Breast Cancer Genetics - Scott Flashcards
What are the normal components of the Luminal layer of the breast?
Contains ductal and alveolar cells
Contains ER-alpha positive and negative cells
Contains progenitor cells
What are the normal components of the Myoepithelial/basal layer of the breast?
Contains primarily contractile cells
Also contains stem cells
All are ER-alpha negative
The normal breast development requires repeated
rounds of proliferation that are dependent on what type of regulation?
Regulated by: Hormonal-dependent proliferation
How does adult breast tissue maintain the capacity for proliferation?
Adult tissue contains stem and progenitor cells which provide capacity for growth throughout the lifespan
Cells maintain signaling pathways leading to proliferation
What are the two key signaling pathways for proliferation in breast tissue?
1) Estrogen/estrogen receptor α
2) EGF/EGFR family
What are the two potential Initiating genetic event in breast cancer?
Germline mutation Somatic mutation (any cell in the body, in every descendent of that cell)
What events promote the development of tumor in the breast?
series of somatic mutations
What host factors contribute to tumor development?
diet + hormones + immune response
How do germline genetic changes contribute to breast cancer initiation?
Familial aggregation studies suggest that about 5-10% of breast cancers are due to inherited breast cancer mutations-
5-10% of women with breast cancer have first degree relative who also has breast cancer.
10-20% have second degree relative.
Mutations in BRCA1/2 genes are the most prevalent breast cancer genetic susceptibility factors:
BRCA1 mutations implicated in 2% of all breast cancers, ~20% of all familial breast cancer
Inheritance of susceptibility is autosomal dominant:
Inheritance of a single copy confers susceptibility
Mutations in BRCA1/2 confer significant risk for what types of cancers?
BRCA1: breast cancer ~65%, ovarian cancer ~40%
BRCA2: breast cancer ~40%, ovarian cancer ~11%
Why does inactivation of BRCA1 or BRCA2 result in
breast cancer susceptibility?
- BRCA1 and BRCA2 are essential components of homologous recombination arm of DNA repair machinery:
- Homologous recombination repairs double stranded DNA breaks
- Failure to repair double stranded DNA breaks results in genomic instability
- Genomic instability creates conditions for oncogenic somatic mutations to arise - BRCA1 is required for differentiation, probably at the luminal progenitor stage
What are the commercial genetic tests for for BRCA 1 and BRCA2 are available?
Standard test = full gene sequence for point mutations, some small insertions/deletions
in BRCA1/2
If no significant mutation found in high risk individual then also do test for rearrangements
- BRCA1 and BRCA2 are essential components of homologous recombination arm of DNA repair machinery
Recognition of break => BRCA1 is recruited as part of
recognition complex
Prepare ends for repair => BRCA1 creates single stranded ends, BRCA1 recruits repair machinery
New DNA synthesis => Single stranded ends invade
Intact DNA, start new synthesis
BRCA2 recruits Rad51 which helps
Promote strand invasion
- BRCA1 is required for differentiation, probably at the luminal progenitor stage
In the absence of BRCA1 luminal
progenitor cells accumulate
BRCA1 mutation tumors often resemble
“basal-like” sporadic tumors
Often triple negative- low ER, low PR,
low ERBB2
As a result, there are limited treatment
options for BRCA1 mutant tumors
Sporadic breast cancers
~90% of breast cancers are sporadic-
No known germline mutation
Oncogenesis is initiated by somatic genetic and
epigenetic changes
What are the implications of somatic genetic changes in sporadic cancers?
Occur in somatic cells, not germline
Inherited from one cell division to next during lifetime of individual
Not passed down from parent to child
How does Sporadic breast Cancer develop through microevolution?
Random somatic mutations that result in dysregulated
proliferation and inappropriate survival
are selected for.
Somatic genetic changes are the in initiating event in most breast cancers and the driving force in the development of both familial and sporadic cancers
What are the two most prevalent types of Sporadic breast cancer?
- Estrogen receptor alpha (ER-alpha) positive (~65% of breast cancers are ER alpha positive at time of diagnosis)
- HER2 overexpressing, usually due to genomic amplification (~25%)
How is E2/ER signaling is implicated in breast cancer?
