Pulmonary cancers across different histotypes share hybrid tuft cell/ionocyte-like molecular features and potentially druggable vulnerabilities

Thymic Mimetic Cells: Evolutionarily Ancient Mirrors of the Periphery

Thymic mimetic cells are hybrids of medullary thymic epithelial cells and diverse peripheral cell types. They are important for imposition of self-tolerance and perform other functions similar to those of their peripheral counterparts. Following early histological observations of “misplaced” stromal cells in thymi from multiple species, mimetic cells were first molecularly investigated in mice. Recent studies have characterized mimetic cells in humans and zebrafish with high-resolution. Many mimetic cell types are conserved across species, although specialized subtypes as well as variable frequencies and levels of specialization are also apparent. Features of the human mimetic cell repertoire, such as the expanded nature of muscle mimetic cells with potential implications in myasthenia gravis and the similarity of tuft mimetic cells and thymic carcinomas, hint at their relevance in human disease. Here we review what is known about mimetic cells across diverse organisms. We discuss potential pressures shaping the composition of the mimetic cell repertoire within and across species, and highlight potential therapeutic applications in human disease.

Clinical Impact of POU2F3 Expression in Surgically Resected Pulmonary High-Grade Neuroendocrine Carcinoma

OBJECTIVE

 High-grade neuroendocrine carcinoma (HGNEC) with dominant POU2F3 expression exhibits non-neuroendocrine features. However, clinical data regarding this subset of pulmonary HGNECs are scarce, and its clinical characteristics remain unclear.

METHODS

 Clinicopathological data from 109 patients who underwent surgery for HGNEC were collected and analyzed based on transcription factor expression. Patients were divided into a POU2F3-dominant group (HGNEC-P) and a non-dominant group (HGNEC-non-P) according to immunohistochemical analysis. The clinicopathological characteristics of the two groups were compared, and univariate and multivariate analyses were conducted to identify prognostic factors.

RESULTS

 The HGNEC-P group comprised 26 patients, while the HGNEC-non-P group comprised 83 patients. The HGNEC-P group showed significantly lower expression of carcinoembryonic antigen (CEA) (p < 0.001) and a lower rate of vascular invasion (p = 0.021) compared to the HGNEC-non-P group. In addition, the HGNEC-P group exhibited a unique tumor marker profile, with lower serum CEA and higher serum cytokeratin antigen (CYFRA) levels (p < 0.001 and p = 0.046, respectively). Complete resection was achieved in all HGNEC-P cases, whereas only 75.9% of HGNEC-non-P cases achieved complete resection. Multivariate analysis identified POU2F3 expression as an independent prognostic factor for recurrence-free survival (RFS) and disease-specific survival (DSS) (p = 0.037 and p = 0.038, respectively). In patients with pathological Stage I disease, the HGNEC-P group showed significantly better RFS (p = 0.010).

CONCLUSIONS

 POU2F3-dominant HGNEC is associated with distinct clinicopathological features and favorable prognosis, particularly in early-stage disease. These findings may support the identification of this subset and inform the development of more effective treatment strategies.

KLF4 promotes a KRT13+ hillock-like state in squamous lung cancer

Lung squamous cell carcinoma (LUSC) is basal-like subtype of lung cancer with limited treatment options. While prior studies have identified tumor-propagating cell states in squamous tumors, the broader landscape of intra-tumoral heterogeneity within LUSC remains poorly understood. Here, we employ Sox2-driven mouse models, organoid cultures, and single-cell transcriptomic analyses to uncover previously unrecognized levels of cell fate diversity within LUSC. Specifically, we identify a KRT13+ hillock-like population of slower-dividing tumor cells characterized by immunomodulatory gene expression signatures. The tumor hillock-like state is conserved across multiple animal models and is present in the majority of human LUSCs as well as head and neck and esophageal squamous tumors. Our findings shed light on the cellular origins of lung hillock-like states: normal club cells give rise to tumors with luminal hillock-like populations, while basal-like tumor-propagating cells transition into basal hillock-like states, resembling homeostatic cellular responses to lung injury. Mechanistically, we identify KLF4 as a key transcriptional regulator of the hillock-like state, both necessary and sufficient to induce KRT13 expression. Together, these results provide new molecular insights into cell fate plasticity that underlies intra-tumoral heterogeneity in LUSC, offering potential avenues for new therapeutic strategies.

