Molecular Basis of Neo

MOLECULAR BASIS OF NEOPLASIA

BY MANJIRI JOSHI

REFERRENCE

• Kumar, Abbas, Fausto, Mitchell. Robbins Basic Pathology. 7th

ed. 2007

• Rubin, Strayer. Rubins pathology-clinicopathological

foundation of medicine. 5th ed. 2008.

• Parmus. Essential clinical pathology.1996

• Porth. Pathophysiology concepts of altered health state. 7th

ed. 2005.

• Current Molecular Concept of Oral Carcinogenesis and

Invasion. 2010 Volume 22 Number 01

• Molecular Biology of Oral Cavity Squamous Cell Carcinoma.

Otolaryngol Clin N Am 39 (2006) 229–247

• Medelson, Howley, Gray, Israel, Thompson. The molecular

basis of cancer. 3rd ed. 2008.

SECRET 3- EVASION OF APOPTOSIS

• Just as cell growth is regulated by growth promoting

and growth inhibiting genes, cell survival is

conditioned by genes that promote and inhibit

APOPTOSIS.

• Apoptosis is a pathway of cell death that is

induced by a tightly regulated intracellular

program in which cells destined to die activate

enzymes that degrade the cells' own nuclear DNA

and nuclear and cytoplasmic proteins

5Kumar, Abbas, Fausto, Mitchell. Robbins Basic Pathology. 7th ed. 2007

APOPTOTIC PATHWAY


Genes regulating apoptosis are altered in cancer and these are:

BCL-2 –

– It is a anti-apoptotic gene. Its mutation results in

malignancy.

– For e.g translocation of BCL2 gene from 18q21 to the

immunoglobulin heavy chain locus on 14q32 causes B-cell

lymphomas of the follicular type.

p53 –

– It increases the transcription of pro-apoptotic gene BAX and

BID. Hence mutation of this gene results in decreased

apoptosis.


SECRET 4- DNA REPAIR DEFECT

• Though humans literally swim in a sea of environmental

carcinogens cancer is relatively rare outcome. This is

fortunate state of affairs results from the ability of normal

cells to repair DNA damage thus preventing mutations.

• a third class of genes in which mutations contribute to

pathogenesis of cancer are genes involved in DNA

mismatch repair., so called mutator genes or caretaker

genes. 10Kumar, Abbas, Fausto, Mitchell. Robbins Basic Pathology. 7th ed. 2007

• Normal versions of DNA repair gene exercise surveillance

over the integrity of genetic information by participating in

the cellular response to DNA damage.

• When a strand of DNA is replicating, mismatch repair genes

act as “spell checkers”. E.g if there is an erroneous pairing

of G with T instead of A with T the mismatch repair protein

corrects the defect. Without these proof readers errors

slowly accumulate in several genes.


• Cells with such defects in DNA repair are said to have the

replication error phenotype readily documented by

examination of microsatellite sequences in the tumor cell

DNA.

• Microsatellites are tandem repeats of one to six nucleotides

scattered throughout the genome.

• Microsatellite instability is hallmark of defective mismatch

repair.12Kumar, Abbas, Fausto, Mitchell. Robbins

Basic Pathology. 7th ed. 2007

• HPNCC-

• Hereditary Nonpolyposis Colonic cancer also known as

Lynch syndrome is a familial predisposition to the

development of colorectal cancer.

• The genes that are mutated in patients with HNPCC have

not yet been completely characterized but include the

genes encoding TGF-B receptor II, the TCF component of

the B-catenin pathway, BAX, and other oncogenes and

tumor suppressor genes.13Kumar, Abbas, Fausto, Mitchell. Robbins


• Xeroderma pigmentosum

• It is an autosomal recessive disease in which increased

sensitivity to sunlight is accomplished by a high incidence

of skin cancer, including basal cell carcinoma, SCC,

malignant melanoma.

• UV light causes cross-linking of pyrimidine residues, thus

preventing normal DNA replication.


