ALT: A Multi-Faceted Phenomenon

Abstract

One of the hallmarks of cancer cells is their indefinite replicative potential, made possible by the activation of a telomere maintenance mechanism (TMM). The majority of cancers reactivate the reverse transcriptase, telomerase, to maintain their telomere length but a minority (10% to 15%) utilize an alternative lengthening of telomeres (ALT) pathway. Here, we review the phenotypes and molecular markers specific to ALT, and investigate the significance of telomere mutations and sequence variation in ALT cell lines. We also look at the recent advancements in understanding the different mechanisms behind ALT telomere elongation and finally, the progress made in identifying potential ALT-targeted therapies, including those already in use for the treatment of both hematological and solid tumors.

Keywords: alternative lengthening of telomeres; break-induced telomere synthesis; cancer; telomere-variant-repeat; therapy.

Figure 1

Mutation processes at telomeres. Examples of the interspersion patterns of the canonical (TTAGGG) and telomere-variant repeats (TVRs) commonly found at the proximal (centromeric) ends of human telomeres. (a) The diagram represents three telomeres on different chromosomes (Chr. 1 to 3) each with different interspersion patterns in the two alleles. While many telomeres comprise a region of degenerate repeats at the proximal end, some telomeres lack sequence variation in this region, as shown in Chr. 3 allele 1. (b) During S phase, the replication fork is prone to pausing or stalling as it passes through telomeric DNA. This is due to secondary structures, such as t-loops (not shown) that must be unwound. In addition, a variety of G-quadruplex structures can form on the G-rich strand. It is not known if the C-strand can also adopt secondary structures that impede replication. These obstacles contribute to high somatic and germline mutation rates that usually result in gains or losses of repeats. (c) Telomere lengthening by the break-induced-replication (BIR) mechanism that underlies the alternative lengthening of telomeres (ALT) moves TVRs between telomeres and can result in the distribution of the variant repeats along the full length of the telomeric DNA. The NR2F2 and NR2C2 orphan nuclear receptors can bind to (TCAGGG)_n containing telomeres in ALT positive cells. Similarly, ZBTB10 has recently been shown to bind to (TTGGGG)_n repeats in ALT positive cells. The binding of these proteins at telomeres may contribute to an altered DNA damage response. (d) The replication-driven high mutation rate at telomeres creates mosaicism within normal tissues, cancers, and the germline such that some cells carry mutated telomere interspersion patterns. In addition, the replication-dependent erosion of telomere length results in short telomeres that trigger a DNA damage response (DDR). Telomere erosion may lead to exposure of sequence-variant repeats at the end of telomeric DNA. It is not known whether this triggers specific proteins to bind to the short telomeres, for example NR2F2, NR2C2, ZBTB10, or other zinc-finger containing proteins that may modify the DDR.

Figure 2

Fast and slow routes to break-induced telomere synthesis. Following double strand break response and end resection at ALT telomeres, two different pathways can be invoked, both leading to break-induced telomere synthesis. One involves the recruitment of RAD51 and HOP2-MND1 to the resected end, followed by RAD51-mediated homology search. This process, however, is believed to be slower than the second pathway, which involves the faster loading of the replication factor C (RFC)-mediated proliferating cell nuclear antigen PCNA, acting as an early sensor of telomere damage. PCNA then recruits Pol δ, with its subunit POLD3 crucial for telomere elongation.