High molecular weight epithelial mucins as markers in breast cancer

Tumor biomarkers for diagnosis, prognosis and targeted therapy

Tumor biomarkers, the substances which are produced by tumors or the body's responses to tumors during tumorigenesis and progression, have been demonstrated to possess critical and encouraging value in screening and early diagnosis, prognosis prediction, recurrence detection, and therapeutic efficacy monitoring of cancers. Over the past decades, continuous progress has been made in exploring and discovering novel, sensitive, specific, and accurate tumor biomarkers, which has significantly promoted personalized medicine and improved the outcomes of cancer patients, especially advances in molecular biology technologies developed for the detection of tumor biomarkers. Herein, we summarize the discovery and development of tumor biomarkers, including the history of tumor biomarkers, the conventional and innovative technologies used for biomarker discovery and detection, the classification of tumor biomarkers based on tissue origins, and the application of tumor biomarkers in clinical cancer management. In particular, we highlight the recent advancements in biomarker-based anticancer-targeted therapies which are emerging as breakthroughs and promising cancer therapeutic strategies. We also discuss limitations and challenges that need to be addressed and provide insights and perspectives to turn challenges into opportunities in this field. Collectively, the discovery and application of multiple tumor biomarkers emphasized in this review may provide guidance on improved precision medicine, broaden horizons in future research directions, and expedite the clinical classification of cancer patients according to their molecular biomarkers rather than organs of origin.

CA-549 Serum Levels in Breast Cancer Monitoring

CA-549 serum levels were assessed in 288 patients, 156 with early breast cancer (after surgery) and 132 with advanced breast cancer.

CA-549 was abnormal (> 12 U/ml) in 25/156 patients (16%) without clinical signs of disease after surgery (median 9 U/ml), in 49/60 patients (82%) with disease in progression (P) (median 50 U/ml), in 19/27 patients (70%) with stationary disease (NC) (median 14 U/ml), in 25/33 patients (76%) with partial remission (PR) (median 18 U/ml) and in 4/12 patients (33%) with complete remission (CR) (median 9 U/ml).

CA-549 serum levels correlated mainly with the extent of disease and secondarily with the prevalent metastatic site, higher values being observed in patients with visceral involvement (median 32.5 U/ml). CA-549 serum levels were also assessed in 51 patients at the start of treatment and at the time of objective evaluation: the results underline the concordance of CA-549 behavior with the clinical outcome in 71% of the cases. We conclude that CA-549 is a useful marker for monitoring breast cancer patients during the advanced stages of the disease.

MUC1 (CD227): a multi-tasked molecule

Mucin 1 (MUC1 [CD227]) is a high-molecular weight (>400 kDa), type I membrane-tethered glycoprotein that is expressed on epithelial cells and extends far above the glycocalyx. MUC1 is overexpressed and aberrantly glycosylated in adenocarcinomas and in hematological malignancies. As a result, MUC1 has been a target for tumor immunotherapeutic studies in mice and in humans. MUC1 has been shown to have anti-adhesive and immunosuppressive properties, protects against infections, and is involved in the oncogenic process as well as in cell signaling. In addition, MUC1 plays a key role in the reproductive tract, in the immune system (affecting dendritic cells, monocytes, T cells, and B cells), and in chronic inflammatory diseases. Evidence for all of these roles for MUC1 is discussed herein and demonstrates that MUC1 is truly a multitasked molecule.

Serum tumor markers in patients with breast cancer

Several serum tumor markers have been investigated in patients with breast cancer for assessing outcome, predicting recurrence and monitoring the therapeutic response. There is a general consensus concerning their limited application in diagnosing malignancy; however, serum tumor markers can be considered for the early detection of recurrence. The most effective markers for this indication are cancer antigens (CA)15-3 and 27.29, and c-erbB-2, although their efficacy in establishing disease progression has not been determined to date. In terms of evaluating prognosis and predicting response to therapy, only the expression of c-erbB-2 has clinical evidence. To conclude, at present, no serum tumor marker is cost effective, and none can be used with confidence in the decision making regarding breast cancer patients.

