Cancer Biochemistry

Specific learning objectives1. List the biochemical alterations in cancer cells 2. Explain what are protooncogenes? 3. Explain mechanisms by which they are activated to cancer-producing oncogenes. 4. List the tumor suppressor gene. 5. Explain the mechanism of action of p53 gene product 6. List the functions of cyclins 7. List the antimetabolites and mention their uses 8. Say what are Tumor markers? Classify tumor markers and mention their use.

What is Cancer ?

Derived from Latin word ‘cancrum’ meaning crab.

It is a group of diseases characterised by uncontrolled cell division leading to the growth of abnormal tissue / tumor.

Cell Proliferation Cell multiplication (proliferation) - normal

physiologic process

response to injury immune responses to replace cells that have died to replace cells that have been shed as a part of

their life cycle (eg: skin, mucous membrane of GI tract, etc.,)

Kept at balance Impairment of this balance leads to cancer

Oncology Oncology branch of medicine deals with

etiology, diagnosis, treatment and prevention of cancer.

Onco is a Greek word meaning tumor

.

Types of Tumor The uncontrolled and rapid proliferation of cells can

lead to either benign tumor or malignant tumor (cancer).

Benign tumors do not spread / invade other parts of the tissues / body, and they are rarely a threat to life.

Malignant tumors can invade other organs, spread to distant locations (metastasis) and become life threatening.

Types of cancer cancers can be classified by the type of cell in

which it originates and by the location of the cell.

.

cell Site / location

cancer

Epithelial Digestive tract

carcinoma

Blood cells WBC leukemialymphatic Lymph node lymphomaConnective tissue

bone sarcoma

melanocyte skin melanomaGerm cells Testes/ovary Terratoma

Etiology of cancer Multifactorial in origin:

Physical Chemical Biological (viruses ) Environmental factors (physical & chemical

agents) Hormonal Genetic Mutation

Physical agents

X-ray Gamma ray UV ray

Chemicals

Asbestos Aflatoxins: synthesised by the fungi “aspergillus

flavus’ Aniline (dye) Nitroso compounds Coloring agents Benzo (a) pyrene Polycyclic aromatic hydrocarbons (PAH)

life style: tobacco chewing, smoking, alcohol

Initiation and Promotion in carcinogenesis Skin tumor in mice benzo(a)pyrene alone – no tumor benzo(a)pyrene followed by several

applications of croton oil – many tumors develop

Application of croton oil alone – no tumors Initiation: benzo(a)pyrene –rapid and

irreversible modification of DNA Promotion: croton oil – slower process Most carcinogens cause initiation and

promotion

Mutation Sudden change in the chemical structure of

the DNA - aberration in chemical structure

The substance which causes mutation is known as ‘mutagen’

Example: X-ray Gamma ray UV ray

Oncogenic virus Viruses which cause cancer are known as

oncogenic virus

They can integrate their gene into the host DNA and take over the entire regulatory mechanism of the host cell. The host cell will produce viral genome instead of host genome (transformed cell).

This leads uncontrolled multiplication of the host cells – transformed cells.

Oncogenic virus..

SI No

Virus cancer

1 Epstein-Barr virus (EBV)

NasopharyngealBurkitt’s lymphoma

2 Human Papilloma virus (HPV)

Uterine Cervical

3 Hepatitis B virus (HBV)

Hepatoma

Oncogenes Genes which can cause cancer are known as

oncognes.

Oncognes are present in normal cells also and in normal cells they are known as proto-oncogenes

We have more than 100 proto-oncogenes on various chromosomes

Example: ras gene (sarcoma virus) c-myc (avian myelocytoma virus)

Oncogenes.. Products of these oncognes are involved in

the regulation of cell cycle

These products may be growth regulating factor / receptor

These oncogens are under the control of regulator genes and expressed only when required.

