Microcribriform Adenocarcinoma of Salivary Glands: A Unique Tumor Entity Characterized by an SS18::ZBTB7A Fusion

Microsecretory adenocarcinoma of the hard palate: a case report and literature review

Microsecretory adenocarcinoma (MSA) is a new type of salivary gland neoplasm identified in the 2022 World Health Organization Classification of Head and Neck Tumour (Skalova et al., Head Neck Pathol 16:40-53, 2022) and is characterized by a unique set of histomorphologic and immunohistochemical features and a recurrent MEF2C::SS18 fusion. MSA was initially misdiagnosed as another salivary gland tumour due to its similar morphology; until recently, only fewer than 50 cases were reported. We present a case of MSA of the hard palate with diverse architectural growth patterns, bland cytological features, abundant basophilic intraluminal secretions and fibromyxoid stroma. The tumour cells were positive for the SOX10, S100, and p63 protein and negative for the p40 protein according to immunohistochemistry. SS18 gene rearrangement was demonstrated via break-apart fluorescence in situ hybridization. We also provided a comprehensive literature review and integrated the clinicopathological features, immunophenotype, and molecular alterations of the disease. A comprehensive understanding of MSA enables us to accurately distinguish and categorize MSA from other salivary gland tumours with analogous morphologies.

Undifferentiated round cell sarcoma with CRTC1::SS18 fusion: Expanding clinicopathologic features of a rare translocation sarcoma with prominent desmoplastic stroma

Undifferentiated round cell sarcomas (URCS) represent a diverse group of tumors, including conventional Ewing sarcoma, round cell sarcoma with EWSR1/FUS-non-ETS fusions, CIC-rearranged sarcoma, and sarcoma with BCOR alterations. Since 2018, three cases of URCS with a novel CRTC1::SS18 gene fusion have been reported in the literature. Herein, we report three additional cases of CRTC1::SS18 sarcoma, thereby doubling the number of described cases and expanding the clinicopathologic features of this rare translocation sarcoma. Together with the previously reported cases, we show that the male-to-female ratio is 1:2 with a median age of 34 years (range: 12 to 42 years). Tumors occurred primarily in intramuscular locations involving the lower extremity. Histologically, all tumors contained uniform round to epithelioid cells with a moderate amount of eosinophilic cytoplasm growing in sheets and nests with prominent desmoplastic stroma reminiscent of desmoplastic small round cell tumor (DSRCT). Immunohistochemical results were non-specific, demonstrating variable expression of CD99 (patchy), ALK, GATA3, and cyclin D1. RNA sequencing revealed CRTC1::SS18 gene fusions in all cases, involving exon 1-2 of CRTC1 (the 5' partner gene) on chromosome 19 and either exon 2 or exon 4 of SS18 (the 3' partner gene) on chromosome 18. The clinical course was variable. While one previously reported case demonstrated aggressive behavior with fatal outcome, two others had a relatively indolent course with gradual growth for 6-7 years prior to resection. Two cases developed metastatic disease, including one case with bilateral lung metastasis and one with locoregional spread to a lymph node. By analyzing the clinicopathologic features, we aim to improve recognition of this rare translocation sarcoma to better understand its biologic potential, optimize patient management, and expand the current classification of URCS.

PON3::LCN1 and HTN3::MSANTD3 Gene Fusions With NR4A3/NR4A2 Expression in Salivary Acinic Cell Carcinoma

