Pathogenesis of Sporadic Nonfamilial Breast Cancer

Slide 1 · Pathogenesis of Sporadic(Nonfamilial) Breast Cancer

Molecular pathways • precursor lesions • tumor phenotype

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Contrast low- and high-grade evolutionary pathways

Explain estrogen-related mechanisms in luminal cancer

Relate HER2 and triple-negative molecular drivers to phenotype

Integrate invasion, histology, biomarkers, and clinical implications

Slide 3 · Breast carcinoma develops by stepwise clonal evolution

Breast epithelium

Susceptible epithelial cells

Driver alterations

Somatic selection of altered clones

Preinvasive disease

Clonal expansion and additional events

Invasive carcinoma

Emergence of malignant behavior

The sequence is a model; Not every lesion progresses.

Slide 4 · The initiating event helps determine the final phenotype

Early influences

Identity/state of the susceptible cell

Nature of initial genomic alteration

Subsequent selection under local signals

Later influences

Accumulation of further genetic alterations

Selection of subclones with growth advantage

Interactions with the tumor microenvironment

A tumor subtype reflects both evolutionary history and current phenotype.

Slide 5 · Histologic grade separates two broad evolutionary routes

Low-grade pathway

Recognizable precursor lesions

Molecular divergence

Distinct genomic alterations

High-grade pathway

Earliest precursors are less defined

These routes are useful models, not biologically sealed compartments.

Slide 6 · Three clinically recognizable molecular phenotypes

Luminal

ER-positive / HER2-negative

Usually includes luminal A and luminal B gene-expression patterns

HER2-positive

HER2 overexpression / gene amplification

ER status can be positive or negative

Triple-negative

ER-negative, PR-negative, HER2-negative

Biologically heterogeneous

Receptor/IHC groups overlap with, but are not identical to, expression-defined classes.

Slide 7 · Receptor assays and molecular profiling answer different questions

Routine tumor markers

ER and PR expression: Hormone receptor phenotype

HER2 protein and/or gene status

Help select biomarker-directed management

Expression profiling

Multigene mRNA patterns

Luminal A, luminal B, HER2-enriched, basal-like

Groupings do not perfectly match receptor results

Do not equate an immunohistochemical subtype with an exact genomic-expression class.

Slide 8 · Low-grade progression has recognizable precursor lesions

Columnar lesions

Often associated with flat epithelial atypia

Atypical hyperplasia

Noninvasive clonal proliferation

Low-grade in situ

DCIS and related lesions

Invasive tumor

Often ER-positive / HER2-negative

Progression is possible, not inevitable.

Slide 9 · Flat epithelial atypia belongs to the low-grade pathway

Columnar cell lesions may show epithelial atypia, termed flat epithelial atypia.

The low-grade pathway often includes flat epithelial atypia before advanced lesions.

This lesion is best understood as part of a proposed stepwise sequence—not a guaranteed precursor.

Interpretive limit

Histologic atypia alone does not establish invasive cancer.

Slide 10 · Atypical hyperplasia is an intermediate clonal lesion

Atypical ductal and lobular hyperplasia participate in the low-grade neoplasia pathway.

These lesions share parts of the genomic pattern found in low-grade invasive cancers.

Only a minority of recognized precursor lesions evolve into malignancy.

Progression risk

Multiple additional events are usually required for invasive cancer.

Slide 11 · Low-grade DCIS precedes some ER-positive invasive cancers

Low-grade precursor

Atypia and hyperplasia

Low-grade DCIS

Neoplastic cells remain in situ

Additional changes

Tumor cells and local tissue interact

Invasive carcinoma

Stromal barrier is breached

There is no single known universal genetic switch that produces invasion.

Slide 12 · Chromosome changes characterize low-grade neoplasia

Biologic event

Chromosome 1q and 16q

Copy-number pattern

1q gain; 16q loss

Luminal lineage

Common low-grade molecular pattern

Low-grade tumors often carry fewer large-scale chromosomal rearrangements than high-grade tumors.

