EGFR Family: Structure and Function
The EGFR family, also known as the ErbB or HER family, comprises four receptor tyrosine kinases: EGFR (HER1/ErbB1), HER2 (ErbB2), HER3 (ErbB3), and HER4 (ErbB4). These receptors sit on the cell surface and, when activated by growth factors, trigger signaling cascades that control cell proliferation, survival, and differentiation. Their extracellular domains contain ligand-binding regions, while their intracellular tails have tyrosine kinase activity that phosphorylates downstream substrates.
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Unlike most family members, HER2 has no known direct ligand and is activated through dimerization with other receptors. HER3 has a kinase-impaired domain but still transmits signals by heterodimerizing, particularly with EGFR or HER2. This architecture allows flexible combinatorial signaling, which becomes dangerous when mutated or overexpressed in cancer.
EGFR Family in Cancer
Dysregulation of the EGFR family is a hallmark of several cancers. EGFR mutations and amplifications drive non-small cell lung cancer (NSCLC), while HER2 overexpression is central to a subset of breast and gastric cancers. HER2 remains the most aggressively targeted member, but EGFR-directed therapies have transformed treatment in lung cancer and colorectal cancer.
Downstream pathways such as RAS-RAF-MAPK and PI3K-AKT-mTOR relay growth signals from the receptors to the nucleus. When upstream receptors are mutated, these pathways can become constitutively active, making tumors dependent on EGFR family signaling for survival. This dependency is what makes targeted therapies effective in biomarker-selected patients.
Common EGFR Family Mutations
In NSCLC, EGFR mutations cluster in the tyrosine kinase domain, with exon 19 deletions and the L858R point mutation being the most common sensitizing alterations. These mutations increase kinase activity and create a dependency on EGFR signaling that can be exploited by tyrosine kinase inhibitors (TKIs).
A separate class of mutations includes the T790M resistance mutation, which emerges after initial TKI treatment, and exon 20 insertions, which are historically harder to target. HER2 mutations in NSCLC and amplifications in breast cancer represent another dimension of EGFR family alteration. In colorectal cancer, EGFR amplification or mutation often predicts response to anti-EGFR antibodies, provided the tumor is RAS wild-type.
| Member | Key Alterations in Cancer | Primary Tumor Types |
|---|---|---|
| EGFR (HER1) | Exon 19 del, L858R, T790M, exon 20 insertions | NSCLC, colorectal, head and neck |
| HER2 (ErbB2) | Amplification, activating mutations | Breast, gastric, NSCLC |
| HER3 (ErbB3) | Rare activating mutations, heterodimer-driven signaling | NSCLC, endometrial |
| HER4 (ErbB4) | Alterations less well characterized | NSCLC, hematologic |
Targeted Therapies Against the EGFR Family
EGFR TKIs such as erlotinib, gefitinib, and osimertinib block the intracellular kinase domain of mutant EGFR. Osimertinib is preferred for T790M-positive disease because it selectively targets mutant EGFR while sparing wild-type receptors, reducing side effects. For HER2-positive cancers, monoclonal antibodies like trastuzumab and pertuzumab bind the extracellular domain, blocking dimerization and flagging cells for immune destruction.
Newer agents address previously difficult targets. Bispecific antibodies and antibody-drug conjugates now engage HER2 and HER3 on the cell surface. For exon 20 insertions in EGFR, newer TKIs and combinations are entering clinical practice, though response depends on the specific insertion subtype. Resistance mechanisms, including bypass signaling through MET or HER3, remain an active area of research.
Testing and Treatment Selection
Molecular testing for EGFR family alterations is now standard in advanced NSCLC and increasingly in other solid tumors. Testing methods include next-generation sequencing for mutations and immunohistochemistry or fluorescence in situ hybridization for HER2 amplification. Results guide the choice between TKIs, antibodies, chemotherapy, and clinical trials.
Treatment selection depends on the specific alteration, tumor type, prior therapies, and the presence of resistance mutations. As the understanding of EGFR family signaling deepens, therapies are becoming more precise, and combination strategies that address bypass pathways are improving outcomes for patients with traditionally hard-to-treat alterations.