biochemistry of cancer, causes of cancer

Cancer is a complex group of diseases characterized by abnormal cell growth that can invade or spread to other parts of the body. It can develop in almost any organ or tissue, disrupting normal body functions. Cancer arises from genetic mutations that allow cells to proliferate uncontrollably, forming tumors or affecting the blood and lymphatic systems. Treatment options include surgery, chemotherapy, radiation therapy, and immunotherapy, tailored to the type and stage of cancer to achieve remission or manage symptoms effectively."

1. BIOCHEMISTRY OF CANCER

2. Cancer is defined as uncontrolled growth and proliferation of cells leading to a malignant transformation. It is the second most common cause of death after cardiovascular disease. Benign tumors differ from malignant ones as it does not invade the healthy surrounding tissue and form secondary tumors . Both types of tumors need a huge amount of nutrients to sustain the rapid growth and cell division.

3. Benign vs malignant

4. Properties of cancer cells : Diminished or unrestricted control of growth Loss of contact inhibition Capability of invasion of local tissues Metastasis – via blood and lymphatic system Independent of growth signal from other cells Ignores STOP signal Stimulates angiogenesis ( formation of blood vessels) No cell suicide (apoptosis)

5. Causes of death in Cancer Figure 16.2

6. Cellular changes in malignancy Morphological changes: Cells become rounded and larger than normal Altered nuclear : cytoplasmic ratio Diminished adhesion Multilayer formation Biochemical changes : Increased DNA and RNA synthesis Increased glycolysis both aerobic and anaerobic Altered permeability and surface charges Alteration in expression of cell surface molecules and cytoskeleton component

7. Change in properties of glycoproteins and glycolipids Increased activity of ribonucleotide reductase and decreased catabolism of pyrimidines . Synthesis of fetal proteins - AFP,CEA Appearance of new antigens and loss of certain antigens Alteration in transport properties. Inappropiate synthesis of certain hormones and growth factors. Alteration of oligosaccharide chain. Alteration of enzyme and isoenzyme profile. Biochemical changes contd …

8. Etiology of cancer The exact etiology is unknown but three factors are found to be responsible in causing cancer. 1. Ionizing radiations : Ionizing radiations are carcinogenic. Ultraviolet rays are responsible for cancers of skin. Particulate radiations eg : Alpha particles and neutrons are more carcinogenic than electromagnetic radiation like X-ray and γ-rays. The effective wavelength was found to be in the range of 2900-3200A

9. Effect of Radiation: a).Direct effect Radiations like UV rays damage DNA by events such as single or double strand breaks, cross linking of strands and formation of pyrimidine dimers . b).Indirect effect Excessive radiations by gamma ray and X-rays lead to release of free radicals like superoxide which interact with DNA and other macromolecules in the cells causing molecular damage ( peroxidations of membrane lipids, denaturations of proteins and injury to DNA).

10. Majority of chemicals act as procarcinogens and is converted to carcinogen in the body. These chemicals gain entry into the body through diet, enviroment and drugs. Some chemicals like methchlorethamine , ß- propiolactone interact directly with the DNA molecule . Aromatic amines and hydrocarbon require some metabolic activator. These chemicals interact with the purine , pyrimidine and phosphodiesterase group of DNA. 2. Chemical carcinogens

11. Chemical Nature Name Polycyclic aromatic hydrocarbon Benzopyrenes Dimethyl-benz-anthracene Aromatic amines 2-acetyl aminofluorine N-methyl 4 amino - azobenzene Nitrosoamines Dimethyl -nitrosamine Diethyl- nitrosoamine Alkylating agent used as drugs Cyclophosphamide Diethyl stilbestrol Food toxins Aflatoxin B Inorganic compounds Arsenic, asbestos, cadmium chromium etc Examples of chemical carcinogens

12. Two stages Initiation : Initiation is the stage which produces the irreversible change in the genome resulting in one or more mutation. Such a cell is predisposed or primed to become cancerous. Promotion : After initiation, all cells undergo cell division and malignant transformation is induced. Most promoting agent act by altering the signal transduction and gene expression. Stages of chemical carcinogenesis-

