Download Genetics of Cancer

Genetics of Cancer
Cancer is a genetic disease
 Cancers result from mutations in the genes that regulate cell growth.
 DNA damage increases the risk of developing cancer.
 Familial cancer syndromes are due to mutations in genes affecting DNA repair or genes that
regulate cell growth.
1. What is an oncogene?
2. How do oncogenes arise and cause cancer?
3. What is a tumor suppressor gene, how does it work?
Causes of Cancer
 Oncogenes-an altered forms of normal genes-proto-oncogenes that have key roles in cell growth
and differentiation.
o Want these methylated
 Tumor Suppressor Genes-normal cells contain genes with tumor suppressor activity, if lost of
inactive, can lead to malignancy much like a recessive trait.
o Don’t want these methylated
 Epigenetics and Cancer-refers to heritable changes to gene expression that are not due to
differences in the genetic code
Cancers are associated with mutations that “activate” proteins that stimulate cell growth
DNA is responsible for transforming normal cells into cancer cells
 The demonstration that certain transduced cellular genes (“oncogenes”) cause cancer earned
Varmus and Bishop the Nobel Prize in Medicine. This provided direct evidence that DNA was the
element of cancerous “transformation.”
 Similarly, Weinberg demonstrated that DNA extracted from human bladder cancer could transform
cultured cells, independently confirmed the that DNA was the element responsible for cancerous
transformation.
4. Clinical example of CML and Philadelphia chromosome
Chronic myelogenous (or myeloid) leukemia (CML)
 Chronic myelogenous (or myeloid) leukemia (CML), also known as chronic granulocytic leukemia
(CGL), is a cancer of the white blood cells. It is a form of leukemia characterized by the increased
and unregulated growth of predominantly myeloid cells in the bone marrow and the
accumulation of these cells in the blood. CML is a clonal bone marrow stem cell disorder in which
proliferation of mature granulocytes (neutrophils, eosinophils, and basophils) and their precursors is
the main finding. It is a type of myeloproliferative disease associated with a characteristic
chromosomal translocation called the Philadelphia chromosome.
 The Philadelphia Ph1 chromosome
 In 1960, Nowell demonstrated that the leukemic cells from CML patients had a characteristic
chromosomal abnormality, t(9:22).
o This 9 to 22 chromosomal rearrangement is seen is over 90% of those patients with
CML.
o The translocation has been found to transfer the cellular ABL (Abelson) resulting in a
abberant transcript c-ABL.
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This was not present in the patient’s normal cells. Thus, the Ph1 chromosome is an acquired
genetic abnormality.
 This was the first example of a genetic abnormality consistently associated with cancer.
Evidence that Ph1 provides a growth advantage
 Ph1 translocation creates a novel protein that fuses Bcr and Abl proteins
 While the function of Bcr is unknown, Abl is a known retroviral oncogene and a tyrosine
kinase
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The Ph chromosome
 Chronic myelogenous leukemia (CML) is a malignant disorder of blood cells that ultimately
evolves into acute leukemia, which is rapidly fatal.
 The Ph1 chromosome
 In CML the abnormal clone of cells (which possess Ph 1) overgrow the normal blood cells.
 This implies that Ph1 provides a growth advantage to the blood cells that acquire it.
BCR-ABL transcript is continuously active and does not require activation by other cellular
messaging proteins. In turn, BCR-ABL activates a cascade of proteins that control the cell cycle,
speeding up cell division. Moreover, the BCR-ABL protein inhibits DNA repair, causing genomic
instability and making the cell more susceptible to developing further genetic abnormalities. The
action of the BCR-ABL protein is the pathophysiologic cause of chronic myelogenous leukemia.
5. Clinical example of Retinoblastoma
Tumour Suppressor Genes- are genes that act to inhibit cell proliferation and tumour development.
Mutation or inactivation of the tumour suppressor gene will cause cancer or cell transformation.
 RB gene is a tumor suppresser protein
 The retinoblastoma protein (abbreviated pRb, RB or RB1) is a tumor suppressor protein that
is dysfunctional in several major cancers. One function of pRb is to prevent excessive cell
growth by inhibiting cell cycle progression until a cell is ready to divide.
 Example; deletion of Rb gene will cause retinoblastoma. The development of retinoblastoma
can be either:
 Hereditary: a defective copy of Rb gene is inherited from the affected parents.
 Nonhereditary: in which 2 normal Rb genes are inherited and develop mutation
during life.
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Retinoblastoma is developed if 2 somatic mutations inactivate both copies of Rb in the same
cell.
A (pediatric) cancer with a strong genetic component. Retinoblastoma and Knudson’s “two
hit” theory hypothesis
Genetic mapping identified RB as the mutant gene in individuals with inherited susceptibility
to retinoblastoma. Retinoblastoma occurs when the second RB allele becomes inactivated.
Somatic mutations in RB occur in a large number of cancers.
Inactivation of Tumour suppressor gene will cause cancer. - If the Rb gene interact with DNA
tumour virus (SV40) it will induce cell transformation.
6. What is the role of telomeres and cancer?
Telomere Length and Cancer- If they become short 2 chromosomes
 Can stick together and some of these cells would die due to this.
 In cancer you normally see long telomeres because cancer cells resemble stem cells, because they
have high telomerase activity to create long telomeres.
 In the lab, it must be confirmed that stems cells have long telomeres before stem cells are given
to a patient. If the stem cells do not have long telomeres they will not be efficient.
