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BIOCHEMISTRY OF CANCER

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INTRODUCTION Cancer cells are characterized by

three properties Unrestrained control of growth Immortal Invasion of local tissues Metastasis

Biomedical importance second most common cause for

death world wide Humans of all the ages affected and

wide variety of organs are affected

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CARCINOGENS Radiant energy- UV rays, X- rays, and γ-rays ■ Pyrimidine dimers ■ DNA cross linking ■ Free radical generation Chemical agents-80% of the cancer is caused by the chemicalsExposure can occur during occupation Diet Life style – cigarette smoking,

tobacco ,alcohol Other ways ( therapeutic drugs may be

carcinogenic)

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CHEMICAL CARCINOGENSclass compound

Polycyclic aromatic hydrocarbon

Benzopyrene, dimethyl benzanthracene

Aromatic amines Acetyl aminofluorene, amino benzene

Nitrosamines Dimethyl and diethyl nitrosamines

Drugs cyclophosphamide

Naturally occurring compounds

Aflatoxin B1, dactinomycin

Inorganic compounds Arsenic, asbestose, beryllium, cadmium,chromium

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MECHANISM OF CHEMICAL CARCINOGENESIS procarcinogen→ proximate

carcinogen→ ultimate carcinogen Bind covalently to

macromolecules including DNA, RNA and proteins

Carcinogens are electrophiles ( deficient in electrons) readily attack nucleophilic groups of DNA

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Mutagenesity – can be diagnosed by Ames test- Salmonella typhimurium( his–ve

)

Salmonella typhimurium( his+ve )

Chemical carcinogen

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ONCOGENIC VIRUSES Oncogenic viruses contain either DNA or

RNA as their genome. Integration of viral genes in to the host

DNA- overrules the regulatory checks and balances of the cellular mechanism- transformation

Virus Abbreviation Associated cancer

Epstein barr virus EBV Burkitt’s lymphomaNasopharyngeal carcinoma

Human papilloma virus

HpV Uterine, cervical ca

Hepatitis B virus HBV hepatoma

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ONCOGENES PLAY A CRUCIAL ROLE IN CARCINOGENES Oncogenes are the genes capable causing

cancer Michel bishop and harold varmus-

demonstrated oncogene in Rous sarcoma virus

The same sequences are also present in humans- cellular oncogenes designated by prefix ‘c’ and viral oncogene as ‘v’ eg, c- src and v- src.

These are also called as protooncogenes. > 100 protooncogenes are present in

humans

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PROTO-ONCOGENES ARE REGULATORY GENES Products of many oncogenes are

polypeptide growth factors e.g. sis gene produces PDGF

Act as receptors for growth factors e.g. erb-B produces receptor for EGF.

Some act on key intracellular pathways e.g. src product tyrosine kinase enzyme phosphorylates tyr residue-activation of intracellular events.

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CELLULAR ONCOGENESoncogene Chromos

ome no. Virus carrying the gene

Oncogene product

Subcellular localization of oncogene product

abl 9 Abelson leukemia virus in mouse

Tyrosine kinase

Plasma membrane

Erb-B 7 Erythroblastosis virus in chicken

Receptor for EGF

membrane

Erb-A 17 do Receptor for TGF

nucleus

myc 8 Myelocytoma virus in chicken

DNA binding protein

nucleus

sis 22 Simian sarcoma virus in monkeys

PDGF membrane

src 20 Rous sarcoma virus in chicken

Tyrosine kinase

membrane

ras 12 Rat sarcoma virus

GTPase cytoplasm

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PROTOONCOGES ARE ACTIVATED TO ONCOGENES BY

VARIOUS MECHANISMS

Five mechanisms has been described

Promoter insertion Enhancer insertion Chromosomal translocation Gene amplification Point mutation

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PROMOTER INSERTION Insertion of viral c DNA near the

oncogene acts as a promoter

A. B. ………

…………….

LTR LTR

PROVIRUS

mycmyc

Myc mRNA

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ENHANCER INSERTION Insertion of viral c DNA down stream of

the oncogene.

A. B. ………

……………

PROVIRUS

mycmyc

Myc mRNA

LTR LTR

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GENE TRANSLOCATIONReciprocal translocation in Burkitt’s

lymphoma Translocation is from short arm of

chromosome 8 to short arm of chromosome 14 and in reverse process translocation occures from short arm of chrom. 14 to chrom. 8

Translocated piece from chrom. 8 contains myc gene which is placed next to gene transcribing H chain of immunoglobulin and itself become activated

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GENE AMPLIFICATION Amplification of genes causing

increased expression in to many folds. Amplification of certain genes are found

in some tumours. Can be induced by certain anticancer drugs which causes drug resistance Eg, treatment with methotrexate

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POINT MUTATION Point mutation is observed in some

cancer c-ras c-ras

P 21

GTP ase activity

Diminishes the activityOf adenyl cyclase

P 21(MUTATION AT 12TH POSITION)

Loss of GTPase activity

Overstimulation of adenyl cyclase

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POLYPEPTIDE GROWTH FACTORS ARE MITOGENIC

Growth factors are polypeptide substances secreted from different cells which causes mitosis.

