ER Negative Breast Cancer and miRNA: There Is More to Decipher Than What the Pathologist Can See!

Breast cancer (BC), the most prevalent cancer in women, is a heterogenous disease. Despite advancements in BC diagnosis, prognosis, and therapeutics, survival rates have drastically decreased in the metastatic setting. Therefore, BC still remains a medical challenge. The evolution of high-throughput technology has highlighted gaps in the classification system of BCs. Of particular interest is the notorious triple negative BC, which was recounted as being heterogenous itself and it overlaps with distinct subtypes, namely molecular apocrine (MA) and luminal androgen (LAR) BCs. These subtypes are, even today, still misdiagnosed and poorly treated. As such, researchers and clinicians have been looking for ways through which to refine BC classification in order to properly understand the initiation, development, progression, and the responses to the treatment of BCs. One tool is biomarkers and, specifically, microRNA (miRNA), which are highly reported as associated with BC carcinogenesis. In this review, the diverse roles of miRNA in estrogen receptor negative (ER−) and androgen receptor positive (AR+) BC are depicted. While highlighting their oncogenic and tumor suppressor functions in tumor progression, we will discuss their diagnostic, prognostic, and predictive biomarker potentials, as well as their drug sensitivity/resistance activity. The association of several miRNAs in the KEGG-reported pathways that are related to ER-BC carcinogenesis is presented. The identification and verification of accurate miRNA panels is a cornerstone for tackling BC classification setbacks, as is also the deciphering of the carcinogenesis regulators of ER − AR + BC.


Introduction
Breast cancer (BC) is depicted as the most common cancer in women, with an estimated number of 2.3 million new cases worldwide in 2020 [1]. This incidence is predicted to increase in the next 15 years due to cancer screening tests, but also because of growing risk factors like increases in excess body weight [2,3]. A recent analysis of United States (US) cancer data, by The American Society of Cancer, revealed a slow increase in BC incidence (0.5% per year) since the mid-2010s. In parallel, for 30 years, female BC mortality has decreased, and this is mainly because of earlier diagnoses and improved treatments; however, this effect has been slowing in the last few years. Thus, BC remained as among the first causes of worldwide cancer deaths in 2020 [1], with 43.2 thousand estimated deaths in the US for 2022 [4]. Although the 5-year relative survival rate of BC is 90%-constituting The luminal A-like tumors have clear prognostic and treatment implications as they proliferate less and are endocrine sensitive, thus it confers better prognosis but have a poor response to chemotherapy [13]. Luminal B-like tumors are of a higher Ki67 expression and grade, and they have less endocrine sensitivity and poorer prognoses [13,14]. HER2 overexpression leads to bad prognosis but also to a better prediction of the response to anti-HER2 therapies, which drastically improves patient survival. However, the nonluminal HER2+ group is fast growing, more aggressive, and presents a worse prognosis than luminal groups [15]. Finally, TNBCs-which account for 20% of BCs and is defined by the absence of the three major receptors of ER, PR, and HER2-present with an aggressive behavior that have a high proliferation and the most pejorative survival rates [13]. Moreover, as defined by what they are not, TNBCs remain a highly heterogeneous subgroup that need to be better characterized.
Importantly, the accurate definition of BC is necessary for proper diagnoses and treatment strategies. The huge heterogeneity of this disease is described in the WHO tumor classification [16], which was updated in 2019 [11].

Triple Negative Breast Cancers: What Are They?
TNBCs are characterized by clinical and pathological differences, as well as by distinct molecular expression profiles that translate into distinct behaviors and responses to chemotherapy. In general, TNBCs exert higher risks of recurrence with the emergence of brain and lung metastases that occur more frequently than bone metastasis when compared to other breast subtypes. Also, TNBC metastatic diseases appears rapidly within the first 3 years after diagnosis, thus leading to bad prognosis. However, when patients do not recur during this time, the survival rate is comparable to ER+ BC. Moreover, 30-40%

