Breast Cancer (C50.919)

Over the past decade, mechanistic understanding of breast cancer has evolved from receptor-based classification toward a network-centric, systems-biology model integrating genomic instability, epigenetic regulation, tumor–immune crosstalk, and metabolic plasticity. Large-scale sequencing efforts such as The Cancer Genome Atlas have clarified subtype-defining drivers—including ER/PR signaling in luminal disease, HER2 amplification, and basal-like DNA repair defects—while exposing actionable nodes such as PI3K–AKT–mTOR, CDK4/6–RB, homologous recombination deficiency, and immune checkpoint pathways. Targeted therapeutics now modulate these nodes with increasing precision: HER2-directed antibody–drug conjugates like trastuzumab deruxtecan extend benefit even to HER2-low tumors; PI3Kα inhibitors such as alpelisib target PIK3CA-mutant disease; CDK4/6 inhibitors including abemaciclib reshape cell-cycle control in HR-positive cancers; and PARP inhibitors such as olaparib exploit synthetic lethality in BRCA1/2-mutated tumors. In triple-negative breast cancer, immune checkpoint blockade with agents like pembrolizumab reflects growing insight into tumor immunogenicity and stromal modulation. Emerging frontiers include ESR1 mutation–selective degraders, AKT inhibitors, antibody–drug conjugates targeting TROP2, and bispecific or cellular immunotherapies, all increasingly guided by minimal residual disease monitoring and multi-omic profiling. Collectively, the therapeutic landscape is shifting toward dynamic, biomarker-stratified intervention across evolving tumor states rather than static histologic categories.

Causes

Breast cancer etiology reflects a convergence of inherited susceptibility, somatic genomic instability, hormonal exposure, and environmental modifiers. Germline mutations in high-penetrance DNA repair genes—most notably BRCA1 and BRCA2—confer substantial lifetime risk by impairing homologous recombination and promoting accumulation of double-strand DNA breaks, while moderate-penetrance variants (e.g., PALB2, CHEK2, ATM) contribute additional risk through checkpoint dysregulation. Beyond genetics, cumulative estrogen and progesterone exposure—shaped by early menarche, late menopause, nulliparity, obesity, and exogenous hormone use—drives mitogenic signaling through ER-mediated transcriptional programs, increasing replication stress and mutation probability. Age-related epigenetic drift, including DNA methylation changes and chromatin remodeling, further alters tumor suppressor and oncogene expression. Environmental factors such as ionizing radiation and lifestyle-associated metabolic inflammation may augment risk by increasing oxidative stress and genomic damage. In aggregate, breast carcinogenesis emerges from iterative genomic and microenvironmental perturbations that progressively select for clones capable of evading apoptosis, bypassing senescence, and sustaining proliferative signaling.

Pathophysiology

Breast cancer pathophysiology is driven by dysregulated proliferative signaling, defective DNA repair, and dynamic tumor–microenvironment interactions that enable clonal expansion and metastatic competence. In hormone receptor–positive disease, aberrant activation of ESR1 sustains transcription of cyclins and growth-promoting genes, often reinforced by cross-talk with the PI3K–AKT–mTOR axis through activating mutations in PIK3CA. HER2-amplified tumors exhibit constitutive signaling via ERBB2, driving MAPK and PI3K pathway activation independent of ligand binding. In triple-negative breast cancer, genomic instability and homologous recombination deficiency—frequently involving BRCA1—promote reliance on alternative DNA repair pathways and adaptive stress signaling. Across subtypes, loss of cell-cycle control through RB pathway disruption, evasion of apoptosis via BCL-2 family modulation, metabolic reprogramming toward glycolysis and lipid synthesis, and immune microenvironment remodeling (including PD-1/PD-L1 axis engagement) collectively facilitate tumor progression. Metastatic dissemination involves epithelial–mesenchymal transition, extracellular matrix remodeling, and survival within distant niches such as bone, liver, lung, or brain, reflecting a temporo-spatial evolution of tumor cell states rather than a static disease phenotype.

Clinical features

Clinical features of breast cancer vary by stage and biologic subtype but most commonly present as a painless, palpable breast mass with irregular borders and firm consistency. Patients may notice asymmetry, localized skin dimpling, nipple retraction, or peau d’orange changes reflecting dermal lymphatic involvement. Nipple discharge—particularly if bloody or unilateral—can be an early sign. Inflammatory breast cancer presents more aggressively with rapid breast enlargement, erythema, warmth, and edema without a discrete mass, often mimicking mastitis. Regional spread may manifest as axillary, supraclavicular, or infraclavicular lymphadenopathy. Advanced disease produces systemic features depending on metastatic site: bone metastases cause pain or pathologic fractures; lung involvement may lead to dyspnea or cough; liver metastases can produce hepatomegaly or abnormal liver enzymes; and brain metastases may result in headaches, focal neurologic deficits, or seizures. Importantly, early-stage breast cancer is frequently asymptomatic and detected through screening mammography before clinical signs develop.

