Abstract 2444: Targeting the cell-bound MUC1-SEA module with monoclonal antibodies and peptides to ablate MUC1+ malignancies

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC The MUC1 oncoprotein is overexpressed in a variety of high-incidence malignancies including breast, colon, and pancreatic carcinoma and by the malignant plasma cells of myeloma. It is a heterodimer composed of a large extracellular α chain that is noncovalently bound to a membrane-bound β subunit. The α-chain's ‘on-again-off-again’ interaction with the β-chain results in intracellular signaling. A variety of anti-MUC1 antibodies have been described to date, almost all of which recognize the immunogenic tandem-repeat-array of the MUC1 α-subunit which is shed into the peripheral circulation. Because it circulates freely, the α subunit actively sequesters anti-α chain Abs, precluding their even reaching and adequately targeting MUC1+ malignant cells. To circumvent this problem we identified the MUC1 SEA module that comprises the junction of the extracellular α-subunit with the cell-bound β-subunit as a valuable cancer cell target since it invariably remains attached to the cell surface. We generated a series of anti-MUC1 α/β junction IgGγ1 monoclonal Abs, denominated DMB mAbs, which bind with exceptionally high affinity to MUC1-expressing human breast cancer cells. The antibodies were non-reactive when used as probes in Western blot analyses of the MUC1 protein resolved on denaturing SDS gels, suggesting that the epitopes recognized by the DMB mAbs are configurational. The novel mAbs bound well to MUC1-expressing cancer cells such as T47D and ZR-75 and to the extracellular domain of the MUC1/X protein which comprises only the SEA module that is fused C-terminally to the thirty N-terminal amino acids of the MUC1 protein. The DMB mAbs were much less reactive with recombinant SEA module alone, which corresponds to the MUC1/X extracellular domain that lacks the 30 N-terminal amino acids. Unexpectedly, all DMB mAbs showed high reactivity with mutant ‘SEA module 4G’ protein that comprises an additional 4 glycine residues upstream to the GSVVV cleavage site resulting in a non-cleaved protein. From these results we conclude that the native structure of the cleaved SEA module associated with the MUC1/X protein is different to that of the SEA module that lacks the N-terminal 30 amino acids, and more similar to mutant ‘SEA module 4G’ protein. To reconcile these seemingly contradictory findings, we hypothesized that the N-terminal portion of the MUC1 protein binds to the SEA module, thereby forming epitopes recognized by the DMB mAbs. Based on these structural findings, small peptides were derived from the MUC1 N-terminus and appear to selectively bind the MUC1 SEA module. We conclude that (1)The anti-MUC1 α/β junction Abs and peptides described here bind to specific configurationally-determined MUC1 α/β junction epitopes, and (2)Abs and peptides targeting those epitopes are likely to prove biologically-significant and clinically useful in selectively targeting and killing MUC1+ malignant cells. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2444.