Reduced 5-hydroxymethyluracil-DNA glycosylase activity in Werner's syndrome cells

Werner's syndrome (WS) is an autosomal recessive disease marked by early symptoms of accelerated aging. There is evidence indicating accumulation of oxidized DNA bases to be a major factor in cellular aging. The first step of excision repair of such bases in human cells is their removal from DNA by glycosylases. 5-Hydroxymethyluracil (HMU)-DNA glycosylase excises HMU from DNA; another glycosylase removes many non-aromatic pyrimidine derivatives. Levels of glycosylases that excise oxidized pyrimidines from DNA were compared between confluent and proliferating populations of WS cells, age-matched controls, and young control cells. They were assayed by measurements of direct release of free bases from their respective DNA substrates. Specific activities of the glycosylase that releases various modified pyrimidines and of uracil-DNA glycosylase (which removes uracil from DNA) were essentially the same in all cell lines. Cell cycle variations of these enzymes also did not differ between WS and control cells. HMU-DNA glycosylase specific activity was reduced in WS cells. Reduction of HMU-DNA glycosylase has been described in senescent human WI-38 cells. Therefore, while neither WS nor senescent cells have overall deficiencies of DNA glycosylase activities, they both might have reduced excision of HMU from DNA. This indicates a possible role of HMU accumulation in the aging process.