Irish scientists have discovered that motor neurone disease (MND) and schizophrenia have a shared genetic origin.

This indicates that the causes of these two conditions are biologically linked, which has major implications for how diseases are classified.

The scientists based at Trinity College Dublin (TCD) analysed the genetic profiles of almost 13,000 people with MND and 30,000 with schizophrenia.

MND is a group of diseases in which there is progressive degeneration of the motor neurones in the brain and spinal cord. Motor neurones are the nerve cells that control muscles, and their degeneration leads to weakness and wasting of these muscles.

This causes an increasing loss of mobility in the limbs and difficulties with speech, swallowing and breathing. There is currently no cure for the disease. Around 110 people are newly diagnosed in Ireland every year and over 300 people and their families are currently living with the disease.

Schizophrenia is a serious mental illness characterised by disturbances in a person's thoughts, perceptions, emotions and behaviour. Symptoms include hallucinations, delusions and disorganised behaviour. Around 3,900 people have the condition in Ireland.

By analysing the genetic profiles, the TCD scientists confirmed that many of the genes that are associated with these two different conditions are, in fact, the same.

Previous research has found an overlap between schizophrenia and other neuropsychiatric conditions, such as bipolar affective disorder. However, this marks the first time an overlap in genetic susceptibility between MND and psychiatric conditions has been found.

"This study demonstrates the power of genetics in understanding the causes of diseases. While neurological and psychiatric conditions may have very different characteristics and clinical presentations, our work has shown that the biological pathways that lead to these diverse conditions have much in common," commented the study's lead author, Dr Russell McLaughlin, of TCD.

According to consultant neurologist, Prof Orla Hardiman, who is also a senior author on the study, the scientists' work over the years has revealed that MND is much more complex than previously thought.

"Our recent observations of links with psychiatric conditions in some families have made us think differently about how we should study MND. When combined with our clinical work and our studies using MRI and EEG, it becomes clear that MND is not just a disorder of individual nerve cells, but a disorder of the way these nerve cells talk to one another as part of a larger network," she noted.

She said that another issue that arises as a result of these findings is that ‘the divide between psychiatry and neurology is a false one'.

"We need to recognise that brain disease has many different manifestations, and the best way to develop new treatments is to understand the biology of what is happening. This will have major implications for how we classify diseases going forward, and in turn how we train our future doctors in both psychiatry and neurology.

"That in itself will have knock on consequences for how society understands, approaches and treats people with psychiatric and neurological conditions," Prof Hardiman insisted.

This latest research was prompted by earlier TCD research, which was led by Prof Hardiman. It found that people with MND were more likely than expected to have family members with schizophrenia, and to have family members who had died by suicide.

The TCD team, collaborating with scientists in the University of Utrect in the Netherlands, will continue to study the link between MND and psychiatric conditions. It is hoped this will lead to the development of more effective treatments for those affected.

Details of these findings are published in the journal, Nature Communications.