Download Tumor Viruses

CLASS: 11:00 – 12:00
DATE: November 22, 2010
PROFESSOR: Engler
I.
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VI.
VII.
TUMOR VIRUSES
Scribe: Adam Baird
Proof:
Page 1 of 5
HOW RNA AND DNA VIRUSES HELP US UNDERSTAND ONCOGENES AND TUMOR SUPPRESSORS [S1]
a. Today’s lecture will begin with a few questions:
1. What are the top three types of cancer that will be diagnosed in 2010?
2. Is there a difference between these top three cancers between men and women?
a. Men
i. Prostate Cancer
ii. Lung Cancer
iii. Colon/Rectal Cancer
b. Women
i. Breast Cancer
ii. Lung Cancer
iii. Colon/Rectal Cancer
3. Will the number of new cases of cancer be smaller/greater this year than in past years?
a. It is expected that there will be about 800,000 new cases of cancer this year for men.
b. It is expected that there will be about 740,000 new cases of cancer this year for women.
c. The number of new cases of cancer may be higher than in past years because of new
research, better screening, more cancers caused by environments, long life spans, etc.
d. The number of new cases of cancer may be lower than in past years because of better
prevention, better nutrition, better diagnosis (quicker diagnosis), etc.
e. The statistics suggest that there are 290,000 – 300,000 new cases of cancer for men and
270,00 new cases of cancer for women. The numbers stay pretty consistent though,
regardless of better research, better prevention, etc. About the same amount of incidence are
happening year after year.
b. Viruses have classically been used to describe how cancers work.
CHARACTERISTICS OF CANCER CELLS [S2]
a. What are cancer cells? What are their characteristics?
1. They undergo unregulated growth.
2. They are immortal (they grow when they’re not supposed to).
3. They have increased nutrient uptake (because they’re growing).
4. They become anchorage independent.
5. They have an active cell cycle.
a. Growth signaling pathways activated (oncogenes – RNA tumor viruses)
b. Pathways to prevent cell proliferation are disrupted (tumor suppressors – DNA tumor viruses)
CANCERS DEVELOP IN MANY STEPS DUE TO MANY MUTATION EVENTS [S3]
a. Cancers grow in many steps. There are a variety of mutations that occur that make “normal” cells become
carcinoma and eventually metastasize.
b. Cancers are characterized by their site and by their type.
SOME TYPES OF CANCER ASSOCIATED WITH TUMOR VIRUSES [S4]
a. Types of cancer:
1. Leukemia (derived from blood cell/lymphoid cells)
2. Carcinoma (derived from epithelial/endothelial cells)
3. Sarcomas (derived from connective tissue cells)
DEFINITIONS [S5]
a. Oncogene: “Gain of function” mutant
1. The gene product causes something to happen in a dominant fashion (“a dominant negative”); it may
be a mutation or an over-expression at the wrong time in the cell cycle.
2. It is usually a mutant form of a normal gene. The normal gene is referred to as the “proto-oncogene”.
b. Tumor Suppressor Gene: “Loss of function” mutant
1. The gene whose normal function actually produces cancer.
2. Enhances the likelihood that a cell can become cancerous.
ONCOGENES CAN INCREASE TUMOR SUSCEPTIBILITY IN TRANSGENIC MICE [S6]
a. There has been a great amount of research on oncogenes and tumor suppressor genes.
b. Oncogenes, specifically, can increase tumor susceptibility in transgenic mice.
c. The blue line shows the “Myc” gene, which, if expressed in the mouse as a transgene, will eventually result in
a formation of a tumor.
d. The red line shows the “Ras” genes, which is more potent thatn the “Myc” gene.
e. The purple line shows the “Myc + Ras” gene.
HOW DO ONCOGENES AND TUMOR SUPPRESSORS WORK? [S7]
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: November 22, 2010
Proof:
PROFESSOR: Engler
TUMOR VIRUSES
Page 2 of 5
a. Oncogenes:
1. Dominant mutations
2. Oncogenes can increase the susceptibility to cancer.
3. Oncogenes express a mutation at the wrong time in the cell cycle.
4. In a single event, it takes a normal cell and makes it into a tumor (and it proliferates abnormally).
b. Tumor Suppressor Genes:
1. Recessive mutations (because both copies of the gene must be “knocked out” in order for the tumor
effect to be shown).
2. DNA tumor viruses can “knock out” both copies of the gene (not genetically, but by “sucking up” all of
the normally functioning proteins, keeping it from doing its normal function).
VIII.
THERE ARE MANY PATHWAYS AFFECTED BY ONCOGENES & TUMOR SUPPRESSOR PROTEINS [S8]
a. Please do not memorize this map.
b. This shows all the places that oncogenes or tumor suppressors are known to affect cells.
c. Each red dot is a place where oncogenes may occur.
d. The point: there are many, many places where “things can go wrong”.
