Study reverses male infertility in mice

LA JOLLA, California (CNN) -- A new study raises hope that a common form of male infertility can be reversed, scientists at the Salk Institute in La Jolla, California, said Monday.

About one out of five couples worldwide are estimated to have fertility problems. One-third to one-half of those problems are due to male infertility, and "the overwhelming majority of male infertility cases arise from the inability [of males] to make sperm cells," said Inder Verma, a genetics professor and senior author of the study.

In the study, Salk researchers injected infertile male mice with a gene that makes sperm grow and mature.

"It's only a mature sperm that can fertilize an egg and eventually produce a baby," said Salk researcher Vinay Tergaonkar.

To get the gene into the mouse, researchers hollowed out the bad genetic material from a virus and replaced it with a gene that makes sperm grow. Researchers then injected the altered virus into nine adult infertile male mice.

At least seven of them produced enough viable healthy sperm for offspring through artificial insemination. The sperm from one of the infertile mice produced nine healthy pups.

Scientists can't say yet if the infertile male mice eventually will produce enough sperm for natural breeding. "We would have to wait for a longer time for more sperm to be produced," Tergaonkar said.

Another breakthrough in the study, according to Tergaonkar, was finding a virus that would safely and effectively deliver the gene to the cell. Researchers tried five viruses before finding one that delivered the gene without side effects.

"Since we used a virus to deliver the gene, there is always concern that virus doesn't transfer to the babies," Tergaonkar said. "We were able to show that the virus was not passed onto the pups."

Even though these research results will take years to apply to humans, Dr. Michael Witt of Reproductive Biology Associates, an Atlanta, Georgia, infertility clinic, called the study "significant since currently we have no mechanisms for treating genetic [infertility] problems."

"People have talked about gene therapy where you could insert missing parts of the DNA and correct an underlying problem, but there haven't been any successful human stories. This would offer hope," Witt said.

"If one day you could find a parallel problem in humans and have sperm develop, with today's infertility methods you wouldn't need a lot of sperm to produce healthy offspring."