In normal life history breast tissue undergoes repeated rounds of proliferation driven by estrogen/ER signaling
Many factors that increase risk of breast
cancer increase lifetime exposure to estrogen ( >30 yr old at first pregnancy, late menopause, early menarche, no breastfeeding)
Treatments to block ER signaling are highly effective
In treating ER+ breast cancer (selective estrogen receptor modulators (SERM) aromatase inhibiters)
What is the role of ER in normal breast tissue?
Extracellular estrogen controls proliferation of luminal cells indirectly:
ER is expressed in minority of differentiated
luminal epithelial cells- 10-20%
estrogen binds ERα => ERα promotes transcription of secreted growth factor, such as amphiregulin => Growth factor acts on nearby cells to promote proliferation => Cells expressing ER do not proliferate, neighboring cells do
What is the role of ER in ER positive breast cancer?
Tumors contain a higher percentage of ER positive cells than normal tissue
In tumor cells estrogen regulates growth directly-
Estrogen promotes transcription of cell cycle regulators such as Cyclin D1 => Cells expressing ER undergo proliferation
How are tumor cells different than normal cells?
Increase in the number of cells expressing ER
Change in which genes are regulated by
ER binding
How are tumor cells NOT different than normal cells?
No mutations or amplifications of genes encoding estrogen or ER
How does ER promote transcription of a different set of genes in breast cancer?
Overexpression of the pioneer transcription factor FOXA1:
ER can not bind to closed chromatin => FOXA1 binds to closed chromatin at select sites and opens it, allows access to ER binding
One of the sites opened by FOXA1 is the CYCLIN D1 promoter => CYCLIN D1 is a key regulator of the G1/S transition in the cell cycle
Increased transcription of CYCLIN D1 => dysregulation of proliferation
First SERM used to block E2/ER signaling in breast cancer (ER antagonist in breast tissue, Synthetic estrogen like molecule, Acts like estrogen- Binds ER, complex dimerizes and binds DNA at estrogen responsive promoter sites)
Tamoxifen
Instead of promoting transcription, tamoxifen/ER complex inhibits promoter sites
How does tamoxifen inhibit E2/ER signaling?
Tamoxifen binding to ER favors interaction with co-repressors, not co-activators
When tamoxifen displaces estrogen, ER-dependent transcription is blocked and ER dependent proliferation is inhibited.
Drug that Blocks synthesis of estrogen in non-ovarian tissues, Removes estrogen from the equation, Does not affect estrogen receptor
Aromatase Inhibitors
ERBB2 (HER2) overexpressing breast cancers:
ERBB2 is a member of the EGF receptor family: Members of this family are receptor tyrosine kinases => When activated they initiate signaling pathways that promote proliferation and survival
ERBB2 overexpressing cancers generally are ER negative
and more aggressive than ER positive cancers
Overexpression occurs by genomic amplification and is a somatic event.
ERBB2 is overexpressed in ~25% of breast cancers.
Why is overexpression of ERBB2 oncogenic?
- ERBB2 does not require ligand binding for activation: Dimerization does not require ligand binding => Increased ERBB2 expression, due to amplification allows dimerization to occur by mass action => The cell cycle can proceed independently of external growth factors.
2.ERBB2 can heterodimerize more efficiently
than other family members: Amplification of ERBB2 => increased signaling from ERBB2-EGFR and ERBB2-ERBB3 receptor heterodimers
Monoclonal antibody that binds extracellular domain of ERBB2 and blocks activity of ERBB2 homodimers?
Trastuzumab
Small molecule inhibitor that binds and blocks kinase active site and blocks EGFR-ERBB2 heterodimer activity?
Lapatinib
How do genetic breast cancer subtypes differ in characteristics that are relevant to treatment? Differ in prognosis?
Luminal A: high levels of estrogen receptor => best prognosis
HER2-amplified: overexpression of HER2, prognosis improved with widespread use of trastuzamab
Basal-like: often triple negative- no ER, PR, no amplification of HER2, often BRCA1 inactivated
Estrogen receptor signaling
Estrogen receptor is a steroid hormone receptor that acts as a transcription factor
Estrogen receptor becomes active when it binds its ligand, estrogen => Ligand binding promotes translocation to the nucleus and dimerization of the E2/ER complex
Ligand-bound ER dimers act as transcription factors for many genes that promote proliferation and survival.
