Virus / ไวรัส

Cancers are the result of a disruption of the normal restraints on cellular proliferation.
It is apparent that the number of ways in which such a disruption can occur is strictly
limited and there may be as few as forty cellular genes in which disruption leads to
unrestrained cell growth. There are two basic classes of these genes in which
mutation can lead to loss of growth control:
(a) Those genes that are stimulatory for growth and which cause cancer when
hyperactive. Mutations in these genes will be dominant.
(b) Those genes that inhibit cell growth and which cause cancer when they are
turned off. Mutations in these genes will be recessive.

Viruses are involved in cancers because they can either carry a copy of one of these
genes or can alter expression of the cell's copy of one of these genes.

There are two classes of tumor viruses: the DNA tumor viruses and the
RNA tumor viruses, the latter also being referred to as RETROVIRUSES.
We shall see that these two classes have very different ways of reproducing
themselves but they often have one aspect of their life cycle in common: the ability to
integrate their own genome into that of the host cell. Such integration is not, however,
a pre-requisite for tumor formation.

If a virus takes up residence in a cell and alters the properties of that cell, the cell is
said to be transformed.

TRANSFORMATION BY A VIRUS MAY BE DEFINED AS: CHANGES IN THE BIOLOGIC FUNCTIONS
OF A CELL THAT RESULT FROM REGULATION OF THE CELL BY VIRAL GENES AND THAT
CONFER ON THE INFECTED CELL CERTAIN PROPERTIES CHARACTERISTIC OF NEOPLASIA.
THESE CHANGES OFTEN (BUT NOT ALWAYS) RESULT FROM INTEGRATION OF THE VIRAL
GENOME INTO THE HOST CELL GENOME

Transformation often includes loss of growth control, ability to invade extracellular
matrix and dedifferentiation. In carcinomas, many epithelial cells undergo an
epithelial-mesenchymal transformation. Transformed cells often exhibit chromosomal
aberrations.

The region of the viral genome (DNA in DNA tumor-viruses or RNA in RNA-tumor
viruses) that can cause a tumor is called an ONCOGENE. This foreign gene can be
carried into a cell and cause it to take on new properties such as immortalization and anchorage-independent growth.

The discovery of viral oncogenes in retroviruses led to the finding that they are not
unique to viruses and homologous genes (called proto-oncogenes) are found in
all cells. Indeed, it is likely that the virus picked up a cellular gene during its evolution
and this gene has subsequently become altered. Normally, the cellular
proto-oncogenes are not expressed in a quiescent cell since they are involved in
growth (which is not occurring in most cells of the body) and development; or they are
expressed at low levels. However, they may become aberrantly expressed when the
cell is infected by tumor viruses that do not themselves carry a viral oncogene. We
shall see later how this happens but it is clear that a virus may cause cancer in two
ways: It may carry an oncogene into a cell or it may activate a cellular proto-oncogene.

The discovery of cellular oncogenes opened the way to the elucidation of mechanisms
by which non-virally induced cancers may be caused. We shall investigate what the
protein products of the viral and cellular oncogenes do in the infected cell and in cells
in which cellular proto-oncogenes are expressed. We shall see that their functions
strongly suggest mechanisms by which cells may be transformed to a
neoplastic phenotype. The discovery of cellular oncogenes led to the discovery
of another class of cellular genes, the tumor repressor (suppressor) genes or
anti-oncogenes.

Initially, the involvement of viral and cellular oncogenes in tumors caused by
retroviruses was much more apparent than the involvement of the DNA tumor virus
oncogenes but the discovery of tumor repressor genes (as a result of our
knowledge of how retroviruses cause cancer) led to the elucidation of the mode of
action of DNA virus oncogenes.

It should be noted that while viruses have been vitally instrumental in elucidation of the
mechanisms of oncogenesis, most human cancers are probably not the result of a
retroviral infection although retroviruses are important in cancers in some animals.

In permissive cells, all parts of the viral genome are expressed. This leads to viral
replication, cell lysis and cell death

In cells non-permissive for replication, viral DNA is integrated into the cell chromosomes
(usually but not always) at random sites. Only part of the viral genome is expressed.
The early, control functions (e.g. T antigens) of the virus, are expressed. Viral structural
proteins are not expressed and no progeny virus is released.

