XERODERMA PIGMENTOSUM

XPF and Nucleotide Excision Repair

Sapna Patel

3-4-04

The Basics• Hereditary, autosomal recessive disorder• Results in a hypersensitivity to UV rays• 1/250,000 have XP in the US

• XP leads to a predisposition to cancer

• Mostly basal and squamous cell carcinomas

• Certain types can lead to neurological disorders

Friedberg, 2001.

What causes XP?• 7 different gene abnormalities identified

that lead to XP

• Complementation groups XPA-XPG

• These genes are crucial for DNA damage repair

Friedberg, 2001.

How genes lead to cancer

UV radiation leads to DNA damage in the form of pyrimidine dimers and photoproducts.

• XP genes respond to damage and carry out NER

• Each complementation group gene has a specific function in NER

Friedberg, 2001.

XPF

• XPF is part of an endonuclease

• Forms a heterodimer complex with ERCC1

• Together, they make 5’ incision ~24 nt away from damage

Friedberg, 2001.

• Mutations in XPF gene prevents removal of DNA damage.

Sijbers et al, 1996

• The damage is transcribed and translated

• If in a gene for controlling cell proliferation then the result is cancer.

• Multiple mutations lead up to cancer.

Xeroderma pigmentosum group F

• Mostly affects Japanese

• Symptoms are milder than in other types of XP, later onset

• Few suffer neurological complications

Matsumura et al, 1998.

Identification of XPF

• ERCC4 had been found to correct NER mutations in hamsters.

• It was hybridized to human chromosome and found on #16.

Identification of XPF• Cosmid clones of

chromosome 16 carrying ERCC4 were mapped using fluorescent hybridization.

• It appeared at 16p13.13-p13.2 as predicted

• Linkage to the neo gene present in vector indicated that the human gene was cloned.

Thompson et al, 1994

• S. cerevisiae has a Rad1-Rad10 complex that makes 5’ incision

• Rad10 was known to be the homolog of ERCC1.

• A polypeptide copurified with ERCC1 and thought to be the Rad1 homolog

Identification of XPF

Identification of XPF

• RT-PCR was used with primers created from Rad1• It was used as a probe to find an ORF on C-terminal similar to that of Rad1.• N-terminal of human gene found in database.• Assembled

Sijbers et al, 1996.

• Using in situ hybridization, they found the RAD1 homolog mapped to 16p13.1-13.2

Identification of XPF

Sijbers et al, 1996.

Homolog encodes ERCC4, XPF

• The homolog was transfected into UV sensitive cells

• 3 types: UV41 (ERCC4), UVS1 ERCC11), and 43-3B (ERCC1)

The Results

• Full correction of UV41

• Partial correction of UVS1

• 43-3B cells did not survive

Sijbers et al, 1996.

What’s next?

• There’s no cure for XP

• Common treatments are excision of carcinomas

• Prevention using sunscreen (physical and chemical)

• Some trial w/ repair enzyme