Docstoc

Lecture 16

Document Sample
Lecture 16 Powered By Docstoc
					                                                                                   R. Werner, Genetics (2011)
                                                                                              MBL Lecture 53

                                        Genetics Lecture 27.
                                          Cancer Genetics

Objectives
After studying this section, the student should be able to answer the following questions:


    1. What is an oncogene?
    2. What proteins are typically encoded by proto-oncogenes?
    3. What is a tumor suppressor gene? Give examples.
    4. What is a check point?
    5. What is the Philadelphia chromosome?
    6. How is aneuploidy related to cancer?
    7. What is a tumor promoter?


Most types of cancer can be regarded as an old-age disease. In a complex multicellular organism such as
the human body it is likely that over a lifespan of 70 or more years several cancerous cells arise that have
lost their proper growth control mechanisms. In younger people, the immune system probably recognizes
these abnormal cells and destroys them before they can grow into a tumor. In older people the immune
system becomes weaker, allowing some cancerous cells to escape immune surveillance and form a life-
threatening tumor. What role does genetics play in cancer? We do not have a clear answer, but it is well
established that some cancers occur in families. It is likely that the susceptibility for developing cancer
can be inherited.
One distinguishes two types of tumor, the benign tumor that grows into a large mass of cells but stays
within its normal tissue boundaries, and the malignant tumor, whose cells can invade other tissues and
form secondary tumors, called metastases, at sites removed from the site of the original tumor. Although
human cancers can occur in almost any tissue, they are generally divided into three types:
    1. Carcinomas are cancers derived from epithelial tissues, i.e. skin, gut, lung, exocrine glands.
    2. Sarcomas arise from cells of bone, cartilage, muscle, fibroblasts.
    3. Leukemias and lymphomas originate from blood cells.
Over 90% of all human cancers are carcinomas. Most likely this is because epithelial cells, whether in the
skin, the gut, or the lungs are exposed to environmental influences, such as carcinogens or radiation,
agents that cause mutations, which in turn may cause the cell to become a cancer cell.
Epidemiological studies suggest that some forms of cancer are clearly caused by environmental agents,
such as diet, life style, or pollution of the air, although the nature of the carcinogenic agent is often not yet
known. For example, colon and breast cancer are most prevalent in the United States but occur much
less frequently in Japan. On the other hand, cancer of the stomach is very common in Japan but much
less frequent in the United States. The evidence that cigarette smoking causes most lung cancers is now
overwhelming, and the role of exposure to excessive sun light is known to increase the frequency of skin
cancer.
In addition to carcinogens, certain viruses are suspected of causing cancer as well. Hepatitis B virus, for
example, is believed to be at the root of some liver cancers. The DNA viruses polyoma and SV40
produce cancers in hamsters. Retroviruses have been shown to cause cancers in some species, but
there is no evidence yet that a viral gene product is sufficient to convert a normal cell into a cancer cell.
There may be other mechanisms that allow viruses to transform a normal cell into a cancer cell.



                               Genetics Lecture 27: Cancer Genetics, Page 1
                                                                                   R. Werner, Genetics (2011)
                                                                                              MBL Lecture 53

Most human tumors consist of a clone of cells that are all derived from a single cell with chromosomal
rearrangements. This clonality of cancer has been deduced from the fact that in females, that are
heterozygous for the X-linked enzyme glucose-6-phosphate dehydrogenase, all tumor cells express the
same isozyme, while in the rest of the body different cells have different X-chromosomes inactivated and
thus express different isozymes.
It appears that the initial event in the formation of a cancer cell usually is the creation of aneuploidy, i.e.,
chromosomes, or parts of them, in these cells are scrambled, duplicated, deleted, or structurally
rearranged. That type of chaos affects thousands of different genes. There is no cancer that is not
aneuploid. Thus, it appears that aneuploidy may be the ultimate cause of cancer, although this has not
been proven. Many people still think that it is the accumulation of multiple mutations in cell regulatory
genes that eventually leads to cancer. However, there are carcinogens that a clearly not mutagenic. One
example is asbestos, an inorganic mineral that has been linked to lung cancer. It is conceivable that the
asbestos fibers disrupt the spindle apparatus during cell division thereby causing aneuploidy, which
subsequently leads to cancer.
The aneuploid cell often remains in a precancerous state for long periods of time. It requires the action
of a tumor promoter for the expression of the cancer phenotype. Tumor promoters are typically agents
that stimulate quiescent cells to divide, a necessary step in carcinogenesis. For example, when the skin
of a mouse is treated with benzopyrene, a potent carcinogen, tumors rarely develop. However, if the skin
is subsequently painted with a phorbol ester, such as 12-O-tetradecanoyl phorbol-13-acetate (TPA),
tumors will develop. TPA by itself is not a carcinogen, instead it is an activator of protein kinase C. The
action of protein kinase C causes the mutagenized cell to proliferate and thereby establish its cancer
phenotype. (Remember that you need DNA replication to establish a mutation.) Tumor promoters are also
potent inhibitors of cell-to-cell communication via gap junctions. It appears that the interruption of this
communication allows normal growth inhibiting molecules to pass from a neighboring normal cell into the
aneuploid cell (pre-cancer stage) thereby phenotypically suppressing the cancer phenotype.