Papilloma virus Copyright 1994 Veterinary Sciences Division, Queens University Belfast	Papilloma virus Copyright Dr Linda M Stannard, 1995 (used with permission)	Papilloma virus Computer colorized EM image. All 72 capsomeres are pentamers of the major structural protein. Copyright Dr Linda M Stannard, 1995 (used with permission)
Figure 2

Papilloma viruses are wart-causing viruses that also certainly cause human
neoplasms and cause natural cancers in animals.

Warts are usually benign but can convert to malignant carcinomas. This occurs in
patients with epidermodysplasia verruciformis (more here). Papilloma viruses
are also found associated with human penile, uterine and cervical carcinomas and
are very likely to be their cause; moreover, genital warts can convert to carcinomas.

Epidermodysplasia verruciformis. This widespread, markedly pruritic, erythematous eruption was eventually found to be caused by human papillomavirus infection. International Association of Physicians in AIDS Care	Verrucous carcinoma. The epithelium shows surface maturation, parakeratosis, and hyperkeratosis. There is little or no cellular atypia. The stroma shows a mild chronic inflammatory infiltrate. The Johns Hopkins Autopsy Resource (JHAR) Image Archive.
Figure 3

Squamous cell carcinomas of larynx, esophagus and lung appear very like cervical
carcinoma histologically and these may also involve papilloma viruses.

There are 51 types of papilloma viruses but, clearly, not all are associated with cancers;
however, papillomas may cause 16% of female cancers worldwide and 10% of all
cancers.

Vulvar, penile and cervical cancers associated with type 16 and type 18
papilloma viruses (and others) but the most common genital human papilloma viruses
(HPV) are types 6 and 11. As might be expected if they are indeed the causes of
certain cancers, types 16 and 18 cause transformation of human keratinocytes. In a
German study, it was shown that 1 in 30 HPV type16-infected women will develop
malignant disease while 1 in 500 infected people develop penile or vulvar cancer.
Since not all infected persons develop a cancer, there are probably co-factors in
stimulating the disease. Such co-factors have been identified in alimentary tract
carcinomas in cattle where a diet containing bracken fern is associated with the
disease.

NOTE, HOWEVER: THE FACT THAT A VIRUS IS USUALLY FOUND IN ASSOCIATION WITH A
NEOPLASM DOES NOT IN ANY WAY PROVE THAT THE TRANSFORMATION OF THE CELLS IS
THE RESULT OF THE PRESENCE OF THE VIRUS. THE ASSOCIATION COULD BE CASUAL NOT
CAUSAL. THE VITAL EXPERIMENT, DONE IN MANY ANIMAL SYSTEMS, WOULD BE TO INJECT
THE VIRUS PURIFIED FROM A TUMOR INTO A HUMAN AND SEE IF THE TUMOR REDEVELOPS.
FOR OBVIOUS REASONS THAT CRITICAL EXPERIMENT HAS NOT BEEN DONE.
Nevertheless, the epidemiological data are very strong.

Simian virus 40
SV 40 is a monkey polyoma virus that causes sarcomas in juvenile hamsters. It was
isolated from normal monkey kidney cells in which it replicates.

Polyoma virus
Polyoma virus was so named because it causes a wide range of tumors in a number
of animal species. It was originally isolated from AK mice and is fully permissive for
replication in mouse cells. It causes leukemias in mice and hamsters.

Human polyoma viruses
There are two human polyoma isolates, known as BK and JC; neither came from a
tumor but they will cause tumors when injected into animals. 70-80% of the population
is seropositive for JC. This virus causes progressive multifocal leukoencephalopathy
(see section on slow virus diseases), a disease associated with immunosuppression.
In 1979, the rate of occurrence of this disease was 1.5 per 10 million population. It has
become much more common because of AIDS and is seen in 5% of AIDS patients.

Note: Polyoma viruses are usually lytic and when transformation occurs, it is because
the transforming virus is defective. After integration into host DNA, only EARLY
FUNCTIONS are transcribed into mRNA and expressed as a protein product.
These are the TUMOR ANTIGENS. Because the expression of the genes for tumor
antigens is essential for transformation of the cells, they may be classified as
ONCOGENES.

