Mechanisms of Mutagenesis & Carcinogenesis

Jamil Momand, Ph.D.California State University at Los Angeles

Outline (in brief)Introduction Carcinogenesis - a definitionPathways to cancer: overviewOncogenesTumor suppressor genesApoptosisTelomeraseAngiogenesisSummary

Maintenance of homeostasis

Adult human maintains ~1015 cellsStem cells undergo ~1012 divisions per dayThere is a balance between cell birth and cell deathRandom mutations disrupt homeostasis

Molecular basis for cancer progression

DNA damage

Failure to repair damage

Failure to destroy cells with DNA damage

Mutation

Selection advantage

Example of DNA damage

From Bertram, Molecular Aspects of Medicine 21, 2001, 161-223

List of carcinogens

ChemicalAsbestosArsenicChromiumPolyaromatic hydocarbonsdichlorodiphenyl-trichloroethane (DDT)

PhysicalGamma radiationUV lightRadonX-raysViruses*

http://www.eur.nl/fgg/ch1/gen_research/dbr-man.html

Hereditary form of colon cancer

Case 1: Beth M.'s father died of colon cancer, as did her grandmother.

Now two of her brothers, both in their 40's, have been diagnosed

with colon cancer. Beth, age 37, feels a curse is hanging over her family

and is worried about her future and that of her children. Case 2: Paul C. was 35 when his doctor told him the grim news: he had advanced colon cancer. As far as he knew, Paul had no family

history of the disease. But after checking, Paul learned that several

aunts and uncles had died of colon cancer at an early age.

Diagnosis: hereditary nonpolyposis colorectal cancer (HNPCC)

Frequency: 1 in 6 colorectal cancer casesCause of the disease: hMLH1 or hMSH2 mutations. Genes

responsible for DNA mismatch repair

From Bertram, Molecular Aspects of Medicine 21, 2001, 161-223

Clonal Nature of Cancer

Cancers are composed of cells that descended from a single cell.

Evidence 1: X-chromosome inactivation

Evidence 2: Ig genes in lymphomas and leukemias are identically rearranged.

Multistage theory of cancer development (Armitage and Doll, 1954)

http://www.hhmi.org/communic/annrep/research/regulate.htm

Viruses and cancer

Viruses account for 15% of all cancersDNA viruses Epstein-Barr virus Human papilloma virus Hepatitis B virus

RNA viruses HIV-1 HTLV-1 HTLV-2

NormalCell

InitiatiedCell

CancerCell

DNA Damage

Progression

Mutation

Proliferation

Immortality

Mutator phenotype

Angiogenesis

Metastasis

Protection

Repair

Cell cycle arrest

Apoptosis

Necrosis

Chemotherapy

Radiation therapy

Mu

tatio

ns

Pro

lifera

tion

Adapted from Bertram, Molecular Aspects of Medicine 21, 2001, 161-223

Definitions of terms

AmplificationImmortalOncogenePoint mutationProto-oncogeneTransformationTranslocation

DNA Amplification

c-myc translocation

DNA Methylation and Demethylation-ways to control expression of genes

Point mutation activation of Ras

http://www.biocarta.com/pathfiles/pdgfPathway.asp

Oncoprotein pathways

Her2/neu/erbB-2

This gene was discovered by three different groups. That is why it has three different names.Its oncogene counterpart is v-erbB-2Dr. Slamon (UCLA) described the role of Her2/neu in breast cancer and ovarian cancer.Overexpression, amplification, rare translocationsNo ligand is known

The Philadelphia Chromosome

BCR-ABL translocation and expression

http://www.kent.k12.wa.us/staff/vhoward/apbio/oncogenes/brc-ablcartoon.gif

STI-571--an inhibitor of BCR-ABL function

http://www.blc.arizona.edu/courses/181gh/Lectures_WJG.01/cell_signaling.01/Applications.html

PET scanning to show efficacy of STI-571 on tumor metastasis

http://www.blc.arizona.edu/courses/181gh/Lectures_WJG.01/cell_signaling.01/Applications.html

Design of experiment to clone the first human cellular oncogene (Shi et al., 1979)

What we have learned

Human cellular oncogenes are dominant-acting genes that transform cells and cause tumors in mice. Activation-abnormality that results in higher than normal level of biochemical activity.