ATTEMPS TO ISOLATE A HUMAN CANCER-CAUSING GENE
When normal mammalian cells are grown in tissue culture, the cells divide until they form a monolayer, a
single sheet of cells that covers the culture dish. At that point cell growth ceases. The culture is said to be
confluent. In order to make the cells grow again, they must be dislodged from their support surface and
from each other with the proteolytic enzyme trypsin and re-plated at a lower concentration in a new
culture dish. For human cells, this process of propagation can be repeated until the cells have divided
about 60 times; then all cells die. Presumably, a biological clock ticks inside each cell that makes the cell
age and eventually die. Only germ cells are immune from aging. The nature of this biological clock is not
known.
Sometimes, however, a cell in a primary culture becomes aneuploid. Aneuploid cells possess a greatly
increased rate of further shuffling of chromosomes (over 1,000 times the normal rate). Most of these
newly generated aneuploid cells are unable to compete with normal cells and are eliminated by defense
mechanisms such as apoptosis (to be discussed below). However, sometimes one of the newly created
aneuploid cells is capable to escape apoptosis. It has acquired a series of mutations that stop the
biological clock and thus makes the cell immortal. Cells derived from such a mutant are called a cell line;
they can be propagated indefinitely but still form a monolayer in culture that must be trypsinized for
growth to resume. Tumor cells behave very differently; they are immortal like cell lines, but instead of
forming monolayers they continue to grow, causing the cells to pile up on top of each other.
One cell line that has been most useful in the isolation of a potential human cancer gene is the mouse
fibroblast 3T3 cell line. Normally, 3T3 cells grow in a monolayer. However, when DNA isolated from a
human tumor is sheared to fragments of about 50 kilobase and added in the form of a calcium phosphate
precipitate onto the 3T3 monolayer, some 3T3 cells incorporate the gene that was responsible for the
cancer phenotype of the tumor cells and, as a consequence, assume the properties of the tumor cell.
Such transformed 3T3 cells form foci, clusters of cells piled up on top of one another, that are readily
recognized within the 3T3 monolayer (Fig. 1). DNA isolated from these primary 3T3 transfectants can be
used to transfect new 3T3 cultures, producing foci, which are called secondary transfectants.



                               Genetics Lecture 27: Cancer Genetics, Page 2
                                                                              R. Werner, Genetics (2011)
                                                                                         MBL Lecture 53

The human gene that was incorporated into the mouse genome of 3T3 cells can be identified by the
presence of the highly reiterated human Alu sequence, which is present on average about once in every
5 kb segment of the human genome . (Mouse DNA does not contain this sequence.) This is done by
making a genomic library from a secondary 3T3 transfectant and screening it with a radioactive Alu
probe. The first human oncogene isolated this way was the H-ras-1 gene expressed in a bladder
carcinoma. It was soon discovered that this gene was identical to the H-ras transforming gene of the
Harvey sarcoma virus, a retrovirus causing tumors in rodents.
By using the Harvey sarcoma virus H-ras gene as a hybridization probe, an H-ras homolog was also
found in normal human cells. When this H-ras-1 gene was cloned and used to transfect 3T3 cells, no foci
formed. Sequence analysis soon revealed that the H-ras-1 oncogene isolated from the bladder carcinoma
contained a single base pair change as compared to the normal cellular H-ras-1 gene. Apparently, the
product of the H-ras-1 gene is somehow involved in converting a normal cell into a cancer cell. The H-ras
gene of the Harvey sarcoma virus was found to have a mutation at the same site in addition to alterations
in other genes.




