WASHINGTON, Aug 15 (Reuters) - The AIDS virus damages the brain in two ways, by not only killing brain cells but by preventing the birth of new cells, U.S. researchers reported on Wednesday.

The study, published in the journal Cell Stem Cell, helps shed light on a condition known as HIV-associated dementia, which can cause confusion, sleep disturbances and memory loss in people infected with the virus.

It is less common in people taking drug cocktails to suppress the virus, and why HIV damages brain function is not clearly understood.

The virus kills brain cells but it also appears to stop progenitor cells, known as stem cells, from dividing, the team at Burnham Institute for Medical Research and the University of California at San Diego found.

"It's a double hit to the brain," researcher Marcus Kaul said in a statement. "The HIV protein both causes brain injury and prevents its repair."

The cocktail of drugs known as highly active antiretroviral therapy or HAART that treats HIV does not infiltrate the brain well, allowing for a "secret reservoir" of virus, said Stuart Lipton, who worked on the study.

HIV-associated dementia is becoming more common, as patients survive into their older years.

Working in mice, the researchers found that the virus directly interferes with the birth of new brain cells from stem cells.

"The breakthrough here is that the AIDS virus prevents stem cells in the brain from dividing; it hangs them up," Lipton said. "It's the first time that the virus has ever been shown to affect stem cells."

The culprit is gp120 -- a protein found on the outside of the AIDS virus, the researchers found.

"Knowing the mechanism, we can start to approach this therapeutically," Lipton said.

"This indicates that we might eventually treat this form of dementia by either ramping up brain repair or protecting the repair mechanism," Kaul added.

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