Taming the tiger by the tail: modulation of DNA damage responses by telomeres

Abstract

Telomeres are by definition stable and inert chromosome ends, whereas internal chromosome breaks are potent stimulators of the DNA damage response (DDR). Telomeres do not, as might be expected, exclude DDR proteins from chromosome ends but instead engage with many DDR proteins. However, the most powerful DDRs, those that might induce chromosome fusion or cell-cycle arrest, are inhibited at telomeres. In budding yeast, many DDR proteins that accumulate most rapidly at double strand breaks (DSBs), have important functions in physiological telomere maintenance, whereas DDR proteins that arrive later tend to have less important functions. Considerable diversity in telomere structure has evolved in different organisms and, perhaps reflecting this diversity, different DDR proteins seem to have distinct roles in telomere physiology in different organisms. Drawing principally on studies in simple model organisms such as budding yeast, in which many fundamental aspects of the DDR and telomere biology have been established; current views on how telomeres harness aspects of DDR pathways to maintain telomere stability and permit cell-cycle division are discussed.

Figure 1

DDR proteins at budding yeast telomeres and DSBs. (A, C, E, G) show the recruitment of DNA damage response proteins to a DSB undergoing HR. (B, D, F, H) show the role of DDR and telomere-capping proteins in forming a capped telomere. (A) A blunt ended DSB. (B) A leading strand telomere after DNA replication. (C) Rapid recruitment of Mre11, Rad50 and Xrs2, Tel1 and Yku70/Yku80 to DSBs. (D) Rapid recruitment of Mre11, Rad50, Xrs2, Tel1 and Yku70/Yku80 to a telomere. (E) Nuclease- and helicase-dependent production of ssDNA generates a substrate for RPA binding. (F) Telomeric (G rich) ssDNA, which is partially Mre11 dependent, provides a substrate for Cdc13, Stn1 and Ten1 binding. (G) RPA-coated ssDNA helps recruite not only HR proteins such as Rad51/Rad52 (not shown) but also checkpoint proteins Rad24, the Rad17, Mec3, Ddc1 heterotrimeric ring. Mec1 and, its partner, Ddc2 bind RPA and help contribute to kinase-dependent signal transduction cascades that can lead to not only cell-cycle arrest, but also a capped telomere (dashed line between G and H). Rad9, essential for signalling cell-cycle arrest at DSBs and cdc13-1 uncapped telomeres, is recruited in part through the interaction with the methylated histone H3 lysine 79. (H) Telomerase is recruited to telomeres, in part, through interactions with Yku80, and with Cdc13.

Figure 2

Telomeres and DSBs. A cartoon showing a DSB, in the centre, and two telomeres. The rays emanating from each type of end illustrate the potency of each type of end for inducing DNA damage responses, such as DNA repair and cell-cycle arrest. Telomeres can be estimated to be at least a 1000-fold less potent inducers of cell-cycle arrest compared with DSBs.

Figure 3

Diverse chromosome end structures. (A) A telomere chromosome end as found in yeast and mammalian cells. The G-rich 3′ strand is maintained by telomerase activity, which overcomes the end replication problem. The complementary C-rich strand is maintained by conventional DNA replication machinery. (B) A DSB in the process of HR, after processing to generate a 3′ overhang, an intermediate in the HR repair process. (C) A t loop, found at the end of mammalian telomeres when the 3′ G-rich overhang at telomeres invades double stranded DNA. (D) C. elegans telomeres, maintained by telomerase, contain both 3′ and 5′ ssDNA overhangs. (E) Drosophila telomeres containing arrays of transposons at chromosome ends. The diagonal lines represent the junctions between individual repeats.

Figure 4

Telomere structure negates the harmful effects of the DDR. Black lines indicate telomeric DNA and dashed green arrows the direction of telomeric repeats. (A) Telomeric DNA sequences are repetitive and orientated in the same direction, centromere to telomere, and therefore should HR take place the effects are neutral. (B) The 3′ overhang at telomeres will inhibit NHEJ pathway of DNA repair. (C) If the 3′ overhang loops back to invade double stranded DNA it will form a t loop, when the 3′ end points towards the chromosome end (See Figure 2) rather than towards the centromere as shown. An invading 3′ end facing the centromere would be dangerous because of its potential to initiate break-induced replication events. (D) Should telomeres fuse, they will form palindromes, which have the potential to from Holliday junctions and be resolved by mechanisms that cleave Holliday junctions.