Analysis of mucins: role in laboratory diagnosis

Mucins are high molecular weight glycoproteins with complex oligosaccharide side chains attached to the apomucin protein backbone byO-glycosidic linkage; they are found in crude mucus gels that protect epithelial surfaces in the major tracts of the body and as transmembrane proteins expressed on the apical cell surface of glandular and ductal epithelia of various organs. Changes in the sequence of glycosylation of mucins in different settings generate a variety of epitopes in the oligosaccharide side chains of mucins, including newly expressed blood-group antigens, distinguishing between normal and diseased states. Tumour-associated epitopes on mucins and their antigenicity make them suitable as immunotargets on malignant epithelial cells and their secretions, creating a surge of interest in mucins as diagnostic and prognostic markers for various diseases, and even influencing the design of mucin-based vaccines. This review discusses the emerging roles of mucins such as MUC1 and MUC4 in cancer and some other diseases, and stresses how underglycosylated and truncated mucins are exploited as markers of disease and to monitor widespread metastasis, making them useful in patient management. Furthermore the type, pattern and amount of mucin secreted in some tissues have been considered in the classification and terminology of neoplasia and in specific organs such as the pancreas. These factors have been instrumental in pathological classification, diagnosis and prognostication of neoplasia.

Immune Responses to the MUC1 Mucin

MUC1 mucins are highly glycosylated glycoproteins expressed on the luminal surfaces of glandular epithelia. In breast and ovarian carcinomas, their expression is frequently upregulated and they may be secreted into the circulation of cancer patients. Early studies aimed at the production of anti-MUC1 monoclonal antibodies revealed that MUC1 was a potent immunogen in mice with many monoclonal antibodies raised defining epitopes within the protein core of MUC1. The immunogenicity of MUC1 has now been extended to human studies and it is apparent that patients with breast and ovarian malignant disease are able to mount immune responses against MUC1. These findings provide information on the mechanisms involved in the recognition of MUC1 expressing tumours. The utilisation of MUC1 related immunogens to stimulate immune responses to tumours could lead to the improved management of patients and the development of new immunotherapeutic strategies aimed at the eradication of MUC1 mucin expressing cancers.

Anti-MUC-1 immunoliposomal doxorubicin in the treatment of murine models of metastatic breast cancer

The fate of breast cancer patients is dependent upon elimination or control of metastases. We studied the effect of antibody-targeted liposomes containing entrapped doxorubicin (DXR) on development of tumours in two models of breast cancer, pseudometastatic and metastatic, in mice. The former used the mouse mammary carcinoma cell line GZHI, which expresses the human MUC-1 gene (L. Ding, E.N. Lalani, M. Reddish, R. Koganty, T. Wong, J. Samuel, M.B. Yacyshyn, A. Meikle, P.Y.S. Fung, J. Taylor-Papadimitriou, B.M. Longenecker, Cancer Immunol. Immunother. 36 (1993) 9--17). GZHI cells seed into the lungs of Balb/c mice following intravenous injection. The latter used the 4T1-MUC1 cell line, a MUC-1 transfectant of the mouse mammary carcinoma cell line 4T1, which metastasizes from a primary mammary fatpad (mfp) implant to the lungs (C.J. Aslakson, F.R. Miller, Cancer Res. 52 (1992) 1399--1405). B27.29, a monoclonal antibody against the MUC-1 antigen, was used to target sterically stabilized immunoliposomes (SIL[B27.29]) to tumour cells. In vitro, SIL[B27.29] showed high specific binding to both GZHI and 4T1-MUC1 cells. The IC(50) of DXR-loaded SIL[B27.29] was similar to that of free drug for GZHI cells. In the pseudometastatic model, mice treated with a single injection of 6 mg DXR/kg in DXR-SIL[B27.29] at 24 h after cell implantation had longer survival times than those injected with non-targeted liposomal drug. In the metastatic model, severe combined immune deficiency mice given weekly injectionsx3 of 2.5 mg DXR/kg encapsulated in either targeted or non-targeted liposomes were almost equally effective in slowing growth of the primary tumour and reducing development of lung tumours. Surgical removal of the primary tumour from mfp, followed by various chemotherapy regimens, was attempted, but removal of the primary tumour was generally incomplete; tumour regrowth occurred and metastases developed in the lungs in all treatment groups. DXR-SL reduced the occurrence of regrowth of the primary tumour, whereas neither targeted liposomal drug or free drug prevented regrowth. We conclude that monoclonal antibody-targeted liposomal DXR is effective in treating early lesions in both the pseudometastatic and metastatic models, but limitations to the access of the targeted liposomes to tumour cells in the primary tumour compromised their therapeutic efficacy in treating the more advanced lesions.

