10 Cancer (DIY)

• Causative Factors (1. Genetic Disposition)

  • Mutations to cancer critical genes that occur in germ line cells can be passed from parent to offspring
  • An individual inheriting an oncogene or mutant allele of a tumour suppressor gene will be one step close to accumulating the necessary mutations for cancer to develop
  • E.g. Mutation of BRCA1/ BRCA2 gene in humans
• Causative Factors (2. Carcingens)

  • Agents/ substances that are directly involved in causing cancer
  • Majority of identified carcinogens are mutagens, which are agents that cause DNA damage and generate mutations
• Causative Factors (Carcinogens - Chemical Carcinogens)

  • E.g. Nitrosamines (some other chemicals in tobacco smoke)
  • Most are metabolically activated into forms that bind covalently to DNA, forming DNA adducts
  • NORMALLY, cellular repair systems remove DNA adducts and maintain DNA structure
  • BUT, damaged DNA repair enzymes are damaged or cannot function efficiently → DNA adducts remain and increase the chance of developing mutations in somatic cells
• Causative Factors (Carcinogens - Radiation)

  • Overexposure to ionising radiation (X-rays and gamma-rays etc.) causes DNA mutations that may lead to cancer
  • Overexposure to ultra-violet (UV) radiation can lead to skin cancer
  • UV-A and UV-B are 2 wavelengths of radiation that can penetrate to different depths of the skin
  • UV-A = oxidative damage to skin cell components
  • UV-B = Breaks the hydrogen bond between nitrogenous bases to break → causes the unbounded base to interact with adjacent bases on the same DNA strand to form dimers
• Causative Factors (Carcinogens - Viruses)

  • Integrate their genetic material into the DNA of infected host cells; contribute to cancer development by:
  • Activating a proto-oncogene by gain-of-function mutation/ inactivating tumour suppressor genes by loss-of-function mutation
  • Producing proteins that inactivate p53 and other tumour suppressor proteins, thus making the cell more prone to becoming cancerous
  • Introducing an oncogene into a normal cell → retroviruses may carry a copy of host-derieved oncogene, which is reverse-transcribed and inserted into the genome of the next infected host cell
• Causative Factors (Carcinogens - Virus E.g.)

  • E.g. Epstein Barr Virus causes Lymphoma
  • Hogdgkin’s lymphoma is a type of cancer which originates from a specific type of WBC called lymphocytes
  • EPV preferentially infects B lymphocytes
  •  Antigens encoded by EBV interfere with important cellular pathways → leads to tumour formation
  • EBC antigens have been found to immortalise B cells by facilitating p53 degradation → enhances transcription of certain host and viral genes → blocks apoptosis and chromatin remodelling
• Causative Factors (3 - Loss of Immunity)

  • Patients with weakened immune systems have specialised immune cells that are unable to carry out their function effectively → increases chances of cancerous growth 
  • Specialised immune cells (Cytotoxic T cells, natural killer cells and macrophages) detect and indue the death of developing tumour cells
• Causative Factors (4 - Other Factors)

  • Reactive Oxygen Species (ROS) can also cause DNA damage, possible leading to cancer development
  • High levels of ROS produced during infection by Helicobacter pylori are a causal factor of gastric cancer
  • High levels of bile acids in the colon, which is a result of a high-fat diet, also causes DNA damage
  • Contributes to causing colon cancer
• Uncontrolled Cell division - GAIN-OF-FUNCTION MUTATIONS

  • a mutation that causes a gene to encode proteins with new or enhanced activity
  • Gene products of protooncogenes can become/ are:
  • Hyperactive
  • Resistant to degradation
  • produced in excessive amounts
  • Only ONE copy of the allele needs to be mutated (dominant mutation)
  • Mutation in just one copy is enough
  • Normal cells are sensitive to the amount of such gene products
  • Overstimulates cell cycle → cell proliferates excessively
  • Affects proto-oncogene (e.g. ras gene)
  • Mutated proto-oncogene is known as oncogene
• Uncontrolled Cell division - Proto-oncogenes