Analysis of ASCL1/NEUROD1/POU2F3/YAP1 Yields Novel Insights for the Diagnosis of Olfactory Neuroblastoma and Identifies Sinonasal Tuft Cell-Like Carcinoma

The diagnosis and treatment of sinonasal small round epithelial/neuroepithelial malignancies depend on the expression of conventional neuroendocrine markers (NEMs), such as synaptophysin, chromogranin A, INSM1, and CD56/NCAM1. However, these tumors remain diagnostically challenging because of overlapping histologic and immunohistochemical features. The transcriptional regulators ASCL1, NEUROD1, POU2F3, and YAP1 are novel NEM (nNEM) used for the subtyping of small-cell lung cancer (SCLC). Here, we assessed the immunoexpression of nNEM in 76 sinonasal malignancies, including 27 olfactory neuroblastomas (ONB), 14 small-cell neuroendocrine carcinomas (SCNEC), 2 large-cell neuroendocrine carcinomas, 12 sinonasal undifferentiated carcinomas (SNUC), 7 olfactory carcinomas (OC), 11 SWI/SNF-deficient carcinomas, and 3 neuroendocrine tumors. We correlated nNEM expression with the extent of neuroendocrine (NE) differentiation, as defined by averaged conventional NEM expression (NE-high: H-score, ≥150; NE-low: H-score, <150). Dominant NE subtypes were defined by the nNEM with the highest H-score. Coexpression of 2 nNEM with <100 H-score difference defined a codominant NE subtype. NE differentiation positively correlated with NEUROD1 and negatively with YAP1 expression (P < .0001). ONB were NE-high (96%), and all were NEUROD1-dominant/POU2F3-negative/ASCL1-negative (low)/YAP1-negative (low). In contrast to ONB, all OC were NE-low, mostly (71%) codominant subtypes, NEUROD1-low (negative) (100%, P = .0001), and YAP1 high (71%; P = .0001). Most notably, all SNUC were POU2F3-(co)dominant/NEUROD1-negative irrespective of the IDH2 mutations. Sinonasal tumors with high POU2F3 expression showed enrichment for "tuft cell carcinoma" and tuft cell signatures (P = .009). Similar to SCLC, SCNEC was heterogeneous in terms of nNEM expression comprising several molecular subtypes, including ASCL1-(co)dominant (43%) cases. All SWI/SNF-deficient carcinomas were consistently ASCL1/NEUROD1/POU2F3-negative and YAP1-positive. ASCL1/NEUROD1/POU2F3/YAP1 are useful markers in the differential diagnosis of ONB, SNUC, OC, and SWI/SNF-deficient carcinomas. Subsets of SNUC and large-cell neuroendocrine carcinomas may represent tuft cell-like carcinomas, suggesting that the tuft cell could be explored as the cell of origin for these tumors. The therapeutic vulnerabilities associated with POU2F3 expression in SCLC suggest that a similar approach might be considered for POU2F3-positive carcinomas of the sinonasal tract. Given their diagnostic and possible therapeutic relevance, nNEM have the potential to transform the way we approach the diagnosis and management of sinonasal small round epithelial/neuroepithelial malignancies.

Comparison of molecular subtype composition between independent sets of primary and brain metastatic small cell lung carcinoma and matched samples

INTRODUCTION

Recent advances in the subclassification of small cell lung carcinomas (SCLCs) may help to overcome the unmet need for targeted therapies and improve survival. However, limited information is available on how the expression of the subtype markers changes during tumour progression. Our study aimed to compare the expression of these markers in primary and brain metastatic SCLCs.

