Receptor tyrosine-protein kinase erbB-2, also known as CD340 (cluster of differentiation 340), proto-oncogene Neu, Erbb2 (rodent), or ERBB2 (human), is a protein that in humans is encoded by the ERBB2 gene. ERBB is abbreviated from erythroblastic oncogene B, a gene isolated from avian genome. It is also frequently called HER2 (from human epidermal growth factor receptor 2) or HER2/neu.
HER2 is a member of the human epidermal growth factor receptor (HER/EGFR/ERBB) family. Amplification or over-expression of this oncogene has been shown to play an important role in the development and progression of certain aggressive types of breast cancer. In recent years the protein has become an important biomarker and target of therapy for approximately 30% of breast cancer patients.
HER2 is so named because it has a similar structure to human epidermal growth factor receptor, or HER1. Neu is so named because it was derived from a rodent glioblastoma cell line, a type of neural tumor. ErbB-2 was named for its similarity to ErbB (avian erythroblastosis oncogene B), the oncogene later found to code for EGFR. Molecular cloning of the gene showed that HER2, Neu, and ErbB-2 are all encoded by the same orthologs.
The ErbB family consists of four plasma membrane-bound receptor tyrosine kinases. One of which is erbB-2, and the other members being epidermal growth factor receptor, erbB-3 (neuregulin-binding; lacks kinase domain), and erbB-4. All four contain an extracellular ligand binding domain, a transmembrane domain, and an intracellular domain that can interact with a multitude of signaling molecules and exhibit both ligand-dependent and ligand-independent activity. Notably, no ligands for HER2 have yet been identified. HER2 can heterodimerise with any of the other three receptors and is considered to be the preferred dimerisation partner of the other ErbB receptors.
Dimerisation results in the autophosphorylation of tyrosine residues within the cytoplasmic domain of the receptors and initiates a variety of signaling pathways.
Signaling pathways activated by HER2 include:
- mitogen-activated protein kinase (MAPK)
- phosphoinositide 3-kinase (PI3K/Akt)
- phospholipase C γ
- protein kinase C (PKC)
- Signal transducer and activator of transcription (STAT)
In summary, signaling through the ErbB family of receptors promotes cell proliferation and opposes apoptosis, and therefore must be tightly regulated to prevent uncontrolled cell growth from occurring.
Amplification, also known as the over-expression of the ERBB2 gene, occurs in approximately 15-30% of breast cancers. It is strongly associated with increased disease recurrence and a poor prognosis. Over-expression is also known to occur in ovarian, stomach, adenocarcinoma of the lung and aggressive forms of uterine cancer, such as uterine serous endometrial carcinoma, e.g. HER-2 is over-expressed in approximately 7-34% of patients with gastric cancer and in 30% of salivary duct carcinomas.
HER2 is colocalised and most of the time, coamplified with the gene GRB7, which is a proto-oncogene associated with breast, testicular germ cell, gastric, and eosophageal tumours.
Furthermore, diverse structural alterations have been identified that cause ligand-independent firing of this receptor, doing so in the absence of receptor over-expression. HER2 is found in a variety of tumours and some of these tumours carry point mutations in the sequence specifying the transmembrane domain of HER2. Substitution of a valine for a glutamic acid in the transmembrane domain can result in the constitutive dimerisation of this protein in the absence of a ligand.
As a drug target
HER2 is the target of the monoclonal antibody trastuzumab (marketed as Herceptin). Trastuzumab is effective only in cancers where HER2 is over-expressed. One year of trastuzumab therapy is recommended for all patients with HER2-positive breast cancer who are also receiving chemotherapy. Twelve months of trastuzumab therapy is optimal. Randomized trials have demonstrated no additional benefit beyond 12 months, whereas 6 months has been shown to be inferior to 12. Trastuzumab is administered intravenously weekly or every 3 weeks.
An important downstream effect of trastuzumab binding to HER2 is an increase in p27, a protein that halts cell proliferation. Another monoclonal antibody, Pertuzumab, which inhibits dimerisation of HER2 and HER3 receptors, was approved by the FDA for use in combination with trastuzumab in June 2012.
It has been found that patients with ER+ (Estrogen receptor positive)/HER2+ compared with ER-/HER2+ breast cancers may actually benefit more from drugs that inhibit the PI3K/AKT molecular pathway.
