Sperm may hold key to cancer, chimp study suggests
Last Updated: May 19, 2005



WASHINGTON (Reuters) - The evolutionary path that separated humans
from chimps 5 million years ago may have made human sperm survive
better but paradoxically may have made humans prone to cancer.


A comparison of chimpanzee genes to human genes shows a concentration
of genes unique to people in areas associated with sperm production
and cancer, and suggests the changes that make humans unique also
make us uniquely prone to cancer.

"If we are right about this, it may help explain the high prevalence
of cancer," said Rasmus Nielsen of the University of Copenhagen in
Denmark, who led the study while at Cornell University in New York.

Nielsen and colleagues were studying the chimpanzee genome, the
collection of all DNA, for clues about what make chimps and humans
different. They used genetic sequences published by Maryland-based
Celera Corp.

For the report, published in the journal Public Library of Science
Biology, Nielsen's team at Cornell studied the 13,731 genetic
sequences that are the most different between humans and chimps.

They knew that genes having to do with smell, making sperm and
fighting bacteria and viruses were likely to be different.

"While we expected to find genes involved in olfaction,
spermatogenesis, and immune defense among the 50 annotated genes ...
we were surprised to find a very large proportion of cancer-related
genes, especially genes involved in tumor suppression, apoptosis, and
cell cycle control sequences," they wrote.

"It is surprising to find such a large proportion of genes that may
be related to tumor development and control."

In cancer, cells lose their ability to self-destruct when they become
faulty, a process called apoptosis. Cell cycling -- the process by
which cells activate, divide, and grow into two separate cells -- is
also disrupted in cancer.

"Eliminating cancer cells by apoptosis is one of the main processes
used by the organism to fight cancer," Nielsen said.

"The connection that we saw that these genes involved in
proliferation may be involved in spermatogenesis," Cornell's Andrew
Clark, who worked on the study, said in a telephone interview.

Apoptosis also kills many developing sperm cells before they mature.
But evolution could have interfered with this process, allowing more
sperm to reach maturity, thus carrying the mutation into the next
generation.

Clark said chimpanzees get cancer, too, but no one has been able to
study enough of them in captivity to see if they do so at the same
rate and in the same ways as humans do.

Cancer in people usually occurs in late adulthood, after they have
reproduced, and thus has not been removed by natural selection -- the
process that leads to evolution.