Knudson's Two-Hit Hypothesis - As Explained in Robbins

What is It?

"Two loss-of-function mutations (hits) involving both alleles of RB are required to produce retinoblastoma."

• Robbins & Cotran Pathologic Basis of Disease, Chapter 7 (Fig. 7.25)

The Classic Example: Retinoblastoma and the RB Gene

The Two Scenarios

1. Familial (Hereditary) Form - "One Hit Already Done"

• ~40% of retinoblastomas
• The child inherits one mutated RB allele in every single cell of the body (this is Hit 1 - the germline mutation)
• The child appears completely normal, because one working copy of RB is still present
• Only ONE additional somatic mutation is needed in any retinal cell to knock out that second copy (Hit 2)
• Because second hits are virtually inevitable in a small fraction of retinoblasts, tumors almost always develop
• Result: bilateral tumors, earlier onset, multiple tumors, transmitted as autosomal dominant trait (because risk is dominant, even though the cellular mechanism is recessive)

2. Sporadic Form - "Both Hits Must Happen by Chance"

• ~60% of retinoblastomas
• The child is born with two perfectly normal RB alleles in all cells
• Hit 1: a somatic mutation in one allele of a single retinal cell
• Hit 2: a second independent somatic mutation knocking out the remaining allele in that same cell
• The probability of two independent hits in the same cell is low - which is why sporadic retinoblastoma is rare in the general population
• Result: unilateral tumor, later onset, single tumor, no family history

The Diagram (Fig. 6.18 / Fig. 7.25 - Robbins)

The Apparent Paradox: Recessive Mutation, Dominant Inheritance

"A child carrying an inherited mutant RB allele in all somatic cells is perfectly normal (except for the increased risk of developing cancer); it follows that one defective RB gene does not have adverse effects on cell behavior. Thus, although the genetic trait (increased cancer risk) associated with germline RB mutations is inherited in an autosomal dominant fashion, at the level of individual cells the phenotype associated with RB loss-of-function behaves like a recessive trait."

Beyond Retinoblastoma - Where Else Does This Apply?

"The major themes that emerged from the study of familial retinoblastoma hold for other familial cancers: the risk of cancer is inherited as an autosomal dominant trait due to a germline mutation in a tumor suppressor gene; tumors have second 'hits' in the sole normal tumor suppressor gene allele; and the same tumor suppressor gene is frequently mutated in sporadic tumors of the same type."

Summary in Simple Terms

• Familial: You're born with one brake already broken. It only takes one more failure.
• Sporadic: Both brakes are fine at birth. Both need to break independently - rare, but it happens.