Tumour marker measurements in the diagnosis and monitoring of breast cancer

Elevation of established blood tumour markers correlates with the stage of breast cancer. The major role of current blood markers is therefore in the diagnosis and monitoring of metastatic disease. A combination of markers is better than a single marker with the most widely adopted combination being CEA and one MUC1 mucin, commonly detected as either CA15.3 or CA27.29. Tumour marker measurement is now used as a complementary test in the diagnosis of symptomatic metastases. In the monitoring of therapeutic response to both endocrine and cytotoxic therapies in advanced disease, biochemical assessment using blood markers not only correlates with conventional UICC criteria but has a lot of advantages which make it a potentially superior way of assessment. In this regard, CA15.3, CEA and ESR are the best validated combination. Studies are ongoing to evaluate the use of sequential blood tumour marker measurements in the follow-up of patients after treatment for their primary breast cancer, in terms of both early detection and early therapeutic intervention. Further randomized studies are also required to ascertain that marker-directed therapy is superior to the current practice for metastatic disease. In line with clinical studies, intensive laboratory work is being carried out to optimize the use of blood markers in advanced disease as well as to exploit their use in screening and diagnosis of early primary breast cancer.

MUC1 mucin as a tumour marker in bladder cancer

OBJECTIVE

To evaluate serum MUC1 levels (a high molecular weight glycoprotein which is upregulated and abnormally glycosylated in bladder cancer and other carcinomas) in patients with a variety of stages and grades of transitional cell carcinoma (TCC) of the bladder, to assess its potential as a tumour marker.

PATIENTS AND METHODS

Blood samples were taken before treatment in 87 patients with TCC of the bladder and in 31 controls undergoing cystoscopy for benign conditions. Serum MUC1 levels were estimated with an enzyme-linked immunosorbent assay using the C595 monoclonal antibody.

RESULTS

Of patients with T4 tumours, 47% had MUC1 levels above the normal range (P<0.001); patients with T3 tumours also had significantly higher MUC1 levels than controls. The overall sensitivity was only 24% for all tumours when the upper limit of normal was defined as 4.8 U/mL; the specificity was 97%.

CONCLUSION

Serum MUC1 is not as useful tumour marker for screening, as it has a low sensitivity. However, MUC1 levels are high in advanced disease and serum MUC1 levels may be useful for disease monitoring.

Specificity analysis of sera from breast cancer patients vaccinated with MUC1-KLH plus QS-21

The mucin MUC1 is expressed on breast cancers in an underglycosylated form compared to normal tissues and is therefore a potential target for cancer immunotherapy. MUC1 contains multiple tandem repeats of the 20 amino acid (aa) peptide (VTSAPDTRPAPGSTAPPAHG). The APDTRPA epitope is particularly immunogenic since it is recognized by a variety of murine monoclonal antibodies and by some sera and cytotoxic T-cells from unimmunized patients with epithelial cancers. We have prepared a 30 aa peptide (C)VTSAPDTRPAPGSTAPPAHGVTSAPDTRPA with cysteine at the N-terminal end, and used the cysteine for chemical conjugation to keyhole limpet haemocyanin (KLH). Six breast cancer patients immunized with this conjugate plus the immunological adjuvant QS-21 have all produced high titre (by ELISA) IgG and IgM antibodies against the 30 aa MUC1 peptide, but these sera reacted moderately, or not at all, with MUC1-positive tumour cells. To understand this specificity better, we prepared a series of smaller peptides to determine the epitopes recognized by these immune sera in inhibition assays. Only peptides containing APDTRPA at the C-terminal end were able to completely inhibit ELISA reactivity for the full 30 aa peptide. No sera were completely inhibited by APDTR, APDTRP, PDTRPA or any other peptides that did not contain the full APDTRPA epitope. Remarkably, sera from all six patients recognized this same epitope and were completely inhibited by only this epitope. The specificity of these sera (1) primarily for C-terminal APDTRPA, and the absence of this epitope at the C-terminal end of any tumour mucins, and (2) the N-terminal APDTRPA alanine, which is normally buried in the β turn MUC1 assumes in its secondary structure may explain the moderate to weak reactivity of these high titer sera against MUC1-positive tumour cells. © 1999 Cancer Research Campaign

