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chapter 26
Regulation of Gene Expression
FIGURE 26-11
Genome of the Rous sarcoma virus (RSV). The single-stranded RNA is
about 10 kb. Four proteins are encoded in this retrovirus: gag, a core
protein; pol, reverse transcriptase; env, glycoprotein of the envelope; and
src, a nonessential viral gene whose product is responsible for cellular
transformation. LTR, Long terminal repeat sequences present at the ends of
each viral RNA molecule.
This single-stranded oncogenic RNA virus infects chicken
cells in which the RNA is copied by
reverse transcriptase
and converted to DNA (Figure 26-11). The DNA copy in-
tegrates into a chromosome, where it becomes a provirus.
Integration of viral DNA is an essential step in the produc-
tion of more virus particles. Expression of the
src
gene,
which is nonessential to multiplication of the virus, causes
tumor formation in infected fowl. Infected chicken fibrob-
last cells cultured
in vitro
become transformed and grow
in an unregulated fashion that is visibly distinct from the
growth of uninfected cells.
Study of DNA from avian tissue has indicated that the
src
gene is present in cells from normal tissue. How-
ever, the cellular
src
gene, denoted c
-src,
contains introns,
whereas the viral gene,
v-src,
does not. This has led to the
conclusion that at some time in the distant past, processed
v-src
mRNA was converted to DNA by reverse transcrip-
tase, a virus-encoded enzyme that makes DNA from a
single-stranded RNA template, and the virus picked up
this DNA copy by genetic recombination. Similar onco-
genes have been found in other viruses, and in each case
a related cellular copy of the gene is present. The cellular
counterpart is called a
proto-oncogene.
The oncogenes
and proto-oncogenes are almost never identical because
they have evolved separately. The product of the
src
gene
is a tyrosine-specific protein kinase, an unusual enzyme
because most protein kinases are active only against ser-
ine. This enzyme is capable of phosphorylating a large
number of cellular proteins, causing either activation or
inactivation.
The first human oncogene was discovered by testing
DNA fragments isolated from human cancer cells and test-
ing for their ability to transform a culture of mouse cells.
This experiment utilized DNA from a line of human blad-
der cancer cells that was applied to a culture of National
Institutes of Health 3T3 cells, an established mouse cell
line. Some of the mouse cells became transformed, which
suggested that an oncogene was being expressed in the
bladder cancer cells. Only a single DNA fragment could
induce the transformation; therefore, this fragment con-
tained the human bladder cancer oncogene, which was
later named
ras.
A search for a matching sequence in non-
cancerous cells (in analogy to the
src
study) uncovered,
a proto-oncogene that differed from
ras
by a single base
pair in the fragment that was sequenced. That is,
ras
is a
mutant form of a normal gene,
ras
has also been found
in many other viruses and eukaryotes, including yeast. A
list of most known oncogenes is given in Table 26-6.Their
names are abbreviations or acronyms for the virus or tissue
of origin.
Among the numerous oncogenes that have been identi-
fied in cancer cells, the
ras
genes are the most common.
Expression of the oncogenic or mutant
ras
gene is found
in 50% of human cancers. The protein produced by the
ras
genes is called p21 (M.W. 21,000) and contains 189 amino
acid residues. The oncogenic
ras
gene proteins differ from
the normal protein by a single amino acid substitution in
most instances, and these changes may be responsible in
some cases for converting a normal cellular gene product
to a cancer-promoting one.
The normal
ras
p21 binds GTP and GDP; it also hy-
drolyzes GTP to GDP. The mutant
ras
p21 proteins show
reduced or complete absence of GTPase activity. In this
regard, normal p
2 1
appears to be similar to the cellular
G (GTP-binding) protein that activates adenylate cyclase
and thereby mediates cellular activities.
Chromosome changes are the hallmark of many can-
cer cells. The most common type of change is
transloca-
tion,
a chromosome alteration in which two chromosomes
have exchanged segments. For example, in
Burkitt’s lym-
phoma,
fragments in chromosomes
8
and 14 are ex-
changed in 90% of affected individuals, between chro-
mosomes
8
and 2 in 5%; and between chromosomes
8
and
22 in another 5%. In each case, a segment of chromosome
8
has been moved. The
proto-oncogene c-myc
is located
on chromosome
8
, and in each of the translocations, c-
m yc
is relocated adjacent to a gene encoding an antibody.
This relocation apparently places the proto-oncogene un-
der control of the genes that regulate antibody synthesis,
so that
c-m yc
is made in large quantities.
Involvement of a different oncogenic protein has been
demonstrated in
chronic myelogenous leukemia (CML).
This form of leukemia is associated with the so-called
Philadelphia (Ph) chromosome and is characterized by a
translocation from chromosome 9 to a new location on the
short arm of chromosome 22. The translocation results in
the production of a chimeric
a b l
oncogenic protein. The
chimeric
a b l
oncogene has enhanced tyrosine protein ki-
nase activity both
in vivo
and
in vitro
that is correlated
with the development of leukemia. The translocated seg-
ment of DNA is inserted into chromosome 22 in a region
consisting of
2 0 0
bp called the
breakpoint cluster region
(bcr).
Consequently, in different CML patients the
b cr/a b l
p
2 1
proteins differ slightly in amino acid sequence, but all
