The retinoblastoma susceptibility gene, RB1 (Genbank sequence accession number L11910), is located on chromosome 13q and is a tumour suppressor gene. Mutations in both copies (alleles) of the RB1 gene are necessary for the development of retinoblastoma.
In about 55% of affected children, both mutations occur in a single retinal cell by chance and cannot be passed to offspring. These children have unilateral (one eye), unifocal (one tumour) disease. In the remaining patients one ‘predisposing’ mutation is present in germ line cells (sperm or ova) and can be transmitted to offspring. This is genetic/hereditary retinoblastoma which generally affects both eyes (bilateral disease) when inherited. Somatic and/or germ line mosaicism, where some but not all cells contain the predisposing mutation, is observed in some founders of new mutation lineages.
In the sporadic (no family history) unilateral form of retinoblastoma, it is not possible to know without genetic testing whether the disease will be heritable unless the patient has affected offspring. It is estimated that around 15% of such sporadic cases do carry germ line mutations. Early identification of these cases requires genetic testing which is most efficiently done using tumour tissue from an enucleated eye.
The spectrum of predisposing RB1 gene mutations includes large structural changes (about 10-20%) some of which are also detectable by cytogenetic analysis. Other changes include single base substitutions (about 50-60% of mutations) and small insertions/deletions (about 30%). 60-70% of tumours exhibit loss of heterozygosity (loss of one copy/allele) with a mutation in the remaining copy. Finally, hypermethylation at the RB1 gene promoter, which inhibits the levels of retinoblastoma protein made by the cell rather than its function, is observed in around 10% of tumours.
Variable penetrance (the probability that a specific mutation will lead to cancer development) and expressivity (the number of tumours occurring) is a feature of retinoblastoma. Much of this variation is due to the different types of RB1 mutation. Mutations that result in premature termination (truncated protein) are most often associated with almost complete penetrance and bilateral retinoblastoma. However, milder disease with incomplete penetrance and reduced expressivity is usually found in families with missense mutations, some splice site changes, small in-frame deletions and promoter region mutations.
The risk of other members of the family being affected depends upon whether or not the affected child (proband) has a germline RB1 mutation. Table 1 below gives the chances of a germline mutation being present in a retinoblastoma patient based on family history and tumour presentation.
Bilateral retinoblastoma is considered to be hereditary. Whether a sporadic unilateral retinoblastoma is heritable can only be known by identification of both RB1 mutations in the retinoblastoma tumour. A blood sample must then be examined for the same pair of mutations. Finding one of the candidate mutations in blood indicates heritable retinoblastoma. The absence of RB1 mutations in blood suggests non-heritable retinoblastoma, although a small risk of mosaicism remains where the mutation is present in too few blood cells to be reliably detected.
Once a heritable mutation is identified, molecular testing can determine which members of a family are at risk of developing retinoblastoma. Children who do not carry the family's RB1 mutation are not at increased risk. Children who do carry the mutation can be examined frequently for tumours and treated early if any are detected. The children of germ line mutation carriers have a 50% risk of inheriting the parent’s RB1 mutation (lower if the parent is mosaic) and developing retinoblastoma. However, in certain families with incompletely penetrant RB1 mutations the risk of developing retinoblastoma is lower.
In cases where a mutation can not be identified, linkage analysis using polymorphic markers within and adjacent to the RB1 gene can be used (exclusion testing) to modify the risk of other family members. Empirical recurrence risk estimates can be used in cases where molecular testing (mutation or linkage) is not possible or informative - see Table 2, below.
Table 2. Empirical risk for retinoblastoma development in family members in the absence of RB1 mutation information
* If there is no unaffected sibling (Draper G.J et al 1992. British Journal of Cancer 66: 211-219)