IX.
GENERAL OVERVIEW OF DNA AND RNA TRANSFORMING VIRUSES [S9]
a. How do DNA and RNA viruses fit into what has been discussed so far? They were the model systems that
helped us figure everything out. They are easy genetic elements to study and manipulate. They helped us
understand how cancers work.
b. There are many kinds of viruses.
c. Role in natural oncogenesis:
1. Rous sarcoma virus forms solid tumors in chickens
d. Potential for involvement in human tumors:
1. DNA viruses
a. HPV – Cervical cancer
i. Gardasil – Drug that is designed to lower the chance of cervical cancer.
b. Herpes or Epstein Barr Virus (EBV)
i. Many people have antibodies
ii. Anyone infected with mononucleosis has been infected by EBV
iii. Immunologic defect allows cancer to occur (Burkitts lymphona, nasophayngeal
carcinoma)
iv. Hepatitis B Virus (DNA virus causing liver cancer)
2. RNA viruses
a. HTLV-1 (T-cell leukemia, mainly in Japan)
b. Hepatitis C Virus (RNA virus causing liver cancer)
3. RNA viruses
4. DNA and RNA viruses have been shown to having some linkages in the formation of cancers.
X.
RETROVIRUS LIFE CYCLE REQUIRES INTEGRATION INTO THE CHROMOSOME [S10]
a. Why study retroviruses? Reverse transcriptase, which takes the RNA genome of the RNA virus, makes a
DNA copy of it, and inserts the DNA copy into the chromosome.
b. Acute viruses carry oncogenes.
c. Chronic viruses don’t carry oncogenes (but instead insert material into the chromosome).
XI.
RETROVIRUSES CAN CAUSE TUMORS IN TWO WAYS [S11]
a. Acute viruses:
1. Carry oncogenes in the virus RNA genome
2. Can transform cells in culture
3. May be able to replicate by themselves
4. Rous sarcoma virus, which can replicate as a virus, even though it carries an oncogene.
5. Many acute viruses require a helper virus in order to grow.
b. Chronic (non-acute) transforming viruses
1. Don’t carry oncogenes
2. Insert material to another chromosome nearby
3. By inference, they can transcribe the oncogene, making it “turn on” at the wrong time in the cell cycle
4. They are still capable of replication (but may take a long time to form a tumor)
XII.
CHRONOLOGY OF UNDERSTANDING ONCOGENES [S12]
a. Note: Some of the following information was written on the chalkboard.
b. The Rous sarcoma virus (RSV) was the model for this (named after Peyton Rous, who discovered it). It
causes tumors in chickens.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: November 22, 2010
Proof:
PROFESSOR: Engler
TUMOR VIRUSES
Page 3 of 5
c. If the tumor in the chickens was “ground up” and “strained out”, the strained liquid from tumor of the first
chicken could infect other chickens.
d. Chickens have a normal body temperature of 41 degrees Celsius. The normal virus will make tumors at that
exact temperature (and at 37 degrees Celsius as well).
e. RSV was clearly a virus (and not a bacteria).
f. Early 1900’s
1. Rous found some mutations in the virus that were temperature sensitive (meaning that the
phenotype can be changed by temperature change).
2. Rous discovered some copies of the virus that resulted in no tumors at 41 degrees Celsius, but
some copies of the virus that resulted in the formation of tumors at 37 degrees Celsius. This was the
first piece of genetic evidence that it might be temperature sensitive.
g. Mid 1950’s
1. Peter Vogue (?) was studying RSV as well. He found that, as he grew RSV, he could identify
defective viruses. So these defective viruses could not make viruses anymore.
2. Vogue discovered that the RNA of a normal virus was longer than the RNA of a defective virus –
there was something missing.
3. Vogues postulated that the oncogene (the thing that makes the virus form a tumor) was absent in the
defective virus (possibly by deletion). From what we know today, this makes perfect sense.
h. 1970’s
1. Reverse transcriptase had been discovered.
2. It was later discovered that reverse transcriptase could be used to ask questions about the genes in
the defective and normal viruses. Reverse transcriptase could be used to make DNA copies of RNA
of functional viruses and make a hybrid experiment (meaning that a pool of RNA could be hybridized
to the RNA of the defective virus). There was always some part of the original RNA pool that was not
hybridized. The hypothesis: the non-hybridized RNA was specific for the oncogene. Their hypothesis
was correct.
3. They later discovered that oncogenes are present in normal DNA (and this is where the idea of the
proto-oncogene comes from). The normally functioning gene in the cell gets inserted into the virus
and causes tumors (because it is expressed inappropriately). They found that this was the case on
chicken DNA (65%), mouse DNA (50%), and on human DNA (40%).
