Chapter-5 Tumor suppressor genes (emerogenes)
The category of genes that can suppress transformation or tumorigenicity may be as
diversified as the oncogenes or even more diversified. The constitutive activation of a
‘growth-factor oncogene’ for example, may be canceled by the loss or dysfunction of
the corresponding receptor or by a roadblock elsewhere within the complex pathway of
signal transmission. Oncogene-induced blocks to cell maturation may be overcome by
strong inducers or circumvented by the use of alternative pathways. This chapter
reviews the fragmentary but firm evidence that shows the existence of such
mechanisms.
Suppression of tumorigenicity by somatic hybridization
A large variety of spontaneous, virally, and chemically induced tumors become low- or
nontumorigenic after fusion with fibroblasts, lymphocytes, or macrophages (7, 37, 38,
50, 54, 91, 97, 100, 120). Reappearance of tumorigenicity after chromosome loss was
found to occur at variable rates, depending os berability of each hybrid combination.
The suppression of tumorigenicity by cell hybridization can be discussed in genetic or
epigenetic terms that are not mutually exclusive. If genetic losses play an essential role
in the evolution of the malignant phenotype, the normal cell genome may act by genetic
complementation. In cases where the neoplastic transformation is due to a blockage of
maturation, e.g. by a dominantly acting oncogene, the normal partner cell may impose
its own differentiation program on the hybrid. Stanbridge has concluded that ‘the hybrid
cell takes on the phenotypic signature of the normal parental cell, regardless of the
origin of the malignant parental cell’.
The identification of chromosomes from the normal parent that are regularly lost in the
highly malignant hybrids, was helpful in mapping the location of the relevant suppressor
genes. Human-human hybrids were studied most extensively. Stanbridge et al. (101)
found that the appearance of tumorigenicity in hybrids of HeLa cells and normal
fibroblasts was associated with the loss of one copy of chr 11 and one copy of chr 14.
Klinger’s group (45, 46, 55) provided similar evidence for human chr 11, whereas
Benedict et al. (9) implicated human chr 1 and possibly chr 4 in the suppression of the
HT 1080 fibrosarcoma by normal fibroblasts. This is not necessarily a contradiction. The
tumorigenic phenotype may be suppressed by functionally different mechanisms,
depending on the transforming gene and the phenotype of the normal partner cell. The 2
malignant partners of these crosses, Hela and HT 1080, produced nontumorigenic
hybrids when fused with each other, suggesting genetic complementation between
cells that carry different genetic lesions. HT 1080 carries a mutationally activated N-ras
allele. Corresponding losses were found in tumors that carry mutated ras, including
chemically induced mouse skin carcinomas (83), thymic lymphomas (35) and a variety
of human tumors and derived cell lines. It is therefore conceivable that the normal ras
may antagonize the tumorigenic effect of the mutated allele. It was particularly
suggestive that the progression of chemically induced mouse skin papillomas to
carcinoma was accompanied by the amplification of the mutated ras or the loss of the
normal allele or both.
The suppression of tumorigenicity in hybrids between normal cells and tumor cells
transformed by activated oncogenes may occur at different levels. Down-regulation of
transcription has been demonstrated for u-src, but it is more the exception than the
rule. It is more frequent that suppression acts beyond the level of oncoprotein
production. This was found in SV40 transformed cells (41, 92) and particularly often in
relation to rastransformed cells.
Geiser et al. (33) fused the human EJ bladder carcinoma line, which carries a
transforming, ras gene, with normal fibroblasts. The hybrids retained the transformed
phenotype in vitro, but did not grow in nude mice. Tumorigenic hybrids that grew in nude
mice appeared on serial cultivation. The mutated ras p21 protein was present at the
same level in tumorigenic and nontumorigenic hybrids. Insertion of the c-H-ras gene
into the cells (transfection) increased the amount of p21, but did not induce
tumorigenicity. Suppression of transformation in the absence of any change in p21
expression was also demonstrated in a Chinese hamster and a mouse system. Isolated
from Kirsten sarcoma virus-transformed murine fibroblasts were found to contain a
functionally intact viral oncogene. Their p21 level was as high as in the original
transformants, but they were resistant to retransformation by activated ras of either
cellular or viral origin. Somatic hybridization of the revertants with both nontransformed
and transformed cells of the same lineage generated nontransformed hybrids. The
revertants could also suppress src, fes, K-, H-, and N-ras and mutated human H-ras
transformants, but not mos, sis, fms, ras, polyoma, SV40, and chemically transformed
cells of the same origin.
Src and fes encode oncoproteins unrelated to ras. The common suppression pattern
suggests that the dominant reversion imposes a block on a transformation pathway that
converges in these. Raf and mos are believed to act at a level beyond the ras-dependant
signalling pathway. The analysis of the suppression patterns provides a new approach
towards the definition of these pathways in cells transformed by different oncogenes.
The mapping of suppressor genes by the relatively cumbersome method of somatic
hybridization will probably be replaced by the more direct microcell-mediated transfer
of single chromosomes (log).
