Werner Syndrome, aging and cancer.

Werner syndrome (WS) is a rare autosomal recessive genetic instability/cancer predisposition disorder that displays many symptoms of premature aging. The mimicry of agerelated phenotypes in WS, as well as its dependence on a single defective gene product, has provided the impetus for studying this fascinating disease as a model system for normative aging and its related pathologies such as atherosclerosis, neoplasia, diabetes mellitus, and osteoporosis. The gene product defective in WS, WRN, is a member of the RecQ DNA helicase family that is widely distributed in all kingdoms of life, and is believed to play a central role in genomic stability by preferentially operating on non-canonical DNA structures. Although there have been considerable advances in our understanding of the biochemistry of WRN and its interacting protein partners, the in vivo molecular function(s) of WRN remain(s) elusive. In addition to summarizing the features and clinical progression of WS, the following chapter details our current understanding of the WRN protein with respect to its biochemistry and its interacting protein partners, and considers its putative in vivo roles in various DNA transactions.

Intrinsic polymerase activities of UmuD'(2)C and MucA'(2)B are responsible for their different mutagenic properties during bypass of a T-T cis-syn cyclobutane dimer.

In wild-type Escherichia coli, translesion replication is largely dependent upon the UmuD'(2)C complex (DNA polymerase V [polV]) or its plasmid-encoded homologs, such as MucA'(2)B. Interestingly, both the efficiency of translesion replication of a T-T cis-syn dimer and the spectra of mutations observed are different in Umu- and Muc-expressing strains. We have investigated whether the polIII core is responsible for these differences by measuring the frequency of dimer bypass, the error rate of bypass, and the resulting mutation spectrum in mutants carrying a deletion of dnaQ (epsilon subunit) or holE (theta subunit) or carrying the dnaQ allele mutD5, which is deficient in proofreading but is competent in the structural function of epsilon, or the dnaE antimutator allele spq-2. The chromosomal copy of the umuDC operon was deleted in each strain, and the UmuDC, UmuD'C, MucAB, or MucA'B proteins were expressed from a low-copy-number plasmid. With only few exceptions, we found that the characteristically different mutation spectra resulting from Umu- and Muc-mediated bypass are maintained in all of the strains investigated, indicating that differences in the activity or structure of the polIII core are not responsible for the observed phenotype. We also demonstrate that the MucA'(2)B complex is more efficient in promoting translesion replication than the UmuD'(2)C proteins and show that, contrary to expectation, the T-T dimer is bypassed more accurately by MucA'(2)B than by UmuD'(2)C. These results are consistent with the view that in a wild-type cell, the polV-like enzymes are responsible for the spectra of mutations generated during translesion replication and that polIII may simply be required to fix the misincorporations as mutations by completing chromosomal replication. Our observations also show that the mutagenic properties of a lesion can depend strongly on the particular enzyme employed in bypass.