Slide 13 · PIK3CA is a major luminal pathway alteration

Biologic event

PIK3CA activation

PI3K signaling

Growth-factor receptor downstream pathway

Clonal selection

Supports growth signaling

PIK3CA encodes a central signaling enzyme; A mutation is not the whole causal story.

Slide 14 · Genomic organization differs by tumor grade

Lower-grade tumors

Relatively “quiet” genomes

1q gain and 16q loss are characteristic

Often strongly ER-positive

Higher-grade tumors

Numerous chromosomal aberrations

Greater structural genomic complexity

High proliferation and more frequent TP53 alterations

These patterns are probabilistic associations, not immutable diagnostic rules.

Slide 15 · High-grade evolution has less certain early steps

Early lesion

Often not identified

High-grade DCIS

Established preinvasive lesion

Genomic complexity

Instability and multiple driver events

Invasion

High-grade carcinoma

Unknown precursor ≠ absence of a precursor.

Slide 16 · Why high-grade precursor lesions are difficult to identify

The earliest lesion in the high-grade pathway remains poorly characterized.

The source proposes that some precursor stages may progress before detection.

High-grade DCIS is a recognized immediate noninvasive precursor.

Evidence boundary

The timing and identity of earliest high-grade precursor lesions remain uncertain.

Slide 17 · High-grade DCIS links high-grade in situ and invasive disease

High-grade DCIS shares many genomic abnormalities with associated invasive carcinoma.

TP53 changes, increased genomic instability, and HER2 amplification occur in relevant high-grade branches.

Molecular similarity does not identify a universal driver of basement membrane invasion.

Ductal confinement

DCIS denotes carcinoma confined within ductal structures.

Slide 18 · Three genomic themes enrich the high-grade route

Genomic instability

Complex copy-number profiles

Chromosomal rearrangements

TP53 alteration

Frequent in several high-grade phenotypes

Loss of genomic surveillance

Selected growth drivers

HER2 amplification in HER2-positive disease

DNA repair pathway impairment in subsets

These processes frequently coexist in individual tumors.

Slide 19 · TP53 alteration favors genomic instability

Biologic event

TP53 dysfunction

Impaired genome control

Mutations accumulate with selection

High-grade phenotype

Aggressive histologic features

TP53 mutation is common in HER2-positive and high-grade triple-negative carcinomas.

Slide 20 · Homologous recombination defects permit chromosomal damage

Biologic event

HR repair impairment

Defective DSB repair

Errors in high-fidelity repair

Unstable tumor genome

Copy-number changes and rearrangements

Sporadic tumors can develop repair deficiencies without an inherited BRCA mutation.

Slide 21 · The high-grade pathway can produce distinct phenotypes

Shared instability

Structural damage and mutations

Branching alterations

Different selected driver programs

Resulting tumors

Luminal B / HER2 / triple-negative

There is no single molecular program shared by all high-grade cancers.

Slide 22 · Low-grade tumors may progress toward high grade

Typical pattern

Low-grade and high-grade pathways differ in molecular profile

Each has characteristic histologic features

Important exception

Low-grade lesions may acquire additional genomic alterations

Some can progress to a more aggressive high-grade phenotype

Separate pathways are a simplification, not a barrier to evolutionary transition.

Slide 23 · In situ–invasive transition is not explained by one new driver

What is observed

Many genomic abnormalities of invasive cancer occur in its DCIS precursor

Major driver mutations can precede invasion

What remains uncertain

No universal invasion-specific genomic alteration has been found

Local tissue and microenvironmental events may be important

Do not present stromal invasion as the consequence of one named mutation.

Slide 24 · A stable ductal boundary resists stromal invasion

Schematic breast duct cross-section (not histology)

Luminal epithelial cells

Intact myoepithelial cell layer

Continuous basement membrane

An intact myoepithelial–basement membrane unit constrains stromal invasion.

