Session Item

A potential synergistic effect between pre-operative radiotherapy and systemic therapies for high-risk breast cancer (BC) is currently being investigated in a handful of ongoing clinical trials. However, the scarce in vivo data on gene expression changes induced by radiotherapy come from low-risk (i.e. Luminal A) breast cancer. The purpose of this work is to investigate radiotherapy-induced gene expression changes in high-risk (Luminal B, HER2-enriched and Basal-like) breast cancer cell lines.

Six different human breast cancer cell lines (BT474, MCF7, MDA-MB-468, MD-MA-453, T47D and ZR751) were investigated (see Table 1). They were irradiated with a single fraction of 6, 8 and 10 Gy using a clinical 6 MV photon beam. RNA was extracted before and 72 hours after irradiation. For the 6 Gy irradiation, RNA was also extracted 6 and 21 days after irradiation. The RNA was profiled using the NanoString nCounter Elements XT panel, which includes the 50 genes within the PAM50 signature. PAM50 intrinsic subtypes and other signatures (e.g. Proliferation score) were computed using R. All experiments were repeated at least 3 times.

Table 1: Description of the 6 human breast cancer cell lines investigated in this work, in terms of intrinsic molecular subtype, hormone receptors status and HER2 status.

We found that radiotherapy induces significant gene expression changes in HR+/HER2-negative cell lines, while HER2+ and triple-negative cell lines did not seem to be affected by the radiation doses investigated in this work. For HR+/HER2-negative cell lines, we identified a strong RT-induced downregulation of proliferative genes, together with a notable upregulation of genes related to the HER2-enriched (HER2-E) subtype, such as ERBB2, GRB7 or TMEM45B (see figure 1), 72 hours after irradiation. Three weeks after irradiation, the upregulation of HER2-E-related genes persisted, while the expression of proliferative genes returned to the baseline level.

Figure 1: Luminal B, HER2-enriched and proliferation signatures, as well as ERBB2, GRB7 and TMEM45 gene expression, as a function of radiation dose, for MCF7 (top) and T47D (bottom) cell lines.

Radiotherapy-induced gene expression changes depend strongly on the breast cancer cell line. For HR+/HER2-negative cell lines, radiotherapy seems to induce a low-proliferative HER2-enriched phenotype. This HER2-enriched phenotype seems to be persistent (at least up to 3 weeks after irradiation), while cell proliferation seems to return to the baseline. If confirmed in patients, these findings could open the door to new therapeutic strategies for high-risk HR+/HER2-negative BC, based on the combination of radiotherapy and anti-HER2 drugs.