By Julie Steenhuysen

CHICAGO, Sept 15 (Reuters) - Just three tumor proteins can indicate lung cancer as much as a year before symptoms emerge, U.S. researchers said on Monday in a finding that may lead to a blood test for lung cancer within five years.

They said an analysis of blood samples taken from smokers found three proteins or antigens were present in more than half of the people who later developed lung cancer.

"The fact that we got a signal like this with just three biomarkers is very significant," Dr. Samir Hanash of the Fred Hutchinson Cancer Research Center in Seattle said in a statement.

"If we can enlarge this panel by adding a few more, we could develop a blood test with sufficient sensitivity and specificity for detecting lung cancer much earlier than current screening methods allow," said Hanash, whose research appears in the Journal of Clinical Oncology.

The lung cancer test uses immune-system signals in the same way as blood tests now used to detect human immunodeficiency virus, or HIV, which causes AIDS. It looks for an immune response.

"What is going on in cancer is the immune system recognizes the presence of tumor antigens as foreign proteins, even though they are made by cancer cells we have in us," Hanash said in a telephone interview.

"The immune system thinks those are aberrant proteins and it needs to respond against them."

Hanash wanted to see if three biomarkers linked with early-stage lung cancer could be detected in the blood of people before any symptoms appeared.

The researchers were looking for two previously identified tumor antigens, annexin1 and 14-3-3 theta, as well as a newly discovered lung cancer antigen, LAMR1.

They tested blood samples from 85 current or former smokers collected within a year of lung-cancer diagnosis and samples from 85 current or former smokers who did not develop cancer.

They found three proteins were present in 51 percent of the people who went on to develop lung cancer.

"This was a critical step to pass to show that, in fact, a set of antigens do show positivity even before a diagnosis of lung cancer, at a time when subjects don't have any symptoms," Hanash said.

The next step is to see if the blood test used in conjunction with computed tomography, or CT scans, can boost early diagnosis of lung cancer, perhaps catching cancers the scan missed.

Eventually, the team wants to have a lung cancer blood test approved by the U.S. Food and Drug Administration.

"That will take maybe five years if every step of the way we are successful," he said.

Lung cancer is the leading cause of cancer death in men and the second-leading cause of cancer death in women worldwide, according to the American Cancer Society. (Editing by Will Dunham and Bill Trott)

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