Review
Telomere dysfunction and chromosome instability

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Abstract

The ends of chromosomes are composed of a short repeat sequence and associated proteins that together form a cap, called a telomere, that keeps the ends from appearing as double-strand breaks (DSBs) and prevents chromosome fusion. The loss of telomeric repeat sequences or deficiencies in telomeric proteins can result in chromosome fusion and lead to chromosome instability. The similarity between chromosome rearrangements resulting from telomere loss and those found in cancer cells implicates telomere loss as an important mechanism for the chromosome instability contributing to human cancer. Telomere loss in cancer cells can occur through gradual shortening due to insufficient telomerase, the protein that maintains telomeres. However, cancer cells often have a high rate of spontaneous telomere loss despite the expression of telomerase, which has been proposed to result from a combination of oncogene-mediated replication stress and a deficiency in DSB repair in telomeric regions. Chromosome fusion in mammalian cells primarily involves nonhomologous end joining (NHEJ), which is the major form of DSB repair. Chromosome fusion initiates chromosome instability involving breakage–fusion–bridge (B/F/B) cycles, in which dicentric chromosomes form bridges and break as the cell attempts to divide, repeating the process in subsequent cell cycles. Fusion between sister chromatids results in large inverted repeats on the end of the chromosome, which amplify further following additional B/F/B cycles. B/F/B cycles continue until the chromosome acquires a new telomere, most often by translocation of the end of another chromosome. The instability is not confined to a chromosome that loses its telomere, because the instability is transferred to the chromosome donating a translocation. Moreover, the amplified regions are unstable and form extrachromosomal DNA that can reintegrate at new locations. Knowledge concerning the factors promoting telomere loss and its consequences is therefore important for understanding chromosome instability in human cancer.

Section snippets

Telomeres and the consequences of telomere loss

Seminal work by Hermann Muller in fruit flies [1] and Barbara McClintock in maize [2] demonstrated long ago that the ends of chromosomes did not fuse with chromosome breaks, and therefore must somehow be “capped.” These chromosome ends, which Muller termed telomeres, are now know to be composed of a short DNA repeat sequence [3] and a large number of proteins, named the shelterin complex [4], which together form the protective cap. Telomeres are maintained in germ line cells and embryonic stem

Mechanisms of chromosome fusion following loss of telomere function

Chromosome fusions can occur through a variety of different mechanisms depending on the cell type and the mechanism of loss of telomere function. As will be discussed in detail below, most chromosome fusions in mammalian cells occur through double-strand break (DSB) repair involving nonhomologous end joining (NHEJ). This is not surprising, because NHEJ is the predominant form of repair of unprotected DNA ends in mammalian cells. There are at least two forms of NHEJ, classical (C-NHEJ), and

Chromosome fusion due to a deficiency in telomeric proteins

Studies of cells deficient in shelterin proteins that are essential for the caps on the ends of chromosomes have been an important source of information on the mechanisms of chromosome fusion. Cells deficient in shelterin proteins undergo chromosome fusion despite the presence of telomeric repeat sequences due to a failure to protect the end of the chromosome. A deficiency in TRF2, which binds to double-stranded telomeric repeat sequences, results in the appearance of DSB repair foci at

Monitoring telomere loss and chromosome fusion by PCR

Telomere shortening and its effect on chromosome fusion have been analyzed in human fibroblasts by single telomere length analysis (STELA). STELA uses polymerase chain reaction (PCR) to amplify individual telomeres utilizing one primer that is specific for the end of a chromosome, and a second primer that binds to the single-stranded 3′ overhang at the end of the telomere [85]. The results with STELA demonstrate that individual telomeres in human fibroblasts shorten gradually during cell

Chromosome fusion due to rapid deletion events

Rapid deletion events similar to those detected by Fusion PCR were first reported in mammalian cells in an immortal human cell line that maintains its telomeres through a telomerase-independent mechanism [8]. These rapid deletion events were proposed to result from recombination, consistent with the demonstration that cell lines utilizing this “ALT” pathway maintain telomeres through recombination [9]. The rapid deletion events in ALT cells can result in complete loss of the telomeric repeat

The role of DSBs in telomere loss

The role of DSBs in telomere loss and chromosome fusion in mouse ES cells and the EJ-30 human tumor cell line has been investigated using DSBs introduced within subtelomeric regions with the I-SceI endonuclease. I-SceI endonuclease recognizes an 18 bp sequence that is not found in the mammalian genome, and has been used extensively to study DSB repair mammalian cells [93], [94], [95], [96]. As with the analysis of spontaneous telomere loss, the consequences of I-SceI-induced DSBs near telomeres

The sensitivity of subtelomeric regions to DSBs

The prevalence of large deletions, sister chromatid fusions, and the addition of telomeric repeat sequences at the site of the DSBs near telomeres in mouse ES cells suggests that DSBs in subtelomeric regions are processed differently from DSBs at interstitial sites. None of these events are common to DSBs generated by I-SceI at interstitial sites [93], [94], [95], [96]. Consistent with this conclusion, a study in S. cerevisiae demonstrated that I-SceI-induced DSBs in subtelomeric regions were

Chromosome healing

An important observation arising from our studies on the types of events resulting from spontaneous telomere loss or DSBs occurring near telomeres is that mammalian cells are capable of adding telomeric repeat sequences on to the ends of broken chromosomes, a process called chromosome healing. Chromosome healing has been extensively studied in yeast, where it has been demonstrated to involve telomerase [102]. Chromosome healing is known to occur in germ line cells in humans, since it is

Mechanism of spontaneous telomere loss

The types and proportions of DNA rearrangements that result from DSBs near telomeres in the EJ-30 human tumor cell line are very similar to what is observed as a result of spontaneous telomere loss, i.e., 95% of the events result in the loss of the plasmid, while approximately 4% result in GCRs, and 1% in chromosome healing [12], [64]. This similarity in spontaneous and I-SceI-induced events suggests that DSBs play a role in spontaneous telomere loss. However, while sister chromatid fusion was

Future directions

The above studies leave little doubt that telomere loss, either during crisis or after the expression of telomerase, contributes to the chromosome instability leading to human cancer. However, much remains to be determined regarding the mechanisms involved in telomere loss and the chromosome instability that results. A primary focus of future studies will be to exploit this knowledge for anti-cancer therapy. Preventing chromosome instability resulting from telomere loss could serve to prevent

Conflict of interest statement

No conflicts of interest to disclose.

Acknowledgement

This work was supported by National Institutes of Health grant CA120205.

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