  • Normal cellular genes that code for proteins that stimulate normal cell growth and proliferation
  • Products are:
  • Proteins derived from proto-oncogenes
  • Involved in stimulating normal cell growth and division
  • E.g. growth factors, growth factor receptors, transcription factors etc.
  • Mutation of proto-oncogene → oncogene (which is a gain-in-function mutation)
  • Results in an increase in the:
  • Amount of proto-oncogene protein product
  • The intrinsic activity of the protein product
  • Mutation mechanisms lead to oncogene formation
• Mutation Mechanism 1: Point Mutation in proto-oncogene itself/ a control element

  • Occurs in the coding sequence of proto-oncogene
  • changes the amino acid sequence of the proto-oncogene protein (affects 3rd and 4th structure)
  • Changes the protein to be either:
  • More active (hyperactive)
  • More resistant to degradation (degradation-resistant gene product) then the normal protein 
  • Occurs in base sequences of regulator elements (the promoter that controls a proto-oncogene)
  • Leads to the upregulation of proto-oncogene expression
  • Excess production of the growth-stimulating protein occurs
• Mutation Mechanism 2: Amplification of the proto-oncogene

  • Results in an abnormal increase in the number of copies of the proto-oncogene in teh cell
  • Excessive production of proto-oncogene protein
  • Promotes excessive cell division
• Mutation Mechanism 3.1: Movement of DNA within the genome

  • Chromosomal translocation
  • Movement/ exchange of an entire segment of chromosome
  • A proto-oncogene is translocated as part of a chromosome segment and ends up near an especially active (hyperactive) promoter
  • Results in the upregulation of proto-oncogene expression
• Mutation Mechanism 3.2: Movement of DNA within the genome

  • Gene Transposition
  • Movement of a single gene from one location to another; may be within the same chromosome/ between 2 chromosomes
  • Transposition of proto-oncogene such that it comes under the control of a more active promoter/ enhancer/ transposition of a more active promoter adjacent to a proto-oncogene
  • Results in the upregulation of proto-oncogene expression
• Mutation Mechanism 3.3: Movement of DNA within the genome

  • Retroviral Integration
  • Integration of viral genome into a proto-oncogene can convert it to an oncogene, causing insertional mutagenesis
  • Alters protein product → excessive cell division
• Proto-oncogene: Ras gene (normal)

  • Normal ras gene encodes Ras proteins, which are involved in signal transduction pathways
  • NORMAL ras gene:
  • Growth factor binds to its receptor embedded on the cell surface membrane
  • Binding triggers a series of reactions inside a cell:

  1. GTP binds to inactive Ras protein and activates it
  2. Active Ras proteins transduce (pass down) signals from the growth factor to downstream signalling processes
  3. Eventually, cell cycle will be stimulated and the cell undergoes cell division
• Proto-oncogene: Ras gene (mutated)

  • MUTATION
  • Ras-GDP loses its ability to hydrolyse GDP → constitutively active Ras protein due to activation by stimulus exchanging GDP for GTP → always bound to GTP and is thus permanently activated
  • Leads to increased cell division even in the absence of growth factor binding to its receptor
• Uncontrolled Cell Division - LOSS-IN-FUNCTION MUTATIONS

  • A mutation that causes a gene product to be non-functional
  • Gene products of tumour-suppressor genes are defective and cannot activate other, genes
  • Both copies of allele need to be mutated (recessive mutation)
  • If one is mutated, there is still another copy of the normal allele that can function normally (products can still activate other genes for DNA repair)
  • Unable to halt cell cycle to repair DNA damage
  • Cells with accumulated mutations keep dividing
• Uncontrolled Cell Division - Tumour Suppressor genes

  • NORMAL cellular genes that code for proteins that normally inhibit cell growth and division
  • Products are:
  • Proteins derived from tumour suppressor genes
  • Activate cell cycle arrest, DNA repair and/ or apoptosis (programmed cell death)
  • MUTATION;
  • Contribute to cancer when there is a loss-of-function mutation
  • Causes the expressed protein to lose its ability to inhibit cell growth and division
• Tumour Suppressor Gene: p53 Gene (normal)

