Telomerase-Independent Stabilization of Short Telomeres in Trypanosoma brucei

Abstract

In cancer cells and germ cells, shortening of chromosome ends is prevented by telomerase. Telomerase-deficient cells have a replicative life span, after which they enter senescence. Senescent cells can give rise to survivors that maintain chromosome ends through recombination-based amplification of telomeric or subtelomeric repeats. We found that in Trypanosoma brucei, critically short telomeres are stable in the absence of telomerase. Telomere stabilization ensured genomic integrity and could have implications for telomere maintenance in human telomerase-deficient cells. Cloning and sequencing revealed 7 to 27 TTAGGG repeats on stabilized telomeres and no changes in the subtelomeric region. Clones with short telomeres were used to study telomere elongation dynamics, which differed dramatically at transcriptionally active and silent telomeres, after restoration of telomerase. We propose that transcription makes the termini of short telomeres accessible for rapid elongation by telomerase and that telomere elongation in T. brucei is not regulated by a protein-counting mechanism. Many minichromosomes were lost after long-term culture in the absence of telomerase, which may reflect their different mitotic segregation properties.

We are grateful to Titia de Lange and Joachim Lingner for helpful discussions and protocols, to Jenny Li for technical assistance with growing trypanosomes in animals, to Sean Rooney for human ALT DNA, to Luisa Figueiredo and Bibo Li for inspiring discussions and probes, and to Veena Mandava and Christian Janzen for discussion and critical reading of the manuscript.

This work was supported by NIH grant AI 50614.