                          Figure 1. Isolation of a human cellular oncogene.


If the development of cancer requires more than a single mutation, why then is a single mutated H-ras-1
gene capable of transforming 3T3 cells? The answer probably is that 3T3 cells are not really normal cells
(they are aneuploid), even though they do not cause tumors yet when injected into animals. It is possible
that the immortalizing mutation, responsible for making mouse fibroblasts a cell line, together with the
mutated H-ras-1 gene is sufficient for transformation. When primary mouse cells are transfected with the
mutated H-ras-1 gene, they do not become transformed.


ONCOGENES
The mutated H-ras gene was called an oncogene because it was thought that it causes cancer. I arose
by mutation from a normal cellular gene called a proto-oncogene. RNA tumor viruses are retroviruses



                             Genetics Lecture 27: Cancer Genetics, Page 3
                                                                                                     R. Werner, Genetics (2011)
                                                                                                                MBL Lecture 53

that usually carry an oncogene. Since the genome of retrovirus viruses integrates into its host's
chromosomal DNA during part of the virus's life cycle, it is likely that a cellular proto-oncogene was picked
up when the provirus genome was transcribed into viral RNA and then packaged into virus particles.
There, the proto-oncogene mutated into an oncogene, and when the virus then infects a normal cell, it
expresses the oncogene, which may lead to the transformation of that cell into a tumor cell.
This was shown to occur in some laboratory animals, but attempts to transform a normal cell with a
mutated oncogene have so far not been successful. Even combinations of mutated oncogenes were
unable to transform a normal cell simply by expressing their gene products. The question, therefore,
remains “what specific event causes a cell to become a cancer cell”. A recent publication by A. Klein et
al., entitled “Transgenic oncogenes induce oncogene-independent cancers with individual karyotypes and
phenotypes” showed that even though oncogenes indeed can transform normal cells into tumor cells the
presence of the oncogene in the tumor cell is not required, since over 50% of the tumor cells had lost
their oncogene. How can this be explained? To understand the sequence of events that might happen we
have to first discuss what cellular proteins normal proto-oncogenes code for.


Many different viral oncogenes, called v-onc, have been isolated and characterized from retroviruses.
Corresponding cellular proto-oncogenes, called c-onc, have been found in every case. One might ask
what the normal cellular function of these genes is. In several cases, the properties of their gene product
      The activities and cellular cases, only of the products and time of expression of
or their cellular function is known. In other locationsthe subcellular locationof the main
the proto-oncogene product is known. Oncogenes which can be grouped into five distinct classes:
                             classes of known proto-oncogenes
Protein kinases, GTP-binding proteins, cellular growth factors, hormone and growth factor receptors, and
nuclear transcription factors. Figure 2 illustrates the activities and cellullar locations of some cellular proto-
oncogenes.

                                           Ras proteins                                         PDGF (sis)
          PDGF receptor                    (H-ras)                                              EGF
          EGF receptor (erbB)              (N-ras)              Src protein kinase              M-CSF
          M-CSF receptor (fms)             (K-ras)                     (src)
                                                                                              growth factors



          growth factor receptors         GTP-binding           membrane/cytoskeleton-
         acting via tyrosine-specific      proteins            associated tyrosine-specific
           protein kinase activity                                                                (myc)
                                                                     protein kinases              (fos)
                                                                                                  (jun)
                                          thyroid hormone
                   (fes)                  receptor (erbA)               (raf)
                                                                                              nuclear proteins

       cytoplasmic tyrosine-specific     steroid-type growth        serine/threonine-
       protein kinases                   factor receptors           specific
                                                                    protein kinases


                 Figure 2. The activities and cellular locations of some proto-oncogenes.


