HER2 FISH results in breast cancers with increased CEN17 signals using alternative chromosome 17 probes - reclassifying cases in the equivocal category.

Abstract

HER2 testing of invasive breast cancer by in-situ hybridization guides therapy decisions. Probing HER2 and centromere of chromosome 17 (cen17) simultaneously is supposed to reveal both a potential HER2 gene amplification and polysomy 17. However, a considerable number of breast cancer patients with quasi polysomy 17 are considered 'equivocal', which is diagnostically meaningless. Moreover, patients with equivocal/false polysomic tumours are prevented from a potentially beneficial anti-HER2 treatment. Here we evaluated the RAI1, D17S122 and TP53 hybridization markers to indicate true polysomy reliably and to reclassify equivocal samples accurately as HER2-positive. Samples with (n = 82) and without (n = 52) increased cen17 copy numbers and 78 evidently HER2-amplified specimens were analysed using dual and triple marker hybridization probes. Selected putative polysomic samples were subjected to array-based comparative genomic hybridization (aCGH). We found that 37.8% samples with putative polysomy 17 did not show any gain in RAI1, D17S122 or TP53. Accordingly, aCGH revealed evidence for the presence of HER2/cen17 co-amplification rather than for true polysomy in those cases. Reflex testing using alternate 17p markers reclassified up to 56.3% equivocal cases as HER2-positive and the combined assessment of a 17p and 17q locus allows the differentiation of true versus false polysomy. An increased cen17 copy number does not necessarily reflect polysomy, and reflex testing facilitates the reclassification of 'equivocals'. Nevertheless, the prognostic and predictive value of a HER2/cen17 co-amplification versus HER2 gene amplification alone must still be evaluated prospectively.