DEFINITION OF AN ONCOGENE: AN ONCOGENE IS A GENE THAT CODES FOR A PROTEIN THAT
POTENTIALLY CAN TRANSFORM A NORMAL CELL INTO A MALIGNANT CELL. IT MAY BE
TRANSMITTED BY A VIRUS IN WHICH CASE WE REFER TO IT AS A VIRAL ONCOGENE.

1) They are true viral genes. There are no cellular homologues in the uninfected cell.

2) They are necessary in lytic infections because they participate in the control of viral
and cellular DNA transcription.

Adenovirus Copyright Dr Stephen Fuller, 1998	Adenovirus Copyright Dr Linda M Stannard, University of Cape Town, South Africa, 1995 (used with permission).
Figure 5

These viruses are highly oncogenic in animals and only a portion of the virus is
integrated into the host genome. This portion codes for early functions (E1A region
contains the oncogenes that code for several T antigens). No humans cancers have
been unequivocally associated with adenoviruses. E1A gene product (early
non-structural protein) binds to the product of Rb gene (see below). Thus polyoma
and adenoviruses seem to cause cell transformation in a similar manner: the
integration of early function genes into the chromosome and the expression of these
DNA synthesis-controlling genes without the production of viral structural proteins.

Herpes virus. Negative stain Copyright Dr Linda M Stannard, University of Cape Town, South Africa, 1995 (used with permsssion).

Liquid-Crystalline, Phage-like Packing of Encapsidated DNA in Herpes Simplex Virus (F.P.Booy, W.W.Newcomb, B.L.Trus, J.C.Brown, T.S.Baker, and A.C.Steven, in CELL, Vol 64 pp 1007-1015, March 8, 1991)

Herpes Simplex Virus (TEM x169,920) Copyright Dr Dennis Kunkel (used with permission)

Figure 6

There is considerable circumstantial evidence that implicates these enveloped
DNA viruses in human neoplasms. They are highly tumorigenic in animals. It is
notable that herpes viruses exist primarily as episomes in the cell and do not
integrate into the host cell genome. By the time that tumors arise, no trace of the
virus can usually be found. Herpes virus DNA is found in only a small number of
herpes-transformed cells. They may have a hit and run mechanism of oncogenesis,
perhaps by causing chromosomal breakage or other damage. See below.

This is the herpes virus that is most strongly associated with cancer. It is causally
associated with:

This herpes virus is frequently associated with Kaposi's sarcoma but this is now
thought probably to be caused by a newly-discovered herpes virus,
human herpes virus 8.

This virus was associated in epidemiological studies with cervical cancer. Now the
evidence for papilloma virus is better

Hepatitis B virions: two exposed cores (indicated by arrows)	Hepatitis B virions	A diagrammatic representation of the hepatitis B virion and the surface antigen components	All four images: Copyright Dr Linda M Stannard, University of Cape Town, South Africa, 1995 (used with permission).
Figure 8

Hepatitis B virus is very different from the other DNA tumor viruses. Indeed, even
though it is a DNA virus, it is much more similar to the oncornaviruses (RNA tumor
viruses) in its mode of replication. Hepatitis B is a vast public health problem and
hepatocellular carcinoma (HCC), which is one of world's most common cancers,
may well be caused by HBV. There is a very strong correlation between HBsAg
(hepatitis B virus surface antigen) chronic carriers and the incidence of HCC.
In Taiwan, it has been shown that HBsAg carriers have a risk of HCC that is 217 times
that of a non-carrier. 51% of deaths of HBsAg carriers are caused by liver cirrhosis or
HCC compared to 2% of the general population.

This woman has hepatitis B and is suffering from liver cancer. She was a Cambodian refugee and died 4 months after she arrived in a refugee camp (average life expectancy after diagnosis of liver cancer is 6 months) Immunization Action Coalition Courtesy of Patricia Walker, MD, Ramsey Clinic Associates, St. Paul, MN

Figure 9

NOTE: Hepatitis B virus is a DNA tumor virus BUT it has a very weird way of
replicating itself. The DNA is transcribed into RNA not only for the manufacture of
viral proteins but for genome replication. Genomic RNA is transcribed back into
genomic DNA. This is called REVERSE TRANSCRIPTION. This is not typical
of DNA tumor viruses but reverse transcription is a very important factor in the
life cycles of RNA-tumor viruses. See below.

