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NUCLEOTIDE EXCISION REPAIR AND HUMAN SYNDROMES
DNA contains the blueprint for the development, proper functioning and
reproduction of every organism. Undesired alterations to the chemical stmcture of
DNA molecules (lesions) can arise spontaneously through intrinsic instability of
DNA (e.g. deamination, depurination etc.) or they can be induced by chemical
compounds and iradiation. DNA lesions are of many different types (see Figure 1)
including single strand and double strand breaks, inter- and intrastrand crosslinks
and different of base modifications. At the cellular level, DNA lesions hamper
processes like transcription and replication resulting in cell cycle arrest,
(programmed) cell death and genomic instability (mutagenesis). At the organismal
level, DNA lesions have been implicated in genetically inherited disease,
carcinogenesis and ageing. A clear example of the deleterious effects of genotoxic
agents in man is the strong conelation between sunlight exposure or smoking
cigarettes and the development of skin and lung cancer respectively [I]. Both
sunlight and cigarette smoke are exogenous sources of DNA damage. DNA base
modifications can also arise endogenously through cellular metabolites, for instance
oxidative DNA damage can result from free radicals generated as by-product of
active oxidative metabolism. Accumulating evidence suggests that modulation of
oxidative stress plays a role in ageing [2].
To prevent the hamlful consequences of DNA damage, multiple DNA repair
mechanisms exist (Figure 1). Briefly, double strand breaks are repaired by
homologous recombination dependent repair or in an end-joining reaction, and most
small base modifications are removed by base excision repair (BER). Nucleotide
excision repair (NER), the repair mechanism investigated in this thesis, removes
primarily bulky, helix-distorting adducts. However, considerable overlap exists in
substrate specificity of repair pathways, and certain proteins are used in more than
Ol1e pathway [3].
NUCLEOTIDE EXCISION REPAIR
Elucidation of the core mechanism of NER in the Gramm-negative bacterium
Escherichia coli served as a paradigm for studies on NER in almost all organisms.
The basic principle is removal of a small single-stranded patch of DNA containing
the lesion by dual incision of the damaged strand (see Figure 2a). Resynthesis of the
gap occurs using the complementary strand as template. The principle is
evolutionary conserved from bacteria to man but the proteins involved share little
homology. Most mammalian NER genes have been identified now and recently, the
NER reaction was reconstituted in vitro using purified proteins [4]. This chapter
summarizes the substrate specificity and the distinct steps of mammalian NER. For
Nucleotide excision repair and human syndromes 9