Virus can carry these gene and transmit it to individuals

Activation of Proto-oncogenes to oncogenes

Promoter Insertion Enhancer Insertion Chromosomal Translocations Chronic myeloid leukemia- Philadelphia chromosome Chromosome 9(abl) and 22 (bcr),Reciprocal

translocations Burkitt’s lymphoma- Chromosome 8 (c-myc)and 14

(IgG)

Gene Amplification Point mutation

Chronic Myeloid Leukemia

bcr-abl gene product - tyrosine kinase activity

Anti-oncognes / onco-suppressor gene

The genes which prevent cancer are known as anti-oncogenes or onco-suppressor genes

The products of these genes act as breaks and regulate cell proliferation

When these genes are deleted / mutated then it leads cancer

Anti-oncogene /onco-suppressor geneAnti-oncogene / onco suppressor gene

Cancer (due to mutation of the gene)

p53 Breast cancer, colon cancer

BRCA1 & BRCA2 Breast cancerRB Retinoblastoma in

childrenWT Wilms’ tumor

P53 tumor suppressor gene 53kDA – nuclear phosphoprotein -

unstable It can activate DNA repair proteins

when DNA has sustained damage. It can arrest growth by holding the

cell cycle at the G1/S regulation point on DNA damage recognition

It can initiate apoptosis P53 P21 G1/S arrest

The mammalian cell cycle

G1

S

G2M

G0

DNA synthesis and histone synthesis

Growth and preparation forcell division

Rapid growth and preparation forDNA synthesis

Quiescent cells

phase

phase

phase

phase

Mitosis

Cyclins in cell cycle progression

Cyclin Kinase Function

D CDK4, CDK6

Progression past G1/S boundary

E,A CDK2 Initiation of DNA synthesis in early S phase

B CDK1 Progression from G2 to M

Telomerase and cancer Active in cancer cells and proliferating

normal cells RNA primer at 5’end of newly synthesized

strands cannot be replaced with DNA Shortening of the ends of chromosomes at

each replication, with loss of important genes

In humans telomeres consist of 1000 or more arrays of TTAGGG repeats at the 3’ ends

Genomic stability in germ-line cells is maintained by Telomerase

Characteristics of cancer cells1. uncontrolled proliferation2. abnormal nucleus3. loss of anchorage 4. disorganized multilayer (loss of contact inhibition)5. forms tumor6. undergoes metastasis & angiogenesis7. lack of differentiation into specialized cells.8. increased rate of anaerobic glycolysis

Cancer cells: Biochemical changes

Increased synthesis of DNA & RNA

Increased rate of aerobic and anaerobic glycolysis to meet the increased demand for energy for more cells.

the rate of anaerobic glycolysis is tremendously increased

Normal cell Cancer cell

Controlled cell proliferation Uncontrolled proliferation

Contact inhibition No Contact inhibition

Normal nucleus Abnormal nucleus

Organised , single layer Disorganised, multilayer

Differentiated cells Non differentiated cells

---- Metastasis & angiogenesis

apoptosisDoes not undergo apoptosis

telomerase activity stops after about 50 cell cycles

Increased & persistent telomerase activity

Standard cancer treatment

Surgery: removal of cancerous tissue

Radiotherapy: destruction of cancer cells using

radioactive rays

Chemotherapy: killing cancerous cells using drugs

Antimetabolites/Anticancer drugsAnticancer drug chemistry Mechanism of actionmethotrexate Folic acid analogue Inhibits

dihydrofolate reductase (THF)

6-mercaptopurine Purine analogue Inhibits formation of AMP

6-thioguanine Purine analogue Inhibits thymidylate synthesis

Mitomycin C antibiotic Linking DNA base pairs

Actinomycin D antibiotic Inhibits transcription

cisplatin Platinum compound Forms DNA adducts

Vinblastine & vincristine

alkaloids Inhibits spindle movement (mitosis)

Cancer prevention: Antioxidants

Prevents / scavenges free radicals. Can detoxify carcinogens

Antioxidant vitamins: Vitamin A Vitamin E Vitamin C Vegetables & fruits

Tumor Markers

Ideal tumor marker Must be produced by tumor cells

Should not be present in normal / healthy / benign condition

Must be detectable in body fluids

It could be used for screening the presence of cancer in asymptomatic patients.

