Lecture 47-48 Flashcards
Evidence for monoclonality of Cancer
- Examination of X-inactivation pattern
- usually all tissue are mosaic for X-inactivation, but all cells from cancers have same copy of X inactivated. - Chromosomal abnormalities of cancer
- Once chromosomal aberration (translocation) occurs, then cancer develops then, all cancer cells contain the aberration. - Normal B-cell produce polyclonal Ig , but multiple myeloma (malignant B-cell) produce the same antibody molecule (Monoclonal Ig)
Difference between oncogene and protooncogene
Protooncogene is normal state, but if there is mutation in protooncogene, it becomes oncogene.
(Mutation in one copy is sufficient to cause cancer - gain of function)
Major cell proliferation pathway
MAP (Mitogen-activated protein pathway)
Describe MAP kinase pathway
major cell proliferation pathway
- Growth factor binds to tyrosine kinase receptor.
- Then, dimerization and phosphorylation of activation lip tyrosine
- Phosphorylation of additional tyrosine residue, which leads to activation of Ras-GTP
- Ras-GTP activates Raf (MEKK)
- Raf activates MEK, which activates MAPK. (Phosphorylation of serine/threonine residue)
- MAPK activates DNA-binding proteins which in turn activate gene expression to drive DNA replication and cell division.
Type of mutation in MAP kinase pathway (4)
1.Constitutive activation of receptor
truncation(deletion) or point mutations at GF receptor which cause constitutive activation= self-sufficiency = dimerzation occur without ligand binding
- Activation of myc
- Burkitt Lymphoma
- Translocation between chromosome 8 and 14 occur. So, myc on chr8 fuse to immunoglobulin locus at chr 14. Since myc is now under regulation of IgH promoter, level of oncogene expression increase, and due to rapid cell division caused by myc, lymphocyte fail to differentiate. = - Bcr-Abl Translocation
- Chronic Myeloid leukemia
- Translocation between chromosome 9 and 22, forming philadelphia chromosome (fusion oncogene)
- Hybrid bcr-abl fusion protein is an unregulated cytosolic tyrosine kinase, so abl is stuck in its active form. - Mutation in Ras protein - single point mutation at Gly12 or Gln 61
- Mutation inhibit GTPase activity of Ras, making Ras constitutively active
What is Myc?
nuclear transcription factor important for G1/S transition., stimulate cell cycle
What is abl
Cytoplasmic tyrosine kinase (MAP kinase pathway)
Imatinib mesylate
a powerful tyrosine kinase inhibitor specific for a few tyrosine kinases including bcr-abl, which inhibit constitutively active tyrosine kinase by binding to active site of fusion protein.
What is Ras
It is a GTPase involved in MAP kinase pathway.
It is activated by binding GTP.
- It is inhibited by intrinsic GTPase activity (GTP>GDP).
What is double minutes and its relation to oncogenesis
Double minutes are extrachromosomal fragments of DNA containing an amplified oncogene (multiple / hundreds of copies of oncogene)
- seen in aggressive tumors
What is homogenously staining regions (hsr)
HSR of abnormal chromosome contain amplified oncogenes (multiple/hundreds of copies of oncogene)
Difference between double minutes and homogenously staining regions
Double minutes extrachromosomal fragments of DNA that contain amplified oncogenes, while homogenously staining regions are amplified oncogenes that are attached to chromosome.
Cause of Wilms Tumor
Results form loss-of-function in WT1 on chromosome 11
- Autosomal dominant inheritance, but actually recessive at cellular level, but dominant in pedigree
Difference between familial and non-familial (sporadic) cancer
Familial
- multiple tumors and bilateral
- present at an earlier age
- family history present
What is two-hit hypothesis of sporadic cancers and familial cancer.
For sporadic cancer, 1st mutation in tumor suppressor gene occur (1st hit)
2nd mutation in same gene should occur (2nd hit) for cancer and tumor initiation
For familial cancer, first mutation (1st hit) is inherited thus present in every cells. So second mutation (2nd hit) can occur in any cell, thus only one more hit is needed for tumor initiation.
Relationship between two-hit hypothesis and loss of heterozygosity (LOH)
When there is 2nd hit, it results in loss of heterozygosity (usually due to deletion of normal allele and left with mutant allele only)
Machenisms producing 2nd hit of two-hit hypothesis
- Loss through non-disjunction
- Mitotic recombination
- Gene deletion
- Point mutation
- Methylation (epigenetics)
How is Rb protein regulated?
