Abstract P6-17-25: PAM50 and CGH-array genomic characterization of HER2-equivocal breast cancers defined by the ASCO/CAP2013 recommendations and response to treatment

Background HER2 breast cancer status determines patients' eligibility for targeted therapy. HER2 level of amplification is associated with a better response to anti-HER2 therapy (Arnould CCR 2007, Singer CCR 2017). Benefit of anti-HER2 therapy for equivocal cases remains debated. Objectives We aimed to better characterize HER2-equivocal breast cancers by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) according to 2013 ASCO/CAP guidelines using PAM50 gene expression-based molecular subtyping. We then investigated genome-wide copy number alterations of HER2-equivocal cases, assessing agreement between genomic alterations of the chromosome (chr) 17 and molecular subtypes. Methods PAM50 (nCounter assay; Nanostring) was performed on RNA from formalin-fixed paraffin-embedded samples of 40 HER2-equivocal cases. These cases were subsequently analyzed by Agilent 60-mer oligonucleotide microarrays for array-based comparative genomic hybridization (aCGH). Results The 40 HER2-equivocal cases were classified as Luminal B in 16 cases (40%), HER2-Enriched in 14 cases (35%), Luminal A in 9 cases (22.5%) and Basal-like in 1 case (2.5%) using PAM50. By IHC, 34 cases (85%) were ER+, 24 (60%) were also PgR+, 26 (65%) were grade III and 33 (82.5%) showed a high Ki67 > 20%. Using aCGH, 10 cases (25%) presented chr 17q large copy number gain, 10 (25%) showed segmental copy number gains including HER2, 9 (22.5%) showed HER2 amplification, one (2.5%) showed a large copy number loss and 10 cases (25%) didn't show any copy number alteration of the chr 17. Out of the 14 PAM50 HER2-Enriched cases, only 5 (35.7%) showed HER2 genomic amplification (Table 1). Four HER2 amplified cases at the genomic level were classified as Luminal B (3 cases, Ki67 > 20%, ER+, PgR- by IHC) or Luminal A (1 case, Ki6710% by IHC) using PAM50, although these luminal B tumors presented strong correlation with the HER2-Enriched centroid. In total, 13 cases (32.5%) were discordant between molecular classification and genomic alteration status of the chr 17. Among patients with early stage HER2-equivocal breast cancers (n=37), 2 received neo-adjuvant chemotherapy (5.4%), 25 received adjuvant chemotherapy (67.6%) and 2 received adjuvant trastuzumab (5.4%). With a median follow up of 5.8 years (3.8-6.9), one controlateral recurrence (2.7%), four metastatic recurrences (10.8%) and three deaths were observed (8.1%). Conclusion Using PAM50, the majority of HER2-equivocal cases were classified as Luminal tumors. At the genomic level, HER2-equivocal cases harbored mostly chr 17 segmental or large copy number gains. These results emphasized the need of HER2 status genomic determination. In line with the new ASCO 2018 recommendations intending to decrease the number of cases considered as HER2 equivocal, we showed that these tumors were mainly HER2 not amplified, ER positive and grade 3. There is no evidence of benefit of anti-HER2 therapy in these cases. Table 1Genomic alterations of chromosome 17Basal-likeHER2-EnrichedLuminal ALuminal BTotalHER2 amplified05139Large copy number gain013610Segmental copy number gain153110No alteration032510Large copy number loss00011Total11491640 Citation Format: Borcoman E, Ngo C, Rapinat A, Simaga F, Mariani O, Fuhrmann L, Jeannot E, Laé M, Pierga J-Y, Gentien D, Pierron G, Morel P, Brauer HA, Vincent-Salomon A. PAM50 and CGH-array genomic characterization of HER2-equivocal breast cancers defined by the ASCO/CAP2013 recommendations and response to treatment [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-17-25.