Molecular biology of retinoblastoma.

Retinoblastoma (Rb), the most common intraocular tumor in childhood, is caused by the loss of function of both retinoblastoma susceptibility gene (RB1 or Rb1) alleles. In 1971, Alfred Knudson proposed his "two-hit" theory based upon empiric observations of the clinical genetics of Rb, revealing the role of tumor-suppressor genes in human cancer. Knudson proposed that: "In the dominant inherited form of Rb, one mutation is inherited via germ line and the second occurs in somatic cells. In the nonhereditary form, both mutations occur in somatic cells." The Knudson hypothesis was validated later with the cloning of RB1, the first tumor-suppressor gene to be identified. A few years later, Harbour extended these findings to small-cell lung cancer, showing that the RB1 locus was disrupted in tumors other than Rb and osteosarcoma. Since then, it has been found that most, if not all, tumors have defects in their RB1 pathway through genetic lesions in the RB1 gene itself or other genes in the pathway. The history of Rb research highlights how basic research on a rare childhood cancer can have a much broader effect on a disease that affects millions of people each year worldwide.