3.3 Retinoblastoma - a rare childhood eye tumor
A rare childhood eye tumor, Retinoblastoma.
This tumor of the retina arising in the precursors of photoreceptor cells, is normally observed in about 1 in 20,000 children
The tumor syndrome appears in two forms.
1. unilateral retinoblastoma
Some children—those who are born into families with no history of retinoblastoma—present with a single tumor in one eye or the other.
If this tumor is eliminated, either by radiation or by removal of the affected eye, then this child has no further risk of retinoblastoma and no risk of tumors elsewhere in the body.
This is a sporadic form of this disease.
Since this form of the disease affects only a single eye, it is often termed unilateral retinoblastoma.
2. bilateral retinoblastoma
The familial form of retinoblastoma appears in children having a parent who also suffered from the disease, there are usually multiple foci of tumors arising in both eyes
Curing the eye tumors does not protect these children from a greatly increased risk of other cancers.
Those who survive these tumors - in half of their offspring, the familial form of retinoblastoma again rears its head
Alfred Knudson concluded in 1971
The rate of appearance of familial tumors was consistent with a single random event
The sporadic tumors behaved as if two random events were required for their formation
If the tumor-predisposing alleles of Rb are recessive, then both copies of the Rb gene must be knocked (unlikely through mutation?)
A solution to this dilemma
Suppose that the first Rb gene was inactivated by mutation 1
10-6 per cell generation. This heterozygous cell would continue to exhibit a wild-type phenotype.
The second, intact Rb is inactivated by an non-mutational event?
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