Mini-symposium: Oxidative DNA damage and repair: Role in cancer and ageing

Thursday, 12 February 2004 at 14.00, Aud. A3.3067, A-building, Rikshospitalet

Repair and stability of exocyclic DNA adducts in human cancers

Prof. Barbara Tudek, Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw, Poland

The role of oxidative DNA damage in the ethiology of human cancers is not clear, although many types of human caners are linked to chronic inflammations and oxidative stress. Reactive oxygen species damage nucleic acids, proteins and lipids. Lipid peroxidation increases formation of promutagenic exocyclic DNA adducts, such as 1,N⁶-ethenoadenine (eA) and 3,N⁴-ethenocytosine (eC). We have studied the DNA repair efficiency of such adducts in blood leukocytes from cancer patients and find that repair is decreased relative to healthy individuals. This effect was much higher in the group of patients developing inflammation-related adenocarcinoma (AD), suggesting that decreased activity of eA and eC repair is linked particularly to chronic inflammation-related lung adenocarcinoma, and may be a risk factor.

One of the adducts, 1,N⁶-ethenoadenine, was found to be chemically unstable and to spontaneously rearrange to form pyrimidine ring-opened forms as well as a deformylated compound, which are handled by different repair enzymes than eA itself. The pyrimidine-ring opened from constitutes a strong block for DNA synthesis in vitro by most of the studied DNA polymerases whereas the deformylated compound was bypassed and caused nucleotides misincorporations initiating AT ® TA and AT ® CG transversions as well as 1-2-nucleotides deletions. This can explain a higher mutagenic potential of the deformylated eA. than its parent adduct.

Selected reading:

• Gackowski D, Speina E, Zielińska M, Kowalewski J, Rozalski R, Siomek A, Paciorek T, Tudek B, Oliński R (2003) Products of oxidative DNA damage and repair as possible biomarkers of susceptibility to lung cancer. Cancer Res. 63: 4899-4902.

• Speina E, Zielińska M, Barbin A, Gackowski D, Kowalewski J, Grąziewicz M.-A., Siedlecki JA, Oliński R, Tudek B (2003) Decreased repair activities of 1,N⁶-ethenoadenine and 3,N⁴-ethenocytosine in lung adenocarcinoma patients. Cancer Res. 63: 4351-4357.

• Speina E, Cieśla JM, Wjcik J, Bajek M, Kuśmierek JT, Tudek B, (2001) The pyrimidine ring-opened derivative of 1,N⁶-ethenoadenine is excised from DNA by the Escherichia coli Fpg and Nth proteins. J Biol Chem., 276:21821-21827.

Mitochondrial repair of oxidative damage and changes with aging

Reactive oxygen species (ROS) are formed in all living organisms as a by-product of normal metabolism (endogenous sources) and as a consequence of exposure to environmental compounds (exogenous sources). Endogenous ROS are largely formed during oxidative phosphorylation in the mitochondria, and therefore, mitochondrial DNA (mtDNA) is at particularly high risk of ROS-induced damage. Mitochondria are essential for cell viability, and oxidative damage to mtDNA has been implicated as a causative factor in a wide variety of degenerative diseases, and in cancer and aging. Changes in mitochondrial function with age have been observed in several organisms and accumulation of DNA lesions in mtDNA with age may be an underlying cause for numerous age-associated diseases. We have used a number of different approaches to explore the mechanisms of DNA damage processing of the mtDNA, and we have been able to demonstrate that mammalian mitochondria efficiently remove e.g. 7,8-dihydro-8-oxoguanine (8-oxoG) and 7,8-dihydro-8-oxoadenine (8-oxoA) from their genome. Furthermore, we have demonstrated that cells from patients suffering from the segmental premature aging syndrome, Cockayne Syndrome, are deficient in mitochondrial repair of these lesions.

Selected reading:

• Stevnsner, T., Nyaga, S., de Souza-Pinto, N.C., van der Horst, G.T.J., Gorgels, T.G.M.F., Hogue, B.A., Thorslund, T., Bohr, V.A. (2002) Mitochondrial repair of 8-oxoguanine is deficient in Cockayne Syndrome group B. Oncogene 21, 8675-8682.

• Stevnsner, T., Thorslund, T., de Souza-Pinto, N.C., Bohr, V.A. (2002) Mitochondrial repair of 8-oxoguanine and changes with aging. Experimental Gerontology 37, 1189-1196.

• Jensen, A., Calvayrac, G., Karahlil, B., Bohr, V.A., Stevnsner, T. (2003) Mammalian 8-oxoguanine DNA glycosylase 1 incises 8-oxoadenine opposite cytosine in nuclei and mitochondria, while a different glycosylase incises 8-oxoadenine opposite guanine in nuclei. J.Biol.Chem. 278, 19541-19548.

• Licht, C.L., Stevnsner, T., Bohr, V.A. (2003) Cockayne's syndrome group B - Cellular and biochemical functions. Am. J. Hum. Genet. 73, 1217-1239.