Recombinant Viral Vectors for Investigating DNA Damage Responses and Gene Therapy of Xeroderma Pigmentosum

Abstract

1.1 The dark side of the sun The genome of all living organisms is constantly threatened by a number of endogenous and exogenous DNA damaging agents. Such damage may disturb essential cellular processes, such as DNA replication and transcription, thereby resulting in double-strand breaks (referred to as ‘replication fork collapse’), which can lead to chromosomal aberrations and/or cell death, ultimately contributing to mutagenesis, early aging and tumorigenesis (Ciccia & Elledge, 2010). One of the most important exogenous sources of DNA damage is the ultraviolet radiation (UV) component of sunlight, since it is responsible for a wide range of biological effects, including alteration in the structure of biologically essential molecules, such as proteins and nucleic acids. Indeed, UV is one of the most effective and carcinogenic exogenous agents that act on DNA, threatening the genome integrity and affecting normal life processes in different aquatic and terrestrial organisms, ranging from prokaryotes to mammals (Rastogi et al., 2010). In addition, UV is the major etiologic agent in the development of human skin cancers (Narayanan et al., 2010). Sunlight is the primary UV source, whose spectrum is usually classified according to its wavelength in UVA (320-400 nm; lowest energy), UVB (280-320 nm) and UVC (200-280 nm; highest energy). Although these three UV bands are present in sunlight, the stratospheric ozone layer entirely blocks the UVC and most of UVB, thus the solar UV spectrum that reaches the Earth’s ground is composed by UVA and some UVB, even though ozone layer depletion can cause changes in this spectral distribution (Kuluncsics et al., 1999). The chemical nature and efficiency in the formation of DNA lesions greatly depend on the wavelength of the incident photons. Despite its lowest energy, UVA light can deeply penetrate into the cells, mostly damaging DNA by indirect effects caused by the generation of reactive oxygen species which may react with nitrogen bases, resulting in base alterations and breaks in the DNA molecule. On the other hand, UVB can be directly absorbed by DNA bases, producing two main types of DNA damage, the cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone-(6-4)-photoproducts (6-4PPs), both resulting from