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Poor Mom. Already responsible for carrying genes
that cause everything from hemophilia to
colorblindness in her sons, she might be to blame
when it comes to her sons' infertility, too.
Scientists have found that almost half the genes
related to sperm production reside in the X
chromosome, universally thought of as the female
sex chromosome. It had been assumed that genes
governing male fertility, if they were specialized
at all, were from the Y, or male sex
chromosome.
This could mean that the X chromosome could
affect male infertility, an area that has never
before been explored.
"The door is now wide open to see if there are
links on the X chromosome to male infertility,"
says Dr. David Page, a biology professor at the
Whitehead Institute for Biomedical Research in
Cambridge, Mass., and lead author of a study
published in this month's Nature Genetics.
"It's rare in science to stumble into a
completely unvisited valley," he says. "The Y
chromosome has an outsize role in sperm production,
but now it looks like the X chromosome has a
specialized role. We certainly didn't anticipate
this."
To date, research on male infertility has
focused primarily on abnormalities in the Y
chromosome, but that only accounts for 5 percent to
10 percent of the causes of male infertility, Page
says.
In an animal study with mice, researchers at the
Whitehead Institute and Howard Hughes Medical
Institute found 25 genes -- including 19 new ones
-- in mouse sperm cells. Of those only three were
linked to the Y chromosome, while 10 were linked to
the X chromosome. The rest were linked to
non-gender specific chromosomes. Later, the
scientists were identified the same pattern of
chromosome links in humans.
A lot more work needs to be done before any
conclusions can be made about how much the X
chromosome can affect male infertility, Page
says.
"We don't yet know that mutations in the X
chromosome cause male infertility, but these
studies open the door to the possibility," he
says.
"This is clearly an important finding because
people have been focusing on the Y chromosome,"
says Dr. Margaret McGovern, associate professor of
genetics at Mount Sinai School of Medicine in New
York City. "Geneticists will start thinking
differently about these patterns of genetics.
Mothers' uncles and brothers are people who haven't
been looked at before [in searching for causes
for male infertility]. Now they will be
encouraged."
The X-linked mode of inheritance is one of three
modes of inheritance that doctors study, Page says.
In this mode, a genetic defect on the X-chromosome
may cause a disease like hemophilia or
colorblindness, one of hundreds of X-linked
diseases. The mother with a defective gene on one
of her two X chromosomes is protected against the
disease herself, because she has two X chromosomes
and the normal X makes up for the defective
one.
However, if her son inherits the defective X
chromosome -- which he has a 50 percent chance of
doing -- he is likely to get the disease because he
doesn't have the balancing chromosome that a woman
would. A daughter inheriting the defective X
chromosome has another healthy X chromosome, like
her mother, to protect her from the disease, but
she can in turn pass the defective chromosome onto
her own sons.
Any breakthroughs in treating male infertility
are way down the road, but this study opens an
exciting avenue of new research.
americanbaby.com/ab/CDA/newsFeedDetail/1,1455,413,00.html
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