The Function of Werner Syndrome Protein

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Werner's syndrome (WS) is a homozygous recessive disease characterized by early onset of many characteristics of normal aging, such as wrinkling of the skin, graying of the hair, cataracts, diabetes, and osteoporosis. Because of the acceleration of aging in WS, the study of this disease will hopefully shed light on the degenerative processes that occur in normal aging. Cells from WS patients grow more slowly and senescence at an earlier population doubling than age-matched normal cells, possibly because these cells appear to lose the telomeric ends of their chromosomes at an accelerated rate. In general, WS cells have a high level of genomic instability, with increased amounts of DNA deletions, insertions, and rearrangements. These effects could potentially be the result of defects in DNA repair, replication, and/or recombination, although the actual biochemical defect remains unknown. The gene that is defective in WS, the WRN gene, has been identified and characterized. We have made purified WRN protein for use in a number of basic and complex biochemical assays. We are pursuing several avenues to identify and characterize the biochemical defect in WS cells. WRN protein has helicase activity and will unwind small and large DNA duplex constructs. It will also unwind unusual DNA structures such as triple helices and DNA forks. We are comparing the Werner helicase activity to that of another helicases in the family of RecQ helicases that are all involved in the maintenace of genome stability. WRNp has another enzymatic activity, a 3-5' exonuclease function. We are searching for pathways in which WRN participates and have discovered a number of new functional and physical protein interactions with Werner protein. Our data strongly suggest that WRN is involved in two of the major DNA repair pathways: base excision repair and recombination. This conclusion is supported by biochemical studies of protein functional interaction and by cell biological data. Further, our observations and results from other work suggest that a major function of WRN is at the telomere end. WRN interacts with key telomeric proteins such as TRF1 and 2 and POT1. These observations of functional protein interactions corroborated by cell biological observations suggest that WRN protein is involved in DNA repair processes and maintenance at the telomere end. WRN is also involved in the DNA repair process. Specifically, we find in vitro and in vivo evidence for a role of WRN in the DNA repair of oxidative DNA base lesions and in the DNA repair of double strand breaks. Further, post-translational modification of WRN protein can changes its activities in the DNA repair process and thus might be involved in the regulation of these processes. For example, the acetylation of WRN enhances its activities in DNA repair and is likely a significant step in DNA damage signaling. We have also characterized the function of the homolog of WRNp found in the simple organism, the nematode. Interestingly, the nematode WRN protein has similar biochemical characteristics to the human WRNp.