Discordant retinoblastoma in monozygotic twins.

Abstract

We report cases of discordant retinoblastoma in twins confirmed to be monozygotic by DNA analysis. Twin A demonstrated severe, bilateral, multifocal retinoblastoma, which was recalcitrant to external beam irradiation and chemoreduction. Twin B has not demonstrated retinoblastoma. DNA analysis was performed with polymorphic microsatellite markers to confirm monozygosity. Single-stranded conformation polymorphism and Southern blot analysis of the retinoblastoma gene were performed. Molecular genetic analyses confirmed monozygosity but failed to disclose a retinoblastoma gene mutation in either twin. The extreme phenotypic discordance may best be explained by an unidentified, postzygotic retinoblastoma gene mutation in early embryonic development of the affected twin.