Alternative Lengthening of Telomeres: A Prognostic Paradox in Cancer.

Telomere maintenance enables unlimited cell proliferation by counteracting telomere erosion. While the majority of tumors activate telomerase, a significant subset-approximately 10-15%-utilizes alternative lengthening of telomeres (ALT), a recombination-based mechanism. ALT-positive cancers are classically associated with genomic instability, anaphase bridges, chromosomal rearrangements, and resistance to DNA-damaging therapies. This process is closely associated with genetic instability, which contributes to chromosomal rearrangements and tumor evolution. Consequently, ALT has traditionally been considered an adverse prognostic marker in aggressive malignancies such as osteosarcoma, pancreatic neuroendocrine tumors, and high-grade sarcomas. Paradoxically, recent evidence demonstrates that ALT positivity correlates with improved survival in glioblastoma (GBM) and chondrosarcoma, two tumor types that have historically been regarded as immune-cold and therapeutically intractable. This favorable outcome likely reflects a convergence of factors, including replication stress and DNA damage that impose a fitness cost in slow-growing or metabolically constrained tumors. Loss of ATRX/DAXX, while enabling ALT, further amplifies chromatin fragility, and ALT-mediated instability may paradoxically enhance immunogenicity within immune-quiescent microenvironments. Moreover, ALT-positive cells exhibit unique therapeutic vulnerabilities, particularly to ATR and PARP inhibitors. Together, these observations support a context-dependent model in which ALT functions as a double-edged sword, acting as a driver of malignant aggressiveness in rapidly proliferating cancers while serving as a relative liability in slower-growing, immune-cold tumors. Understanding this duality not only refines prognostic stratification but also opens opportunities for precision oncology. By integrating ALT-specific biomarkers into clinical workflows and exploiting ALT-related DNA repair dependencies, clinicians may transform a once uniformly negative prognostic factor into an actionable therapeutic target.