Molecule of the Month: HER2/neu and Trastuzumab

Breast cancer is the leading cause of cancer-related deaths in women worldwide. Approximately 20% of invasive breast cancers have excess amounts of human epidermal growth factor receptor 2 (HER2), also known as neu or ErbB2. This can happen when the HER2 gene makes too many copies of itself (HER2 gene amplification), which can lead to overproduction of HER2 receptors (HER2 protein overexpression). Patients with too much HER2 have decreased survival rates and poorer responses to chemotherapy and hormone treatment, due to the uncontrolled growth and division of breast cancer cells. Understanding the mechanism of action and three-dimensional structure of HER2 is helping scientists discover new and effective treatments to fight these problems.

HER2 belongs to a family of four tyrosine kinase receptors (HER1, HER2, HER3, and HER4) found embedded within the plasma membranes of many cells around the body. These receptors normally regulate important biological processes such as cell growth and proliferation, metabolism, survival, and differentiation. HER receptors can exist in two conformations: tethered and extended. In the tethered form, the protein is folded on itself and unable to partner with itself or another receptor. The open, extended shape, however, is formed through interaction with their ligands and is free to form complexes and participate in signaling. HER2 is unique among this class of tyrosine kinases because it always adopts an extended shape without the need for a ligand, making it an ideal binding partner and major contributor in tumor development. In normal signaling pathways, HER2 generally forms complexes with other HER receptors. In cancerous cells, however, the higher concentration of HER2 causes the formation of active dimers with itself in addition to more of the normal complexes with other HER receptors, sending stronger signals to the cell to reproduce.

Structural understanding of HER2 is helping to discover new effective therapies for breast cancer. Trastuzumab, a laboratory-created monoclonal antibody, is a HER2-targeted therapy approved by the FDA (Food and Drug Administration). Trastuzumab binds to the HER2 receptor, blocking its homodimerization. This inhibits cell proliferation and replication, and ultimately slows the growth of HER2-positive breast cancer. The illustration shown here is built from three PDB entries: the extracellular domain with the Fab portion of trastuzumab is from 1n8z, the kinase domain inside the cell is from 3pp0, and the transmembrane domain is from 2ks1. Unfortunately, despite this clinical benefit, the overall impact of the antibody is limited due to resistance that is often acquired after administration of trastuzumab.

Trastuzumab has drastically improved the prognosis of HER2-positive breast cancer patients. However, approximately 50-60% of HER2-positive breast cancer patients acquire resistance and stop responding to the antibody. Scientists have tackled this issue by introducing Pertuzumab, a new antibody that also targets HER2, but in a different way. As seen in PDB entry 1s78, it binds farther up the molecule away from the membrane, blocking a binding pocket that HER2 uses to heterodimerize with other HER receptors. Pertuzumab particularly prevents formation of HER2:HER3 complexes, which play a key role in oncogenesis. Pertuzumab is also FDA-approved, and by blocking the HER2 signaling cascade at its source, it has contributed to overall higher survival rates and better responses, especially in patients who failed to respond to treatments previously.