Abstract
Telomeres, the ends of linear chromosomes, have a critical role in protection against chromosome end-to-end fusion. Telomeres shorten in every cell division due to the end replication problem. Telomerase is a reverse transcriptase that adds telomeric DNA repeats to the ultimate chromosome end. In vitro models of long-term fibroblast cultures have identified two sequential mortality stages, senescence (M1) and crisis (M2). Senescence can be bypassed by loss of p53 or Rb function, whereas escape from crisis can only be achieved by activating a telomere maintenance mechanism, mostly telomerase reactivation. Cells that bypass senescence (M1) did not reactivate telomerase, resulting in further telomere shortening to a critical telomere length upon reaching crisis (M2). In these models, critical telomere shortening induces extensive chromosome instability, most likely via chromosome end-to-end fusions. Dicentric chromosomes lead to anaphase breakage-fusion-bridges resulting in multiple chromosomal aberrations. To investigate whether similar mechanisms may be involved in the development of genetic instability in human cancer, we studied telomere length and expression of critical telomeric proteins (TRF2 and POT1) in acute myeloid leukemia (AML) patients. AML is a good model for these studies since distinct subgroups of AML are characterized by either exchanges along chromosome arms (translocation or inversion), or by a complex karyotype with multiple chromosome aberrations. Groups were age-matched. Telomere length was studied in metaphase arrested leukemic cells using quantitative fluorescence in situ hybridization (Q-FISH) using a telomere-specific probe. Subsequently, metaphase spreads were hybridized with a leukemia-specific probe to confirm leukemic origin of each metaphase. Telomeres were significantly shorter in AML samples with multiple chromosomal abnormalities in comparison to AML samples with a reciprocal translocation/inversion or no abnormalities (mean±SEM=16±1.7 AFU, n=12 versus 29±4.3 AFU, n=18; p=0.015). Interestingly, telomerase activity level is significantly higher in AML samples with multiple chromosomal abnormalities, compared to AML samples with a reciprocal translocation or inversion (mean±SEM=330±95, n=11 versus 70±21, n=13; p=0.02). Expression levels of telomeric proteins TRF2 and POT1 were similar in these AML groups. Our observations suggest that, consistent with previous in vitro models in fibroblasts, critical telomere shortening may have a role in the development of genetic instability in human AML. Critically short telomeres in association with high levels of telomerase activity suggest that AML cells with multiple chromosomal abnormalities have bypassed crisis (M2). The longer telomeres and low levels of telomerase activity in AML cells with a reciprocal translocation or inversion suggest that they originate from an earlier stage, preceding crisis. Consequently, telomere length modulation may have a role in cancer prevention.
The temporal dynamics of chromosome instability in ovarian cancer cell lines and primary patient samples