13. Procarcinogen is not chemically active and require prior metabolism to become reactive and produce ultimate carcinogens. Ultimate carcinogens are electrophiles or electron rich nucleophilic molecules like DNA, RNA, protein etc. Proximate carcinogen is an intermediate compound formed during metabolic activation. The metabolism of procarcinogen involves the action of mono- oxygenases ( Cyt P450) and transferases . Mechanism of chemical carcinogenesis

14. Chemical carcinogens Pro-carcinogens Proximate carcinogen Ultimate carcinogens Cancers Direct carcinogens Chemical Carcinogens

15. Ames assay Ames assay is a simple and sensitive screening test for detecting the mutagenicity of chemical carcinogens. It uses several strains of bacteria Salmonella typhimurium which undergo mutation in the gene coding histidine .These strains cannot synthesize histidine on their own. The culture medium contains all the required nutrients ,except histidine which is present in minimal amount to sustain multiplication for few cycles but not sufficient to divide rapidly and make colonies. Addition of chemical mutagens to the plate results in new mutations and histidine is synthesised . This results in appearance of discrete colonies in the plate after incubating for 2days.

16. Virus Genome Cancer Hepatitis B(HBV) DNA Hepatocellular carcinoma Hepatitis C(HCV) RNA Hepatocellular carcinoma Human herpes virus type 1 DNA Kaposi ’s sarcoma Human papilloma virus Type-8 DNA Cancer of uterine cervix Epstein- Barr virus DNA Burkitt’s lymphoma B-cell lymphoma Nasopharyngeal cancer Human T-cell leukemia Virus type 1 RNA Adult T-cell leukemia 3. Viruses associated with human cancers DNA virus – it binds to the DNA and alter gene expression. RNA virus – they convert RNA genome to DNA with the help of reverse transcriptase enzyme.

17. Mutations Spontaneous mutation occurs at a frequency of 10 7 bps/cell division. The rate increases rapidly in dividing cell (hair follicles, mucosal cells, blood cells).The damaged DNA or the mutations make the cell genetically abnormal and primed to develop cancer. Oxidative stress produces reactive oxygen species, thus increasing mutation rate. Mutation may also correct the already mutated genes into normal gene i.e. restore the mutated genes to normal phenotypes. This is called reverse mutation. .

18. DNA of a normal cell When the parent cell divides to create two cells, the cell's DNA is also divides, creating two identical copies of the original DNA.

19. Mutation of DNA This DNA has suffered a mutation , either through mis -copying (when its parent cell divided) or through the damaging effects of exposure to radiation or a chemical carcinogen .

20. Genetically altered cell Body cells replicate through mitosis. Sometimes a genetic mutation will cause a cell and its descendants to reproduce replacement cells. The DNA of the above cell has a mutation that causes the cell to replicate even though this tissue doesn't need replacement cells at this time or place.

21. Spread and second mutation The genetically altered cells are reproduced unchecked , crowding out the surrounding normal cells. At this point the cells continue to look the same as the surrounding healthy cells. After about a million divisions, there's a chance that one of the new cells will have further mutation. This cell, now carrying two mutant genes , could have an altered appearance and be even more prone to reproduce unchecked.

22. Third mutation Not all mutations lead to formation of cancerous cells. All normal cells have surveillance mechanisms that look for damage or for problems with their own control systems. If such problems are found, the cell destroys itself. After many cell divisions, a third mutation may arise. That cell will grow more vigorously than its predecessors and thus speed up the growth of the tumour .

23. Fourth mutation The new type of cells grow rapidly, allowing for more opportunities for mutations. The next mutation paves the way for the development of an even more aggressive cancer . At this point the tumour is still contained .

24. Breaking through the membrane The newer, wilder cells created by another mutation are able to push their way through the epithelial tissue's basement membrane , which is a meshwork of protein that normally creates a barrier. The invasive cells in this tumour are no longer contained . At this point the cancer is still too small to be detected .

25. Angiogenesis When the tumor breaks through the basement membrane, angiogenesis takes place. Angiogenesis is the recruitment of blood vessels from the network of neighbouring vessels. Without blood and nutrients, a tumor would not be able to continue growing. With the new blood supply, however, the growth of the tumour accelerates ; it soon contains thousand million cells and is large enough to be detected as a lump.

26. Invasion and dispersal The tumour has now invaded the tissue beyond the basement membrane. Individual cells from the tumour enter into the network of newly formed blood vessels , using these vessels as highways by which they can move to other parts of the body.

