Before a benign tumor turns malignant, it must progress through a series of stages — starting with early changes in cellular activity and the surrounding microenvironment. But exactly what these changes are, and what causes them, remains a mystery.
Peter Westcott, a cancer geneticist and immunologist at Cold Spring Harbor Laboratory, is studying the full evolution of cancer and the role the immune system plays. “If you can capture those transitory events, I think many things may be different,” he said. “As a cell transitions from a benign tumor cell to a cancer cell, it may transit through distinct cell states and transcriptional states that may actually be different than what you are when you’re a full-blown cancer.”
Using spatial transcriptomics, Westcott and colleagues recently studied samples of stage 0 polyps contained within a precancerous polyp (known as a carcinoma in situ) from human colons. What they found was surprising: Early-stage malignant tumors had a higher presence of T cells and other adaptive immune cells compared to benign tumors. This stands in contrast to the later stages of colorectal cancer, when the microenvironment is immunosuppressed and very few T cells are present. “That raises the question, are T cells holding cancer in check, or promoting it, or both?” said Westcott.
To explore that question, his team developed mouse models that allowed them to dive deeper and separate out the events that cause the early initiation of cancer from the events that cause progression. To induce a benign tumor, they used CRISPR-Cas9 to knock out a tumor suppressor gene, and then used a split-Cre recombinase system to knock out another tumor suppressor gene and switch on an oncogene in a few rare colon cells within the tumor — but not all. They labeled these cells with a red fluorescent marker for tracking. The choice to only genetically modify some cells was important to accurately model disease, Westcott said, because these mutations don’t just suddenly happen in every cell of the tumor. “If you were to mutate the entire tumor with turning on a KRAS mutation, that’s going to profoundly change the microenvironment in ways that I don’t think would be physiologically relevant.”
When they depleted T cells in the mouse models, the researchers observed that many more benign polyps formed in the colon — a finding that aligns with past research showing that T cells are protective against early tumor formation. However, as they continued to study the polyps, they found that the ones without T cells were less likely to progress into a cancerous tumor. “So, T cells are actually playing a role in facilitating the benign to malignant transition,” said Westcott. Further experiments in a colon organoid model supported that idea. When they added T cells to precancerous organoids, they found that the ones that harbored a KRAS mutation grew better than the ones without the mutation. “With T cells there, they’re actually unmasking this malignant potential of KRAS,” said Westcott.
Their findings highlight the importance of breaking down the distinct steps of cancer evolution. Although T cells are involved in boosting the transition to a malignant state in colorectal tumors, Westcott and his team emphasized that they’re not trying to claim that T cells are bad — just that it matters at what stage you study them.
In future work, Westcott and his team plan to identify the molecular mechanisms behind T cell’s involvement in the transition from benign to malignant tumors, and apply their mouse models to study risk factors for colorectal cancer like high-fat diets and pathogenic microbial strains. But Westcott knows he may be surprised again. “These models constantly prove me wrong,” he said. “I have these simple hypotheses, but it’s always way more complicated than that. It’s extremely rewarding.”