Acinic cell carcinoma of the salivary gland (AciCC) is a low-grade carcinoma characterized by the overexpression of the transcription factor nuclear receptor subfamily 4 group A member 3 (NR4A3). AciCC has been the subject of a few molecular research projects. This study delves into AciCC's molecular landscape to identify additional alterations and explore their clinical implications. RNA sequencing and immunohistochemical staining for markers NR4A3/NR4A2, DOG-1, S100, and mammaglobin were utilized on 41 AciCCs and 11 secretory carcinoma (SC) samples. NR4A3 was evident in 35 AciCCs, while the residual 6 were NR4A3-negative and NR4A2-positive; SC samples were consistently NR4A3-negative. A novel fusion, PON3 exon 1- LCN1 exon 5, was detected in 9/41 (21.9%) AciCCs, exhibiting a classical histologic pattern with serous cell components growing in solid sheets alongside the intercalated duct-like component. Clinical follow-up of 39 patients over a median of 59 months revealed diverse prognostic outcomes: 34 patients exhibited no disease evidence, whereas the remaining 5 experienced poorer prognosis, involving local recurrence, lymph node, and distant metastasis, and disease-associated death, 4 of which harbored the PON3::LCN1 fusion. In addition, the HTN3::MSANTD3 fusion was recurrently identified in 7/41 AciCC cases. SC patients lacked both fusions. Immunohistochemistry uncovered differential expression of DOG-1, S100, and mammaglobin across samples, providing nuanced insights into their roles in AciCC. This study accentuates PON3::LCN1 and HTN3::MSANTD3 fusions as recurrent molecular events in AciCC, offering potential diagnostic and prognostic utility and propelling further research into targeted therapeutic strategies.

Detection of salivary gland and sinonasal fusions by a next-generation sequencing based, ligation-dependent, multiplex RT-PCR assay

AIMS

The discovery of tumour type-specific gene fusion oncogenes in benign and malignant salivary gland and sinonasal (SGSN) tumours has significantly increased our knowledge about their molecular pathology and classification.

METHODS AND RESULTS

We developed a new targeted multiplexed next-generation sequencing (NGS)-based method that utilizes ligation dependent reverse-transcriptase polymerase chain reaction (LD-RT-PCR) to detect oncogenic fusion transcripts involving 116 genes, leading to 96 gene fusions known to be recurrently rearranged in these tumours. In all, 180 SGSN tumours (formalin-fixed, paraffin-embedded samples, 141 specimens and 39 core needle biopsies) from the REFCORpath (French network for rare head and neck cancers) with previously identified fusion genes by fluorescent in situ hybridisation (FISH), RT-PCR, or molecular immunohistochemistry were selected to test its specificity and sensitivity and validate its diagnostic use. Tested tumours encompassed 14 major tumours types, including secretory carcinoma, mucoepidermoid carcinoma, adenoid cystic carcinoma, salivary gland intraductal carcinoma, clear cell carcinoma, pleomorphic adenoma, adamantinoma-like Ewing Sarcoma, EWSR1::COLCA2 sinonasal sarcoma, DEK::AFF2 sinonasal carcinoma, and biphenotypic sinonasal sarcoma. In-frame fusion transcripts were detected in 97.8% of cases (176/180). Gene fusion assay results correlated with conventional techniques (immunohistochemistry [IHC], FISH, and RT-PCR) in 176/180 tumours (97.8%).

CONCLUSION

This targeted multiplexed NGS-based LD-RT-PCR method is a robust, highly sensitive method for the detection of recurrent gene fusions from routine clinical SGSN tumours. It can be easily customized to cover new fusions. These results are promising for implementing an integrated NGS system to rapidly detect genetic aberrations, facilitating accurate, genomics-based diagnoses, and accelerate time to precision therapies in SGSN tumours.

Comprehensive Molecular Characterization of Polymorphous Adenocarcinoma, Cribriform Subtype: Identifying Novel Fusions and Fusion Partners

Polymorphous adenocarcinoma (PAC) is a common, usually low-grade salivary gland carcinoma. While conventional PACs are most associated with PRKD1 p.E710D hotspot mutations, the cribriform subtype is often associated with gene fusions in PRKD1, PRKD2, or PRKD3. These fusions have been primarily identified by fluorescence in situ hybridization (FISH) analysis, with a minority evaluated by next-generation sequencing (NGS). Many of the reported fusions were detected by break-apart FISH probes and therefore have unknown partners or were negative by FISH altogether. In this study, we aimed to further characterize the fusions associated with PAC with NGS. Fifty-four PACs (exclusively cribriform and mixed/intermediate types to enrich the study for fusion-positive cases) were identified and subjected to NGS. Fifty-one cases were successfully sequenced, 28 of which demonstrated gene fusions involving PRKD1, PRKD2, or PRKD3. There were 10 cases with the PRKD1 p.E710D mutation. We identified a diverse group of fusion partners, including 13 novel partners, 3 of which were recurrent. The most common partners for the PRKD genes were ARID1A and ARID1B. The wide variety of involved genes is unlike in other salivary gland malignancies and warrants a broader strategy of sequencing for molecular confirmation for particularly challenging cases, as our NGS study shows.