Triple Negative Breast Cancers: What Are They?
TNBCs are characterized by clinical and pathological differences, as well as by distinct molecular expression profiles that translate into distinct behaviors and responses to chemotherapy. In general, TNBCs exert higher risks of recurrence with the emergence of brain and lung metastases that occur more frequently than bone metastasis when compared to other breast subtypes. Also, TNBC metastatic diseases appears rapidly within the first 3 years after diagnosis, thus leading to bad prognosis. However, when patients do not recur during this time, the survival rate is comparable to ER+ BC. Moreover, 30-40% of TNBC patients experience a pathological complete response (pCR) after neoadjuvant chemotherapy, and this constitutes a strong surrogate marker for overall survival. Therefore, it is clear that TNBCs are not a single clinico-pathological entity, but they need a better characterization of their more homogenous entities for the optimization of treatment.
Several gene expression studies have tried to dissect this heterogeneous group [17][18][19]. Initially, Lehmann et al. described six subgroups of TNBCs: basal-like 1 (BL1), basal-like 2 (BL2), immunomodulatory (IM), mesenchymal-(M), mesenchymal stem-like (MSL) and luminal androgen receptor (LAR) [17,20]. Finally, after the removal of immunological and stromal expression signals, this classification was refined into four tumor-specific subtypes (TNBCtype-4): BL1, BL2, M, and LAR. These subtypes have clear differences in their responses to chemotherapy [20]. Nevertheless, this subtyping is not currently used in routine practice. Moreover, the LAR subtype, with luminal characteristics but androgen receptor (AR) overexpression, should certainly be considered differently. In addition, the 2019 WHO classification recognized the existence of an ER− subtype, but AR+ mammary carcinoma was categorized as a distinct type of BC [7].

Apocrine Carcinoma: Just a Histology or a Molecular Entity?
Historically, breast apocrine carcinomas were defined by their particular morphological and histological appearances, with their tumor cells possibly presenting abundant granular cytoplasm, central nuclei positions, prominent nucleoli, and gross cystic disease fluid protein-15 (GCDFP-15) positive expressions by IHC [21,22]. This particular histology is also described in rare malignant adnexal neoplasms, which most commonly arise in areas with high-apocrine-gland densities, such as the axilla.
In 2005, after the transcriptomic profiling of BC, Farmer et al. described a new subtype of BC that is characterized by a luminal expression profile without ER but AR overexpression, as well as with a morphological apocrine differentiation (which was designated by the term molecular apocrine breast cancer (MABC) [23]). Subsequently, different groups have identified the MABC in non-redundant BC datasets [24]; these MABC tumors were recurrently found to specifically overexpress the AR gene and its consecutive pathway in an ER negative context with frequent expression/amplification of HER2 [23,24]. This led to the proposal of a new BC classification by Guedj et al., who split the HER2-like subtype of Perou and Sorlie into luminal B and MABC [25]. In parallel, Lehmann et al. published the TNBC subclassification described above and defined the LAR subtype as ER−/HER2−/AR+ [17,20]. Some confusion could be induced by these different descriptions, but it can be assumed that LARs probably converge on the HER2− part of the initially described MABC [25,26] (even if this has yet to be formally proven). Altogether, these data recently contributed to the consideration of these invasive MABC/LAR carcinomas as a subgroup of its own [27], leading to its inclusion in the WHO categorization of BC. This individualization of a subtype makes sense if distinct diagnoses, prognoses, or treatments are allowed by its identification as such.

MABC/LAR: How, and Why Are they Not Identified in Routine Practice?
MABC/LAR definition is based on the gene signatures obtained by messenger RNA (mRNA) expression profiling when they are not routinely performed. Some groups, including ours, have proposed MABC mRNA signatures or surrogate immunohistochemistry (IHC) markers as they are easier to apply [24,26]. However, currently, MABC/LAR profiling is not yet systematically performed.
Nevertheless, MABC/LARs are characterized by AR overexpression, and this can be easily evaluated by pathologists. Thus, MABC/LARs are essentially characterized by AR positive IHC in the context of an absence of ER and PR expressions. AR is a member of the sex steroid hormone receptor family (like ER, PR, etc.), and it is expressed in several human tissues including the breast [28]. In the context of BC, AR is overexpressed in more than 70% of cases, so it represents the greatest largely expressed hormone receptor [29]. However, it seems clear that AR plays a different role if associated with the presence or absence of ER overexpression [30].
In the ER− MABC/LAR context, the proof of concept and clinical trials supporting the targeting of AR by anti-AR drugs has come away with modest and controversial results [31][32][33][34][35][36][37][38]. Some inconsistencies could be explained by the lack of standardized AR evaluation, which is an obstacle that constitutes a major limitation for the proper definition of the subtype. Indeed, no consensus exists for the use of specific anti-AR antibodies, protocols, and positive cut-off scores. Moreover, the comparison of AR IHC evaluation and mRNA MABC signatures has demonstrated a weak concordance between these two classification tools [26]. Finally, the identification of this subtype remains a challenge, and better means for identifying it are hence needed to refine its diagnosis, prognosis, and treatment. With respect to novel and potentially useful biomarkers, microRNA (miRNA) appears to be a promising diagnostic biomarker. Moreover, the miRNA network could also help to better define the carcinogenesis of MABC/LARs and their behavior. Accordingly, in this review, we will focus on the potential role of specific dysregulated miRNA profiles in TNBC. More interestingly (in that of the less known ER−AR+ subtypes), we will also explore new approaches in order to understand and diagnose MABC/LAR breast tumors.

