Web edition: October 22, 2009
When geneticists talk about junk DNA, they don’t usually mean that you can safely jettison large chunks of the stuff from the genome. But researchers have discovered just that — big chunks of the human genome are often missing in otherwise healthy people.
These losses (and some duplications) of the genome are known as copy number variations and are a hot area of genetic research. Missing chunks of DNA have been associated with diseases such as schizophrenia and autism. In most cases, copy number variations mean someone is missing chunks from one chromosome, but have a full-length version of the other chromosome in the pair. In most cases, the "healthy" chromosome is preventing you from getting sick from missing DNA on one chromosome. In cases such as schizophrenia, the missing DNA is too big a deficit to cover for.
New work shows that many people are missing the same chunks of DNA from both chromosomes, but these people aren't sick, indicating that those parts of the genome are really not all that essential.
Healthy young people can do without, on average, about 2.8 million bases — the chemical units that are the building blocks of DNA — with no obvious problems, says Terry Vrijenhoek of Radboud University Nijmegen Medical Center in the Netherlands. He and his colleagues studied 600 healthy university students and found that 96 percent of the students had a least one part of their genome that was missing on both chromosomes.
These deletions on both chromosomes were found in 58 regions, Vrijenhoek reported at a press conference at the American Society of Human Genetics meeting on October 21. He will present the results October 24. Those 58 regions contain 39 different genes, many of which are immune system genes or genes involved in smell (or, olfaction, as scientists say it). The work shows that most of the commonly deleted genes are members of a large family of related genes, so the missing genes may have relatives that can compensate for the loss. The researchers don’t know the function of some of the other apparently disposable genes.
The deleted regions usually do not contain noncoding RNA genes, perhaps indicating that these RNAs are important for health, Vrijenhoek says. Discovered relatively recently, noncoding RNAs may help control many important processes in cells.