Xeroderma Pigmentosum: The moon child
Xeroderma Pigmentosum (XP) is a rare, autosomal-recessive, hereditary skin disease. It is caused by defective DNA repair mechanisms (i.e., nucleotide excision repair). The affected individuals get skin ulcers and skin burns with minimal UV radiation (Sunlight contains UV Radiation so they can’t tolerate sunlight.
The UV light causes carcinogenic effects. XP was first described in 1874 by dermatologist Moritz Kaposi and he coined the term "xeroderma" in reference to the dry or xerotic skin quality of its sufferers. Incidences are higher in consanguineous marriages because it is an autosomal recessive disease. Individuals with the disease have been referred to as "children of the night" or "moon children".
Pathogenesis
Defective Nucleotide Excision Repair Defect → defective DNA damage repair (UV light from Sunlight Damages DNA and causes Thymine dimerization) → increased cell injury → skin ulcers and skin pigmentations
Increased DNA damage → increased Mutations → increased cancers
Clinical findings
The disease onset – early childhood
- Extreme sensitivity to sunlight with pigmentary changes
- corneal ulcers
- sunburn
- Development of multiple lesions actinic keratosis and eventually malignant skin cancer (basal cell carcinoma, squamous cell carcinoma, melanoma).
Management
- Protection from light and avoidance of sun exposure
- Sunscreen and protective clothing
- Frequent skin cancer screening
Prognosis: severely limited life expectancy (Average life expectancy Nearly 20 years)
A 5-year-old male born from a consanguineous marriage presents with severe sunburns and freckling in sun-exposed areas. The mother explains that he experiences these sunburns every time when he goes outside despite applying copious amounts of sunscreen. Which of the following DNA repair mechanisms is defective in this child?
A. Nucleotide excision repair
B. Base excision repair
C. Mismatch repair
D. Homologous recombination
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