Overexpression of the EGFR is commonly caused by EGFR gene amplification and is sometimes associated with expression of a variant EGFR (de2-7 EGFR or
EGFRvIII) bearing an internal deletion in its extracellular domain.
mAb 806 is a novel EGFR antibody with significant antitumor activity that recognizes both the de2-7 EGFR and a subset of the wild-type (wt) EGFR when overexpressed, but does not bind the EGFR expressed in normal tissues. Recently, we demonstrated that the
mAb 806 epitope is restricted to a short
cysteine loop of the EGFR (
amino acids 287-302) that is only available for antibody binding in a transitional form of the receptor, which occurs as the receptor changes from its inactive tethered conformation to a dimeric untethered form. The truncation associated with the de2-7 EGFR mutation renders this receptor constitutively untethered, leading to increased binding of
mAb 806. We now show that
mAb 806 preferentially binds the immature high-
mannose wt and de2-7 EGFR precursors normally located in the endoplasmic reticulum, indicating that this form of the wt EGFR is also constitutively untethered. Using the unique specificity of
mAb 806, we clearly demonstrated the presence of these high-
mannose EGFR precursors on the cell surface. Given that the high-
mannose forms of the wt EGFR must be untethered they may contribute to the spontaneous EGFR signaling reported in cells overexpressing the receptor. These precursor forms of the EGFR thus represent novel
tumor targets and contribute to the exceptional selectivity of
mAb 806 for EGFR when overexpressed in
cancer cells. As our observations are likely to apply to other receptors overexpressed in
cancer, they suggest a strategy for developing antitumor
antibodies even when the target receptor is expressed in normal tissue.