Werner syndrome (WS) is a recessive disorder characterized by premature senescence. Bloom syndrome (BS) is a recessive disorder characterized by short stature and immunode®ciency. A common characteristic of both syndromes is genomic instability leading to tumorigenesis. WRN and BLM genes causing WS and BS, encode proteins that are closely related to the RecQ helicase. We produced WRN 7/7, BLM 7/7 and WRNmutants in the chicken B-cell line DT40. WRN 7/7 cells showed hypersensitivities to genotoxic agents, such as 4-nitroquinoline 1-oxide, camptothecin and methyl methanesulfonate. They also showed a threefold increase in targeted integration rate of exogenous DNAs, but not in sister chromatid exchange (SCE) frequency. BLM 7/7 cells showed hypersensitivities to the genotoxic agents as well as ultraviolet (UV) light, in addition to a 10-fold increase in targeted integration rate and an 11-fold increase in SCE frequency. In WRN 7/7 /BLM 7/7 cells, synergistically increased hypersensitivities to the genotoxic agents were observed whereas both SCE frequencies and targeted integration rates were partially diminished compared to the single mutants. Chromosomal aberrations were also synergistically increased in WRN 7/7 / BLM 7/7 cells when irradiated with UV light in late S to G 2 phases. These results suggest that both WRN and BLM may be involved in DNA repair in a complementary fashion.

Werner syndrome (WS) is a rare autosmomal recessive genetic disorder causing premature aging. The gene (WRN) responsible for WS encodes a protein homologous to the RecQ-type helicase. WRN has a nucleolar localization signal and shows intranuclear tra cking between the nucleolus and the nucleoplasm. WRN is recruited into the nucleolus when rRNA transcription is reactivated in quiescent cells. Inhibition of mRNA transcription with a-amanitin has no e ect on nucleolar localization of WRN whereas inhibition of rRNA transcription with actinomycin D releases WRN from nucleoli, suggesting that nucleolar WRN is closely related to rRNA transcription by RNA polymerase I (RPI). A possible function of WRN on rRNA transcription through interaction with RPI is supported by the results described here showing that WRN is coimmunoprecipitated with an RPI subunit, RPA40. Here we show that WS ®broblasts are characterized by a decreased level of rRNA transcription compared with wild-type cells, and that the decreased level of rRNA transcription in WS ®broblasts recovers when wild-type WRN is exogenously expressed. By contrast, exogenously expressed mutant-type WRN lacking an ability to migrate into the nucleolus fails to stimulate rRNA transcription. These results suggest that WRN promotes rRNA transcription as a component of an RPIassociated complex in the nucleolus.

We studied tumorigenic and phenotypic characteristics of pre- and postimmortal human B-lymphoblastoid cell lines (LCLs) transformed by Epstein-Barr virus (EBV): preimmortal LCLs showed low telomerase activity and a normal diploid karyotype while postimmortal LCLs showed much higher telomerase activity and maintained a clonal aneuploidic state. Among five postimmortal LCLs tested, LCLs N0005 and N6803 formed colonies in agar medium and showed marked aneuploidy, and N6803 was transplantable into nude mice indicating that it had a complete malignant phenotype, but all preimmortal LCLs and the remaining three postimmortal LCLs lacked these characteristics. The products of tumor suppresser genes, p16(INK4A) and pRb, were downregulated in these two LCLs, and the p53 gene was mutated in N0005 LCL. We believe these results showed for the first time that some postimmortal EBV-transformed LCLs can become tumorigenic, contrary to previous reports, and that these LCLs provide an in vitro model of tumorigenesis induced by EBV.