Jordan A. Mays

Key Points: 8 XP family proteins repair DNA damaged by sunlight through Nucleotide Excision Repair. XPF-ERCC1 complex is a structure-specific endonuclease (incises the 5’ end of DNA damage) Without XPF, mutations accumulate in cells exposed to UV radiation. The mislocalization of XPF (usually localized in the nucleus) leads to skin that is sensitive to sunlight and more susceptible to skin cancer.

XPF is one of eight complementary XP proteins which carry out Nucleotide Excision Repair (NER) following the formation of UV-induced thymine dimers

XPF-/- XPA-/- Sensitive to UV Resistant to UV damage Cell fusion experiments identified each of the 8 XP proteins which complement each other to perform NER. Sensitive to UV

The Process of NER XP proteins are recruited to the DNA damage site to form a recognition intermediate TFIIH unwinds the damage DNA, allowing the binding of more NER proteins XPF-ERCC1 and XPG make incisions at each end of the damage New DNA is synthesized and ligated

Molecular Level XPF-ERCC1 complex forms a highly conserved, structure-specific endonuclease  XPF contains the nuclease catalytic domain  ERCC1 recruits the complex to NER proteins and interacts with XPA at the DNA damage site The nuclease domain makes an incision at the 5’ end of the damaged DNA segment

Cellular & Organismal Level The XPF-ERCC1 complex is normally localized in the nucleus to quickly bind to proteins recruited to sites of DNA damage. The XP protein-mediated process of NER is essential to prevent the accumulation of mutations on genes which are necessary for regulation. -The accumulation of mutations on proto- oncogenes or tumor suppressor genes can cause cellular transformation.

Incidence: 1 out of 250,000 Less than 40% of patients live beyond 20 years of age. Symptoms:  extreme sensitivity to sunlight of skin/eyes causing: Extreme sunburns Keratoses Freckles Blisters Dry/Scaly skin  1000x increased risk of skin cancers, with earlier onsets  18% develop neurodegeneration Xeroderma Pigmentosum

Catalytic activity of XPF mutants remains unchanged Mutations of XPF are autosomal recessive: Heterozygotes have WT phenotype; Homozygotes have abnormal skin sensitivity and increased rates of skin cancer

Differential Immunofluorescence: Mutant XPF found to be cytoplasmic or pancellular. WT only localized in nucleus Mutations in XPF are found to cause a mislocalization of the protein. Nuclear XPF is needed to perform its endonuclease function using damaged DNA substrate.

Treatment Prevention includes completely avoiding exposure to sunlight. No current cure for Xeroderma Pigmentosum:  protein therapy: currently undergoing clinical trials using lotion infused with WT XP proteins mutated in the patient  gene therapy: introduction of a normal gene not yet attempted Topical fluorouracil can treat keratoses and some basal cell carcinomas -chemotherapy agent -pyrimidine analogue promotes apoptosis after incorporation into replicated DNA of cancer cells

References Ahmad A, Enzlin JH, Bhagwat NR, Wijgers N, Raams A, et al. (2010) Mislocalization of XPF- ERCC1 Nuclease Contributes to Reduced DNA Repair in XP-F Patients. PLoS Genet 6(3): e doi: /journal.pgen Das, D., Folkers, G. E., van Dijk, M., Nicolaas, G. J., Hoeijmakers, J., Kaptein, R., & Boelens, R. (2012). The Structure of the XPF-ssDNA Complex Underscores the Distinct Roles of the XPF and ERCC1 Helix-Hairpin-Helix Domains in ss/ds DNA Recognition. Structure, 20(4), 667– 675. Cleaver, J. E. (2005). Cancer in xeroderma pigmentosum and related disorders of DNA repair. Nature Reviews, 5, 564–573. Shell, S. M., & Chazin, W. J. (2012). XPF-ERCC1: On the bubble. Structure, 20(4), 566–568. Su, Y., Orelli, B., Madireddy, A., Niedernhofer, L. J., & Sharer, O. D. (2012). Multiple DNA binding domains mediate the function of ERCC1-XPF in nucleotide excision repair.Journal of Biological Chemistry, 1–13.