MATERIALS AND METHODS

Immunohistochemical analysis of the subtype markers was performed on 120 SCLCs (including 10 matched samples) and SCLC xenografts.

RESULTS

Compared to primary SCLCs, there was a significant increase in the proportion of mixed subtypes in brain metastases, with a rate of ASCL1high/NeuroD1high and ASCL1high/NeuroD1high/YAP1high subtypes increasing to 48 % and 18 %, respectively. The subtype of the paired samples matched in only one-third of the cases. Although we did not observe a significant change after chemotherapy, a continuous decrease in ASCL1 expression coupled with an increase in the NeuroD1 expression was detected in the xenografts in a long-term experiment.

DISCUSSION

Our results indicate that the expression of subtype markers frequently changes during disease progression, and subtype analysis of the primary SCLC may not provide accurate information about the characteristics of the recurrent or metastatic tumour. Therefore, repeated sampling and subtyping may be necessary for subtype-specific targeted therapy.

Expression of POU2F3 Transcription Factor and POU2AF2, POU2F3 Coactivator, in Tuft Cell-like Carcinoma and Other Tumors

Epithelial chemosensory cells in hollow organs, also known as tuft cells, were implicated in tumorigenesis, including a tuft cell-like small cell lung carcinoma. Expression of the POU2F3 transcription factor is a marker of tuft cell lineage. However, tuft cell development, differentiation, and proliferation are controlled by the expression of the complex formed by POU2F3 and POU2AF2 or POU2AF3 transcriptional coactivators. A cohort of epithelial (n=6064) and mesenchymal/neuroectodermal (n=2730) tumors was screened for POU2F3 expression by immunohistochemistry. Variable immunoreactivity ranging from diffuse to scattered positive cells was found in ∼12.4% of epithelial and 4.6% of mesenchymal/neuroectodermal tumors. Cases with predominantly diffuse or patchy POU2F3 positivity representing various types of malignant tumors (n=43) were selected for further study, including POU2AF2 immunohistochemistry. Thirteen of 15 tumors with neuroendocrine differentiation originating from the lung, colon, head and neck, skin, and bladder revealed diffuse POU2F3 positivity. Most of those tumors (n=9) co-expressed POU2AF2, usually extensively. Seven squamous and basal cell carcinomas from the oral cavity, skin, lung, and thymus with diffuse POU2F3 immunostaining except one, lacked POU2AF2 expression. Other variably POU2F3-positive carcinomas (n=13) from the colon, pancreas, liver, kidney, testis, endometrium, ovary, and breast lacked POU2AF2 immunoreactivity. All POU2F3-positive mesenchymal and neuroectodermal tumors (n=8), including synovial sarcoma, solitary fibrous tumor, glioblastoma, Wilms tumor, and melanoma were POU2AF2-negative. POU2F3 expression is a highly sensitive but nonspecific indicator of tuft cell differentiation. Co-expression of POU2F3 and POU2AF2 appears to be a more specific marker, although it may not pinpoint tumors driven by the POU2F3-POU2AF3 complex.

Basal cell of origin resolves neuroendocrine-tuft lineage plasticity in cancer

Neuroendocrine and tuft cells are rare, chemosensory epithelial lineages defined by expression of ASCL1 and POU2F3 transcription factors, respectively1,2. Neuroendocrine cancers, including small cell lung cancer (SCLC), frequently display tuft-like subsets, a feature linked to poor patient outcomes3-13. The mechanisms driving neuroendocrine-tuft tumour heterogeneity, and the origins of tuft-like cancers are unknown. Using multiple genetically-engineered animal models of SCLC, we demonstrate that a basal cell of origin (but not the accepted neuroendocrine origin) generates neuroendocrine-tuft-like tumours that highly recapitulate human SCLC. Single-cell clonal analyses of basal-derived SCLC further uncovers unexpected transcriptional states and lineage trajectories underlying neuroendocrine-tuft plasticity. Uniquely in basal cells, introduction of genetic alterations enriched in human tuft-like SCLC, including high MYC, PTEN loss, and ASCL1 suppression, cooperate to promote tuft-like tumours. Transcriptomics of 944 human SCLCs reveal a basal-like subset and a tuft-ionocyte-like state that altogether demonstrate remarkable conservation between cancer states and normal basal cell injury response mechanisms14-18. Together, these data suggest that the basal cell is a plausible origin for SCLC and other neuroendocrine-tuft cancers that can explain neuroendocrine-tuft heterogeneity-offering new insights for targeting lineage plasticity.

