proteins.
The results suggest that the cells without TSC1 are more efficient at loading RNA onto ribosomes, which is puzzling, says Sahin. “It almost seems as if the cell is trying to rebalance itself by trying to increase the protein expression of these transcripts.”
But this supposed compensatory effort falls short: In the cerebellar cells that lack TSC1, the protein level of three FMRP target genes is reduced, the researchers found. One protein in particular, SHANK2, is located at the signaling end of synapses, and mutations in the gene that encodes SHANK2 have been linked to autism and intellectual disability.
Mice lacking TSC1 in their Purkinje cells display characteristics similar to autism, such as:
The new study suggests possible molecular bases for these traits.
“It suggests that some of the dysregulated transcripts may be involved in some of the neurological deficits,” says Bordey, but it will be key to test whether restoring these levels of RNA saves animal behavior.
Taken together, the results complement the growing evidence that TSC mutations lead to lower protein levels in some cases.
“It goes against the real expectation that translation will increase globally, but I think people sometimes jump to conclusions,” says Bordey.
For example, live mice lacking TSC2 show decreased protein production throughout the brain, according to a 2018 study. But the effects of TSC mutations vary by cell type, says Carolyn Beebe Smith, senior researcher at the US National Institute of Mental Health who led this work.
Another team discovered a different pattern in the hippocampus two years later: Mice that lack TSC2 have an increased expression of FMRP-linked genes in this brain region.
“I think we also need to take into account that isolated neurons may not be representative of what is going on in an intact nervous system,” says Beebe Smith.
Regardless, the new study strengthens the links between TSC and fragile X syndrome. The results suggest that the two conditions share not only behavioral traits but also molecular pathways.
Studying this convergence in humans could help researchers uncover the molecular origins of the diseases, as well as the overlap between their various treatment options, Sahin says. For example, upcoming clinical trials will test whether drugs that help some people with Fragile X syndrome, such as mGluR5 antagonists, will also benefit those with TSC.
“Until we actually go to the patient and test some of these hypotheses, unfortunately by drawing conclusions from the section culture or such transcriptome experiments.” [is] will be very difficult, ”he says. “Ultimately, it is the patients who count.”