XPD Helicase
Xeroderma pigmentosum group D (XPD) helicase is a subunit of transcription factor IIH (TFIIH), which aids in transcription initiation and DNA repair. XPD helicse unwinds DNA, allowing other DNA repair enzymes to access and correct damaged regions in the DNA. XPD helicase helps to fix DNA damaged by ultraviolet (UV) light radiation, therefore mutations in XPD helicase results in diseases characterized by light sensitivity.
Function
XPD helicase is an essential subunit of the general transcription factor IIH (TFIIH), which is a complex that, along with other general transcription factors, helps to initiate transcription and repair damaged DNA [1]. XPD helicase helps to stabilize the structure of TFIIH but also plays a functional role in repairing DNA as a helicase enzyme [2]. Helicases are enzymes that unwind double-stranded DNA into single-stranded DNA so that other enzymes, like polymerases, can act upon the DNA [3]. In the context of DNA repair, helicases unwind DNA and other enzymes remove the damaged DNA and replace it with the complementary nucleotides based on the other DNA sequence. When DNA is exposed to UV radiation, adjacent nucleotide bases, often thymines, can react and form bulky pyrimidine dimers, which can block enzymes that work on DNA [4]. For example, during DNA replication, thymine dimers do not fit into the active site of DNA polymerases smoothly, sometimes resulting in mismatched nucleotides. To fix this type of damage on single strands of DNA, cells employ a process called nucleotide excision repair (NER) [2]. This is the type of DNA repair that TFIIH, with the help of the XPD helicase subunit, carries out to remove the damaged DNA.
Breaking the hydrogen bonds that hold the two DNA strands together requires energy, so XPD helicase is dependent on ATP [5]. The ATP-dependent helicase activity of XPD helicase, however is only required for NER, even though TFIIH participates in both repair and transcription initiation [6]. XPD helicase not only unravels the DNA around the damage, but also helps TFIIH in recognizing bulky lesions in DNA [7]. The DNA is then threaded through the central pore of XPD helicase, which then opens up the double helix.
The first step in repairing a mutation is identifying that one has occurred. Just as electrons can travel the length of a wire so can electrons travel the length of a DNA molecule. This phenomenon is known as charge transport. When DNA is exposed to UV light it is possible for certain amino acids, specifically guanine, to become oxidized. Occasionally, DNA charge transport allows for electrons to subsequently travel to the oxidized amino acid and reduce it to its original state. However, if an electron is not available to replace the one lost the DNA molecule will hold a positive charge which can also travel the length of the molecule. It is theorized that the accumulation of unnatural charges in the genome due to mutagens acts as an intracellular signal. The XPD helicase subunit of TFIIH contains a 4Fe-4S cluster that expresses a redox potential when bound to DNA. This functions to coordinate the transcription factor complex to the site of the mutation marked by charge transport.
Disease
The NER pathway consists of 28 genes, three of which are part of TFIIH, and mutations in many of these are associated with a set of diseases that are similar but have marked differences [8]. Mutations in XPD helicase are associated with three distinct diseases: Cockayne Syndrome (CS), Xeroderma Pigmentosum (XP), and trichothiodystrophy (TTD) [9]. The common symptom between these diseases is sensitivity to UV light because of defects in the repair system that fixes mutations caused by UV radiation [2].
CS is characterized by short stature, signs of premature aging, failure to gain weight, impaired development of the nervous system, and photosensitivity [10]. XP is characterized by extreme sensitivity to sunlight and a higher risk of skin cancer. Some XP patients have neurological degeneration, which can be explained by the fact that neurons do not divide, and mutations that are not corrected by NER could accumulate and eventually lead to cell death [8]. TTD is characterized by sparse and brittle hair, pregnancy-induced high blood pressure, intellectual disabilities, a higher risk of recurrent respiratory infections, and photosensitivity [11]. It has been proposed that specific mutations in XPD helicase affect the transcription activities of TFIIH more than its repair activities, resulting in development issues that lead to intellectual disabilities [8]. Interestingly, only XP has been found to be associated with an increased risk of skin cancer; studies are being conducted to determine why some mutations in XPD helicase result in a higher risk of skin cancer and others do not. Different types of mutations in XPD helicase as well as interactions between XPD helicase mutations and defects in other NER proteins can result in these different diseases. Due to the complexity of these interactions, little is known about the molecular basis for the differences in these diseases [8].
