Alternative mechanisms of telomere lengthening: Permissive mutations, DNA repair proteins and tumorigenic progression

Abstract

Telomeres protect chromosome termini to maintain genomic stability and regulate cellular lifespan. Maintenance of telomere length is required for neoplastic cells after the acquisition of mutations that deregulate cell cycle control and increase cellular proliferation, and can occur through expression of the enzyme telomerase or in a telomerase-independent manner termed alternative lengthening of telomeres (ALT). The precise mechanisms that govern the activation of ALT or telomerase in tumor cells are unknown, although cellular origin may favor one or the other mechanisms. ALT pathways are incompletely understood to date; however, recent publications have increasingly broadened our understanding of how ALT is activated, how it proceeds, and how it influences tumor growth. Specific mutational events influence ALT activation, as mutations in genes that suppress recombination and/or alterations in the regulation of telomerase expression are associated with ALT. Once engaged, ALT uses DNA repair proteins to maintain telomeres in the absence of telomerase; experiments that manipulate the expression of specific proteins in cells using ALT are illuminating some of its mechanisms. Furthermore, ALT may influence tumor growth, as experimental and clinical data suggest that telomerase expression may favor tumor progression. This review summarizes recent findings in mammalian cells and models, as well as clinical data, that identify the genetic mutations permissive to ALT, the DNA repair proteins involved in ALT mechanisms and the importance of telomere maintenance mechanisms for tumor progression. A comprehensive understanding of the mechanisms that permit tumor cell immortalization will be important for identifying novel therapeutic targets in cancer.

The Role of Double-Strand Break Repair Pathways at Functional and Dysfunctional Telomeres

“…4D). The idea that the telomere maintenance in these cells is dependent on HR is consistent with the finding that diminished activity of BRCA2, MUS81, FEN1, FANCD2, FANCA, SMC5/6-dependent sumoylation, RAD50, and other recombination factors affects telomere maintenance in ALT cells (reviewed in Gocha et al 2013).…”

DNA helicases involved in DNA repair and their roles in cancer

“…5). In addition, RecQ helicases have a role in the alternative lengthening of telomeres (ALT) pathway, which affects proliferation in 5–10% of tumours 195 . Characterization of small molecules that modulate helicase function in the ALT pathway may be valuable for devising strategies to treat ALT-dependent tumours (for example, osteosarcomas) that are resistant to conventional anticancer therapies.…”

Distribution of TERT promoter mutations in pediatric and adult tumors of the nervous system

“…ALT has been demonstrated in gliomas and other tumors [7,8]. However, TERT promoter mutations appear to be the single most frequently occurring mechanism to affect telomere length.…”