Activating ERBB2/HER2 mutations indicate susceptibility to pan-HER inhibitors in Lynch and Lynch-like colorectal cancer.

Mismatch repair (MMR)-deficient colorectal cancer (CRC) with underlying Lynch or Lynch-like syndrome is generally characterised by the absence of BRAFV600E. Unresponsiveness of BRAFV600E-mutated CRC to BRAF inhibition could be overcome by combined inhibition of the BRAF/EGFR-axis. Concomitant microsatellite and mutational analyses enable a more comprehensive characterisation in routine clinical practice. Lynch and Lynch-like CRC frequently harbour activating ERBB2 mutations, but also specific mutational patterns in PIK3CA and KRAS. ERBB2-mutated microsatellite unstable CRC is susceptible to irreversible pan-HER blockade. Our findings suggest that a more comprehensive characterisation of Lynch and Lynch-like CRC needs to include mutational analysis of ERBB2. Patients suffering from this molecular subtype should be included in ongoing studies evaluating the effectiveness of HER-targeting inhibitors.

OBJECTIVE: Microsatellite instability (MSI) is detected in approximately 15% of all colorectal cancers (CRC) and virtually in all cases with Lynch syndrome. The MSI phenotype is caused by dysfunctional mismatch repair (MMR) and leads to accumulation of DNA replication errors. Sporadic MSI CRC often harbours BRAF(V600E); however, no consistent data exist regarding targeted treatment approaches in BRAF(wt) MSI CRC. DESIGN: Mutations and quantitative MSI were analysed by deep sequencing in 196 formalin fixed paraffin embedded (FFPE) specimens comprising Lynch and Lynch-like CRCs from the German Hereditary Nonpolyposis Colorectal Cancer registry. Functional relevance of recurrent ERBB2/HER2 mutations was investigated in CRC cell lines using reversible and irreversible HER-targeting inhibitors, EGFR-directed antibody cetuximab, HER2-directed antibody trastuzumab and siRNA-mediated ERBB2/HER2 knockdown. RESULTS: Quantification of nucleotide loss in non-coding mononucleotide repeats distinguished microsatellite status with very high accuracy (area under curve=0.9998) and demonstrated progressive losses with deeper invasion of MMR-deficient colorectal neoplasms (p=0.008). Characterisation of BRAF(wt) MSI CRC revealed hot-spot mutations in well-known oncogenic drivers, including KRAS (38.7%), PIK3CA (36.5%), and ERBB2 (15.0%). L755S and V842I substitutions in ERBB2 were highly recurrent. Functional analyses in ERBB2-mutated MSI CRC cell lines revealed a differential response to HER-targeting compounds and superiority of irreversible pan-HER inhibitors. CONCLUSIONS: We developed a high-throughput deep sequencing approach for concomitant MSI and mutational analyses in FFPE specimens. We provided novel insights into clinically relevant alterations in MSI CRC and a rationale for targeting ERBB2/HER2 mutations in Lynch and Lynch-like CRC.