Analysis of pathogenic variants in retinoblastoma reveals a potential gain of function mutation

Retinoblastoma (Rb1) is a gene that codes for a tumour suppressor protein involved in various types of cancer. It was first described in retinoblastoma and is segregated as an autosomal dominant trait with high penetrance. In 1971, Knudson proposed his hypothesis of the two hits, where two mutational events are required to initiate tumour progression. We analysed three different point mutations present in patients' retinoblastoma. We produced three cell lines with retinoblastoma protein (RB) mutated in various regions: the missense pN328H, pD718N, and the nonsense early stop codon pR552*. We studied the effect of these point mutations on levels of mRNA and protein expression, proliferation, viability, localisation, and migration using an RBKO cell line. All three affected their localisation patterns and proliferation. However, the pR552* mutation also increases viability and migration. Moreover, when this mutation is simultaneously expressed with a wild-type RB, the phenotype and proliferation parameters are as with the mutant alone, suggesting that maybe only one mutated allele is needed to trigger the characteristic cancer phenotype. In other words, the pR552* mutant behaves more like a gain-of-function or oncogenic mutant. Indeed, a family carrying this mutation showed complete penetrance and high expressivity.