Diagnosis

Diagnosis of breast cancer integrates imaging, histopathology, and molecular characterization to define both stage and biologic subtype. Screening and diagnostic imaging—most commonly mammography supplemented by ultrasound or breast MRI in high-risk patients—identify suspicious lesions based on architectural distortion, calcifications, or mass characteristics. Definitive diagnosis requires image-guided core needle biopsy, allowing histologic classification (e.g., invasive ductal or lobular carcinoma) and immunohistochemical profiling for estrogen receptor (ER), progesterone receptor (PR), and HER2 expression. In situ hybridization may be used to confirm HER2 gene amplification when immunohistochemistry is equivocal. Proliferation indices such as Ki-67 and multigene expression assays (e.g., recurrence score panels) further refine prognostic and therapeutic decision-making in hormone receptor–positive disease. Staging is performed using the TNM system established by the American Joint Committee on Cancer, incorporating tumor size, nodal involvement, and distant metastases, often assessed with cross-sectional imaging in higher-stage cases. Clinical management algorithms are guided by evidence-based recommendations from organizations such as the National Comprehensive Cancer Network, ensuring integration of anatomic stage with tumor biology to inform precision therapy.

Mechanism of action videos

Biological pathways

p53 signaling pathway
Toll-like receptor signaling pathway
Targeted therapy in breast cancer
Targeted agents in triple negative breast cancer
TGFBR1 KD Mutants in Cancer
Notch signaling pathway
NF-kappa B signaling pathway
Loss of Function of TGFBR1 in Cancer
JAK-STAT signaling pathway
Integrated breast cancer pathway
Insulin signaling pathway
Signaling by ERBB2 KD Mutants
Homologous recombination
Glycolysis / Gluconeogenesis
Glucose metabolism in triple-negative breast cancer cells
FoxO signaling pathway
Focal adhesion
Fanconi anemia pathway
Estrogen signaling pathway
ErbB signaling pathway
Cyclin-dependent kinase 4/6 inhibitors in breast cancer
Chemokine signaling pathway
Cell cycle
Breast cancer pathway
Impaired BRCA2 binding to PALB2
Breast cancer
B cell receptor signaling pathway
Apoptosis
TP53 Regulates Transcription of DNA Repair Genes
Resolution of D-loop Structures through Synthesis-Dependent Strand Annealing (SDSA)
RNA Polymerase II Transcription
Generic Transcription Pathway
Diseases of signal transduction by growth factor receptors and second messengers
Diseases of DNA repair
Defective homologous recombination repair (HRR) due to PALB2 loss of function
Defective HDR through Homologous Recombination Repair (HRR) due to PALB2 loss of BRCA2/RAD51/RAD51C binding function
Transcriptional Regulation by TP53
DNA Double-Strand Break Repair
Gene expression (Transcription)
Resolution of D-loop Structures through Holliday Junction Intermediates

