XPF
& ITS ROLE IN
XERODERMA
PIGMENTOSUM
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-/-
XPF -/-
Sensitive to UV
XPA-/-
XPA-/-
Sensitive to UV
XPF -/-
XPA-/-
Resistant to UV
damage
Cell fusion experiments identified each of the 8 XP
proteins which complement each other to perform NER.
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 protooncogenes or tumor suppressor genes can cause
cellular transformation.
Xeroderma Pigmentosum
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
Mutations of XPF are autosomal recessive:
Heterozygotes have WT phenotype;
Homozygotes have abnormal skin sensitivity and
increased rates of skin cancer
Catalytic activity of
XPF mutants
remains unchanged
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 XPFERCC1 Nuclease Contributes to Reduced DNA Repair in XP-F Patients. PLoS
Genet 6(3): e1000871.doi:10.1371/journal.pgen.1000871
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.