Molecular Basis of Cancer 1

• Normal cell proliferation

  Binding of growth factor to specific receptor -> transient & limited activation of growth factor receptor -> activates several signal-transducing proteins on the inner leaflet of plasma membrane -> transmission of the transduced signal across the cytosol to the nucleus (via 2nd messengers or cascade of signal transduction molecules) -> induction & activation of nuclear regulatory factors that intiate DNA transcription -> progression of cell in cell cycle -> cell division
• What is the cell cycle clock?

  Molecular circuitry in cell nucleus

  Processes & integrates a variety of afferent signals

  Decides wether or not the cell should enter the active cell cycle or to retreat into non-proliferative state
• What happens to the cell cycle clock if active proliferation is decided?

  Programs sequence of biochemical changes in cell
• What are the 2 types of regulatory proteins involved in the cell cycle?

  Cyclins

  Cyclin-dependent kinases (CDKs)
• The cell cycle has specific checkpoints where the cell will stop until given a go-ahead signal.
• What CDKs do D-type cyclins bind to?


  CDK4

  CDK6
• What CDKs do E-type cyclins bind to?

  CDK2
• What CDKs do A-type cyclins bind to?

  CDK2

  CDC2
• What CDKs do B-type cyclins bind to?

  CDC2
• p53 is the most common target for genetic alteration in human tumours.
• What does p53 do?

  Tumor suppressor

  Prevents propagation of genetically damaged cells
• How does p53 inhibit neoplastic transformation?

  Activation of temporary cell cycle arrest -> quiescence

  Induction of permanent cell cycle arrest -> senescence

  Triggering of programmed cell death -> apoptosis
• What is penetrance?

  The proportion of individuals with a specific genotype who exhibit the associated phenotype.
• What are the factors that affect penetrance?

  Age

  Sex

  Environment

  Modifier genes

  Mutations that affect expression

  Tissue where the mutation occurs
• Penetrance does not equal severity.
• What is NF-1?

  Neurofobromatosis type 1

  Complex mult-system human disorder -> due to mutation of neurofibromin 1 (NF1) gene

  Autosomal dominant

  Causes tumours along NS
• What is epigenetics?

  An epigenetic trait is a stably heritable phenotype resulting from changes in chromosomes without alterations in the DNA sequence
• In order to multiply, cells required extracellular signals that drive…

  Cell cycle progession

  Cell growth
• Role of cyclins in cell cycle.

  G1/S

  • Cyclin D activates CDK4/6 -> complex phosphorylates Rb -> inhibtion of E2F1 -> induces expression of cyclin E -> cyclin E/CDK2 hyperphosphorylates Rb (inactivation) & induces expression of cyclin A

  S

  • CDK2/cyclin A -> starts DNA replication

  G2

  • CDK1/cyclin A -> activates CDC25 phosphatase -> activates CDK1/cyclin B -> inactivates Wee1 & activates CDC25 (+ve feedback)

  M

  • CDK1/cyclin B -> activates condensin -> DNA folding
  • CDK1/cyclin B -> disassembles nuclear envelop & activates centrosomes
• Fill in the blanks

  A) cyclin B

  B) nuclear D1

  C) cyclin E

  D) cyclin A

  E) R point

  
• What is the role of Rb in the cell cycle?

  M/G1 transition = Rb (protein) is unphosphorylated

  As cell progresses through G1 -> Rb is hypophosphorylated

  R point = Rb is hyperphosphorylates
• What is the role of unphosphorylated Rb?

  Binds to transcription factors (E2Fs)

  Prevents E2F-mediated transcriptional activation
• What is the role of phosphorylated Rb?

  Dissociates E2Fs -> turns of genes required for transition to S and DNA synthesis -> irreversibly commits to DNA synthesis
• What can deregulation of the cell cycle & genome maintenance pathways cause?

  Cancer
• What do CDK inhibitors do?

  Cell cycle regulation
• What is the role of p16?

  Tumour suppresor gene

  Inhibits Cyclin D/CDK4/6 kinase activity

  Loss of function -> commmon in several cancers
• Give 5 examples of tumour viruses.

  H.pylori

  HPV

  Hep B

  Hep C

  EBV
• Tumour viruses often seek to inactivate Rb & p53 (TSGs).
• How can HPV contribute to cancer?

  2 viral genes - E6 & E7

  E6 -> binds to p53 & BAX -> mediates degradation -> acitvates telomerase

  E7 -> binds to Rb -> displaces E2F transcription factors that are normally sequestered by Rb -> promotes progression through cell cycle
• What is HTLV-1?


  Human T cell Leukaemia Virus Type 1

  causes a form of T cell leukaemia/lymphoma

  Viral DNA integrates into host & Tax (protein) inactivates p53 -> dysregulation of cell cycle
• What are the multiple mechanisms of viral integration (critical step in tumourgenesis)?

  Chronic expression of proteins interfereing with TSG function

  Expression of viral proto-oncogenes

  Transcriptional activation of endogenous proto-oncogenes

  Inactivation of endogenous TSG
• What are the 7 proteins that when mutant forms are expressed, can lead to cancer?

  Extra & intracellular signalling molecules

  Signal receptors

  Signal transducing proteins

  Transcription factors

  Cell cycle control proteins (restrain cell proliferation)

  DNA repair proteins

  Apoptotic proteins (TSG)
• What is the two hit hypothesis?

  Mutations on both alleles is required for loss of function of tumour suppressor
• What is the function of BRCA1?


  Checkpoint activation

  DNA repair
• What is the function of BRCA2?

  Mediator of homologous recombination
• What is the most commonly occuring epigenetic changes in mammalian genome?

  DNA methylation

  Modifies DNA structure
• What is the process of DNA methylation?

  Addition of methyl group to C-5 position of cytosine residues

  Methyl groups are transferred from S-adenosyl methionine (SAM) in a reaction catalysed by a DNA methyl transferases (DNMT)

  SAM then converted to SAH (S-adenosyl homocysteine)
• What is the function of DNMT3a, DNMT3b & DNMT3L?

  Add methyl groups to CpG sites

  Re-establish the methylation pattern
• What are the functions of DNMT1?

  Maintains pattern of DNA methylation after DNA replication

  Requires hemi-methylated DNA substrate -> will reproduce pattern of DNA methylation on new strand
• What are the roles of DNA methylation?

  Long term silencing of genes

  X chromosome inactivation

  Establishment & maintencance of imprinted genes

  Silencing of repetitive elements

  Suppression of expression of viral genes & other deleterious elements that are incorporated into genome of host over time

  Carcinogenesis??