Comparison of the effects of UV irradiation on 5-methyl-substituted and unsubstituted pyrimidines in alternating pyrimidine-purine sequences in DNA

5-methylcytosine at HpaII sites in p53 is not hypermutable after UVC irradiation

Using a yeast based p53 functional assay we previously demonstrated that the UVC-induced p53 mutation spectrum appears to be indistinguishable from the one observed in Non Melanoma Skin Cancer (NMSC). However, position 742 (codon 248, CpG site) represented the major hot spot in NMSC but was not found mutated in the yeast system. In order to determine whether UVC-induced mutagenic events may be facilitated at methylated cytosine (5mC), a yeast expression vector harbouring a human wild-type p53 cDNA (pLS76) was methylated in vitro by HpaII methylase. Methylation induced 98% protection to HpaII endonuclease. Unmethylated and methylated pLS76 vectors were then UVC irradiated (lambda(max): 254 nm) and transfected into a yeast strain containing the ADE2 gene regulated by a p53-responsive promoter. The results revealed that: (i) 5mC at HpaII sites did not cause any difference in the UVC-induced survival and/or mutagenicity; (ii) none of the 20 mutants derived from methylated pLS76 showed p53 mutations targeted at HpaII sites; (iii) the UVC-induced p53 mutation spectra derived from methylated and unmethylated pLS76 were indistinguishable not only when classes of mutations and hot spots were concerned, but also when compared through a rigorous statistical test to estimate their relatedness (P = 0.85); (iv) the presence of 5mC did not increase the formation of photo-lesions at codon 248, as determined by using a stop polymerase assay. Although based on a limited number of mutants, these results suggest that the mere presence of 5mC at position 742 does not cause a dramatic increase of its mutability after UVC irradiation. We propose that position 742 is a hot spot in NMSC either because of mutagenic events at 5mC caused by other UV components of solarlight and/or because not all the NMSC are directly correlated with UV mutagenesis but may have a "spontaneous" origin.

Insulin and insulin-like growth factor-I promote rabbit blastocyst development and prevent apoptosis

Insulin as well as insulin-like growth factor-I (IGF-I) promote early embryo development, and IGF-I binds to the coats of preimplantation rabbit embryos. As the IGF-I receptor is expressed from the morula stage onwards, the embryos are capable of responding to insulin and IGF-I, which is present in the oviductal and uterine secretions that surround them. The embryonic coats were removed to exclude any influence by IGF-I bound to the coats. The in vitro development of such embryos under classical conditions appears to be retarded. Addition of IGF-I (68 pM-6.8 nM) or insulin (68 nM-6.8 microM), however, promotes blastocyst formation. Embryo development under such conditions is not significantly different from that of embryos cultured with intact coats. In contrast, coat-free embryos cultured without IGF-I or insulin supplementation show apoptosis. Because IGF-I stimulates cell proliferation and prevents apoptosis, we investigated whether insulin or IGF-I may act as "survival factors" in preimplantation development. Therefore, apoptosis was induced by slight UV irradiation (254 nm wave length; 11.8 W/m2). Compared to the untreated controls, embryos displaying retarded development or degeneration were increased by 22% and 14%, respectively. Addition of IGF-I or insulin to the culture medium of UV-irradiated embryos improved [3H]thymidine incorporation and blastocyst formation significantly. By immunohistochemistry we could show that addition of insulin (0.68-68 nM) decreased apoptosis and increased cell proliferation in a dose-dependent manner, supporting blastocyst development significantly.

Formation and processing of UV photoproducts: effects of DNA sequence and chromatin environment

Cyclobutane pyrimidine dimers and (6-4) photoproducts are the two major classes of lesions produced in DNA by UVB and UVC irradiation. Their distribution along genes is nucleotide sequence-dependent. In vivo, the frequency of these lesions at specific sites is modulated by nucleosomes and other DNA binding proteins. Repair of UV photoproducts is dependent on the transcriptional status of the sequences to be repaired and on the chromatin environment. The formation of DNA photolesions by UV light is responsible for the induction of mutations and the development of skin cancer. To understand the mechanisms of UV mutagenesis, it is important to know how these lesions are formed, by which cellular pathways they are repaired and how they are dealt with by DNA polymerases.

Molecular cloning and functional analysis of a Schizosaccharomyces pombe homologue of Escherichia coli endonuclease III

The Escherichia coli endonuclease III (Nth-Eco) protein is involved in the removal of damaged pyrimidine residues from DNA by base excision repair. It is an iron-sulphur enzyme possessing both DNA glycosylase and apurinic/apyrimidinic lyase activities. A database homology search identified an open reading frame in genomic sequences of Schizosaccharomyces pombe which encodes a protein highly similar to Nth-Eco. The gene has been subcloned in an expression vector and the protein purified to apparent homogeneity. The S.pombe Nth homologue (Nth-Spo) is a 40.2 kDa protein of 355 amino acids. Nth-Spo possesses glycosylase activity on different types of DNA substrates with pyrimidine damage, being able to release both urea and thymine glycol from double-stranded polymers. The eukaryotic protein removes urea more efficiently than the prokaryotic enzyme, whereas its efficiency in excising thymine glycol is lower. A nicking assay was used to show that the enzyme also exhibits an AP lyase activity on UV- and gamma-irradiated DNA substrates. These findings show that Nth protein is structurally and functionally conserved from bacteria to fission yeast.