• Ataxia Telangiectasia

• A rare hereditary syndrome that features cerebellar

degenration, immunological abnormalities, occulocutaneous

telangiectasia. And predisposition to cancer.

• Gene ATM on 11q22-q23 codes for a nuclear

phosphoprotein that participates in multiple responses to

DNA damage.


• Bloom syndrome

• It is an autosomal recessive syndrome.

• Clinical features include small stature, sensitivity to

sun, immunodeficiency, and predisposition to array of

cancers.

• BS gene encodes a protein that has helicase activity

involved in repair of DNA damage.16Kumar, Abbas, Fausto, Mitchell. Robbins


• BRCA-1 & BRCA-2 gene:

• BRCA-1 is located on 17q21 and BRCA-2 is located on

13q12-13.

• Mutation of both the genes causes 80% of familial breast

cancer.



SECRET 5- TELOMERASE

• After a fixed number of divisions, normal cells become arrested

in a terminally nondividing state known as replicative

senescence.

• It has been noted that with each cell division there is some

shortening of specialized structures, called telomeres, at the

ends of chromosomes.

• Once the telomeres are shortened beyond a certain point, the

loss of telomere function leads to activation of p53-dependent

cellcycle checkpoints, causing proliferative arrest or apoptosis .

thus, telomere shortening functions as a clock that counts cell

divisions.


• In germ cells, telomere shortening is prevented by the

sustained function of the enzyme telomerase, thus explaining

the ability of these cells to self-replicate extensively. This

enzyme is absent from most somatic cells,

• and hence they suffer progressive loss of telomeres.

Introduction of telomerase into normal human cells causes

considerable extension of their life span


• Telomerase activity and maintenance of telomere length

are essential for the maintenance of replicative potential in

cancer cells. Reactivation of telomerase in cells with

abnormal genomes confers an unlimited proliferative

capacity to cells that have tumorigenic potential.


SECRET- 6 ANGIOGENESIS

• Tumors stimulate the growth of host blood vessels, a

process called angiogenesis, which is essential for

supplying nutrients to the tumor. Even with genetic

abnormalities that dysregulate growth and survival of

individual cells, tumors cannot enlarge beyond 1-2 mm

diameter or thickness unless they are vascularized.

Presumably the 1- to 2-mm zone represents the maximal

distance across which oxygen and nutrients can diffuse

from blood vessels. 22Kumar, Abbas, Fausto, Mitchell. Robbins


AngiogenesisVessel dilation

New TheoryOld Theory

• Beyond this size, the tumor fails to enlarge without

vascularisation because of hypoxia-induced cell death

• Tumor angiogenesis can occur by recruitment of endothelial

cell precursors or by sprouting of existing capillaries, as in

physiologic angiogenesis.


THE ANGIOGENESIS SIGNALING CASCADE

Genes are activated in cell nucleus

Cancer cell

VEGF (or bFGF)

Endothelial cell surface

Relay proteins

Receptor protein

Proteins stimulate new

endothelial cell growth

ENDOTHELIAL CELL ACTIVATION

SecretesMMPs that

digest surroundin

g matrix

Cell migrates

and divides

Matrix

Activated endothelial cell

Blood vessel

Tumor that can grow and spreadSmall localized tumor

Signaling molecule

Angiogenesis

• However, tumor blood vessels differ from the normal

vasculature by being tortuous and irregularly shape and by

being leaky. The leakiness is attributed largely to the

increased production of VEGF." In contrast to normal mature

vessels, which are quiescent structures, tumor vessels may

grow continuously


• Tumor growth is controlled by a balance between

angiogenic and anti-angiogenic factors.

• The important angiogenic factors are VEGF and bFGF.

• The anti-angiogenic factors are thrombospondin-1,

angiostatin, endostatin and tumstatin


SECRET -7 “INVASION AND METASTASIS”

• Invasion and metastasis are biologic hallmarks of malignant

tumors. They are the major cause of cancer-related

morbidity and mortality and hence are the subjects of

intense scrutiny.

