20.20: The Retinoblastoma Gene

Tumor suppressor genes are normal genes that can slow down cell division, repair DNA mistakes, or program the cells for apoptosis in case of irreparable damage. Hence, they play an essential role in preventing the proliferation of damaged cells.

The first-ever tumor suppressor gene called Rb was identified in retinoblastoma - a rare eye tumor in children. In inherited forms of the disease, a child inherits one defective copy of the Rb gene, which predisposes them to retinoblastoma. However, when a somatic cell loses the second copy of the gene as well, it triggers the development of tumors. Such individuals develop multiple tumors in both eyes. In the disease's nonhereditary form, a cell undergoes two independent somatic mutations and loses both functional copies of the Rb gene. Such cells become cancerous and lead to the development of a tumor in only one eye.

Retinoblastoma is one of the most common intraocular malignant tumors in children, with an incidence rate of 1 in 15,000 to 1 in 18,000. While the hereditary form comprises 25-35% of the total retinoblastoma cases, the nonhereditary or sporadic form comprises 65-75% of the cases.

Rb gene was the first-ever tumor suppressor gene discovered. Loss of function mutations in the Rb gene leads to retinoblastoma - cancer of the retinal cells of the eye.

A healthy individual possesses two functional copies of the Rb gene, which encode a retinoblastoma protein that regulates cell cycle progression.

In the case of hereditary retinoblastoma, a child inherits one functional and one mutated Rb gene from their parents. Every somatic cell of these individuals will therefore have only one normal Rb gene, but the amount of Rb protein produced is enough to maintain the normal cell cycle.

However, if any of these somatic cells lose the other functional copy of the gene, the cell stops producing the Rb protein altogether and becomes cancerous.

In rare cases, even when an individual inherits two functional Rb gene copies, both copies of the Rb gene can be lost or inactivated in a single cell lineage by two independent events over time.

Such double loss of function mutations in the retinal cells leads to non-hereditary or sporadic retinoblastoma.

• Tumor Suppressor Genes - Genes that can slow cell division, repair DNA mistakes, or program cell apoptosis.
• Rb Gene - The first tumor suppressor gene identified, related to retinoblastoma.
• Retinoblastoma - A rare intraocular malignant tumor in children often linked to the Rb gene.
• Hereditary Retinoblastoma - Results from inherited mutation in Rb gene, leads to multiple tumors in both eyes.
• Sporadic Retinoblastoma - Non-hereditary, results from two independent somatic mutations in the Rb gene.

• Define Tumor Suppressor Genes – Explain their function in cell division and DNA repair (e.g., Rb gene).
• Contrast Hereditary vs Sporadic Retinoblastoma – Highlight the differences in their genetic etiology (e.g., Rb gene mutations).
• Explore Retinoblastoma – Provide insights about the disease's prevalence, inheritance, and significance (e.g., Rb gene relevance).
• Explain Rb Gene Mechanism – Summarize its tumor suppressing function and related mutations.
• Apply in Genetic Context – Understand the implications and applications of Rb gene and its mutations in diagnosing and treating retinoblastoma.

• What is the function and significance of tumor suppressor genes such as the Rb gene?
• How do hereditary and sporadic retinoblastoma differ genetically?
• How does mutation in the Rb gene lead to retinoblastoma?

• Students – Learn about tumor suppressor genes, their role in preventing cancer, and their relation to diseases like retinoblastoma.
• Educators – Provides a basis for teaching about genetic disease mechanisms and cancer biology.
• Researchers – Crucial for studying genetic diseases, cancer biology, and potential therapeutic targets.
• Healthcare Professionals – Better understanding reveals opportunities for disease diagnosis and treatment.