Over-expression of HER2 can also be suppressed by the amplification of other genes. Research is currently being conducted to discover which genes may have this desired effect.
The expression of HER2 is regulated by signaling through eostrogen receptors. Normally, estradiol and tamoxifen acting through the eostrogen receptor down-regulate the expression of HER2. However, when the ratio of the coactivator AIB-3 exceeds that of the corepressor PAX2, the expression of HER2 is upregulated in the presence of tamoxifen, leading to tamoxifen-resistant breast cancer.
HER2 testing is performed in breast cancer patients to assess prognosis and to determine suitability for trastuzumab therapy. It is important that trastuzumab is restricted to HER2-positive individuals as it is expensive and has been associated with cardiac toxicity. For HER2-negative tumours, the risks of trastuzumab clearly outweigh the benefits.
Tests are usually performed on biopsy samples obtained by either fine-needle aspiration, core needle biopsy, vacuum-assisted breast biopsy, or surgical excision. Immunohistochemistry is used to measure the amount of HER2 protein present in the sample. Examples of this assay include HercepTest, Dako, Glostrup, and Denmark. The sample is given a score based on the cell membrane staining pattern. Specimens with equivocal IHC results should then be validated using fluorescence in situ hybridisation (FISH). FISH can be used to measure the number of copies of the gene which are present and is thought to be more reliable than IHC.
The extracellular domain of HER2 can be shed from the surface of tumour cells and enter the circulation. Measurement of serum HER2 by enzyme-linked immunosorbent assay (ELISA) offers a far less invasive method of determining HER2 status than a biopsy and consequently has been extensively investigated. Results so far have suggested that changes in serum HER2 concentrations may be useful in predicting response to trastuzumab therapy. However, its ability to determine eligibility for trastuzumab therapy is less clear.
HER2/neu has been shown to interact with:
- "ERBB2 erb-b2 receptor tyrosine kinase 2 [Homo sapiens (human)] - Gene - NCBI". www.ncbi.nlm.nih.gov. Retrieved 2016-06-14.
- Reference, Genetics Home. "ERBB2". Genetics Home Reference. Retrieved 2016-06-19.
- Barh D, Gunduz M (2015-01-22). Noninvasive Molecular Markers in Gynecologic Cancers. CRC Press. p. 427. ISBN 9781466569393.
- Mitri Z, Constantine T, O'Regan R (2012). "The HER2 Receptor in Breast Cancer: Pathophysiology, Clinical Use, and New Advances in Therapy". Chemotherapy Research and Practice. 2012: 743193. doi:10.1155/2012/743193. PMC 3539433. PMID 23320171.
- Coussens L, Yang-Feng TL, Liao YC, Chen E, Gray A, McGrath J, Seeburg PH, Libermann TA, Schlessinger J, Francke U (December 1985). "Tyrosine kinase receptor with extensive homology to EGF receptor shares chromosomal location with neu oncogene". Science. 230 (4730): 1132–9. Bibcode:1985Sci...230.1132C. doi:10.1126/science.2999974. PMID 2999974.
- Keshamouni VG, Mattingly RR, Reddy KB (June 2002). "Mechanism of 17-beta-estradiol-induced Erk1/2 activation in breast cancer cells. A role for HER2 AND PKC-delta". The Journal of Biological Chemistry. 277 (25): 22558–65. doi:10.1074/jbc.M202351200. PMID 11960991.
- Rusnak DW, Affleck K, Cockerill SG, Stubberfield C, Harris R, Page M, et al. (October 2001). "The characterization of novel, dual ErbB-2/EGFR, tyrosine kinase inhibitors: potential therapy for cancer". Cancer Research. 61 (19): 7196–203. PMID 11585755.
- Olayioye MA (2001). "Update on HER-2 as a target for cancer therapy: intracellular signaling pathways of ErbB2/HER-2 and family members". Breast Cancer Research. 3 (6): 385–9. doi:10.1186/bcr327. PMC 138705. PMID 11737890.
- Roy V, Perez EA (November 2009). "Beyond trastuzumab: small molecule tyrosine kinase inhibitors in HER-2-positive breast cancer". The Oncologist. 14 (11): 1061–9. doi:10.1634/theoncologist.2009-0142. PMID 19887469.