Human immunodeficiency virus Copyright Department of Microbiology, University of Otaga, New Zealand.	Structure of a retrovirus: (The virus shown is human immunodeficiency virus-1) From the Harvard AIDS Institute Library of Images, courtesy of Critical Path AIDS Project, Philadelphia.
Figure 10

Retroviruses are different from DNA tumor viruses in that their genome is RNA but
they are similar to many DNA tumor viruses in that the genome is integrated into host
genome.

Since RNA makes up the genome of the mature virus particle, it must be copied to
DNA prior to integration into the host cell chromosome. This life style goes against
the central dogma of molecular biology in which DNA is copied into RNA.

Coat proteins (surface antigens) are encoded by env (envelope) gene. One primary
gene product is made but this is cleaved so that there are more than one surface
glycoprotein in the mature virus (cleavage is by host enzyme in the
Golgi apparatus).

Inside the membrane is an icosahedral capsid containing proteins encoded by the
gag gene (Group- specific AntiGen). Gag- encoded proteins also coat the genomic
RNA. Again there is one primary gene product. This is cleaved by a
virally-encoded proteins (from the pol gene)

There are two molecules of genomic RNA per virus particle with a
5' cap and a 3' poly A sequence. Thus, the virus is diploid. The RNA is plus sense
(same sense as mRNA).

About 10 copies of reverse transcriptase are present within the mature virus,
these are encoded by the pol gene.

Pol gene codes for several functions (again, as with gag and env, a polyprotein is
made that is then cut up)

1) ONCOVIRINAE
These are the tumor viruses and those with similar morphology. The first member of
this group to be discovered was Rous sarcoma virus (RSV)- which causes a slow
neoplasm in chickens.

HTLV-1 (human T-cell lymphotropic virus) (Information Box): Causes Adult T-cell
leukemia (Sezary T-cell leukemia) which is found in some Japanese islands,
the Caribbean, Latin America (Information Box) and Africa. HTLV-1 is sexually
transmitted (Information Box)

2) LENTIVIRINAE
These have a long latent period; they are mainly associated with diseases of
ungulates (e.g. visna virus) but HIV (formerly HTLV-III) which causes AIDS belongs
to this group. It is much more closely related to some Lentivirinae than it is to HTLV-I
and HTLV-II which are Oncovirinae

3) SPUMAVIRINAE
There is no evidence of pathological effects of these viruses.

2) Uptake by endocytosis or by direct fusion to the plasma membrane. The virus
may need entry into a low pH endosome before fusion can occur although some
(e.g. HIV) can fuse directly with the plasma membrane

3) RNA (plus sense) is copied by reverse transcriptase to minus sense DNA. Here,
the polymerase is acting as an RNA-dependent DNA polymerase. Note: reverse
transcriptase is a DNA polymerase and it therefore needs a primer. This is a tRNA
that is incorporated into the virus particle.

4) RNA is displaced and degraded by a virus-encoded RNase H activity. Reverse
transcriptase now acts as a DNA-dependent DNA polymerase and copies the
new DNA into a double strand DNA. This is the provirus.

5) Double strand DNA is integrated into host cell DNA (see below) using a virally
encoded integrase enzyme. This DNA is copied every time cellular DNA is copied.
Thus, at this stage the provirus is just like a normal cellular gene.

6) Full length, genomic RNA (plus sense) is copied from integrated DNA by
host RNA polymerase II. It is capped and poly adenylated

Note: mRNA comes from splicing genomic RNA or is the genomic RNA. As a
result, both mRNA and genomic RNA must be the same sense - since mRNA
must be plus sense, the genomic RNA of all retroviruses must also be plus sense.

An advantage of this mode of replication is that it allows growth in terminally
differentiated cells since the only host cell polymerase usurped by the virus is
RNA polymerase II which is present in all cells.