Search Strategy
A search strategy was adopted for the following part of the study, and two approaches were applied. The miRNAs in TNBCs were targeted by using the PubMed medical subject heading (MeSH) database. PubMed was searched for the following: "Breast Neoplasms" [MeSH] AND "MicroRNA" [MeSH] AND biomarkers AND prognosis AND diagnosis. For miRNA-AR interaction, the following terms were searched: "MicroRNA"[MeSH Terms] AND ("receptors, androgen"[MeSH Terms] OR ("receptors"[All Fields] AND "androgen"[All Fields]) OR "androgen receptors"[All Fields] OR ("androgen"[All Fields] AND "receptors"[All Fields])) AND ("breast neoplasms"[MeSH Terms].

microRNA
miRNAs are small non-coding RNAs of about 18-25 nucleotides in length. Most of these miRNAs bind to the 3 untranslated regions of target mRNAs, thus regulating gene expression at the post-transcriptional level and leading to mRNA cleavage, translational suppression, or deadenylation [39][40][41]. In humans, it is estimated that almost a third of mRNAs are controlled by miRNAs. In fact, this is a complex network of interactions where one miRNA may bind to as much as 200 targets, and a single gene can be regulated by various miRNAs [42,43]. Rarely does a miRNA activate mRNA translation and elevate target protein levels [44]. The miRNA-mediated regulation of gene expression was highlighted by a number of studies that revealed that miRNAs play a pivotal role in physiological and pathological processes [45,46]. miRNA dysregulation is implicated in a number of diseases, including cancer [46][47][48][49][50][51]. miRNAs are associated with cancers that are generally referred to as either oncomiRs (which are highly expressed often and can promote tumor development by the targeting of tumor suppressor genes) or tumor suppressive miRNA (which are often downregulated and inhibit cancer by regulating oncogenes [52]). Some cancer-associated miRNAs are known as context-dependent miRNAs. This is highly attributed to the fact that they can act in a tissue-specific manner so that single miRNAs can have either oncogenic or tumor suppressive roles in different cancers. Collectively, a surfeit of studies has reported alterations in miRNA expression in different types of cancers. Of particular interest, some miRNAs are related to cancer development, progression, and the response of the tumor to therapy [53-55]. Moreover, miRNAs can be secreted into body fluids and are referred to as circulating miRNAs [56]. They are highly stable and exist as free miRNA, or are released in exosomes [57,58]. The underlying mechanism of the relationship between tissue and circulating miRNA is not well known; yet, it seems that the extracellular miRNA levels reflect deregulated signaling pathways in cancer cells [59]. Finally, these small molecules, considered as one of the largest groups of gene regulators [60,61], are easily accessible, sensitive, specific, and stable; furthermore, they accordingly have a great potential to be considered as diagnostic, prognostic, and predictive biomarkers [46,49,62-64].  [71,72]. The association of miRNA activity with BC biology and its behavior was further supported by the proof that miRNAs are implicated in the regulation of ER and HER2 [73]. Moreover, there is good evidence that miRNA expression differs between primary and metastatic BCs [74,75]. This consequently led researchers to consider miRNA signatures as potential biomarkers that would help to further the understanding of BC subtypes, as well as help to predict metastasis or therapeutic resistance, thus leading to prolonged patient survival [74,76,77].
In an effort to better understand how these miRNAs are having such an impact on TNBC carcinogenesis, we executed in-silico analysis to determine which pathways these miRNAs are regulating. First of all, we had to identify the predominant miRNAs in cases where they were not reported in the literature as 3p or 5p. This was conducted through the MiRBase Converter, which is embedded in the online miRNA Enrichment and Annotation Analaysis (miEAA) tools. We also checked the miRNA annotations through using the miRbase. After which, an over-representation analysis was performed for the dysregulated miRNAs by using (miEAA), as well as by selecting the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways database as a reference. Then, we manually filtered the results to include pathways that are solely associated to BC initiation, progression, and response to therapy. Also, only the significantly deregulated pathways were accounted for, whereby significance was determined based on there being a minimum of two miRNAs present in a pathway and those which had an adjusted p-value < 0.05 ( Figure 2). Afterward, we identified the pathways that were found to be deregulated by a common set of more than 20 miRNAs (Figure 3). Out of the fifty-eight identified miRNA, twenty-one miRNA (hsa-miR-34a-5p; hsa-miR-93-5p; hsa-miR-124-3p; hsa-miR-15a-5p; hsa-miR-15b-5p; hsa-miR-16-5p; hsa-miR-195-5p; hsa-miR-145-5p; hsa-let-7e-5p; hsa-let-7b-5p; hsa-miR-301b-3p; hsa-miR-301a-3p; hsa-miR-30a-5p; hsa-miR-30c-5p; hsa-miR-9-5p; hsa-miR-210-3p; hsa-miR-19a-3p; hsa-miR-24-3p; hsa-miR-92a-3p; hsa-miR-222-3p; and hsa-miR-155-5p) were implicated in all of the pathways that are presented in Figure 3.   