A 3-year-old Indian child with a family history of retinoblastoma is found to have bilateral intraocular masses on screening. Which is the most common genetic abnormality in hereditary retinoblastoma?

Explanation

## Most Common Genetic Abnormality in Hereditary Retinoblastoma ### RB1 Gene: The Primary Culprit **Key Point:** RB1 gene mutations account for >90% of all hereditary retinoblastoma cases and approximately 40% of unilateral sporadic cases. ### Genetics of Retinoblastoma | Genetic Feature | RB1 | TP53 | BRCA1 | NF1 | | --- | --- | --- | --- | --- | | **Frequency in hereditary RB** | >90% | <5% (Li-Fraumeni) | Rare | <1% | | **Inheritance pattern** | Autosomal dominant | Autosomal dominant | Autosomal dominant | Autosomal dominant | | **Bilateral involvement** | 40–50% | Unilateral typically | Unilateral | Unilateral | | **Associated malignancies** | Osteosarcoma, soft tissue sarcoma | Multiple cancers (breast, lung, brain, sarcomas) | Breast, ovarian cancer | Optic nerve glioma, pheochromocytoma | | **Penetrance** | ~90% | High | Moderate | High | ### The RB1 Gene: Structure and Function **High-Yield:** The RB1 gene encodes the retinoblastoma protein (pRb), a tumour suppressor that: 1. Binds and inactivates E2F transcription factors 2. Prevents progression from G1 to S phase of the cell cycle 3. Maintains genomic stability When both copies of RB1 are inactivated (Knudson's two-hit hypothesis), cells lose cell-cycle control and undergo malignant transformation. ### Knudson's Two-Hit Hypothesis ```mermaid flowchart TD A[RB1 Gene]:::outcome --> B{Hereditary or Sporadic?}:::decision B -->|Hereditary| C[First hit: Germline mutation]:::action B -->|Sporadic| D[First hit: Somatic mutation in one cell]:::action C --> E[Present in all cells]:::outcome D --> E E --> F[Second hit: Somatic mutation in retinal cell]:::action F --> G[Loss of pRb function]:::outcome G --> H[Uncontrolled cell proliferation]:::urgent H --> I[Retinoblastoma]:::outcome ``` ### Clinical Pearls **Clinical Pearl:** Hereditary retinoblastoma (RB1 germline mutation) presents earlier (mean age 18 months) and is bilateral in 40–50% of cases. Sporadic retinoblastoma (somatic RB1 mutation) is typically unilateral and presents at a mean age of 24 months. **Warning:** TP53 mutations (Li-Fraumeni syndrome) are associated with retinoblastoma but account for <5% of hereditary cases. Patients with TP53 mutations have a much higher risk of secondary malignancies (breast cancer, sarcomas, brain tumours) and should undergo lifelong surveillance. ### Mnemonic: "RB1 Rules" **R** = **Retinoblastoma** protein (pRb) **B** = **Both hits** required (Knudson's two-hit) **1** = **>90%** of hereditary cases **Rules** = Controls cell cycle (G1 checkpoint) ### Screening and Management Implications - **Germline RB1 mutation carriers:** Require bilateral eye screening every 4–6 weeks until age 5 years - **Bilateral disease:** Suggests hereditary form; genetic counselling and family screening essential - **Secondary malignancies:** RB1 mutation carriers have ~50% lifetime risk of secondary cancers (especially osteosarcoma, soft tissue sarcoma) [cite:Boyd & Kaliki, Retinoblastoma: Epidemiology and Management; Lohmann & Gallie, Retinoblastoma Genetics]