Gene for human sex reversal syndrome identified

        (Release at 2 p.m. EST)
         By Patricia Reaney - LONDON (Reuters)

British and Italian researchers said Wednesday they had 
identified a gene they believe causes sex reversal syndrome,
in which people have the chromosomes of one sex but the
physical attributes of the other.
Scientists had already suspected that the DAX1 gene played 
a role in the troubling condition. Now they believe they have
proof.
``The evidence we have strongly suggests that DAX1 is 
the gene responsible for human sex reversal syndrome. Just a
simple duplication of the gene in a double dose apparently
leads to sex reversal,
'' Robin Lovell-Badge, of the MRC National
Institute for Medical Research, told Reuters.
DAX1 lies on the X chromosome. When it duplicates it causes
an individual who is genetically male to develop physically
as a female. Another gene called Sry, on the Y chromosome, is the
main sex gene which determines whether testes or ovaries
will develop in the embryo.
     
       Lovell-Badge and researchers at the University
of Pavia in Italy conducted experiments on transgenic mice to determine
how DAX1 and Sry work. The research offers new insights into how
sex is determined and identifies the underlying cause of the
disorder.
     
In a report published in the scientific journal Nature, they
showed that the interaction of Sry and DAX1 is vital in the
early stages of sex-determination.
``We know that Sry is the gene critical for male development.
If it's there you get a male, if it isn't you get a female,
'' Lovell-Badge explained.
``Dax1 and Sry act in the same point in the pathway (chain of
events) but in a normal male Sry wins. The normal function of
DAX1, we believe, is to ensure that secondary male genes are
turned off. It's an anti-male gene.''
The XY chromosome configuration determines males and the XX
females, but in sex reversal syndrome a male with XY will
develop female sex organs or women with XX will have male
characteristics. 
``In this case with these duplications of the X chromosome
including DAX1 the individuals are essentially XY so they
should have developed as males but they developed (physically) as
females,
'' Lovell-Badge said.
The same things happens with Sry. If you have a mutation in
that gene you develop as an XY female. In most cases those
with sex reversal syndrome are not aware of the condition until
puberty, when they fail to ovulate or have a menstrual
cycle.
      
``They are women but they are not fertile,'' he added.
       
There are also individuals who are chromosomal females with
XX chromosomes but who develop as males. Again, the condition
may not be evident until puberty.
       
Complete sex reversal occurs in about one in 5,000 people
and partial reversal, producing a hermaphrodite with both
testes and ovaries, happens in about one in 1,000 individuals.
      
The researchers' work was limited to physical sex reversal,
but Lovell-Badge said that since DAX1 was found in other
parts of the embryo, including the brain, it could have an affect
on sex-specific behavior and might explain why some females
feel trapped in a male body or vice versa.

REUTERS@

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