A series of repeated DNA sequences unique to humans may be linked to the development of schizophrenia and bipolar disorder, according to a new study by researchers at the School of Medicine.

The finding suggests that the rapid evolutionary changes that led to the extraordinary complexity of the human brain may have predisposed our species to psychiatric diseases not found in other animals. It also outlines a possible way to one day identify people at risk for, and ways to intervene in, these disorders.

Although the sequences exist within a small stretch of DNA that has been previously linked to schizophrenia and bipolar disorder, they represent a kind of genomic stutter that is particularly difficult to detect using conventional sequencing methods. As a result, they’ve been effectively hidden from researchers attempting to pinpoint a specific mutation that contributes to risk for the diseases.

“The human genome reference sequence shows only 10 repeats of this 30-nucleotide sequence, but we’ve found that individuals actually have from 100 to 1,000 repeats, and that the sequence itself can vary,” said professor of developmental biology David Kingsley, PhD. “In contrast, chimpanzees and other primates have just one repeat of the sequence, indicating that the region has greatly expanded during human evolution. Some of the sequence variants now found in people are also closely associated with the development of schizophrenia and bipolar disorder.”

Kingsley, who is a Howard Hughes Medical Institute investigator, is the senior author of the research, which published todfay in the Journal of Human Genetics. Graduate student Janet Song and former postdoctoral scholar Craig Lowe, PhD, share lead authorship of the study.