title: Telomerase expression is sufficient for chromosomal integrity in cells lacking p53 dependent G1 checkpoint function.
abstract: BACKGROUND
  Secondary cultures of human fibroblasts display a finite lifespan ending at senescence. Loss of p53 function by mutation or viral oncogene expression bypasses senescence, allowing cell division to continue for an additional 10-20 doublings. During this time chromosomal aberrations seen in mitotic cells increase while DNA damage and decatenation checkpoint functions in G2 cells decrease.
  
  
  METHODS
  To explore this complex interplay between chromosomal instability and checkpoint dysfunction, human fibroblast lines were derived that expressed HPV16E6 oncoprotein or dominant-negative alleles of p53 (A143V and H179Q) with or without the catalytic subunit of telomerase.
  
  
  RESULTS
  Cells with normal p53 function displayed 86-93% G1 arrest after exposure to 1.5 Gy ionizing radiation (IR). Expression of HPV16E6 or p53-H179Q severely attenuated G1 checkpoint function (3-20% arrest) while p53-A143V expression induced intermediate attenuation (55-57% arrest) irrespective of telomerase expression. All cell lines, regardless of telomerase expression or p53 status, exhibited a normal DNA damage G2 checkpoint response following exposure to 1.5 Gy IR prior to the senescence checkpoint. As telomerase-negative cells bypassed senescence, the frequencies of chromosomal aberrations increased generally congruent with attenuation of G2 checkpoint function. Telomerase expression allowed cells with defective p53 function to grow >175 doublings without chromosomal aberrations or attenuation of G2 checkpoint function.
  
  
  CONCLUSIONS
  Thus, chromosomal instability in cells with defective p53 function appears to depend upon telomere erosion not loss of the DNA damage induced G1 checkpoint.
authors.affiliation: Department of Pathology and Laboratory Medicine, Lineberger Comprehensive Cancer Center, University of North Carolina, CB 7295, Chapel Hill, NC 27599, USA. dennis@email.unc.edu
authors.forename: Dennis A
authors.initials: DA
authors.lastname: Simpson
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Livanos
authors.forename: Timothy P
authors.initials: TP
authors.lastname: Heffernan
authors.forename: William K
authors.initials: WK
authors.lastname: Kaufmann