Our analysis reflects the complexity of miRNA interactions in TNBC carcinogenesis, i.e., where the existence of a set of signaling pathways that are reported to be implicated in TNBC hostility is indicated. Indeed, Javier Martinez et al. described epigenetic modifications as pivotal in TNBC development, as they appear to impact both oncogenes and tumor suppressor factors, which influence various molecular pathways such as WNT/β-catenin, MAPK, and PI3K-mTOR [237]. Another implication of WNT/β-catenin alongside JAK/STAT is that they regulate BC stem cell survival and thus raise the risk of TNBC relapse [238]. TNBCs' genomic instability, metabolic plasticity, and mutation in genes (including p53 and MAPK influence signaling pathways) are associated with the immune response [239]. Also, several studies have described deregulated lipid metabolism as a contributor in cancer cell survival, and these studies also further showed that it was mediated by PPAR-α signaling pathway [240]. A major glitch in the treatment of TNBCs is reportedly chemoresistance. It is suggested that the EGFR-K-RAS-SIAH pathway activation is a major tumor driver in chemoresistant TNBC patients [241]; another pathway that is being investigated is cAMP and its anti-proliferative role [242]. Also, oxidative phosphorylation (OXPHOS) is associated with several cancers; however, TNBC patients with a higher expression of OXPHOS have been reported to have the worst outcome [243]. In addition, checkpoint inhibitor therapy holds promise, especially in the context of metastatic TNBCs where programmed death ligand 1 (PD-L1) and PD-1 pathways are being targeted by inhibitors in combination with other adopted treatments to try to alleviate patient response [244]. Finally, it is interesting to note that the ferroptosis pathway is largely represented. This type of cell death is increasingly studied in the context of cancer [245] in line with non-coding RNAs [246], as well as recently-in particular-in the ER-/AR+ BC subtype [247].  Our analysis reflects the complexity of miRNA interactions in TNBC carcinogenesis, i.e., where the existence of a set of signaling pathways that are reported to be implicated in TNBC hostility is indicated. Indeed, Javier Martinez et al. described epigenetic modifications as pivotal in TNBC development, as they appear to impact both oncogenes and tumor suppressor factors, which influence various molecular pathways such as WNT/βcatenin, MAPK, and PI3K-mTOR [237]. Another implication of WNT/β-catenin alongside JAK/STAT is that they regulate BC stem cell survival and thus raise the risk of TNBC relapse [238]. TNBCs' genomic instability, metabolic plasticity, and mutation in genes (including p53 and MAPK influence signaling pathways) are associated with the immune response [239]. Also, several studies have described deregulated lipid metabolism as a contributor in cancer cell survival, and these studies also further showed that it was mediated by PPAR-α signaling pathway [240]. A major glitch in the treatment of TNBCs is reportedly chemoresistance. It is suggested that the EGFR-K-RAS-SIAH pathway activation is a major tumor driver in chemoresistant TNBC patients [241]; another pathway that is being investigated is cAMP and its anti-proliferative role [242]. Also, oxidative phosphorylation (OXPHOS) is associated with several cancers; however, TNBC patients with a higher expression of OXPHOS have been reported to have the worst outcome [243]. In addition, checkpoint inhibitor therapy holds promise, especially in the context of metastatic TNBCs where programmed death ligand 1 (PD-L1) and PD-1 pathways are being targeted by inhibitors in combination with other adopted treatments to try to alleviate patient response [244]. Finally, it is interesting to note that the ferroptosis pathway is largely represented. This type of cell death is increasingly studied in the context of cancer [245] in line with non-coding RNAs [246], as well as recently-in particular-in the ER−/AR+ BC subtype [247].
The predicted pathways in Figure 3 are not novel in terms of TNBC; yet, those pathways have also not been studied in terms of miRNA interaction. This sheds light on the importance of investigating the panels of miRNAs in the context of studying carcinogenesis pathways.

