Aims HER2NEU gene amplification is present in the majority of invasive breast carcinomas that have HER2 protein overexpression. A subset of breast cancers harbour an increased chromosome 17 (CEP17) copy number (polysomy 17). We investigated the clinicopathologic significance of polysomy 17 in correlation with various histological parameters and HER2NEU gene amplification.

Methods We collected the surgical specimens of 266 consecutive cases of primary invasive breast carcinomas. HER2NEU gene status and CEP17 copy numbers were assessed by fluorescent in situ hybridisation (FISH). Chromosome 17 polysomy was determined by the presence of ≥3 average CEP17 signals per nucleus.

Results 63 tumours (23.7%) harboured polysomy 17. Carcinomas with polysomy 17 were associated with adverse histological indicators including high histological grade, high nuclear grade, poor Nottingham Prognostic Index, advanced local tumour extent and progesterone receptor negativity. Polysomy 17 was common to HER2NEU amplified and unamplified tumours, and more frequently observed in HER2NEU unamplified (71.4%) cases.

Conclusions In the absence of the gene amplification, HER2 protein overexpression may be explained by other mechanisms including polysomy 17.