Human cancer cells utilize mitotic DNA synthesis to resist replication stress at telomeres regardless of their telomere maintenance mechanism.

Özgün Özer,Ying Liu,Rahul Bhowmick,Ian D Hickson

doi:10.18632/oncotarget.24745

Abstract

Telomeres resemble common fragile sites (CFSs) in that they are difficult-to-replicate and exhibit fragility in mitosis in response to DNA replication stress. At CFSs, this fragility is associated with a delay in the completion of DNA replication until early mitosis, whereupon cells are proposed to switch to a RAD52-dependent form of break-induced replication. Here, we show that this mitotic DNA synthesis (MiDAS) is also a feature of human telomeres. Telomeric MiDAS is not restricted to those telomeres displaying overt fragility, and is a feature of a wide range of cell lines irrespective of whether their telomeres are maintained by telomerase or by the alternative lengthening of telomeres (ALT) mechanism. MiDAS at telomeres requires RAD52, and is mechanistically similar to CFS-associated MiDAS, with the notable exception that telomeric MiDAS does not require the MUS81-EME1 endonuclease. We propose a model whereby replication stress initiates a RAD52-dependent form of break-induced replication that bypasses a requirement for MUS81-EME1 to complete DNA synthesis in mitosis.

Highlights

Telomeres are the specialized DNA structures that cap the ends of linear chromosomes in eukaryotes
It has been shown previously that cells exposed to APH-induced DNA replication stress conduct BIRlike DNA repair synthesis (MiDAS) at common fragile sites (CFSs) loci in early mitosis [14,15,16]
To ensure that we omitted examples of mitotic DNA synthesis (MiDAS) occurring at CFSs fortuitously located close to a telomere, we only scored those EdU foci that colocalized precisely with a telomeric fluorescence in situ hybridization (FISH) signal or that lay at the very tip of the chromosome distal to the FISH signal (Figure 1A)

Summary

Introduction

Telomeres are the specialized DNA structures that cap the ends of linear chromosomes in eukaryotes. Each telomere comprises a long dsDNA tract of TTAGGG repeat units that terminates on the G-rich 3′-strand with a ssDNA overhang of variable length [1]. This 3′-overhang is proposed to loop back and invade into the doublestranded telomeric repeat DNA to form a so-called T-loop, which resembles the D-loop generated as an intermediate during homologous recombination [2, 3]. Shelterin proteins serve to maintain the T-loop at telomeres and to ensure that chromosome ends are not mistaken for pathological dsDNA breaks, which suppresses excessive recombination between telomeres [1]

Methods

Results

Conclusion