DNA secreted by tumor cell extracellular vesicles prompts anti-metastatic immune response

Specially packaged DNA secreted by tumor cells can trigger an immune response that inhibits the metastatic spread of the tumor to the liver, according to a study led by researchers at Weill Cornell Medicine, Memorial Sloan Kettering Cancer Center and Korea’s Yonsei University. The discovery improves the scientific understanding of cancer progression and anticancer immunity, and could yield new clinical tools for assessing and reducing metastasis risk.

In the study, reported Dec. 3 in Nature Cancer, the researchers examined cancer cells’ secretion of short stretches of DNA packaged on tiny capsules called extracellular vesicles (EVs). All cells use EVs to secrete proteins, DNA and other molecules, and tumor cells are particularly active EV secreters.

The biological functions of these EV-packaged molecules are still being explored, but in this case, the researchers discovered that in various cancer types, EV-DNA secreted by tumor cells works as a “danger” signal that activates an anti-tumor response in the liver, reducing the risk of liver metastasis.

“Initially we hypothesized that more tumor EV-DNA would mean a worse prognosis, but we were surprised to find the opposite,” said study co-senior author Dr. David Lyden, the Stavros S. Niarchos Professor in Pediatric Cardiology and a professor of Pediatrics and of Cell and Developmental Biology at Weill Cornell Medicine.

The other co-senior authors of the study are Dr. Han Sang Kim, associate professor at Yonsei University College of Medicine and a visiting associate professor of Molecular Biology Research in Pediatrics at Weill Cornell Medicine; and Drs. Yael David and Jacqueline Bromberg of Memorial Sloan Kettering Cancer Center. The first author of the study is Dr. Inbal Wortzel, a research associate in the Lyden laboratory.

Dr. Lyden and colleagues have found in prior research that tumor cells secrete snippets of EV-DNA from across their genomes, including pieces with cancer-associated mutations. In the new study, they took a more comprehensive look at tumor EV-DNA, and in early experiments found something unexpected.

“We had assumed that this DNA is in the form of ‘naked’ strands inside EVs, but we were surprised to find that it is mostly on the EV surface, wrapped around support proteins called histones, much as it would be in a chromosome,” said Dr. Wortzel.

With the help of Dr. David’s lab, the researchers determined that these histone proteins have a unique set of modifications, hinting that EV-DNA has a specific signaling function.

The team also identified several genes that help regulate EV-DNA packaging, and found that when one of these, APAF1, was absent, the amount of EV-DNA secreted by tumor cells was sharply reduced.

Other recent work by Dr. Lyden and colleagues have shown that cancer cells can secrete EV-packaged proteins and fatty acids that make the liver more hospitable for metastatic tumor development. The team therefore suspected that tumor-secreted EV-DNA would also promote metastasis.

However, in animal models of pancreatic and colorectal cancer, boosting tumor EV-DNA levels lowered metastasis risk—whereas reducing tumor EV-DNA levels through genetic deletion of APAF1 greatly raised that risk.

“We also found that in colorectal cancer patients, those with low levels of EV-DNA in their tumors at the time of diagnosis were more likely to develop liver metastases later on, compared to those with high EV-DNA levels,” Dr. Kim said.

The researchers discovered that tumor EV-DNA is taken up by liver-resident immune cells called Kupffer cells, which are spurred by marks of damage in the EV-DNA to organize immune cell clusters that resist liver metastases.

“This is a tumor suppressor mechanism that had not been described before, and we think the cancers that can turn it off, by secreting less EV-DNA, have a better chance of spreading,” Dr. Wortzel said.

The researchers now hope to develop an EV-DNA-based prognostic test for metastasis risk, as well as a vaccine-like therapy to enhance tumor EV-DNA signaling and suppress metastasis in patients with early stage cancer.