PROJECT #3

Telomeres Contain Multiple Topo II Sites.

Eukaryotic topoisomerase II:  A Telomere binding protein.  Biochim. Biophys. Acta. 1395:110-120 (1998)

ABSTRACT

In this work, the association of endogenous topoisomerase II with chromosomal telomeric DNA has been analyzed in HeLa cells.  We report that DNA topoisomerase II is catalytically active in cleaving the telomere DNA repeat both in vitro and in vivo.  The major topoisomerase II cleavage site is 5' T T A G G*G 3' (cleavage marked by the asterisk) and since telomere DNA is a tandem array of the above sequence, topoisomerase cleavage sites exist every six base pairs. Telomeres represent moderate affinity topoisomerase II cleavage substrates compared to known high affinty sites like alternating purine/pyrimidine elements.  Furthermore, detection of topoisomerase cleavages in vitro and in vivo required the topoisomerase II poison etoposide.  Another poison, m-AMSA, did not support cleavage activity at the telomere repeat.  These results are consistent with the presence of eukaryotic topoisomerase II in nuclear matrix/chromosome scaffold structures to which telomere sequences bind in vivo.  The findings also raise the interesting possibility of directing novel topoisomerase II inhibitors against telomerase positive malignant cells which possess extended telomeric DNA repeats.  In addition, recent data suggest that telomeres are actually looped back structures in a hairpin arrangement called a "T-Loop" (Griffith et al.  Cell 97, 503-514 1999).  There may a need for topo II during decatenation of such structures.

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