Multiplex Ligation-dependent Probe Amplification Can Clarify HER2 Status in Gastric Cancers with "Polysomy 17"

Therapy with trastuzumab confers a survival benefit in HER2 positive advanced gastric and gastroesophageal adenocarcinoma. HER2 status is evaluated by immunohistochemistry (IHC) and in situ hybridization (ISH). An ISH ratio of HER2 to centromere 17 (CEP17) ≥2.0 is considered amplified. This assumes that CEP17 reflects chromosomal copy number. Cases where CEP17 exceeds 3 are classified as polysomic, but it's unknown if they represent true polysomy or centromeric amplification. This has implications on the validity of current ISH criteria. Multiplex ligation-dependent probe amplification (MLPA) allows simultaneous quantification of multiple loci and can distinguish between true polysomy and centromeric amplification. We selected 13 gastric cancers with CEP17 counts ≥3.0 (polyCEP17), and 8 non-polyCEP17 gastric cancer controls. Silver ISH for HER2 and CEP17 were performed and scored by manufacturer guidelines. We also performed an MLPA HER2 assay that evaluates 22 genes on chromosome 17. MLPA identified HER2 amplification in 7 polyCEP17 cases compared to 2 identified by ISH. Overall, 9 of 13 polyCEP17 cases had amplification of the peri-centromeric gene WSB1, compared to 1 of 8 non-polyCEP17 controls (p=0.02). This could account for ISH CEP17 counts ≥3.0. MLPA did not show any cases of complete chromosome 17 duplication and peri-centromeric amplification can explain most cases of ISH polyCEP17. Current ISH criteria may under-diagnose HER2 amplification in polyCEP17 cases due to flawed assumptions about polysomy. MLPA can detect HER2 amplification missed by IHC and ISH, and thus may be an effective ancillary technique in evaluating HER2 status.