27. metastasis What makes most tumours so lethal is their ability to metastasize -- that is, establish new tumour sites at other locations throughout the body. Most of these cells die soon after entering the blood or lymph circulation. To form a secondary tumour , a tumour cell needs to leave the vessel system and invade tissue. The cell must attach itself to a vessel's wall and then enter the tissue.

28. The genes present in normal cells are called proto - oncogenes or cellular oncogenes . These have role in cellular growth and differentation . Proto- oncogenes due to mutation become cancerous and are called oncogenes . Oncogenes are cancer susceptibile genes present in tumor cells. Oncogenes may also be accidentally acquired from viruses during infection and then retained in the host genome. These genes help the virus in surviving within the host cells. Origin of oncogenes

29. Action of proto- oncogenes Proto-oncogene have crucial role in cell proliferation and differentiation which is regulated by growth factors. The signal generated by the membrane receptors are transmitted by the cascade involving kinases , G protein and other regulatory proteins. These signals finally affect the transcription factors in the nucleus which regulate expression of genes in cell proliferation, differentiation and cell death.

30. Activation of proto-oncogene to oncogene Five different mechanism leads to activation of proto- oncogenes to oncogenes . 1 Gene Amplification: DNA sequence amplification is a mechanism of increased gene expression. The amplified genes is detected as ‘ ’homogenously stained regions ’’ on specific chromosomes. For eg : Gene amplification of N- myc is seen in Neuroblastoma .

31. 2 Promoter insertion: When a retrovirus infects a host, a copy of an genomic RNA is transcribed by reverse transcriptase into complementary DNA. cDNA is flanked on both sides by long terminal repeat sequences( LTR) which has two functions They are required for integration of cDNA and they can act as promoters for transcription . For eg : when chicken B-lymphocytes are infected by avian leukemia virus, provirus is integrated near the c- myc gene of lymphocytes. The LTR placed immediately upstream of the c- myc gene acts as promoter and initiates the transcription of the gene. Its product act as a transcription factor and ultimately produces the B-cell tumor.

32. LTR V- onc LTR C-MYC LTR V - onc LTR tumor MYC-mRNA Host genome Provirus Promoter insertion Viral genome

33. 3 Enhancer insertion: When a retroviral genome like the genome of avian leukemia virus is inserted downstream of the C-MYC gene, the activation of C-MYC gene transcription still occurs. In such a situation LTR of provirus acts as an enhancer and not as promoter, for which promoter has to be located upstream of C-MYC gene. Increased DNA binding protein produced by C-MYC gene activation drives the cell towards malignant transformation. .

34. LTR V- onc LTR C-MYC Host cell Provirus Viral genome LTR as an enhancer Enhancer insertion C- myc V- onc

35. 4 Chromosomal translocation In chromosomal translocation a portion of the chromosome is split off and joined to another chromosome. If the second chromosome also donates material to it, it is called reciprocal translocation. Burkitt’s lymphoma, a B lymphocyte carcinoma is an example of reciprocal translocation between chromosome 8 and 14. Previously inactive C-MYC gene on chromosome 8 is activated after translocation because it is placed under the influence of enhancer of the heavy chain gene on chromosome 14.This results in increase synthesis of C-MYC coded DNA binding protein.

36. Leukemic cells from all patients with chronic myelogenous leukaemia (CML )contain the so-called Philadelphia chromosome , which results from a translocation between chromosomes 9 and 22. The abnormality involves translocation of c- abl oncogene which encodes a tyrosine kinase on chromosome 9 to be inserted to the bcr gene on chromosome 22. The bcr -c- abl gene product activates the signal transduction pathways and there is over expression of leukaemia cells.

37. 5 Point mutation : It is a common mechanism of oncogene activation. Single point mutation differs from each other in one base only resulting in an amino acid substitution. For eg : V-RAS gene product RAS-P 21 shows point mutation at several positions as compared to C-RAS proto-oncogene. RAS-P21 is a monomeric,GTP -binding protein. This mutation affects the conformation and decrease the activity of GTPase . The lowered GTPase activity results in chronic stimulation of adenylate cyclase which modify a large number of cellular processes to cause malignancy .