miRNA-Implications in AR+ Tumors
Recent investigations highlighted that AR expression may be regulated by a variety of miRNAs either directly or indirectly by affecting the expression of co-activators or corepressors. The latter would shape the AR functions [248][249][250][251]. AR is a nuclear receptor made up of a single gene that is located on the X-chromosome [252][253][254]. Androgens are usually depicted as male hormones, yet they were found to also play important biological roles in female development and physiology [255]. Dehydroepiandrosterone sulphate (DHEAS), dehydroepiandrosterone (DHEA), androstenedione (A4), testosterone, and dihydrotestosterone (DHT) are kinds of androgenic hormones that are present in the blood stream [256].
First of all, a correlation between AR expression and miRNA is particularly depicted in prostate cancer (PC) [257,258]. This interaction was found to be associated with tumor initiation and development in PC. The androgen regulation of miRNAs was examined by Waltering et al. in 2011, where DHT was found to positively regulate 17 miRNAs, out of which only 4 (miR-10a, miR-141, miR-150, and miR-1225-5p) exhibited similar androgen regulation in both in vitro and in vivo studies [259]. AR activation in PC patients reduces miR-190a expression, thus enhancing tumor-free survival [250].
By contrast, the impact of AR in BC tumorigenesis remains controversial, for it was reported that women with increased levels of androgens have increased risk of BC, while it was also reported that AR expression is a favorable BC prognostic indicator (but it has to be noticed that this is mainly true in ER+ contexts [260][261][262]). The imbalance of miRNA levels in AR+ BC cells compared to AR− BC cells implies that miRNA has a crucial role in the function of AR in BCs [263]. However, studies on the miRNA-AR interactions in BCs are limited [257,258]. Some data indicate that miR-21, an oncomiR, is upregulated in hormone-dependent neoplasms including PC and BCs [264,265], and this is reported to reduce BC cell proliferation [130]. Interestingly, AR was found to repress the transcription of miR-21 expression [266]. This suggests that more has to be evaluated in this context.
Nevertheless, some studies have focused on BCs, especially ER− ones. Shi et al. performed miRNA expression profiling in ER−/AR+ BC and revealed a total of 153 differentially expressed miRNAs in AR+ compared to AR− BC. The most significantly upregulated miRNAs were miR-933 and miR-5793, and the most downregulated was miR-4792 [263]. miR-221 and 222 that are upregulated in BC and PC are considered as oncogenes where they promote proliferation. Of interest are the miRs that are repressed by AR [130]. Another miRNA that plays an essential role in ER−/AR+ cells is miR-30b, which has been reported to inhibit cell growth [267]. miR-9-5p has an inverse relationship with AR in BCs where it exerts an anti-proliferative role [268]. miR-328-3p suppression by DHT in MDA-MB-231, suppressed CD44 expression and consequently cell adhesion. Conversely, an opposite effect was obtained upon transfection with an AR antagonist, whereby the idea that miRNAs regulate BCs was emphasized [269]. miR-190a was previously reported to be implicated in BC metastasis [270]. miR-135b, a direct regulator of AR in PC cells, was shown to have a lower expression in ER+ breast tumors when compared to ER−, as well as a higher expression in AR-low BC patient samples. It also reduces proliferation in AR+ PC cells [260]. A study conducted by Guo et al. depicted that miR-520g-3p and miR-520h are both downregulated, and that they have a significant potential in AR+ TNBC diagnosis and prognosis [271]. miR-3163 that is downregulated in AR+ ER− tumors was found to have good prognostic value [272].
MABC/LARs, i.e., the scope of this review, are characterized by AR overexpression and hyperactivation. Little is known about the miRNAs associated with this subtype. This subtype has been investigated, in vitro, via BC cell lines, in which AR expression was shown to promote their growth [273]. Of interest, in the MDA-MB-453 cell line, is an MABC model, whereby the miRNA expression that was investigated by Lyu et al. in 2014 was found to reveal four upregulated miRNAs (let-7a, let-7b, let7-c, and let7-d), where let-7a decreased cell proliferation, invasion, and migration, as well as self-renewal capacities when treating cells with DHT. In addition, this process showed a better outcome in patients with invasive BCs [274,275]. AR activity is repressed indirectly by miR-let-7c [276]. Another study investigated the role of miR-30a in MDA-MB-453, after DHT treatment, and revealed that the stimulation of AR expression inhibits miR-30a and consequently suppresses cell growth [277]. In response to AR agonists, the miR-100 and miR-125 expression was significantly reduced in MDA-MB-453 BC cells, consequently leading to the increased expression of miR-100 and miR-125 target metalloprotease-13 (MMP13) [278].
A summary of the miRNAs implicated in AR+ BC and PC is summarized in Table 2.

