From morphology to methylome: epigenetic studies of Müllerian mesonephric‐like adenocarcinoma reveal similarities to cervical mesonephric adenocarcinoma†

Mesonephric adenocarcinomas (MAs) and mesonephric-like adenocarcinomas (MLAs) are rare, aggressive neoplasms that arise in the gynecologic tract and show overlapping morphologic, immunohistochemical, and molecular features. While MAs occur in the cervix and are thought to arise from mesonephric remnants, MLAs occur in the endometrium and ovary and are believed to originate from transdifferentiation of Müllerian lesions. Both MAs and MLAs show a variety of architectural patterns, exhibit frequent expression of GATA3 by immunohistochemistry, and harbor KRAS mutations. In a recent article published in The Journal of Pathology, Kommoss and colleagues used DNA methylation profiling to extend these similarities and showed that MLAs and MAs cluster together based on their epigenetic signatures and are epigenetically distinct from other Müllerian adenocarcinomas. They also showed that MLAs and MAs harbor a high number of global copy number alterations. This study provides evidence that MLAs more closely resemble MAs than Müllerian carcinomas on an epigenetic level. As a result, the authors argue that MLA should be renamed ‘mesonephric-type adenocarcinoma.’ Further research is needed to establish the relationship between these two entities, their etiology, and pathogenesis. © 2024 The Pathological Society of Great Britain and Ireland. Mesonephric adenocarcinomas (MAs) are aggressive and rare HPV-independent carcinomas that occur in the uterine cervix [1, 2]. They are thought to arise from mesonephric remnants, due to their association with mesonephric hyperplasia and localization to the lateral cervical wall. Mesonephric-like adenocarcinomas (MLAs), first described in 2016 by McFarland et al [3], display morphologic, immunohistochemical, and molecular overlap with MAs, but typically occur in the endometrium and ovary and are not associated with mesonephric remnants [1, 2]. MAs and MLAs show similar morphologic features, with diverse growth patterns, including glandular, solid, papillary, retiform, and tubular architectures with colloid-like eosinophilic material within lumens. Cells are often cuboidal with nuclear clearing, nuclear grooves, and variable mitotic activity. Spindle cell morphology has also been described. Immunohistochemically, both MLAs and MAs are typically positive for PAX8, CK7, CD10 (apical), GATA3, and TTF-1, and negative for ER and PR [1, 2]. Despite the overlap in their immunoprofiles, MLAs more frequently show TTF-1 positivity and focal ER/PR positivity when compared with MAs [2]. Molecularly, the majority of MAs and MLAs are characterized by driver mutations in KRAS, and less frequently in NRAS. MLAs and MAs show recurrent copy number variations with loss of chromosome 1p and gain of 1q and variable gains in chromosomes 2, 10, 12, and 20 [2, 4]. MLAs and MAs also show some genetic differences: MLAs often harbor mutations associated with endometrioid neoplasia, such as PIK3CA, PTEN, or CTNNB1 [2]. Most cases would be categorized as copy number-low or no specific molecular profile (NSMP) according to the endometrial TCGA classification. MLA has a deceptively bland appearance, often with predominantly glandular architecture and low-grade cytology. As such, before the recognition and description of MLAs, these tumors had largely been diagnosed as low-grade endometrioid carcinomas. They comprise ~1% of endometrial carcinomas and are likely underrecognized [4]. It has become increasingly clear that the importance of correctly recognizing MLAs is two-fold. First, they have an aggressive disease course characterized by a propensity to metastasize to the lung. Although the morphology (low-grade cytology and absence of significant solid component) and molecular profile (NSMP) of most MLAs are suggestive of a low-grade carcinoma, the clinical behavior of MLA is more similar to grade 3 endometrioid or serous carcinoma [2]. Second, the presence of a KRAS mutation allows for potential targeted therapy and the diagnosis of MLA alerts clinicians to this possibility. Indeed, there is currently a phase 2 trial of avutometinib (a RAF/MEK inhibitor) and defactinib (a FAK inhibitor) in MAs and MLAs (NCT05787561). As pathologists become more familiar with the diagnosis, the incidence of MLAs will likely increase, and we will have a better understanding of their clinicopathologic features and responses to targeted therapies. In a recent study published in The Journal of Pathology, Kommoss et al [5] investigated copy number changes and methylation profiles of 19 MLAs (5 uterine and 14 ovarian) and 7 cervical MAs and compared them with various Müllerian adenocarcinomas. They first confirmed the previously reported characteristic copy number changes found in MLAs and MAs. They then revealed that MLAs and MAs showed an intermediate global burden of copy number alterations, much higher than other NSMP endometrial carcinomas but lower than aggressive TP53-mutant tumors such as ovarian high-grade serous carcinoma. While the mechanism underlying the high frequency of copy number alterations is unclear, it provides a potential explanation for the aggressive behavior of MLAs and MAs. The authors then compared the methylation profiles of MLAs and MAs with other Müllerian carcinomas. Interestingly, MLAs and MAs showed the closest epigenetic profile among the studied neoplasms, suggesting that these two tumors not only harbor overlapping morphology, immunohistochemical, and mutational profiles but also a similar epigenome. MLA formed a methylation cluster distinct from other ovarian and