Chapter-6 Family Cancer Syndromes
When a gene change that greatly increases cancer risk runs in a family, it is often
referred to as a family cancer syndrome. Other terms that you might hear include
inherited cancer syndrome or genetic cancer syndrome.
It’s important to understand that not every cancer that seems to run in a family is
caused by a family cancer syndrome. About 1 in 3 people in the United States will
develop cancer during their lifetime, so it’s not uncommon to have many cancers in a
family. Sometimes, cancer might be more common in certain families because family
members share certain behaviors or exposures that increase cancer risk, such as
smoking, or because of other factors that can run in some families, like obesity.
But cancer can sometimes be caused by an abnormal gene that is passed from
generation to-generation. Although these cancers are often referred to as inherited
cancers, what is actually inherited is the abnormal gene that can lead to cancer, not the
cancer itself. Only about 5% to 10% of all cancers are known to be strongly linked to
gene defects (called mutations) inherited from a parent.
How do you recognize an inherited or family cancer syndrome?
Certain things make it more likely that cancers in a family are caused by a family cancer
syndrome, such as:
• Many cases of the same type of cancer (especially if it is an uncommon or rare
type of cancer)
• Cancers occurring at younger ages than usual (like colon cancer in a 20-year-old)
• More than one type of cancer in a single person (like a woman with both breast
and ovarian cancer)
• Cancers occurring in both of a pair of organs (like both eyes, both kidneys, or
both breasts)
• More than one childhood cancer in siblings (like sarcoma in both a brother and a
sister)
• Cancer occurring in the sex not usually affected (like breast cancer in a man)
• Cancer occurring in many generations (like in a grandfather, father, and son)
When trying to determine if cancer might run in your family, first collect some
information. For each case of cancer, look at:
• Who has the cancer? How are you related? Which side of the family are they on
(mother’s or father’s)?
• What type of cancer is it? Is it rare?
• How old was this relative when they were diagnosed?
• Did this person get more than one type of cancer?
• Did they have any known risk factors for their type of cancer (such as smoking for
lung cancer)?
• Has anyone in the family with or without cancer had genetic testing, and did that
testing show any abnormal genes?
Cancer in a close relative, like a parent or sibling (brother or sister), is more likely to be a
cause for concern for you than cancer in a more distant relative. Even if the cancer in a
distant relative was from a gene mutation, the chance of the abnormal gene being
passed on to you is less likely than with a closer relative.
It’s also important to look at each side of the family separately. Having 2 relatives with
cancer is more concerning if they are on the same side of the family. For example, it’s
more concerning if both relatives are your mother’s brothers (because they share some
of the same genes) than if one was your father’s brother and the other was your
mother’s brother.
The type of cancer matters, too. It is more concerning if many relatives have the same
type of cancer than if they have several different kinds of cancer. Still, in some family
cancer syndromes, there’s an increased risk of different types of cancer. For example,
the risk of breast cancer and ovarian cancer is increased (as well as some other
cancers) in families with inherited breast and ovarian cancer syndrome. Colon and
endometrial cancer risk are increased in Lynch syndrome (also known as hereditary
non-polyposis colorectal cancer, or HNPCC).
Likewise, more than one case of the same rare cancer is more worrisome than cases of
a more common cancer. For some rare cancers, the risk of a family cancer syndrome is
relatively high with even one case.
The age of the person when the cancer was diagnosed is also important. For example,
colon cancer is rare in people younger than 30. Having close relatives under 30 with
colon cancer could be a sign of a family cancer syndrome. On the other hand, prostate
cancer is very common in elderly men, so if both your father and his brother were found
to have prostate cancer when they were in their 80s, it is less likely to be due to an
inherited cancer syndrome.
Certain kinds of benign (not cancer) tumors and medical conditions are sometimes also
part of a family cancer syndrome. For example, people with the multiple endocrine
neoplasia, type II syndrome (MEN II) have a high risk of a certain type of thyroid cancer.
They also may develop benign tumors of the parathyroid glands and can also get tumors
in the adrenal glands called pheochromocytomas, which are usually benign.
When many relatives have the same type of cancer, it’s important to note if the cancer
could be related to a risk factor like smoking. For example, lung cancer is commonly
caused by smoking, so having several cases of lung cancer in a family of people who all
smoke is more likely to be due to smoking than to an inherited or family cancer
syndrome.
Examples of family cancer syndromes
There are many family cancer syndromes. Some of these are discussed briefly here as
examples, but this is not a full list. See our information on specific cancer types to learn
more about their possible causes.
Hereditary Breast and Ovarian Cancer (HBOC) syndrome
Families with hereditary breast and ovarian cancer syndrome (HBOC) have family
members who have developed breast cancer and/or ovarian cancer. Often these
cancers are found in women who are younger than the usual age these cancers are
found, and some women might have more than one cancer (such as breast cancer in
both breasts, or both breast and ovarian cancer)..
Most often, HBOC is caused by an inherited mutation in either the BRCA1 or BRCA2
gene. Some families have HBOC based on cancer history, but don’t have mutations in
either of these genes. Scientists believe that there might also be other genes that can
cause HBOC that are not yet known.
The risk of breast and ovarian cancer is very high in women with mutations in either
BRCA1 or BRCA2.
