A common "genetic signature" that defines dormant cancer cells hiding for years or even decades in a patient's skeleton has been identified by a team led by Australian researchers. When these dormant cells awaken, the result is disease relapse and progression. With the identification of this unique genetic signature in mouse models, the researchers say it may be possible to target and eradicate these cells in patients, or simply keep them asleep, providing a potential new weapon against the recurrence of cancer.
"Dormancy is an elusive and deadly component of cancers," said Peter Croucher, PhD, Deputy Director of the Garvan Institute of Medical Research in Sydney and Head of the institute's Bone Biology Lab. Croucher presented results during a meeting on the "Evolving Tumor Microenvironment in Cancer Progression," held virtually January 11-12, 2021, by the American Association for Cancer Research. "Eradicating dormant cancer cells is one of the keys to curing cancers and represents an unrealized opportunity," he added.
Searching for Dormant Cells
Frequently, dormant cancer cells hide out in distant niches, such as the inner lining of bones, where they can elude the impact of conventional chemotherapy. The goal of Croucher's research was to figure out where these dormant cells hide, why they're reawakened, and how best to target them to prevent the relapse of cancer. Instead of focusing on the cancer cell itself, Croucher and colleagues have been examining other cells in their microenvironment that determine their fate.
Initially, the team hypothesized that dormant cells occupy a common niche somewhere in skeletal bone, where they find a safe harbor for long-term dormancy. They then decided to test this notion in the most common cancers, including breast and prostate cancer, as well as one of the most difficult-to-treat cancers, multiple myeloma.
Until recently, this task proved almost impossible. But Croucher and colleagues developed a technology to track and analyze dormant cancer cells from different types of tumor types, as well as the compartment in the skeleton where they've taken up residence.
"Dormant cancer cells can be as rare as one in a million and distinguishing them from growing cells was not previously possible," he said. "We leveraged the fact that dormant cells do not proliferate to find these cells in the skeleton of mouse models."
Study Details
As a first step, the team labeled cancer cells with a fluorescent dye, which is shared with daughter cells every time they divide. Cells that retained the dye-the dormant cancer cells-were isolated.
Then, with a new technology developed in their lab called intravital two-photon microscopy, the team imaged the cells deep inside the bone. Results showed that dormant cells from a variety of cancers were hiding in the inner lining of bone, suggesting different cancers may occupy a common niche. Having isolated these rare dormant cells, the researchers then analyzed their transcriptome-a snapshot of all the genes that are switched on in the cell and control dormancy.
Single-cell RNA sequencing showed these cells expressed a distinct gene signature that was enriched for myeloid genes. Sequencing of more than 130,000 cells isolated from this bone compartment and bone marrow isolated 32 distinct cell clusters. Their findings were independently confirmed at the Weizmann Institute of Science in Rehovat, Israel.
"We investigated which cells in bone bound most with all the molecules found in dormant tumor cells from myeloma, breast, and prostate cancer," Croucher said. "We then used computer modeling as there are many thousands of molecules and scores of different types in bone.
"This investigation showed us that there are some cell types in bone that have large numbers of molecular binding partners for the molecules found on dormant cancer cells."
Also, the team discovered that non-hemopoietic cells, particularly cells of the osteoblast lineage called LepR-high/CXCL12-high mesenchymal cells, showed greatest enrichment for dormant cell-binding partners. This was common across all three dormant tumor types examined. For example, the gene Gas6-expressed by LepR-high/CXCL12-high mesenchymal cells-has the binding partner Ax1 expressed by dormant myeloma cells, Mertk in dormant breast cancer cells, and Mertk and Tyro3 in dormant prostate cancer.
Results from their prostate cancer study were published in EMBO Reports in 2020 (https://doi.org/10.15252/embr.202050162). Their work on breast cancer has yet to be published.
"We were surprised by the set of genes that are switched on when cells engage in specific niches in the body, which was common to each of three different tumor cell types," Croucher said. "This suggests there may be a common mechanism that is dependent on the niche environment and not the tumor cell."
To further test their hypothesis, Croucher and team injected mice bearing myeloma cancer cells with small inhibitors of Ax1. Following this treatment, the number of dormant cells decreased, while tumor growth increased, suggesting that Ax1/Gas6 interaction is important for controlling dormancy in this cancer.
"Together, these data show that single-cell sequencing can be used to define the cells and molecular pathways that facilitate dormant cancer cell niche formation in the skeleton," the researchers wrote in an abstract of their work. "This approach suggests that cancer cell-specific molecules interact with common molecules in the endosteal niche, including LepR-high/CXCL12-high mesenchymal cells, to switch on common molecular pathways to control dormancy."
Croucher said there are several potential pathways toward clinical relevance: keeping cells in dormant state long-term to prevent them from reactivating to cause disease; waking them up and targeting with conventional therapy; or targeting with therapies aimed at delivering agents to the dormant cells based on their unique cellular profile.
As for next steps, Croucher said the team is beginning to investigate whether their observations in experimental mouse models can be replicated in patients. In addition, his team is studying if they can target and eradicate dormant cells in experimental models of different cancers, using drugs already in use in other settings, as well as immunotherapy approaches.
"We ultimately hope to test new interventions arising from the steps above in a clinical setting," Croucher concluded.
Warren Froelich is a contributing writer.