Lecture_10_Cancer_and_Development.pdf

🐣 Lecture 10: Cancer and Development Part 1: Cancer Developments LO: Characteristics of cancer Cancer Development Tissue Regeneration and Stem Cells: Adult tissues, such as...

🐣 Lecture 10: Cancer and Development Part 1: Cancer Developments LO: Characteristics of cancer Cancer Development Tissue Regeneration and Stem Cells: Adult tissues, such as skin, blood, and testes, constantly regenerate from populations of stem cells. Examples of continual production include hair, red and white blood cells, and sperm. In the intestinal epithelium (the layer of cells facing the lumen of the tube), the entire epithelial layer is renewed every four to five days; ie. being continuously shed Structure of the Intestinal Epithelium: Lecture 10: Cancer and Development 1  The epithelium is organized into two compartments: Crypts (at the bottom, forming pockets) contain stem cells. Villi (finger-like projections) are where cells migrate and eventually die at the top. Stem cells in the crypts proliferate and differentiate to form all the different cell populations in the epithelium. Cells migrate from the crypts to the villi, where they are shed. Cell Production and Maintenance: In humans, 10 billion cells are produced daily to maintain the epithelial pool. There is a balance between cell proliferation (at the bottom of the crypts) and cell death (at the top of the villi) to maintain tissue size. Mutations disrupting this balance can lead to uncontrolled proliferation and ultimately, cancer. Cancer as a Genetic Disease Genomic Alterations: Lecture 10: Cancer and Development 2  Cancer is a genetic disease characterized by genomic alterations, including: Single-cell substitutions, indels, amplifications, deletions, and translocations. These alterations can either: Cause hyperactivation of a gene. Lead to loss of function of a gene. The most common genomic alteration is a single base substitution. Progression of Cancer: Cancer is a progressive disease resulting from the accumulation of mutations. Example: In bowel cancer: Mutations in the APC gene (a tumor suppressor involved in regulating the cell cycle) lead to loss of APC function, resulting in hyperproliferation and formation of small adenomas. Further mutations (e.g., in K-Ras) lead to larger adenomas. Additional mutations in cell cycle or cell death pathways eventually result in cancer formation (carcinoma). Cancer as a Clonal Disease Clonal Nature of Cancer: Cancer is a clonal disease, meaning a single clone (a single cell) is responsible for tumor formation. Lecture 10: Cancer and Development 3  Example: Normal epithelial cells can acquire mutations that give them a proliferative or anti-apoptotic advantage, leading to tumor formation. These initial cells are termed the "cells of origin" of the tumor. Stem Cells in Cancer Development: In the gut, stem cells are located at the bottom of the crypts and are marked by LGR5. These stem cells divide and give rise to transit amplifying cells, which divide multiple times before becoming fully differentiated cells. Stem cells remain in the tissue for the organism's lifetime, meaning mutations in these cells are more likely to result in cancer, as opposed to mutations in transit amplifying or absorptive cells, which are shed and eliminated quickly. Research on Cancer and Stem Cells Mouse Model Study: A mouse model was use to study the effects of APC mutations in either stem cells or transit amplifying cells. Lecture 10: Cancer and Development 4  Results: Mutations in APC in transit amplifying cells formed only small, non-progressive tumors. Mutations in APC in stem cells led to the formation of large polyps, which could progress to cancer. Conclusion: Colon cancer is a disease of stem cells. LO: Hallmarks of cancer Lecture 10: Cancer and Development 5  (1) Self-sufficiency in growth signals Mechanism: Activation of the Ras oncogene In normal cells, Ras positively regulates the cell cycle. In cancer, Ras undergoes an activating mutation, resulting in hyperactivation of the cell cycle and uncontrolled cell proliferation, leading to self-sufficiency in growth signals. (2) Insensitivity to anti-growth signals Mechanism: Loss of function in retinoblastoma (RB) or APC genes Both RB and APC are involved in the negative regulation of the cell cycle, ensuring proper control. In cancer, the loss of these genes removes this control, allowing unregulated cell cycle progression and proliferation. Lecture 10: Cancer and Development 6  (3) Tissue invasion & metastasis Inactivation of E-cadherin: E-cadherin is crucial for maintaining cell-cell adhesion in epithelial tissues. When lost, cells gain the ability to move and invade surrounding tissues. Activation of SNE1: SNE1 plays a role in the epithelial-to-mesenchymal transition (EMT), a process that enables cells to become more mobile and invasive. (4) Litmitless replicative potential