Specificity analysis of sera from breast cancer patients vaccinated with MUC1-KLH plus QS-21

The mucin MUC1 is expressed on breast cancers in an underglycosylated form compared to normal tissues and is therefore a potential target for cancer immunotherapy. MUC1 contains multiple tandem repeats of the 20 amino acid (aa) peptide (VTSAPDTRPAPGSTAPPAHG). The APDTRPA epitope is particularly immunogenic since it is recognized by a variety of murine monoclonal antibodies and by some sera and cytotoxic T-cells from unimmunized patients with epithelial cancers. We have prepared a 30 aa peptide (C)VTSAPDTRPAPGSTAPPAHGVTSAPDTRPA with cysteine at the N-terminal end, and used the cysteine for chemical conjugation to keyhole limpet haemocyanin (KLH). Six breast cancer patients immunized with this conjugate plus the immunological adjuvant QS-21 have all produced high titre (by ELISA) IgG and IgM antibodies against the 30 aa MUC1 peptide, but these sera reacted moderately, or not at all, with MUC1-positive tumour cells. To understand this specificity better, we prepared a series of smaller peptides to determine the epitopes recognized by these immune sera in inhibition assays. Only peptides containing APDTRPA at the C-terminal end were able to completely inhibit ELISA reactivity for the full 30 aa peptide. No sera were completely inhibited by APDTR, APDTRP, PDTRPA or any other peptides that did not contain the full APDTRPA epitope. Remarkably, sera from all six patients recognized this same epitope and were completely inhibited by only this epitope. The specificity of these sera (1) primarily for C-terminal APDTRPA, and the absence of this epitope at the C-terminal end of any tumour mucins, and (2) the N-terminal APDTRPA alanine, which is normally buried in the beta turn MUC1 assumes in its secondary structure may explain the moderate to weak reactivity of these high titer sera against MUC1-positive tumour cells.

The epithelial mucin, MUC1, of milk, mammary gland and other tissues

MUC1 is a mucin-type glycoprotein that is integrally disposed in the apical plasma membrane of the lactating epithelial cell and protrudes from the cell surface into the alveolar lumen where milk is stored. Envelopment of milk fat globules by this membrane accomplishes their secretion and conveys MUC1 into milk. The human form of this mucin has been detected in many other organs, tissues and body fluids. It projects from the cell surface as long filaments. In the human and a number of other species, MUC1 is polymorphic due to variable numbers of a tandemly repeated segment 20 amino acids in length. The individual codominantly expresses two alleles for the mucin so that differences in its size among individuals and between the two forms of an individual are observed. The tandem repeats are rich in serines and threonines which serve as O-glycosylation sites. Carbohydrate content of MUC1, as isolated from milk of human, bovine and guinea pig, is approximately 50%. The oligosaccharides carry substantial sialic acid at their termini and this accounts for two putative functions of this mucin, i.e., to keep ducts and lumens open by creating a strong negative charge on the surface of epithelial cells which would repel opposite sides of a vessel, and to bind certain pathogenic microorganisms. MUC1 is protease resistant (trypsin, chymotrypsin and pepsin) and large fragments of it can be found in the feces of some but not all breast-fed infants. MUC1 has a highly varied structure because of its polymorphism, qualitative and quantitative variations in its glycosylation between tissues, individuals and species, and differences due to divergence in the nucleotide sequences among species. Sequencing of the MUC1 gene for various species is showing promise of revealing unique evolutionary relationships and has already indicated conserved aspects of the molecule that may be functionally important. Among these are positions of serine, threonine and proline in the tandem repeats and a high degree of homology in the transmembrane and cytoplasmic segments of the molecule.

