xeroderma pigmentosum(xp) 着色性干皮病. we owe our lives to light from the sun,which provides...
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Xeroderma Pigmentosum(XP)
着色性干皮病
• We owe our lives to light from the sun,which provides the energy captured during photosynthesis ( 光合作用 ).
• But the sun also emits a constant stream of ultraviolet rays that ages and mutates the cells of our skin.
• The hazardous effects of the sun are most dramatically illustrated by the rare recessive genetic disorder , Xeroderma Pigmentosum(XP) .
Patients with XP possess a dificient repair system that cannot remove segments of DNA damaged by ultraviolet( 紫外线 ) radiation.
• As a result ,person with XP are extremely sensitive to sunlight
• Even very limited exposure to the direct rays of the sun can produce large numbers of dark-pigmented spots on exposed areas of the body and a greatly elevated risk of developing disfiguring and fatal skin cancers.
The mechanism about XP------nucleotide excision repair deficiency( 核苷酸切除修复缺陷 )
• When subjected to ultraviolet radiation ,adjacent( 相邻的 ) pyrimidines( 嘧啶 ) on a DNA strand have a tendency to interact with one another to form a covalent( 共价的 ) dimer complex.(example as TT--- 胸腺嘧啶二具体 )
• Once TT is formed, ultraviolet radiated DNA will not be repaired correctly.
• The replication and the transcription of DNA will be influenced seriously.
• THF II H--- a crucial link between transcription and DNA repair
• XPB and XPD are two subunits of THF II H
nucleotide excision repair
• In persons with XP who carry specific mutations in the XPD gene.
• XPD gene encodes a subunit of the TFII H required for transcription initiation( 起始 )
• So, Mutations in XPD could lead to defect( 缺陷 ) in both DNA repair and transcription
How to help XP patients?• Some help for XP patients may be on the way in the f
orm of skin creams that contain DNA repair enzymes.
• The enzyme are contained in liposomes( 脂质体 ) that can apparently penetrate ( 穿过 ) the outer layer of the skin and participate in repair pathways
• XP is a very rare condition ,but colon cancer( 结肠癌 ) is relatively common.
• It is estimated that up to 15 percent of colon cancer cases can be attributed to mutations in the genes that encode the proteins required for mismatch repair.
• Mutations that cripple the mismatch repair system inevitably lead to higher mutation rate in other genes because mistakes made during replication are not corrected.