Question

A 6-year-old boy presents to his pediatrician with skin lesions all over his body. For several years he has been very sensitive to sunlight. Neither the boy’s parents nor his siblings have the same skin lesions or sun sensitivity. Biop- sies of several of the boy’s lesions reveal squa- mous cell carcinoma. Which mutation would one expect to see in this patient’s DNA?

1. (A)  Methylation of the gene

2. (B)  Missense mutation in the gene

3. (C)  Nonsense mutation in the middle of the

   gene

4. (D)  Point mutation within the enhancer region

5. (E)  Point mutation within the operator region

6. (F)  Point mutation within the promoter region

7. (G)  Thymidine dimers

Answer 1

The correct answer is G. This patient has xe- roderma pigmentosa, an autosomal recessive disease characterized by a defect in excision repair. This disease results in an inability to repair thymidine dimers that can form in the presence of ultraviolet light. This can lead to the development of skin cancer and photosen- sitivity.

Answer A is incorrect. Methylation of a partic- ular gene does not cause xeroderma pigmento- sum.

Answer B is incorrect. A missense mutation does not cause xeroderma pigmentosum.

Answer C is incorrect. A nonsense mutation does not cause xeroderma pigmentosum.

Answer D is incorrect. A mutation in the en- hancer region of a gene does not cause xero- derma pigmentosum.

Answer E is incorrect. A mutation in the oper- ator region of a gene does not cause xeroderma pigmentosum.

Answer F is incorrect. A mutation in the pro- moter region of a gene does not cause xero- derma pigmentosum.