Slide 25 · Myoepithelial loss and membrane gaps may facilitate invasion

Schematic breast duct cross-section (not histology)

Luminal epithelial cells

Myoepithelial depletion

Membrane thinning / gaps

Disrupted ductal support structures may help permit stromal entry.

Slide 26 · Cancer-associated fibroblasts reshape the tumor environment

Stromal fibroblasts interact with adjacent neoplastic epithelium.

Cancer-associated fibroblasts can facilitate adhesion and extracellular matrix remodeling.

The stromal response may also contribute to local immunosuppression.

Stromal contribution

Invasion is a tissue-level process, not just a tumor-cell genotype.

Slide 27 · PD-L1 is an example of tumor immune evasion

Biologic event

Tumor PD-L1

Immune-checkpoint ligand

Suppression of antitumor response

Survival advantage

Persistence of selected clones

Checkpoint ligand expression can weaken antitumor immunity.

Slide 28 · Estrogen exposure is central to luminal cancer risk

The dominant ER-positive pathway produces many luminal breast carcinomas.

Estrogen acts through multiple biologic mechanisms rather than a single mutation.

Cumulative exposure and cell proliferation help explain age-related risk.

Exposure and growth

Luminal cancer as the largest major subgroup.

Slide 29 · Estrogen increases locally acting growth factors

Biologic event

Estrogenic stimulation

Local growth factors

TGF-α, PDGF, and FGF

Epithelial response

Enhanced proliferative signaling

Growth-factor induction is one mechanism linking estrogen to luminal growth.

Slide 30 · Estrogen regulates epithelial transcriptional programs

Biologic event

Estrogen–ER action

Gene expression

Many regulated epithelial transcripts

Growth propensity

Changes supporting growth and development

Luminal tumors preserve a recognizably estrogen-regulated transcriptomic program.

Slide 31 · Estrogen also expands the at-risk epithelial population

Puberty and cycles

Physiologic epithelial proliferation

More divisions

Repeated DNA replication

Mutation opportunity

Replication-associated errors

Clonal selection

Survival of altered cells

Proliferation increases opportunities for mutation; It is not itself malignancy.

Slide 32 · Repeated menstrual cycling may promote mutation fixation

Cell division

DNA synthesis during proliferation

Replication error

Occasional genetic lesion

Cell-cycle changes

Repair may be imperfect

Fixed alteration

Descendants inherit the mutation

Proposed mechanism: Imperfect repair can leave permanent DNA alterations.

Slide 33 · Cumulative estrogenic exposure links age and risk

Exposure-associated factors

A greater lifetime number of menstrual cycles

Proliferative stimulation across reproductive life

Postmenopausal hormone therapy in some contexts

Mechanistic interpretation

More cells experience DNA replication

Somatic alterations can accumulate over time

ER-driven cell growth can favor selected clones

Association does not mean estrogen exposure alone causes every luminal carcinoma.

Slide 34 · Precursor clones require additional evolution

ER-positive clone

Acquired somatic alteration

Estrogenic milieu

Supports growth of some clones

Further events

Genomic and tissue changes

Malignancy

A minority of precursors progress

Most low-grade precursor lesions do not become invasive cancers.

Slide 35 · Endocrine epidemiology supports a causal estrogen role

Risk reduction evidence

Estrogen antagonists lower development of luminal cancers in high-risk females

Therapeutic benefit implicates ER signaling

Risk increase evidence

Luminal cancer incidence is higher in women receiving postmenopausal hormone therapy

Pattern is consistent with hormonal promotion

These are convergent lines of evidence, not molecular proof of a single pathway.

Slide 36 · ER is a ligand-regulated transcription factor

Biologic event

Estrogen receptor

Gene regulation

Estrogen-responsive transcription

Luminal phenotype

ER-driven growth-related program

ER immunoreactivity indicates a potential endocrine-treatment target.