- Burstein HJ (October 2005). "The distinctive nature of HER2-positive breast cancers". The New England Journal of Medicine. 353 (16): 1652–4. doi:10.1056/NEJMp058197. PMID 16236735.
- Tan M, Yu D (2007). "Molecular mechanisms of erbB2-mediated breast cancer chemoresistance". Advances in Experimental Medicine and Biology. Advances in Experimental Medicine and Biology. 608: 119–29. doi:10.1007/978-0-387-74039-3_9. ISBN 978-0-387-74037-9. PMID 17993237.
- Kumar V, Abbas A, Aster J (2013). Robbins basic pathology. Philadelphia: Elsevier/Saunders. p. 697. ISBN 9781437717815.
- Kumar V, Abbas A, Aster J (2013). Robbins basic pathology. Philadelphia: Elsevier/Saunders. p. 179. ISBN 9781437717815.
- Santin AD, Bellone S, Roman JJ, McKenney JK, Pecorelli S (August 2008). "Trastuzumab treatment in patients with advanced or recurrent endometrial carcinoma overexpressing HER2/neu". International Journal of Gynaecology and Obstetrics. 102 (2): 128–31. doi:10.1016/j.ijgo.2008.04.008. PMID 18555254.
- Buza N, Roque DM, Santin AD (March 2014). "HER2/neu in Endometrial Cancer: A Promising Therapeutic Target With Diagnostic Challenges". Archives of Pathology & Laboratory Medicine. 138 (3): 343–50. doi:10.5858/arpa.2012-0416-RA. PMID 24576030.
- Rüschoff J, Hanna W, Bilous M, Hofmann M, Osamura RY, Penault-Llorca F, van de Vijver M, Viale G (May 2012). "HER2 testing in gastric cancer: a practical approach". Modern Pathology. 25 (5): 637–50. doi:10.1038/modpathol.2011.198. PMID 22222640.
- Meza-Junco J, Au HJ, Sawyer MB (2011). "Critical appraisal of trastuzumab in treatment of advanced stomach cancer". Cancer Management and Research. 3 (3): 57–64. doi:10.2147/CMAR.S12698. PMC 3085240. PMID 21556317.
- Chiosea SI, Williams L, Griffith CC, Thompson LD, Weinreb I, Bauman JE, Luvison A, Roy S, Seethala RR, Nikiforova MN (June 2015). "Molecular characterization of apocrine salivary duct carcinoma". The American Journal of Surgical Pathology. 39 (6): 744–52. doi:10.1097/PAS.0000000000000410. PMID 25723113.
- Nagy P, Jenei A, Kirsch AK, Szöllosi J, Damjanovich S, Jovin TM (June 1999). "Activation-dependent clustering of the erbB2 receptor tyrosine kinase detected by scanning near-field optical microscopy". Journal of Cell Science. 112 (11): 1733–41. PMID 10318765.
- Kaufmann R, Müller P, Hildenbrand G, Hausmann M, Cremer C (April 2011). "Analysis of Her2/neu membrane protein clusters in different types of breast cancer cells using localization microscopy". Journal of Microscopy. 242 (1): 46–54. doi:10.1111/j.1365-2818.2010.03436.x. PMID 21118230.
- Yonesaka K, Zejnullahu K, Okamoto I, Satoh T, Cappuzzo F, Souglakos J, et al. (September 2011). "Activation of ERBB2 signaling causes resistance to the EGFR-directed therapeutic antibody cetuximab". Science Translational Medicine. 3 (99): 99ra86. doi:10.1126/scitranslmed.3002442. PMC 3268675. PMID 21900593.
- Brandt-Rauf PW, Rackovsky S, Pincus MR (November 1990). "Correlation of the structure of the transmembrane domain of the neu oncogene-encoded p185 protein with its function". Proceedings of the National Academy of Sciences of the United States of America. 87 (21): 8660–4. Bibcode:1990PNAS...87.8660B. doi:10.1073/pnas.87.21.8660. PMC 55017. PMID 1978329.
- Mazières J, Peters S, Lepage B, Cortot AB, Barlesi F, Beau-Faller M, et al. (June 2013). "Lung cancer that harbors an HER2 mutation: epidemiologic characteristics and therapeutic perspectives". Journal of Clinical Oncology. 31 (16): 1997–2003. doi:10.1200/JCO.2012.45.6095. PMID 23610105.