Foxi1 regulates multiple steps of mucociliary development and ionocyte specification through transcriptional and epigenetic mechanisms

Foxi1 is a master regulator of ionocytes (ISCs / INCs) across species and organs. Two subtypes of ISCs exist, and both α- and β-ISCs regulate pH- and ion-homeostasis in epithelia. Gain and loss of FOXI1 function are associated with human diseases, including Pendred syndrome, male infertility, renal acidosis and cancers. Foxi1 functions were predominantly studied in the context of ISC specification, however, reports indicate additional functions in early and ectodermal development. Here, we re-investigated the functions of Foxi1 in Xenopus laevis embryonic mucociliary epidermis development and found a novel function for Foxi1 in the generation of Notch-ligand expressing mucociliary multipotent progenitors (MPPs). We demonstrate that Foxi1 has multiple concentration-dependent functions: At low levels, Foxi1 confers ectodermal competence through transcriptional and epigenetic mechanisms, while at high levels, Foxi1 induces a multi-step process of ISC specification and differentiation. We further describe how foxi1 expression is affected through auto- and Notch-regulation, how Ubp1 and Dmrt2 regulate ISC subtype differentiation, and how this developmental program affects Notch signaling as well as mucociliary patterning. Together, we reveal novel functions for Foxi1 in Xenopus mucociliary epidermis formation, relevant to our understanding of vertebrate development and human disease.

Molecular subtypes and prognostic factors of lung large cell neuroendocrine carcinoma

Background

Lung large cell neuroendocrine carcinoma (LCNEC) is an aggressive disease with poor prognosis and short-term survival, which lacks effective prognostic indicators. The study aims to investigate the molecular subtypes and prognostic markers of lung LCNEC.

Methods

Patients diagnosed with lung LCNEC at Sun Yat-sen University Cancer Center (SYSUCC) between November 2007 and January 2021 were screened. Baseline clinical data were collected and routine blood indexes including lymphocyte-to-monocyte ratio (LMR), neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR) and systemic immune-inflammation index (SII) were calculated. Immunohistochemistry (IHC) of ASCL1, NEUROD1, POU2F3, YAP1 were done to perform molecular subtyping, while CD56, Syn, CgA, CD3, CD8, CD20, CD68, and CD163 were also stained on tissue samples. Then prognostic factors of lung LCNEC were explored.

Results

One hundred and fifty-one lung LCNEC patients were identified, 103 of whom had complete clinical information, available routine blood and biochemical indexes were eventually included in the present study. Tumor tissue specimens were available from 64 patients. Positive expression rates of ASCL1, NEUROD1, and YAP1 were 82.8%, 50.0%, and 28.1%, respectively. No POU2F3+ cases were detected. Forty (62.5%) patients co-expressed with two or three markers. High LMR (>3.3) was an independent predictor of favorable prognosis of disease-free survival (DFS) [hazard ratio (HR), 0.391; 95% confidence interval (CI): 0.161-0.948; P=0.04] and overall survival (OS) (HR, 0.201; 95% CI: 0.071-0.574; P=0.003). Notably, high LMR was correlated with higher intra-tumoral CD3+ (P=0.004), CD8+ (P=0.01), and CD68+ (P<0.001) immune cell infiltration compared to low LMR in lung LCNEC.

