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Reading
the Book of Y (continued)
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About one in every 4,000 boys is born with a piece
of his Y chromosome missing. When these boys grow up,
they produce few or no sperm.
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Fortunately,
a crack team of sequencers at the Genome Sequencing Center
in St. Louis was willing to tackle the job. Working closely
with gene mapping experts in Page's group, the St. Louis team
read every letter of every sentence of not just the Azoospermia
Factor C Region but of most of the rest of the Y chromosome
in only a few years.
This
effort showed that within the Azoospermia Factor C Region
large sections of 100s of thousands or even millions of letters
are duplicated almost exactly. We don't yet know why. It could
be a back-up redundancy system; if you have four copies of
a gene, and one gets damaged, you still have three good copies.
This repetition made sequencing the Azoospermia Factor C and
similar Y regions nightmarishly difficult. Think of running
this chapter with huge repeated passages through a paper shredder,
and then trying to paste it back together in the right order.
Although
it made things difficult for Page's group and the St. Louis
sequencers, repetition in Y's Azoospermia Factor C Region
turned out to be the key to its frequent deletion. In recombination,
chromosome pairs trade chunks of DNA with each other. This
is an essential and healthy process. But when huge sections
of a chromosome are repeated, as on the Y, this recombination
process sometimes becomes confused by the repeated sections
and deletes everything in between them.
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Think of running this chapter with huge repeated passages
through a paper shredder, and then trying to paste it
back together in the right order.
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Not
only did the Y sequence show that the Azoosperima Factor C
Region's repetitive "hall of mirrors" sequence structure makes
it vulnerable to deletions; it also showed that the region
is more than three times larger than originally thought and
provided vital clues to the identities of the region's genes.
Some of these genes are now themselves the target of focused
research efforts.
Thanks
to the Human Genome Project, researchers like myself now have
a detailed understanding of the Y chromosome that would have
been inconceivable even ten years ago. We now know that the
Y is, on the one hand, a shrunken version of the X chromosome.
But we've also learned that it contains genes that are vital
to survival as well as genes that are vital for fertility.
The Y sequence will pave the way to a fuller understanding
of both the normal processes of sperm production and the causes
of male and female infertility.
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3
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Steve
Rozen is a researcher at the Whitehead Institute for Biomedical
Research, where he has worked on genomics and bioinformatics
since 1993.
Photo:
Sam Ogden.
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