Nucleotide excision repair

Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals (e.g. intercalating agents), radiation and other mutagens. Three excision repair pathways exist to repair single stranded DNA damage: Nucleotide excision repair (NER), base excision repair (BER), and DNA mismatch repair (MMR). While the BER pathway can recognize specific non-bulky lesions in DNA, it can correct only damaged bases that are removed by specific glycosylases. Similarly, the MMR pathway only targets mismatched Watson-Crick base pairs. Nucleotide excision repair (NER) is a particularly important excision mechanism that removes DNA damage induced by ultraviolet light (UV). UV DNA damage results in bulky DNA adducts — these adducts are mostly thymine dimers and 6,4-photoproducts. Recognition of the damage leads to removal of a short single-stranded DNA segment that contains the lesion. The undamaged single-stranded DNA remains and DNA polymerase uses it as a template to synthesize a short complementary sequence. Final ligation to complete NER and form a double stranded DNA is carried out by DNA ligase. NER can be divided into two subpathways: global genomic NER (GG-NER or GGR) and transcription coupled NER (TC-NER or TCR). The two subpathways differ in how they recognize DNA damage but they share the same process for lesion incision, repair, and ligation. The importance of NER is evidenced by the severe human diseases that result from in-born genetic mutations of NER proteins. Xeroderma pigmentosum and Cockayne's syndrome are two examples of NER associated diseases.

In eukaryotes Nucleotide excision repair is more complex in eukaryotes than prokaryotes, which express enzymes like the photolyase. In humans and other placental animals, there are 9 major proteins involved in NER. Deficiencies in certain proteins leads to disease; protein names are associated with the disease. XPA, XPB, XPC, XPD, XPE, XPF, and XPG all derive from xeroderma pigmentosum and CSA and CSB represent proteins linked to Cockayne syndrome. Additionally, the proteins ERCC1, RPA, RAD23A, RAD23B, and others also participate in nucleotide excision repair. A more complete list of proteins involved in NER is found below. Eukaryotic nucleotide excision repair can be divided into two subpathways: global genomic NER (GG-NER) and transcription coupled NER (TC-NER). Three different sets of proteins are involved in recognizing DNA damage for each subpathway. After damage recognition, the three subpathways converge for the steps of dual incision, repair, and ligation.

Damage recognition

Global genomic NER (GG-NER)

Global genomic NER repairs damage in both transcribed and untranscribed DNA strands in active and inactive genes throughout the genome. This process is not dependent on transcription. This pathway employs several "damage sensing" proteins including the DNA-damage binding (DDB) and XPC-Rad23B complexes that constantly scan the genome and recognize helix distortions: the XPC-Rad23B complex is responsible for distortion recognition, while DDB1 and DDB2 (XPE) can also recognize some types of damage caused by UV light. Additionally, XPA performs a function in damage recognition that is as yet poorly defined. Upon identification of a damaged site, subsequent repair proteins are then recruited to the damaged DNA to verify presence of DNA damage, excise the damaged DNA surrounding the lesion then fill in the repair patch.

GG-NER associated diseases Mutations in GG-NER machinery are responsible for multiple genetic disorders including:

Xeroderma pigmentosum (XP): severe photosensitivity, high cancer rates in areas of the body exposed to the sun (e.g. skin)

Transcription coupled repair (TC-NER)

At any given time, most of the genome in an organism is not undergoing transcription; there is a difference in NER efficiency between transcriptionally silent and transcriptionally active regions of the genome. For many types of lesions, NER repairs the transcribed strands of transcriptionally active genes faster than it repairs nontranscribed strands and transcriptionally silent DNA. TC-NER and GG-NER differ only in the initial steps of DNA damage recognition. The principal difference between TC-NER and GG-NER is that TC-NER does not require XPC or DDB proteins for distortion recognition in mammalian cells. Instead TC-NER initiates when RNA polymerase stalls at a lesion in DNA: the blocked RNA polymerase serves as a damage recognition signal, which replaces the need for the distortion recognition properties of the XPC-RAD23B and DDB complexes. CS proteins (CSA and CSB) bind some types of DNA damage instead of XPC-Rad23B. Other repair mechanisms are possible but less accurate and efficient.

TC-NER associated diseases TC-NER initiates when RNA polymerase stalls at a lesion in DNA, whereupon protein complexes help move the polymerase backwards. Mutations in TC-NER machinery are responsible for multiple genetic disorders including:

Trichothiodystrophy (TTD): some individuals are photosensitive, ichthyosis, mental/physical retardation Cockayne syndrome (CS): photosensitivity, intellectual disability, progeria-like features, microcephaly

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