Palisading Adenocarcinoma: A Morphologically Unique Salivary Gland Tumor With a Neuroendocrine-like Appearance and a Predilection for the Sublingual Glands of Women

Adenocarcinoma, not otherwise specified (NOS) is a heterogenous group of salivary gland tumors that likely contains distinct tumors that have not yet been characterized. Indeed, in recent years, cases previously diagnosed as adenocarcinoma, NOS have been recategorized into novel tumor designations such as secretory carcinoma, microsecretory adenocarcinoma, and sclerosing microcystic adenocarcinoma. We sought to describe a distinctive, hitherto-undescribed salivary gland tumor encountered in the authors' practices. Cases were pulled from the surgical pathology archives of the authors' institutions. Histologic, immunohistochemical, and clinical findings were tabulated, and targeted next-generation sequencing was performed on all cases. Nine cases were identified, arising in 8 women and 1 man ranging from 45 to 74 years (mean, 56.7 y). Seven tumors (78%) arose in the sublingual gland, while 2 (22%) arose in the submandibular gland. The cases shared a distinctive morphologic appearance. They were biphasic, with ducts scattered among a predominant polygonal cell with round nuclei, prominent nucleoli, and pale eosinophilic cytoplasm. These cells were arranged as trabeculae and palisaded as pseudorosettes around hyalinized stroma and vessels, resembling a neuroendocrine tumor. Four of the cases were well-circumscribed, while the remaining 5 showed infiltrative growth including perineural invasion in 2 (22%) and lymphovascular invasion in 1 (11%). Mitotic rates were low (mean, 2.2/10 HPFs); necrosis was absent. By immunohistochemistry, the predominant cell type was strongly positive for CD56 (9 of 9) and variably positive for pan-cytokeratin (AE1/AE3) (7 of 9) with patchy S100 (4 of 9), but negative for synaptophysin (0 of 9) and chromogranin (0 of 9), while the ducts were strongly positive for pan-cytokeratin (AE1/AE3) (9 of 9) and CK5/6 (7 of 7). Next-generation sequencing did not reveal any fusions or obvious driver mutations. All cases were resected surgically, with external beam radiation also done in 1 case. Follow-up was available in 8 cases; there were no metastases or recurrences after 4 to 160 months (mean, 53.1 mo). A dual population of scattered ducts with a predominance of CD56-positive neuroendocrine-like cells characterizes a unique salivary gland tumor which is often encountered in the sublingual glands of women, for which we propose the term "palisading adenocarcinoma." Although the tumor was biphasic and had a neuroendocrine-like appearance, it lacked convincing immunohistochemical evidence of myoepithelial or neuroendocrine differentiation. Although a subset showed unequivocally invasive growth, this tumor appears to behave in an indolent manner. Moving forward, recognition of palisading adenocarcinoma and its separation from other salivary adenocarcinomas, NOS will facilitate a better understanding of the characteristics of this previously unrecognized tumor.

NKX3.1 Expression and Molecular Characterization of Secretory Myoepithelial Carcinoma (SMCA): Advancing the Case for a Salivary Mucous Acinar Phenotype

BACKGROUND

Secretory myoepithelial carcinomas (SMCA) are rare, mucinous, signet ring predominant tumors with primitive myoepithelial features. While many mucinous salivary gland tumors have now been molecularly characterized, key drivers in SMCA have yet to be elucidated. Recently, NKX3.1, a homeodomain transcription factor implicated in salivary mucous acinar development was also shown in a subset of salivary mucinous neoplasms, salivary intraductal papillary mucinous neoplasms (SG-IPMN). To date, NKX3.1 expression has not been characterized in other mucinous salivary lesions. Here, we report molecular and extended immunophenotypic findings in SMCA and NKX3.1 expression in the context of other head and neck lesions.