Mechanism: Activation of telomerase Telomerase allows cells to maintain their telomeres, the protective caps on chromosomes, which are normally shortened with each division. This enables cancer cells to divide indefinitely. (5) Sustained angiogenesis Mechanism: Production of vascular endothelial growth factor (VEGF) VEGF stimulates the formation of new blood vessels, ensuring that the tumor is well-supplied with oxygen and nutrients via the bloodstream. (6) Evasion of apoptosis Mechanism: Cancer cells evade programmed cell death (apoptosis) by utilising factors such as IGF (Insulin-like Growth Factor). Cancer cells acquire these hallmark traits in different orders depending on the tumor type. In one case, the first step might be self-sufficiency in growth signals via a KRAS mutation, followed by insensitivity to anti-growth signals through loss of APC, then inactivation of E-cadherin, production of VEGF, and activation of telomerase, eventually leading to cancer formation. In another case, the sequence might begin with a mutation in APC, leading to insensitivity to anti-growth signals, followed by a mutation in KRAS, and then production of VEGF and activation of telomerase, ultimately resulting in cancer. Lecture 10: Cancer and Development 7  LO: Oncogenes and tumor suppressor genes Two classes of genes frequently altered in cancer → oncogenes & tumour suppressor genes Oncogenes Function in Normal Cells: Oncogenes, such as KRAS, play a role in activating the cell cycle and promoting cell proliferation. They have a growth-stimulatory function. Role in Cancer: When oncogenes are mutated, they become hyperactivated or overproduced, leading to uncontrolled growth. A mutation in a single copy of an oncogene can have a dominant effect, driving abnormal cell behavior. Eg) Ras, Myc, FGF, b-catenin Tumor suppressor genes Lecture 10: Cancer and Development 8  Function in Normal Cells: Tumor suppressor genes, such as retinoblastoma (RB) and APC, have a growth-inhibitory function. They regulate and control cell division. Role in Cancer: In cancer, tumor suppressor genes are inactivated, allowing unchecked cell growth. Both copies of a tumor suppressor gene must be inactivated or deleted to see a significant effect on cell division. Eg) p53, Rb, APC, AXIN1 When both copies of a tumor suppressor gene are functional, they produce proteins that maintain normal growth inhibition, keeping cell division under control. However, when both copies are mutated, the protein produced becomes non-functional, leading to uncontrolled cell division and activation of the cell cycle. Lecture 10: Cancer and Development 9  Loss of one copy of a tumour suppressor gene can create a hereditary predisposition to cancer Normal Healthy Individual: In a normal cell, there are two functional copies of the APC gene. Occasionally, a cell may inactivate one of its two good APC genes, leaving one functional allele. In this case, the remaining allele still maintains normal APC function, so no tumor forms. Hereditary Familial Adenomatous Polyposis (FAP): In FAP patients, all cells inherit a mutant copy of the APC gene. Occasionally, a cell may inactivate its only good APC gene copy, resulting in the loss of APC function and excessive cell proliferation, which leads to the development of colon cancer. Most FAP patients develop tumors in the bowel. Sporadic Colon Cancer: A normal cell may occasionally inactivate one of its two good APC genes, leaving one functional copy. Over time, the second copy of the APC gene may become inactivated in the same cell, leading to the complete loss of APC function, uncontrolled cell division, and the development of colon cancer. This process occurs in 80% of sporadic colorectal cancers, which contribute to the fact that 1 in 12 people worldwide will develop colorectal tumors through this specific pathway. Lecture 10: Cancer and Development 10  Summary of cancer development Stem Cells and Mutation Accumulation Lecture 10: Cancer and Development 11  At the bottom of the crypts in tissues like the colon, there is a population of stem cells. Due to their long lifespan, these stem cells can accumulate mutations over time. Mutations may occur in both alleles of the APC tumor suppressor gene, or there may be an activating mutation in the oncogene beta- catenin. These mutations lead to the formation of aberrant crypts and early adenomas, marked by excessive proliferation. Progression of Adenomas to Cancer As mutations accumulate, additional changes, such as the activation of the KRAS oncogene, drive the development of larger adenomas. Later, the loss of function of p53 occurs when both alleles of the gene are mutated, which pushes the progression toward cancer. Finally, the cells may acquire the ability to invade other organs, especially when genes involved in cell-cell adhesion or the epithelial-to- mesenchymal transition (EMT) are inactivated. LO: