Telomere exchange and asymmetric segregation of chromosomes can account for the unlimited proliferative potential of ALT cell populations
Introduction
Normal human somatic cells enter a senescent state after a finite number of doublings [1], [2] (Hayflick limit). Lacking telomerase activity, these cells lose ∼100–400 bp of telomeric DNA, composed of TTAGGG repeats, from chromosome ends in every cell division [3]. This loss has been attributed to the end replication problem and also to post-replicative processing to create the 3′ overhangs that are necessary to cap chromosome ends [4], [5]. The maximum proliferative potential of normal somatic cells (i.e., the number of cell divisions before senescence) has been related to the time of triggering of a double-strand break checkpoint response when one or more telomeres become shorter than the length necessary to create a chromosome protective structure [5]. The inability of the cell to repair the “damage” leads to its inability to divide further and the cell senesces. In contrast to normal cells, cancer cells must proliferate indefinitely, a necessary condition for tumorigenesis [6]. Inactivation of the p53 and Rb pathways allows proliferation beyond the Hayflick limit of cells with critically short telomeres, but at the expense of escalating chromosomal instability. A crisis stage is reached when massive instability prohibits the generation of sufficient viable cells to sustain proliferation. A small percentage of cells avoid crisis by re-establishing chromosome end protection. Often these cells have activated telomerase allowing them to maintain adequate telomere length to form protective end structures [7]. However, some telomerase-negative cells escape crisis by an as yet poorly understood mechanism referred to as “alternative lengthening of telomeres” (ALT) [8], [9]. This phenotype is mostly found in sarcomas. Occasionally tumours are found to possess either both mechanisms of telomere length maintenance [10] or none [11]. The ALT phenotype is characterized by a lack of detectable telomerase activity and heterogeneous telomere lengths [12]. Increasing evidence suggests that ALT is recombination based. For example, ALT cells contain ALT-associated promyelocytic leukaemia (PML) bodies (APBs), and these APBs contain telomeric DNA, telomere-associated proteins (e.g., the telomere-binding proteins TRF1 and TRF2), and recombination-associated proteins (e.g., RAD50, RAD51, RAD52, MRE11, NBS1, BLM and WRN) [13]. Telomere exchange between sister chromatids has been suggested as a possible mechanism of ALT. It was recently found [14] that in non-ALT cells the rates of sister chromatid exchange in telomeres (T-SCE) are 10-fold higher compared to other DNA sequences. T-SCE rates are higher in ALT cells compared to normal cells [15], [13], [16] and has been suggested they play an important role in determining the proliferative potential of telomerase-negative cells [14], [15], [13], [16], [17].
In the absence of T-SCE and other recombination events, the proliferative potential of a cell is determined by the telomere lengths of individual chromosomes and the amount of telomere loss at each cell division (basal telomere loss). Small variations can occur as a result of the fluctuations in the size of the basal telomere loss (BTL). In contrast when T-SCEs are present, and if sister chromatids exchange unequal amounts of DNA [14], proliferative potential becomes a dynamic quantity dependent on the T-SCE frequency and the exchange size asymmetry. Because T-SCE are thought to result from DNA damage they are believed to be random events.
Section snippets
Results
To quantify the role played by elevated T-SCE rates in determining the proliferative potential of somatic cells we developed a mathematical model that incorporates the processes of telomere basal loss as well as T-SCE. The telomere basal loss decreases all telomeres at approximately the same rate, and telomere sister chromatid exchange leads to heterogeneity in the telomere sizes among cells with a common ancestor. In our mathematical model we also included the possibility of telomere
Conclusions
In this paper we investigated the influence of T-SCE on cells’ proliferative potential. To increase the proliferative potential of a real cell, all telomeres must be elongated. Our results show that high T-SCE rates alone are insufficient to overcome telomere-driven colony senescence. Instead they support the hypothesis that transition to immortality requires co-segregation of longer telomeres together with a T-SCE rate in excess of a critical value. These results help put ALT into a
Methods
The simulations in this paper were performed on an agent-based model using the TElomere DYnamics SIMulator (TEDYSIM) code written in Java. Each cell is implemented as Cell class. The class has a two dimensional array representing the telomeres on different chromosomes and different chromosome arms, and a state variable labelling the cell as dividing or senescent. In each simulation we started with 64 initial cells with the same telomere size in each cell on each chromosome end. The basal loss
Conflict of interest
All authors have no conflict of interest.
Acknowledgement
We would like to thank Susan Bailey and Hank Bass for useful discussions. E.H. Goodwin was partly supported by a grant from the U.S. Department of Energy W-7405-ENG-36.
References (28)
- et al.
The serial cultivation of human diploid cell strains
Exp. Cell Res.
(1961) The limited in vitro lifetime of human diploid cell strains
Exp. Cell Res.
(1965)- et al.
Accumulation of short telomeres in human fibroblasts prior to replicative senescence
Exp. Cell Res.
(2000) The biology of replicative senescence
Eur. J. Cancer
(1997)Cellular senescence as a tumor-suppressor mechanism
Trends Cell Biol.
(2001)- et al.
A survey of telomerase activity in human cancer
Eur. J. Cancer
(1997) - et al.
Telomere dynamics and telomerase activity in in vitro immortalised human cells
Eur. J. Cancer
(1997) - et al.
Analysis of telomeric single-strand overhang length in human endometrial cancers
FEBS Lett.
(2005) Principle of marginotomy in template synthesis of polynucleotides
Dokl Akad Nauk SSSR
(1971)- et al.
Evidence for an alternative mechanism for maintaining telomere length in human tumors and tumor-derived cell lines
Nat. Med.
(1997)