TRANSCRIPTIONAL ACTIVATION OF PROTOONCOGENES
Karyotype analysis of human tumor cells revealed that most forms of cancer are always associated with
chromosomal rearrangements. In some cases a segment from one chromosome was found translocated
onto the end of another chromosome, in other cases mitotic nondisjunction led to uneven distribution of
chromosomes among daughter cells: One cell received both copies, the other none, of a particular
chromosome giving rise to trisomy and monosomy, respectively. Two examples of cancers involving
chromosomal rearrangements are Burkitt's lymphoma and chronic myelogenous leukemia.
In Burkitt's lymphoma, a cancer of B lymphocytes found commonly in Africa, the c-myc proto-oncogene is
translocated from the long arm of chromosome 8, to the long arm of chromosome 14 where the genes for
the heavy immunoglobulin chains are located. This translocation brings the myc oncogene under the


                                        Genetics Lecture 27: Cancer Genetics, Page 4
                                                                                  R. Werner, Genetics (2011)
                                                                                             MBL Lecture 53

influence of a powerful enhancer, located near the genes for the immunoglobulin heavy chain constant
region (Ig-Cm), resulting in transcriptional activation of the myc gene. In a few cases of Burkitt's
lymphoma, the c-myc gene remained on chromosome 8, but segments from chromosomes 2 (lambda
light chain locus) or 22 (kappa light chain locus) were found translocated next to the myc locus. This
activation of the c-myc gene involves no change in the protein sequence. Tumor formation appears to be
solely the result of overexpression of c-myc. In normal cells, c-myc is induced in response to mitogenic
stimuli; it is not expressed in quiescent cells.
Chronic myelogenous leukemia (CML) is a cancer of myeloid stem cells. It is caused by a reciprocal
crossing-over event between the long arms of chromosomes 9 and 22. The altered chromosome 22 is
called the Philadelphia chromosome.
Protooncogenes can also be activated by insertion of a strong viral promoter. This explains why some
retroviruses are capable of transforming cells even though they do not carry a v-oncogene. Typically it
takes longer for these viruses to produce a tumor in the organism they infect. As will be discussed in the
next section, all retroviruses carry strong enhancers within their LTR termini. Thus, if the virus is inserted
near a cellular protooncogene, this gene is transcribed at much higher levels, which may lead to a
cancerous phenotype. For example, B-cell leukemia cells in chicken, caused by infection with avian
leukosis virus (ALV), have the virus genome, often in partially deleted form, integrated into the first exon
(nontranslated) of the c-myc gene. The c-myc gene is now under transcriptional control of the strong
promoter located at the 3'-end of the integrated ALV genome.


TUMOR VIRUSES
As discussed in the previous section, many animal viruses are capable of transforming a cell into a tumor
cell. The first such virus was the Rous sarcoma virus, isolated from the tumor of a chicken by Peyton
Rous in 1909. It is a retrovirus containing RNA as its genome. However, there are also DNA viruses that
are known to lead to cancer.
SV40 and polyoma are two small viruses with circular double-stranded DNA that induce tumors in
newborn hamsters. Certain “permissive” cells allow replication of the virus and are killed in the process.
Other cells, such as newborn hamster cells are non-permissive for viral replication, and a small fraction of
such cells become permanently transformed by the virus. In these transformed cells, the virus DNA has
become integrated at random locations into the host genome. SV40 was originally discovered in the
rhesus monkey cell lines used in the preparation of the Salk vaccine against polio. It has never been
demonstrated to cause tumors in humans.
Human Papilloma virus (hPV) is another DNA virus. It has been implicated in cervical cancer, and there is
now a vaccine available that suppresses infection by certain strains of hPV. What is the evidence that
hPV causes cervical cancer? It is circumstantial. According to the CDC 80% of all women 50 years and
older have been infected by hPV. However, the virus cannot be detected in 90% of those women. It has
become integrated into the human genome and is essentially silent (remember phage l). I have not found
evidence that women who develop cervical cancer have a higher degree of hPV infection than women
without cancer. I guess we will have to wait about 30 years to see if the vaccination reduces cervical
cancer.
RNA tumor viruses are retroviruses, i.e., their life cycle involves reverse transcription of the RNA genome
into complementary DNA, which is then integrated into the host genome. Since cells normally do not
contain a reverse transcriptase, the virus must carry this enzyme packaged together with the RNA in the
virion. Most retroviruses have three genes. The gag gene (for group-specific antigen) codes for a
precursor protein from which all capsid proteins are made. The product of the pol gene is cleaved into
reverse transcriptase and an enzyme required for the integration of the virus into the host genome. This
cleavage is catalyzed by a virus-encoded protease, the sequence of which is also contained in the gag-
pol polyprotein. The env gene codes for the envelope glycoprotein that covers the surface of the virus
particle. Gag and pol are transcribed into a single mRNA which contains a stop codon at the end of gag.
In a few percent of the mRNAs the stop codon is ignored, and a gag-pol polyprotein is made that is