This syndrome can also lead to fallopian tube cancer, primary peritoneal cancer, male
breast cancer, pancreatic cancer, and prostate cancer, as well as some others. Some
people might have more than one cancer. For example, a woman might have breast
cancer in both breasts, or both breast and ovarian cancer, or a man might have both
pancreatic and prostate cancer. Male breast cancer, pancreatic cancer, and prostate
cancer can be seen with mutations in either gene, but are more common in people with
BRCA2 mutations. In the US, mutations in the BRCA genes are more common in people
of Ashkenazi Jewish descent than in the general population.
Women with a strong family history of breast cancer and/or ovarian cancer may choose
to get genetic counseling to help estimate their risk for having a mutation in one of the
BRCA genes. The genetics professional can estimate the risk based on a person’s
history of cancer, the history of cancer in their family, and other factors. If they have a
high risk, they might choose to be tested for BRCA mutations (see Understanding
Genetic Testing for Cancer). If a BRCA mutation is present, the woman has a high risk of
developing breast cancer and ovarian cancer (as well as some other cancers). She can
then consider taking steps to find cancer early with screening tests and to lower her risk
of getting cancer.
Because breast cancer is rare in men, men with this cancer are often offered genetic
counseling and testing for BRCA mutations. Having a BRCA mutation can also affect a
man’s risk of some other cancers, such as prostate and pancreatic cancer. It can also
be helpful for a man’s close relatives to know that he has a mutation and that they might
be at risk.
If someone has a BRCA mutation, it means that their close relatives (parents, siblings,
and children) have a 50% chance of having the mutation, too. These relatives may wish
to be tested for the mutation, or even without testing may want to start screening for
certain cancers early or take other precautions to lower their risk of cancer.
HBOC is not the only family cancer syndrome that can cause breast or ovarian cancer.
For information about other genes and syndromes that raise the risk of these cancers.
Lynch syndrome (hereditary non-polyposis colorectal cancer)
The most common inherited cancer syndrome that increases a person’s risk for colon
cancer is Lynch syndrome, also called hereditary non-polyposis colorectal cancer
(HNPCC). People with this syndrome are at high risk of developing colorectal cancer.
These cancers are more likely to develop at earlier ages, often before the age of 50.
Lynch syndrome also leads to a high risk of endometrial cancer (cancer in the lining of
the uterus), as well as cancers of the ovary, stomach, small intestine, pancreas, kidney,
brain, skin, breast, prostate, ureters (tubes that carry urine from the kidneys to the
bladder), and bile duct.
Lynch syndrome is caused by a mutation in any of several mismatch repair (MMR)
genes, including MLH1, MSH2, MSH6, PMS2, and EPCAM. These genes are normally
involved in repairing damaged DNA. When one of these genes isn’t working, cells can
develop mistakes in their DNA, which might lead to other gene mutations and
eventually cancer.
Doctors and genetics professionals can check if you are likely to have Lynch syndrome,
based on your personal and family cancer history using certain criteria known as the
Amsterdam criteria and the revised Bethesda guidelines. Mutations in the genes that
cause Lynch syndrome can then be tested for with genetic testing.
For people who have colorectal, endometrial, or other cancers that are linked with
Lynch syndrome, the cancer cells can be tested for microsatellite instability (MSI).
Having MSI means that one of the MMR genes probably isn’t working properly. Having
normal findings (no MSI or MMR gene changes) suggests that a person probably does
not have Lynch syndrome. But if the MSI tests shows that some of the MMR genes are
not working, the person may have Lynch syndrome, and should be referred for genetic
counseling and possible testing. For more information about genetic testing.
Someone who is known to carry a gene mutation linked to Lynch syndrome may be
advised to start screening for colorectal cancer when they are younger (such as during
their early 20s), or take other steps to try to lower their risk of colorectal cancer. Women
with Lynch syndrome may be advised to start screening for endometrial cancer or take
other steps to try to lower their risk of this cancer.
If someone has Lynch syndrome, it means that their close relatives (parents, siblings,
and children) have a 50% chance of having the mutation that casues it, too. They may
wish to be tested, or even without testing they may want to start screening early for
certain cancers or take other precautions to help lower their risk of cancer.
Li-Fraumeni syndrome
Li-Fraumeni syndrome (also called the sarcoma, breast, leukemia, and adrenal gland
[SBLA] cancer syndrome) is a rare inherited syndrome that can lead to an increased risk
of a number of cancers, including sarcoma (such as osteosarcoma and soft-tissue
sarcomas), leukemia, brain (central nervous system) cancers, cancer of the adrenal
cortex, and breast cancer. These cancers often develop in relatively young adults or
even children.
People with Li-Fraumeni syndrome can develop more than one cancer in their lifetime.
They also seem to have a higher risk of getting cancer from radiation exposure, so
doctors treating these patients might try to avoid giving them radiation therapy when
possible.
This syndrome is most often caused by inherited mutations in the TP53 gene, which is a
tumor suppressor gene. A normal TP53 gene makes a protein that helps stop abnormal
cells from growing.
