Cancer Biology Lecture 9 2024/2025 PDF

CANCER BIOLOGY TUMOR SUPPRESSOR GENES Dr. HAYTHAM MOHAMED Lecture ACADEMIC YEAR 2024/2025 Previously DISCOVERY OF HOW 5 Mechanism CELLULAR CELLULAR CELLULAR ONCOGENES ONCOGENES ONCOGENES...

CANCER BIOLOGY TUMOR SUPPRESSOR GENES Dr. HAYTHAM MOHAMED Lecture ACADEMIC YEAR 2024/2025 Previously DISCOVERY OF HOW 5 Mechanism CELLULAR CELLULAR CELLULAR ONCOGENES ONCOGENES ONCOGENES ARISE Objectives 1 2 3 ONCOGENES TUMOR TUMOR SIGNALING SUPPRESSOR SUPPRESSOR PATHWAYS GENES GENES DISCOVERY EXAMPLES WHAT ARE THE PROTEINS PRODUCED BY ONCOGENES?? 1. Growth Factors One of the first oncogenes found to trigger such a scenario was v-sis, a gene from the simian sarcoma virus that causes sarcomas in monkeys. The v-sis oncogene codes for a mutant form of the growth factor PDGF. The mutant PDGF produced by the v-sis oncogene continually stimulates the cell’s own proliferation. In certain human sarcomas, a chromosomal translocation creates a fusion gene in which part of the PDGF gene is joined to part of an unrelated gene that codes for collagen, a protein component of the extracellular matrix. The fused gene (COL1A1-PDGFB) behaves as an oncogene because it produces PDGF in an uncontrolled fashion, thereby causing cells containing the gene to continually stimulate their own proliferation 2. Receptor Proteins v-erb-b oncogene, found in the avian erythroblastic leukemia virus that causes a cancer of red blood cells in chickens. The v-erb-b oncogene produces an altered version of the receptor for EGF that retains tyrosine kinase activity but lacks the EGF binding site. Therefore, the receptor is constitutively active oncogene permanently stimulates the Ras-MAPK pathway even in the absence of EGF. (Important) Another group of oncogenes produces normal receptors but in excessive quantities, The ERBB2 gene, which codes for a member of the EGF receptor family. The multiple copies of the ERBB2 gene produce excessive amounts of a normal receptor protein. The presence of so many receptor molecules at the cell surface leads to a magnified response to growth factor binding and hence excessive cell proliferation. (Important) One oncogene that codes for a receptor involved in the Jak-STAT pathway has been detected in the myeloproliferative leukemia virus, which causes leukemia in mice. The oncogene, called v-mpl, codes for a mutant version of the receptor for thrombopoietin, which is a growth factor that uses the Jak-STAT pathway to stimulate the production of blood platelets. (Important) 3. Plasma Membrane G Proteins The Ras protein is a member of a class of molecules called G proteins because their activity is regulated by the two small nucleotides GTP (guanosine triphosphate) and GDP (guanosine diphosphate). G proteins are molecular switches whose “on” or “off” state depends on whether they are bound to GTP or to GDP. In the absence of receptor stimulation, Ras is normally bound to GDP and is inactive. To become active, it must release GDP and acquire GTP in a reaction that requires the help of another protein called a guanine Nucleotide exchange factor (GEF). In contrast, GTPase activating protein convert GTP to GDP, then G protein become inactive.  Human cells possess three closely related RAS proto-oncogenes known as HRAS, KRAS, and NRAS; each can incur point mutations that produce oncogenes coding for abnormal Ras proteins. Such mutations are detected in roughly 30% of all human cancers, making RAS oncogenes the most commonly encountered type of human oncogene. Of the three types of RAS genes, KRAS is the most frequently mutated in human cancers. Point mutations in KRAS are present in about 30% of lung cancers, 50% of colon cancers, and up to 90% of pancreatic cancers. (For your information only) Mutations in NRAS are less frequent in epithelial cancers but are detected in about 25% of acute leukemias. (For your information only) Finally, HRAS mutations are encountered primarily in bladder cancers, where they appear in about 10% of cases. (For your information only) 4. Some Oncogenes Produce Intracellular Protein Kinases Two types: 1. serine and threonine 2. Nonreceptor tyrosine kinases 1. Unlike receptor tyrosine kinases, the intracellular kinases involved in this cascade of protein phosphorylation reactions attach phosphate groups mainly to the amino acids serine and threonine in target proteins, rather than to tyrosine. An example is the BRAF oncogene, which codes for mutant forms of the Raf kinase in roughly two-thirds of human melanomas and at a lower frequency in a variety of other cancers. 2. Additional type Intracellular tyrosine kinases may be nuclear, cytoplasmic, or associated with the inner surface of the plasma membrane. Intracellular tyrosine kinases do not possess receptor sites and are therefore referred to as nonreceptor tyrosine kinases. 