Mechanisms of telomere loss and their consequences for chromosome instability

Abstract

The ends of chromosomes in mammals, called telomeres, are composed of a 6-bp repeat sequence, TTAGGG, which is added on by the enzyme telomerase. In combination with a protein complex called shelterin, these telomeric repeat sequences form a cap that protects the ends of chromosomes. Due to insufficient telomerase expression, telomeres shorten gradually with each cell division in human somatic cells, which limits the number of times they can divide. The extensive cell division involved in cancer cell progression therefore requires that cancer cells must acquire the ability to maintain telomeres, either through expression of telomerase, or through an alternative mechanism involving recombination. It is commonly thought that the source of many chromosome rearrangements in cancer cells is a result of the extensive telomere shortening that occurs prior to the expression of telomerase. However, despite the expression of telomerase, tumor cells can continue to show chromosome instability due to telomere loss. Dysfunctional telomeres in cancer cells can result from oncogene-induced replication stress, which results in double-strand breaks (DSBs) at fragile sites, including telomeres. DSBs near telomeres are especially prone to chromosome rearrangements, because telomeric regions are deficient in DSB repair. The deficiency in DSB repair near telomeres is also an important mechanism for ionizing radiation-induced replicative senescence in normal human cells. In addition, DSBs near telomeres can result in chromosome instability in mouse embryonic stem cells, suggesting that telomere loss can contribute to heritable chromosome rearrangements. Consistent with this possibility, telomeric regions in humans are highly heterogeneous, and chromosome rearrangements near telomeres are commonly involved in human genetic disease. Understanding the mechanisms of telomere loss will therefore provide important insights into both human cancer and genetic disease.

Keywords: chromosome instability; double-strand break repair; gross chromosomal rearrangement; sister chromatid fusion; telomere.

FIGURE 1

Contribution of telomere loss to chromosome instability in cancer. Oncogene expression causes unregulated cell division, resulting in replication stress and excessive telomere shortening. The very short telomeres or DSBs near telomeres that are caused by replication stress result in cell senescence. Mutations in the p53 and p16 proteins that are required for cell cycle regulation can allow cells to continue to divide, leading to cell crisis as a result of dysfunctional telomeres and extensive chromosome fusion. The activation of telomerase expression or the ALT pathway in rare cells allows for continued cell division, although the surviving cells will contain chromosome rearrangements as a result of telomere loss during crisis. Cells expressing telomerase will continue to experience a low rate of telomere loss due to a combination of replication stress causing DSBs near telomeres and a deficiency in DSB repair in subtelomeric regions.

FIGURE 2

The types of events resulting from interstitial and telomeric I-SceI-induced DSBs. DSB repair at interstitial sites occurs primarily by C-NHEJ, although some DSBs are also repaired by HRR, involving error-free repair, and A-NHEJ, which is associated with large deletions and GCRs. Direct ligation of the ends of DSBs without the loss of the I-SceI site by C-NHEJ is the most common event at interstitial DSBs. Small deletions of a few base pairs that eliminate the I-SceI site are the next most common event, followed by HRR, large deletions, and GCRs. The deficiency in NHEJ near telomeres is proposed to be due to inhibition of C-NHEJ, while the efficiency of HRR is unchanged. As a result, most DSBs near telomeres are repaired by A-NHEJ. Consistent with a predominant role for A-NHEJ in repair of telomeric DSBs, large deletions are the most common event at I-SceI-induced DSBs near telomeres. Most of these large deletions would also result in GCRs, because they cause the loss of the telomere. GCRs that occur without large deletions are also greatly increased at DSBs near telomeres, while small deletions of a few base pairs occur at the same frequency as at interstitial DSBs.

FIGURE 3

Model for processing DSBs near telomeres. The presence of TRF2 at the telomere (red horizontal lines) promotes chromosome healing by inhibiting ATM, which is required to activate PIF1 and/or other redundant proteins that prevent chromosome healing by binding to TERT, the catalytic subunit of telomerase. TRF2 also promotes the inappropriate processing of DSBs near telomeres by Apollo, which is then followed by extensive resection by EXO1. This function of Apollo is normally involved in the generation of single-stranded 3′ overhangs required for telomere formation. However, unlike telomeres (red horizontal lines), in which resection is limited by POT1/TPP1 binding to the single-stranded telomeric DNA, resection of DSBs in subtelomeric regions (black horizontal lines) continues unabated due to the inability of POT1/TPP1 to bind to non-telomeric DNA. The inappropriate processing of the DSB results in large deletions and/or GCRs with other chromosomes (green horizontal lines) through a mechanism involving A-NHEJ.