Table 1 List of molecular alterations in MpBC

Epithelial-to-mesenchymal transition (EMT)

-EMT core genetic signature shares similarity to core geneticsignatures of claudin-low and metaplastic breast cancers [20].

-MpBC tumors have more stem-like features, express highlevels of EMT markers, and share a similar genetic signature totumor-initiating cell (TIC) genetic signature [21].

-TICs are prominent following endocrine/chemotherapy, morechemoresistant, exhibit EMT, and can undergo self-renewal [22].

Taube et al. [20]

244

Hennessy et al. [21]

28

Epidermal growth factor receptor(EGFR) signaling pathway

-34% of MpBC cases exhibit EGFR gene amplification associatedwith gene overexpression and no EGFR activating mutations [23, 24].

-Fluorescent in situ hybridization showed high EGFR copynumber secondary to aneusomy (22%) and amplification (4%) [25].

-Majority of MpBC is positive for p63 (59%), cytokeratin 5/6 (58%),KIT (24%), and EGFR (66%) overexpression [25].

Reis-Filho et al. [23]

25

Reis-Filho et al. [24]

47

Gilbert et al. [25]

38

Phosphoinositide 3-kinase (PI3K)signaling pathway

-47% of MpBC tumors harbor PIK3CA mutations and 5% have PTENdeletions [21]. 36.4% of HR+ breast cancers have PIK3CA mutations [26].

-Whole-exome sequencing analysis of MpBC tumors showedthe most altered genes were PIK3CA (29%), PIK3R1 (11%), FAT1(11%), ARID1A (11%), and PTEN (11%) [27].

-Next-generation sequencing of MpBC tumors showed themost commonly altered genes were TP53 (68.4%), PIK3CA(42.1%), and PTEN (15.8%) [28].

Hennessy et al. [21]

28

Razavi et al. [26]

1918

Ng et al. [27]

35

Afkhami et al. [28]

21

Nitric oxide synthase (NOS)signaling pathway

-TNBC expresses high levels of nitric oxide (NO) than HER2+or luminal breast cancers and enhanced inducible nitric oxidesynthase (iNOS) expression is associated with worse prognosisand may confer resistance to chemotherapy [29].

-Inhibition of iNOS via L-NMMA in combination with docetaxelis more effective than docetaxel alone in enhancing tumorapoptosis, cell proliferation/migration, and reducing tumor-initiating capacity in TNBC and MpBC models.

-39/40 (97.5%) of MpBC tumors harbor a RPL39 A14V oncogenicmutation, which is associated with enhanced NO activity, cancercell stemness, and lung metastasis [30].

Granados-Principalet al. [29]

83

Dave et al. [30]

40

Wnt/β-catenin signaling

-Immunohistochemistry (IHC) of MpBC samples showed aberrant β-catenin expression in 33/36 (92% of cases), and mutational analysisshowed that 25.9% of MpBC tumors had CTNNB1 missense mutations,7.4% tumors had APC mutations, and 18.5% tumors had WISP3mutations [31].

-IHC of MpBC tumor samples reveals that β-catenin expressionhas more focal nuclear localization [32].

-MpBC tumors commonly harbor mutations in Wnt/β-cateninsignaling and PI3K/Akt signaling than TNBC tumors [27].

-The levels of CCN6 are low in MpBC, leading to enhancedinsulin-like growth factor 1 levels, EMT, invasion, metastasis, andbone morphogenic-4 signaling [8].

-A mouse model of mammary epithelium-specific Ccn6 proteindeletion (MMTV-cre;Ccn6^fl/fl) has been developed, which canrecapitulate many features of human spindle MpBC tumors [17, 33].

Hayes et al. [31]

26

Lacroix-Triki et al. [32]

52

Ng et al. [27]

35

Martin et al. [33]

–

Programmed cell death protein 1 (PD-1)/programmed death ligand-1 (PD-L1)

-PD-L1 is expressed more in MpBC tumors (46%) relative to otherbreast tumor types (6% in HR+ and 9% in HER2+ breast cancers [34].

-Another study performed PD-L1 immunohistochemical staining of 21MpBC tumor samples and found that PD-L1 expression was associatedwith a worse RFS and OS [28].

-A patient with metastatic MpBC (PD-L1+ and with PIK3CAH1047L mutation) showed a dramatic response topembrolizumab in combination with nab-paclitaxel [35].

Joneja et al. [34]

290

Afkhami et al. [28]

21

Adams 2017 [35]

1

Cell cycle regulation

-MpBC tumors harbor a high frequency of TP53 (64%) and TERT(catalytic subunit of telomerase) promoter mutations (25%) [36]

-TERT mutations are commonly found in the spindle andsquamous MpBC [36]

-Myoepithelial MpBC shows a 9p21.3 chromosomal loss, including lossof genes CDK2NA and CDK2NB, which code for cyclin-dependent kinaseinhibitors p16^INK4a and p15^INK4b [37].

-64.3% of myoepithelial MpBC tumors with 9p21.3 loss alsohad a PIK3CA mutation [37].

Krings and Chen [36]

28

Bartels et al. [37]

34