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.