If someone has Li-Fraumeni syndrome, their close relatives (especially their children)
have an increased chance of having the mutation, too. Close relatives may wish to be
tested, or even without testing they may want to start screening for certain cancers early
or take other precautions to help lower their risk of cancer.
Other family cancer syndromes
You can learn more about the family cancer syndromes listed above, along with other
inherited syndromes and gene mutations that might affect a person’s risk for cancer, by
reading:
• Genetic Counseling and Testing for Breast Cancer Risk: Hereditary Breast and
Ovarian Cancer Syndrome (HBOC), BRCA1 and BRCA2 mutations, and other
specific gene mutations
• Genetic Testing, Screening, and Prevention for People with a Strong Family
History of Colorectal Cancer: Lynch syndrome, familial adenomatous polyposis
(FAP), and other specific gene mutations
• Genetic Counseling and Testing for People at High Risk of Melanoma: Specific
gene mutations
• Ovarian Cancer Risk Factors: Hereditary Breast and Ovarian Cancer Syndrome
(HBOC), Lynch syndrome, Peutz-Jeghers syndrome, MUTYH-associated
Polyposis, BRCA1 and BRCA2 mutations, and other specific gene mutations
• Risk Factors for Retinoblastoma: Hereditary Retinoblastoma
• Risk Factors for Kidney Cancer: von Hippel-Lindau disease, Cowden syndrome,
Birt-Hogg-Dube (BHD) syndrome
• Risk Factors for Soft Tissue Sarcomas: Neurofibromatosis, Tuberous sclerosis,
Gorlin syndrome
Genetic counseling and testing
People with a strong family history of cancer may want to learn more about their genes.
This may help the person or other family members plan their health care for the future.
Since inherited mutations affect all cells of a person’s body, they can often be found by
genetic testing done on blood or saliva (spit) samples. Still, genetic testing is not helpful
for everyone, so it’s important to speak with a genetic counselor first to find out if testing
might be right for you. For more information, see Understanding Genetic Testing for
Cancer.
Chapter-7 Gene Therapy for Cancer
Cancer occurrs by the production of multiple mutations in a single cell that causes it to
proliferate out of control. Cancer cells often different from their normal neighbors by a
host of specific phenotypic changes, such as rapid division rate, invasion of new
cellular territories, high metabolic rate, and altered shape. Some of those mutations
may be transmitted from the parents through the germ line. Others arise de novo in the
somatic cell lineage of a particular cell. Cancer-promoting mutations can be identified
in a variety of ways. They can be cloned and studied to learn how they can be
controlled.
Several methods such as surgery, radiation, and chemotherapy have been used to treat
cancers. The cancer patients who are not helped by these therapies may be treated by
gene therapy. Gene therapy is the insertion of a functional gene into the cells of a
patient to correct an inborn error of metabolism, to repair an acquired genetic
abnormality, and to provide a new function to a cell.
Two basic types of gene therapy have been applied to humans, germinal and somatic.
Germinal gene therapy, which introduces transgenic cells into the germ line as well as
into the somatic cell population, not only achieve a cure for the individual treated, but
some gametes could also carry the corrected genotype. Somatic gene therapy focuses
only on the body, or soma, attempting to effect a reversal of the disease phenotype by
treating some somatic tissues in the affected individual.
One of the most promising approaches to emerge from the improved understanding of
cancer at the molecular level is the possibility of using gene therapy to selectively target
and destroy tumor cells, for example, the loss of tumor suppressor genes (e.g. the P53
gene) and the over expression of oncogenes (e.g. K-RAS) that have been identified in a
number of malignancies. It may be possible to correct an abnormality in a tumor
suppressor gene such as P53 by inserting a copy of the wild-type gene; in fact, insertion
of the wild-type P53 gene into P53-deficient tumor cells has been shown to result in the
death of tumor cells. This has significant implications, since P53 alterations are the
most common genetic abnormalities in human cancers. The over expression of an
oncogene such as K-RAS can be blocked at the genetic level by integration of an
antisense gene whose transcript binds specifically to the oncogene RNA, disabling its
capacity to produce protein. Experiments in vitro and in vivo have demonstrated that
when an antisense K-RAS vector is integrated into lung cancer cells that over express K
RAS their tumorigenicity is decreased.
Despite the promise of such approaches, a number of difficulties remain to be
overcome, the most important of which is the need for more efficient systems of gene
delivery. No gene transfer system is 100% efficient, unless germ-line therapy is
contemplated. During the past two decades, there have been major advances in our
understanding of how cancer develops, proving that cancer has a genetic basis. A series
of genetic abnormalities that accumulate in one cell may result in a pattern of abnormal
clonal proliferation. Our growing understanding of the genetic basis of cancer offers
new opportunities for the molecular prevention and treatment of cancer. There has
been a substantial growth in gene therapy, especially in the field of oncology since the
first experiment in human gene therapy began in 1990, with the aim of treating
adenosine deaminase deficiency. By the end of 1993, there were 45 approved trials by
US Recombinant DNA Advisory Committee, 30 of which are for the treatment of cancer.
