Strand break-induced replication fork collapse leads to C-circles, C-overhangs and telomeric recombination.

Tianpeng Zhang,Yong Zhao,Haiying Liu,Zepeng Zhang,Xiaocui Li,Gong Shengzhao,Jin-Qiu Zhou

doi:10.1371/journal.pgen.1007925

Abstract

Telomerase-independent ALT (alternative lengthening of telomeres) cells are characterized by high frequency of telomeric homologous recombination (HR), C-rich extrachromosomal circles (C-circles) and C-rich terminal 5' overhangs (C-overhangs). However, underlying mechanism is poorly understood. Here, we show that both C-circle and C-overhang form when replication fork collapse is induced by strand break at telomeres. We find that endogenous DNA break predominantly occur on C-rich strand of telomeres in ALT cells, resulting in high frequency of replication fork collapse. While collapsed forks could be rescued by replication fork regression leading to telomeric homologous recombination, those unresolved are converted to C-circles and C-overhang at lagging and leading synthesized strand, respectively. Meanwhile, multiple hallmarks of ALT are provoked, suggesting that strand break-induced replication stress underlies ALT. These findings provide a molecular basis underlying telomeric HR and biogenesis of C-circle and C-overhang, thus implicating the specific mechanism to resolve strand break-induced replication defect at telomeres in ALT cells.

Highlights

Linear chromosome ends are capped by telomeres, which are composed of TTAGGG/ CCCTAA tandem DNA repeats and a protein complex called shelterin [1,2,3]
We find that endogenous C-strand breaks predominantly exist in telomeres of alternative lengthening of telomeres (ALT) cells, which induce high frequency of replication fork collapse
These findings suggest that the formation of C-circle and C-overhang represents a unique manner for ALT cells to prevent

Summary

Introduction

Linear chromosome ends are capped by telomeres, which are composed of TTAGGG/ CCCTAA tandem DNA repeats and a protein complex called shelterin [1,2,3]. It has been proposed that telomeric DNA damage, double-stranded breaks (DSBs), promotes C-circles generation in ALT cells [19, 20], and that defects in telomere replication related proteins, such as SMARCAL1 (SWI/SNF-related, matrix associated, actin-dependent, regulator of chromatin subfamily A-like 1) or the CST (CTC1/STN1/TEN1), changes the level of C-circles in ALT cells [21,22,23]. These results imply a potential connection between C-circles formation and DNA damage repair and/or replication defect at telomeres. The question regarding whether and how the formation of C-circle and C-overhang is coordinated and their relationship with high frequency of telomeric HR and ALT remains to be elucidated

Methods

Results

Discussion

Conclusion