4. This research used one simple observation from the early 1900’s about RSV and turned it into a
paradigm, showing that the normal genes in the DNA “lose control “by being captured by the RNA in
the virus and transmitted from cell to cell to cause tumors.
5. In summary, they determined that it was normal genes that “lost control” that caused cancer.
XIII.
ONCOGENE-ENCODING VIRUSES [S13]
a. There are many different kinds of retroviruses; some are replication competent, but many are replication
defective.
b. Notice that the RSV has an oncogene called “src” (for sarcoma). The virus could infect the cell, express the
“src” gene, and cause the cell to become a tumor. (Fortunately, this only infects chickens.)
c. Other oncogenes that were discovered were “v-myb”, “v-ets”, “v-erb B”, etc. These were often non-defective
viruses that carry an oncogene but can’t grow by themselves (so they are found in mixtures of wild type virus
and the oncogenic virus). Notice the large deletions. Notice that the oncogenes here have taken over
essential parts of the genome, meaning that it can only grow as defective viruses. For this reason, they are a
little slower to grow and to cause tumors.
XIV.
CHRONOLOGY OF UNDERSTANDING ONCOGENES [S14]
a. Exam material: proto-oncogenes and the four classes of oncogenes…
XV.
GENETIC CHANGES CONVERT A PROTO-ONCOGENE INTO AN ONCOGENE [S15]
a. These are both cell service receptors.
b. “erbB” is the receptor for epidermal growth factor, which is part of what the body needs to regulate itself (it
senses the hormone). In “erbB”, the entire sensor is deleted (that senses whether “egf” is present), which
confuses the protein (so that it appears that “egf” is still there). This causes the cell to continuously grow,
even if it’s not supposed to.
c. Another change that can occur is seen in the receptor for the “neu” oncogene. A single mutation (down in the
membrane) changes the protein so that it’s always sending a “on” or “keep growing” signal, even if growth is
not needed.
d. This is typical with oncogenes; many times, there are mutations that activate the cells to grow at the wrong
times.
XVI.
PROTO-ONCOGENES MAINLY ENCODE COMPONENTS OF… [S16]
a. Proto-oncogenes mainly encode components of growth factor signal transduction pathways.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: November 22, 2010
Proof:
PROFESSOR: Engler
TUMOR VIRUSES
Page 4 of 5
XVII. FOUR CLASSES OF ONCOGENES [S17]
a. Class One: Oncogene that mimics a growth factors. It is secreted by the virus, gets out of the cell, and signals
other cells to grow (even though they don’t need to). Example:
1. “Sis”:
a. Acts like a platelet-derived growth factor
XVIII. PROTO-ONCOGENES MAINLY ENCODE… [S18]
a. Exam material: Things that are “phony” growth factors can be oncogenes because they send a signal to grow
at the wrong time.
XIX.
CLASS TWO: MUTATED RECEPTORS [S19]
a. Class Two: Oncogenes that are mutated receptors for growth factors. It doesn’t matter if the growth factor is
there or not; it seems like it is present, and so the cells continuously grow. Examples:
1. “neu”
2. “erbB”
XX.
GENETIC CHANGES CONERT A PROTO-ONCOGENE INTO AN ONCOGENE [S20]
a. This is the same slide that we talked about earlier.
b. These are examples of oncogenes, converted by mutation, that always send an “on” signal whether or not the
growth factor is actually present or not.
XXI.
CLASS THREE: INTRACELLULAR TRANSDUCERS [S21]
a. Class Three: Intracellular transducers
b. The receptors are sitting on the surface of the cell. There are many molecules inside the cell that take the
initial surface signal and send it into the nucleus. There are four types of oncogene transducers:
1. Protein-tyrosine kinases
2. Protein-serine/threonine kinases
3. G-Protein (Ras) proteins
4. Phospholipase C (PKC)
c. All of these transducers send a signal to the cell telling it to grow, even if it isn’t supposed to. They are
sending an incorrect “on” signal.
XXII. PROTO-ONCOGENES MAINLY ENCODE… [S22]
XXIII. CLASS FOUR: TRASNCRIPTION FACTOR ONCOGENES [S23]
a. Class Four: Transcription factors
b. There are transcription factors, when mutated, start expressing genes involved with cell growth at a time that
is inappropriate. Examples:
1. “Jun”
2. “Fos”
3. “Myc”
4. “Myb”
5. “Rel”
6. “erbA”
c. Transcription factor AP1 (a commonly used transcription factor) consists of both “Jun” and “Fos” (and each of
these can be an oncogene). If either “Jun” or “Fos” is “on”, then there a lot of AP1 present, and there will be a
lot of gene expression, whether it’s needed or not.
d. Summary of the four kinds of oncogenes: there can be a “phony” growth factor, there can be a defective
“always on” receptor, there can be an “always on” transducer, or there can be a mutated transcription factor
(that is always working). All of these are known because of RNA tumor viruses.
