This week's Nature celebrates the tenth anniversary of the publication of the human genome sequence. Two new articles surveyed Copy Number Variations (CNVs) in hundreds of human individuals. CNVs are kinds of mutations that occur when (typically small) regions of the human genome are duplicated or deleted. CNVs can be a few hundred nucleotides up to thousands of nucleotides, and they are fairly common. It might surprise you to learn that most of us are walking around a few thousand nucleotides short of a genome (though I did have my suspicions about some people I know).
Conrad et al. looked at nearly 5000 CNVs in 450 European, African, and East Asian individuals. They found only 30 that might influence susceptibility to disease. The rest do not seem to be associated with diseases. In the other article, the Wellcome Trust Case Control Consortium examined 3400 CNVs in 19,000 individuals (16,000 with diseases, 3000 without). They found three CNVs associated with conditions like Crohn's disease, types 1 and 2 diabetes, and arthritis. They concluded, "common CNVs that can be typed on existing platforms are unlikely to contribute greatly to the genetic basis of common human diseases."
So... all mutations are harmful? I don't think so. Sounds like human genomes are designed well enough to withstand all sorts of punishment.
Conrad et al. 2010. Origins and functional impact of copy number variation in the human genome. Nature 464:704-712.
Wellcome Trust Case Control Consortium. 2010. Genome-wide association study of CNVs in 16,000 cases of eight common diseases and 3,000 shared controls. Nature 464:713-720.