WASHINGTON – The largest genetic study of mental illnesses to date finds five major disorders may not look much alike but they share some gene-based risks. The surprising discovery comes in the quest to unravel what causes psychiatric disorders and how to better diagnose and treat them.

The disorders — autism, attention deficit-hyperactivity disorder or ADHD, bipolar disorder, major depressive disorder and schizophrenia — are considered distinct problems. But findings published online Wednesday suggest they’re related in some way.

“These disorders that we thought of as quite different may not have such sharp boundaries,” said Dr. Jordan Smoller of Massachusetts General Hospital, one of the lead researchers for the international study appearing in The Lancet.

That has implications for learning how to diagnose mental illnesses with the same precision that physical illnesses are diagnosed, said Dr. Bruce Cuthbert of the National Institute on Mental Health, which funded the research.

The Psychiatric Genomics Consortium, a collaboration of researchers in 19 countries, analyzed the genomes of more than 61,000 people, some with one of the five disorders and some without. They found four regions of the genetic code where variation was linked to all five disorders.