This is, in part, became tumor cells can be manipulated ex vivo, while the affected
tissues from individuals with other genetic diseases often cannot.
More than 100 clinical applications of gene transfer into human patients for both
therapeutic and cell-marking purposes have now been approved in the USA and a
number of other countries. Trials for cancer gene therapy that have been approved in
the USA have involved malignancies that are considered incurable. This clinical
situation, which is unlike many genetic diseases for which life expectancy is measured
in years rather than weeks or months, has been considered more appropriate ethically
for untested technologies. For these reasons, applications of gene therapy to cancer
will continue to be the fastest growing area of human gene therapy.
TECHNOLOGIES
Gene therapy for the treatment of cancer has a wide variety of potential uses. There are
several potential strategies for gene therapy in the treatment of cancer.
Strategies of gene therapy for cancer
1. Enhancing the immunogenicity of the tumor, for example by introducing genes
that encode foreign antigens.
2. Enhancing immune cells to increase anti-tumor activity, for example by
introducing genes that encode cytokines.
3. Inserting a “sensitivity” or suicide’ gene into the tumor, for example by
introducing the gene that encodes HSVtk.
4. Blocking the expression of oncogenies, for example by introducing the gene that
encodes antisense K-RAS message.
5. Inserting a wild-type tumor suppressor gene, for example P53 or the gene
involved in Wilm’ tumor.
6. Protecting stem cells from the toxic effects of chemotherapy, for example by
introducing the gene that confers MDR-1.
7. Blocking the mechanisms by which tumors evade immunological destruction,
for example by introducing the gene that encodes antisense IGF-1 message.
8. Killing tumor cells by inserting toxin genes under the control of a tumor-specific
promoter, for example the gene that encodes diphtheria A chain.
Approaches to ex vivo gene transfer
1. Genetically engineered tumor cells
Various groups are investigating the production of autologous cellular vaccines for the
treatment and prevention of cancer. This is most commonly attempted by surgically
removing tumor cells from the patient, growing them in tissue culture and inserting
immunostimulatory genes in vitro. These cells are then reinjected into the patient in an
effort to induce a significant systemic immune response that will both destroy tumor
cells and protect the patient against a recurrence of the tumor. Treating cells that
produce cytokines has been shown to result in systemic immunity in mice. Alteration of
syngeneic tumors with the genes that encode IL-1 b, IL-2, IL-4, IL-6, TNFa, GM-CSF or r
interferon (6, 7) results in immunological destruction of the tumor cells in vivo.
In human gene therapy trials, patients are injected with either autologous or allogeneic
genetically modified tumor cells. These trials involved the insertion of retroviral vectors
carrying the gene that encodes either IL-2, TNFa or GM-CSF into melanoma, colorectal
renal cell carcinoma, neuroblastoma or breast cancer cells in vitro. One modification of
this technique is the insertion of the gene for either IL-2 or IL-4 into autologous
fibroblasts, which are then mixed with irradiated tumor cells from the patient and
reinjected. This approach has the advantage that growing fibroblasts in vitro is much
easier than culturing tumor cells from a large number of individuals. Besides modifying
tumor cells to produce immune activating cytokines, another strategy is to block the
production of insulin-like growth factor-1 (IGF-1). Many tumors such as breast cancer
produce high levels of IGF-1. Insertion of an antisense gene that stops production of
IGF-1 in the tumor allows immunological rejection of the genetically altered tumor after
reimplantation (8). Destruction of the tumor is mediated by cytotoxic T lymphocytes.
The precise mechanism by which IGF-1 mediates tumor protection in vivo remains
unclear.
2. Genetically engineered T lymphocytes
T lymphocytes have the capacity to hone in on tumor tissue. This property has been
used to deliver cytokines directly to tumor masses for human gene therapy. The
secretion of cytokines locally at the tumor site by the effector T lymphocytes will
enhance their anti-tumor activity and avoid the side-effects that result from the
systemic administration of cytokines. For the trial of TNF-modified tumor infiltrating T
lymphocytes, T lymphocytes are difficult to transduce with retroviral vectors and tend to
downregulate expression of the cytokine gene carried by the vector (9).
These two problems of poor gene transfer efficiency and poor cytokine expression have
so far limited the application of this approach, and have shifted the emphasis from
modification of T lymphocytes toward the genetic alteration of tumor cells, which are
much easier to grow in culture and more readily engineered.
3. Insertion of a sensitivity gene
Gene therapy uses the genes to activate a relatively nontoxic pro-drug to form a highly
toxic agent. The most widely studied system uses the thymidine kinase gene of the
Herpes simplex virus (HSVtk). The HSVtk gene confers sensitivity to the anti-herpes
drug, ganciclovir (GCV), by phosphorylating GCV to a monophosphate form (GCV-MP).
Phosphorylation to the triphosphate form (GCV-TP) by cellular kinases results in
inhibition of DNA polymerase, and leads to cell death. In this procedure, GCV kills
tumor cells which express KSVtk, and the adjacent cells that lack the gene are also
destroyed. This is termed the bystander effect phenomenon. To use the bystander effect
to kill human cancer in vivo, the irradiated ovarian tumor cells that contain the HSVtk
gene will be injected into the peritoneal cavity of patients, who will be given GCV. These
HSVtk-expressing cells will destroy bystander tumor cells in vivo.
