Book of Abstracts: Albany 2011
June 14-18 2011
©Adenine Press (2010)
The Good and the Bad of DNA Damage DNA Damage and Epigenetic Marks Drive Transcription and Repair
Methylation is an epigenetic modification of the DNA, which can silence genes. By using a sophisticated genetic system, we have induced a break in the double helix of DNA at a single site in mouse or human cells. This important lesion is repaired by a precise mechanism: the damaged chromosome pairs and retrieves genetic information from the homologous chromosome partner. The repair of the double strand break leaves a series of methyl groups on the DNA base C (methylation) in a fraction of the repaired molecules flanking the break and, as consequence, the underlying genetic information may be packed and silenced. DNA methylation represents a scar marking damaged and repaired DNA. This processi s a strong evolutionary adaptive response used by the cell to protect the genome. Packed chromatin DNA is more resistant to damage, and silencing protects the cell from expressing a damaged gene, since the repair may introduce some changes in the code.
During these experiments we found that selective and local DNA damage is an essential step governing transcription of genes. Estrogens are hormones that associated to the specific receptor penetrate into chromatin-DNA and bind a specific DNA sequence present in several places in the chromosomes. We have found that the receptor bound to the hormone induces oxidation and single strand breaks in the DNA, where the receptor binds. The relaxed DNA chromatin bends and loops out bringing in close proximity non-contiguous regions of chromosomes. The loops of DNA promote the association of the receptor-hormone and RNA polymeraseII. This enzyme normally is poised for activation of transcription at 5’ end of genes across the genome. As soon as the RNA polymerase touches the receptor, the enzyme receives directions and starts transcription. We found that DNA untwisting, which renders the strands flexible, is caused by nicks in the DNA induced by the receptor. These “holes” open an entry site for RNA polymerase, which now can find the bound receptor and eventually are sealed rapidly by repair enzymes. At the end of transcription, the DNA is packed back into condensed chromatin. This mechanism was unexpected, since “nicking” of DNA is rather a dangerous event, which shall be quickly repaired. However, the price to pay for estrogen, sex differentiation, and maintenance of the gnetic information may be high, because repeated nicks and sealing events may result in imperfect repair and change of the genetic code. This mechanism may explain the occurrence of breast cancer, which is considered strictly dependent on estrogens. Also, it suggests that transcription is a costly process that deteriorate the whole genomic machine in the long run.
Enrico V. Avvedimento
Dept Biologia e Patologia Molecolare e Cellulare,
Facolta’ di Medicina,
Universita’ “Federico II”,