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Each time a cell divides, a small section of each chromosome's protective cap—the telomere—is worn away. Most cells use an enzyme called telomerase to help mitigate this loss, but 10% to 15% of cancers have another mechanism called the alternative lengthening of telomeres (ALT) pathway.

"ALT is found in some of the worst cancers, such as pancreatic neuroendocrine tumors, osteosarcomas and subsets of glioma," said Roderick O'Sullivan, Ph.D., professor in the Department of Pharmacology and Chemical Biology at the University of Pittsburgh and UPMC Hillman Cancer Center. "Interfering with ALT in these cancers may be a novel therapeutic approach, which represents a huge biomedical window of opportunity, but ALT has been a black box."

In a new study, published today in , senior authors O'Sullivan and Kyle Miller, Ph.D., professor in the Department of Radiation Oncology at Emory University, and their team describe a novel tool called BLOCK-ID that offers a glimpse into the black box of ALT, bringing them one step closer to developing cancer therapies that target this process.

During cell division, the double helix of DNA unwinds to create a replication fork, allowing replication proteins access to do their job. But sometimes, this process stalls and the proteins become stuck on DNA, creating what is known as a protein barrier.

"A replication fork is like a train coming along train tracks, but if the line ends suddenly due to a protein barrier, the train will collide," said O'Sullivan. "BLOCK-ID is like an artificial barrier that allows us to monitor a collision event at one very specific part of the cell."

BLOCK-ID uses an enzyme to add a molecule called biotin to all proteins that play a role at the collision event.

"Biotin acts like a tag that says, 'This protein has been in contact with the protein barrier,'" said O'Sullivan. "Even though proteins may move to another part of the cell, we can tell they were at the collision event because they are marked for life, allowing us to trace brief interactions that would normally be missed."

Using BLOCK-ID, the researchers identified a protein called TRIM24 as an essential player in the ALT pathway of cancer cells.

"If you remove TRIM24 from normal cells, they can tolerate it, but if you remove this protein from ALT cells, they don't like it," said O'Sullivan. "Without TRIM24, telomeres in ALT cells become a mess: they shorten, and they become destabilized and nonfunctional."

Until now, it had been thought that a protein called PML was essential for ALT because it forms a shell around telomeres, creating a specialized environment that attracts other repair proteins.

However, when the researchers molecularly tethered TRIM24 to telomeres in cancer cells lacking PML, they found that, surprisingly, these telomere repair shells still formed.

"This experiment tells us how important TRIM24 is to the ALT mechanism," said O'Sullivan. "It also tells us that ALT has inbuilt redundancies. This is really important because if we are going to target ALT, we need to understand its complexities."

Other authors are listed in the manuscript.