- Mates M, Fletcher GG, Freedman OC, Eisen A, Gandhi S, Trudeau ME, Dent SF (March 2015). "Systemic targeted therapy for her2-positive early female breast cancer: a systematic review of the evidence for the 2014 Cancer Care Ontario systemic therapy guideline". Current Oncology. 22 (Suppl 1): S114–22. doi:10.3747/co.22.2322. PMC 4381787. PMID 25848335.
- Jameson; et al. (2018). Harrison's Principles of Internal Medicine, 20th ed. McGraw-Hill Education. pp. Chapter 75: Breast Cancer. ISBN 978-1-259-64403-0.
- Le XF, Pruefer F, Bast RC (January 2005). "HER2-targeting antibodies modulate the cyclin-dependent kinase inhibitor p27Kip1 via multiple signaling pathways". Cell Cycle. 4 (1): 87–95. doi:10.4161/cc.4.1.1360. PMID 15611642.
- Jiang H, Rugo HS (November 2015). "Human epidermal growth factor receptor 2 positive (HER2+) metastatic breast cancer: how the latest results are improving therapeutic options". Therapeutic Advances in Medical Oncology. 7 (6): 321–39. doi:10.1177/1758834015599389. PMC 4622301. PMID 26557900.
- Loi S, Sotiriou C, Haibe-Kains B, Lallemand F, Conus NM, Piccart MJ, Speed TP, McArthur GA (2009). "Gene expression profiling identifies activated growth factor signaling in poor prognosis (Luminal-B) estrogen receptor positive breast cancer". BMC Medical Genomics. 2: 37. doi:10.1186/1755-8794-2-37. PMC 2706265. PMID 19552798. Lay summary – ScienceDaily.
- "Study sheds new light on tamoxifen resistance". Cordis News. Cordis. 2008-11-13. Retrieved 2008-11-14.
- Hurtado A, Holmes KA, Geistlinger TR, Hutcheson IR, Nicholson RI, Brown M, Jiang J, Howat WJ, Ali S, Carroll JS (December 2008). "Regulation of ERBB2 by oestrogen receptor-PAX2 determines response to tamoxifen". Nature. 456 (7222): 663–6. Bibcode:2008Natur.456..663H. doi:10.1038/nature07483. PMC 2920208. PMID 19005469.
- Telli ML, Hunt SA, Carlson RW, Guardino AE (August 2007). "Trastuzumab-related cardiotoxicity: calling into question the concept of reversibility". Journal of Clinical Oncology. 25 (23): 3525–33. doi:10.1200/JCO.2007.11.0106. PMID 17687157.
- Giuliano AE, Hurvitz SA (2019). "Breast Disorder". In Papadakis MA, McPhee SJ, Rabow MW. Current Medical Diagnosis & Treatment. New York, NY: McGraw-Hill.
- Ali SM, Carney WP, Esteva FJ, Fornier M, Harris L, Köstler WJ, Lotz JP, Luftner D, Pichon MF, Lipton A (September 2008). "Serum HER-2/neu and relative resistance to trastuzumab-based therapy in patients with metastatic breast cancer". Cancer. 113 (6): 1294–301. doi:10.1002/cncr.23689. PMID 18661530.
- Lennon S, Barton C, Banken L, Gianni L, Marty M, Baselga J, Leyland-Jones B (April 2009). "Utility of serum HER2 extracellular domain assessment in clinical decision making: pooled analysis of four trials of trastuzumab in metastatic breast cancer". Journal of Clinical Oncology. 27 (10): 1685–93. doi:10.1200/JCO.2008.16.8351. PMID 19255335.
- Schroeder JA, Adriance MC, McConnell EJ, Thompson MC, Pockaj B, Gendler SJ (June 2002). "ErbB-beta-catenin complexes are associated with human infiltrating ductal breast and murine mammary tumor virus (MMTV)-Wnt-1 and MMTV-c-Neu transgenic carcinomas". The Journal of Biological Chemistry. 277 (25): 22692–8. doi:10.1074/jbc.M201975200. PMID 11950845.
- Bonvini P, An WG, Rosolen A, Nguyen P, Trepel J, Garcia de Herreros A, Dunach M, Neckers LM (February 2001). "Geldanamycin abrogates ErbB2 association with proteasome-resistant beta-catenin in melanoma cells, increases beta-catenin-E-cadherin association, and decreases beta-catenin-sensitive transcription". Cancer Research. 61 (4): 1671–7. PMID 11245482.
