In December 2008, Bruno Reversade travelled to India in pursuit of
some spit. The journey took him first to the northeastern city of
Allahabad to Mohammad Pur Umri, a
farming village somewhat famous for its prolific production of identical, monozygotic twins. Globally, only 1 in every 250 to 300 births are identical
twins. In Umri, roughly one in ten is of this type, births that the
villagers — including the twin village leaders — call "gifts from God".
Reversade
is looking for genes that might be responsible for this gift. "Every 50
seconds a pair of natural clones is born. It's more frequent than some
of the most frequent genetic diseases," says Reversade, a developmental
biologist at Singapore's Institute of Medical Biology. "It can't be
random." Many scientists disagree, arguing that chance could fully
account for this cluster of cases and for every twin birth besides.
Reversade is
gathering samples from three 'twin towns', and using genomic analyses
that, he hopes, will point to a common molecular pathway involved in
twinning. Embryologists and obstetricians are looking for clues in
assisted reproduction,
which is known to promote monozygotic twinning as well as high rates of
dizygotic twins, which result from the transfer of multiple embryos.
Reversade
became interested in monozygotic twinning after cutting frog embryos
into halves and watching through a microscope as they developed into
identical embryos. "Pure awe," he says. His goal is to understand why
cells that are acting together to form an embryo split off and start
building a whole new organism, something they can do early, when the
embryo is just a few cells big, or as late as two weeks into
development. Conjoined twins can result if the embryo splits too late
or incompletely.
Reversade says that twinning offers the best way to
study 'regulative development' — the interaction between cells that
informs each one when to follow the pack and when to act alone. He
moved to Singapore in February 2008 after landing the government's
first A *STAR Investigatorship, a US$500,000 per year grant modelled on
the Howard Hughes Medical Institute awards, having convinced the
funders of the feasibility and importance of finding a twinning gene or
genes.
Twinning is often assumed to run in families, and in the
case of dizygotic twins, caused by the release and then fertilization
of two eggs, scientists agree that this is the case. So far though,
researchers have only found genes that are weakly correlated with
dizygotic twinning. The picture for monozygotic twins is even less
definitive. In large-scale studies, family members of the mothers of
monozygotic twins do not seem more likely to have monozygotic twins
themselves.
But some families and some communities do produce identical twins in
numbers that seem to defy this interpretation. Reversade is visiting
them one by one.
Two
years ago he travelled to Jordan to collect saliva samples from members
of a family with 15 pairs of monozygotic twins. The family tree fits a
pattern in which a dominant gene — one that would cause monozygotic
twinning even if only one copy is present — is on one of the 22
autosomal chromosomes. But to make the hereditary pattern work, the
gene must have 'variable penetrance', such that some women would not
bear monozygotic twins even though they carried a copy of the gene.
"Variable penetrance is of course a 'black box'," says Reversade. "Why
don't we see more twins?" One reason, he suggests, is that some twins
'vanish' — meaning that at least one of the two dies — during pregnancy.
Founder effect
In Jordan, Reversade used genetic tests to search for shared patterns
of single-letter variations in the genomes of the twins that might
point to a twinning gene. He found one candidate region, on chromosome
four. One of the genes in the region has some promising
characteristics: it is conserved through vertebrate evolution, it codes
for a protein that is expressed in the blastocyst stage of mice
embryos, and its activity drops once the cells that make it have
differentiated. Reversade thinks that mutations in the gene or other
genes working in the same signalling pathway might have been present in
the founders of each twin town and then spread through the population.
He says he won't publish the work "until I have the full story, namely
the gene, and a possible mechanism".
Reversade hopes that newer technologies — he plans
to use next-generation genetic sequencing machines on the candidate
regions in all his subjects — will give him the necessary sensitivity
to find genes, but he has a long way to go to convince his peers. Reversade's next stop, probably
this summer, will be the village of Linha São Pedro in Brazil, a town
predominantly of blond-haired, blue-eyed people founded by German
immigrants. In the 1990s, 10% of the births there were twins, and
almost half of those were monozygotic.