4. Protection of hematopoietic stem cells
Protection of hematopoietic stem cells (HSCs) from the toxic effects of chemotherapy
by using the gene that confers multiple drug resistance type 1(MDR-1) is another
possible strategy for human cancer therapy. The MDR-1 gene (10) will be isolated from
tumor cells, where it functions to pump chemotherapy drugs (including daunorubicin,
doxorubicin, vincristine, vinblastine, VP-16, VM-26, taxol and actinomycin-D) from
within the cell. Transfer of a retroviral vector carrying the MDR-1 gene into bone marrow
stem cells and their subsequent reintroduction will protect stem cells in vivo from the
effects of large doses of taxol.
Genetic alteration of cancer cells in situ
1. Liposome-mediated gene transfer
The genetical modification of tumors in situ involves the direct injection of liposomes
containing an allogene that encoded HLA-B7, a foreign antigen that is transiently
expressed on the cell surface and includes an immune reaction against the altered
tumor cells. Anti-tumor immune response is significantly increased when some of the
tumor cells express foreign antigens on their cell surface. The transient expression of
immunostimulatory genes in tumors might have potential as a treatment and as a
vaccination against certain malignancies.
2. Retrovirus-mediated gene transfer
In vivo gene transfer using murine retroviral vectors has been applied to the treatment of
brain tumors. In this process, murine fibroblasts that are actively producing retroviral
vectors, so-called retro viral vector producer cells or VPCs, are implanted directly into
growing tumors. The gene transferred by the retroviral vectors into the surrounding
tumor cells is the HSVtk gene. The HSVtk gene should integrate only into the
proliferating tumor cells because retrovirus-mediated gene transfer is limited to
mitotically active cells. This technique resulted in transfer of the gene for HSVtk into 30
60% of brain tumor cells and was capable of mediating complete tumor destruction in
80% of patients.
More than 50% of the cancers can be eliminated completely, At least 10% of cells in a
tumor contain HSVtk, adjacent tumor cells that do not contain HSVtk are destroyed
through the bystander effect. No associated systemic toxicity or evidence of systemic
spread of the retroviral vectors is seen with this form of in vivo gene transfer. So far,
however, it is not clear whether this gene delivery system will suffice to eradicate the
larger, infiltrative human tumors.
Two protocols for in vivo gene transfer for cancer therapy have been approved for
clinical trials. Both entail the direct injection of a supernatant containing a retroviral
vector (RV) into tumor deposits. One group will inject two different RVs into
endobronchial non-small-cell lung cancers (4). The vectors will carry genes that target
the genetic mechanisms responsible for the malignancy: for example, if the lung tumors
are deficient in expression of the tumor suppressor gene, this gene will be used. In lung
cancers that overexpress the K-RAS oncogene, a vector containing an antisense K-RAS
gene will be used. Experiments in vitro have demonstrated that the introduction of both
such vectors can result in decreased tumorigenicity. Another group will inject a RV
containing a vector that encodes r-interferon directly into melanoma deposits.
PUBLIC PROS, CONS AND ARGUMENTS
Civic, religious, scientific, and medical group have all accepted, in principle, the
appropriateness of gene therapy for cancer of somatic cells in humans for specific
cancer. Somatic cell gene therapy is seen as an extension of present methods of
therapy that might be preferable to other technologies. It is considered that patients
should not be subjected to unreasonable risk of harm, excessive discomfort, or
unwanted of privacy, and that they should receive special care, monitoring, and
consideration.
Scientists and physicians acknowledge the fact that somatic-cell gene transfer is the
only form that is technically feasible and ethically acceptable for human use, and gene
therapy will revolutionize medicine. The development of gene transfer strategies for
human cancer is limited as much by our imagination as by current technology. It is
thought that a large number of projects about gene therapy for cancer will be needed to
provide very important information on the safety of the biologies, efficiency of gene
transfer and efficacy in humans.
It has been suggested that the researches in preclinical and clinical studies will be
necessary to evaluate the therapeutic benifit of such gene-based therapies for cancer.
For example, the main advantage that retroviral vectors have over vaccinia-based
vectors is that they are non-replicating, minimizing the concerns regarding the use of a
freely replicating virus in immunocompromised patients. Disadvantages of retroviral
gene delivery system are the complexity and high cost of the transfer procedure. People
are concerned with the safety of retroviral delivery system associated with the use of
viral vectors. All the evaluations of these therapeutic benefit of these methods await
imminent clinical trials.
The gene therapy tools for cancer currently in use are blunt, and will remain so until
solutions are found to the technical problems posed by the need for persist transgene
expression, specific targeting of foreign genes to the appropriate tissues, site-specific
integration of retroviral vectors and adeno-associated vectors, stable gene transfer into
post-mitotic cells, and the need to overcome the immune response to the gene transfer
vector and the transience of gene expression from non-integrating vectors.
