HER2 Gene Function
Human Epidermal Growth Factor Receptor 2 (ERBB2)
Overview
HER2 amplification produces constitutively active homodimers and heterodimers (preferentially with ERBB3) that drive PI3K/AKT and MAPK/ERK without ligand, occurring in ~20% of breast cancers and ~15% of gastric cancers. Trastuzumab transformed HER2+ breast cancer outcomes (5-year survival now >90%). Trastuzumab deruxtecan (T-DXd) — carrying 8 topoisomerase I inhibitor molecules per antibody — extends benefit into the HER2-low category (IHC 1+ or 2+/FISH-), transforming ~40% of breast cancers into a newly actionable category. Resistance to trastuzumab arises through PIK3CA mutations/PTEN loss, MUC4 epitope masking, and p95-HER2 truncation; T-DXd's bystander effect overcomes most trastuzumab resistance mechanisms.
Molecular Mechanism
Mechanism Summary
HER2 lacks a known ligand and preferentially signals as a heterodimer partner. Amplification produces surface receptor density sufficient for ligand-independent constitutive homodimerisation, driving PI3K/AKT and MAPK without growth factor input.
Step-by-Step Mechanism
In HER2-amplified cells, receptor density on the cell surface increases 10–30-fold. At high concentration, HER2 spontaneously forms homodimers through stochastic collision, or heterodimerises with EGFR, HER3, or HER4 upon ligand binding to the partner receptor.
The HER2 kinase domain acts exclusively as an 'activator' in the asymmetric kinase dimer, phosphorylating the partner ('receiver') kinase. HER2 lacks intrinsic autophosphorylation capacity; it activates its partner to be phosphorylated in return.
HER2 homodimers and HER2/EGFR heterodimers activate the RAS/MAPK cascade through GRB2/SHC/SOS, driving cyclin D1 expression and cell cycle entry.
Trastuzumab binds HER2 extracellular domain IV, sterically blocking dimerisation arm exposure and preventing HER2 from acting as an activator kinase. Additionally, trastuzumab recruits NK cells via ADCC and promotes HER2 receptor internalisation and degradation.
Trastuzumab deruxtecan (T-DXd) attaches a topoisomerase I inhibitor (DXd) payload to an anti-HER2 antibody. HER2-targeted cell internalisation delivers high intracellular DXd concentrations, enabling a bystander effect that kills HER2-low neighbouring tumour cells.
Upstream Regulators
Ligand-bound partner receptors drive HER2 transphosphorylation through asymmetric dimer kinase mechanism
Sterically shield HER2 from trastuzumab binding; promote constitutive activation
Phosphorylates HER2 Tyr877, promoting kinase activity independent of dimerisation
Downstream Targets
AKT/mTOR survival and proliferation
ERK-driven proliferation, cyclin D1
Anti-apoptotic BCL-XL, MCL1 expression
Key Post-Translational Modifications
GRB2 and SHC docking; MAPK pathway activation
HER3-mediated PI3K recruitment; dominant survival signal
Releases p95-HER2 (truncated membrane fragment); trastuzumab-resistant
Disease Mechanism
HER2 amplification occurs in ~20% of breast cancers and ~10% of gastric/GEJ cancers. Before trastuzumab, HER2+ breast cancer had ~50% 5-year survival; current HER2-directed regimens (trastuzumab, pertuzumab, T-DM1, T-DXd) achieve >90% 5-year survival in early-stage disease. T-DXd ('HER2-low' trials) has expanded HER2-targeted therapy to tumours previously considered HER2-negative.
Database References
Key Pathways
- ·ERBB/HER2 signaling
- ·PI3K-AKT signaling
- ·MAPK signaling
- ·Antibody-drug conjugate mechanisms
Disease Associations
- ·HER2-positive breast cancer
- ·HER2-low breast cancer
- ·Gastric/GEJ cancer
- ·Lung adenocarcinoma (HER2 exon 20 insertions)
Research Activity
HER2 is an actively studied target: about 1000+ clinical trials that mention it are currently recruiting on ClinicalTrials.gov. Trial activity reflects research interest, not proven benefit — designs, endpoints and populations vary widely.
