Rescue of a telomere length defect of Nijmegen breakage syndrome cells requires NBS and telomerase catalytic subunit

Abstract

Nijmegen breakage syndrome (NBS) is a rare human disease displaying chromosome instability, radiosensitivity, cancer predisposition, immunodeficiency, and other defects [1, 2]. NBS is complexed with MRE11 and RAD50 in a DNA repair complex [3-5] and is localized to telomere ends in association with TRF proteins [6, 7]. We show that blood cells from NBS patients have shortened telomere DNA ends. Likewise, cultured NBS fibroblasts that exhibit a premature growth cessation were observed with correspondingly shortened telomeres. Introduction of the catalytic subunit of telomerase, TERT, was alone sufficient to increase the proliferative capacity of NBS fibroblasts. However, NBS, but not TERT, restores the capacity of NBS cells to survive gamma irradiation damage. Strikingly, NBS promotes telomere elongation in conjunction with TERT in NBS fibroblasts. These results suggest that NBS is a required accessory protein for telomere extension. Since NBS patients have shortened telomeres, these defects may contribute to the chromosome instability and disease associated with NBS patients.

Nijmegen breakage syndrome gene, NBS1, and molecular links to factors for genome stability

“…Although the clinical observation of premature aging is not observed in NBS, blood cells from NBS patients similarly have the reduced telomere length (Ranganathan et al, 2001) and primary cells from skin fibroblast of these patients show accelerated telomere shortening during in vitro growth. The reduction of telomere length in NBS cells, approximately 220 -260 bp/replication, is comparable with that of AT primary cells (unpublished data).…”

“…This interaction is detected only at early S phase when telomere elongation occurs, whereas the M/R complex binds to TRF2 throughout the cell cycle, suggesting that NBS1 functions in telomere replication after formation of the N/M/R complex, similar to that described for DNA repair. On the other hand, telomere length in NBS cells is restored to the normal level only after co-expression of NBS1 with hTERT (Ranganathan et al, 2001). This evidence implies that the role of NBS1 in telomere maintenance is to generate Gstrands (telomeric 3'-overhang), and hTERT could replicate the telomeres by using this G-strand as a primer.…”

Telomerase is not an oncogene

“…Finally, a great deal more is known about telomere associated proteins, the overall telomere structure, and the sensitivity of telomeres to DNA damaging agents. These ®ndings point to additional factors which regulate the rate of telomere loss in the absence of telomerase, the equilibrium telomere length in telomerase positive cells, and provide insights into the consequences of aberrant (or`uncapped') telomere structures or critical telomere loss (Blackburn, 2001;Goytisolo et al, 2000;Gri th et al, 1999;Hemann et al, 2001;Ranganathan et al, 2001;Smith and de Lange, 2000;Smogorzewska et al, 2000;von Zglinicki, 2000;Wong et al, 2000).…”

New ways not to make ends meet: telomerase, DNA damage proteins and heterochromatin

“…In human cells, ATM kinase and the MRX complex seem to have an additional role in telomere end protection; cells lacking ATM or the Xrs2p homologue NBS1 show rapid telomere shortening, even in the presence of endogenous levels of telomerase (Metcalfe et al, 1996;Vaziri et al, 1997). Interestingly, overexpression of hTERT can counteract the loss of ATM or NBS1 in human ®broblasts (Ranganathan et al, 2001;Wood et al, 2001).…”