Currently, breast cancer patients are classified uniquely according to the expression level of
hormone receptors, and human epidermal growth factor receptor 2 (HER2). This coarse classification
is insufficient to capture the phenotypic complexity and heterogeneity of the disease. A methodology
was developed for absolute quantification of receptor surface density ρR, and molecular interac tion (dimerization), as well as the associated heterogeneities, of HER2 and its family member, the
epidermal growth factor receptor (EGFR) in the plasma membrane of HER2 overexpressing breast
cancer cells. Quantitative, correlative light microscopy (LM) and liquid-phase electron microscopy
(LPEM) were combined with quantum dot (QD) labeling. Single-molecule position data of receptors
were obtained from scanning transmission electron microscopy (STEM) images of intact cancer cells.
Over 280,000 receptor positions were detected and statistically analyzed. An important finding was
the subcellular heterogeneity in heterodimer shares with respect to plasma membrane regions with
different dynamic properties. Deriving quantitative information about EGFR and HER2 ρR, as well as
their dimer percentages, and the heterogeneities thereof, in single cancer cells, is potentially relevant
for early identification of patients with HER2 overexpressing tumors comprising an enhanced share
of EGFR dimers, likely increasing the risk for drug resistance, and thus requiring additional targeted
therapeutic strategies