And Alain Sarasin

Introduction

Oxidative damage to cellular DNA arises from attack by free radicals produced by cellular metabolism or by exogenous agents such as UV-A and ionizing radiation. Among the numerous types of base damage produced, 8-oxoguanine (8-oxoG or GO) is one of the most abundant. Because 8-oxoG readily mispairs with A, it is highly mutagenic, inducing G-to-T transversions in the absence of efficient repair. The base pair GO: C can be repaired in human cells by base excision repair (BER) initiated by the hOGGl gene product, a bifunctional DNA glycosylase with the dual function of lesion removal and incision of the DNA by AP (apurinic)-lyase activity.1

Although there are no known naturally occurring human mutations that specifically confer defects in BER, several human diseases are associated with deficiency in the nucleotide excision repair (NER) pathway, a versatile repair process that removes UV damage and other bulky lesions from DNA. These include the sun-sensitive disorders xeroderma pigmentosum (XP), which is characterized by skin changes and extreme predisposition to UV-induced skin cancer, and Cockayne syndrome (CS), a severe postnatal developmental disorder. Cells from CS individuals are specifically deficient in the preferential removal of lesions in template strands of genes transcribed by RNA polymerase II. Although this transcription-coupled repair (TCR) was initially presumed to be specific for DNA lesions

1 S. Boiteux and J. P. Radicella, Biochimie 81, 59 (1999).

repaired by NER,2 it is now clear that some lesions induced by ionizing radiation and other agents that generate free radicals are also removed by TCR. Although the full spectrum of oxidative lesions processed by TCR is not yet known, one of these is the oxidatively damaged base thymine glycol, which, like most other kinds of oxidative base damage, is primarily removed by BER.3 We were interested in determining whether 8-oxoG could also be repaired by TCR. We addressed this question by the use of a single, site-specifically placed 8-oxoG paired with C in a defined location in a shuttle vector transfected into human cells, as previously described.4,5

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