Abnormal Cell Growth

1. Abnormal Cell Growth

2. I. OVERVIEW • Cells are often lost through: • Death (apoptosis and necrosis), • Sloughing (e.g., shedding of cells lining the gastrointestinal tract and skin), or • Injury (e.g., bleeding). • New cells replace cells at the same rate they are lost, a highly regulated state of balance known as homeostasis. • If normal cellular regulatory mechanisms malfunction, unregulated and unchecked cell division may result, i.e., cancer.

3. Protooncogenes regulate or produce proteins that regulate normal cell growth and development. • Mutations in protooncogenes may convert them from regulatory genes into cancer-causing oncogenes. • In addition, mutations that create a loss of function in genes known as tumor suppressor genes may also induce cancer. • Most genetic changes that occur during carcinogenesis (transformation of normal cells to cancer cells) are somatic mutations. Each time a cell divides, there is a chance of somatic mutation; therefore, there is always a low background risk for cancer. A far more prevalent cause of cancer is environmental exposure.

4. I. GENES AND CANCER • Protooncogenes and oncogenes - Protooncogenes are genes whose protein products control cell growth and differentiation. - These genes undergo mutations causing qualitative and quantitative changes in gene products and are then called oncogenes.

5. 1. Protooncogenes • Protooncogenes stimulate the cell cycle and have been identified at all levels of the various signal transduction cascades that control cell growth, proliferation, and differentiation. • As normal regulatory elements, proto-oncogenes function in a wide variety of cellular pathways . • Mutations can occur in any of the steps involved in regulating cell growth and differentiation. When such mutations accumulate within a particular cell type, the progressive deregulation of growth eventually produces a cell whose progeny forms a tumor.

6. Figure 1. Protooncogenes and their roles in regulating growth.

7. 2. Several ways of activating protooncogenes to oncogenes: • Point mutations, insertion mutations, gene amplification, chromosomal translocation, and/or changes in expression of the oncoprotein can all result in deregulated activity of these genes.

8. Figure 2. Mechanism of conversion of protooncogenes to oncogenes.

9. Tumor Suppressor Genes • Tumor suppressor genes are important for maintaining normal cell growth control by curtailing unregulated progression through the cell cycle. Situations that diminish tumor suppressor gene function may lead to neoplastic changes. • Loss of function: Loss of tumor suppressor genes predisposes cells to cancer. • Protein products of tumor suppressor genes repress cell growth and division. • Loss of gene function can lead to cell transformation

10. 2. p53-guardian of the genome: • Most frequently inactivated tumor suppressor gene and most often implicated in cancer development • More than half of human cancers show p53 mutations • Loss of p53 function can contribute to genomic instability within cells p53 Functions • Regulates gene expression and controls several key genes involved in growth regulation • Facilitates DNA repair. p53 senses the damage and causes G1 arrest of the cells until damage is repaired • Activates apoptosis of damaged cells when DNA is beyond repair • p53 suppresses telomerase activity

11. p53 -- guarding the genome

13. Dominant and recessive nature of oncogenes and tumor suppressor genes • When proto-oncogenes undergo mutations they are “activated” to oncogenes. Because these genes normally regulate growth, mutations in them often favor the unregulated growth of cancer. • Generally, tumor suppressor genes are “inactivated” by mutations and deletions, resulting in the loss of function of the protein and in unregulated cell growth. • Both copies of the tumor suppressor genes have to be mutated or lost for loss of growth control; therefore, these genes act recessively at cell level. Oncogenes, on the other hand, are dominant in action requiring mutation of only one copy of a protooncogene

14. Oncogenes are dominant in action at the cell level and tumor suppressor genes are recessive.

15. MicroRNAs as oncogenes and tumor suppressors • MicroRNAs (miRNAs) regulate gene expression by controlling the levels of target RNA posttranscriptionally. • They are encoded within the noncoding and intron regions of different genes and are transcribed into RNA, but not protein. • These single-stranded RNA molecules are approximately 21 to 23 nucleotides in length and are processed from primary transcripts known as pri-miRNA into short stem-loop structures, pre-miRNA, and finally to functional miRNA. • Mature miRNA molecules are partially complementary to one or more messenger RNA (mRNA) molecules and function to downregulate gene expression.

16. Canonical miRNA biogenesis and function. miRNAs are transcribed as capped and polyadenylated primary transcripts (pri-miRNAs) containing a hairpin structure that is excised by the endonuclease Drosha. This liberated stem-loop (pre-miRNA) is transported to the cytoplasm by exportin 5 to be processed by the Dicer endonuclease. RNA-induced silencing complex (RISC) selectively handles one strand of the resulting duplex and triggers target mRNA silencing through cleavage or translational repression

17. miRNAs are thought to affect the expression of critical proteins within the cell, such as cytokines, growth factors, transcription factors, etc. • The expression profiles of miRNAs are frequently altered in tumors. • When miRNA’s targets are oncogenes, their loss of function results in increased target gene expression. • Conversely, overexpression of certain miRNAs may decrease the levels of protein products of target tumor suppressor genes. • Therefore, miRNAs behave as oncogenes and tumor suppressor genes.

18. II. MOLECULAR BASIS OF CANCER • Cancer is a stepwise process. Often, several genetic alterations must occur at specific sites before malignant transformation is seen in most adult cancers. • Cancers of childhood appear to require fewer mutations before manifestation of overt cancer. • Rare inherited mutations can pre-dispose individuals to cancer at one or more sites. This type of mutation is present virtually in all somatic cells of the body.

19. A. Growth regulation • Normal cells respond to a complex set of biochemical signals, which allow them to develop, grow, differentiate, or die. • Cancer results when any cell is freed from these types of restrictions and the resultant abnormal progeny of cells are allowed to proliferate.

