| Research Abstract |
In human genome, five genes --- WRN, BLM, RECQ1, Q4, and Q5 --- have been found to encode DNA helicases. These genes share homology with the RecQ family including E. colt RecQ, cerevisiae Sgs1, pombe Rqh1, so that they are called as human RecQ-type DNA helicases. As far three of them, WRN, BLM and RECQ4, have been identified in human genetic disorders, Werner's (WS), Bloom's (BS) and Rothmund-Thomson syndromes (RTS). These three genetic disorders are associated with cancer predisposition and/or premature aging. However, the relationship between, the deficiency of DNA helicase and phenotype in human is not clarified. In order to analyze molecular mechanisms of aging and tumor formation in vivo, we have established four different mutant mouse lines (RecQ1, Wrn, RecQ4, and RecQ5) by targeted mutagenesis.
RECQ1, which may encode a helicase with most homologous to E.coli. RecQ, was inactivated in mice. Homozygous RecQ1-null mutant mice, however, were born without any apparent phenotype and d
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eveloped normally. They were healthy and fertile in spite of its ubiquitous expression in mice, suggesting that RecQ1 is not essential for mouse development.
Wemer's syndromes (WS) is an autosomal recessive disorder characterized by many features of premature aging and early onset of age-related diseases including neoplasm. Although WRN, which is responsible gene for WS, assumes to work in DNA replication, repair, or maintaining chromosomal integrity, its physiological function remains unknown. To analyze molecular mechanisms of aging and tumor formation in mice, we also established a mutant mouse line for WRN, which was designed to reproduce the major mutation of Japanese WS patients. Wm mutant mice, however, showed no signs of premature aging. They were healthy and fertile with quite subtle phenotype. By a year observation, loss of Wrn protein had no effect on lifespan and apparent cancer predisposition. Although mutant mice deficient for both Wm and RecQ1 did not show any significant phenotype, more continuous detailed analyses on these mutant mice are necessary to elucidate their physiological functions.
In 1999, RECQ4, the other member of the human RecQ heiicase family, was identified as a gene responsible for some cases of Rothmund-Thomson syndrome (RTS). RTS is also a rare autosomal recessive genetic disorder characterized by growth deficiency, photosensitivity with poikilodermatous skin changes and some clinical similarities to WS and BS. RecQ4 mutant mice were also established and analyzed. Homozygous mice for RecQ4-nulI mutation suffer embryonic lethality in early development (E3.5 - E6.S). This result shows the RecQ4 gene is essential for passing through peri-implantation stage of development in mice in spite of the recessive inheritance of RTS. Detailed analyses using several different RecQ4 alleles suggested that RecQ4 helicase activity might be necessary for growth or differentiation of inner cell mass in vivo. Establishment of conditional RecQ4 knock-out mice is in process.
RECQ5, another member of the human RecQ helicase family, was cloned and mapped to chromosome 17q25. So far, this gene is not identified as a gene responsible for any kinds of syndromes. Establishment of RecQ5-null mutant mice is in process. Less
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