Retinoblastoma is almost always caused by a mutation in both copies of the tumor suppressor gene RB1, located on the long arm of chromosome 13 at locus 14 (13q14); much less commonly, it is due to a deletion of part of this chromosomal arm. RB1 was the first tumor suppressor gene to be identified. The wild-type protein product of RB1, pRB, serves as a cell-cycle regulator by inhibiting cell-cycle progression; thus, without a functional protein the cell cycle is unregulated. Both copies of RB1 must be mutated (ie, “two hits”) for retinoblastoma to occur, and additional mutations may be needed to promote tumorigenesis. Retinal cells are not terminally differentiated at birth; therefore, mutations in RB1 can still occur in retinal cells over the first few years of life as the retina matures, resulting in tumor formation. Mutations in RB1 and dysfunction of pRB can also be found in many different tumor types throughout the body because of their role in cell-cycle regulation.
Retinoblastoma may be inherited (10% of cases) or sporadic, owing to a new mutation (90%). Children with sporadic disease may have a solitary tumor related to a somatic event within a single retinal cell or may harbor a new spontaneous germline mutation, which can be inherited by their future children. Of the patients with sporadic disease, two-thirds have nonhereditary mutations in which both RB1 mutations occur as a somatic event in a single retinal cell. The remaining one-third of patients have a new germline mutation present in all retinal cells or a mutation that occurred during embryogenesis in which the first RB1 mutation is present in many but not all retinal cells (mosaicism). Thus, although these patients may have a family history of retinoblastoma (10% of them), it is more likely that the disease developed from a new sporadic mutation in a parental germ cell or in the developing embryo. These patients have a risk of passing a mutated copy of RB1 to their offspring.
Patients with germline mutations in RB1 often have bilateral or multifocal disease and an earlier presentation than do those with non-germline mutations. In addition, approximately 15% of patients with unilateral disease also have a germline mutation. Unless there are multiple tumors in the affected eye, clinical examination findings cannot distinguish patients with unilateral disease caused by germline versus non-germline mutations. Thus, it is crucial for children with retinoblastoma to undergo molecular testing for a germline RB1 mutation.
Analysis of RB1 mutations is done on DNA from peripheral leukocytes. Methods used for this analysis include next-generation sequencing, Sanger sequencing, karyotyping, fluorescence in situ hybridization (FISH), multiplex ligation-dependent probe amplification (MLPA), and RNA analysis. With these screening methods, there is a 96% chance of finding a tumor mutation in the peripheral blood, if one exists. This detection rate is higher when both blood and freshly harvested tumor are available for tumor-directed mutational analysis, as the results of each cell type can be compared in order to identify clinically relevant mutations.
In children younger than 6 months, 1%–3% of unilateral retinoblastomas are caused by amplification of the N-myc proto-oncogene (MYCN) and not by mutation in RB1. On clinical examination of a young child, it can be difficult to differentiate tumors driven by MYCN amplification from advanced unilateral disease owing to RB1 mutation; enucleation is recommended if MYCN is the suspected etiology. When enucleation is performed, distinct histologic features of MYCN-associated tumors can be observed, including undifferentiated cells with prominent nucleoli and little calcification. These cellular features are similar to those seen in other MYCN-amplified tumors such as neuroblastoma. Tumors driven by MYCN amplification appear to be nonheritable. The exact mechanism of the amplification is still the subject of study.
Counseling with a genetic specialist is recommended for all families affected by retinoblastoma, and it is reasonable to examine parents and siblings of the patient for evidence of untreated retinoblastoma or retinocytoma, which would represent a hereditary predisposition to the disease. Genetic counseling for retinoblastoma can be complex. Owing to 90% penetrance of the disease-causing allele, a survivor of bilateral retinoblastoma has a 45% chance of having an affected child, whereas a survivor of unilateral disease (with a germline mutation) has a 7%–15% likelihood of having an affected child. Unaffected parents of a child with bilateral retinoblastoma have less than a 5% risk of having another child with retinoblastoma. If 2 or more siblings are affected, there is a 45% chance that another child will be affected, as this represents hereditary disease. Systematic screening for retinoblastoma by an ophthalmologist is recommended for children in families with retinoblastoma. In 2018, the first national guidelines for retinoblastoma screening in the United States were published (see the Skalet et al reference); these recommendations addressed practices and frequencies for screening for children at various risk levels. See also BCSC Section 6, Pediatric Ophthalmology and Strabismus, Table 25-8.
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Skalet AH, Gombos DS, Gallie BL, et al. Screening children at risk for retinoblastoma: consensus report from the American Association of Ophthalmic Oncologists and Pathologists. Ophthalmology. 2018;125(3):453–458.
Thériault BL, Dimaras H, Gallie BL, Corson TW. The genomic landscape of retinoblastoma: a review. Clin Exp Ophthalmol. 2014;42(1):33–52.
Excerpted from BCSC 2020-2021 series: Section 4 - Ophthalmic Pathology and Intraocular Tumors. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.