The characteristics and clinical significance of atypical mitosis in breast cancer

Abstract

Atypical mitosis is considered a feature of malignancy, however, its significance in breast cancer (BC) remains elusive. Here, we aimed to assess the clinical value of atypical mitoses in BC and to explore their underlying molecular features. Atypical and typical mitotic figures were quantified and correlated with clinicopathological variables in a large cohort of primary BC tissue sections (n = 846) using digitalized hematoxylin and eosin whole-slide images (WSIs). In addition, atypical mitoses were assessed in The Cancer Genome Atlas (TCGA) BC dataset (n = 1032) and were linked to the genetic alterations and pathways. In this study, the median of typical mitoses was 17 per 3 mm² (range 0-120 mitoses), while the median of atypical mitoses was 4 (range 0-103 mitoses). High atypical mitoses were significantly associated with parameters characteristic of aggressive tumor behavior. The total number of mitoses, and a high atypical-to-typical mitoses ratio (>0.27) were associated with poor BC specific survival (BCSS), (p = 0.04 and 0.01, respectively). The atypical-to-typical mitoses ratio dichotomized triple negative-BC (TNBC) patients into two distinct groups in terms of the association with the outcome, while the overall number of mitoses was not. Moreover, TNBC patients with high atypical-to-typical mitoses ratio treated with adjuvant chemotherapy were associated with shorter survival (p = 0.003). Transcriptomic analysis of the TCGA-BRCA cohort dichotomized based on atypical mitoses identified 2494 differentially expressed genes. These included genes linked to pathways involved in chromosomal localization and segregation, centrosome assembly, spindle and microtubule formation, regulation of cell cycle and DNA repair. To conclude, the atypical-to-typical mitoses ratio has prognostic value independent of the overall mitotic count in BC patients and could predict the response to chemotherapy in TNBC.

Fig. 1. Features of mitosis.

Morphological features of typical and atypical mitosis (40×); a a prophase shows round hyperchromatic structure with hairy-like appearance, b metaphase shows curvilinear structure with slightly eosinophilic cytoplasm, c anaphase shows 2 pulled away uniform chromatin masses, d telophase shows 2 uniform completely separate cells; e lag atypical mitosis shows 2 unattached chromosomes at each side of metaphase plate; f tripolar atypical mitosis shows three poles with hairy outline and slightly granular eosinophilic cytoplasm; g polar asymmetry shows either 2 pulled- away chromatid unequal in size, enlarged size and slightly eosinophilic cytoplasm or h two non-uniform separated cells; i anaphase bridge shows a string of chromatin extending from one pole of the anaphase to the other connected poles; j dispersed mitosis shows non-clumped multiple chromosomes with slightly eosinophilic cytoplasm and enlarged in size; k ring atypical mitosis shows ring-like structure with hairy outline; l lag atypical mitosis shows one un-attached chromosomes at one side of metaphase plate; m–t other forms of atypical mitoses showing asymmetry of the chromatin masses within the cells.

Fig. 2. Association of atypical-to-typical mitosis ratio with outcome.

Kaplan–plots showing associations of atypical-to-typical mitoses ratio with a Breast Cancer Specific Survival (BCSS) and with b Distant Metastasis Free Survival (DMFS).

Fig. 3. Association of atypical-to- typical mitosis ratio with outcome in different molecular classes.

a, b Kaplan–Meier plots showing associations of atypical-to-typical mitoses ratio with BCSS in luminal while c, d association of high ratio with both poor BCSS and DMFS in triple negative breast cancer subtype.

Fig. 4. Association of atypical-to- typical mitosis ratio with chemotherapy response.

Kaplan–Meier analysis showing a significant association between BCSS and atypical-to-typical mitoses ratio in TNBC patients who received chemotherapy while there was no association between overall mitoses and BCSS in those patients (b).

Fig. 5. Differential expression genes.

Venn diagram of obtained Differentially expressed genes (DEG) shows a the overlap between the higher expressed genes associated with atypical and typical mitoses while b) the overlap between the lower expressed genes (DEGs) associated with atypical and typical mitoses. Differentially expressed genes were chosen based on fold change (≥±1) combined with False discovery rate (FDR) < 0.05.