If host RNA polymerase II is used to copy the DNA back to RNA, there are major
problems with having a DNA provirus form but an RNA genome in the mature virus
particle

Failure to copy the entire gene
The problem is that, when transcribing genes, RNA polymerase II needs control and
recognition sites upstream from the transcription initiation site. The upstream
site at which the polymerase molecule binds is called the PROMOTOR. Promotors
are not themselves copied into mRNA since they have no function in the translation
of protein. After binding to the promotor, the polymerase begins transcription at a
downstream site, the RNA initiation site. The polymerase continues to transcribe
DNA into RNA until it reaches a termination/polyadenylation signal, part of which
is not copied since it also has no function in the making of the protein. Furthermore,
both up and downstream from the transcribed region are control sequences that
modulate the transcription of the gene. These are called ENHANCERS.
These are essential parts of any gene and must be present for RNA polymerase II
to work but they are not copied to RNA. This is because RNA polymerase II in the
host cell has the function of making messenger RNA which is dispensed with after
translation. To make a protein, the actual mRNA molecule does not need the controls
equences of the original gene. Thus, the use of host RNA polymerase II means
that the control sequences in the original genome will not get into the RNA genome
of progeny virions.

This means that either the DNA copy of the viral RNA genome virus must integrate
into host DNA downstream from a host promotor and upstream from host
termination sites (a tall order indeed!) or it must find a way of providing its own
control sequences (which, as we said, are not copied into progeny genome). It does
the latter in a most complex manner.

1) The viral RNA is composed of three regions. At each end are repeats (called, not
surprisingly, terminal repeats). The repeat sequences (R) (shown in green) do not
code for proteins. In between the two repeats, there is a unique (not repeated) region
that contains the viral genes that code for the proteins (GAG, POL and ENV) plus other
unique sequences at either end that do not code for protein. At the 5' end of the RNA
genome is the U5 region and at the 3' end is the U3 region. PBS (in above diagram)
is the primer binding site. The tRNA binds here when RT starts copying. PPT is a
polypurine tract.

2) In the integrated form (when transcribed into DNA and inserted into the host cell chromosome), the provirus is more complicated. We find that part of the 3' unique
region (called U3) of the RNA genome has been copied and transposed to the
opposite end of the genome. Conversely, part of the 5' end of the unique region
(called U5) has been copied and transposed to the other end. This gives the
integrated DNA the structure shown in figure 15B.

For convenience, only one strand of the DNA is shown. Now, of course, there are
larger terminal repeats since the U3 and U5 regions are also repeated.
The U3-R-U5 regions are known as long terminal repeats or LTRs.
The U3 region contains all of the promotor information that is necessary to start
RNA transcription at the beginning of the R (repeat) region while the U5 region
contains all of the information necessary to terminate after the other R repeat.
In addition, the LTRs contain information that enhances the degree of transcription of
the three retroviral genes (enhancer regions). These enhancers can be up or
downstream from the protein-encoding part of the genes.

Host RNA polymerase II copies the provirus DNA to genomic RNA which can be
also spliced to mRNAs. Since the polymerase starts after the promotor (in U3), at
the transcription initiation site, it begins exactly at the beginning of the R region (see
diagram below). Thus we get a faithful (almost-see below) copy of the RNA that
entered the cell. The termination sequences and poly A signal are in U5 which is
also not copied.

Note: Because of this mechanism, there can be only one promotor site (from U3)
for all three viral genes so they must be all transcribed together. Splicing
(enzymes from the host cell nuclear splicing machinery) gives the individual mRNAs
where necessary. See notes on HIV in which this has been well elucidated. Unlike the
situation with DNA tumor viruses, there is no distinction between early/late functions.

Note: You may ask why, if U5 contains the termination and polyadenylation sites,
does the transcript not just terminate after the first R region in the above diagram and
never get into the structural genes. The termination site in the first U5 is suppressed,
often by complex secondary structure mechanism. In some retroviruses there is a
sequence in the gag gene that provides the context to suppress the termination
activity of the first U5. Clearly the second U5 does not have a gag gene following it.

Note: The copying of the RNA and the synthesis of the complementary DNA strand
are carried out by reverse transcriptase. Reverse transcriptase is an
RNA-dependent DNA polymerase and, like all DNA polymerases, it needs a
primer. This is a cellular tRNA that is packaged within the viral particle.

