3 - Genetics Flashcards
At what level does the malignant phenotype occur?
Genetic!
At the DNA level
Type of origin - Most tumors
Clonal! You can trace the lineage back to the original heritable mutation in one cell.
Polyclonal tumors
They exist, but they’re rare!! It means independently, separate cells have mutated to their malignant form.
How many steps are in the process of cancer development?
Multi
Phenotypic properties of cancer cells
Loss of control over cell growth Failure of cellular differentiation Inappropriate resistance to cell death Acquisition of angiogenic capacity Acquisition of metastatic potential
Steps of Metastasis
Destruction of basal lamina Infiltration of local connective tissue Intravasation Extravasation Distal colonization
Each step of cancer development is due to
A specific genetic or epigenetic alteration
These accumulate and work together
They are subject to clonal selection
Some of these are rate-limiting!!
Multiple genetic alterations leading to cancer development
Some are inherited.
MOST are acquired somatically.
Alterations that increase the rate of cell division
C-Myc activation
Rb inactivation
Alterations that decrease genomic stability
Inactivation of mismatch repair genes
p53 mutations
Mismatch repair genes
hMSH2
hMLH1
What typically induces the genetic alterations associated with malignancy?
Viruses
Chemicals
Radiation
Random Errors
What genes, when altered, promote cancer?
Proto-oncogenes
Tumor suppressor genes
What is an example of a gene altered in a restricted set of tumor types?
APC tumor suppressor in colorectal carcinoma
What is an example of a gene altered in a broad spectrum of tumor types
p53 tumor suppressor
Ras proto-concogene
Proto-oncogenes
Promote cancer when malignantly ACTIVATED
Gain-of-function (eg Ras)
“Dominant” at the cellular level. Can elicit a tumor even in the presence of the wild type allele.
Tumor suppressor genes
Promote cancer when malignantly INACTIVATED
Loss-of-function
“Recessive” at the cellular level. Typically will not promote a tumor in heterozygotes unless the other allele (the wild type) is also lost.
Exception: Dominant negative mutations (eg p53)
Mechanisms for oncogene activation
Coding mutations (leading to altered protein function) eg Ras
Chromosomal rearrangements (eg translocations, leading to gene dysregulation or overexpression) eg c-Myc gene translocation (Burkitt's Lymphoma)
Gene amplification (leading to overexpression) eg MDM2 gene amplification (Sarcomas)
How many human cancers are heritable?
Fewer than 10%!!
How are hereditary syndromes of cancer susceptibility usually caused?
Germline mutations of tumor suppressor genes
Hereditary syndromes of cancer susceptibility
Familial retinoblastoma (Rb)
Li-Fraumeni syndrome (p53)
Familial adenomatous polyposis coli (APC)
Hereditary non-adenomatous cc (MLH1, MSH2)
Familial breast & ovarian cancer (BRCA1, BRCA2)
Fully penetrant mutations
Segregate as dominant traits in mendelian fashion
2 forms of Retinoblastoma
Sporadic
Heritable
Sporadic Retinoblastoma
60% of cases
~6 years
Single tumor (only one eye)
Their kids have the same rate of retinoblastoma as the general population
Both Rb alleles normal in the germline
Both Rb alleles inactivated or lost in tumors
Heritable Retioblastoma
40% of cases
~2 years
Multiple tumors (both eyes)
Their kids have a 50% chance of having a retinoblastoma
Transmit an “Rb susceptibility gene” in a dominant mendelian fashion
One Rb gene lesion in the germline
Second Rb allele inactivated or lost in tumors
Sporadic Retinoblastoma - Two required rate-limiting lesions
Both alterations acquired somatically
Incidence: 1 in 10^5 (random probability)
Very rare, involves only one eye
Heritable Retinoblastoma - Two required rate-limiting lesions
One alteration inherited in the germline (eg “Rb susceptibility gene”)
Second alteration acquired somatically.
Incidence: 10 tumors per person (lifetime). With 10^7 cell divisions, there are just too many opportunities for mutation!
Fully penetrant (transmitted via mendelian dominance)
Tumor suppressor gene (thus this is an exception to mendelian trends for tumor suppressor genes
Affects both eyes
Rb gene - Two rate-limiting genetic alterations
Cytogenetic abnormalities of Chromosome 13
A second hit on the other allele
Cytogenetic abnormalities of Chromosome 13
Interstitial deletions (variable length) ALL involve material from 13q14
Sporadic patients - Deletions in tumor cells only
Heritable patients - Deletions in both normal & tumor cells (that means this is the inherited first hit!)
Both alleles of a gene on 13q14 are knocked out
Retinoblastoma!
How do we inactivate the second Rb allele?
De novo mutation
Chromosome loss
Chromosome loss & replication
Gene conversion
Some carriers of hereditary retinoblastoma will also develop
Osteosarcoma (low/incomplete penetrance)
When is Rb normally hypophosphorylated?
G0 (resting cells) Early G1 (cycling cells)
When is Rb normally hyperphosphorylated?
S phase
G2
When does Rb get phosphorylated?
Before the G1/S transition
At the restriction point of the cell cycle
What phosphorylates Rb?