endometrial adenocarcinomas, supporting MLA as a unique entity, such as it is categorized in the current WHO 2020 classification of female genital tumors. Methylation profiling has emerged as a useful tool in tumor diagnosis and classification. The study by Kommoss et al [5] shows the potential role of methylation analysis to aid in the diagnosis of MLA and MA, while also highlighting the epigenetic similarity of these neoplasms. Interestingly, MAs formed a separate cluster from MLAs, which suggests that there are still differences in the epigenome between these two neoplasms, perhaps reflecting their site of origin or subtle immunohistochemical and molecular distinctions (Figure 1). In addition, methylation analysis appeared unable to discriminate between ovarian and endometrial MLAs, which supports the notion that MLAs in these two sites represent similar disease processes. The pathogenesis of MLA is still unclear. MLA was coined as ‘mesonephric-like’ by McFarland et al [3] in its original description, given the absence of associated mesonephric rests and its anatomic location in the endometrium (where there are no mesonephric remnants), which would argue against a true mesonephric origin. Accumulating evidence has shown that associated Müllerian lesions are clonally related to MLA (e.g. ovarian MLA associated with endometriosis, ovarian MLA occurring as mixed tumors with serous and mucinous neoplasms, and endometrial MLA associated with endometrial intraepithelial neoplasia and endometrioid adenocarcinomas) [2, 6]. MLAs, in contrast to MAs, often harbor mutations typically identified in Müllerian adenocarcinomas, such as PIK3CA, PTEN, and CTNNB1. These findings support the theory that MLAs originate from transdifferentiation of Müllerian lesions [2]. An alternative theory of the origin of MLA is based on the presence of stem cells in the genital tract that could differentiate into a mesonephric-type neoplasm. The intimate relationship and common origin of Müllerian (paramesonephric) and Wolffian (mesonephric) ducts during embryonic development could potentially explain a common progenitor stem cell giving rise to clonally related Müllerian and mesonephric(-like) lesions. Even though MAs are currently believed to be bona fide mesonephric neoplasms due to their anatomic location and close association with mesonephric remnants/hyperplasia, limited molecular data support the morphologic association between MAs and mesonephric remnants. In a series of mesonephric hyperplasia not associated with MAs, no KRAS or NRAS mutations were identified [7]. In another case series of cervical mesonephric proliferations, KRAS mutations and chromosome 1q gains were demonstrated in three cases of atypical mesonephric hyperplasia associated with endocervical adenocarcinomas [8]. However, MAs can show a diverse range of morphologic patterns with variable atypia; hence, it is reasonable to consider the possibility that atypical mesonephric hyperplasia co-occurring with MA could represent a well-differentiated component of MA and not a true precursor lesion. Further research is warranted to better understand the pathogenesis of both MA and MLA as well as their putative precursor lesions. Methylation analyses can detect tissue-specific patterns and also predict the primary site of metastatic carcinomas; therefore, it can capture signatures of the cell of origin of neoplasms. However, additional evidence has shown that the methylation profile can change in cells that exhibit transdifferentiation and also appears to shift depending on the site in cases of synchronous tumors [9]. Therefore, the similarity between the methylation profiles of MLA and MA is not necessarily evidence of a mesonephric origin for MLA. Additionally, while the study by Kommoss et al [5] compared MLAs with NSMP endometrial carcinomas, it is important to note that the NSMP group comprises a heterogeneous group of tumors. Further research evaluating ER/PR-negative NSMP tumors compared with MLAs are needed, since hormonal status greatly influences epigenetic features including global methylation profiles [10]. As a recently described entity, the diagnostic criteria for MLA are broadly understood, but not precisely defined. Uncertainty surrounds the boundaries of morphologic, immunohistochemical, and molecular features that are required for diagnosis. For example, does any squamous or mucinous differentiation exclude MLA? How much hormone receptor positivity can be tolerated in MLA? Do all MLAs need to be NSMP by TCGA molecular classification? Methylation analysis provides an intriguing technique to investigate these questions by providing an orthogonal method to classify cases with otherwise ambiguous features. Finally, we would exert caution on the proposition to rename MLAs from ‘mesonephric-like’ to ‘mesonephric-type’ adenocarcinomas [5]. While we appreciate the points raised by the authors, we are apprehensive of changes in disease terminology and feel that there is insufficient evidence or reason for revised nomenclature. MA in the cervix is currently classified as mesonephric-type adenocarcinoma by the WHO, and MLA is already a well-stablished term within the literature. Therefore, renaming MLA to ‘mesonephric-type’ adenocarcinoma could lead to confusion between these two entities. This change seems premature as more research is needed to elucidate the pathogenesis of cases with mesonephric differentiation, including MA, MLA, and tumors with mixed morphology.

BEH and DLK were supported by the Mesonephric-like Adenocarcinoma Consortium (MLAC). Author contributions statement LHL, BEH and DLK wrote and edited manuscript. All authors read and approved the final paper.