Europe
— Scientists say they may have found genes that
help explain why a tiny percentage of men see themselves
as women, cruelly trapped in the wrong body.
The researchers say the
findings are very preliminary and should be
“interpreted with the utmost caution,” due to
the small sample size used in their study.
Nonetheless, they say,
the results might shed some light on the rare condition,
transsexualism. It is estimated to afflict about one in
30,000 men, some of whom follow through on their sense of
their correct gender by getting sex-change
operations.
More broadly, the
research could help clarify one of the most contentious
and poorly understood questions in biology: what creates
“gender identity”—the sense most people
have that they are either a man or a woman.
The feeling is normally
rather deep-seated; people don’t need to examine
their body shapes to confirm it. It is also considered
distinct from the issue of whom a person is sexually
attracted to.
The question is how
genes, culture or both conspire to produce gender
identity.
Transsexualism
“raises important questions as to how the gender
identity is moulded in humans,” wrote the
researchers, who included Susanne Hennigsson of
Göteborg University <http://www.gu.se/>
and Mikael Landén of the the Karolinska Institute
<http://ki.se/>
in Göteborg and Stockholm, Sweden, respectively.
They describe the
research in the August issue of the scientific journal
Psychoneuroendocrinology. <http://www.ispne.org/>
If their findings are
correct, the risk of becoming a transsexual may depend
partly on variations in the length of certain segments of
DNA where the genetic code “stutters,” that is,
a few “letters” of the code repeat themselves
in the same order many times.
Notably, scientists found
in a study published last December that these repeat
sequences may be the sites of some of the most common
genetic mutations, and thus may underlie some of the
fastest evolutionary changes in life’s history.
Evolutionary theory holds that mutations produce
evolution, because the occasional mutations that are
advantageous spread through populations, changing these
populations’ characteristics, and over time
gradually create new species.
In that study of last
year, researchers found that the muzzle length of dogs
depends on the length of certain repeat
sequences.
In the transsexualism
study, the researchers examined a repeat sequence in each
of three genes known to affect the sexual development of
the brain, in hopes that one or more of these might shed
light on transsexualism. They studied several common
variants affecting the length of these repeats in
different people.
These variants “are
all much more common than is transsexualism” itself,
they wrote. “Hence, the goal of this study was not
to reveal the primary cause of transsexualism,” but
rather to help explain “whether the studied genes
may facilitate or prevent” it.
One particular variant
seemed significantly associated with the frequency of
transsexualism, they found. This variant was in a gene
responsible for producing a molecule called ER-Beta.
ER-Beta acts as a minuscule gateway that controls the
flow of estrogen, a hormone, through the brain during
fetal development.
Estrogen is thought to be
responsible for wiring the brain in a
“masculine” way in males before
birth—although, paradoxically, the substance is
better known as a female sex hormone. This is because
after birth, it influences the development of female
sexual characteristics.
The gene that produces
the ER-beta receptor contains a section called a CA
repeat sequence, so called because C and A are names for
two “letters” of the genetic code which, here,
are repeated many times in a row.
The researchers found
that longer CA repeats were associated with a greater
risk of transsexualism in the the study, which included
29 male-to-female transsexuals (men who wish they were
women) and 229 healthy males.
It is unknown exactly how
this change in the ER-beta gene might contribute to
transsexualism, the researchers said. The gene may
produce different variants of the molecular gateway,
which transmit estrogen more or less easily; but
it’s unknown whether one of these might be the
reason for the effect, or whether the reason is something
else.
Moreover, the researchers
found that the two other genes that they studied also
seem to influence the risk of becoming a transsexual. But
neither of these genes on its own predicted that risk,
they found. Rather, specific combinations of all three
variants seemed to be more common among
transsexuals.
The other two genes
studied were genes encoding the production of molecules
called aromatase and androgen receptor. These genes, too,
are believed to help determine how “masculine”
the brain becomes.
More masculine, in this
context, means that certain brain structures are
relatively smaller or larger in males than in females,
possibly reflecting the relative importance of those
brain structures in each sex.
Researchers have found
that parts of the frontal cortex, the seat of many
reasoning functions, and the limbic cortex, involved in
emotions, are bigger compared to other brain areas in
women than in men. Parts of the parietal cortex, which
contributes to spatial perception, and the amygdala,
which responds to emotionally arousing information, are
bigger in men. A part of the hypothalamus, a brain region
that regulates reproductive behavior, is also believed to
be bigger in males.
At least one brain region
has also been found to be different in male heterosexuals
and homosexuals, also a part of the hypothalamus. And
more recently, gay and straight men have been found to
differ in how their brain responds to a scent in male
sweat.
But scientists don’t
know whether homosexuality and transsexualism have
anything in common biologically, beyond the fact that
some people view both as aberrations—and that now,
both are being found to have a possible genetic
basis.
The idea that genes
underlie transsexualism, at least among men who want to
be women, has gained support from reports on twin and
non-twin siblings who both have “this very rare
condition, and from reports on families with more than
one member” having it, Henningsson and colleagues
wrote.
Gender identity is
typically established by 18 to 24 months of age, when
“boys come to know they are boys, and girls come to
know they are girls,” according to the Merck Manual
of Diagnosis and Therapy, 17th Edition.
<http://www.merck.com/mrkshared/mmanual/>
Some people act in ways
typically considered more appropriate for the other sex,
but this doesn’t make them transsexuals, as long as
they’re comfortable with their physical gender.
Rather, transsexuals “believe that they are victims
of a biologic accident,” the book says, “and
that they are cruelly imprisoned within a body
incompatible with their gender identity.”
