Which of the following statements about xeroderma pigmentosum (XP) is FALSE? Mut
ID: 79367 • Letter: W
Question
Which of the following statements about xeroderma pigmentosum (XP) is FALSE?
Mutations in one of several genes can contribute to the XP phenotype.
Cells from XP patients are deficient in unscheduled DNA synthesis.
The XP phenotype is caused by defects in double-stranded break repair pathways.
XP patients are extremely sensitive to UV radiation in sunlight.
a.Mutations in one of several genes can contribute to the XP phenotype.
b.Cells from XP patients are deficient in unscheduled DNA synthesis.
c.The XP phenotype is caused by defects in double-stranded break repair pathways.
d.XP patients are extremely sensitive to UV radiation in sunlight.
Explanation / Answer
B. Cells from XP patients are deficient in unscheduled DNA synthesis.
Xeroderma pigmentosum (XP) is a rare autosomal recessive genetic disorder of DNA repair in which the ability to repair damage caused by ultraviolet (UV) light is deficient. In extreme cases, all exposure to sunlight must be forbidden, no matter how small; as such, individuals with the disease are often colloquially referred to as "Moon child". Multiple basal cell carcinomas (basaliomas) and other skin malignancies frequently occur at a young age in those with XP; metastatic malignant melanoma and squamous cell carcinoma are the two most common causes of death in XP victims. This disease is present in both genders and in all races, with an incidence of 1:250,000 in the United States. XP is roughly six times more common in Japanese people than in other groups.
Normally, damage to DNA in epidermal cells occurs during exposure to UV light. The absorption of the high-energy light leads to the formation of pyrimidine dimers, namely cyclobutane-pyrimidine dimers and pyrimidine-6-4-pyrimidone photoproducts. In a healthy, normal human being, the damage is first excised by endonucleases. DNA polymerase then repairs the missing sequence, and ligase "seals" the transaction. This process is known as nucleotide excision repair.
The XPA protein acts during NER as a scaffold for assembly of other DNA repair proteins at sites of DNA damage to ensure appropriate excision of the damage.
The XPB(ERCC3) protein is employed in unwinding the DNA double helix after DNA damage is initially recognized. Mutations in the XPB(ERCC3) gene can lead to XP or XP combined with Cockayne syndrome.
The XPC protein forms a complex with RAD23B protein to form the initial damage recognition factor in global genomic nucleotide excision repair (GG-NER).This complex recognizes a wide variety of damages that thermodynamically destabilize DNA duplexes.
The XPD(ERCC2) protein, in combination with the XPB helicase-containing transcription/repair complex TFIIH, is employed in unwinding the DNA duplex after damage is initially recognized. Mutations in the XPD(ERCC2) gene cause a variety of syndromes; XP, trichothiodystrophy (TTD), or a combination of XP and Cockayne syndrome (XPCS). Both trichothiodystrophy and Cockayne syndrome display features of premature aging, suggesting an association between deficient DNA repair and premature aging (see DNA damage theory of aging).
XPE is a heterodimeric protein composed of two subunits. The larger subunit DDB1 primarily functions as a core component of CUL4A- and CUL4B-based E3 ubiquitin ligase complexes. Substrates that are ubiquitinnated by these complexes include proteins employed in DNA repair.
The XPF(ERCC4) protein together with the ERCC1 protein forms a complex usually designated ERCC1-XPF. This complex separates the DNA helix for a short distance on either side of the site of damage. It then acts as a endonuclease to incise the damaged DNA strand on the 5’ side of the damaged site. Mutant cells with deficient ERCC1-XPF are not only defective in NER, but also in the repair of double-strand breaks and inter-strand crosslinks.
The XPG protein is an endonuclease that incises DNA during NER at the 3’ side of the damaged nucleotide. Mutations in the XPG(ERCC5) gene can lead to XP alone, or in combination with Cockayne syndrome (CS), or in combination with infantile lethal cerebro-oculo-facio-skeletal syndrome.
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