CEA, MCA, CA 15.3 and CA 549 and their combinations in expressing and monitoring metastatic breast cancer: a prospective comparative study

Serum levels of carcinoembryonic antigen (CEA), mucin-like carcinoma-associated antigen (MCA), CA 15.3 and CA 549 were concurrently assayed in patients with metastatic breast cancer. Overall sensitivity in detecting metastatic breast cancer (201 pts) was CEA 45%, MCA 59%, CA 15.3 71% and CA 549 72% (P < 0.01). Sensitivity increased by only 6% to 8% when two or more antigens were simultaneously considered. An overall sensitivity of correlation with objective response (n = 71) was observed in the range of 53-67% (P = n.s.) in patients with abnormal baseline marker values, and in the range of 42-87% (P < 0.05) in patients with normal baseline values. The combination of two or more markers did not improve sensitivity, but decreased specificity of correlation with objective response. In conclusion, CA 15.3 and CA 549 have individually higher sensitivity in detecting metastatic breast cancer. No clinical advantage was observed for using two or more markers concurrently over CA 15.3 or CA 549 alone in the monitoring of metastatic breast cancer.

Assessment of five serum marker assays in patients with advanced breast cancer treated with medroxyprogesterone acetate

This study concerns five different tumour marker assays examined in the context of 94 patients with advanced breast cancer treated in a prospectively randomised trial of different doses of medroxyprogesterone acetate (MPA). MPA was administered at doses of 500 or 1000 mg daily and clinical evaluation of patients was carried out according to UICC criteria. Carcinoembryonic antigen (CEA) was selected as a standard marker, with three assays for MUC1 mucins (epithelial mucin core antigens (EMCA), EMCA2 and BR-MA immunoradiometric assay) differing in antibody specificities for different mucin epitopes. An additional novel assay for soluble cytokeratin was also evaluated as an example of an independent marker with a different nature and biology. Sensitivity of individual assays ranged between 44 (EMCA2) and 69% (cytokeratin) and the use of two assays in combination led to sensitivities as high as 84% (cytokeratin+BR-MA). The proportion of patients found to be assessable by each assay ranged between 51 (EMCA2) and 76% (cytokeratin). Of those patients whose marker changes were assessable, those receiving the higher dose of MPA displayed significant falls in marker levels after 12 weeks of treatment. This effect was not observed in patients receiving 500 mg. The change in cytokeratin levels in patients undergoing high dose MPA therapy proved to be most marked. Using the cytokeratin assay, 91% (of 23 patients) of patients with progressive disease showed at least a 25% rise in serum marker levels. Of these, 66% showed increases before disease progression was detected clinically with a mean lead time of 14 weeks. There was very little difference between the responses of the five tumour marker assays in patients with stable or responding disease, the proportion of these patients with stable or falling tumour marker levels ranging between 58% (CEA) and 77% (EMCA). We conclude that the cytokeratin assay has an application in monitoring response to therapy and predicting tumour progression in advanced breast cancer patients with assessable tumour marker profiles, especially if used in combination with a MUC1 mucin assay.

Circulating tumour-associated mucin concentrations, determined by the CASA assay, in healthy women

This investigation was undertaken to establish a reference range for tumour-associated MUC1 mucins in the serum of healthy women of the ages at risk for adenocarcinoma of the ovary and breast. Blood samples and clinical information were obtained from 5,000 women attending a breast screening mammography clinic. Data from women diagnosed with breast carcinoma and those subsequently diagnosed with other cancers were omitted from the reference range. Mucin concentrations were measured using the CASA assay which detects the protein core of MUC1 encoded mucins. Multiple linear regression analysis showed no effect on CASA concentrations by non-malignant changes to the breast, menopausal status, presence/absence of the reproductive tract, parity or history of hormone use. However, CASA concentrations were significantly increased in smokers (P < 0.001) and progressively increased with age (P < 0.001). These data show that these factors must be given consideration when setting upper limits of normal using MUC1 protein core binding assays.