XXIV. THE SAME ONCOGENES CAN BE FOUND IN MORE THAN ONE VIRUS ISOLATE [S24]
a. Don’t memorize this. These are just examples…
XXV. RETROVIRUSES CAN CAUSE TUMRON IN TWO WAYS… [S25]
a. So, the viruses we just talked about were acute viruses. Those are the one that carry the oncogene itself.
XXVI. ONCOGENESIS BY VIRUS INSERTION [S26]
a. There are also non-acute (chronic) viruses. They can’t transform in culture because they don’t carry an
oncogene, but they work “at a distance”. They insert themselves nearby and they “turn on” a proto-oncogene
at the wrong time. They can often replicate themselves too. But they are much slower to form tumors
(because it starts with a single event and then must branch afterwards). These viruses transform cells by
insertional mutagenesis – and there are many ways that they can do this.
XXVII. RETROVIRUS LIFE CYCLE… [S27]
a. Here is an example of one of the ways that non-acute transforming viruses work.
b. The simplest way: when certain proto-oncogenes and viruses have “enhancers” (which are transcription
controlled elements that signal others that it’s time to transcribe).
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: November 22, 2010
Proof:
PROFESSOR: Engler
TUMOR VIRUSES
Page 5 of 5
c. Another way: they can insert themselves so that their promoter can transcribe a proto-oncogene and turn it
“on” at the wrong time.
d. A third way: altering the transcription of the gene by putting in different splice sites, resulting in hybrid RNAs
that live longer in the cell (leading to over-production).
e. In some cases, they can inactive another gene (like a tumor suppressing gene, for example, which usually
keeps a cell shut “off”). This, however, is rare and is difficult to observe.
XXVIII. HOW DO TUMORS (AND VIRUSES) OVERPRODUCE ONCOGENE PROTEINS? [S28]
a. DNA viruses have a different mechanism than RNA viruses. Rather than carrying oncogenes, DNA viruses
inactivate the components that keep growth “off”. The work of RNA tumor viruses, for example, is similar to
putting gas in a car (signaling it to go faster), whereas the work of DNA tumor viruses is similar to removing
your foot from the break (signaling it to also go faster). In both cases, the cells grow.
XXIX. INSERTIONAL ACTIVATION OF PROTO-ONCOGENES [S29]
XXX. SOME DNA TUMOR VIRUSES BLOCK TUMOR SUPPRESSOR PATHWAYS [S30]
a. Human adenovirus is an example of a DNA tumor virus.
b. Human papillomavirus (HPV) is another example of DNA tumor virus.
c. There are a number of papovaviruses too.
d. They have proteins that sequester “Rb” (retinoblastoma), which is a tumor of the eye that comes from a very
specific set of mutations in the “Rb” gene. One way to get retinoblastoma is to have a genetic mutation in the
“Rb” gene. Another way to get retinoblastoma: DNA tumor viruses make virus proteins that bind to “Rb” and
take it away from its original location (they “suck up” the “Rb”).
XXXI. MANY DNA TUMOR VIRUSES ENCODE PROTEINS THAT BIND TO AND SEQUESTER RB [S31]
XXXII. RETINOBLASTOMA [S32]
a. Retinoblastoma is hereditary (deletion in Chromosome 13).
b. It is recessive.
c. About 1 child in 20,000 is afflicted.
d. It can also be non-hereditary.
XXXIII. RETNOBLASTOMA GENE (RB) [S33]
a. “Rb” encodes an anti-proliferation protein (tumor repressor).
b. Normally, “Rb” inactivates a protein involved in turning on all the replication genes in the cell. So at the right
point of the cell cycle, it happens to get inactivated by phosphorylation and the proteins required to make
replication are made, and the cell moves into the synthesis phase of the cell cycle.
XXXIV. RB SHUTS OFF CELL PROLIFERATIONS BY BINDING TO E2F [S34]
a. However, in the case of infection with a tumor virus, there is a protein that will bind up the “Rb” so that it can’t
hold onto the E2F, so the E2F is released. The cell thinks that it’s time to enter the synthesis phase of the cell
cycle, but it’s not ready. Cell proliferation, then, is activated because of the virus infection.
b. The virus wants many DNA replication genes surrounding it, because that’s what it uses to grow.
XXXV. VIRAL PROTEINS SEQUESTER TUMOR SUPPRESSORS TO PROMOTE CELL PROLIFERATION [S35]
XXXVI. HOW DO ONCOGENES AND TUMOR SUPPRESSORS WORK? [S36]
a. There are two ways to control cells (to start tumors). One way is dominant (gain of function mutation called an
oncogene). The other way is non-dominant (loss of function mutation called tumor suppressor).
XXXVII. SUMMARY [S37]
a. This is the summary of what was discussed in the lecture today. Review the different types, as well as their
examples.
[End XX:XX mins]