Conclusions

Our study validated molecular subtypes by IHC in lung LCNEC, and co-expression was found among different subtypes, with no prognostic effect. High blood LMR level was associated with a favorable prognosis in lung LCNEC, which might partly reflect a hot tumor tissue immune microenvironment. Our findings may benefit clinical practice, and further studies are warranted.

Characterization of two transcriptomic subtypes of marker-null large cell carcinoma of the lung suggests different origin and potential new therapeutic perspectives

Pulmonary large cell carcinoma (LCC) is an undifferentiated neoplasm lacking morphological, histochemical, and immunohistochemical features of small cell lung cancer, adenocarcinoma (ADC), or squamous cell carcinoma (SCC). The available molecular information on this rare disease is limited. This study aimed to provide an integrated molecular overview of 16 cases evaluating the mutational asset of 409 genes and the transcriptomic profiles of 20,815 genes. Our data showed that TP53 was the most frequently inactivated gene (15/16; 93.7%) followed by RB1 (5/16; 31.3%) and KEAP1 (4/16; 25%), while CRKL and MYB genes were each amplified in 4/16 (25%) cases and MYC in 3/16 (18.8%) cases; transcriptomic analysis identified two molecular subtypes including a Pure-LCC and an adenocarcinoma like-LCC (ADLike-LCC) characterized by different activated pathways and cell of origin. In the Pure-LCC group, POU2F3 and FOXI1 were distinctive overexpressed markers. A tuft cell-like profile and the enrichment of a replication stress signature, particularly involving ATR, was related to this profile. Differently, the ADLike-LCC were characterized by an alveolar-cell transcriptomic profile and association with AIM2 inflammasome complex signature. In conclusion, our study split the histological marker-null LCC into two different transcriptomic entities, with POU2F3, FOXI1, and AIM2 genes as differential expression markers that might be probed by immunohistochemistry for the differential diagnosis between Pure-LCC and ADLike-LCC. Finally, the identification of several signatures linked to replication stress in Pure-LCC and inflammasome complex in ADLike-LCC could be useful for designing new potential therapeutic approaches for these subtypes.

Expression of FOXI1 and POU2F3 varies among different salivary gland neoplasms and is higher in Warthin tumor

PURPOSE

Salivary gland tumors are histologically diverse. Ionocytes and tuft cells, rare epithelial cells found in normal salivary glands, might be associated with salivary tumors. Here, we explored the expression of FOXI1 and POU2F3, master regulators of ionocytes and tuft cells, respectively, for common salivary neoplasms using immunohistochemistry.

METHODS

We analyzed normal salivary tissues and nine salivary gland tumors; Warthin tumors (WT), pleomorphic adenomas (PA), basal cell adenomas, and oncocytomas were benign, whereas mucoepidermoid, adenoid cystic, acinic cell, salivary duct carcinomas, and polymorphous adenocarcinomas were malignant.

RESULTS

Normal salivary glands contained a few FOXI1- and POU2F3-positive cells in the ducts instead of the acini, consistent with ionocytes and tuft cells, respectively. Among the benign tumors, only WTs and PAs consistently expressed FOXI1 (10/10 and 9/10, respectively). The median H-score of WTs was significantly higher than that of PAs (17.5 vs. 4, P = 0.01). While WTs and PAs harbored POU2F3-positive cells (10/10 and 9/10, respectively), the median H-score was higher in WTs than in PAs (10.5 vs 4, respectively). Furthermore, WTs exhibited a unique staining pattern of FOXI1- and POU2F3-positive cells, which were present in luminal and abluminal locations, respectively. Whereas none of the malignant tumors expressed FOXI1, only adenoid cystic carcinoma consistently expressed POU2F3 (5/5), with a median H-score of 4.

CONCLUSION

The expression patterns of the characteristic transcription factors found in ionocytes and tuft cells vary among salivary gland tumor types and are higher in WT, which might be relevant for understanding and diagnosing salivary gland neoplasms.

