Homologous recombination is elevated in some Werner-like syndromes but not during normal in vitro or in vivo senescence of mammalian cells

Werner syndrome (WS) is a recessive genetic condition associated with markedly reduced replicative lifespans of cells in culture, high chromosomal instability in vivo and in vitro, and premature appearance of many characteristics of normal aging, including an increased incidence of cancer. We have monitored plasmid homologous recombination frequencies in diploid fibroblasts from 6 Werner or Werner-like syndrome patients, following transfection with a plasmid substrate containing 2 overlapping fragments of the TN5 Neor gene. Plasmid DNA recovered from these cells was then assayed for homologous recombination by (a) transformation of recA- bacteria to Ampr (indicating total viable plasmid) or Neor (indicating viable recombinant plasmid), and (b) by limited-cycle polymerase chain reaction (PCR) to co-amplify a recombinant fragment containing the overlap region, and a control region of the same plasmid, without bacterial transformation. Bacterial assay data indicated that recombination rates in 3 of the 6 WS strains were significantly elevated above normal controls; 4 of 6 appeared elevated by PCR assay. The highest-recombination WS strain showed evidence of reduced degradation of transfected plasmid DNA. For this small sample of WS strains, clinical severity of WS was not well correlated with recombination rate as determined by either assay (Pearson r = 0.78, not significant, for PCR assay); elevated recombination may, however, define a subset of WS at greatest risk for cancer and/or atherosclerosis. PCR assay of a hyperoxia-resistant HeLa cell line, displaying substantially increased chromosome breakage, indicated increased recombination between direct-repeat fragments. Nevertheless, elevated recombination in WS strains is unlikely to be secondary to impaired replicative capacity characteristic of WS cells, or to defective repair of chromosome damage which is increased in WS, since recombination in non-WS strains was unaffected by passage level or repeated UV irradiation.