  • Most commonly mutated gene in human cancers about 50% of all human cancers are associated with mutations in this gene)
  • Normal p53 gene encodes a p53 protein product that functions as a specific transcription factor (activator) protein
  • NORMAL function:
  • P53 protein binds to DNA (at the enhancer region) to promote synthesis of cell cycle-inhibiting proteins (proteins that act to cause cell cycle arrest, repair DNA damage/ apoptosis if DNA damage is beyond repair)
• Tumour Suppressor Gene: p53 Gene (mutated)

  • MUTATION:
  • P53 protein fails to stop cell division and repair DNA → cell divides without repair to damaged DNA
  • Uncontrolled division of damaged cells → damages accumulates → cell turns cancerous
• Tumour Suppressor Gene: p53 Gene (functions - 1)

  1. Cell cycle arrest

  • P53 protein activates a specific gene whose product halts the cell cycle

  1. Checkpoint proteins are activated to halt cell cycle at G1, G2 or M phase checkpoints in response to defects

  • So cell with damaged DNA has more time to repair its DNA to prevent the production of mutated daughter cells
• Tumour Suppressor Gene: p53 Gene (functions - 2)

  1. DNA repair

  • p53 protein activates genes whose protein products can directly repair DNA
  • This preserves genomic integrity and prevents mutations that may lead to the formation of oncogenes and inactivation of other tumour suppressor genes
• Tumour Suppressor Genes: p53 Gene (functions - 3)

  1. Apoptosis (Programmed Cell Death)

  • P53 protein activates other genes whose protein products cause cell death → removes damaged cells
  • Important as it removes cells with potential to cause cancer
• Cellular Immortality - GENES ENCODING TELOMERASE

  • Normal somatic cells have a natural limit on the number of times they can divide, due to the shortening of the telomeres with each round of DNA replication (end replication problem)
  • Cancer cells only have the ability to undergo indefinite cell proliferation
• Cellular Immortality - GENES ENCODING TELOMERASE (what allows them to keep dividing repeatedly?)

  • the gene encoding telomerase in cancer cells is activated (inactivated in normal somatic cells)
  • Hence, cancer cells can lengthen shortened telomeres with the expression of telomerase, thus compensating for the end replication problem
  • Prevents critical length from even being reached
  • Confers cancer cells immortality and enables them to divide repeatedly
  • Does not undergo replicative senescence
• Invasion and Metastasis - GENES ENCODING PROTEINS INVOLVED IN ANGIOGENESIS (Intro)

  • Latter stages of cancer development → tumours will undergo angiogenesis
  • ANGIOGENESIS: process where new blood vessels form within and around a growing mass of tumour cells
  • New blood vessels are a network of new capillaries that allow O2 and nutrients to reach cells/ tissues more effectively
• Invasion and Metastasis - GENES ENCODING PROTEINS INVOLVED IN ANGIOGENESIS (Process)

  • Due to expression of genes encoding proteins involved in angiogenesis (angiogenic factors) is upregulated
  • As tumour mass grows, cells in the centre receive less oxygen and nutrients (via passive diffusion) than cells on the outside
  • Network of blood vessels enable the tumour mass to grow beyond the limits imposed by passive diffusion
  • Tumour cell mass can now obtain nutrients and oxygen more efficiently + remove metabolic waste products
• Invasion and Metastasis - GENES ENCODING PROTEINS INVOLVED IN METASTASIS (Intro)

  • METASTASIS: the process whereby cells from the localised primary tumour
  • Leave the tumour cell mass
  • Invades adjacent tissues
  • Travel to another part of the body by the bloodstream/ lymphatic system
  • Establishes themselves as secondary tumours
• Invasion and Metastasis - GENES ENCODING PROTEINS INVOLVED IN METASTASIS (Process)

  • Tumour cells also alter the expression of certain genes:
  • Downregulates expression of genes encoding proteins responsible for cell-cell adhesion
  • Up-regulates expression of genes encoding extracellular proteases
  • Proteases expressed are required to breakdown the cytoskeleton and filaments that hold the cells together
  • This facilitates invasion of cancer cells into the bloodstream and lymphatic system
• Describe how dysregulation of the checkpoints of cell division may lead to cancer [Part 1]

