updated 5:35 p.m. ET Nov. 17, 2008

CHICAGO - Tiny sacs released from tumor cells and circulating in the blood carry genetic information about the tumor, offering a new way to track and treat the cancer, U.S. researchers said on Sunday.

“They contain a little piece of the tumor cell in the blood stream. If you just look at these packets, you basically know what kind of mutations are in the tumor cell,” said Xandra Breakefield of Massachusetts General Hospital in Boston, whose study appears in the journal Nature Cell Biology.

These membrane-covered packets, called exosomes, represent a new way of getting information about a cancer, offering a means of choosing the best therapy, seeing how a patient responds to treatment, and possibly offering a way to deliver therapies back to the tumor, Breakefield said.

“It’s a whole new concept of cell communication we didn’t know tumors used,” Breakefield said in a telephone interview.

She said for most forms of cancer, there is no good way to know what genetic mutations are in a tumor, short of taking a sample and doing a biopsy.

Many current blood tests, such as the prostate specific antigen, or PSA, test for prostate cancer, simply check for elevated levels of a specific protein.

By using exosomes, doctors might be able to get specific information about a cancer from a simple blood test.

“It’s a blood biomarker,” she said.

Cell communication
Johan Skog, who works in Breakefield’s lab and led the study, said many types of cells release exosomes as part of normal cell-to-cell communication, and several types of tumors are known to shed exosomes containing proteins that can alter the environment to make it more favorable to tumor growth.

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“It’s a form of cell communication that normal cells use but tumor cells use with a vengeance,” Skog said in a telephone interview.

For the study, the researchers carefully analyzed the contents of the exosomes shed from glioblastoma cells. Glioblastoma is a type of aggressive brain cancer.

Inside, they found fragments of genetic messenger ribonucleic acid, or RNA, including cell messengers related to cell growth, immune response and the construction of new blood vessels.

When they exposed these exosomes to normal cells in the lab, the tumor RNA delivered its genetic message into the cells.

“That is important to know. We didn’t realize they had this external means of communicating with their surroundings,” Breakefield said.

The team also analyzed blood and tissue samples from 25 brain cancer patients. They were able to find tumor exosomes in both. In two patients, this analysis turned up a specific genetic mutation in the epidermal growth factor receptor, or EGFR, gene that a surgical biopsy had missed.

The first step would be to develop a blood test, but eventually Skog thinks it may be possible to use the exosomes to deliver therapies to the cancer.

Massachusetts General Hospital has licensed the technology to Exosome Diagnostics Inc, where Skog is director of research in addition to his duties at the hospital.

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