38. Growth factor- receptor binding stimulates tyrosine kinase activity which stimulates the RAS protein. RAS is active when GTP is bound to it and inactive when GDP is bound. Activated   RAS triggers a phosphorylation cascade  of protein kinases , which relay and distribute the signal. 42

39. Tumor suppressor genes Also called “anti oncogenes or recessive oncogenes .” These genes suppresses the proliferation of cell. They prevent tumor formation. Mutation of this gene results in loss of its function i

40. Tumor suppressor gene include the following: P53 gene. Retinoblastoma gene(RB-1) Neurofibromatosis gene -1(NF-1) Adenopolyposis of colon gene-1(APC-1) BRCA-1 P53 Gene - P53 is described as the “Guardian of the genome” as it prevents genome mutation. It induces cell cycle arrest, apoptosis, DNA repair and also regulates cellular metabolism. P53 gene is located on chromosome 17 which encodes a nuclear phosphoprotein . If P53 is absent or inactive due to mutation apoptosis does not occur and the damaged DNA persists. Li Fraumeni syndrome occurs due to mutation in one copy of p53 gene. Over 90% of small-cell lung cancers and 50% of breast and colon cancers have been shown to be associated with mutations in p53.

41. Retinoblastoma Gene-1: Hereditary retinoblastoma is a rare childhood cancer in which tumor develops from neural precursor cells in the immature retina. The affected child has inherited a mutated Rb allele. RB-1 is located on the chromosome 13q 14. Its product is a nuclear protein (P RB ) which regulates the cell cycle by binding with the elongation factor E2F and thus preventing the progression of cells from G1 to S phase.

42. Tumor markers Tumor cell produce certain substances whose detection in blood indicates the presence of tumor in the body. Such substances include surface antigens, cytoplasmic proteins, enzymes and hormones. 1. Alpha -fetoprotein: It is a glycoprotein produced by yolk sac during early fetal life. Normal range of AFP is 0-20 ng /ml . This level rises considerably in patients with hepatomas and non-seminal testicular carcinoma . AFP level may also be raised in some non-malignant conditions, such as cirrhosis, hepatitis and other forms of liver damage .

43. 2. Carcinoembryonic antigen : CEA is a membrane glycoprotein found on gastrointestinal and liver cells of 2 to 6month-old fetus . CEA levels in normal people range up to 2.5ng/ml, They increase significantly in certain malignancies, particularly colo -rectal cancers. They may also rise in some non-malignant conditions (such as alcoholic cirrhosis, pulmonary emphysema and heavy smoking). Levels that are 4-5 times normal have been used to predict recurrence of colo -rectal tumors . CEA is not considered as a specific marker for the detection of cancer. But its detection in serum is considered reliable in monitoring the established cancer patients.

44. 3.Prostate specific antigen : It is produced by secretory epithelium of prostate gland. It is normally secreted into seminal fluid. It is increased in prostate cancers. 4. β - hcG : It is synthesized by normal syncytotrophoblast of placenta and levels of β -subunit of hcG are increased in hydatiform mole, choriocarcinoma and germ cell tumors.

45. Application of tumor marker : Detection : Screening asymptomatic persons Diagnosis : Differentiating malignant from benign conditions. Monitoring : Predicting therapy and detecting recurrent cancer. Classification: Choosing therapy and predicting tumor behaviour . Staging : Defining the extent of disease Localization : Nuclear scanning of injected radioactive antibodies. Therapy : Cytotoxic agents directed to marker containing cells

46. Anticancer drug Chemical nature Mechanism of action Methotreaxate Folic acid analogue Competitive inhibitor of the enzyme dihydrofolate reductase 6-Mercaptopurine Purine analogue Inhibits the formation of AMP from AMP from IMP 6-Thioguanine Purine analogue Blocks the synthesis of purine nucleotides Mitomycin C Antibiotic Blocks transcription Cyclophosphamide Alkylating agents Interfares with DNA strand Separation during replication

47. Anticancer drug Chemical nature Mechanism of action Actinomycin D A ntibiotic Blocks transcription Vinblastin and Vincristin Alkaloids Inhibit spindle movement And interfares with cytoskeletal formation Etoposide Phylotoxin Interfares with DNA replication

48. New Hope In Cancer Treatments Remove less surrounding tissue during surgery Combine surgery with radiation or chemotherapy Immunotherapy Cancer-fighting vaccines Gene therapy Stem cell research