3 examples is Src kinase Jak kinase and Abl kinase 5. Some Oncogenes Produce Transcription Factors ▪ Oncogenes that produce altered forms or excessive quantities of specific transcription factors have been detected in a broad range of human and animal cancers. ▪ Among the most common are oncogenes coding for Myc transcription factors, which control the expression of numerous genes involved in cell proliferation and survival. ▪ For example, amplification of the MYC gene is frequently observed in small cell lung cancers and, to a lesser extent, in a wide range of other carcinomas, including 20% to 30% of breast and ovarian (For your information only)  Two other members of the MYC gene family, which code for slightly different versions of the Myc protein, have also been implicated in cancer development. (For your information only) One is called MYCN (because it was first discovered in neuroblastomas), and the other is MYCL (first discovered in lung cancers). About 30% to 40% of small cell lung cancers exhibit amplification of the MYC, MYCN, or MYCL gene. MYCN is also amplified in other tumor types, including neuroblastomas and glioblastomas. In neuroblastomas. (For your information only) 6. Some Oncogenes Produce Cell Cycle or Cell Death Regulators One example is the BCL2 gene, which codes for a protein called Bcl2. In non-Hodgkin’s lymphomas, a common chromosomal translocation causes the BCL2 gene to produce too much Bcl2. The excessive amounts of Bcl2 block the pathway for apoptosis, thereby leading to a progressive accumulation of cells that would otherwise have been destroyed. Another gene that affects cell death, called MDM2, is amplified in some human sarcomas and produces excessive amounts of a protein (Mdm2) that inhibits the ability of cells to self-destruct by apoptosis. Oncogenes such as BCL2 and MDM2 help cancer cells evade the apoptotic pathways that would otherwise trigger their destruction. The activated genes include those coding for cyclin-dependent kinases (Cdks) and cyclins. Several human oncogenes produce proteins in this category. For example, a cyclin-dependent kinase gene called CDK4 is amplified in certain sarcomas and glioblastomas, and the cyclin gene CYCD1 is often amplified in breast cancers and is altered by chromosomal translocation in some lymphomas. (For your information only) The presence of such oncogenes causes the production of excessive amounts or hyperactive versions of Cdk-cyclin complexes, which then stimulate progression through the cell cycle. (For your information only) TUMOR SUPPRESSOR GENES DISCOVERY - Whereas proto-oncogenes undergo gain-of- function mutations that can lead to cancer, tumor suppressor genes undergo loss of- function mutations that can likewise lead to cancer.  The loss of tumor suppressor genes is not restricted to hereditary cancers. These genes may also be lost or inactivated through random mutations that strike a particular target tissue Loss of Heterozygosity. After a tumor suppressor gene has undergone mutation in one chromosome, the normal copy present in the other homologous chromosome may be disrupted through a phenomenon called loss of heterozygosity. Three mechanisms for loss of heterozygosity are illustrated here: (a) mitotic nondisjunction, (b) mitotic recombination, and (c) gene conversion. A chromosome region that has undergone loss of heterozygosity in cancer cells but not in normal cells is likely to contain a tumor suppressor gene within it. TUMOR SUPPRESSOR GENES CLASSIFICATION The terms gatekeepers and caretakers are used to distinguish between these two classes of tumor suppressor genes. The tumor suppressors described in the first part of this chapter, which exert direct effects on cell proliferation and survival, are considered to be “gatekeepers” because the loss of such genes directly opens the gates to tumor formation. Tumor suppressors involved in DNA maintenance and repair, on the other hand, are “caretakers” that preserve the integrity of the genome and whose inactivation leads to mutations in other genes (including gatekeepers) that actually trigger the development of cancer. (For your information only) RB Gene Role of the Rb Protein in Cell Cycle Control. In its normal, dephosphorylated state, the Rb protein binds to the E2F transcription factor. This binding prevents E2F from activating the transcription of genes coding for proteins required for DNA replication, which are needed before the cell can pass through the restriction point and into S phase. In cells that have been stimulated by growth factors, signaling pathways such as the Ras-MAPK pathway trigger the production of Cdk cyclin complexes that catalyze Rb phosphorylation. The phosphorylated Rb can no longer bind to E2F, which allows E2F to activate gene transcription and trigger the onset of S phase. At the time of the subsequent mitosis (not shown), the phosphate groups are removed from Rb so that it can once again inhibit E2F. P53 Gene One of the most important is the p53 gene (also called TP53 in humans), which produces the p53 protein. The p53 gene is mutated in a broad spectrum of different tumor types, and almost half of the close to the ten million people diagnosed worldwide with cancer each year will have p53 mutations, making it the most commonly mutated gene in human cancers The p53 protein is sometimes called the “guardian of the genome” because of the central role that it plays in protecting cells from the effects of DNA damage.  