Challenges
Despite the fact that BC is a highly investigated research topic, and that miRNAs can serve as a biomarker for BCs, the reports on MABC are not frequent, and-in most cases-not clear. MABC is often described as under TN in the literature but also as an ER− subtype with AR overexpression, yet the mention of the name itself is not stated. This also has an impact on the search for miRNA-MABC reports. Another obstacle with most of the miRNAs reported in the literature is the lack of full miRNA annotation. This requires the use of in silico programs to predict the isoforms of miRNAs, and these might not always end up in providing the isoform investigated in the literature. Moreover, miRNAs' specificity is often questioned, since in many cases the data are unreproducible in different datasets. This could be explained by ethnic differences, age groups, or the standardization of miRNA quantification assays in all studies. In addition to this, pathway analysis is mostly dependent on algorithms and predictions. It is worthwhile to note that all the predicted actors need to be experimentally validated before clinical utility; however, this kind of analysis could be highly valuable for new hypotheses, and could promote further pathway explorations that could help with deciphering these poorly understood BCs. Furthermore, this inventory could be a starting point through which to develop new approaches for MABC/LAR BC subtypes by including the miRNA network in the picture.

Conclusions
Differential gene expression, epigenetic modification, IHC along with other current techniques in BC classification have revealed the huge heterogeneity of this disease. Therefore, understanding the different subtypes of BCs may benefit its diagnosis, prognosis, and therapeutics. This is essential in understanding poorly diagnosed and misclassified subtypes such as MABC/LARs, as well as the consequent impact on the health management of its corresponding patients. miRNAs are reported to be deregulated in various cancers, specifically in BC and in different BC subtypes (including ER−/AR+ ones). Hence, miRNAs are a highly stable and easily detectable molecule, and they may assist in a better understanding of MABC carcinogenesis. Thus, the verification of miRNA panels in MABC patients might create a distinctive definition of this subtype, and could depict an improved understanding of the signal networks driving the biology of MABCs. In addition to this, there is piling evidence of miRNA-AR interactions in development, as well as the progression of cancer that might elucidate on MABC initiation and progression. Moreover, specific miRNAs might actually serve as diagnostic or prognostic biomarkers, but more research needs to be conducted to verify the potential clinical application of these findings. Therefore, the search for ideal biomarkers necessitates the standardization of panels in different groups, and this is subject to continuous updates that are based on advances in research and molecular technology. In this context, exploring the state-of-the-art developments of miRNAs in the MABC/LAR subtype, and attempting to extract the main miRNAs of interest could shed light on this other level of complexity, as well as help to generate new hypotheses from new angles for approaching this BC subtype that is still poorly understood.