Fine specificity of antibody recognition of carcinoma-associated epithelial mucins: antibody binding to synthetic peptide epitopes

The protein core of polymorphic epithelial mucins consists predominantly of a repeating 20 amino acid peptide motif. Many monoclonal antibodies reactive with breast carcinomas recognise determinants located within the mucin protein core, and epitope mapping techniques have demonstrated that these antibodies bind to epitopes of three, four or five amino acids within the hydrophilic sequence, P D T R P A P. Each of these mucin core-reactive antibodies map to epitopes containing the central arginine residue. The fine specificity of a panel of anti-mucin antibodies binding to the tetrameric peptides P D T R or R P A P (synthesised on the heads of polyethylene pins) was examined by systematically replacing each amino acid in turn with all other 19 natural amino acids, and then testing these analogues for antibody binding. We have (i) identified those amino acids in epitopes which are essential for antibody binding, (ii) shown that for each epitope there is a hierarchy of residues required for immune recognition--certain amino acids may be replaced with little or no loss of antibody binding, while the presence of others is essential, and (iii) concluded that antibody specificity is further regulated by the residue(s) flanking an epitope motif which may impose conformational constraints upon the presentation of the epitope to an antibody.

Clinical value of a mucin-like carcinoma-associated antigen in monitoring breast cancer patients in comparison with CA 15-3

A new tumour marker, mucin-like carcinoma-associated antigen (MCA), was evaluated in 176 breast cancer patients classified either as free of tumour (NED, n = 141) or as having metastases (PD, n = 35). During the 5 year follow-up, 842 measurements of MCA and 363 measurements of CA 15-3 were done. MCA levels were significantly increased in the PD group (P = 0.0001) but not in the NED group. The sensitivities of the MCA and the CA 15-3 assays were 84% and 78% and the specificities were 81% and 78%, respectively. The negative predictive value of 97% for MCA was significantly higher (P = 0.0001) than the 88% for CA 15-3. Thus the MCA enzyme immunoassay is at least equivalent to the CA 15-3 test and is recommended in the assessment of metastatic spread or tumour recurrence in breast cancer patients.

Reactivity of an anti-(human gastric carcinoma) monoclonal antibody with core-related peptides of gastrointestinal mucin

A murine anti-(human gastric carcinoma) monoclonal antibody, GL-013 (IgG1), which reacts with a high-molecular-mass glycoprotein from colorectal tumour tissue [Yang and Price (1989) Anticancer Res 9: 1707], was examined for reactivity against a panel of purified and partially purified antigens associated with tumours of the gastrointestinal tract. These included carcinoembryonic antigen (CEA), normal cross-reacting antigen, Y-hapten glycoproteins, and perchloric acid extracts and glycolipid preparations from colorectal tumours. While the GL-013 antibody failed to bind to these antigens, it was found to react strongly with synthetic peptides with sequences based upon that reported for the protein core of a human gastrointestinal mucin [Barnd et al. (1989) Proc Natl Acad Sci USA 86: 7159; Gum et al. (1989) J Biol Chem 264: 6480]. In control tests, a series of other anti-(colorectal tumour) antibodies (IgG1 and IgG3), with broad reactivity towards gastrointestinal carcinomas, as well as an anti-CEA antibody, (IgG1) failed to react with the synthetic peptides. It is concluded that the anti-(gastric carcinoma) monoclonal antibody GL-013 binds to a threonine-rich peptide epitope expressed within the protein core of gastrointestinal mucins.

Scanning tunneling microscopy imaging of the tumour associated antigenic 20 amino acid human polymorphic epithelial mucin core peptide fragment

Human polymorphic epithelial mucins (PEM) are high molecular weight glycoproteins that are associated with breast cancer. Recent structural studies have identified that the protein core of PEM contains a 20 amino acid tandem repeat that has elements of secondary structure which coincide with the epitopes for a number of tumour reactive antibodies. In our continuing structural studies we have now investigated the use of the scanning tunneling microscope (STM) to directly image the conformation of the twenty amino acid PEM core peptide. High resolution STM images reveal that the peptide has an overall topography similar to that predicted by molecular modelling. The images identify directly that the free peptide is conformationally non-restricted and can adopt a number of discrete conformations in the solid state.

