[Rushtalk] Scientists find DNA differences between gay men and their straight twin brothers

Carl Spitzer lynux at keepandbeararms.com
Sun Nov 8 13:25:31 MST 2015

Scientists find DNA differences between gay men and their straight twin
Homosexuality epigenome markers
Could the molecular signals that turn genes on and off reveal a person's
sexual orientation? New research identifies epigenomic "marks" linked to
homosexuality. But experts say the origins of partner preference remain
a mystery.

(Chuck Nigash ,Stephen Sedam / Los Angeles Times)
Melissa HealyMelissa Healy

For men, new research suggests that clues to sexual orientation may lie
not just in the genes, but in the spaces between the DNA, where
molecular marks instruct genes when to turn on and off and how strongly
to express themselves.

On Thursday, UCLA molecular biologist Tuck C. Ngun reported that in
studying the genetic material of 47 pairs of identical male twins, he
has identified "epigenetic marks" in nine areas of the human genome that
are strongly linked to male homosexuality.

In individuals, said Ngun, the presence of these distinct molecular
marks can predict homosexuality with an accuracy of close to 70%.

That news, presented at the 2015 meeting of the American Society of
Human Genetics on Thursday, may leave the genetically uninitiated
scratching their heads.

But experts said the results -- as yet unpublished in a peer-reviewed
journal -- offer preliminary new evidence that a man's genetic
inheritance is only one influence on his sexual orientation. Through the
epigenome, the results suggest, some facet of life experience likely
also primes a man for same-sex attraction.

DNA from 4,500-year-old Ethiopian reveals surprise about ancestry of
DNA from 4,500-year-old Ethiopian reveals surprise about ancestry of
Over a person's lifetime, myriad environmental factors -- nutrition,
poverty, a mother's love, education, exposure to toxic chemicals -- all
help shape the person he will become.

Researchers working in the young science of epigenetics acknowledge they
are unsure just how an individual's epigenome is formed. But they
increasingly suspect it is forged, in part, by the stresses and demands
of external influences. A set of chemical marks that lies between the
genes, the epigenome changes the function of genetic material, turning
the human body's roughly 20,000 protein-coding genes on or off in
response to the needs of the moment.

While genes rarely change over a lifetime, the epigenome is constantly

Geneticists suggest that together, the human genome and its epigenome
reflect the interaction of nature and nurture -- both our fixed
inheritance and our bodies' flexible responses to the world -- in making
us who we are.

Ngun's study of twins doesn't reveal how or when a male takes on the
epigenomic marks that distinguish him as homosexual. Many researchers
believe that a person's eventual sexual preferences are shaped in the
uterus, by hormonal shifts during key stages of fetal brain development.

FROM THE ARCHIVES: A closer look at scientific inquiry into sexual

By imprinting themselves on the epigenome, though, environmental
influences may powerfully affect how an individual's genes express
themselves over the course of his life. Ngun's findings suggest they may
interact with genes to nudge sexual orientation in one direction or the

"The relative contributions of biology versus culture and experience in
shaping sexual orientation in humans continues to be debated," said
University of Maryland pharmacology professor Margaret M. McCarthy, who
was not involved in the current study. "But regardless of when, or even
how, these epigenetic changes occur," she added, the new research
"demonstrates a biological basis to partner preference."

To find the epigenomic markers of male homosexuality, Ngun, a
postdoctoral researcher at UCLA's Geffen School of Medicine, combed
through the genetic material of 47 sets of identical male twins.
Thirty-seven of those twin sets were pairs in which one was homosexual
and the other was heterosexual. In 10 of the pairs studied, both twins
identified as homosexual.

In identical twins, DNA is shared and overlaps perfectly. But the
existence of twin pairs in which one is homosexual and the other is not
offers strong evidence that something other than DNA alone influences
sexual orientation. Ngun and his colleagues looked for patterns of DNA
methylation -- the chemical process by which the epigenome is encoded --
to identify the missing factor in partner preference.

Their analysis generated a dataset far too large for a team of humans to
make sense of. So they unleashed a machine learning algorithm on the
data to search for regularities that distinguished the epigenomes of
homosexual twin-pairs from twins in which only one was homosexual.

In nine compact regions scattered across the genome, they found patterns
of epigenomic differences that would allow a prediction far more
accurate than a random guess of an individual's sexual orientation, Ngun
reported Thursday.

McCarthy and other experts cautioned that the discovery of epigenomic
marks suggestive of homosexuality is a far cry from finding the causes
of sexual preference.

The distinctive epigenomic marks observed by Ngun and his colleagues
could result from some other biological or lifestyle factor common to
homosexual men but unrelated to their sexuality, said University of Utah
geneticist Christopher Gregg. They could correlate with homosexuality
but have nothing to do with it.

“Epigenetic marks are the consequence of complex interactions between
the genetics, development and environment of an individual," said
University of Cambridge geneticist Eric Miska. "Simple correlations --
if significant -- of epigenetic marks of an individual with anything
from favorite football player to disease risk does not imply a causal
relationship or understanding.”

One longtime researcher in the field of sexual orientation praised
Ngun’s use of identical twins as a means of teasing apart the various
biological factors that influence the trait.

“Our best guess is that there are genes” that affect a man’s sexual
orientation “because that’s what twin studies suggest,” said
Northwestern University psychologist J. Michael Bailey, who  has
explored a range of physiological markers that point to homosexuality’s
origins in the womb. But the existence of identical twin pairs in which
only one is homosexual “conclusively suggest that genes don’t explain
everything,” Bailey added.

While Ngun’s research needs to be replicated in larger studies of twins,
it advances the fitful process of better understanding how — and when —
a boy’s sexual orientation develops, Bailey said.

Follow me on Twitter @LATMelissaHealy and "like" Los Angeles Times
Science & Health on Facebook.


So why not test for it and adjust insurance premiums according to risk??
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