More than 24
mutations in the XPD/ERCC2 gene have been identified in people
with Xeroderma Pigmentosum (XP). In USA, mutations in XPD gene are the second
most common cause of XP.
The XPD/ERCC2 gene mutations prevent the
TFIIH complex from repairing damaged DNA effectively. As a result,
abnormalities accumulate in DNA, causing cells to malfunction and eventually the
cells become cancerous or die. These problems with DNA repair cause people with
XP to be extremely sensitive to UV rays from sunlight. When UV rays damage
genes which control cell growth and division, cells can grow too fast and in an
uncontrolled way. As a result, people with XP have a greatly increased risk of
developing cancer. These cancers occur most frequently in areas of the body exposed
to the sun such as the skin and eyes.
When XP is caused by XPD/ERCC2 gene mutations, it is
often associated with progressive neurological abnormalities such as hearing
loss, poor coordination, difficulty walking, movement problems, loss of
intellectual function, difficulty swallowing & talking, seizures etc. The
neurological abnormalities are thought to result from a buildup of DNA damage,
although the brain is not exposed to UV rays. Researchers suspect that other
factors damage DNA in nerve cells. It is unclear why some people with XP
develop neurological abnormalities while others do not.
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