  1. Dysregulation of the cell cycle checkpoints mean that cell division continues even when the cell fails to meet the criteria of having adequate size, replicated its DNA and having all chromosomes attached to the mitotic spindle
  2. Thus, even in the absence of growth factors, uncontrolled cell division still occurs
• Describe how dysregulation of the checkpoints of cell division may lead to cancer [Part 2]

  1. Daughter cells from such cell division will have problems in their genetic composition, which may lead to further accumulation of genetic mutations that could result in cellular immortality, metastasis, and hence cancer
• Outline the development of cancer including the effects of this causative agent [Part 1]

  1. The causative agent increases chances of DNA damage and mutations in the genes which control regulatory checkpoints of the cell cycle in a single cell.
  2. Loss-of-function mutation in both alleles of tumour suppressor genes will result in  the inability to inhibit cell cycle, repair damaged DNA and promote apoptosis.
• Outline the development of cancer including the effects of this causative agent. [Part 2]

  1. Gain-in-function mutation in just one allele of proto-oncogenes to form oncogenes will result in overexpression of proteins/growth factors OR production of hyperactive/degradation resistant proteins/growth factors.
  2. Both of which result in uncontrolled cell division to form a primary tumour.
  3. Loss of contact inhibition enables cells to grow into a tumour (Metastasis)
  4. Activation of genes coding for telomerase means that tumour cells can divide indefinitely.
• Outline the development of cancer including the effects of this causative agent [Part 3]

  1. Production of angiogenetic factors induces growth of new blood vessels that transport oxygen and nutrients for tumour growth.
  2. Regulation of metastasis-related genes (upregulation of extracellular proteases and downregulation of cell-cell adhesion proteins) results in the formation of a malignant tumour capable of metastasizing to other parts of body to form secondary tumours.
• State the meaning of the term mutation

  1. Refers to the change in the specific DNA sequence
  2. Can be a deletion/ insertion mutation where one or several nucleotides are removed from/ added into a DNA nucleotide sequence
  3. Can be a substitution mutation where one nucleotide is replaced by another
  4. Can be an inversion mutation, where a segment of nucleotide sequences separates from the allele and rejoins at the original position, but is inverted
• Suggest why the p53 protein only binds to certain parts of the DNA molecule

  1. P53 protein has a DNA-binding domain (p53 is NOT an enzyme → CANNOT use active site)
  2. That is complementary in conformation and charge to specific parts of DNA molecule
  3. P53 only recognises a specific sequence of bases due to its specific 3D conformation
• Suggest why the mutations of the p53 gene have no effect on the activity of the p53 protein [Part 1]

  1. May be due to a silent mutation: genetic code is degenerate where more than one type of codon codes for the same amino acid.
  2. Hence, a substitution mutation at the 3rd base/nucleotide of codon may still result in the same amino acid being coded for.
  3. Alternatively, it may be a conservative missense mutation: a point mutation results in a different codon that codes for a different amino acid with an R group of similar chemical properties to the original amino acid.
• Suggest why the mutations of the p53 gene have no effect on the activity of the p53 protein [Part 2]

  1. Mutations may also occur in the introns, which are excised during splicing and not translated, hence no change in amino acid sequence.
  2. Mutation could affect non-essential amino acids that are not at the DNA binding site, and hence there is no change in conformation of the DNA-binding site of p53.
  3. Hence p53 still complementary in conformation and charge and is able to bind to the same specific DNA sequences.
• Suggest how this multi-drug resistance in a tumour may be overcome

  1. Anti-sense RNA that will bind to mRNA transcripts of Mdr1 gene (context of this question)
  2. To prevent translation of mRNA coding for membrane-bound transporter so that number of transporters will decrease

  OR

  1. Molecular inhibitors that bind directly to membrane-bound transporters
  2. To prevent their pumping activities so that there will be no efflux of the anticancer drugs