                               Genetics Lecture 27: Cancer Genetics, Page 5
                                                                                R. Werner, Genetics (2011)
                                                                                           MBL Lecture 53

subsequently cleaved into gag and pol proteins. This translational frameshifting accounts for the smaller
number of reverse transcriptase molecules made as compared to the number of gag proteins.
Each retrovirus particle contains two identical copies of linear RNA. The sequence at the ends of the RNA
molecules consists of short direct repeats and longer non-translated segments, U5 and U3. In addition, a
tRNA molecule is annealed to a sequence (primer binding site, or PB) near the 5'-end of the RNA
molecule. It serves as a primer for the synthesis of the complementary DNA after infection of a cell. The
replication of the retroviral RNA is complex and leads to the generation of two identical long terminal
repeats (LTRs) at the ends of the provirus before it is inserted into the host genome. The LTR regions
contain powerful enhancer and promoter sequences.
Most retroviruses do not contain oncogenes. As discussed in the previous section some of these
retroviruses can nevertheless cause tumors in animals, but only after very long incubation periods. In the
tumor cell, the retrovirus LTR is usually found inserted near a cellular proto-oncogene, which becomes
activated by the viral LTR (strong promoter).
Only retroviruses carrying oncogenes can transform cells in culture. They can pick up cellular proto-
oncogenes when the provirus inserts near a proto-oncogene. These retroviruses are called acute
transforming viruses. The Rous sarcoma virus is unique among the retroviruses in that it carries the v-src
gene as an additional fourth gene. All other known acute transforming viruses are replication defective
because the oncogene replaces part of a viral gene. Thus, these viruses can only replicate in cells that
are simultaneously infected with a nondefective helper virus. There are several pathways by which a
proto-oncogene can be activated to an oncogene by a retrovirus. Usually the oncogene arises from a
mutation in the proto-oncogene. In the case of cell-surface receptors, the mutation might cause the
activation of the receptor without the need for a ligand. In other cases, the strong retroviral LTR promoter
may activate a nearby proto-oncogene.
As briefly mentioned earlier, retroviruses have been used as shuttle vectors for the delivery of
recombinant DNA to live cells.




TUMOR SUPPRESSOR GENES
The discovery of cellular oncogenes and their mutational or transcriptional activation in tumor cells led to
the hypothesis that the activation of one or more cellular oncogenes in a cell was sufficient to transform
the cell into a malignant cell. This theory was further supported by the finding that many proto-oncogenes
code for growth factors, growth factor receptors, signal transducers, protein kinases, and transcriptional
activators; the abnormal expression of such proteins would be expected to interfere with normal cellular
growth control. Thus, it was generally assumed that the activation of an oncogene is dominant in nature.
When tumor cells were fused with normal cells of the same type, however, the hybrid cells were not
tumorigenic; in other words, the original mutation responsible for the tumor phenotype was recessive.
This phenomenon was called tumor suppression. It indicated that normal cells contain a gene product
that can suppress the expression of an activated oncogene. Soon after this discovery, it was shown that
different tumors could be suppressed by the introduction into the cell of specific chromosomes from
normal cells.
Tumor suppressor gene products function in cell cycle checkpoints and in the control of
apoptosis (Fig. 3). In the following paragraphs I will discuss a few known tumor suppressor genes and
their functions.




                              Genetics Lecture 27: Cancer Genetics, Page 6
                                                                                R. Werner, Genetics (2011)
                                                                                           MBL Lecture 53




                              Figure 3. The checkpoints in the cell cycle.