Slide 37 · Luminal A has a strong estrogen-regulated gene signature

Expression profiling shows high levels of estrogen-responsive transcripts.

The estrogen receptor gene itself is part of this luminal program.

Proliferation-associated gene expression is generally lower than in luminal B.

Expression-defined group

“Luminal A” is a gene-expression category with clinical-pathologic correlates.

Slide 38 · Luminal A often carries PIK3CA mutation and 1q/16q change

Biologic event

Luminal A genomic pattern

Recurrent events

PIK3CA mutation; Gain 1q; Loss 16q

Cellular phenotype

Strong ER program, lower proliferation

Somatic PI3K-pathway activation complements estrogen-dependent growth.

Slide 39 · Luminal A morphology tracks with differentiation

Luminal A cancers tend to be well differentiated.

Strong ER and progesterone receptor expression is typical.

They commonly show low cellular proliferation.

These are associations; Grade and Ki-67 can vary within luminal tumors.

Histologic correlate

Relatively organized histology reflects the less proliferative molecular phenotype.

Slide 40 · Luminal B expresses a stronger proliferation program

Luminal B is defined by higher expression of proliferation-related genes.

The luminal/estrogen-related expression program is usually less prominent than in luminal A.

ER and PR protein levels tend to be lower, although ER expression remains relevant.

Classification caution

Expression-defined luminal B is not synonymous with HER2 positivity.

Slide 41 · Luminal B may evolve with genomic destabilization

Luminal precursor

ER-positive clonal lesion

Additional changes

E.g., TP53 alteration in a subset

Genomic complexity

Greater chromosomal burden

Luminal B tumor

More proliferative and higher grade

Luminal A-to-B progression is possible but not required.

Slide 42 · Luminal A and luminal B: Major discriminators

Feature

Luminal A pattern

Luminal B pattern

ER-related genes

Prominent expression

Lower relative expression

Proliferation

Usually lower

Usually higher

Differentiation

Typically well-differentiated

Often less differentiated

Genomic burden

Relatively quiet

More chromosomal abnormalities

These are broad transcriptomic–pathologic correlations, not standalone assay cutoffs.

Slide 43 · Several special histologies commonly map to luminal disease

Well-differentiated types

Tubular carcinoma

Cribriform carcinoma

Mucinous and papillary

Mucinous carcinoma

Some papillary carcinomas

Lobular pattern

Most lobular carcinomas are luminal

A minority may show more aggressive biology

Morphologic subtype is informative but cannot replace ER, PR and HER2 testing.

Slide 44 · HER2-positive carcinomas are centered on ERBB2 amplification

Chromosome 17q

ERBB2 locus

Gene amplification

Increased HER2 copy number

Protein overexpression

Abundant HER2 receptor

Growth signaling

Cell proliferation and survival

ERBB2 is the gene encoding the HER2 receptor tyrosine kinase.

Slide 45 · HER2 amplification is the usual route to HER2 protein excess

Biologic event

ERBB2 amplification

HER2 protein

Increased receptor abundance

Tumor behavior

Strong mitogenic signaling

Gene amplification is the predominant mechanism of HER2 overexpression.

Slide 46 · HER2 promotes growth through RAS and PI3K–AKT

Two signaling branches downstream of amplified HER2

HER2

Receptor tyrosine kinase

RAS pathway

Proliferative signaling

PI3K–AKT pathway

Growth and antiapoptotic signaling

Both pathways contribute to the HER2-driven phenotype.

Parallel outputs from HER2 signaling support proliferation and survival.

Slide 47 · HER2 tumors combine growth signaling with genomic complexity

Amplification

High-level ERBB2 copy-number gain

Strong HER2 overexpression

Genome

Complex rearrangements

High burden of mutations

Coalterations

TP53 frequently involved

PIK3CA also altered

A high-grade genomic pattern frequently coexists with the principal HER2 driver.