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Functional Partners
Common Questions About HER2
What is HER2 overexpression in breast cancer?
HER2 gene amplification leads to receptor overexpression on the tumour cell surface, forming constitutively active homodimers and heterodimers that signal through PI3K/AKT and MAPK without requiring ligand binding. HER2 amplification occurs in ~20% of breast cancers.
Is HER2-positive breast cancer aggressive?
HER2-positive breast cancer was historically aggressive with poor prognosis, but targeted HER2 therapies have transformed outcomes. Five-year survival rates for HER2-positive early breast cancer now exceed 90% with trastuzumab, pertuzumab, and T-DM1, compared to ~50% in the pre-trastuzumab era.
How does trastuzumab (Herceptin) work?
Trastuzumab binds the extracellular domain IV of HER2, blocking downstream signalling, promoting receptor internalisation and degradation, and recruiting natural killer cells via antibody-dependent cellular cytotoxicity (ADCC). The combination of direct signalling blockade and immune activation explains its clinical efficacy.
Can HER2 be targeted in cancers other than breast cancer?
HER2-directed evidence and regulatory indications differ substantially by tumour type and by biomarker definition—amplification, overexpression, mutation and 'HER2-low' are not interchangeable. A breast- or gastric-cancer result should not be copied into colorectal, lung or biliary cancer without checking the current tumour-specific label, test and regimen.
What is the difference between HER2-positive, HER2-low, and HER2-zero breast cancer?
HER2 status is traditionally scored by immunohistochemistry (IHC 0–3+) and FISH amplification: HER2-positive is IHC 3+ or IHC 2+/FISH-amplified (~20% of breast cancers). HER2-low is IHC 1+ or IHC 2+/FISH-non-amplified (~40–50% of breast cancers) — previously treated as HER2-negative and ineligible for HER2-targeted therapy. Trastuzumab deruxtecan (T-DXd), an antibody-drug conjugate carrying 8 molecules of topoisomerase I inhibitor per antibody, showed substantial activity in HER2-low disease (DESTINY-Breast04, HR 0.64 for PFS), transforming the HER2-low category from a biological distinction without treatment implication to a clinically actionable category.
How does trastuzumab deruxtecan (T-DXd) differ from trastuzumab and T-DM1?
All three are trastuzumab-based antibody-drug conjugates (ADCs) that deliver cytotoxic payload to HER2-expressing cells, but they differ critically in drug-to-antibody ratio and payload mechanism. Trastuzumab carries no cytotoxic payload (pure antibody — ADCC and signalling blockade only). T-DM1 (ado-trastuzumab emtansine) carries ~3.5 molecules of DM1 (microtubule inhibitor) per antibody via stable linker. T-DXd carries ~8 molecules of DeruxTecan (topoisomerase I inhibitor, similar to irinotecan) per antibody via cleavable linker — the high drug load and cleavable linker enable a 'bystander effect' where released payload kills adjacent HER2-low or HER2-negative tumour cells. This bystander effect is why T-DXd is active even in HER2-low disease.
What causes trastuzumab resistance in HER2-positive breast cancer?
Trastuzumab resistance arises through several mechanisms: (1) PI3K/AKT pathway activation via PIK3CA mutations or PTEN loss bypasses HER2 signalling dependence and is the most common mechanism (~30–40% of resistant tumours); (2) MUC4 mucin overexpression sterically masks the HER2 domain II epitope that trastuzumab binds, preventing antibody contact; (3) p95-HER2 truncation removes the trastuzumab-binding extracellular domain, leaving a constitutively active transmembrane fragment; (4) EGFR or MET co-receptor overexpression provides parallel RTK signalling. Pertuzumab (blocking HER2-HER3 dimerisation), T-DM1, and T-DXd overcome most of these resistance mechanisms by exploiting different HER2 binding sites or delivering payload directly into HER2-expressing cells.