“The data presented here and in the literature are consistent with the hypothesis that at least one cancer, retinoblastoma, can be caused by two mutations…. One of these mutations may be inherited as a result of a previous germinal mutation…. Those patients that inherit one mutation develop tumors earlier than do those who develop the nonhereditary form of the disease; in a majority of cases those who inherit a mutation develop more than one tumor.

1. The two-hit hypothesis
Bui Dac Chi,MD
Medic Center

4. • Much of what scientists know about the origins of
cancer and the role of tumor suppressors can be
traced back 28 years to the elegant theory of cancer
researcher Alfred G. Knudson. Widely thought to be
one of the most significant theories in modern biology,
Knudson's "two-hit .
• This hypothesis is regconized as an important
contribution to our understanding of oncogenesis.

5. • Thanks to the advent of genetic and molecular
technologies which made it possible to trace the
origins of certain cancers with high degree of
accuracy and contribute to the development of
cancer treatments. But have you ever wondered
what used to happen several decades back where
there was no technology? The scientists could only
hypothesise the basis of cancer development and
that is how the ‘two-hit hypothesis of cancer’ came
into existence by the study of retinoblastoma caused
by mutations in the tumour suppressor protein,
retinoblastoma protein (pRB).

6. • “Cancer is, in essence, a genetic disease. Although
cancer is complex, and environmental and other
nongenetic factors clearly play a role in many stages of
the neoplastic process, the tremendous progress made
in understanding tumorigenesis in large part is owing to
the discovery of the genes, that when mutated, lead to
cancer.”

7. • Cancer is caused by the accumulation of
genetic and epigenetic mutations in genes
whose products have a role in: 1) directly
regulating cell proliferation could be either
promoting or inhibiting 2) controlling
apoptosis 3) repairing of damaged DNA.
These types of genes are grouped into
two general categories: tumour
suppressor genes (growth inhibiting) and
proto-oncogenes (growth promoting).

9. • During the 1950s and 1960s, cancer epidemiologists
were preoccupied with trying to understand the
environmental causes of the disease. In a 1953 paper,
cancer biologist C. O. Nordling noted that in developed
nations, the incidence of cancer seemed to increase with
age (C. O. Nordling. Nordling's proposal that the
occurrence of cancer needed the accumulation of at
least six sequential mutations was ultimately proved
wrong. But his idea that cancer is caused by a certain
number of 'hits' to the genome paved the way for
Knudson's key insight.

13. • Knudson developed the two-hit hypothesis by statistical methods,
without any experimental approaches. He simply compared the time
at which both eyes would be affected by retinoblastoma if one or two
hits were required and predicted the relative chance of this
occurring Knudson’s hypothesis clearly postulated the recessive
nature of tumor-initiating gene mutations and the mode of
inheritance in familial cancer. This led to the concept of the
existence of tumor suppressor genes and loss-of-heterozygosit
(LOH) as relevant to carcinogenesis. Notably, Knudsonalready
mentioned the possibility of “delayed mutation” that may correspond
to germline-mosaic mutations. Including this possibility, his insights
facilitated the development ofthe field of cancer genetics. The first
molecular cloning of the tumor suppressor gene RB1, the
predisposing gene for retinoblastoma, was achieved in 1986.

15. • “The data presented here and in the literature are
consistent with the hypothesis that at least one cancer,
retinoblastoma, can be caused by two mutations…. One
of these mutations may be inherited as a result of a
previous germinal mutation…. Those patients that inherit
one mutation develop tumors earlier than do those who
develop the nonhereditary form of the disease; in a
majority of cases those who inherit a mutation develop
more than one tumor

17. Individuals will develop cancer of the retina if they
either inherit one mutated retinoblastoma (Rb)
gene and incur a second mutation (possibly
environmentally induced) after conception, or if
they incur two mutations or hits after
conception.3 If only one Rb gene functions
normally, the cancer is suppressed. Knudson
dubbed these preventive genes anti-oncogenes;
other scientists renamed them tumor
suppressors

19. • 1998 Lasker~DeBakey Clinical Medical
Research Award - Alfred Knudson, Jr.