A 5-month-old girl has bilateral retinoblastoma. Neither parent has a history of having had retinoblastoma. Chromosomal analysis of the patient’s stimulated peripheral blood lymphocytes is done; the photograph is of a representative karyotype. Which of the following critical events has most likely resulted from an aberration involving chromosome 13?
A. Proto-oncogene activation
B. Proto-oncogene amplification
C. Proto-oncogene loss
D. Tumor-suppressor gene activation
E. Tumor-suppressor gene loss
Correct answer : E. Tumor-suppressor gene loss
Karyotype shows loss of a part of 13th chromosome. RB gene is located in chromosome 13q. It is a tumour supressor gene. Both copies of RB gene must be lost / inactivated for retinoblastoma to develop. (Two hit hypothesis). One copy of RB gene was lost due to loss of a part of 13th chromosome. Some mutation may have inactivated the remaining copy. This is the reason for bilateral retinoblastoma in the child.