Growth factors may beEndocrineParacrine Autocrine

Growth factors acts on mitosis via transmembrane signal transduction

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GROWTH FACTORSGrowth factors

Source Function

EGF Mouse salivary gland

Stimulates growth of many epidermal and epithelial cells

Erythropoietin Kidney, urine Development of early erythropoietic cells

IGF-1 and IGF-2

Serum So4 incorporation into cartilage, mitogenic for chondrocytes and exert insulin like effects on many cells

Transforming growth factor-a

Tumor cells, placenta

Similar to EGF

TGF-b Placenta, platelets

Inhibition of fibroblasts

Platelet derived growth factor

platelets Accelerated wound healing

Nerve growth factor

Submaxillary gland

Growth of sensory neurons

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GROWTH FACTORS CONT.,Growth factors Source Function

Granulocyte macrophage colony stimulating factor

Endothelial cells and T-cells

Stimulates granulocytes, monocytes, megakaryocytes

Granulocyte colony stimulating factor

Endothelial cells and fibroblasts

Stimulates granulocytes

Monocyte colony stimulating factor

Endothelial cells Stimulates monocytes

Tumour necrosis factor- alpha(TNF-α )

monocyte Necrosis of tumour cells, proliferation of leukocytes

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GROWTH FACTORS AND ONCOGENES INTERACT IN SEVERAL WAYS

The products of several oncogene act as growth factors or receptors for growth factors

v-sis codes 100 a.a acids for B chain of PDGF

v-erb codes for truncated receptor for EGF which causes continuous activation.

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TUMOUR SUPPRESSOR GENES

Genes which prevents the causation of cancer

These sometimes called as recessive oncogenes or anti oncogenes

Oncogenes Tumour suppressor genes

Mutation in one of the allele is sufficient

Gain of function of a protein that signals cell division

Mutation in somatic cells which is not inherited

Some tissue preference

Mutation in both the alleles is required

Loss of function of a protein

Mutation in germ cells which is Inherited

Strong tissue preference

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IMPORTANT ONCOSUPRESSOR GENES

Oncosupressor gene Abbreviation

Chromosome no.

Retinoblastoma RB 13

Wilm’s tumour WT 11

Familial adenomatous polyposis

FAP 5

Deleted in colon cancer DCC 18

Gene for protein-53 p53 17

Familial breast cancer BRAC 3

Von hippel lindau gene VHL 3

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Mitosis: division ofthe nucleus

Cytokinesis:division of cytoplasm

Daughtercells

M-phase

Interphase = G1, S, G2

S-phase

G2-phase G1-phase

Prep.for division: organelles duplicate

Cell growth + normal cell activities

Synthesis of DNA(chromosomes replicate)

Cells divide

The Cell Cycle

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RB 1 GENEIMPORTANT PROPERTIES Gene is located on chrom. 13q14 Familial retinoblastoma occurs after

identical mutations in both the alleles Product of RB( pRB) gene is a

phosphoprotein p RB binds certain viral proteins and

forms inactive complexes pRB binds to certain transription

factors that are active in S phase thus slowing cell cycle

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Properties of p53 gene Gene is located on the chrom. No 17 Product is a nuclear phosphoprotein It binds to specific DNA sequences It acts as a transcriptional regulator It binds to various viral proteins

forming inactive oligomeric complexes Mutations in p53 gene are the most

common genetic alteration in cancer and are frequent in colon, breast and lung cancer

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TUMOR MARKERS These are the substances

released by the cancer cells and detectable in blood

useful for the following purposes Diagnosis of cancer Follow up of cancer and to

monitor effectiveness of therapy. Prognosis

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common tumor markersName Increased in

Alfa fetoprotein Hepatoma, germ cell tumors

Carcinoembryonic antigen Colorectal, gastrointestinal and lung cancer

Beta HCG choriocarcinoma

Prostate specific antigen Prostrate cancer

calcitonin Medullary carcinoma of thyroid

CA-125 Overian cancer

Alkaline phosphatase Bone secondaries

Neuronal specific enolase Nervous system cancer

Venyl mandelic acid pheochromacytoma

Hydroxy indole acetic acid Carcinoid syndrome

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ALPHA FETOPROTEIN Chemical nature : is a oncofetal protein Sources: in embryonic life mainly

produced by liver and yolk sac Normal serum levels: <10µg/L Clinical use: Diagnosis: of hepatocellular cancer and

germ cell tumor (testicular carcinoma).

Prognosis: if AFP > 10µg/L and bilirubin > 2mg/dl indicates bad prognosis.

Monitoring of therapy

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CARCINOEMBYONIC ANTIGEN

Chemical nature: it is a glycoprotein Sources: present in fetal

gastrointestinal tract Clinical use:A. Diagnosis of adenocarcinoma of

colon and levels are increased in smokers and aged people

B. Main use is monitoring of the colon cancer

C. CEA may also be raised in 10-15% of breast cancer

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PROSTATE SPECIFIC ANTIGEN Chemical nature: it is a extracellular

protease Source : prostate gland Normal serum levels: it is usually present

in serum either in free form or complex with anti protease ( alpha-2 macroglobulin). Normal serum level- 0 to 0.4 µg/L in 40 – 70 years of age

Clinical use: PSA along with the digital examination is

used for screening the prostate cancer in 50-75 years of age group

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HUMAN CHORIONIC GONADOTROPIN

Chemical nature: is a glycoprotein Source: trophoblastic tissues of

placenta and testes Normal serum levels : < 5 IU/L Clinical use: markedly elevated in

choriocardcinoma and germ cell tumors

Mainly used as diagnostic, therapeutic and prognostic tool for germ cell tumors

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CALCITONIN

Chemical nature: polypeptide containing 32 a.a

Source: secreted from parafollicular cells of thyroid in response to hypercalcemia

Normal serum level: 8.8 ng/L Clinical uses: very useful for screening and

diagnosis of medullary carcinoma of thyroid gland

Also used to assess severity and monitoring the therapy

It is also increased in breast, liver and lung cancers