Slide 48 · HER2-positive tumors are frequently poorly differentiated

HER2-positive breast carcinomas are typically high grade.

A solid sheet-like growth pattern is described.

Marked pleomorphism and brisk proliferation are common.

Apocrine histology is a notable HER2-associated subtype.

Testing implication

Histologic suspicion requires biomarker confirmation.

Slide 49 · HER2 protein and HER2 gene tests interrogate different levels

Immunohistochemistry

Measures HER2 protein overexpression

Provides localization in tumor cells

A routine component of phenotyping

In situ hybridization

Assesses ERBB2/HER2 gene amplification

Assesses DNA-level copy-number state

Complements protein-based assessment

Protein and gene-based assays measure complementary aspects of HER2 biology.

Slide 50 · HER2-positive cancers are not uniformly ER-negative

ER-negative / HER2-positive

Often classified near HER2-enriched expression pattern

HER2-driven signaling predominates

ER-positive / HER2-positive

Can overlap with a luminal B expression pattern

Hormone receptor coexpression has biologic implications

HER2 status alone does not fully determine expression-based subtype.

Slide 51 · Triple-negative cancer is defined by absent routine markers

Triple-negative breast cancers lack ER, PR, and HER2 expression by clinical classification.

They do not follow the estrogen-dependent luminal pathway.

HER2 gene amplification is not the defining mechanism.

Heterogeneous category

Triple-negative is a receptor-defined umbrella, not one molecular disease.

Slide 52 · Triple-negative tumors are biologically heterogeneous

Most TNBC

High grade and rapidly proliferative

Frequent genome instability

Often basal-like at the RNA level

Uncommon TNBC variants

Some rare histologic types are low grade

Some behave much more favorably

Not all use the same driver mechanisms

Avoid attributing uniform aggressiveness to every triple-negative histology.

Slide 53 · A basal-like expression program overlaps substantially with TNBC

Biologic event

Basal-like mRNA

Myoepithelial-associated genes

Expression of basal keratins

Routine phenotype

Frequently triple-negative

“Basal-like” and “triple-negative” are related but nonidentical categories.

Slide 54 · Defective homologous recombination creates a mutator phenotype

Double-strand damage

DNA lesions threaten genome integrity

Faulty HR repair

Deficient high-fidelity repair

Chromosomal changes

Copy-number variation / rearrangement

Tumor heterogeneity

Clones accumulate abnormalities

High-grade TNBC often shows this pattern.

Slide 55 · Sporadic TNBC can inactivate BRCA1 without inherited mutation

Familial context

Germline BRCA1 / BRCA2 variants can cause HR deficiency

The inherited setting is taught separately

Sporadic context

BRCA1 coding mutations are uncommon in sporadic TNBC

Epigenetic silencing with allelic loss can impair BRCA1 function

Distinguish germline susceptibility from tumor-acquired repair deficiency.

Slide 56 · TP53 is the most consistently mutated TNBC cancer gene

TP53 is the only cancer gene described as mutated in a majority of TNBCs.

Additional oncogenic effects may result from copy-number alterations and rearrangements.

Mutational diversity contributes to a biologically heterogeneous group.

Mutation pattern

Do not assert that a particular single mutation is present in every TNBC.

Slide 57 · Structural genome disruption is a hallmark of high-grade TNBC

Biologic event

Defective genome maintenance

Chromosomal rearrangements

Complex structural alterations

Aggressive phenotype

High proliferation / high grade

Copy-number alteration can contribute alongside point mutations.

Slide 58 · Genomic instability can increase immune visibility

Unstable genome

Altered tumor proteins

Neoantigens

Potential immune recognition

Immune infiltrates

Often prominent in TNBC

Immunotherapy

Some TNBCs are more responsive

Greater immunogenicity does not mean universal immunotherapy benefit.

