Targeting 'enabler cells' to stop the spread of triple-negative breast cancer

Triple-negative breast cancer is aggressive and linked to worse prognoses than other types of the disease. Now, a research team led by scientists at Georgetown University Medical Center has identified a small subset of early breast cancer cells that drives the invasive spread to other parts of the body.

By using CRISPR editing and RNA sequencing, the Georgetown-led team found that early-stage triple-negative breast cancer cells expressing the AIB1-Delta4 protein can recruit neighboring cells and enable metastasis. The finding was published online in Cancer Research, a journal of the American Association for Cancer Research.

The detection of these AIB1-Delta4 “enabler cells” could be used to predict the likelihood the cancer will metastasize, and drugs could be developed to suppress this subpopulation of cells, the researchers suggested.

AIB1-Delta4 is made from a truncated version of the AIB1 oncogene during a single RNA splicing event. Previous studies have noted its increased expression in early-stage breast cancers called ductal carcinoma in situ (DCIS), but scientists haven't been able to discern whether AIB1-Delta4 has distinct functions.

For the new study, the researchers used CRISPR-based gene editing to create triple-negative DCIS that only expressed the AIB1-Delta4 protein to examine its role in driving invasive cancer.

The AIB1-Delta4 cells showed increased movement and migration as well as an enhanced ability to invade the extracellular matrix in lab dishes. What’s more, mixing a small population of these cells with their DCIS counterparts made the early-stage breast cancer more invasive, the team found.

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The AIB1-Delta4 breast cancer cells were then implanted into animals. In mice, mixing the edited cells with the parental DCIS cells significantly enhanced tumor growth compared with an equivalent number of pure DCIS cells alone, the researchers reported. Animals that got the mixed tumors also had a much higher rate of lung metastases that correlated with growth of the primary tumor.

Surprisingly, tumors with only AIB1-Delta4 cells grew significantly slower than the normal DCIS tumors did. And the metastases from the mixed model consisted largely of normal DCIS cells but not AIB1-Delta4 ones. Thus, the AIB1-Delta4 cells “appear to enable invasion of neighboring cells yet are not selected as a dominant population that takes over during progression,” the researchers wrote in the study.

This finding probably explains why these “enabler cells” only make up about 20% or less of the cells in an aggressive tumor and could be overlooked in tissue analyses, the team figured.

Using RNA sequencing, the researchers found that AIB1-Delta4 exhibited distinct genomic features related to those involved in breast cancer progression. For example, the PPAR signaling pathway was notably dialed down in the AIB1-Delta4 cells and appeared to play a significant role in the recruitment of metastasis-enabling cells, the team found. Treating the breast cancers cells with a PPAR-gamma agonist called efatutazone didn’t affect the invasion of AIB1-Delta cells but significantly decreased the enabler effect on other cancer cells.

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Other research groups have studied cell subpopulations for clues to treating aggressive breast cancers. Researchers at Baylor College of Medicine recently pinpointed the enzyme MAP kinase as the culprit behind slow-growing estrogen receptor-positive breast cancer cells transforming into more aggressive cells that lack the estrogen receptor.

A team led by Dartmouth College found that activating the protein kinase A in early breast cancer could block the emergence of aggressive subpopulations. The team suggested that promoting PKA by inhibiting a protein called SOX4 represents a possible adjuvant to chemotherapy for breast cancers.

The Georgetown-led group believes the new insights into the AIB1-Delta4 enabler cells could inform both diagnosis and treatment of breast cancers.

“We propose that the detection of these enabler cells in early-stage breast cancers could predict which tumors are more aggressive and destined to metastasize,” Ghada Sharif, Ph.D., the study’s first author, said in a statement. “Therapeutic targeting of vulnerabilities uncovered in the enabler cells, such as the splice variants, could represent a new approach to preventing malignant progression of breast cancer.”