Clinical trials

A Modular Phase I/IIa, Open-label, Multi-centre Study to Assess the Safety, Tolerability, Pharmacokinetics, Pharmacodynamics and Preliminary Efficacy of Ascending Doses of AZD9574 as Monotherapy and in Combination With Anti-cancer Agents in Patients With Advanced Solid Malignancies (CERTIS1)
A Multicentre, Single Arm, Non-interventional, Observational, Prospective Study to Assess the BRCA1/2m Prevalence, Treatment Approaches and Outcomes in HER2-negative High-risk Early Breast Cancer Patients in Russia
A Phase 1 Trial of the ATR Inhibitor BAY 1895344 in Combination With Cisplatin and With Cisplatin Plus Gemcitabine in Advanced Solid Tumors With an Emphasis on Urothelial Carcinoma
Evaluating the Effectiveness of Supervised Practice or Distance Supervised Physical Activity (Walking) in the Overall Management of Fatigue in Breast Cancer Patients During Hormonal Therapy: Pilot Study
Standard Chemotherapy Plus Oral Everolimus as Adjuvant Therapy for LAR Breast Cancer Patients: A Randomized, Open-Label, Phase III Trial （POLARIS）
Study AKRA: Correlation of Immune-Related Tumor Stroma Factors With Complete Pathologic Remission in Early Breast Cancer After Neoadjuvant Chemotherapy and With Circulating Tumor Cells/Circulating Tumor DNA
A User-centered Mobile Health App to Promote Participation of Black Women in Breast Cancer Clinical Trials - Clinical Trial mHealth Study
The 'SWIVEL' Study (Switch Vs Effects Relief): Effectiveness of a Medication 'Switch' vs Guideline-Directed Interventions for Relieving Side Effects of Aromatase Inhibitors Among Breast Cancer Patients
Phase 2 Study of Low Dose Tamoxifen +/- High Dose Omega-3 Fatty Acids in Overweight Postmenopausal Women at Increased Risk for Breast Cancer
A Real-world Study on the Treatment of HR+/HER2- Advanced Breast Cancer Patients With CDK4/6 Inhibitors and Treatment Options After Drug Resistance.
Efficacy and Safety of Dexmedetomidine Added to Modified Pectoral's Block in Patient Undergoing Breast Cancer Surgery :A Comparative Study
Cardiotoxicity Assessment and Reduction Through Exercise in BREAST Cancer
Effect of Laser Acupuncture on Carpal Tunnel Syndrome Post Aromatase Inhibitors in Breast Cancer Patient .
Phase II, Prospective, Randomized, Proof-of-Concept Study to Evaluate the Effects of a Personalized Dietary Intervention in Women With Advanced Gynecologic or Breast Tumors Treated With Antibody-Drug Conjugates (ADCs).
[18F] Fluoroestradiol (FES) PET as a Predictive Measure for Endocrine Therapy in Patients With Newly Diagnosed Metastatic Breast Cancer
EPIK-B5: A Phase III, Randomized, Double-blind, Placebo-controlled Study of Alpelisib in Combination With Fulvestrant for Men and Postmenopausal Women With HR-positive, HER2-negative Advanced Breast Cancer With a PIK3CA Mutation, Who Progressed on or After Aromatase Inhibitor and a CDK4/6 Inhibitor
Understanding and Addressing Rejection of Personalized Breast Cancer Risk Information in Women
A Phase 1a/1b Study of NRM-823 as Monotherapy and in Combination With Immune Checkpoint Inhibition in Participants With Locally Advanced or Metastatic Refractory Solid Tumors
Phase 1b Study of Pidnarulex and Trastuzumab Deruxtecan in Patients With HER2 Expressing Solid Tumors
Toxicity Outcomes of Simultaneous Integrated Boost as Part of Adjuvant Radiotherapy in Breast Cancer Patients After Breast-Conserving Surgery
A Phase III Trial of Rx Therapy Guided by Genomic Risk Assessment For High Anatomic Stage ER-pos/HER2-neg Breast Cancer With RS</=25 (RxFINE-Low)
Phase I Study Targeting DNA Methyltransferases in Metastatic Triple-Negative Breast Cancer
DART: Dual Anti-CTLA-4 and Anti-PD-1 Blockade in Rare Tumors
Phase 1/1B Study of DS-8201a in Combination With ATR Inhibition (AZD6738) in Advanced Solid Tumors With HER2 Expression (DASH Trial)
A Phase I Study of GCAR1, a Chimeric Antigen Receptor (CAR) T-CELL Therapy for Participants With Selected Relapsed/Refractory GPNMB-Expressing Solid Tumours
Treatment of Elderly Patients With Early Stage Breast Cancer (CRYO-1): The Efficacy of Cryoablation In The Elderly - A Phase II Multicentre, Feasibility Study.
A Phase 1b Trial of ZEN003694 (ZEN-3694) With Pembrolizumab and Nab-Paclitaxel in Patients With Metastatic Triple-Negative Breast Cancer
Phase III Trial Of Neoadjuvant Durvalumab (NSC 778709) Plus Chemotherapy Versus Chemotherapy Alone For Adults With MammaPrint High 2 Risk (MP2) Hormone Receptor (HR) Positive / Human Epidermal Growth Factor Receptor (HER2) Negative Stage II-III Breast Cancer
MICRO-VERZ: MICRObiome Changes During VERZenio Therapy
Cancer Moonshot Biobank Research Protocol
A Phase I Study of Concurrent Abemaciclib and Radiation Therapy (RT) for Patients With Metastatic Hormone Receptor Positive (HR+), HER2 Negative (HER2-) Breast Cancer
A Phase Ib/IIa, Open-Label, Two-Cohort, Dose-Escalation and Dose-Expansion Clinical Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Preliminary Anti-tumor Activity of MR001 in Patients With Locally Recurrent or Metastatic Advanced Triple-Negative Breast Cancer (TNBC) Who Have Progressed After First-Line or Later-Line Therapy
A Randomized, Open Label, Phase III Trial to Evaluate the Efficacy and Safety of Palbociclib + Anti-HER2 Therapy + Endocrine Therapy vs. Anti-HER2 Therapy + Endocrine Therapy After Induction Treatment for Hormone Receptor Positive (HR+)/HER2-Positive Metastatic Breast Cancer
Study on the Impact of Electroacupuncture Combined With Self-Acupressure on the Quality of Life of Patients With Early-Stage Breast Cancer Undergoing Chemotherapy
Adaptation and Implementation of an Evidence-Based Patient Navigation Intervention for Adolescent and Young Adult Cancer Survivors
A Phase 1 Study in Patients With Clinically Node-Positive Breast Cancer to Assess the Safety, Ultrasound Conspicuity, and Migration of an Optimized Ultrasound Twinkling Marker Observed for Sonographic Targeting (UTMOST2 Trial)
Audiovisual Didactic Experience for Latinx Patient Treatment Adherence and Non-English Speaker Trial Enrollment (ADELANTE) Study in Radiation Oncology
An Umbrella, Randomized, Controlled, Pre Operative Trial Testing Integrative Subtype Targeted Therapeutics in Estrogen Receptor Positive, HER2-Negative Breast Cancer
Refining Tamoxifen Dose for Premenopausal Breast Cancer Risk Reduction (RENAISSANCE): A Phase II Single Arm Trial
Pilot Trial of Nivolumab Plus Cabozantinib for Advanced Solid Tumors in Patients With HIV Infection

Therapeutic area: Oncology