In vitro methylation of the BsuRI (5'-GGCC-3') sites in the E2a region of adenovirus type 2 DNA does not affect expression in Xenopus laevis oocytes

Presence and role of cytosine methylation in DNA viruses of animals

Nucleotide composition varies greatly among DNA viruses of animals, yet the evolutionary pressures and biological mechanisms driving these patterns are unclear. One of the most striking discrepancies lies in the frequency of CpG (the dinucleotide CG, linked by a phosphate group), which is underrepresented in most small DNA viruses (those with genomes below 10 kb) but not in larger DNA viruses. Cytosine methylation might be partially responsible, but research on this topic has focused on a few virus groups. For several viruses that integrate their genome into the host genome, the methylation status during this stage has been studied extensively, and the relationship between methylation and viral-induced tumor formation has been examined carefully. However, for actively replicating viruses—particularly small DNA viruses—the methylation status of CpG motifs is rarely known and the effects on the viral life cycle are obscure. In vertebrate host genomes, most cytosines at CpG sites are methylated, which in vertebrates acts to regulate gene expression and facilitates the recognition of unmethylated, potentially pathogen-associated DNA. Here we briefly introduce cytosine methylation before reviewing what is currently known about CpG methylation in DNA viruses.

Induction of a step in carcinogenesis that is normally associated with mutagenesis by nonmutagenic concentrations of 5-azacytidine

The permanent cell line BHK-21/cl 13 can be transformed by mutagenic carcinogens as the result of the induction of a recessive somatic mutation. Yet when these cells were treated with 5-azacytidine under conditions in which no mutants resistant to either ouabain or 6-thioguanine could be detected, they were transformed efficiently. These transformants were induced, not selected. 6-Azacytidine was ineffective at transforming BHK cells; 2'-deoxy-5-azacytidine was exceptionally effective. When tested by cell fusion, transformants induced by 5-azacytidine fell into the same complementation group as those induced by highly mutagenic carcinogens, but they were phenotypically distinct in that they were unstable during prolonged passage and rarely displayed the temperature-limited phenotypes so common among BHK transformants induced by strongly mutagenic carcinogens. These results raise the possibility that a cell can be induced by either genetic or epigenetic means to traverse the same single step in carcinogenesis.

Induction of a step in carcinogenesis that is normally associated with mutagenesis by nonmutagenic concentrations of 5-azacytidine

The permanent cell line BHK-21/cl 13 can be transformed by mutagenic carcinogens as the result of the induction of a recessive somatic mutation. Yet when these cells were treated with 5-azacytidine under conditions in which no mutants resistant to either ouabain or 6-thioguanine could be detected, they were transformed efficiently. These transformants were induced, not selected. 6-Azacytidine was ineffective at transforming BHK cells; 2'-deoxy-5-azacytidine was exceptionally effective. When tested by cell fusion, transformants induced by 5-azacytidine fell into the same complementation group as those induced by highly mutagenic carcinogens, but they were phenotypically distinct in that they were unstable during prolonged passage and rarely displayed the temperature-limited phenotypes so common among BHK transformants induced by strongly mutagenic carcinogens. These results raise the possibility that a cell can be induced by either genetic or epigenetic means to traverse the same single step in carcinogenesis.

DNA methylation of three 5' C-C-G-G 3' sites in the promoter and 5' region inactivate the E2a gene of adenovirus type 2

The E2a gene of human adenovirus type 2 (Ad2) encodes the 72-kilodalton DNA-binding protein. We previously described perfect inverse correlations between the methylation of all 5' C-C-G-G 3' sequences in the Ad2 E2a gene in virus-transformed hamster cells containing viral DNA sequences in an integrated state and the extent to which this gene is expressed. We subsequently showed that in vitro methylation of all 14 5' C-C-G-G 3' sequences in the cloned E2a gene by prokaryotic Hpa II DNA methyltransferase leads to transcriptional inactivation after microinjection into Xenopus laevis oocytes. The unmethylated cloned E2a gene is expressed in these cells. We report here the construction of partly methylated clones of the E2a gene. In the promoter (5')-methylated construct, three 5' C-C-G-G 3' sequences at the 5' end of the subclone were methylated. One of these sites is located 215 base pairs (bp) upstream (bp 26,169 of Ad2 DNA), and two sites are located 5 and 23 bp downstream from the cap site (bp 25,931 and 25,949 of Ad2 DNA) of the E2a gene. This construct was transcriptionally inactive upon microinjection into nuclei of X. laevis oocytes. In the gene (3')-methylated construct, 11 5' C-C-G-G 3' sequences in the main part of the transcribed gene region were methylated in vitro. This construct was transcribed in X. laevis oocytes, and at least some of the Ad2-specific RNA synthesized was initiated at the same sites as in Ad2-infected human KB cells. Both mock-methylated constructs were transcribed into Ad2-specific RNA in X. laevis oocytes. These results demonstrate that DNA methylations at or close to the promoter and 5' end of the E2a gene cause transcriptional inactivation. Perhaps only one methyl group would be adequate for inactivation; in vivo methylation of more than one cytosine may be a form of safeguard or redundancy.

Replication and expression of Xenopus laevis globin genes injected into fertilized Xenopus eggs

Cloned Xenopus adult alpha 1- and beta 1-globin genes were injected into fertilized Xenopus eggs, and the eggs were allowed to develop into swimming tadpoles. The injected DNA replicated during early Xenopus development but did not become methylated de novo. When DNA was modified with Hpa II methylase before injection, methylation was maintained during replication. Although the Xenopus adult globin genes are not normally expressed until metamorphosis, both the unmethylated and in vitro methylated adult alpha- and beta-globin genes were transcribed at low levels, but from the correct promoters, during early development.