[Biological characteristics of triple negative breast cancer with low expression of HER2].

<b>Objective:</b> To investigate the biological characteristics of triple negative breast cancer (TNBC) with low expression of HER2 (HER2-low). <b>Methods:</b> A total of 93 TNBC cases in Shanxi Cancer Hospital from 2017 to 2019 were collected and divided into HER2-negative and HER2-low groups according to HER2 expression status. The clinicopathological features and prognostic differences between the two groups were retrospectively analyzed and compared, and genetic detection of tumor tissues was performed to clarify somatic mutation status and differences between the two groups. <b>Results:</b> Ninety-three patients aged 26 to 86 years were enrolled, including 60 patients in the HER2-negative group and 33 patients in the HER2-low group. The distribution of HER2-low in luminal androgen receptor (LAR) subtype (14/23, 60.87%) and non-LAR subtype (19/70, 27.14%) was significantly different (<i>P=</i>0.005). There were no significant differences in age, pT stage, histological grade, infiltration mode, lymph node metastasis and survival analysis. The expression of HER2-low in the tumor was heterogeneous, including different proportions of weak, weak to moderate intensity, and incomplete to intact membrane staining. With the change of the proportion of HER2-positive cells, the different distribution of those cells in the total tumor cells was noted, including cluster, mosaic and scattered patterns. The concentration and quality of DNA extracted from 71 of the 93 samples met the requirements for making libraries, including 43 in the HER2-negative group and 28 in the HER2-low group. Genetic mutations were mainly missense mutations, single nucleotide mutations, and point mutations in which base C was replaced by base T. There was no significant difference in genes with mutation frequency>3 times between the two groups. CTNNB1 and FGFR3 genes were only mutated in HER2-low group; while ALK, CYP2D6 and FAT1 genes were only mutated in HER2-negative group. HER2-low group included 18 HER2 1+ cases and 10 HER2 2+ cases. Genes with mutation frequency>3 times between the two groups included PIK3CA, TP53, SLX4, ATM and BRCA1. The mutation frequency of PIK3CA in HER2 2+ was significantly higher than that in HER2 1+ group (<i>P</i><0.05), and SLX4 gene was only mutated in HER2 1+ group. <b>Conclusions:</b> There are some differences of histological morphology and genetic variation between HER2-negative group and HER2-low group, and also differences in genetic variation between HER2 1+ and HER2 2+ in HER2-low group, which are helpful for more accurate stratification of TNBC and useful for finding the therapeutic target and precise treatment of HER2-low TNBC.