This strategy of virus replication in which viral RNA is first copied to DNA (by reverse transcriptase) which then gives rise to mRNA and protein poses a problem for the
virus. The initial step (RNA to DNA) is carried out by a viral enzyme which is not
normally in the cell. Yet this transcription step must occur before any mRNA
transcription or protein translation can occur. The problem is solved by the virus
carrying about 10 copies of the reverse transcriptase protein into the cell with it.
These were packaged when the virus was assembled in the previous host cell.

The structure shown in figure 15A and the upper part of figure 17 is that of a typical
retrovirus with three structural genes (gag, pol and env) but none of these is
oncogenic
If the virus is to transform a cell, in addition to, or instead, of part of the gag/pol/env
genome, it must have sequences that alter cellular DNA synthesis and provide the
other functions that are typical of a transformed cell. Thus we also find an
ONCOGENE (onc) in the viral genome of many retroviruses that transform cells to
neoplasia (figure 17).

Definition of virally-induced transformation: The changes in the biologic
function and antigenic specificity of a cell that result from integration of viral genetic
sequences into the cellular genome and that confer on the infected cell certain
properties of neoplasia. Note, however, that transformation can be induced by
factors other than viruses e.g. carcinogens.

In retroviruses, these were first discovered as an extra gene in Rous sarcoma
virus (RSV). This gene was called src (for sarcoma).
src is not needed for viral replication. It is an extra gene to those (gag/pol/env)
necessary for the continued reproduction of the virus. RSV has a complete
gag/pol/env genome. Deletions/mutations in src abolish transformation and
tumor promotion but the virus is still capable of other functions. RSV is unusual in that
it has managed to retain its whole genome of gag/pol/env.

In sharp contrast to RSV, many retroviruses have lost part of
their genome to accommodate an oncogene. This has two consequences:

1) The protein encoded by the oncogene is often part of a fusion protein with other
virally-encoded amino acids attached

2) Virus is in trouble as cannot make all of itself. To replicate and bud from the host
cell needs products of another virus, that is a helper virus

About forty oncogenes have now been identified. Note that they are referred so by a
three letter code (e.g. src, myc) often reflecting the virus from which they were first
isolated. Some viruses can have more than one oncogene (e.g. erbA, erbB). Here
are a few of the most studied:

Once retroviral oncogenes had been discovered, a surprising observation was
made: Unlike the situation with DNA virus oncogenes which are true viral genes,
there are homologs of all retrovirus oncogenes in cells that are not infected by a
retrovirus. These cellular homologs are often genes involved in growth control and development/differentiation (as might be expected) and have important
non-transforming functions in the cell; some can cause cancer under certain
circumstances and, presumably, those not shown to cause cancer have the ability to
do so under the correct conditions. The cellular homologs of viral oncogenes are
called proto-oncogenes. To distinguish viral oncogenes from
cellular proto-oncogenes, they are often referred to as v-onc and c-onc respectively.
Note: c-oncs are not identical to their corresponding v-oncs. It appears that the virus
has picked up a cellular growth controlling or differentiation gene and, after the gene
was acquired by the virus, it has been subject to mutation.

Definition of a proto-oncogene: A host gene that is homologous to an oncogene
that is found in a virus but which can induce transformation only after being altered
(such as mutation or a change of context such as coming under the control of a highly
active promotor). It usually encodes a protein that functions in DNA replication or
growth control at some stage of the normal development of the organism.

1) These are typical cellular genes with typical control sequences. As with most
eucaryotic genes, most have introns (retroviral oncogenes do not)

3) As with all genes in the eucaryotic genome, they are always at same place in
genome (cf. what would be expected of endogenous retroviruses that had, over time,
become incorporated into the cellular genome)

5) Viral oncogenes are most like the c-onc of the animal from which the virus is thought
to have acquired the gene. Thus, v-src of RSV is more like chicken src than human
src. Note: v-onc was picked up accidentally by the virus from the genome of a
previous host cell

6) Cellular oncogenes are expressed by the cell at some period in the life of the cell,
often when the cell is growing, replicating and differentiating normally. They are
usually proteins that are involved in growth control.