Cyclin D/Cdk 4,6 hyperphosphorylates Rb thus not bound to E2F. So E2F can activate S-phase genes»_space; cell division
When there is no cyclin D/cdk, Rb is hypophosphorylated, thus bound to E2F. E2F still binds to DNA, but recruits histone methyl transferase and histone deacetylase, blocking transcription.
4 mutations that can lead to dysregulation of G1-S checkpoint
mutation in these 4 genes
- CDKN2A gene (p16) : CDK Inhibitor
- cyclin D gene : complex with cdk inhibit Rb by phosphorylating it
- cdk4 gene :complex with cyclin inhibit Rb by phosphorylating it
- Rb gene : when hypophosphorylated, inhibit E2F thus block transcription
Mutation of Rb gene on chromosome 13 leads to which disease condition?
Retinoblastoma (a childhood cancer)
What disease is caused by inherited mutation in p16 (CDKN2A)?
malignant melanoma
Role of p53 in signaling through G1 checkpoint
when there is double strand break in DNA, ATM and ATR (DNA damage signals) activate Chk1 and 2, which phosphorylates serine residue of p53. P 53 induce either G1 phase arrest or cell apoptosis.
3 roles of p53
- Slows cell cycle for repair (G1 arrest)
- Increase DNA repair capability
- If damage is too great, initiate apoptosis and eliminate cell
What is p21?
similar to p16, it inhibits Cyclin/cdk complex at G1.
How does p53 influence apoptosis?
It activates intrinsic apoptosis via 3 ways
- increase expression of pro-apoptotic bcl-2 family members (inhibit bcl2 and bclxl , but activate bax and bak)
- increase expression of fas receptor
- increase expression of IGFBP-3, which sequesters cell survival proteins (IGF1/2) that inhibit apoptosis.
What is Li-Fraumeni syndrome
inherited mutation in p53 (TP53 Gene)
- increase risk of cancer in young age
- results in several kinds of cancer
- responsible LOH (Loss of heterozygosity)
2 types of colorectal cancer that is commonly inherited
Familial Adenomatous Polyposis (FAP) - autosomal dominant
Hereditary Non-polyposis Colon Cancer (HNPCC) : Lynch syndrome
What causes Familial Adenomatous Polyposis (FAP)
- mutation in APC gene (early gate-keeper). Both alleles has to be mutated (1st hit & 2nd hit). ]
- Mutation in Ras
3a. no mutation in p53 leads to oncogene-induced senescence
3b. mutation in p53 (late event) leads to carcinoma
- Early mutation in APC gene Late mutation in p53
- Ras is not usually first mutation.
Mode of inheritance of Familial Adenomatous Polyposis (FAP)
Autosomal inheritance with allelic heterogeneity (multiple mutations of APC genes)
What is APC?
APC encodes for tumor suppressor which down-regulate growth promoting signals. Acts as a gatekeeper in WNT signalling pathway.
Describe WNT signaling pathway
When WNT signal is present, APC is inactivated, thus Beta-catenin is not degraded, thus it moves to nucleu, forming complex with TCF-4, which activates growth promoting genes.
But when WNT signal is absent, APC interacts with Beta-catenin, triggering phosphorylation of beta-catenin, which leads to ubiquination and degradation of beta-catenin, thus no transcription of growth-promoting gene.
What causes hereditary Non-polyposis colon cancer
Defect in mismatch DNA repair (MMR genes)
- MSH2, and MLH1
- Care-taker role
Difference between Familial adenomatous polyposis and hereditary Non-polyposis colon cancer
FAP
- many polyps
- initiation faster (in APC)
but progress slowly
HNPCC - Few polyps - initiation slower but progress rapidly - show microsatellite instability
Pattern of inheritance of hereditary Non-polyposis colon cancer (HNPCC) or lynch syndrome
autosomal dominant (loss-of-function)
Releationship between breast cancer and locus heterozygosity and allelic heterogeneity
Breast cancer caused by mutation in two different genes, BRCA1 and 2 = locus heterozygeneity
Hundreds of mutations in BRCA1 gene = allelic heterogeneity
Where is BRCA1 and 2 involved?
in DNA repair (homologous recombinant repair ) and apoptosis after duplication, thus it is care-taker gene
What is usually discovered genomically in sporadic breast cancer?
