Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis
ReviewTelomere dysfunction and chromosome instability
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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