HER2-low breast cancers: incidence, HER2 staining patterns, clinicopathologic features, MammaPrint and BluePrint genomic profiles.

Recently, clinical trials have demonstrated promising efficacy for novel HER2-targeted therapies in HER2-low breast cancers, raising the prospect of including a HER2-low category (immunohistochemical [IHC] score of 1+, or 2+ with non-amplified in-situ hybridization [ISH]) in the HER2 evaluation of breast cancers. In order to better understand this newly-proposed HER2 category, we investigated the incidence, HER2 staining patterns, clinicopathologic features, and genomic profile of HER2-low breast cancers. HER2-stained slides of 281 consecutive breast cancers were re-reviewed and the clinicopathologic information, MammaPrint, and BluePrint results of these cases were retrospectively analyzed. HER2-low breast cancers were identified in 31% of cases and were more common in estrogen receptor (ER)-positive than ER-negative breast cancers (33.6% vs 15%, p = 0.017). HER2-low cancers were generally clinical stages I-II (79%), ER-positive (93.1%), had homogenous HER2 staining (59.2%), HER2 IHC score of 1+ (87.4%), ductal phenotype (81.6%), histologic grades of 1 or 2 (94.2%) and luminal molecular subtypes (94.3%). Three HER2-low patients received neoadjuvant chemotherapy and none of them achieved pathologic complete response. When compared to HER2-negative (IHC of 0+) and HER2-positive (IHC of 3+ or IHC of 2+ with amplified ISH) cancers, HER2-low breast cancers had significantly lower Ki-67 (p = 0.03 and p < 0.01, respectively) and higher ER positivity (p = 0.01 and p = 0.03, respectively). HER2-low breast cancers were less likely to be basal molecular subtype when compared to HER2-negative cancers (p < 0.01) and were less likely to have a HER2 molecular subtype when compared to HER2-positive cancers (p < 0.01). When adjusted for ER status, there was no significant difference on all the examined variables between HER2-low and HER2-negative groups. Our study provides valuable baseline characteristics of HER2-low breast cancers deriving from consecutive, real-world cases with a consensus confirmation of HER2 status, and would help to increase our understanding of this newly-proposed HER2 category in breast cancers.