The retinoblastoma protein
The observation that many tumor cells contain chromosomal rearrangements, including deletions, was
the basis for the isolation of the first tumor suppressor gene, the retinoblastoma suppressor gene Rb-1.
This tumor arises from cells of the embryonal neural retina and occurs in young children. Cytogenetic
studies of cells from retinoblastoma tumors often were found to have deletions on the long arm of
chromosome 13. Chromosome-walking from a nearby known locus led to the isolation of a clone
containing the Rb-1 locus. This clone hybridized to a 4.7-kb mRNA found in normal cells but not in
retinoblastoma cells. Retinoblastoma protein (Rb), the tumor suppressor product of the retinoblastoma
suppressor gene, is a 110 kDa protein that functions as a negative regulator of the cell cycle by arresting
cells in the G1 phase and halting inappropriate cell proliferation. At the transcriptional level, Rb protein
exerts its growth suppressive function by binding to transcription factors including E2F1, PU.1, ATF2,
UBF, Elf-1 and c-Abl. The protein is unphosphorylated during the G0 and G1 phases but becomes
multiply phosphorylated during the S and G2/M phases of the cell cycle.
The p53 protein
Another example of a tumor suppressor gene is the gene that codes for the p53 protein, a 53-kd cellular
phosphoprotein first identified because of its ability to bind to SV40 T antigen (a DNA tumor virus).
Normal p53 protein is thought to be involved in controlling cell entry into S phase. When normal cells are
co-transfected with the activated ras oncogene and a mutated p53 gene, the cells become transformed.
Co-transfection with a wild-type p53 gene, however, suppresses the transformed phenotype. Deleted or
mutated p53 genes are frequently found in human tumors. In fact, mutations in p53 are the most common
genetic change found in human cancers.
Chronic myelogenous leukemia (CML)

Almost all cases of CML show a typical reciprocal translocation between chromosomes 9 and 22. The
result is that part of the bcr ("breakpoint cluster region") gene from chromosome 22 (region q11) is fused
with part of the abl gene on chromosome 9 (region q34). Abl is a protooncogene that was originally
discovered in the Abelson leukemia virus. The result of the translocation is a fusion protein of 210 kDa
resulting from the fusion of the abl gene with the bcr gene. The fused "bcr-abl" gene is located on the
resulting, shorter chromosome 22. Because abl carries a domain that can add phosphate groups to
tyrosine residues (tyrosine kinase) the bcr-abl fusion gene is also a tyrosine kinase. In contrast to the
wildtype abl gene, the bcr-abl fusion gene is constitutively transcribed and does not require activation by
other cellular messaging proteins. The bcr-abl protein activates a number of cell cycle-controlling proteins



                              Genetics Lecture 27: Cancer Genetics, Page 7
                                                                                R. Werner, Genetics (2011)
                                                                                           MBL Lecture 53

and enzymes, speeding up cell division. Moreover, it appears to inhibit DNA repair, causing genomic
instability, which eventually leads to blast crisis in CML.

A new highly effective drug (imatinib) therapy has been developed that is based on the inhibition of the
tyrosine kinase activity of the fusion protein. This often produces remission, but since the CML cells are
aneuploid they continue to shuffle their chromosomes until this inhibition no longer is effective.




   Figure 4. Reciprocal crossing over                        Figure 5. Conversion of the abl proto-
   between chromosomes 9 and 22                              oncogene into an oncogene patients with
   produces the Philadelphia chromosome,                     chronic myelogenous leukemia.
   which is in part responsible for chronic
   myelogenous leukemia (CML).


Human breast cancer
In the US the incidence of breast cancer is one in eight women. Most breast cancers do not appear to be
hereditary. However, some breast cancers clearly run in families. Thus, it is likely that there is a genetic
component for some forms of breast cancer. 25% of women with breast cancer have mutations in one of
two genes, BRCA1 and BRCA2. Both proteins are very large proteins and can have mutations in many
different exons. Despite extensive search for additional breast cancer susceptibility genes, no such genes
have been found yet, even in families with breast cancer histories. It is possible that these forms of breast
cancer are multifactorial.
Colon cancer
There are two forms of familial colon cancer, adenomatous polyposis (FAP) and hereditary non-
polyposis colon cancer (HNPCC).
    1. FAP is associated with a mutation in the APC gene on chromosome 5. APC is a classical tumor
       suppressor that is involved in signal transduction in the Wnt pathway, cell adhesion and
       interactions with the cytoskeleton. APC protein down-regulates b-catenin levels in the cell. An
       increase in b-catenin production has been noted in those people who have Basal Cell Carcinoma
       and leads to the increase in proliferation of related tumors.

