Is circulating HER-2 more than just a tumor marker?

HER2/neu (also known as neu and as c-erbB-2) is a protooncogene of the EGF receptor family of receptor tyrosine kinases (1). HER2/neu encodes a Mr185,000 transmembrane glycoprotein receptor (HER2, or c-erbB-2) that has partial homology with the other members of the EGF receptor family, and which also includes the EGF receptor (also called HER1), HER3, and HER4. These receptors are composed of an extracellular binding domain, a transmembrane lipophilic segment, and an intracellular protein tyrosine kinase domain with a regulatory carboxyl terminal segment (2). HER2 is overexpressed in 25–30% of breast cancers and its overexpression is associated with a high risk of relapse and death (3). ECD/HER-2 can be released by proteolytic cleavage from the full-length HER2 receptor and detected in the serum of patients. A humanized antibody directed against the extracellular domain of HER2 has shown clinical activity against HER2overexpressing breast tumors and has been recently approved for clinical use given alone or in combination with chemotherapy (4 – 6). Taking these data into consideration, tissue HER2 determination in breast cancer by either immunohistochemistry or fluorescence in situ hybridization has become increasingly important to provide optimal care to patients with breast cancer (7). The issues remain which of the available methods to determine HER2 overexpression/ amplification may be used and whether a plasma-based assay may have a role in the clinic. Levels of circulating ECD/ HER-2 are easily detectable in the serum of breast cancer patients, and the Food and Drug Administration has recently approved an ELISA-based HER-2 serum test for use in the follow-up and monitoring of patients with metastatic breast cancer (Bayer Diagnostics, Tarrytown, NY). Over the last years, the measurement of serum ECD/HER-2 levels in patients with breast cancer has been suggested as useful for several clinical applications. These include: the monitoring of women with metastatic breast cancer to aid in patient management; detecting the early appearance of recurrent breast cancer; predicting response to hormonal therapy or chemotherapy; and the selection patients for trastuzumab therapy and the monitoring of response to trastuzumab (8, 9). In this regard what does the study by Hayes et al. (10) add to our current knowledge? In this study, 242 patients who were enrolled in Cancer and Leukemia Group B (CALGB) prospective therapeutic trials for metastatic breast cancer were assayed for circulating ECD/HER-2 using a commercially available sandwich enzyme immunoassay (10). In their study, elevated ECD/HER-2 levels were observed in 37% of patients and were associated with a shorter overall survival. However, in a multivariate analysis, ECD/HER-2 did not independently correlate with survival. Furthermore, ECD/HER-2 levels were not predictive for time to progression and for response to megestrol acetate or chemotherapy, including a subgroup of patients treated with an anthracycline-containing regimen. The authors concluded appropriately that, like most other circulating tumor markers, circulating levels of ECD/HER-2 are most likely associated with a high tumor burden, and that their utility may be restricted to monitoring the clinical course in patients undergoing therapy. Is there more to ECD/HER-2 than just another marker of tumor burden? The answer is probably yes. This notion stems from the very nature of ECD/HER-2 generation and release from the tumor cells into the serum. A large body of experiments with cultured cells indicate that transmembrane, fulllength HER2 undergoes a proteolytic event that results in the release of the soluble ECD/HER-2 fragment and, concomitantly, in the production of an amino-terminally truncated, cell-associated, HER2 fragment that contains the kinase domain (designated as HER2 p95 because of its molecular weight) with potentially enhanced signaling activity (11–14). It is tempting to speculate that the adverse prognosis observed in patients with high levels of ECD/HER-2 may be related, at least in part, to the fact that it should reflect a concomitant increase of truncated, signaling-competent, HER2 p95. Studies measuring serum ECD/HER-2 and tumor HER2 p95 are needed to fully support this possibility. As examples of the signaling activity of truncated receptors, it has been shown that the cleavage of HER4, a receptor of the same family that HER2, or the TrkA neurotrophin receptor, produces active tyrosine kinase fragments that resemble the activated full-length receptors (15, 16). These results suggest that the extracellular domains of these receptors prevent spontaneous activation of the intracellular kinase domain. Conceivably, ligand-binding or ectodomain cleavage might counteract this inhibitory effect on the kinase domain. This hypothesis is strengthened by studies showing that an engineered deletion of the ECD/HER-2 markedly increases the tyrosine kinase activity and transforms efficiency of the resulting NH2-terminally truncated HER2 protein (17). In support of the in vivo signaling activity of the truncated HER2 p95 fragment, we have found that it is phosphorylated in human breast cancer tumors (14). In addition, the level of HER2 p95 in primary breast tumors is associated with the presence of lymph node metastases, whereas the level of full-length HER2 did not Received 6/6/01; accepted 6/15/01. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 To whom requests for reprints should be addressed, at Medical Oncology Service, Vall d’Hebron University Hospital, Paseo Vall d’Hebron 119-129, Barcelona 08035, Spain. Phone: 34-93-274-6077; Fax: 34-93-2-74-6059; E-mail: [email protected]. 2 The abbreviations used are: EGF, epidermal growth factor; ECD/ HER-2, extracellular domain of HER2. 2605 Vol. 7, 2605–2607, September 2001 Clinical Cancer Research