• The metastatic cascade will be divided into two phases:

• (1) invasion of the extracellular matrix and

• (2) vascular dissemination and homing of tumor cells.


• Invasion ofthe ECM is an active process that can be

resolved into several steps

• ■ Detachment ("loosening up") of the tumor cells from

each other

• ■ Attachment to matrix components

• ■ Degradation of ECM

• ■ Migration of tumor cells33Kumar, Abbas, Fausto, Mitchell. Robbins


• Vascular Dissemination and Homing of Tumor Cells

– Once in the circulation, tumor cells are particularly

vulnerable to destruction by innate and adaptive immune

defenses.

– Within the circulation, tumor cells tend to aggregate in

clumps. This is favoured by homotypic adhesions among

tumor cells as well as heterotypic adhesion between tumor

cells and blood cells, particularly platelets.

– Formation of platelet–tumor aggregates may enhance tumor

cell survival and implantability.


• Arrest and extravasation of tumor emboli at distant sites

involve adhesion to the endothelium, followed by egress

through the basement membrane.

• The site at which circulating tumor cells leave the

capillaries to form secondary deposits is related, in part, to

the anatomic location of the primary tumor.

• Chemokines have a very important role in determining the

target tissues for metastasis


36

MICROENVIRONMENT IN CANCER DEVELOPMENT

• Microenvironment of cancer plays a critical role in determining its development.

• Supportive tissues (stroma) of cancer actively collaborate with cancer cells.

• Composition of the stroma- fibroblast- myofibroblast- inflammatory white blood cells- endothelial cells of blood and

lymphatic vessels

• Cancer cells and stromal cells evolve together.

http://www.lbl.gov/LBL-Programs/lifesciences/BissellLab/main.html

EPIGENETICS

• Prior to the middle of the twentieth century, before DNA

was given a special status in biology, the developmental

biologist and evolutionist Conrad H. Waddington (1905-

1975) emphasized that genetics and developmental biology

were related, hypothesizing that patterns of gene

expression, turning genes on and off, and not the genes

themselves, define each cell type, thus linking genes and

gene action to development.

• Waddington coined the term ‘epigenetics’ from the Greek

word epigenesis, referring to embryology and genetics as

“a gradual coming into being of newly formed organs and

tissues out of an initially undifferentiated mass”.

• Epigenetic changes set the stage for alterations in gene

expression and have been identified as important

components of carcinogenesis.

• It has become evident during the past few years that

certain tumor suppressor genes may be inactivated not

because of structural changes but because the gene is

silenced by hypermethylation of promoter sequences

without a change in DNA base sequence’ .

• Methylation also participates in the phenomenon called

genomic imprinting, in which the maternal or paternal allele

of a gene or chromosome is modified by methylation and is

inactivated.

FIELD CANCERIZATION

• Some oral cancer patients develop SCC over a broad

area of the oral mucosa, with multiple lesions arising

simultaneously or over a period of time

• The mechanisms of ‘field cancerization are unkown ,

although three basic hypotheses were proposed

recently by OgdenMolecular Biology of Oral Cavity Squamous Cell Carcinoma. Otolaryngol Clin N Am 39 (2006) 229–247

• Firstly, ‘field changes’ (molecular changes throughout

the oral mucosa of oral cancer patients) may

predispose to the development of multiple primary

cancers.

• In this scenario, a large region of the oral mucosa

may be exposed to the etiological agent(s) which

causes independent transformation of multiple

epithelial cells at separate sites. Molecular Biology of Oral Cavity Squamous Cell Carcinoma. Otolaryngol Clin N Am 39 (2006) 229–247

• A single etiological agent acting at different sites would

cause multiple separate cancers with identical genetic

defects, each arising as a separate clone within the oral

mucosa.

• Subsequent genetic modification (due to spontaneous

mutation or continued exposure to exogenous mutagens)

may render the separate clones genetically distinct.