 Cell Senescence- telomeres are shorter in disease state and aging
Epigenetics
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and Different Aspects of Life
Means “on/over” the genetic information encoded in DNA
The study of reversible heritable changes in gene function that occur without a change in the
sequence of nuclear DNA
Gene-regulatory information that is not expressed in DNA sequences is transmitted from one
generation (of cells or organisms) to the next
Molecular Mechanisms that Mediate Epigenetic Phenomena
 DNA methylation (CpG dinucleotides)- turn “off” genes
o First DNA methylation
 It is the covalent addition of methyl group to 5th Position of cystosine with in CPG dinucleotides which are frequently located in the promoter region of genes.
 It is a complex process catalyzed by DNA methyl transferases, DNMTS.
o Deoxycytosine  5-methylcytosine via the enzyme DNA-methyltransferase
o Most embryonic genes are unmethylated
o Terminal cells are heavily methylated
o CpG island: a cluster of CpG residues often found near gene promoters (at least 200 bp and
with a GC percentage that is greater than 50% and with an observed/expected CpG ratio
that is greater than 0.6).
 ~29,000 CpG islands in human genome (~60% of all genes are associated with CpG
islands)
 Most CpG islands are unmethylated in normal cells.
 Histone modifications- acetylation turns genes “on”
Epigenetics and Cancer
 Chromosomal infrastructure is essential for gene control, determining both active and repressed
states
 It is important not only to turn the right genes on but also to turn the right genes off
 Histones and chromatin components have key roles in this decision making process
 If as few as three inappropriate genes are turned off, a normal cell can be converted into a cancer
cell
 This epigenetic silencing of genes underlies a new approach to cancer therapy
 Mistargeting of these enzymes leads to tumorigenesis, but inhibition of their activity presents a
novel approach to therapy
Neoplastic Transformation means “new” transformation
 It is a complex multi-event and multi-stage process.
 The process can be divided into two requisite sequences
o Neoplastic conversion – DNA alteration
o Neoplastic development
Alternative Sequencing Methods:
 Bisulfite sequencing is used to detect methylation in DNA.
o Bisulfite deaminates cytosine, making uracil.
o Methylated cytosine is not changed by bisulfite treatment.
o The bisulfite-treated template is then sequenced.
o The sequence of treated and untreated templates is compared.
“We want a cure for cancer, not to just cut it out.” – Dr. Brenner
Recent Cancer methylation studies predict that hundred (100) of CPG islands could be methylated in a tumor cell.
However, it is clear that both the genome-wide methylation studies and candidate gene approaches that each tumor
type may have its own set of cancer cell type specific genes that are more susceptible to methylation. Thus each
cancer type may have the potential to be typed or classified according to methylation profile.
In colorectal cancer:
 Progressive methylation of DNA and subsequent silencing of a subset of genes occurs in normal tissues along
side age and time dependent events which predispose these normal cells to neoplastic transformation.
 Cytosine methylation of CPG loci underlies many of the genetic events in colorectal carcinogenesis which can
be discovered in very early stage.
 The inactivation of adenomatous polyposis coli (Apc):
o The gene is believed to initiate colorectal carcinogenesis in 80% of the cases therefore, it has potential
as sensitive marker of on early stage of the disease.
In lung cancer
 It has been shown that a panel of markers, for aberrant methylation that detects lung cancer at the early
stages of development has been observed.
This panel includes the following genes:
-P 16
-APC
-G-ST
-E-cadhrin
In breast cancer
 BRCA gene is a breast cancer susceptibility gene, that is tumor suppressor gene responsible for both normal
development and carcinogenesis in breast.
 BRCA1, reveals multi functional protein involved in DNA repair. Cell cycle regulation, transcription and
apoptosis.
 BRCA1 mutations may play a significant role in the tumor-genesis of familial breast cancer.
 Aberrant methylation of BRCA1 CPG island Promoter is associated with decreased BRCA1 mRNA in sporadic
breast cancer cells.
 There is a strong relationship between BRCA1 promoter hyper-methylation and the existence of LOH (loss of
hetero-zygosity) at the BRCA locus. This finding suggested that one allele has been lost by deletion and the
other is inactivated by aberrant methylation. Both events simultaneously leading to the bi-allelic inactivation
and complete lack of function of the BRCA1 gene.
Epigenome in relation to cancer drug resistance
 Intrinsic and acquired drug resistance remain the most unpredictable factor affecting chemotherapy. DNA
hypermethylation has been found to be associated with drug resistance acquired during cancer
chemotherapy and therefore, re-expression of methylation-silenced genes resulted in increased sensitivity to
existing chemotherapy.
CONCLUSION
 Hyper methylation of CPG islands associated With tumor suppressor genes has been proposed to contribute
to carcinogenesis.
 Epigenetic markers open new avenues for an early detection, diagnosis , prognosis as well as therapeutic
targets in cancers.
 DNA demethylating agents and histone deacetylase inhibitors are underway to awake those silent tumor
suppressor genes for a better treatment of patients suffering from cancers .
 Focus of Epigenetic Cancer Lecture
 Understand cytosine methylation and CpG island and gene expression.
 Understand alternative DNA sequencing methods to determine if a gene is methylated in a specific cancer
 Understand how epigenetics can cause lung and breast cancers.
 Understand how epigentic inhibitors may be used to develop new cancer therapeutics