20. B. Cancer genesis—a multistep process • Mutations in the key genes have to accumulate over time to create a progeny of cells that have lost most control over growth. • Each individual mutation contributes in some way to eventually producing the malignant state. • The accumulation of these mutations spans several years and explains why cancers take a long time to develop in humans.

21. Both exogenous (environmental insults) and endogenous (carcinogenic products generated by cellular reactions) processes may damage DNA. • DNA damage that goes unrepaired may lead to mutations during mitosis. • Increased errors during DNA replication or a decreased efficiency of DNA repair may favor increased frequency of genetic mutations • Cells become cancerous when mutations occur in proto-oncogenes and tumor suppressor genes.

22. Colon cancer progression

23. Cancer cells are genetically unstable and specific genes are responsible for this instability. • About 200 oncogenes and 170 tumor suppressor genes identified • Additional genes that aid in breaking down basement membranes are also important for oncogenesis • Certain combinations of mutant genes were found in distinct cancers and also different types of cancer from the same tissue • From these observations came different theories of cancer formation

24. 1. Clonal evolution model • Initial damage (a genetic mutation) occurs in a single cell, giving it a selective growth advantage and time to outnumber neighboring cells. • Within this clonal population, a single cell may acquire a second mutation, providing an additional growth advantage and allowing it to expand and become the predominant cell type. • Repeated cycles followed by clonal expansion eventually lead to a fully developed malignant tumor. • Accumulated mutations within key genes trigger a single transformed cell to eventually develop into a malignant tumor

25. Theory of clonal evolution

26. 2. Hallmarks of cancer According to this theory, oncogenesis requires cells to • Acquire self-sufficiency of growth signals • Become insensitive to growth inhibitory signals • Evade apoptosis • Acquire limitless replicative potential, and • Sustain angiogenesis

28. According to this model, the type of genetic insult may vary with different cancers. • All cancers, however, should acquire damage to these different classes of genes until a cell loses a critical number of growth control mechanisms and initiates a tumor.

30. 3. Stem cell theory of cancer • Tumors contain cancer stem cells with indefinite proliferative potential • Cancer stem cells are self-renewing and responsible for all components of a heterogeneous tumor • These tumor-initiating cells tend to be drug resistant and to express markers typical of stem cells • Despite the small number of cancer stem cells, they may be responsible for tumor recurrence years after treatment.

31. Cell Surface Phenotypes of Cancer Stem Cells in Different Tumor Types

32. D. Tumor progression • Cancer cells gain metastatic abilities as they evolve. Among these gene products that allow breakdown of tissue structure and invade basement membrane  migration to other sites • As tumors accumulate in cellular mass they induce growth of blood vessels (angiogenesis) to supply the growing tumor with adequate nutrition and oxygen

33. Stem cell theory of cancer

34. IV. Inherited mutations of cancer genes • Number of individuals with inherited predisposition is low compared to total number of human cancers • Risk for cancer is several-fold higher in an individual carrying a mutation in the cancer causing gene

35. A. Inherited mutations mostly affect tumor suppressor genes • A large % of genes mutated in familial cancers are due to tumor suppressor genes. • Mutation in proto-oncogenes during development may not be compatible with life.

36. Examples of familial cancer syndromes

37. Inherited mutations and cancer risk • Cancer risk in individuals carrying a mutation in a cancer-causing gene is several fold greater, because the presence of the mutation in every cell in their bodies makes it highly likely for other mutations to occur

38. V. Mutations in drug-metabolizing enzymes and cancer susceptibility • Environmental chemicals may be classified as genotoxic or non-genotoxic. • Genotoxic chemicals interact with DNA, causing mutations in critical genes • Non-genotoxic (carcinogens) mechanisms differ depending on nature of compound • Chemical carcinogenesis is a multistep process. • Chemicals that have no carcinogenic potential but greatly enhance tumor development when exposed to them for long periods of time mediate tumor promotion • In terms of lifestyle, exogenous hormones, high-fat diet etc. are known to promote cancer and therefore can be an important determinant of cancer risk.

39. Chemical carcinogenesis

40. While genetic predisposition, ethnicity, age, gender, and, to some extent, health and nutritional impairment are cancer susceptibility factors, recent studies are showing polymorphisms in certain drug-metabolizing enzymes to be associated with this inter-individual variation. • Variations in the expression or form of the drug-metabolizing genes, such as cytochrome P450, glutathione transferase, and N-acetyl transferase genes, strongly influence individual biologic response to carcinogens. • While inheritance of cancer-causing genes will increase the cancer risk, its occurrence in the population is low. • On the other hand, the carcinogen-metabolizing enzymes exist in different forms within the population at a high rate and will increase the cancer risk in some individuals carrying the form that allows activation of certain carcinogens.

42. Chapter Summary • Cancer is a multistep process. • There are several characteristic features that a cell has to attain before it undergoes neoplastic transformation. • Protooncogenes are normal counterparts of oncogenes and usually have a role in growth regulation. • Tumor suppressor genes normally restrain growth. • Protooncogenes undergo mutations that cause them to be overactive or function without regulation. • Tumor suppressor genes lose function when mutated

43. Oncogenes are dominant in action and tumor suppressor genes are recessive • Mutation in DNA repair genes can cause cancer • Inherited predisposition to cancer accounts for 5-10% of all human cancers • Lifestyle factors influence cancer risk in the general population • Polymorphism in drug-metabolizing enzymes explains cancer susceptibility in the general population • Germ line mutations in cancer-causing genes occur infrequently • Polymorphism in drug-metabolizing enzymes occurs more frequently • Mutations in p53 gene are the most common cancer-associated mutations and its function underscores its importance in the prevention of cancer