The Tumor-Specific Expression of L1 Retrotransposons Independently Correlates with Time to Relapse in Hormone-Negative Breast Cancer Patients

Background: Long-Interspersed Nuclear Element (L1) retrotransposons are silenced in healthy tissues but unrepressed in cancer. Even if L1 reactivation has been associated with reduced overall survival in breast cancer (BC) patients, a comprehensive correlation with clinicopathological features is still missing. Methods: Using quantitative, reverse-transcription PCR, we assessed L1 mRNA expression in 12 BC cells, 210 BC patients and in 47 normal mammary tissues. L1 expression was then correlated with molecular and clinicopathological data. Results: We identified a tumor-exclusive expression of L1s, absent in normal mammary cells and tissues. A positive correlation between L1 expression and tumor dedifferentiation, lymph-node involvement and increased immune infiltration was detected. Molecular subtyping highlighted an enrichment of L1s in basal-like cells and cancers. By exploring disease-free survival, we identified L1 overexpression as an independent biomarker for patients with a high risk of recurrence in hormone-receptor-negative BCs. Conclusions: Overall, L1 reactivation identified BCs with aggressive features and patients with a worse clinical fate.