Immunological and structural features of the protein core of human polymorphic epithelial mucin

The protein core of high mol. wt polymorphic epithelial mucin (PEM--approximately 400 kDa glycoprotein) which is associated with breast carcinomas, consists of a repeating 20 amino acid peptide motif [Gendler et al. (1988) J. biol. Chem. 263, 12,820-12,823]. Monoclonal antibodies C595 (anti-urinary mucin) and NCRC-11 (anti-breast carcinoma cells), and other antibodies against human milk fat globule membranes, were found to recognize determinants present within this 20 amino acid peptide. A model of the peptide was developed based on hydropathicity and structure prediction calculations and these indicated that the repeated structure is dominated by a hydrophilic domain of seven amino acids, extending into two flanking beta turns. NMR analysis of the 20 amino acid peptide was undertaken to probe the secondary structure. Epitope mapping experiments involving solid phase synthesis of overlapping heptapeptides in the repeat unit identified the minimum structures for antibody binding as Arg-Pro-Ala-Pro and Arg-Pro-Ala for the C595 and NCRC-11 antibodies, respectively. These determinants were found within the predicted hydrophilic turn region domain of the peptide. The epitopes for six other PEM-reactive monoclonal antibodies were also determined to reside within the predicted hydrophilic turn domain. This evidence is in accord with the disposition of this region of the PEM peptide core being at the exterior of the glycoprotein where it would be accessible to antibody recognition and binding events.

C595--a monoclonal antibody against the protein core of human urinary epithelial mucin commonly expressed in breast carcinomas

Urinary mucins which express determinants for the anti-breast carcinoma monoclonal antibody, NCRC-11 (IgM), closely resemble the mammary mucins found in milk fat globules and carcinomas. An IgG3 monoclonal antibody, C595, was prepared against urinary mucins isolated on a NCRC-11 antibody affinity column, and this 'second generation' antibody was shown to have a very similar pattern of reactivity to the original NCRC-11 antibody. By immunohistology, the profile of reactivity of both antibodies with tumour and normal tissue specimens was virtually identical. Both antibodies reacted with epithelial mucins isolated from breast tumours or normal urine using an NCRC-11 antibody affinity column, although the antibodies were unreactive with other antigen preparations. Heterologous immunoradiometric assays ('sandwich' tests) confirmed that NCRC-11 and C595 epitopes were co-expressed on the same molecule. C595 antibodies inhibited the binding of radiolabelled NCRC-11 antibodies to antigen, suggesting that the two epitopes were in close topographical proximity. The protein core of the mammary mucins has recently been shown to consist predominantly of a repeated 20 amino acid sequence (Gendler et al., 1988). Peptides with this complete sequence and small fragments were synthesised, and the C595 antibody was found to recognise an epitope within this repeat. The ability to identify and synthesise monoclonal antibody-defined determinants, as well as those in the adjacent or overlapping sequences within the protein core of epithelial mucins, is viewed as a strategy for facilitating the production of antibodies of new and novel specificity to complement the panels of existing anti-breast cancer reagents.

Elements of secondary structure in a human epithelial mucin core peptide fragment

The protein core of human epithelial mucin has previously been shown to consist of tandem repeats of a 20-amino-acid sequence that carries the epitopes for a number of tumour-marking monoclonal antibodies. High-field n.m.r. studies have now been undertaken on an 11-amino-acid fragment of this sequence dissolved in dimethyl sulphoxide. The studies reveal elements of secondary structure to be present: a type I beta-turn has been identified from Asp2 to Arg4 of this peptide, and this turn is extended by Pro5 being in the trans form. The observed turn region extends into the known epitopes for the antibodies C595 and NCRC-11 and may form the basis for how the antibodies recognize these peptides.