The Scientific advisory panel to the National Institutes of Health’ public upbraiding of
gene therapy for cancer was mingled with the message that gene therapy is “not a
failure,” but a technology, which likes most innovative technologies, progresses slowly.
They also suggested that human experiments should be set up so that both positive and
negative results give us an answer, but so far, in human gene therapy experiments, there
is no certain answer.
Perhaps the biggest hurdle to better results with gene therapy for cancer are the gene
transfer vectors, which ferry the gene of interest into target cells. Although most of these
vectors are retrovirues, none is ideal, or perhaps even nearly ideal. It was also believed
that the low frequency of gene transfer as a major shortcoming in virtually all of the
existing trials and other failings were a lack of suitable controls in experiments.
Therefore, a suitable control is necessary for experiments.
The public thinks that researchers and media have oversold the current research in the
field of gene therapy for cancer, leading to an inaccurate perception of its success. This
problem will threaten the field and future public support of the field. One immediate
problem is the public’s inflated expectations about gene therapy for cancer may lead
some patients to relax compliance with treatment plans or alternative reproductive
choices in the belief that a gene treatment is close at hand.
Other genetics experts argue that the time has come to re-evaluate the approach taken
by most gene therapists, and perhaps even to redirect their effects. They are also a bit
concerned that they were not fulfilling the promise of gene therapy for cancer in any
obvious way at this point, and they suggested that more basic aspects of gene therapy
research for cancer will need to be emphasized. Some of patients with cancer made a
reproductive decision based on the feeling that gene therapy for cancer is “right around
the corner”.
Currently, NIH spends an estimated $200 million of its $11 billion annual budget on
gene therapy, while industry pumps an estimated $200 million annualy into the field. As
of June 1995, the majority of the 106 approved clinical protocols involved proposed
therapies for cancer. There are now 600 Americans enrolled in 100 clinical trials. Yet
after all the tests and all the hype, there is still no unambiguous proof that gene therapy
for cancer has cured, or even helped, a single patient. Gene therapy, because it is a
modification of the human genome, generates a level of concern different from other
therapies and therefore is deserving of continued public scrutiny.
No one denies that gene therapy for cancer holds extraordinary promise or that it will
eventually yield results. But critics have grown increasingly concerned that the initial
excitement led to a premature rush to get unproved gene therapies for cancer out of the
laboratory and into human patients. Researchers are still not sure which are the best
methods to transport genes into cancer cells. Nor have they figured out how to stop a
person’s own immune systems from rejecting what are, in effect, microscopic
transplants of foreign material.
Even more troubling are signs that financial considerations may have replaced scientific
rigor in determining how and when to use gene therapy for cancer. Some critics charge
that businessmen are pushing researchers too hard in order to get a quick return on
their investment, and that some doctors have been too hasty, launching clinical trials
early in the hopes of “cashing in when a large drug company buys their firm.
A number of therapies such as drug and chemotherapy treatments have been
developed over the years in an attempt to treat patients with cancer, most of them can
only ameliorate the symptoms rather than effect a cure. The possibility of gene therapy
opens a new area of therapeutics and hope for individuals afflicted with these cancer.
But several technical hurdles must be overcome before successful and complete cures
are possible for the cancers, and technologies must continually be improved upon if the
cancers are to be treated. Like all medical therapies, certain gene therapy will
ameliorate some, but not all, symptoms of a particular cancer. It seems likely that no
single vector system will be appropriate for treating all cancers or will cure all cancers.
Scientists think that clinical trials of gene therapies for cancer has been made possible
by two major technological advances: the ability to cle genes that constitute the genetic
basis of cancinogenesis or. That have therapeutic potential and the development of an
increasing number of gene transfer methods. Increasing numbers of cytokine and
sensitivity genes will present new opportunities for the selective destruction of cancer
cells, but the primary factor hampering the wide spread application of gene therapy for
human cancer is the lack of an efficient method of delivering genes in situ. Therefore,
developing strategies to deliver genes to a sufficient number of tumor cells to induce
complete tumor regression or restore genetic health remains a challenge. However, as
these obstacles are overcome, gene therapy will become a standard part of the practice
of oncology.
Before gene therapy can become the strategy of choice in a wide variety of clinical
settings, improvements in the efficiency of gene transfer into target cells and in the
maintenance of expression from the relevant transferred gene must occur. The problem
of efficient gene transfer will require not only further research to improve delivery
systems and vector constructions but also a parallel effort to understand the biology of
the target cells. Germline therapy would change the genetic pool of the entire human
species and future generations would have to live with that change. Because of these
ethical problems, a number of technical difficulties would make it unlikely that germline
therapy would be tried on human in future.
Chapter-8 Understanding Genetic Testing for Cancer Risk
What is genetic testing?
Genetic testing is the use of medical tests to look for certain mutations (changes) in a
person’s genes. Many types of genetic tests are used today, and more are being
developed.
Genetic testing can be used in many ways, but here we’ll focus on how it is used to look
for gene changes that are linked to cancer.