Heterogeneity of tumors Tumors are highly heterogeneous, with multiple distinct cell types → this heterogeneity suggests that cancer progression is not strictly a linear process but is more likely driven by a branch process. In the branch model: The cancer originates from the same cell of origin (a stem cell) with the accumulation of mutations in the APC gene, leading to the formation of hyperproliferative cells. However, the microenvironment and interactions with other cells (e.g., the microbiome or other cell populations) influence the trajectory of each cell's development, causing them to accumulate distinct mutations. Branching Pathways of Mutations Lecture 10: Cancer and Development 12  For example, after the first APC mutation, some cells might acquire a KRAS mutation, followed by a p53 mutation. Alternatively, another set of cells with the same APC mutation might follow a different path, accumulating a p53 mutation first, and then other mutations. This leads to a highly complex and heterogeneous cell population, where each cell has its own mutation profile. Types of tumour heterogeneity: Primary tumours refer to the original site where the tumor develops → taking colon cancer as an example, the primary tumor consists of different clonal populations, each with its own specific set of mutations - eg) C1 to C6. Lecture 10: Cancer and Development 13  Intratumoural heterogeneity: Within a single tumor (eg. the primary tumour), there are diverse, genetically distinct populations of cancer cells Intermetastic heterogeneity: When the tumor progresses to an advanced stage, it has the potential to metastasise. In colon cancer, the most common site for metastasis is the liver. If clones C1 and C2 metastasize to the liver, they can form two distinct tumors in the liver ^ These two metastases will have different genetic profiles. Intrametastatic heterogeneity: Over time, these metastatic tumors can accumulate new mutations, further increasing their complexity and creating intrametastatic heterogeneity within each metastatic tumor/lesions Interpatient heterogeneity: Lecture 10: Cancer and Development 14  Patient 1's primary tumor may be very different from patient 2's, with each patient's metastases likely to be genetically distinct from one another as well. This heterogeneity poses significant challenges in cancer treatment: A treatment that is effective against C1 subclones and their specific mutations may not affect other clones, such as C2 or C3, in the primary tumor. Similarly, even if the treatment targets metastases dependent on C1 mutations, other metastatic clones (e.g., C2) may remain unaffected. This is why cancer, especially at advanced stages, is so difficult to treat and often lethal. The genetic diversity within and between tumors allows some cancer cells to survive, even when others are targeted by therapy. Study’s findings that supports the concept of tumour heterogeneity & the branch model of tumour evolution: In this study, scientists collected and analyzed samples from both the primary tumor (located in the kidney) and its metastases from various locations, including the perinephric metastasis, chest wall metastasis, and lung metastasis Findings: Ubiquitous mutations across all tumor regions. Shared mutations between specific primary tumor locations and metastases. Private mutations unique to individual tumor regions or metastases. Lecture 10: Cancer and Development 15  Part 2: Does cancer resemble development? LO: Human developmental syndromes Wnt Signalling Pathway Lecture 10: Cancer and Development 16  INACTIVE In the inactive state, when the Wnt pathway is not activated: The destruction complex (Axin, APC, GSK3) phosphorylates B- catenin. Phosphorylated beta-catenin is degraded by the proteasome. As a result, there is no Wnt target gene activation. ACTIVE When the pathway is activated, for example in the human colon, the Wnt ligand (such as Wnt3) binds to the Frizzled receptor: The destruction complex dissociates, preventing phosphorylation of beta-catenin. B-catenin accumulates in the cytoplasm, migrates to the nucleus, and binds to TCF4. This binding leads to the activation of Wnt target genes such as MYC, AXIN2, and ASCL2. Lecture 10: Cancer and Development 17  Wnt signaling needs to be regulated at the right levels—not too low and not too high. This balance is maintained by a negative feedback loop, which rapidly regulates the pathway. In Familial Tooth Agenesis Syndrome - AXIN2 is mutated. Specifically: Both copies of the AXIN2 allele are lost. The negative feedback loop is disrupted because the function of AXIN2 is not maintained. Consequently, Wnt signaling is aberrantly activated, leading to a strong phenotype. In affected families: Male patients (indicated by black circles) are affected with oligodontia (permanent missing teeth). Female patients (also indicated by black circles) are similarly affected with