Alveolar Organoids in Lung Disease Modeling

Lung organoids display a tissue-specific functional phenomenon and mimic the features of the original organ. They can reflect the properties of the cells, such as morphology, polarity, proliferation rate, gene expression, and genomic profile. Alveolar type 2 (AT2) cells have a stem cell potential in the adult lung. They produce and secrete pulmonary surfactant and proliferate to restore the epithelium after damage. Therefore, AT2 cells are used to generate alveolar organoids and can recapitulate distal lung structures. Also, AT2 cells in human-induced pluripotent stem cell (iPSC)-derived alveolospheres express surfactant proteins and other factors, indicating their application as suitable models for studying cell-cell interactions. Recently, they have been utilized to define mechanisms of disease development, such as COVID-19, lung cancer, idiopathic pulmonary fibrosis, and chronic obstructive pulmonary disease. In this review, we show lung organoid applications in various pulmonary diseases, drug screening, and personalized medicine. In addition, stem cell-based therapeutics and approaches relevant to lung repair were highlighted. We also described the signaling pathways and epigenetic regulation of lung regeneration. It is critical to identify novel regulators of alveolar organoid generations to promote lung repair in pulmonary diseases.

POU2F3-Expressing Small Cell Lung Carcinoma and Large Cell Neuroendocrine Carcinoma Show Morphologic and Phenotypic Overlap

Considering the differences in protein expression in small cell lung carcinoma (SCLC) by molecular classification, it is likely that there are differences in morphology, but the relationship between molecular classification and morphology has not been examined. Furthermore, there are limited reports concerning this molecular classification for large cell neuroendocrine carcinoma (LCNEC) and SCLC simultaneously. Therefore, we investigated the relationship between immunohistochemistry-based molecular classification and morphology, protein expression, and clinical features of 146 consecutive resection specimens of pulmonary neuroendocrine carcinoma (NEC), focusing mainly on POU2F3, the master transcription factor involved in tuft cell generation. POU2F3-dominant SCLC (n=24) and LCNEC (n=14) showed overlap in cytomorphology, while non-POU2F3-dominant SCLC (n=71) and LCNEC (n=37) showed distinct differences in cytomorphology. In addition, POU2F3-dominant NEC exhibited significantly more abundant tumor stroma, more prominent nest formation, more frequent bronchial intraepithelial involvement, and less frequent background fibrosis than non-POU2F3-dominant NEC. Immunohistochemically, POU2F3-dominant SCLC and LCNEC were characterized by lower expression of TTF-1, CEA, and neuroendocrine markers and higher expression of bcl-2, c-Myc, and c-kit. Clinically, POU2F3-dominant NEC had a significantly better prognosis than non-POU2F3-dominant NEC for recurrence-free survival. POU2F3-dominant NEC had a higher smoking index than non-POU2F3-dominant NEC. POU2F3-dominant NEC forms a unique population, exhibiting intermediate morphologic features between SCLC and LCNEC, with distinct protein expression as tuft cell-like carcinoma. Recognition of this unique subtype may provide clues for solving the long-standing issues of NEC and appropriate therapeutic stratification. It is important to accurately identify POU2F3-expressing carcinomas by immunohistochemistry and to analyze their clinicopathological features.

Thymic Carcinoma: Unraveling Neuroendocrine Differentiation and Epithelial Cell Identity Loss

BACKGROUND

The histogenesis of thymic epithelial tumors (TETs) has been a subject of debate. Recent technological advancements have revealed that thymic carcinomas often exhibit a phenotype akin to tuft cells, which is a subset of medullary TECs. Here, we further explored the gene expression signatures of thymic carcinomas in relation to tuft cells and their kinships-ionocytes and neuroendocrine cells (neuroendocrine group).

METHODS

We analyzed a single-cell RNA sequencing dataset from the normal human thymus. Concurrently, we examined publicly available datasets on the mRNA expression and methylation status of TECs and lung cancers. Real-time quantitative PCR was also conducted with our tissue samples.