Point mutation Overexpression

Proto-oncogenes-The wild-type non-cancerous form of the oncogene

How are oncogenes activated?

Point mutation-Eg. K-ras, Amplification-Eg. N-myc, MDM2, Her2/neu/ErbB2Chromosome translocation-Eg. c-myc, bcr-ablOverexpression due to DNA demethylation

Tumor suppressor genes

•Somatic cell hybrids•Knudsen’s hypothesis•Mutations and mechanisms thatlead to tumor suppressor gene loss of function•RB•p53

Somatic cell hybrids

Harris et al., 1969

Transformed

Normal

NormalCell fusion

Genetics of Retinoblastoma

Pedigree of Rb-prone family

Alfred Knudson

Knudson’s hypothesis

Mechanisms of tumor suppressor gene inactivation

DeletionPoint mutationMutation followed by duplicationLoss of heterozygosityDNA methylationPost-translational mechanism-binding to DNA viral oncoproteins

Genetic mapping of Rb susceptibility gene

RB function

http://p53.curie.fr/p53%20site%20version%202.0/p53%20in%20cancer/p53_databaseANAL.html

p53 mutation spectrum

p53 structure

[MDM2/p53] complex (transient)

p53 Latent

degradation

synthesis

p53 signalp53 signaltransduction pathway transduction pathway

DNA damage

p53Stabilization Transcription

3. WAF1/CIP1 Cell cycle arrest

DNA Repair1. MDM2

2. GADD45

n

4.5.

Cell Death}Other p53 functions

Functional domains of BRCA1

Ring finger

Dimerization

NLS

Transcriptionalactivation

Proteins that bind BRCA1:BARD1 Rb BRCA2 p300BAP1 p53 RHAE2F1 Rad51 RNA PolIIcMyc Rad50 CREB binding protein

BRCA1 C-Term. domains

BRCA1

Mapped to chromosome 17q by Mary Clair King in 1990Linkage was also found in ovarian cancer families.More than 90% of women with germline BRCA1 mutations lose the wild-type allele in breast tumor.The gene encodes a nuclear phosphoprotein of 220 kD (1863 aa’s)The mRNA is 5711 bases long24 exons, 22 of which is coding

BRCA1 (cont. 1)

BRCA1-deficient ES cells are hypersensitive to oxidative reagentsBRCA1-deficient ES cells are defective in transcription-coupled repairExpression of BRCA1 leads to p21CIP1/WAF1 upregulation and G1-S cell cycle arrest (is this through p53?)BRCA1del11 maintain G1-S cell cycle arrest but not G2-M arrest.

http://www.biocarta.com/pathfiles/atmPathway.asp

What have we learned about tumor suppressor proteins?

Both alleles are deleted in the cancerIf one allele is mutated at birth then patients have increased susceptibility to cancer at an early ageThey often serve as cell cycle checkpoints or DNA repair activities

Apoptosis-programmed cell death

BCL-2 is a protooncogene that gets overexpressed insome B-cell leukemias

Other processes that affect tumor formation and growth

Telomerase expressionAngiogenesis

The BIG picture-a molecular analysis of mutations in colorectal cancers

Correlation of tumor morphology to specific allelic deletion

(Vogelstein et al., 1988)

Future studies-profiling cancers with DNA arrays

http://cmgm.stanford.edu/pbrown/

http://www.pnas.org/cgi/content/full/241500798

Dilbert by Scott AdamsSan Gabriel Valley Tribune, Sept. 10, 2000

References

Bertram, J.S. (2001) The molecular biology of cancer. Molecular Aspects of Medicine 21, 167-223.Gelehrter et al. Principles of Medical Genetics, 2nd ed. pp 245-272, Williams & Wilkins, Baltimore, 1998.Levine and Lane, (2000) Surfing the p53 network. Nature 408, 307-310Angier, N., Natural Obsessions: Striving to Unlock the Deepest Secrets of the Cancer Cell, Mariner Books/Houghton Mifflin Co, 1999.Bazell,R., Her-2: The Making of Herceptin, a Revolutionary Treatment for Breast Cancer, Crown Publishing Group, 1998.