Molecular Biology of Oral Cavity Squamous Cell Carcinoma. Otolaryngol Clin N Am 39 (2006) 229–247

Different etiological agents acting at different sites

would cause multiple separate cancers with different

genetic defects, each arising as a separate clone within

the oral mucosa.

• Secondly, the etiological agent(s) may transform a single

oral epithelial cell.

• The expanding clone of cancer cells may spread through the

oral mucosa via local tissue spread, regional blood vessels,

seeding via the saliva into a mucosal erosion or seeding due

to the trauma of surgery.

• This would give rise to geographically distinct but

genetically identical cancers.


• The clinical significance of p53 mutation within the normal oral

epithelium of oral cancer patients is unclear.

• Some reports suggest an association with the development of

second primary cancers while others find no such association.

• Recent molecular studies have shown that oral cancer is a clonal

proliferation of neoplastic keratinocytes (that is, oral cancers arise

from a single genetically altered cell) and that multiple primary

tumours result from the migration of clonally-related pre-neoplastic

cells through the oral epitheliumMolecular Biology of Oral Cavity Squamous Cell Carcinoma. Otolaryngol Clin N Am 39 (2006) 229–247

• Thirdly, a tumor may have a paracrine effect on the

adjacent oral mucosa. Of great recent interest is that

tumors have been found to secrete tumor inhibitory factors

including inhibitors of neovascularization.

• Removal of the primary tumor would remove these

inhibitors of cancer development and hence promote

second primary tumor formation.


• Alternatively, tumors may secrete promoters of apoptosis.

• Removal of the primary tumors would reduce the level of

apoptosis in adjacent tissue and hence promote second

primary tumor formation.

Molecular Biology of Oral Cavity Squamous Cell Carcinoma. Otolaryngol Clin N Am 39 (2006) 229–

247

GENOMICS AND PROTEOMICS

• The advent of technology and high-throughput analytical tools

has facilitated the dissection of the genetic pathways that

govern tumor biology. One such analytical tool is genomics.

• Genomic is the study of the patterns of gene expression in a

cellular system, which generally refers to the field of biology

that seeks to understand biologic processes from a global view,

evaluating all the transcriptional activity of a particular system

under certain conditions.

Current Molecular Concept of Oral Carcinogenesis and Invasion. 2010 Volume 22 Number 01

• Its counterpart, proteomics, is the evaluation of the entire

network of proteins that contribute to cellular function.

• These two complementary fields brought tremendous

advances in the understanding of tumor biology, primarily

by allowing scientists to study the changes that occur in

thousands of genes or proteins in a single experiment.

Current Molecular Concept of Oral Carcinogenesis and Invasion. 2010 Volume 22 Number

01

• One particular tool, which has identified genomic signatures

of lymphatic metastasis for OCSCC and may allow the early

detection of occult metastases in selected patients.

• The clinical usefulness of these technologies is currently

under evaluation in other tumor systems

GENE THERAPY

• Gene therapy is a technique for correcting defective genes

which are responsible for genetic abnormalities and

diseases. The idea of gene transfer for treating human

diseases was put forward by CUSACK and TARNER in 1998.

The idea envisages the transfer of a therapeutic gene into

cancer cells via a vector. These processes delete the

mutant alleles’ and are replaced by the therapeutic or

functional gene.50Oral & Maxillofacial Pathology Journal [ OMPJ ] Vol 1 No 2 Jul- Dec 2010

51

RATIONAL TREATMENT OF CANCER

• More specific strategies based on genetic instability of cancer cells.

• More specific delivery of anticancer drugs using monoclonal antibodies.

• Development of specific small molecules.

52

RATIONAL TREATMENT OF CANCER

• Cancer treatment by targeting angiogenesis.

• Cancer treatment by inducing immune responses.

• Cocktail approaches to suppress drug resistance.

• Genomic profiling makes specific treatment strategies possible.

• No magic solution. Still a long way to go…

Molecular Basis of Neo

Documents

robbins basic pathology

dna mismatch repair

repair of dna damage

dna repair defect

nuclear dna

tumor cell dna

mutator genes

caretaker genes