METHODS

We retrieved 4 previously reported SMCA, performed additional immunohistochemical and targeted next-generation sequencing (NGS). We also investigated the use of NKX3.1 as a marker for SMCA in the context of its prevalence and extent (using H-score) in a mixed cohort of retrospectively and prospectively tested head and neck lesions (n = 223) and non-neoplastic tissues (n = 66).

RESULTS

NKX3.1 positivity was confirmed in normal mucous acini as well as in mucous acinar class of lesions (5/6, mean H-score: 136.7), including mucinous adenocarcinomas (3/4), SG-IPMN (1/1), and microsecretory adenocarcinoma (MSA) (1/1). All SMCA were positive. Fluorescence in situ hybridization for SS18 rearrangements were negative in all successfully tested cases (0/3). NGS was successful in two cases (cases 3 and 4). Case 3 demonstrated a PTEN c.655C>T p.Q219* mutation and a SEC16A::NOTCH1 fusion while case 4 (clinically aggressive) showed a PTEN c.1026+1G>A p.K342 splice site variant, aTP53 c.524G>A p.R175H mutation and a higher tumor mutation burden (29 per Mb). PTEN immunohistochemical loss was confirmed in both cases and a subset of tumor cells showed strong (extreme) staining for P53 in Case 4.

CONCLUSION

Despite a partial myoepithelial phenotype, SMCA, along with mucinous adenocarcinomas/SG-IPMN and MSA, provisionally constitute a mucous acinar class of tumors based on morphology and NKX3.1 expression. Like salivary mucinous adenocarcinomas/SG-IPMN, SMCA also show alterations of the PTEN/PI3K/AKT pathway and may show progressive molecular alterations. We document the first extramammary tumor with a SEC16A::NOTCH1 fusion.

A Subset of Salivary Intercalated Duct Lesions Harbors Recurrent CTNNB1 and HRAS Mutations: A Molecular Link to Basal Cell Adenoma and Epithelial-Myoepithelial Carcinoma?

BACKGROUND

Intercalated duct lesions (IDLs) are benign salivary gland proliferations that resemble normal intercalated ducts and are subdivided into hyperplastic, adenoma or hybrid types depending on circumscription. While IDLs were historically regarded as non-neoplastic, frequent association with basal cell adenoma (BCA) and epithelial-myoepithelial carcinoma (EMC) has raised the possibility that they are neoplastic precursors.

METHODS

In this study, we performed β-catenin immunohistochemistry and targeted molecular analysis on IDLs to clarify their pathogenesis.

RESULTS

We identified 15 IDLs from the parotid glands of eight men and six women with a median age of 65 years (range 42-85 years). These lesions included nine hyperplastic, three adenoma, and three hybrid types. Nuclear β-catenin localization was present in 7 of 13 lesions tested (54%). Next generation sequencing was successfully completed in 12 IDLs, of which seven (58%) had likely oncogenic mutations. These included three recurrent CTNNB1 mutations in hyperplastic (n = 2) and hybrid (n = 1) lesions and two recurrent HRAS hotspot mutations in adenomas.

CONCLUSION

Despite substantial heterogeneity, these findings confirm that a majority of IDLs are genuinely neoplastic, and some demonstrate molecular overlap with both BCA and EMC, supporting their theorized role as precursors to these tumors. Nevertheless, no oncogenic drivers were present in a significant subset of cases, suggesting that some IDLs may be truly reactive and hyperplastic. As such, IDL appear to represent a diverse morphologic and molecular spectrum that include both neoplastic and hyperplastic lesions. Reconsideration of the boundary between IDL and BCA in the future may be necessary to simplify classification.