Posted on 01/16/2005 5:11:46 PM PST by 4mor3
Researchers in Iceland have discovered a region in the human genome that, among Europeans, appears to promote fertility, and maybe longevity as well.
Though the region, a stretch of DNA on the 17th chromosome, occurs in people of all countries, it is much more common in Europeans, as if its effect is set off by something in the European environment. A further unusual property is that the region has a much more ancient lineage than most human genes and the researchers suggest, as one possible explanation, that it could have been inserted into the human genome through interbreeding with one of the archaic human lineages that developed in parallel with that of modern humans.
The genetic region was discovered by scientists at DeCode Genetics of Reykjavik who have made the Icelandic population, with its comprehensive genealogy and medical records, a prime hunting ground for the genetic roots of common diseases. Their finding is published in Monday's issue of Nature Genetics in a report by Dr. Kari Stefansson, Dr. Augustine Kong, Dr. Hrein Stefansson and other Decode scientists.
The report seems likely to receive considerable attention, even though it raises as many questions as it answers. "I thought it was one of the most interesting papers in population genetics I have ever read," said Dr. Nick Patterson, a mathematician at the Broad Institute in Cambridge, Mass., who advised Decode on the article but has no other connection with the company.
The region came to light in the search for a schizophrenia-causing gene, which turned out not to be there. But the DeCode researchers noticed that the DNA sequences they had examined did not seem to agree with those in the standard human genome sequence, said Dr. Kari Stefansson, Decode's chief executive.
The lack of agreement turned out to be caused by the fact that the region exists in two forms in the Icelandic population. The region is not a single gene but a vast section of DNA, some 900,000 units in length, situated in the 17th of the 23 pairs of human chromosomes. In some Icelanders, the Decode team found, the section runs in the standard direction but in others it is flipped. Looking for any physical consequence, the Decode researchers found that women carrying the flipped or inverted section tend to have slightly more children.
The section carries several known genes, none of which have any obvious connection with fertility. It is not clear why inverting the section should have any effect on the number of children, Dr. Stefansson said. But the inversion does increase the rate of recombination, the shuffling of genes between generations that is a major source of genetic novelty. That could account for some of the increase in fertility.
The Decode scientists found that the chromosome 17 inversion is rare in Africans, almost absent in Asians, but is possessed by 20 percent of Europeans, the same frequency as in Iceland. The inversion seems to have been favored by natural selection among Europeans in fairly recent times, perhaps the last 10,000 years. "Maybe something switched it on in the European environment, such as an interaction with diet," said Dr. David Reich, a population geneticist at the Broad Institute.
Fertility is doubtless affected by different genes in different populations and Decode has found one special to Europeans because that is where it was looking. The increased frequency of the inversion in Europeans is one of a growing number of examples of recent human evolution.
The inversion itself, however, is surprisingly ancient. Its age is revealed by its counterpart, the standard or non-inverted section of chromosome 17. The standard and inverted regions cannot exchange genetic elements during recombination because their DNA sequences do not match. Hence, unlike most of the rest of the genome, which gets shuffled in each generation, the two forms have enjoyed a separate existence ever since their creation. This event presumably happened when the region came adrift from its parent chromosome and got knitted back in the wrong way round.
When all the known versions of a human gene are compared, in most cases they turn out to have had a single common ancestor about a million years ago. But the standard and flipped version of the chromosome 17 region last shared a common ancestor three million years ago. It is highly unusual for two different versions of a gene to endure for so long, because one will usually get lost by a natural random process of elimination.
The Decode researchers propose two explanations. One is that the standard and flipped versions confer different advantages, so it is beneficial for a person to have inherited a copy of each from different parents. This balancing selection can keep two versions of a gene around in a population indefinitely.
Decode's alternative proposal is that the flipped version was carried for many years in a different human lineage, one of the archaic populations that preceded the emergence of anatomically modern humans in Africa 150,000 years ago. Then, in some episode of rape or interbreeding, a single copy of the flipped version entered the modern human lineage some time before humans left Africa 60,000 years ago.
In support of this view, the flipped version carries far fewer mutations than the standard version, as if it had been accumulating them from a much more recent date. There have been other recent hints of modern human interaction with archaic hominids, notably the finding last October that a lineage of modern human body lice seems to have been inherited from a different human species.
Dr. Stefansson said that another property of the inversion, though one not described in today's article, is that it is associated with longevity. Decode scientists have located two sites on Icelanders' genomes where there is some genetic variant that promotes longer lifespan. The chromosome 17 inversion, it turns out, lies at one of these sites. It occurs at much higher frequency in women over 95 and in men over 90 than in the normal population. "It seems to confer on people the ability to live to extreme old age," Dr. Stefansson said.
It is particularly surprising that the same genetic element should promote fertility and longevity since most organisms are obliged to follow a strategy either of breeding fast during short lives or of living longer and having fewer children. "Usually people think of there being a trade-off between fertility and longevity," said Dr. Alan Rogers, a population geneticist at the University of Utah. "So we are getting a free lunch here."
Dr. Stefansson said his findings were empirical observations for which functional explanations have yet to emerge. It is not clear why the inversion should affect fertility or longevity, why it is favored in Europeans or how it has endured for three million years.
Though a surprising claim with so many loose ends would usually be greeted with skepticism, other researchers seem impressed with the solidity of Decode's findings. "It's a startling and amazing claim and it's actually pretty convincing and compelling evidence at the same time," Dr. Reich said.
Dr. Rogers, who also has no connection with Decode, described it as a "cool paper."
Redheads are always makin' trouble. ;')
"In some Icelanders, the Decode team found, the section runs in the standard direction but in others it is flipped. Looking for any physical consequence, the Decode researchers found that women carrying the flipped or inverted section tend to have slightly more children."
Thanks. I haven't read it in entire yet, but if this study was done in Iceland, even with the Vikings sailing all devil-may-care in the northern Atlantic (until the Little Ice Age hit and put on the brakes), islands tend to concentrate mutations in the population.
It also occurs to me that, if this is at all an old mutation, then the "slightly more children" factor, over generations, would tend to make it a fairly commonplace snip of code. :')
Stay on topic:
Redheads are always makin' babies...
okay, let's go back to trouble.
What, right now? [ducks, runs]
You got the duck gene instead?
I've got a little of the goose gene too. ;')
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I said your review was helpful on about 10 of your reviews. Then, my ADD kicked in and I lost interest.
You should tell people in your FR tagline you review salad dressing. I bet you'll get interesting FReepmails.
Just the one time. And it was Green Goddess, which is GGG-ping-list related. ;')
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