Role of the p53 Protein in Responding to DNA Damage. Damaged DNA activates the ATM protein kinase, leading to phosphorylation of the p53 protein. Phosphorylation stabilizes p53 by blocking its interaction with Mdm2, a protein that would otherwise mark p53 for degradation by attaching it to ubiquitin When the interaction between p53 and Mdm2 is blocked by p53 phosphorylation, the phosphorylated p53 protein accumulates and triggers two events. ➀ The p53 protein binds to DNA and activates transcription of the gene coding for the p21 protein, a Cdk inhibitor. The resulting inhibition of Cdk-cyclin prevents phosphorylation of the Rb protein, leading to cell cycle arrest at the restriction point. ➁When the DNA damage cannot be repaired, p53 activates genes coding for a group of proteins involved in triggering cell death by apoptosis. A key protein is Puma (“p53 upregulated modulator of apoptosis”), which promotes apoptosis by binding to, and blocking the action of, the apoptosis inhibitor Bcl2.  Individuals who inherit a mutant p53 gene from one parent have an elevated risk of developing cancer because they only require one additional mutation to inactivate the second copy of the gene. This high-risk hereditary condition is called the Li-Fraumeni syndrome. Most p53 mutations, however, are not inherited; they are caused by exposure to DNA-damaging chemicals and radiation When a mutation in one copy of the p53 gene causes the p53 protein to be inactivated in this way, even in the presence of a normal copy of the gene, it is called a dominant negative mutation.  In some cases, however, mutation of one copy of the p53 gene may be sufficient to disrupt the p53 protein, even when the other copy of the gene is normal. The apparent explanation is that the p53 molecule is constructed from four protein chains bound together to form a tetramer. The presence of even one mutant chain in such a tetramer can be enough to prevent the p53 protein from functioning normally. APC Gene ▪ APC gene, is the tumor suppressor. Individuals with this condition inherit a defective APC gene that causes thousands of polyps to grow in the colon and imparts a nearly 100% risk of developing colon cancer for individuals who live to the age of 60. Two-thirds of all colon cancers involve APC mutations. PTEN Gene The PI3K-Akt Signaling Pathway. Growth factors that bind to receptor tyrosine kinases activate several pathways in addition to the Ras-MAPK pathway. (For your information only) The PI3K-Akt pathway shown in this diagram leads to the activation of Akt, a protein kinase that suppresses apoptosis and inhibits cell cycle arrest by phosphorylating several key target proteins. The PI3K-Akt pathway is inhibited by PTEN, an enzyme that reverses step ➁ by catalyzing the breakdown of PIP3 to PIP2. When loss-of-function mutations disrupt the ability to produce PTEN, the cell cannot degrade PIP3 efficiently and its concentration rises. The accumulating PIP3 in turn activates Akt, thereby leading to enhanced cell proliferation and survival (even in the absence of growth factors). Mutations that reduce PTEN activity are found in up to 50% of prostate cancers and glioblastomas, 35% of uterine endometrial cancers, and to varying extents in ovarian, breast, liver, lung, kidney, thyroid, and lymphoid cancers. (For your information only) TGFβ Gene Growth factors are usually thought of as being molecules that stimulate cell proliferation, but some growth factors have the opposite effect: They inhibit cell proliferation. An example is transforming growth factor (TGF), TGF is especially relevant for tumor development because it is a potent inhibitor of epithelial cell proliferation, and roughly 90% of human cancers are carcinomas—that is, cancers of epithelial origin. important Components of the TGF-Smad signaling pathway are frequently inactivated in human cancers. For example, loss-of-function mutations in the TGF receptor are common in colon and stomach cancers, and occur in some cancers of the breast, ovary, and pancreas as well. (For your information only) Loss-of-function mutations in Smad proteins are likewise observed in a variety of cancers, including 50% of all pancreatic cancers and about 30% of colon cancers. Such evidence indicates that the genes coding for TGF receptors and Smads both qualify as tumor suppressors. (For your information