Structure
XPD helicase is made up of alpha helices and beta sheets and contains one main domain, a DNA interaction interface. There are two motifs, one of which performs the function of unwinding the DNA strand. The second motif, located at residues 682-695, directs XPD helicase to the nucleus, as it is the nuclear localization signal. Residues 7-283 form the Helicase ATP binding domain(), where ATP itself binds at residues 42-49, (). Interaction with the gene MMS19 is mediated by region consisting of residues 438-637. Residues 234-237 form the motif which is the DEAH box of this transcription factor which is where the unwinding of DNA is performed (). Iron Sulfur bonding residues consist of C116, C134, C155, and C160,(). Features of Cockayne Syndrome and Xeroderma pigmentosum have been associated with the point mutation G602D, and the point mutation L461V is associated with TTD1 [12].
Structural highlights
References
- ↑ Mydlikova Z, Gursky J, Pirsel M. Transcription factor IIH - the protein complex with multiple functions. Neoplasma. 2010;57(4):287-90. PMID:20429618
- ↑ 2.0 2.1 2.2 Fan L, Fuss JO, Cheng QJ, Arvai AS, Hammel M, Roberts VA, Cooper PK, Tainer JA. XPD helicase structures and activities: insights into the cancer and aging phenotypes from XPD mutations. Cell. 2008 May 30;133(5):789-800. PMID:18510924 doi:10.1016/j.cell.2008.04.030
- ↑ Tuteja N, Tuteja R. Unraveling DNA helicases. Motif, structure, mechanism and function. Eur J Biochem. 2004 May;271(10):1849-63. PMID:15128295 doi:http://dx.doi.org/10.1111/j.1432-1033.2004.04094.x
- ↑ Vink AA, Roza L. Biological consequences of cyclobutane pyrimidine dimers. J Photochem Photobiol B. 2001 Dec 31;65(2-3):101-4. PMID:11809365
- ↑ Buechner CN, Heil K, Michels G, Carell T, Kisker C, Tessmer I. Strand-specific recognition of DNA damages by XPD provides insights into nucleotide excision repair substrate versatility. J Biol Chem. 2014 Feb 7;289(6):3613-24. doi: 10.1074/jbc.M113.523001. Epub 2013, Dec 14. PMID:24338567 doi:http://dx.doi.org/10.1074/jbc.M113.523001
- ↑ Kuper J, Braun C, Elias A, Michels G, Sauer F, Schmitt DR, Poterszman A, Egly JM, Kisker C. In TFIIH, XPD helicase is exclusively devoted to DNA repair. PLoS Biol. 2014 Sep 30;12(9):e1001954. doi: 10.1371/journal.pbio.1001954., eCollection 2014 Sep. PMID:25268380 doi:http://dx.doi.org/10.1371/journal.pbio.1001954
- ↑ Constantinescu-Aruxandei D, Petrovic-Stojanovska B, Penedo JC, White MF, Naismith JH. Mechanism of DNA loading by the DNA repair helicase XPD. Nucleic Acids Res. 2016 Feb 20. pii: gkw102. PMID:26896802 doi:http://dx.doi.org/10.1093/nar/gkw102
- ↑ 8.0 8.1 8.2 8.3 Kraemer KH, Patronas NJ, Schiffmann R, Brooks BP, Tamura D, DiGiovanna JJ. Xeroderma pigmentosum, trichothiodystrophy and Cockayne syndrome: a complex genotype-phenotype relationship. Neuroscience. 2007 Apr 14;145(4):1388-96. Epub 2007 Feb 1. PMID:17276014 doi:http://dx.doi.org/10.1016/j.neuroscience.2006.12.020
- ↑ Liu J, Fang H, Chi Z, Wu Z, Wei D, Mo D, Niu K, Balajee AS, Hei TK, Nie L, Zhao Y. XPD localizes in mitochondria and protects the mitochondrial genome from oxidative DNA damage. Nucleic Acids Res. 2015 Jun 23;43(11):5476-88. doi: 10.1093/nar/gkv472. Epub 2015, May 12. PMID:25969448 doi:http://dx.doi.org/10.1093/nar/gkv472
- ↑ Nance MA, Berry SA. Cockayne syndrome: review of 140 cases. Am J Med Genet. 1992 Jan 1;42(1):68-84. PMID:1308368 doi:http://dx.doi.org/10.1002/ajmg.1320420115
- ↑ Hashimoto S, Egly JM. Trichothiodystrophy view from the molecular basis of DNA repair/transcription factor TFIIH. Hum Mol Genet. 2009 Oct 15;18(R2):R224-30. doi: 10.1093/hmg/ddp390. PMID:19808800 doi:http://dx.doi.org/10.1093/hmg/ddp390
- ↑ . UniProt: a hub for protein information. Nucleic Acids Res. 2015 Jan;43(Database issue):D204-12. doi: 10.1093/nar/gku989. , Epub 2014 Oct 27. PMID:25348405 doi:http://dx.doi.org/10.1093/nar/gku989