HER-2 overexpression in form of double minute and homogenously staining region (HSR)
(Human epidermal growth factor receptor 2)
Treatment on HER-2 overexpression
Use herceptin drug (antibody to HER2), which prevent binding of growth factor to HER2, leading to decreased tumor cell proliferation.
* effective for Her 2+ tumors, but not effective for Her2- tumors
Possible direct role of epigenetics in tumorigenesis
- Hypermethylation of tumor suppressor genes
- loss of imprinting causing activation of growth-associated genes or oncogenes
- micro RNAs
- reduction of miRNA that could inhibit oncogene RNA
- increase of miRNA that inhibit tumor suppressor RNA
Two sources of spontaneous mutation
- Errors of replication
- during S phase
- by DNA pol - Spontaneous lesion
- anytime during cell cycle
- due to chemical changes that occur spontaneously
How can errors of replication occur?
DNA mismatch by DNA polymerase
1. due to tautomerism
Thymine can exist as two form : keto form (pair with A)
and rare enol form (pair with G)
- Can be detected by proofreading by DNA pol
Or frameshift mutation by slippage of DNA pol at base repeats, which leads to DNA loops or kink and one or more bases are not copied or are copied twice.
What is spontaneous lesion?
Changes that occur in a resting cell due to the chemical nature of DNA (common mutation)
- increased by exposure to mutagen such as sunlight
3 types of spontaneous lesion
- Depurination (most common)
- Deamination
- Oxidative damage
What is depurination
breaking of glycosidic bond between sugar and base in purine nucleotide.
- only base is lost
What is deamination and example
loss of amine group from base, particularly cytosine
ex) cytosine deaminates to form uracil, which pairs with A
* easy to fix
ex) 5’ methylcytosine deaminates to thymidine, which abnormally pairs with G
* it is mutational hotspot because it is not readily detected by DNA pol due to methylation (promoter region- epigenetics)
What is oxidative damage on DNA?
Addition of oxygen groups to nucleotide bases
ex) 8-oxo-7-hydroxyguanosine mispairs with A and also results in transversion.
How does UV ray cause mutation?
It cause dimerization of pyrimidine (esp. thymine dimer) via covalent linkages between bases on the same strand. This create bulky adduct and interfere with normal pairing and block replication.
What is DNA glycosylase?
It removes damaged DNA base pair during base excision repair.
ex) uracil glycosylase specific for removal of U.
* Sugar phosphate is removed by endonuclease (nonspecific)
Difference between base excision repair and nucleotide excision repair
Base excision repair = 1 base pair
Nucleotide excision repair = remove bulky distortions/adducts in DNA
Inheritance of xeroderma pigmentosum
Ausotomal recessive with locus heterogeneity
cause of xeroderma pigmentosum
mutation in Nucleotide excision repair genes
such XPA, ERCC3
Phenotype of xeroderma pigmentosum
Sun sensitivity Ocular involvement (conjunctivities)
What is post-replicative repair mechanism?
mismatch repair, which is very important in removing small repeats that tend to expand such as triplet expansion disorder
- occur G2
- MMR proteins recognizes mismatch that are missed by proofreading
- excise few bases not one base
What is microsatellite instability
Variation in size of simple repetitive DNA sequences due to MMR protein not working (due to mutation)
* phenotype of mismatch repair mutation = hypermutable phenotype
2 types of repair of double strand breaks
- Non-homologous end joining
- G1/S
- More common method
- prone to more error than recombinational repair - Recombinational repair
- G2/M
Cause of Ataxia telangiectasia
mutation in ATM, a serine/threonine kinase, stimulating homologous recombination
- affecting cerebellum and immune system
- increased incidence of cancer
Since ATM is not working, homologous recombination is not working, but non-homologous end joining is still working to fix ds break.
Mode of inheritance of Ataxia telangiectasia
autosomal recessive
Which repair mechanism is BRAC involved in?
Homologous recombination repair
However BRCA1 is actually central to a large number of repair processes
What is bloom syndrome?
caused by defect in BLM gene (DNA helicase), which is required for replication repair and recombination
* It result in chromosomal instability resulting in chromosomal breaks and sister chromatid exchanges.