- Kanai Y, Ochiai A, Shibata T, Oyama T, Ushijima S, Akimoto S, Hirohashi S (March 1995). "c-erbB-2 gene product directly associates with beta-catenin and plakoglobin". Biochemical and Biophysical Research Communications. 208 (3): 1067–72. doi:10.1006/bbrc.1995.1443. PMID 7702605.
- Huang YZ, Won S, Ali DW, Wang Q, Tanowitz M, Du QS, Pelkey KA, Yang DJ, Xiong WC, Salter MW, Mei L (May 2000). "Regulation of neuregulin signaling by PSD-95 interacting with ErbB4 at CNS synapses". Neuron. 26 (2): 443–55. doi:10.1016/s0896-6273(00)81176-9. PMID 10839362.
- Jaulin-Bastard F, Saito H, Le Bivic A, Ollendorff V, Marchetto S, Birnbaum D, Borg JP (May 2001). "The ERBB2/HER2 receptor differentially interacts with ERBIN and PICK1 PSD-95/DLG/ZO-1 domain proteins". The Journal of Biological Chemistry. 276 (18): 15256–63. doi:10.1074/jbc.M010032200. PMID 11278603.
- Bilder D, Birnbaum D, Borg JP, Bryant P, Huigbretse J, Jansen E, Kennedy MB, Labouesse M, Legouis R, Mechler B, Perrimon N, Petit M, Sinha P (July 2000). "Collective nomenclature for LAP proteins". Nature Cell Biology. 2 (7): E114. doi:10.1038/35017119. PMID 10878817.
- Huang YZ, Zang M, Xiong WC, Luo Z, Mei L (January 2003). "Erbin suppresses the MAP kinase pathway". The Journal of Biological Chemistry. 278 (2): 1108–14. doi:10.1074/jbc.M205413200. PMID 12379659.
- Schulze WX, Deng L, Mann M (2005). "Phosphotyrosine interactome of the ErbB-receptor kinase family". Molecular Systems Biology. 1: 2005.0008. doi:10.1038/msb4100012. PMC 1681463. PMID 16729043.
- Bourguignon LY, Zhu H, Zhou B, Diedrich F, Singleton PA, Hung MC (December 2001). "Hyaluronan promotes CD44v3-Vav2 interaction with Grb2-p185(HER2) and induces Rac1 and Ras signaling during ovarian tumor cell migration and growth". The Journal of Biological Chemistry. 276 (52): 48679–92. doi:10.1074/jbc.M106759200. PMID 11606575.
- Olayioye MA, Graus-Porta D, Beerli RR, Rohrer J, Gay B, Hynes NE (September 1998). "ErbB-1 and ErbB-2 acquire distinct signaling properties dependent upon their dimerization partner". Molecular and Cellular Biology. 18 (9): 5042–51. doi:10.1128/mcb.18.9.5042. PMC 109089. PMID 9710588.
- Xu W, Mimnaugh E, Rosser MF, Nicchitta C, Marcu M, Yarden Y, Neckers L (February 2001). "Sensitivity of mature Erbb2 to geldanamycin is conferred by its kinase domain and is mediated by the chaperone protein Hsp90". The Journal of Biological Chemistry. 276 (5): 3702–8. doi:10.1074/jbc.M006864200. PMID 11071886.
- Jeong JH, An JY, Kwon YT, Li LY, Lee YJ (October 2008). "Quercetin-induced ubiquitination and down-regulation of Her-2/neu". Journal of Cellular Biochemistry. 105 (2): 585–95. doi:10.1002/jcb.21859. PMC 2575035. PMID 18655187.
- Grant SL, Hammacher A, Douglas AM, Goss GA, Mansfield RK, Heath JK, Begley CG (January 2002). "An unexpected biochemical and functional interaction between gp130 and the EGF receptor family in breast cancer cells". Oncogene. 21 (3): 460–74. doi:10.1038/sj.onc.1205100. PMID 11821958.
- Li Y, Yu WH, Ren J, Chen W, Huang L, Kharbanda S, Loda M, Kufe D (August 2003). "Heregulin targets gamma-catenin to the nucleolus by a mechanism dependent on the DF3/MUC1 oncoprotein". Molecular Cancer Research. 1 (10): 765–75. PMID 12939402.
