The retrotransposon LINE-1: a double-edged sword in tumorigenesis

Transposons, also known as jumping genes, are DNA sequences capable of relocating within or between chromosomes. Long interspersed element-1 (LINE-1), the only autonomously active retrotransposon in the human genome, plays a critical role in maintaining genomic stability through its dynamic regulation. Under normal physiological conditions, the host employs epigenetic and other mechanisms to maintain LINE-1 in a silenced state. However, when this precise regulatory control is disrupted, aberrant LINE-1 activation can lead to insertional mutations, resulting in genomic instability and the development of various genetic disorders and malignant tumors. Recent evidence has demonstrated elevated LINE-1 expression in multiple cancers, such as breast, esophageal, lung, and colorectal cancer, suggesting a close association between LINE-1 dysregulation and tumorigenesis. This review summarizes the multi-layered regulatory network governing LINE-1, encompassing epigenetic modifications, non-coding RNAs, and various host restriction factors. It also explores the molecular mechanisms underlying LINE-1 aberrant activation in the tumor microenvironment and outlines the diverse pathways through which LINE-1 influences tumor development, such as compromising genomic stability, triggering inflammation and immune responses, and participating in cellular immortalization. This review not only provides a theoretical foundation for utilizing LINE-1 as a molecular biomarker in cancer diagnosis but also offers new perspectives for developing novel anti-tumor therapeutic strategies based on LINE-1 regulation.