Enzymatic complex
CDK4 / Cyclin D
The Restriction Point
Late G1
Major control point of cell cycle progression
Mediated by E2F family of transcription factors
E2F binds the promoters of genes required for the progression of the cell cycle
S phase genes regulated by E2F
Thymidine Kinase Dihydrofolate Reductase (DHFR) DNA Polymerase α ORC1 Histone H2A Cyclin E Cyclin A
Thymidine Kinase - Function
Nucleotide Synthesis
Dihydrofolate Reductase (DHFR) - Function
Nucleotide Synthesis
DNA Polymerase α - Function
DNA Synthesis
ORC1 - Function
DNA Synthesis
Histone H2A - Function
Chromosome Assembly
Cyclin E - Function
Cell Cycle Progression
Cyclin A - Function
Cell Cycle Progression
Hypophosphorylated Rb
Restrains cell proliferation
How: Binds to promoter-bound E2F in early G1 Inactivates E2F-controlled transcription S phase genes are repressed G1/S transition is blocked
CDK4/Cyclin D
Phosphorylates Rb in its “pocket” causing it to dissociate from E2F
E2F-controlled transcription remains active
S phase begins
This process is a common focal point of major signal transduction pathways controlling normal cell growth
E2F controls
Transcription of proteins needed for S phase
Hyperphosphorylated Rb
Allows cells to proliferate
How:
CDK4/Cyclin D phosphorylates Rb in its “pocket” causing it to dissociate from E2F
E2F-controlled transcription remains active
S phase begins
Loss of Rb Function
Uncontrolled growth
How:
Deregulation of E2F (and G1/S transition)
Mutations leading to Inactive Rb Function - Direct
Rb gene deletion (retinoblastoma)
Point mutations in the Rb pocket (retinoblastoma)
Occupancy of the Rb pocket by early proteins of DNA tumor viruses (HPV)
HPV
Encodes 2 proteins required for tumorigenesis
E7 (one of those 2) binds the pocket of hypophosphorylated Rb
E2F is deregulated
G1/S transition is deregulated
p16
Inhibits CDK4/Cyclin
Tumor Suppressor
Rb
Oncoproteins
E2F
CDK4
Cyclin D
p16
Mutations leading to Inactive Rb Function - Indirect
Overexpression of Cyclin D1 (Breast cancer, B Cell Lymphoma)
Loss of p16, a CDK4 inhibitor (Many human cancers)
Inherited point mutation in CDK4, rendering it insensitive to p16’s inhibition (Familial melanoma)
Inactivation of Rb Function
Occurs in most, if not all human tumors
p53 encodes
Transcription factor
The most broadly-altered gene in human cancer
How is the p53 gene usually altered in human tumors?
Missense mutations
Example of a dominant-positive mutation
Ras proto-oncogenes
Example of a recessive-negative mutation
Rb loss
Example of a dominant-negative mutation
p53 missense mutation
Only one allele needs to mutate for a malignant phenotype
The mutation leads to loss-of-function of that tumor suppressor
Dominant-Negative Mutations
One allele mutates
The protein products of BOTH alleles are functionally inactivated.
How does p53 normally function in the cell?
Homo-tetramer which serves as a transcription factor
How does a p53 mutation become dominant?
Mutant p53 is much more stable than wild-type p53
With one mutant allele, levels of mutant p53 much higher than that of wild-type
Li-Fraumeni Syndrome (LFS)
Rare hereditary condition
Germline mutations of p53
Carriers develop many forms of cancer
p53 sporadic cancers
Often have somatic mutations of p53 (dominant-negative)
Very common in human cancer
Found in many different forms of cancer
What is the half life of normal p53 polypeptides?
~30 minutes
What genotoxic stresses can damage the p53 gene?
UV light
Ionizing radiation
Chemical carcinogens
Errors in replication
What does a damaged p53 gene lead to?
Post-translational modifications of p53 polypeptides
ESPECIALLY Phosphorylation & Acetylation
This leads to a half life of ~150 min (stabilized peptide!)
Higher steady state levels
Increased transcriptional activity of p53
Increased transcriptional activity of p53 - Normal Fibroblasts
G1 arrest
DNA repair
Increased transcriptional activity of p53 - Certain epithelial cells
G1 arrest
DNA repair
Increased transcriptional activity of p53 - Thymocytes
Apoptosis
p53’s ultimate job
Ceasing the replication of damaged DNA
Preventing oncogenic mutation accumulation
Maintaining genetic integrity in cells under genotoxic stress
Transcriptional targets of p53
p21 CDK Inhibitor 14-3-3σ PUMA p53R2 Nuclear Ribonucleotide Reductase p48 Subunit of the XPA Complex and more!!
p21 CDK Inhibitor
Activated p53 binds its promoter
Leads to G1 & G2 arrest (fibroblasts)
14-3-3σ
Induced by p53
Leads to G2 arrest (epithelial cells)
PUMA
Induced by p53 Promotes apoptosis (thymocytes, fibroblasts, neurons)
p53R2 Nuclear Ribonucleotide Reductase
Induced by p53
Required for DNA repair
p48 Subunit of the XPA Complex
Induced by p53
Required for nucleotide excision repair
ATM
One of the tumor suppressor kinases that detects DNA damage and activates p53 via phosphorylation
Mdm2
One of the oncoproteins that ubiquitinates & targets p53 for destruction
