Breast cancer ranks second in cancer-related deaths among women. Anti-estrogen therapy is used to prevent recurrence in ~70% of breast cancer patients who are estrogen receptor positive (ER+). However, approximately 30% of patients develop resistance to these therapies, and the cancer recurs. Telomeres are the protective caps at the ends of chromosomes, and cancer cells maintain their immortality through sustaining their telomeres. In ER+ breast cancers, the enzyme telomerase is activated to extend the telomeres, promoting cancer cell longevity.
We sought to understand if telomeres are maintained in ER+ breast cancer that are treated with anti-estrogen therapy. We employed long-term engineered resistance models that closely mimic the gradual development of drug resistance observed in patients. Cells were treated with two anti-estrogen therapies, tamoxifen and fulvestrant and collected at different time points during the development of drug resistance, and compared to passage matched control cells. Untreated cells (MCF-7) had high telomerase activity, hTERT expression, and active proliferation. By contrast, tamoxifen and fulvestrant treatments resulted in downregulation of ER, hTERT, significant suppression of telomerase activity and entry into senescence. Tamoxifen resistant cells restored expression of estrogen receptor (ER) in the resistant state. We observed concurrent re-expression of hTERT with resistance, and an increase in telomerase activity. In contrast, the fulvestrant resistant cells failed to re-express ER or hTERT and showed low telomerase activity.
We next used whole genome sequencing (WGS) to sequence telomeric regions and mapped the reads to our created telomere reference genome using our custom pipeline. Telomere length decreases following tamoxifen and fulvestrant treatment, but it was completely restored in tamoxifen resistant cells with the reactivation of telomerase. By contrast, fulvestrant resistant cells, which do not restore hTERT expression and telomerase activity, continued to undergo telomere shortening.
Our data reveal that telomerase is active in ER+ breast cancer cells, suppressed during anti-estrogen therapy, and becomes reactivated in drug resistance when ER is re-expressed. This coincides with telomere length changes known as telomere remodelling. As ~90% of breast cancers re-express ER following anti-estrogen drug resistance, telomerase reactivation and telomere elongation could be a significant mechanism of drug resistance in ER+ breast cancer. This knowledge suggests potential therapeutic strategies targeting telomerase activation or telomere length to overcome anti-estrogen therapy resistance in breast cancer patients.