RESULTS

Thymic carcinomas displayed a neuroendocrine phenotype biased toward tuft cells and ionocytes. When exploring the possible regulators of this phenotype, we discovered that HDAC9 and NFATC1 were characteristically expressed in the neuroendocrine group in adult TECs and thymic carcinomas. Additionally, the pan-thymic epithelium markers, exemplified by PAX9 and SIX1, were significantly suppressed in thymic carcinomas.

CONCLUSIONS

Thymic carcinomas might be characterized by unique neuroendocrine differentiation and loss of identity as thymic epithelial cells. Future studies investigating the role of HDAC9 and NFATC1 in thymic epithelium are warranted to explore their potential as therapeutic targets in TETs.

Histogenetic and disease-relevant phenotypes in thymic epithelial tumors (TETs): The potential significance for future TET classification

Thymic epithelial tumors (TETs) encompass morphologically various subtypes. Thus, it would be meaningful to explore the expression phenotypes that delineate each TET subtype or overarching multiple subtypes. If these profiles are related to thymic physiology, they will improve our biological understanding of TETs and may contribute to the establishment of a more rational TET classification. Against this background, pathologists have attempted to identify histogenetic features in TETs for a long time. As part of this work, our group has reported several TET expression profiles that are histotype-dependent and related to the nature of thymic epithelial cells (TECs). For example, we found that beta5t, a constituent of thymoproteasome unique to cortical TECs, is expressed mainly in type B thymomas, for which the nomenclature of cortical thymoma was once considered. Another example is the discovery that most thymic carcinomas, especially thymic squamous cell carcinomas, exhibit expression profiles similar to tuft cells, a recently discovered special type of medullary TEC. This review outlines the currently reported histogenetic phenotypes of TETs, including those related to thymoma-associated myasthenia gravis, summarizes their genetic signatures, and provides a perspective for the future direction of TET classification.

An exploratory study for tuft cells in the breast and their relevance in triple-negative breast cancer: the possible relationship of SOX9

BACKGROUND

Breast cancer is highly heterogeneous, suggesting that small but relevant subsets have been under-recognized. Rare and mainly triple-negative breast cancers (TNBCs) were recently found to exhibit tuft cell-like expression profiles, including POU2F3, the tuft cell master regulator. In addition, immunohistochemistry (IHC) has identified POU2F3-positive cells in the normal human breast, suggesting the presence of tuft cells in this organ.

METHODS

Here, we (i) reviewed previously identified POU2F3-positive invasive breast cancers (n = 4) for POU2F3 expression in intraductal cancer components, (ii) investigated a new cohort of invasive breast cancers (n = 1853) by POU2F3-IHC, (iii) explored POU2F3-expressing cells in non-neoplastic breast tissues obtained from women with or without BRCA1 mutations (n = 15), and (iv) reanalyzed publicly available single-cell RNA sequencing (scRNA-seq) data from normal breast cells.

RESULTS

Two TNBCs of the four previously reported invasive POU2F3-positive breast cancers contained POU2F3-positive ductal carcinoma in situ (DCIS). In the new cohort of invasive breast cancers, IHC revealed four POU2F3-positive cases, two of which were triple-negative, one luminal-type, and one triple-positive. In addition, another new POU2F3-positive tumor with a triple-negative phenotype was found in daily practice. All non-neoplastic breast tissues contained POU2F3-positive cells, irrespective of BRCA1 status. The scRNA-seq reanalysis confirmed POU2F3-expressing epithelial cells (3.3% of all epithelial cells) and the 17% that co-expressed the other two tuft cell-related markers (SOX9/AVIL or SOX9/GFI1B), which suggested they were bona fide tuft cells. Of note, SOX9 is also known as the "master regulator" of TNBCs.

CONCLUSIONS

POU2F3 expression defines small subsets in various breast cancer subtypes, which can be accompanied by DCIS. The mechanistic relationship between POU2F3 and SOX9 in the breast warrants further analysis to enhance our understanding of normal breast physiology and to clarify the significance of the tuft cell-like phenotype for TNBCs.