When scientists finished sequencing the human genome, the answers to diseases were supposed to follow. Six years later, that promise has gone unfulfilled. Genetics just isn't that useful for predicting who gets sick, and why. The blueprint of life turned out to be an intriguing parts list.
"It's much more complex than we had thought. There aren't going to be easy answers," said Teri Manolio, director of the National Human Genome Research Institute's Office of Population Genomics. "The genome is constantly surprising us. There's so much that we don't know about it."
Manolio is the lead author of a Nature article entitled "Finding the missing heritability of complex diseases." Published Wednesday, it's part of a major change in how scientists see the genome.
In April, several articles in the New England Journal of Medicine featured researchers arguing over why genome-wide association studies — in which thousands of genomes are compared in a hunt for disease-linked patterns — had found so little. Several months later, a massive hunt for schizophrenia genes was described as the field's "Pearl Harbor." At a conference this summer at the Jackson Laboratories, the shortcomings of gene-centered explanations were a starting point for talks by some of the world's most prominent geneticists.
It's not that genes are suddenly unimportant. Researchers are just acknowledging their variations as pieces of an extraordinarily complicated puzzle, along with how genes are turned on, how many copies are made of each, the shape of the genome itself, and how all of the genome's protein products mix and interact.
Wired.com: What do you mean by "missing heritability"?
Teri Manolio: We know that diseases cluster in families. In some diseases, the risk might be two or three times higher than normal, or 30 times higher, for a relative of someone with a disease. But when we do these genomic studies, we find maybe a 50 percent increase in risk. That gap is what's missing.
Wired.com: The numbers can get tricky. If you've found that someone with a certain genetic variant has double the risk of developing a disease, but the heritable risk is a hundred-fold, then we've only connected two percent of the heritability to genetics?
Manolio: That's a fair way of putting it. The gap varies. In some diseases, we're describing half of the genetic heritability. But that's unusual. Only macular degeneration has numbers that high. In many diseases, it's around five percent.
Wired.com: How much of the gap is caused by our inability to link genetics to conditions, and how much has non-genetic causes?
Manolio: There's a lot of thought that this might be DNA and environment together. If you're not exposed to adverse environmental factors, then you may never develop a given disease. With a bad enough environmental exposure, you may get a disease regardless of your genetic makeup.
Wired.com: What about aspects of our DNA that we're just starting to study, like variations in the number of copies we have of each gene, or how genes are activated or physically arranged inside a cell?
Manolio: All of those have been suggested. At least so far, it doesn't look like copy numbers explain a huge amount of this. But there are other places to look, and I suspect that the answer is going to be, "all of the above."
__Wired.com: __How does all this fit with what the public expected of genomics? It seems we had different expectations than the scientific community.