Alternative lengthening of telomeres in mammalian cells

Abstract

Some immortalized mammalian cell lines and tumors maintain or increase the overall length of their telomeres in the absence of telomerase activity by one or more mechanisms referred to as alternative lengthening of telomeres (ALT). Characteristics of human ALT cells include great heterogeneity of telomere size (ranging from undetectable to abnormally long) within individual cells, and ALT-associated PML bodies (APBs) that contain extrachromosomal telomeric DNA, telomere-speci®c binding proteins, and proteins involved in DNA recombination and replication. Activation of ALT during immortalization involves recessive mutations in genes that are as yet unidenti®ed. Repressors of ALT activity are present in normal cells and some telomerase-positive cells. Telomere length dynamics in ALT cells suggest a recombinational mechanism. Inter-telomeric copying occurs, consistent with a mechanism in which singlestranded DNA at one telomere terminus invades another telomere and uses it as a copy template resulting in net increase in telomeric sequence. It is possible that t-loops, linear and/or circular extrachromosomal telomeric DNA, and the proteins found in APBs, may be involved in the mechanism. ALT and telomerase activity can co-exist within cultured cells, and within tumors. The existence of ALT adds some complexity to proposed uses of telomererelated parameters in cancer diagnosis and prognosis, and poses challenges for the design of anticancer therapeutics designed to inhibit telomere maintenance.

High level of telomerase RNA gene expression is associated with chromatin modification, the ALT phenotype and poor prognosis in liposarcoma

“…However, telomere length maintenance can also be achieved in some tumour types in the absence of telomerase activity through an alternative recombination-based mechanism termed ALT (alternative lengthening of telomeres) (Bryan et al, 1995(Bryan et al, , 1997. Alternative lengthening of telomeres is characterised phenotypically by heterogeneous telomeres, ranging from less than 3 kb to more than 50 kb in length (Henson et al, 2002), and the presence of ALT-associated promyelocytic leukaemia (PML) bodies (APBs), which contain telomeric DNA and the telomere binding proteins, TRF1 and TRF2, in addition to several other proteins associated with DNA recombination and repair, including RAD51 and RAD52 (Yeager et al, 1999;Henson et al, 2002).…”

“…The ALT pathway is detected in some tumours of epithelial origin, including carcinomas of the breast, lung and kidney; however, it is found at higher frequency in tumours of mesenchymal origin, such as liposarcomas, osteosarcomas and glioblastomas (Henson et al, 2002). This is thought to be due to tighter repression of telomerase expression in mesenchymal tissues, which have a slower cell turnover and less telomere shortening than many epithelial tissues (Henson et al, 2002).…”

“…This is thought to be due to tighter repression of telomerase expression in mesenchymal tissues, which have a slower cell turnover and less telomere shortening than many epithelial tissues (Henson et al, 2002). Several studies have highlighted an association of the ALT phenotype with sarcomas that have complex karyotypes with many chromosomal losses and gains and evidence of anaphase bridge formation, more consistent with chromosomal instability, while those that have generally more simple karyotypes, including fusion genes created from chromosomal translocations, are associated with telomerase activity (Montgomery et al, 2004;Ulaner et al, 2004).…”

Analysis of alternative lengthening of telomere markers in BRCA1 defective cells

“…In addition to the CC assay another indicator of ALT is detection of APBs (ALT‐associated PML Bodies) (Yeager et al, 1999; Henson et al, 2002; Neumann and Reddel, 2006). We observed a significant difference between the 408 and E14 cell lines in the levels of APBs ( P  = 0.0002) (Fig.…”

Immortalized cells as experimental models to study cancer

“…These cells acquire the capacity to maintain stable long telomeres in a telomeraseindependent manner, termed 'alternative lengthening of telomeres' or ALT (Bryan et al 1997). Some human tumors also show evidence of ALT, suggesting that ALT occurs in vivo (reviewed by Henson et al 2002). However, the mechanism(s) responsible for the ALT phenotype in human cells are not fully understood.…”