If v-onc and c-onc are so alike, why does the viral oncogene (v-onc) introduced by a
virus cause havoc in the cell? This is due to differences in the genes, mutations that
have occurred in the gene once it was picked up by the virus. Such changes include:

3) V-oncs are inserted into the host genome along with LTRs which contain promotors/enhancers. This is likely to result in over expression of a gene that we
know is probably involved in control of DNA transcription and replication!

The observation that an acutely transforming virus such as RSV contains an extra
gene, the oncogene, explains their high neoplastic potential but, in contrast,
chronically transforming retroviruses only produce tumors slowly and they carry
no gene equivalent to a v-onc. At best, these viruses have just the three usual
viral genes (gag/pol/env). An example is avian leukosis virus (ALV).

How do chronically transforming viruses induce a tumor if they do not have an
oncogene?

A seminal observation was made: Just as any other retrovirus does, ALV can
integrate into the cell genome at many different sites but, in ALV-induced tumors,
the virus is ALWAYS found in a similar position (very important!). This means that
the crucial transforming event must be rare and that the cells that form the tumor are
a clone (cf. the acute transformers which integrate all over the place). In all cases of
ALV-induced tumors, the viral genome is inserted near a cellular gene called
c-myc. This is the cellular proto-oncogene that, in an altered form (i.e. as a v-onc),
is carried by some acutely transforming retroviruses (e.g. avian
myelocytoma virus which causes carcinoma, sarcomas and leukemias).
In addition, the level of translation of c-myc in the ALV-transformed cell is much
greater than in uninfected cells. Thus, inserting the genome of ALV and other
chronically transforming retroviruses next to a c-onc has the same effect
as carrying in a v-onc.

So, in integration, the virus comes to lie upstream from c-myc which
then comes under the influence of the strong LTR promotors of the virus which leads
to over expression of c-myc. This is called oncogenesis by promotor insertion

But in some tumors the virus is downstream from the c-myc gene.
However, we saw that LTRs also have enhancers in addition to promotors. We
know that enhancer sequences can be upstream or downstream to have their effect.
This is called oncogenesis by enhancer insertion

Why is insertion near c-myc important? The protein coded for by this gene is found in
the nucleus of normal cells and is involved in control of DNA synthesis. It can be
shown that over-expression of c-myc leads to rapid DNA replication.

Once it had been shown that viruses can either bring an oncogene into the cell or can
take control of a cellular proto-oncogene to give rise to a tumor, the question arose of
whether cellular proto-oncogenes could give rise to tumors in the absence of
retroviral infection. The answer is yes! Other chromosomal rearrangements can bring
a c-onc under the control of the wrong promotor/enhancer. Alternatively, the c-onc
might be mutated in a particular way so that it was over-expressed or it might code
for a mutant protein with an altered function.

Chromosomal mapping allows the precise localization of the site
of a gene on a particular chromosome and many cancers are associated with
alterations in chromosomes, particularly translocations (the breakage of a
chromosome so that the two parts associate with two parts of another chromosome).

Many break sites in tumor cells are very close to a known c-onc.
This is highly suggestive and unlikely to have occurred by chance!

Disease	C-onc	translocation
Burkitt's lymphoma *	myc	8 to 14
Acute myeloblastic leukemia	mos	8 to 21
Chronic myelogenous leukemia	abl	9 to 22
Acute promyelocytic leukemia	fes	15 to 17
Acute lymphocytic leukemia	myb	6 deletion
Ovarian cancer	myb	6 to 14

* In Burkitt's lymphoma the c-myc on chromosome 8 is brought to a site on
chromosome 14 close to the gene for immunoglobulin heavy chains.
It seems that the proto-oncogene may thus be brought under the control of the
Ig promotor, which is presumably very active in B lymphocytes. This explains why
this tumor arises in B cells. In other lymphomas, a c-onc is brought next to the
immunoglobulin light chain promotor. These are also B cell lymphomas.

Epstein-Barr virus is probably the cause of Burkitt=s lymphoma. This is a
herpes virus and herpes viruses commonly cause chromosomal breaks.

IS THERE EVIDENCE THAT MUTATIONS IN CELLULAR ONCOGENES
MIGHT ALSO RESULT IN TRANSFORMATION?

The best evidence comes from the cellular oncogene that is the homologue of the
viral oncogene found in the Harvey