Retinoblastoma

Sixty percent of retinoblastomas are sporadic and unilateral,
with 40% being hereditary and usually bilateral. Hereditary
retinoblastomas follow an autosomal dominant pattern of
inheritance with incomplete penetrance. About 80–90% of
children inheriting the abnormal gene will develop
retinoblastomas. Molecular studies indicate that two events are
involved in the development of the tumour, consistent with
Knudson’s original “two hit” hypothesis. In bilateral tumours
the first mutation is inherited and the second is a somatic event
with a likelihood of occurrence of almost 100% in retinal cells.
In unilateral tumours both events probably represent new
somatic mutations. The retinoblastoma gene is therefore acting
recessively as a tumour suppressor gene.