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Chapter 17
Altering the genetic message:
Mutation
Cancer.
MUTATIONS
= change in the genetic message
= change in the nucleotide sequence of a gene
= DNA Damage + DNA Repair.
1. Types of mutations
2. Causes of DNA damage
3. DNA (damage) repair
Types of mutations.
A. Gene mutationsa. point mutations
or base-pair substitutionb. frame-shift mutations
B. Chromosomal mutations(duplication, inversion, deletion, translocation)
Causes of DNA damage.
1. Spontaneous (tautomeric shift, deamination, depurination, looping-out)2. Oxidation3. Chemical - base-pair analogs - base-modifying agents - intercalating agents - agents that cause ‘bulky’ lesions4. Physical: - UV light
- X-ray
Causes of DNA damage.
1. Spontaneous - tautomeric shift
- deamination- depurination- strand slippage/looping-out
Tautomeric shift of bases
Common Form Uncommon Form
Tautomeric shift leads to altered base pairing.
Depurination of DNA
Apurinic sites
De-amination of cytosine or methyl-cytosine.
5’TACGGAC 3’3’ATGCCTGACTTTGC 5’
5’TACGGACTG 3’3’ATGCCTGACTTTGC 5’
T
5’TACGGACTGAAACG 3’3’ATGCCTGACTTTGC 5’
T
Strand slippage during DNA replication
Newly synthesized DNATemplate DNA
Newly synthesized DNA loops out, …
… resulting in the addition of one nucleotide in the new strand.
5’TACGGAC 3’3’ATGCCTGACTTTGC 5’
5’TACGGACTG 3’3’ATGCCTGCTTTGC 5’
A
5’TACGGACGAAACG 3’3’ATGCCTGCTTTGC 5’
A
Strand slippage during DNA replication
Newly synthesized DNATemplate DNA
Template DNA loops out, …
… resulting in the omission of one nucleotide in the new strand.
Causes of DNA Damage 2. Oxidative damage
(Respiration, Mixed function oxidases, Inflammation)
Causes of DNA damage. 3. Physical
UV-Light(254 – 260 nm)
Pyrimidine dimers
Gross distortionDNA molecule
Malignant Melanoma
Causes of DNA damage. 3. Physical
Causes of DNA damage.
4. Chemical - base-pair analogs - base-modifying agents
- alkylating agents- base deaminating
- intercalating agents - agents that cause ‘bulky’ additions
Causes of DNA damage
4. Chemical:
Base Analogs
Miss-pairing
Alkylating agents add alkyl groups Cause altered base pairing
E.g., Ethyl-methane-sulfonate (mustard gas),Nitroso-guanidine.
Base deamination
E.g., nitrous acid, bisulfite
ActinomycinEthidium BromideAcriding OrangeTetracyclineProflavine
Intercalating agents
Bulky additions grossly distort the DNA
Aflatoxins
… ‘bulky’ additions, gross distortions of
the DNA
Benzo ( )a pyrene
Cellular Responses to DNA damage
Error-free DNA repair Apoptosis
Error-prone DNA repair
Mutation
DNA Repair:
1. Direct correction of DNA repair
a. Proofreading of DNA polymeraseb. Repair of alkylating damage
2. Repair involving excission of base pairs
a. General excission repair system (UvABC)b. Repair by glycosylases and
AP endonucleasesc. Mismatch-repair systemd. SOS-repair system
Proofreading Capacity of DNA Polymerase
Repair by Alkyl-transferases
General Excision repair system
Xeroderma pigmentosa:Deficiency of the general excision repair system.
Glycosylase/AP endonuclease
Repair system
Repair by DNA glycosylaseand AP endonucleases.
Mismatch Repair System
Hereditary Non-Polyposus Colon-rectal Cancer (HNPCC)
Error-prone repair by end-joining.
Chapter 17
Altering the genetic message:
Cancer.
Independent of growth signals
Insensitive to growth inhibitors
Changed energy metabolism
Avoid apoptosis
Limitless replicative potential
Sustained angiogenesis
Tissue invasion metastasis
Cancer
Cancer cells loose contact inhibition, grow on top of each other, and become
rounded.
Normal cells
Normal Cancer
Normal(contact inhibition)
Cancer
Rous sarcoma virus
Nobel Prize 1989
Chicken Rous Sarcoma Virus (RSV) carried an oncogene called v-src and this gene was an intronless version of a normal chicken gene called c-src.
Proto-oncogene(s)
Oncogene(s)
Activetumor suppressor
gene(s)
Inactivetumor suppressor
gene(s)
CANCER
MUTATIONS
Gain-of-function
Loss-of-functionDominant phenotype
Recessive phenotype
DNA Damage+
DNA Repair
+
Rb EF2
Cdk2-cyclin E
P
DNA
mRNA
DNA polymerase
S-phase
Retinoblastoma.
A multi-hit process of mutations accumulating in proto-oncogenes and tumor suppressor genes.
CANCER:
Familial Adenomatous Polyposis Coli
Colonoscopy Results
JB Weitzmann and Nosh YanivNature 1999, 400 p401
What causes cancer?
1. Environmental carcinogens
- chemical (e.g., cigarette smoke)- physical (e.g., UV radiation)
2. Host carcinogens (e.g., inflammation)
3. Viruses:
TUMOR VIRUSES
Papiloma virus
Hepatitis B virus
Human Herpes virus 8 (Kaposi)
Human Herpes virus 4 (Epstein Bar)
Human T lymphotropic virus
This woman has hepatitis B and is suffering from liver cancer. She was a Cambodian refugee and died 4 months after she arrived in a refugee camp (average life expectancy after diagnosis of liver cancer is 6 months)
Kaposi syndrome: Human Herpes Virus 8)
Cutaneous B cell lymphoma
HTLV Leukemia
The End.