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by Sharon Begley
Wall Street Journal, Science
Journal, June, 2002
It wasn't the kind of passage you
usually encounter in a strait-laced science journal: "I have had to spend
periods of several weeks on a remote island in comparative isolation,"
Anonymous wrote in Nature. Curiously, he continued, the day before he was
due for shore leave his beard grew noticeably: "I have come to the
conclusion that the stimulus for this growth is related to the resumption
of sexual activity."
Neither Anonymous nor his fellow scientists were surprised that the
aforementioned activity would loose a flood of testosterone, which affects
beards the way Miracle-Gro affects tomato plants. No, the weird part is
that merely anticipating female companionship did the trick.
Just as stress in the med students I wrote about last week altered the
expression of genes in their immune systems, so libidinous thoughts seem
to affect gene expression, says developmental psychologist David Moore of
Pitzer College in Claremont, California. Thoughts can cause the release of
hormones that can bind to DNA, "turning genes 'on' or 'off.'"
If something as will-o'-the-wisp as a thought can tweak genes, it's no
surprise that more substantial influences can, too. For instance, when R.
Adron Harris and his team at the University of Texas, Austin, screened
10,000 genes in the frontal and motor cortexes of alcoholics, they found
changes in the expression of 191, they reported in last month's Journal of
Neurochemistry.
Alcohol seems to cause "a selective reprogramming" of brain genes in areas
involved in judgment and decision making, says Dr. Harris. Among them,
genes that code for myelin, whose loss may impair cognition and judgment.
Antidepressants may also alter genes. The conventional wisdom is that
drugs such as Prozac work by blocking re-uptake by brain neurons of the
neurotransmitter serotonin. But Prozac starts doing that in 24 hours. Why,
then, do such drugs typically take weeks to lift depression? "The hunch is
that Prozac works by altering gene expression, maybe causing sprouting of
new neurons and remodeling of synapses," Dr. Harris says.
Experience, too, can affect gene expression. How much a mother rat handles
and licks her offspring -- an environmental influence if ever there was
one -- has an astonishing effect: It determines whether genes that code
for receptors for stress hormones in the brain are expressed or not. And
the level of those receptors affects how a rat reacts to stress. Rats with
attentive moms were much less fearful and more curious, finds Michael
Meaney of McGill University in Montreal. Rats that got less maternal
handling grew up to be timid and withdrawn in novel situations.
Rats are not long-tailed people, so you can't infer that maternal
affection affects gene expression and thus temperament in babies, too. But
something sure does. There is no shortage of evidence that intelligence,
shyness, impulsivity, risk-taking and illnesses have a genetic component.
But identical twins, who have the same genes, don't have identical traits:
One twin might be schizophrenic and the other not, one might be shy and
the other outgoing, one might get a "gene-based" cancer and the other not.
The difference between identical twins is the experiences they have and,
if I may speculate, which of their genes are expressed.
What signal from the environment keeps schizophrenia-related genes silent?
What activates IQ-lifting genes? Whatever it is, even a short-lived
environmental signal might turn on genes that tell neurons how, and how
much, to grow. That would leave an enduring mark: Neural circuits would be
complex or simple, and different brain regions would be strongly linked or
not. From such neuronal differences arise differences in intelligence and
personality, health and temperament.
Linking specific environmental influences to gene activity would have been
a pipe dream only a few years ago. But the new technology of microarray
analysis, in which "gene chips" reveal which DNA in a sample of tissue is
expressed and which is quiescent, is making such discoveries possible.
This past April, in one of their coolest uses so far, gene chips showed
that the difference between human brains and chimp brains is not which
genes each brain has. Those are nearly identical. The difference is which
genes are turned on and which are switched off.
Ironically, the recognition that genes depend on the environment follows
hard on the heels of genetics' greatest triumph: sequencing the human
genome. But what's now clear is that the more we learn about genetics, the
more we'll see that genes are not destiny.
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