A founder population reveals genetic variation with major effect on serious mental illness

For decades, scientists have tried to understand why some people develop serious mental illnesses like schizophrenia or bipolar disorder. The prevailing view has been that these conditions arise from two very different kinds of genetic trouble. On one side, there are common genetic variations found in many people; each one is so small that it barely changes a person's risk, but when you add them all up, they explain most of the danger in the general population. On the other side, there are ultra-rare genetic mistakes that happen in very few people but cause massive damage, often leading to severe illness in just a handful of individuals. The challenge has been that most studies look at only one diagnosis at a time, making it hard to see if a specific genetic error causes a broad range of mental health problems or just one specific label. Furthermore, the gap between those tiny common risks and those massive rare ones has remained difficult to bridge, leaving researchers unsure how these different forces interact to shape a person's mind.

A team of researchers has now stepped into this gap by studying a unique group of people living in a genetic isolate in Colombia known as the Paisa. Because this population descended from a small number of original settlers, they carry a specific set of genetic traits that are much more common there than anywhere else in the world. The scientists examined the DNA of nearly 22,000 people from this region who had been diagnosed with serious mental illness or severe depression, comparing them against almost 20,000 healthy neighbors. Instead of looking for the tiny, common variations or the extremely rare ones found only in single families, they searched for a middle ground: genetic errors that are rare globally but have become more frequent in this specific community because of their shared ancestry. This approach allowed them to spot patterns that other studies missed, specifically looking for genetic changes that affect multiple types of mental illness at once.

The study uncovered seven genes that appear to play a major role in serious mental illness, regardless of whether a person is diagnosed with schizophrenia, bipolar disorder, or severe depression. Five of these genes had never been linked to these conditions before. What makes this discovery particularly powerful is that four of these genes carry genetic errors that are enriched in the Paisa population due to a founder effect, a phenomenon where a specific genetic trait becomes common because it was carried by the original ancestors of a group. This finding fills a crucial missing piece in the puzzle, showing how a genetic variant can be rare enough to be distinct but common enough to be studied in a large group, sitting right between the tiny common risks and the ultra-rare catastrophic ones.

The most striking discovery centers on a single genetic change in a gene called ADAP1. This gene produces a protein that is abundant in the brain and helps shape how neurons grow and connect. In the Paisa population, a specific error in this gene cuts the protein short, rendering it useless. This single mistake increases a person's risk of developing schizophrenia or bipolar disorder by more than three times. The researchers found that this genetic error is so significant that it stands out clearly even when looking at the entire human genome. People carrying this specific error do not just have a higher risk of illness; they experience the illness differently. When the scientists looked at medical records and tested their thinking skills, they found that carriers of this error struggled more with psychotic symptoms, manic episodes, and cognitive tasks than other patients with the same diagnosis.

Interestingly, while this genetic error does not significantly increase the risk of major depressive disorder on its own, the people who carry it and happen to have depression show symptoms that look much more like bipolar disorder. This suggests that the error pushes the brain toward a specific type of dysfunction that cuts across traditional diagnostic labels. Because this genetic variant is present in about four out of every 1,000 people in this Colombian population, it provides a rare opportunity for scientists to study how a single, clear cause of mental illness plays out in a large group. By separating patients based on whether they carry this specific genetic cause, researchers can now begin to identify other factors that might protect some people from the worst effects of the illness or worsen it for others, offering a clearer path to understanding the complex machinery of the human mind.