The idea that everybody’s genes are pretty much the same, a central conceit of the Human Genome Project, may need some revising. A group of researchers has accidentally discovered that our DNA contains substantial areas of repeated genes—and the location and number of these genetic echoes vary considerably from person to person.
Biologist Michael Wigler of Cold Spring Harbor Laboratory, who led the study, started out studying genes in cancer cells but soon realized he was seeing unexpected patterns in the healthy cells he examined for reference. Although he does not know exactly what the large-scale repeats mean, he suspects that the number of genes a person is carrying may be just as important as the codes those genes contain. “More copies probably means more expression of whatever that gene is coding for,” he says. Several of the repeated regions involve genes regulating appetite and body weight; others occur in regions associated with breast cancer, leukemia, and nerve development, leading Wigler and his teammates to speculate that differences in these copied genes could help explain why some people are particularly susceptible to obesity, cancer, or neurological diseases.
If the disparities are so large, why has nobody found them before? Part of the answer is simply that the most commonly used DNA probes tell whether or not a given sequence is present, not how many times it appears in an individual cell. As a result, researchers have tended to focus on point variations, such as the mutation or deletion of a single DNA letter within a gene. “People have known isolated cases where this sort of multiple copying existed,” Wigler says. “But everyone assumed that these were just anomalies, and so it wasn’t something anyone was looking for.”