A study led by Massachusetts General Hospital (MGH) investigators reveals how spontaneous changes in the molecular characteristics of tumours can lead to tumours with a mixed population of cells requiring treatment with several types of therapeutic drugs.

In their report in the Sept. 1 issue of Nature, the research team describes finding a mixture of HER2-positive and HER2-negative circulating tumours cells (CTCs) in blood samples from patients who developed metastatic disease after originally being diagnosed with estrogen-receptor (ER)-positive/HER2-negative breast cancer.

"Not only did we observe the acquisition of HER2 positivity in patients with ER-positive/HER2 negative breast tumours, we also found that this population of tumour cells is able to spontaneously oscillate between HER2-positive and HER2-negative states, which contributes to tumour progression and resistance," says Shyamala Maheswaran, PhD, of the MGH Cancer Center, co-senior author of the report. "We also showed in mouse models the types of therapies that may be most useful for patients with these difficult-to-treat tumours."

Molecular heterogeneity of tumours has become a confounding factor in cancer treatment in recent years, requiring the use of multiple drugs that specifically target all the different cell populations driving tumour growth.

The current study was designed to investigate further the differences in HER2 expression that can occur in individual patients' tumours and how they affect tumor growth and treatment.

Using the CTC-iChip - a microfluidic device developed at the MGH Center for Engineering in Medicine that isolates CTCs from blood samples - the researchers found both HER2-positive and HER2-negative CTCs in samples from 16 out of 18 patients who had developed metastases after treatment for ER-positive/HER2-negative breast cancer.

CTCs isolated from patients with ER-positive/HER2-negative breast cancer and grown in culture also showed a similar pattern of HER2 expression, in which some of the tumour cells expressed HER2 and some did not.

Closer examination of these HER2-positive tumour cells showed elevated expression of proteins in several growth signalling pathways, but the level of HER2 expression was not as high as seen in HER2-amplified primary tumours.

These HER2-positive CTCs were no more sensitive to treatment with a HER2-inhibiting drug than were HER2-negative CTCs, but combined treatment with both the HER2 inhibitor and an IGFR1 (insulin-like growth factor receptor 1) inhibitor was toxic to HER2-positive CTCs. In contrast, HER2-negative CTCs had elevated expression of proteins in the Notch developmental pathway and in pathways that respond to DNA damage.

Reflecting those differences, HER2-positive CTCs were found to proliferate more rapidly and respond to treatment with standard chemotherapy drugs, while HER2-negative CTCs were more resistant to chemotherapy drugs but sensitive to gamma secretase inhibitors, which are known to suppress Notch signalling.

Injecting either HER2-positive or HER2-negative breast tumour cells into the mammary tissue of mice led to the development of tumours with both types of cells.

Treatment of tumours in which HER2-positive cells were predominant with the chemotherapy drug paclitaxel led to rapid tumour shrinkage, followed by recurrence with a greater number of HER2-negative cells, while paclitaxel treatment of tumours with more HER2-negative cells did not have any effect.

Treating mice in which tumours had been initiated by a mixture of HER2-positive and HER2-negative tumor cells with a combination of paclitaxel and a gamma secretase inhibitor did delay tumour recurrence significantly, suggesting the potential utility of a combination treatment strategy to eliminate this mixed population of tumour cells.

"The ability of these two populations of tumor cells to convert back and forth highlights the importance of treating tumours with drugs that would simultaneously target both populations," says Maheswaran, who is an associate professor of Surgery at Harvard Medical School. "Now we need to investigate the mechanisms responsible for this interconversion."

Source: Nature