WASHINGTON (Reuters) - Bone marrow cells can be used to repair a heart damaged by a heart attack, U.S. researchers said on Sunday.

The cells, genetically engineered to make them stronger and more likely to survive, restored the heart's pumping capacity by 80 percent to 90 percent in animal models in rats, the team at Brigham and Women's Hospital in Boston said.

Writing in the September issue of the journal Nature Medicine, they said they hoped their experiments will someday lead to a treatment for human patients whose hearts are irreparably damaged by heart attacks.

"This is a very positive development that we think holds immense promise," Dr. Victor Dzau, who led the study, said. "But there is certainly more work to do."

His team used bone marrow cells called mesenchymal stem cells. These cells are already used to repair cartilage and bone defects.

They should, in theory, be able to generate new heart tissue but experiments in pigs have failed so far because the cells die.

So the team added a gene called Akt1, which can prevent transplanted cells from dying.

It worked.

"The results were truly remarkable," Dzau said in a statement.

"The hearts that received the stem cells modified with Akt1 exhibited an amazing amount of reparative growth, significantly, if not completely, restoring cardiac function."

When injected into the hearts or rats given artificial heart attacks, the stem cells hooked up with the heart cells and generated more heart-like cells, the team reported.

"The hope is this sort of process can be turned into a gene therapy for humans," added Dr. Abeel Mangi, formerly of Brigham and Women's and now at Massachusetts General Hospital.