Determining true HER2 status in breast cancers with polysomy using alternative chromosome 17 reference genes: Implications for trastuzumab therapy.

Abstract

10549 Background: Detection of HER2 gene amplification by FISH is a key test to predict response of breast cancer to trastuzumab. The ratio of HER2 to CEP17 signals is often used to determine HER2 status, based on the assumption that centromeric probe to CEP17 indicates the chromosome 17 copy number. This study tests the hypothesis that increased copy numbers of CEP17 may not reflect the presence of polysomy and that the use of other genes on chromosome 17 might more accurately serve as reference genes to determine HER2 gene status. Methods: 122/5683 (2%) of HER2 FISH cases performed between October 2007 and December 2009 with CEP17 copy numbers 2.6 and above were selected for additional gene mapping studies using probes for other chromosome 17 genes, including SMS, RARA, and TP53. Revised HER2/chromosome-17-reference ratios utilizing these alternative loci were calculated. Results: 95/122 cases (78%) demonstrated at least one chromosome 17 gene locus with eusomic copy number. Using these new reference genes, 63 of 95 cases (66%) would have HER2 amplification status changed (negative to positive, equivocal to positive or negative to equivocal), with 46 cases (48%) now considered eligible for trastuzumab therapy. The cases were further divided into three groups based on level of HER2 signals (high [>6], intermediate [4-6], borderline/low [<4]) for analysis. The number of cases having HER2 status upgraded in these groups (high, intermediate and border/low) were 14/45 (31.1%), 43/50 (86%) and 4/27 (14.8%) respectively. Conclusions: Our results support recent findings of CGH studies suggesting that true polysomy 17 is rare in breast cancer, and that the CEP17 region is subject to chromosomal gains similar to other regions. Performing additional FISH using probes to the SMS, RARA and TP53 genes in order to obtain a HER2/chromosome-17-reference ratio is an effective way to determine the true HER2 amplification status in these