- Schroeder JA, Thompson MC, Gardner MM, Gendler SJ (April 2001). "Transgenic MUC1 interacts with epidermal growth factor receptor and correlates with mitogen-activated protein kinase activation in the mouse mammary gland". The Journal of Biological Chemistry. 276 (16): 13057–64. doi:10.1074/jbc.M011248200. PMID 11278868.
- Gout I, Dhand R, Panayotou G, Fry MJ, Hiles I, Otsu M, Waterfield MD (December 1992). "Expression and characterization of the p85 subunit of the phosphatidylinositol 3-kinase complex and a related p85 beta protein by using the baculovirus expression system". The Biochemical Journal. 288 (2): 395–405. doi:10.1042/bj2880395. PMC 1132024. PMID 1334406.
- Peles E, Levy RB, Or E, Ullrich A, Yarden Y (August 1991). "Oncogenic forms of the neu/HER2 tyrosine kinase are permanently coupled to phospholipase C gamma". The EMBO Journal. 10 (8): 2077–86. PMC 452891. PMID 1676673.
- Arteaga CL, Johnson MD, Todderud G, Coffey RJ, Carpenter G, Page DL (December 1991). "Elevated content of the tyrosine kinase substrate phospholipase C-gamma 1 in primary human breast carcinomas". Proceedings of the National Academy of Sciences of the United States of America. 88 (23): 10435–9. Bibcode:1991PNAS...8810435A. doi:10.1073/pnas.88.23.10435. PMC 52943. PMID 1683701.
- Wong L, Deb TB, Thompson SA, Wells A, Johnson GR (March 1999). "A differential requirement for the COOH-terminal region of the epidermal growth factor (EGF) receptor in amphiregulin and EGF mitogenic signaling". The Journal of Biological Chemistry. 274 (13): 8900–9. doi:10.1074/jbc.274.13.8900. PMID 10085134.
- Ross JS, Fletcher JA, Linette GP, Stec J, Clark E, Ayers M, Symmans WF, Pusztai L, Bloom KJ (2003). "The Her-2/neu gene and protein in breast cancer 2003: biomarker and target of therapy". The Oncologist. 8 (4): 307–25. doi:10.1634/theoncologist.8-4-307. PMID 12897328.
- Zhou BP, Hung MC (October 2003). "Dysregulation of cellular signaling by HER2/neu in breast cancer". Seminars in Oncology. 30 (5 Suppl 16): 38–48. doi:10.1053/j.seminoncol.2003.08.006. PMID 14613025.
- Ménard S, Casalini P, Campiglio M, Pupa SM, Tagliabue E (December 2004). "Role of HER2/neu in tumor progression and therapy". Cellular and Molecular Life Sciences. 61 (23): 2965–78. doi:10.1007/s00018-004-4277-7. PMID 15583858.
- Becker JC, Muller-Tidow C, Serve H, Domschke W, Pohle T (June 2006). "Role of receptor tyrosine kinases in gastric cancer: new targets for a selective therapy". World Journal of Gastroenterology. 12 (21): 3297–305. doi:10.3748/wjg.v12.i21.3297. PMC 4087885. PMID 16733844.
- Laudadio J, Quigley DI, Tubbs R, Wolff DJ (January 2007). "HER2 testing: a review of detection methodologies and their clinical performance". Expert Review of Molecular Diagnostics. 7 (1): 53–64. doi:10.1586/1473718.104.22.168. PMID 17187484.
- Bianchi F, Tagliabue E, Ménard S, Campiglio M (March 2007). "Fhit expression protects against HER2-driven breast tumor development: unraveling the molecular interconnections". Cell Cycle. 6 (6): 643–6. doi:10.4161/cc.6.6.4033. PMID 17374991.
- ERBB2 expression across human cancerous and healthy tissues[permanent dead link]
- AACR Cancer Concepts Factsheet on HER2
- Breast Friends for Life Network - A South African Breast Cancer Support Forum for HER2 Positive Women
- HerceptinR : Herceptin Resistance Database for Understanding Mechanism of Resistance in Breast Cancer Patients. Sci. Rep. 4:4483
- Receptor,+erbB-2 at the US National Library of Medicine Medical Subject Headings (MeSH)