Slide 59 · High-grade TNBC has characteristic—not unique—morphology

Many tumors grow in solid sheets.

Marked nuclear pleomorphism is common.

Necrosis and very high proliferation are frequently seen.

Some tumors show medullary-pattern features.

Diagnostic overlap

These features overlap with other high-grade carcinomas; Markers are needed.

Slide 60 · A small subset of triple-negative tumors is relatively indolent

Special histologies

Adenoid cystic carcinoma

Secretory carcinoma

Other distinct forms

Low-grade adenosquamous carcinoma

Some salivary gland-like tumors

Interpretation

Clinical behavior differs from usual high-grade TNBC

Specific diagnosis matters

High-grade TNBC generalizations should not be applied to these rare entities.

Slide 61 · Major phenotype comparison: Molecular drivers

Feature

Luminal ER+/HER2−

HER2-positive

High-grade TNBC

Receptors

ER+ / HER2−

HER2+; ER variable

ER− / HER2−

Growth pattern

ER program; PIK3CA

ERBB2 gain; PI3K / RAS

Genomic instability / HR defect

Genome

Quieter (A), complex (B)

Amplified ERBB2; complex

Large-scale instability

Typical grade

Lower (A), higher (B)

Usually high

Usually high

Phenotype groups differ in driver programs and genome architecture.

Slide 62 · Predictive and prognostic information must be separated

Predictive relevance

ER expression: Endocrine vulnerability

HER2 protein/gene status: HER2-directed therapy eligibility

TNBC immune biology: Potential immunotherapy relevance

Prognostic association

Grade and proliferation correlate with aggressiveness

Luminal A generally less proliferative than luminal B

Triple-negative category includes uncommon favorable subtypes

Prediction of treatment response is different from natural-history prognosis.

Slide 63 · Grade, phenotype and genomic classification are distinct axes

Pathologist-observed features

Differentiation and nuclear features

Mitoses, solid growth and necrosis

In situ versus invasive anatomy

Molecular readouts

ER and PR protein expression

HER2 protein and ERBB2 copy number

Gene-expression profiles and other alterations

A rigorous diagnosis integrates morphology with appropriately validated biomarkers.

Slide 64 · Teaching case: Indolent luminal pattern

Hypothetical teaching case

An invasive carcinoma is well differentiated, strongly ER/PR positive, HER2 negative and shows low proliferation.

Which proposed pathway and molecular subtype best fit?

Formulate a mechanistic explanation using the supplied findings.

Slide 65 · Teaching case: Signaling amplification

Hypothetical teaching case

A poorly differentiated tumor shows marked HER2 protein overexpression with ERBB2 amplification; ER expression is low.

Which genomic event best explains the receptor phenotype?

Formulate a mechanistic explanation using the supplied findings.

Slide 66 · Teaching case: High-grade repair-deficient pattern

Hypothetical teaching case

A high-grade carcinoma lacks ER, PR and HER2, with extensive chromosomal complexity and conspicuous immune infiltrates.

Which mechanistic framework is most compatible?

Formulate a mechanistic explanation using the supplied findings.

Slide 67 · Important uncertainties in sporadic breast cancer evolution

Supported conclusions

Distinct low- versus high-grade pathways are useful models

Certain driver and expression patterns cluster by subtype

Microenvironmental remodeling can contribute to progression

Open questions

Earliest high-grade precursor lesions are not well defined

A universal invasion-specific driver is not established

Not every lesion or tumor follows the canonical pathway

Mechanistic diagrams should distinguish inferred sequences from observed alterations.

Slide 68 · Synthesis: three ways evolution shapes tumor phenotype

Selective signals

Estrogenic transcription and proliferation

ER-positive luminal programs

Driver signaling

PIK3CA and ERBB2 pathway activation

Genomic alterations affecting growth

Genome maintenance

TP53 and DNA repair defects

Instability and tumor heterogeneity

Breast cancer subtype emerges from interacting evolutionary processes.