(Reuters) - A single treatment with gene therapy, an experimental technique for fixing faulty genes, has been shown for the first time to boost output of a vital blood clotting factor, possibly offering a long-term solution for people with hemophilia B.

Researchers said the same technology also is being studied as a treatment for hemophilia A, the far more common type of the inherited bleeding disorder.

"It is a technique for potentially permanently curing patients," said Dr. Charles Abrams, ASH secretary and associate chief of hematology/oncology at the University of Pennsylvania in Philadelphia.

The strategy involves replacing the defective gene that causes the bleeding disorder with a correct version delivered via a virus to the patient's liver cells -- the only cells in the body capable of producing certain clotting factors missing or deficient in people with hemophilia.

The factors are numbered using Roman numerals. The two main forms of the disease are hemophilia A, caused by a lack of clotting factor VIII, and hemophilia B, caused by a lack of clotting factor IX.

Researchers from the University College London Cancer Institute and St. Jude Children's Research Hospital in Memphis, Tennessee, studied six men with severe hemophilia B who were producing clotting factor IX, also called FIX, at less than 1 percent of normal levels. The general goal of current treatment with manufactured FIX is to achieve factor levels greater than 1 percent of normal.

Four of the six trial participants have stopped routine treatment and remain free of spontaneous bleeding. The other two have increased the interval between FIX infusions to once every 10 days to two weeks from a previous two to three times a week, said Dr. Andrew Davidoff, chairman of the department of surgery at St. Jude's and co-author of the study.

Frequent treatments with manufactured FIX can cost hundreds of thousands of dollars a year, making hemophilia a tempting target for gene therapy.

"This gene therapy trial ... is truly a landmark study," Dr. Katherine Ponder, hematology and oncology professor at Washington University in St. Louis, said in a New England Journal of Medicine editorial.

"If further studies determine that this approach is safe, it may replace the cumbersome and expensive protein therapy currently used for patients with hemophilia B," she wrote.

The trial results were reported on Saturday at a meeting of the American Society of Hematology in San Diego and published in the New England Journal of Medicine.

The approach involved the use of a novel viral "vector," designed to specifically target the liver.

The six trial subjects were broken into three groups with each group receiving a different concentration of new genes.

FIX levels in the first subject have remained at 2 percent for nearly two years, while the two patients treated with the highest dose have seen FIX levels rise to between 3 and 12 percent, researchers said.

One high-dose subject developed elevated levels of transaminases, an indicator of possible liver damage, and another had a slight increase in liver enzymes. Both cases were resolved with steroids, the researchers said.

Plans are to treat more patients with the highest dose used so far, and if research continues to succeed, the treatment could be widely available "in the next five years or so," said Dr. Amit Nathwani, co-lead study author of the Department of Hematology at UCL Cancer Institute in London.

He also said the team is working now to use the technique for treating hemophilia A.

People with hemophilia bleed more following trauma than people without the disease, and those with severe disease may bleed spontaneously.

"If future studies support these findings, it would bring a significant improvement in the quality of life for those living with the disease," said Dr. Susan Shurin, acting director of the National Heart, Lung and Blood Institute, which partially funded the study.

Hemophilia B is much less common than hemophilia A, a deficiency of clotting factor VIII. About 1 in 5 hemophilia patients has hemophilia B, according to the National Institutes of Health.

The global market for Factor VIII products is about $5 billion, while the market for Factor IX is worth about $1 billion.

Hemophilia is an inherited condition that affects men more frequently. Worldwide, about 1 in 5,000 men are born with hemophilia A and 1 in 25,000 men are born with hemophilia B each year.