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.