only) CDKN2A Gene The CDKN2A gene, exhibits the rather unusual property of coding for two different proteins that act independently on two of these pathways, the Rb pathway and the p53 pathway CDKN2A gene, however, a shift in the reading frame leads to the production of an alternative protein that is fully functional. Loss-of-function mutations in CDKN2A have been observed in numerous human cancers, including 15% to 30% of all cancers originating in the breast, lung, pancreas, and bladder. (For your information only) Deletion of both copies of the CDKN2A gene, which leads to complete absence of both the p16 and ARF proteins, is common in such cases. (For your information only) BRCA1 and BRCA2 Genes Chromosomal instabilities can be caused by defects in a variety of different tumor suppressors, including the BRCA1 and BRCA2 genes. Women who inherit a mutation in one of the BRCA genes typically exhibit a lifetime cancer risk of 40% to 80% for breast cancer and 15% to 65% for ovarian cancer. (For your information only) The two main ways of repairing double strand breaks are nonhomologous end joining and homologous recombination. Of the two mechanisms, homologous recombination  BRCA2 binds tightly to and controls the activity of Rad51, the central protein responsible for carrying out strand invasion during repair by homologous recombination. BRCA1 is associated with both the Rad50 exonuclease complex and the Rad51 repair complex. Moreover, it is known that ATM phosphorylates BRCA1 in response to DNA damage, suggesting that BRCA1 plays an early role in activating the pathway for repairing double-strand breaks. Cells deficient in either BRCA1 or BRCA2 are extremely sensitive to carcinogenic agents that produce double-strand DNA breaks. Mutations in Mitotic Spindle A critical moment occurs at the end of metaphase, when the chromosomes line up at the center of the mitotic spindle just before being parceled out to the two new cells. If chromosome movement toward opposite spindle poles were to begin before the chromosomes are all attached to the spindle, a newly forming cell might receive extra copies of some chromosomes and no copies of others.  To prevent premature separation, chromosomes that are not yet attached to the mitotic spindle send a “wait” signal that inhibits the anaphase-promoting complex, thereby blocking the activation of separase. The “wait” signal is transmitted by proteins that are members of the Mad and Bub families. The Mad and Bub proteins bind to chromosomes that are unattached to the mitotic spindle and are converted into a Mad- Bub multiprotein complex, which inhibits the anaphase- promoting complex by blocking the action of one of its essential activators, the Cdc20 protein. After the chromosomes have all become attached to the spindle, the Mad and Bub proteins are no longer converted into this inhibitory complex and the anaphase-promoting complex is free to initiate the onset of anaphase.  A lack of Mad or Bub proteins caused by loss-of-function mutations in these tumor suppressor genes disrupts the “wait” mechanism and impedes the ability of the spindle checkpoint to operate properly. The result is a state of chromosomal instability in which cell division creates aneuploid cells lacking some chromosomes and possessing extra copies of others.  The Anaphase-Promoting Complex and the Spindle Checkpoint. (a) The anaphase-promoting complex triggers the onset of anaphase by activating separase, an enzyme that degrades the cohesin proteins that hold duplicated chromosomes together. After cohesin breakdown, the duplicated chromosomes are free to move toward opposite spindle poles. (b) The spindle checkpoint ensures that anaphase does not begin until all chromosomes are attached to the mitotic spindle. Chromosomes that are not yet attached to the mitotic spindle send a “wait” signal (“checkpoint on”) by converting Mad and Bub proteins into a Mad-Bub complex, which inhibits the anaphase-promoting complex by blocking one of its essential activators, the Cdc20 protein. After the chromosomes have become attached to the spindle, this “wait” signal ceases (“checkpoint off”) and the anaphase- promoting complex becomes active.  Cancer cell with three centrosomes that have assembled a spindle with three poles. Stepwise Series of Mutations Can Lead to Malignancy of colon cancers ▪ The most common pattern to be detected is the presence of a KRAS oncogene (a member of the RAS gene family) accompanied by loss-of-function mutations in the tumor suppressor genes APC, p53, and SMAD4. ▪ Rapidly growing colon cancers tend to exhibit all four genetic alterations. In contrast, benign tumors have only one or two, suggesting that mutations in the four genes occur in a stepwise fashion that correlates with increasingly aggressive behavior. Summary of main concepts Next Lecture CANCER SCREENING, DIAGNOSIS, AND TREATMENT END OF LECTURE 9