The basic act of thinking can increase the growth of deadly brain tumours, according to a new study. Scientists have found that high-grade gliomas - the most fatal type of brain tumour - increase in size by hijacking the process of creating thoughts.

These tumours often spread to the cerebral cortex, which is the brain’s folded outer layer that helps us perceive the world, form conscious thoughts and use language. Here, they hijack a process known as myelination, which creates a protective layer around nerve fibres, allowing them to carry thoughts more quickly.

By looking at mouse models, the researchers found that the brain tumours grew faster when they were closer to these highly active nerve cells. It is rare for an organ’s main function to drive the growth of tumours within it, said Michelle Monje, the assistant professor of neurology at the Stanford University School of Medicine. ‘We don’t think about bile production promoting liver cancer growth, or breathing promoting the growth of lung cancer,’ she said.

‘But we’ve shown that brain function is driving these brain cancers.’ High-grade gliomas are the leading cause of brain-tumour death in children and adults. Survival rates have scarcely improved in the last 30 years. The team identified a specific protein, called neuroligin-3, which is largely responsible for the increase in tumour growth associated with thoughts in the cerebral cortex.

In healthy tissue, neuroligin-3 helps to direct the formation and activity of synapses, playing an important role in the brain’s ability to remodel itself. But the new study showed that a secreted form of neuroligin-3 promotes tumour growth. ‘This group of tumors hijacks a basic mechanism of neuroplasticity,’ Monje said.

Into the cerebral cortex of mice with these light-sensitive proteins, the team implanted cancer cells from a human. After the tumors became established, neurons near the tumors were activated with light. Scientists then compared tumour growth between these mice and a control group with implanted tumors but without the nerve activation.

Increased tumour size in the mice that received stimulation using light revealed that brain activity fed the brain tumors. In theory, doctors could slow the growth of these tumors using sedatives that reduce mental activity but there are serious ethical implications involved in this treatment.

The team hope their work will lead to the development of drugs that specifically block the tumour-stimulating activities of neuroligin-3.