    2. HNPCC is associated with a defect in the DNA mismatch repair system.




                              Genetics Lecture 27: Cancer Genetics, Page 8
                                                                                 R. Werner, Genetics (2011)
                                                                                            MBL Lecture 53

The multistage development of cancer
The human body has many defense mechanisms to combat the development of a cancer cell. One of the
most important is apoptosis. Apoptosis stands for programmed cell death. If one of the checkpoints
detects a problem with the duplication of a cell in invokes apoptosis, and the cell self-destructs. Since the
normal cell contains many checkpoints where mishaps during replication and division are detected, the
development of a cancer cell by necessity is a multistage process that requires the inactivation of several
tumor suppressor genes. This often takes many years after the initial precancerous mutation. We still do
not know the exact sequence of events that eventually lead to cancer, and there are many competing
theories.
A lot of money is being spent on characterizing new genes that are found mutated in cancer cells.
However, the problem is much more complicated. It is probably not sufficient to mutate a set of tumor
suppressor genes in order to make a normal cell into a cancer cells. Many of the known mutant tumor
genes when introduced into normal cells have no visible effect, even when several such mutants are
combined. As the paper quoted earlier on many cancers don’t even express the oncogene anymore. That
observation has led some people to propose that it is genetic instability rather than mutation of individual
genes that causes cancer. We know that every cancer cell is aneuploid, i.e., it does not have the normal
complement of chromosomes. Some chromosomes, or regions thereof, are duplicated, others are
deleted. This leads to unbalanced expression of thousands of genes, which causes further chromosome
disruption and may account for the properties of malignant cells that cannot be explained by altered
activity of specific genes. Precancerous cells are already characterized by chromosomal aberrations.
Thus, by determining cellular ploidy in suspicious cells it might be possible to surgically remove these
cells before they develop into malignant cells that will metastasize throughout the body. The following
features of cancer cells cannot be easily explained by simple gene mutation alone:
    1. Cancer risk grows with age.
       For example, if a child is born with all six hypothetical colon cancer mutations, it should develop
       cancer at an early age. However, colon cancer is never seen in children.
    2. Carcinogens take a very long time to cause cancer.
       Even the strongest carcinogens never cause cancer right away.
    3. Carcinogens, which include both mutagens and non-mutagens (e.g., asbestos), may instead
       facilitate development of aneuploidy.
    4. The pattern of aneuploidy seen in different cancers is tumor specific. When a cancer develops it
       is almost as if a new species has formed.
    5. Cancer cells change their phenotype much faster than genes can mutate.
       Even though the cellular mutation rate is not increased in most tumor cells, cancer cells change
       their karyotype at the rate of 1 in 100 generations; which is a very high rate.


Can we treat cancer?
Since all cancers are different from each other (because of the different sets of genes that are expressed
in response to the aneuploidy), there probably will never be a drug that specifically attacks one type of
cancer cell. Sometimes a specific protein is found to be overexpressed in a certain cancer. If that protein
is essential for the survival of the cancer cell, it can be attacked. This could be done by RNAi
complementary to the protein’s gene or by a specific drug that was engineered to inhibit the function of
the protein. Such an approach has worked in the short run. However, because of the genetic instability of
the aneuploid cancer cell there will always be some cells that escape the treatment and continue to
develop into cells that are resistant to any drug treatment. Nevertheless, the drug might cause remission
of the cancer, thereby extending the life of the patient.
In many cases chemotherapy is an approach that is directed at killing only dividing cells. Since cancer
cells typically divide more rapidly than normal cells, the drug kills preferentially cancer cells. Usually,
chemotherapy is applied in the form of short shocks followed by a period of recovery. This allows normal
cells to repair the damage sustained by the chemicals and begin to divide again. After the recovery, a
second shock of chemotherapy is applied. Chemotherapy is been successful in the treatment of many
cancers. However, because of the aneuploid nature of the cancer cell, drug resistant cells will eventually



                              Genetics Lecture 27: Cancer Genetics, Page 9
                                                                            R. Werner, Genetics (2011)
                                                                                       MBL Lecture 53

develop that limit chemotherapy. Also, the drugs used in chemotherapy are very toxic and may act as
carcinogens on other cells in the human body. It is not uncommon to find new types of cancer develop 10
or 15 years after successful chemotherapy of the primary cancer.
The best way to deal with cancer is prevention. This can be done by frequent screening, e.g.,
colonoscopy, visual skin inspection, mammography, Pap smears, etc. Before precancerous cells develop
into dangerous cancer cells they already exhibit aneuploidy. If pathologists were to analyze suspicious
cells by FISH analysis, they would be able to identify pre-cancerous cells and remove them surgically
before they develop into cancer cells.




                            Genetics Lecture 27: Cancer Genetics, Page 10

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:14
posted:12/15/2011
language:Latin
pages:10