RB1 gene: mechanisms of unconventional tumour predisposition

Since its first discovery, a multitude of studies have focused on RB1 as a tumour suppressor gene. Despite the wide literature on the topic, some mechanisms of tumorigenesis remain to be clarified. RB1 mutations have been primary correlated to hereditary retinoblastoma (RB) predisposition, a dominant inherited disease with high penetrance. However, further studies led to the observation of pedigrees characterized by low penetrance (healthy “carriers”) or variable expressivity (individuals with benign retinoma) that deviate from the previous described model. Another important question is whether RB1 can be the predisposing gene for other cancers besides retinoblastoma since there are few studies investigating this topic. The aim of this thesis was to study these mechanisms of “unconventional” tumorigenesis mediated by RB1. The first part of the project focused on the characterization of the molecular mechanisms underlying low penetrance and variable phenotypic presentation of RB. These studies led to identify somatic mosaicism of amorphic variants and parent-of-origin effect of hypomorphic variants as the main mechanisms explaining these phenomena. Subsequently, we focused on the epimutation as initiating event in RB, possibly explaining other cases of low penetrance. Methylation analysis of RB1 promoter performed in a subset of mutation-negative cases allowed to find aberrant methylation leading to allele silencing in a unilateral patient, with important implications in genetic counselling. The last part of the project pinpointed on RB1 as a predisposing gene for other neoplasms. In particular, the discovery of an RB1 variant with splicing effect in a patient carrying a malignant ovarian germ cell tumour led us to focus on this rare type of cancer, of whom little is known about the genetic landscape. In conclusion, this work i) provided further evidence supporting somatic mosaicism and “parent-of-origin effect” as mechanisms explaining phenotypic variability in RB, ii) reported for the first time an epimutation acting as the first hit in knudson’s model of tumorigenesis and iii) opened for the study of RB1 mutational state in a rare group of cancers, namely ovarian germ cell tumours.