Abstract / Summary
HER2-directed therapies are expanding across solid tumors, but ERBB2 amplification incompletely captures HER2 expression and pan-tumor IHC lacks standardization. We developed ERBB2 mRNA, bulk HER2 protein and RNA-signature screening benchmarked against IHC/FISH. Screening identified HER2-enriched populations beyond amplification in a pan-tumor discovery cohort of 1,331 tumor biopsies and validated RNA-based screening in an external cohort of 3,898 tumor biopsies. All six objective responses among 51 treated patients in the discovery cohort occurred in tumors positive by ERBB2 mRNA and the combined RNA-signature. In the external dataset (n=102), ERBB2 mRNA positivity was associated with longer time on treatment (hazard ratio 0.42, 95% confidence interval 0.27-0.64) and overall survival (0.41, 0.25-0.65), with concordant associations for the combined RNA signature. Spatial proteomics comprising 2,721 tiles across 88 samples captured HER2 hetero-geneity and identified HER2-enriched profiles below bulk thresholds. These findings support prospective evaluation of RNA and proteomic screening for treatment selection.