Retinoblastoma is an extremely rare childhood form of cancer of the retina of th

Question

Retinoblastoma is an extremely rare childhood form of cancer of the retina of the eye. In some cases, both eyes are affected (bilateral retinoblastoma) and in others only one eye is affected (unilateral retinoblastoma). Bilateral cases occur almost exclusively in hereditary retinoblastoma. An informative difference between bilateral and unilateral retinoblastoma is age of onset as depicted in the figure below. Here a population of children, all of whom ultimately developed retinoblastoma, is represented. The fraction of the population that was tumor free is plotted against age. The curve for unilateral retinoblastoma has a “shoulder” compared to the linear curve for bilateral retinoblastoma.

(1) Which of the following best explains the age of onset data from the graph?

A. In hereditary retinoblastoma, both eyes are affected so the age of onset is later to allow more time for the cancer to develop.

B. The genome of the eye mutated more rapidly than other cells so hereditary retinoblastoma is observed earlier.

C. Individuals with hereditary retinoblastoma inherit one mutant copy and one normal copy of Rb, which makes it more likely that these individuals will incur a second or third cancer causing mutation earlier than children that do not inherit the mutation.

D. Hereditary retinoblastoma is an inherited mutation in a tumor suppressor, which generally leads to an earlier age of onset than oncogenes.

(2) Which of the following best explains why both eyes are usually affected in retinoblastoma?

A. In hereditary retinoblastoma, the age of onset is later which allows time for the cancer to develop on both eyes.

B. In patients with hereditary retinoblastoma, the genome of eye is the primary target so both eyes are more likely to develop cancer

C. In hereditary retinoblastoma, cells of both eyes as well as all other cells of the body carry the mutation so it is more likely that both eyes will be affected.

D. The eyes are especially sensitive to mutations in tumor suppressors so it is more likely that both will be affected.

Explanation / Answer

Answer:

1. (C) Individuals with hereditary retinoblastoma inherit one mutant copy and one normal copy of Rb, which makes it more likely that these individuals will incur a second or third cancer causing mutation earlier than children that do not inherit the mutation.

2. (C) In hereditary retinoblastoma, cells of both eyes as well as all other cells of the body carry the mutation so it is more likely that both eyes will be affected.

Children born with a mutation in the RB1 gene usually develop retinoblastoma in both eyes (known as bilateral retinoblastoma), and there are often several tumors within the eye (known as multifocal retinoblastoma).

Because all of the cells in the body have the changed RB1 gene, these children also have a higher risk of developing cancers in other areas as well.

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