Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1106 Chapter 20: Cancer
Different Searches for Oncogenes Converged on the Same
Gene—Ras
In an attempt to answer the above question, other researchers searched directly
for oncogenes in the genomes of human cancer cells. They did this by searching
for DNA fragments from cancer cells that could provoke uncontrolled proliferation
when introduced into noncancerous cell lines. As tester cells for the assay,
cell lines derived from mouse fibroblasts were used. These cells had been previously
selected for their ability to proliferate indefinitely in culture, and they
are thought to already contain alterations that take them part of the way toward
malignancy. For this reason, the addition of a single oncogene can sometimes be
enough to produce a dramatic effect.
When DNA was extracted from the human tumor cells, broken into fragments,
and introduced into the cultured cells, occasional colonies of abnormally proliferating
cells began to appear in the culture dish. These cells showed a transformed
phenotype, outgrowing the untransformed cells in the culture and piling up in
layer upon layer (see Figure 20–11). Each colony was a clone originating from a
single cell that had incorporated a DNA fragment that drove cancerous behavior.
This fragment, which carried markers of its human origin, could be isolated from
the transformed cultured mouse cells. And once isolated and sequenced, it could
be recognized: it contained a human version of a gene already known from study
of a retrovirus that caused tumors in rats—an oncogene called v-Ras.
The newly discovered oncogene was clearly derived by mutation from a normal
human gene, one of a small family of proto-oncogenes called Ras. This discovery
in the early 1980s of the same oncogene in human tumor cells and in an
animal tumor virus was electrifying. The implication that cancers are caused by
mutations in a limited number of cancer-critical genes transformed our understanding
of the molecular biology of cancer.
As discussed in Chapter 15, normal Ras proteins are monomeric GTPases that
help transmit signals from cell-surface receptors to the cell interior (see Movie
15.7). The Ras oncogenes isolated from human tumors contain point mutations
that create a hyperactive Ras protein that cannot shut itself off by hydrolyzing its
bound GTP to GDP. Because this makes the protein hyperactive, its effect is dominant—that
is, only one of the cell’s two gene copies needs to change to have an
effect. One or another of the three human Ras family members is mutated in perhaps
30% of all human cancers. Ras genes are thus among the most important of
all cancer-critical genes.
Genes Mutated in Cancer Can Be Made Overactive in Many Ways
Figure 20–18 summarizes the types of accidents that can convert a proto-oncogene
into an oncogene. (1) A small change in DNA sequence such as a point
proto-oncogene
DELETION OR POINT
MUTATION IN
CODING SEQUENCE
REGULATORY
MUTATION
GENE AMPLIFICATION
CHROMOSOME REARRANGEMENT
DNA
X
or
DNA
RNA
RNA
hyperactive
protein made in
normal amounts
normal protein
greatly
overproduced
normal protein greatly
overproduced
Figure 20–18 The types of accidents that can convert a proto-oncogene into an oncogene.
nearby regulatory
DNA sequence causes
normal protein to
be overproduced
fusion to actively
transcribed gene
produces hyperactive
fusion protein