Retinoblastoma Genetics

Retinoblastoma (RB) is the prototypical hereditary cancer in humans — understanding which cases are heritable versus sporadic directly changes surveillance, family counseling, and screening for siblings and future children.

The RB1 gene and Knudson's two-hit model

RB arises from biallelic inactivation of the RB1 tumor suppressor gene (chromosome 13q14.2), following the classic two-hit model:

• Heritable RB (30-40% of cases): the first RB1 mutation (M1) is present in the germline — inherited from a parent or arising de novo — and is present in every cell of the body. A second, somatic mutation (M2) in a retinal cell then triggers tumor formation
• Sporadic RB (60-70% of cases): both mutations occur somatically, confined to the retinal tumor cells only — not present in the germline, and therefore not transmissible

Germline RB1 mutations confer over 90% penetrance for intraocular disease, inherited in an autosomal dominant pattern.

Clinical clues to heritable disease

• Bilateral RB is presumed hereditary essentially by definition
• Unilateral RB: roughly 15% still carry a germline mutation even without bilateral disease or family history — this is why germline genetic testing is now recommended for all children with RB, unilateral or bilateral, not just bilateral cases
• True multifocal disease within one eye should raise suspicion for hereditary disease (though tumor seeding can sometimes mimic true multifocality)
• Family history of RB or retinoma in a relative

Why genetic testing matters beyond the eye

• Children with hereditary RB are at risk for a midline intracranial tumor, most commonly pineoblastoma — warranting neuroimaging surveillance
• Long-term, survivors of hereditary RB carry an elevated lifetime risk of second primary cancers (including osteosarcoma and other sarcomas) — roughly 20% risk of a second primary cancer over their lifetime
• Genetic counseling for reproductive planning — families with a known RB1 mutation can consider prenatal or preimplantation genetic testing

Current surveillance recommendations for at-risk infants

For infants known or suspected to carry a germline RB1 mutation (e.g., a sibling of an affected child, pending their own testing):

Screening intensity is stratified by documented genetic risk (high/intermediate/low/general population risk based on germline testing results) — a child with a confirmed negative germline test in a known-mutation family can often be safely stepped down to general population screening, avoiding unnecessary repeated sedated exams.

A genuinely current point: genetic testing access remains uneven globally

Despite clear consensus that germline testing should now be offered to all RB patients regardless of laterality, testing availability remains sparse in many low-resource settings — an important real-world limitation to keep in mind when counseling families where comprehensive genetic testing may not be locally accessible.

Key references: Update on Retinoblastoma Predisposition and Surveillance Recommendations for Children. Clin Cancer Res. 2025;31(9):1573.  |  Genetics of Retinoblastoma: An Overview and Significance of Genetic Testing in Clinical Practice. 2025.  |  American Cancer Society, Hereditary Retinoblastoma (RB1).