oligodontia. Given the crucial role of Wnt signaling in tooth formation and homeostasis, its aberrant regulation can: Disrupt Wnt signaling, leading to severe effects on tooth formation. Wnt signaling is also vital for stem cell maintenance in the colon. Aberrant activation of Wnt signaling can induce cancer formation. Mutation in AXIN2 leads to loss of permanent teeth and predisposes individuals to colorectal cancer. Lecture 10: Cancer and Development 18  Hedgehod Signalling Pathway Lecture 10: Cancer and Development 19  INACTIVE PTCH1 is located in the cilia and inhibits SMO. SMO is therefore unable to migrate to the cilia. SUFU binds to GLI and, in conjunction with other kinases, induces the phosphorylation of GLI. Phosphorylated GLI acts as a repressor, inhibiting the expression of target genes. ACTIVE The SHH ligand (e.g., EDGARC) binds to PTCH1, causing its removal from the cilium. This allows SMO to accumulate in the cilium. SMO inhibits SUFU, permitting GLI to migrate to the nucleus. GLI then activates the expression of specific target genes. Lecture 10: Cancer and Development 20  Gorlin Syndrome and PTCH1 Mutations PTCH1 is a tumor suppressor gene. In Gorlin Syndrome: Both alleles of PTCH1 are mutated. This inactivation results in SMO being constantly active in the primary cilium - inhiting SUFU This inhibition of SUFU allows GLI to migrate to the nucleus. GLI then activates target genes associated with SHH signaling. —> Hyperactivation of the SHH pathway Consequences and Effects Developmental Abnormalities: Facial malformations Brain defects in the neural tube Vertebral fusion Polydactyly (extra digits Mutations in the human Patched gene (PTCH1) are associated with families with Gorlin’s Syndrome - resulting in developmental abnormalities and a high incidence of basal cell carcinoma. Lecture 10: Cancer and Development 21  LO: Human cancers and developmental signaling pathways Wnt Signaling Pathway 1. Role in Development Wnt Signaling is critical during intestinal development. It affects: Endodermal tube formation Patterning of the intestinal tract Cell differentiation and villous formation 2. Role in Cancer Wnt Signaling is also critical in cancer formation, particularly in colorectal cancer. Table Summary (from a study by Gives and Cleaver): Genes Mutated: APC, β-catenin Lecture 10: Cancer and Development 22  Cancer Types: Colorectal cancer, breast cancer, prostate cancer, liver cancer, pancreatic cancer Wnt Pathway Mutations in Colorectal Cancer 1. APC Mutation Frequency: 80% of colorectal cancer cases Impact: APC is a tumor suppressor gene. Mutation leads to the dissociation of the destruction complex. Results in the accumulation of β-catenin in the nucleus. Activates Wnt target genes like MCC and Cyclin D1. These genes are associated with cell cycle activation. 2. β-catenin Mutation Frequency: 10% of colorectal cancer cases Impact: β-catenin is an oncogene. Mutation causes loss of phosphorylation sites. β-catenin is not degraded by the destruction complex. Leads to the accumulation and activation of Wnt signaling. Lecture 10: Cancer and Development 23  Sonic Hedgehog signalling pathway Hedgehog is expressed in the nervous system, gut and limb buds Mutations in the Hedgehog/Patched pathway are associated with tumour formation: Inactivating mutations of Patched 1 are associated with: Gorlin Syndrome Lecture 10: Cancer and Development 24  Basal Cell Carcinoma Bladder cancer… Activating mutations of Smoothened lead to pathway activation and are involved with: Basal Cell Carcinoma Medulloblastoma Inactivation of SuFu is also associated with Basal Cell Carcinoma Medulloblastoma Amplification of GLI1 can result in: Basal cell carcinoma Glioblastoma Osteosarcoma Rhabdomyosarcoma. Lecture 10: Cancer and Development 25  Cyclopamine: from teratogen to anti-cancer drug Discovery of Cyclopic Lambs A third of sheep giving birth to lambs with severe congenital deformities. Notable deformities included severe brain defects and the presence of only one eye (cyclopic lambs). Investigation and Findings Initial Investigation focused on the correlation between the deformities and the consumption of corn lily by the ewes. Scientists identified a compound in corn lily that interfered with the Hedgehog (Shh) signaling pathway. This compound was named cyclopamine, referencing the cyclopic deformities. Mechanism of Cyclopamine Inhibition of Hedgehog Signaling: Cyclopamine binds directly to Smoothened. Prevents Smoothened from migrating to the primary cilium. Results in strong inhibition of Hedgehog signaling. Application in Cancer Treatment Aberrant Hedgehog signaling is associated with several cancers, including skin cancer. Cyclopamine confirmed to block Hedgehog signaling and inhibit tumor growth. Derivatives of cyclopamine like Patidegib are being investigated as treatments for basal cell carcinoma. Lecture 10: Cancer and Development 26