Study of the relationship genotype/phenotype in retinoblastoma
Retinoblastoma is the most common intraocular childhood cancer and occurs when both alleles of the RB1 gene are inactivated in the retina. In patients without genetic predisposition, the two mutations occurred in a single unique retinal cell. In subjects with a genetic predisposition to retinoblastoma, the first RB1 mutation is found in the germline and the second appears as a somatic mutation. Germline carriers usually develop bilateral or multifocal tumors and the diagnosis is earlier. However, some rare families exhibit low penetrance and variable expressivity of the disease because bilaterally affected, unilaterally affected, and unaffected mutation carriers are known to coexist. The existence of genetic modifiers in retinoblastoma therefore appears highly probable and must be considered. The lack of penetrance and the variable expressivity could be the sum of three independent causes. The presence of a mosaic can affect the phenotype, the nature of the mutation can drive low penetrance and particular phenotype like psychomotor delay in case of large genomic deletions and genetic modifier factors could modulate the phenotype. These three major keys have been studied in order to highlight the relations between the phenotype and the genotype. Firstly, the consequences of mosaicism have been illustrated by a prenatal diagnosis concerning a couple with a bilateral retinoblastoma-affected male patient who requested five successive prenatal diagnoses and in whom RB1 mutation mosaicism had important implications. Implications of mosaicism in genetic counseling have been discussed and taken into consideration in order to limit bias in the two following genotype/phenotype studies. Secondly, the association between whole germline monoallelic deletions of the RB1 gene and psychomotor delay was studied by a high-resolution CGH array focusing on RB1 and its flanking region. Comparative analysis detected a four megabase critical interval including a candidate gene, protocadherin 8 (PCDH8). PCDH8 is thought to function in signaling pathways and cell adhesion in a central nervous system-specific manner, making loss of PCDH8 one of the probable causes of psychomotor delay in RB1-deleted patients. Thirdly, a candidate gene approach based on partners that are necessary for the development of the tumor attempted to find possible genetic modifiers. MDM2, which increases p53 and pRB catabolism, was therefore a prominent candidate. The minor allele of MDM2 that includes a 309T>G transversion (SNP rs2279744) in the MDM2 promoter is known to enhance MDM2 expression. In family-based association analyses performed in 70 retinoblastoma families, the MDM2 309G allele was found to be statistically significantly associated with incidence of bilateral or unilateral retinoblastoma among members of retinoblastoma families under a recessive model (Z = 3.305, two-sided exact P = .001). The strong association of this genotype with retinoblastoma development designates MDM2 as the first modifier gene to be identified among retinoblastoma patients