Genetic testing to help evaluate cancer risk
Predictive genetic testing is a type of testing used to look for inherited gene mutations
that might put a person at higher risk of getting certain kinds of cancer. This type of
testing might be suggested for:
• A person with a strong family history of certain types of cancer, to see if they
carry a gene mutation that increases their ist. If they do have an inherited
mutation, they might want to have screening tests to look for cancer early, or
even take steps to try to lower their risk. An example is testing for changes in the
BRCA1 and BRCA2 genes (which are known to increase the risk of breast cancer
and some other cancers) in people with several family members who have had
breast cancer.
• A person already diagnosed with cancer, especially if there are other factors to
suggest the cancer might have been caused by an inherited mutation (such as a
strong family history, if the cancer was diagnosed at a young age, or if the cancer
is uncommon, such as breast cancer in a man). Genetic testing might show if the
person has a higher risk of developing some other cancers. It can also help other
family members decide if they want to be tested for the mutation.
• Family members of a person known to have an inherited gene mutation that
increases their risk of cancer. Testing can help them know if they need screening
tests to look for cancer early, or if they should take steps to try to lower their risk.
Most people (even people with cancer) do not need this type of genetic testing. It’s
usually done when family history suggests that a cancer may be inherited (see below) or
if cancer is diagnosed at an uncommonly young age.
Who might benefit from genetic testing?
Genetic counseling and testing may be recommended for people who have had certain
cancers or certain patterns of cancer in their family. If you have any of the following, you
might consider talking to a genetic counselor about genetic testing:
• Several first-degree relatives (mother, father, sisters, brothers, children) with
cancer
• Many relatives on one side of the family who have had the same type of cancer
• A cluster of cancers in your family that are known to be linked to a single gene
mutation (such as breast, ovarian, and pancreatic cancers, which are
sometimes linked to BRCA gene mutations)
• A family member with more than 1 type of cancer
• Family members who had cancer at a younger age than normal for that type of
cancer
• Close relatives with cancers that are linked to rare hereditary cancer syndromes
• A rare cancer (in you or a family member), such as breast cancer in a man or
retinoblastoma
• A particular race or ethnicity (such as Ashkenazi Jewish ancestry, which is linked
to a higher risk of BRCA gene mutations)
• A physical finding that’s linked to an inherited cancer (such as having many colon
polyps)
• A known genetic mutation in one or more family members who have already had
genetic testing
• Lab tests of your cancer cells that show features that might be linked to an
inherited gene mutation
If you are concerned about a pattern of cancer in your family, cancer you’ve had in the
past, or other cancer risk factors, you may want to talk to a health care provider about
whether genetic counseling and testing might be a good option for you.
You need to know your family history and what kinds of tests are available. For some
types of cancer, no known mutations have been linked to an increased risk.
For more information on the types of cancer that may be linked to inherited genes.
What is genetic counseling?
Genetic counseling gives you information that you and your family can use to make
decisions about whether to get genetic testing
Genetic counselors have special training in the field of genetic counseling. Most are
board-certified, and some might have a license depending on the rules in their state.
Some doctors, advanced practice oncology nurses, social workers, and other health
professionals may also provide genetic counseling, although they might have different
levels of training in this field. If you are offered genetic counseling, it’s fair to ask about
their training in this area.
Before and after genetic testing, genetic counseling can help you understand what your
test results might mean, your risk of developing cancer, and what you can do about this
risk. It is your decision to have testing and what steps you take after.
Before you get tested
It’s important to find out how useful genetic testing might be for you before you do it.
Talk to your health care provider and plan on getting genetic counseling before the
actual test. This will help you know what to expect. Your counselor can also tell you
about the risks and benefits of the test, what the results might mean, and what your
options are.
Other types of genetic tests
Testing cancer cells for gene changes
Sometimes after a person has been diagnosed with cancer, the doctor will order tests
on a sample of cancer cells to look for certain gene or protein changes. These tests can
sometimes give information on a person’s outlook (prognosis), and they might also help
tell if certain types of treatment may be useful.
These types of tests look for acquired gene changes only in the cancer cells. These tests
are not the same as the tests used to find out about inherited cancer risk.
Home-based genetic tests
Some tests that look for gene changes can be bought without needing a doctor’s order.
For this type of testing, you purchase a test kit and send a sample of your DNA (often
from saliva) to a lab for testing. If you are considering using a home-based genetic test
(also known as a direct-to-consumer genetic test), you need to know what it’s testing
for, what it can (and can’t) tell you, and how reliable the test is.
Home-based tests do not provide information on a person’s overall risk of developing
any type of cancer. Sometimes these tests can sound much more helpful and certain
than they have been proven to be. It may sound like the test will provide an answer to
your specific health concern, such as your risk of hereditary cancer, but the test may
not be able to answer that question completely. For example, a test may look for
mutations in a certain gene, but it might not test for all of the possible mutations. So a
negative test result, even if accurate, may miss the bigger picture regarding your cancer
risk and what you can do to manage it. And you might not be provided with the
important context about the test results that a genetic counselor could provide.
Home-based genetic tests should not be used instead of cancer screening or genetic
counseling that may be recommended by a medical professional based on your
individual risk for cancer. Always consult with your doctor if you are considering or have
questions about genetic testing. Trained genetic counselors can help you know what to
expect from your test results.