[Paleopsych] NYT: Scientists Find DNA Region That Affects Europeans' Fertility

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Scientists Find DNA Region That Affects Europeans' Fertility
NYT January 17, 2005
By NICHOLAS WADE

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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 genetic region has a much more ancient lineage
than most human genes, and the researchers suggest, as one
possible explanation, that it could have entered 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, a biotechnology company in Reykjavik, Iceland,
which has 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 today's issue of Nature
Genetics in a report by Dr. Kari Stefansson, Dr. Augustine
Kong, Dr. Hreinn 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 during 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 tended to
have slightly more children.

The section carries several known genes, not one of which
has 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
present in 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 found a genetic element
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
noninverted 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 existed
separately 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
process of elimination.

The DeCode researchers propose two explanations. One is
that the standard and flipped versions confer different
advantages, so that 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 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 does 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 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 because 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 tradeoff between fertility and longevity. So we are
getting a free lunch here," said Dr. Alan Rogers, a
population geneticist at the University of Utah.

Dr. Stefansson said his findings were empirical
observations for which functional explanations have yet to
emerge. Despite the loose ends, 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.

http://www.nytimes.com/2005/01/17/science/17gene.html