Tumor marker: Criteria for diagnostic marker

1. Highly specific

2. Highly sensitive

3. able to differentiate between neoplastic & non neoplastic diseases

4. increase in level should precede neoplastic process

Tumor markers: uses

Diagnosis

Prognosis

Localization

Monitoring the treatment

Tumor marker: classification Tumor markers are classified into various groups:

Enzymes Hormones Proteins Oncofoetal antigen Genes Carbohydrate antigen Blood group antigen Receptors

Oncofoetal antigens Oncofoetal antigens are proteins produced

during foetal life and present in high concentration during foetal life and decreased to low concentration and disappears after birth.

This protein will reappear in cancer patients

Alpha foetoprotein (AFP) Carcinoembryonic antigen (CEA)

Alpha foetoprotein (AFP) It’s molecular weight is 70000 It is foetal albumin & resembles with adult

albumin It’s normal level in adult: less than 10ug / L More than 300 ng / L is associated with cancer

It’s level is Elevated in: Hepatocellular carcinoma Pregnancy with foetal abnormality (neural tube

defect) Germ cell tumor

Elevated level is also seen in: Hepatitis, cirrhosis & pregnancy.

Carcinoembryonic antigen (CEA) It consists of large family of cell surface

glycoprotein. It’s molecular weight is ranging from 150 to 300

KD It is normally produced by the embryonic tissue

of liver, pancreas & gut.

It level is elevated in: Colorectal cancer, Gastrointestinal cancer,

Pancreatic cancer Lung cancer, Breast cancer, Ovarian cancer,

Uterine cancer

Tumor marker: Enzymes first group of tumor markers identifiedALP: primary / secondary involvement of liver cancer.High level of ALP is also seen in secondary involvement of bone cancer

Gamma glutamyl transferase (y-GGT) & 5’- Nucleotidase (5-NT) are also used to diagnose liver cancer.

Prostatic Acid Phosphatase (PAP): it is elevated in prostatic cancer. Prostate specific antigen (PSA) is more specific and sensitive than Acid phosphatase

Tumor marker: Hormones Hormone is used as tumor marker in the

following conditions:

High level of hormone is secreted by the endocrine tissues where it is normally secreted.

secreted by a non endocrine tissues where it is normally not secreted - ectopic syndrome

Hormones as tumor markerHormone cancer site

ACTH (pituitary)

lung ectopic

Calcitonin (C cells of thyroid)

Medullary thyroid ectopic

Parathyroid hormone (parathyroid gland)

Breast, liver, lung, etc.

ectopic

ADH (posterior pituitary)

Adrenal cortex, pancreatic & duodenal

ectopic

Epinephrine(adrenal medulla)

Pheochromocytoma normal

β Human Chorionic Gonadotropin ( β -hCG)

It is synthesised by trophoblast of placenta It is a glycoprotein and its molecular weight is 45 KD It has two dissimilar subunits It’s alpha submit is identical with FSH, LH & TSH Beta subunit is specific for HCG

Normal value is less than 20 IU / L

It’s level is elevated in: trophoblastic tumour choriocarcinoma Germ cell tumor

Tumor marker: Protein

Prostate specific antigen (PSA): it is useful in diagnosis of prostate cancer

Bence- Jones Proteins: plasma cell from B-lymphocyte proliferates and produces light chain immunoglobulin known as M-protein (paraprotein).

It is used for diagnosis of multiple myeloma It has characteristic property towards heat

treatment. It is precipitated at 50o - 60o C and re dissolves at

90o C. On cooling, it again precipitates

Carbohydrate antigen (CA 125) It is a glycoprotein & Mol wt : 10 million Normal level is less than 35 U / ml.

It is elevated in: Ovarian cancer Pancreatic cancer GI tract cancer

Tumor marker: gene

Oncogenes can be used as tumor markers:oncogene cancerras Sarcoma

c-myc Leukemia & lymphoma

Tumor marker: gene

Antioncogenes as tumour markers:

Tumor suppressor gene cancer

p53 Breast cancer & colon cancer

BRCA1 & BRCA2 Breast cancer

RB retinoblastoma

Receptors Estrogen & Progesterone receptors (ER & PR) are

useful as prognostic indicator and also to decide hormonal therapy in breast cancer

ER positive tumor tissue necessitate hormonal treatment for effective treatment

PR assay is a useful adjunct to ER assay for breast cancer.