DNA methylation profiling and histologic analysis of sellar TTF-1-positive papillary epithelial tumor supports a novel CNS entity

To the Editor

The vast majority of sellar masses (85%–90%) are pituitary adenomas (1). Other common lesions found in the sellar and suprasellar region include Rathke’s cleft cysts, meningiomas, and craniopharyngiomas. Apart from these more common entities, there is a subset of non-neuroendocrine neoplasms of which approximately 1% express thyroid transcription factor-1 (TTF-1). These reported TTF-1-positive tumors of the sella region include ectopic choroid plexus papillomas (eCPPs), salivary gland-like tumors, pituicytomas, primary sellar melanocytic neoplasms, and primary sellar atypical teratoid rhabdoid tumors (1–4). TTF-1, also known as NKX2.1 or thyroid-specific enhancer-binding protein (T/EBP) (5), is an NKX2 homeodomain-containing transcription factor that regulates gene expression involved in cell differentiation, organogenesis, and embryogenesis (6). In adults, TTF-1 is also selectively expressed in thyroid, lung, basal ganglia, cortical interneurons, hypothalamic neurons, ependymal cells, tanycytes, and pituicytes (5, 7). In cancer, TTF-1 has been shown to perform both pro- and antioncogenic functions (8). While its role in cancer is not fully understood, TTF-1 is differentially expressed in neoplasms compared to benign tissue, and its expression is also varied among numerous tumor types (8).

Roncaroli et al (9) recently described 4 primary epithelial sellar tumors with papillary architecture, focal hemorrhage, and diffuse, nuclear TTF-1 expression. They categorized these tumors under the descriptive term “primary papillary epithelial tumor of the sella” (PPETS) as a novel tumor subtype that would be the intracranial equivalent of thyroid-like low-grade nasopharyngeal papillary adenocarcinoma. Furthermore, they suggest that the tumors previously classified as sellar eCPP are actually PPETS (9). Interestingly, sellar eCPPs (possible PPETS) differ from conventional choroid plexus papillomas (CPPs) in their diffuse expression of CK7, absence of CK20 and S-100, and focal expression of EMA, findings that further support their potential classification as a separate and possible novel entity (10, 11).

Here, we present the case of a 59-year-old man with dizziness, progressively worsening headaches and panhypopituitarism. Magnetic resonance imaging (MRI) revealed a 16 mm × 19 mm × 15 mm sellar mass abutting the optic chiasm suggestive of a pituitary macroadenoma. T1- and T2-weighted sequences showed increased signal on the right side of the sella mass. T2-weighted sequences revealed hyperintensity on the right side of the mass suggesting hemorrhage within the tumor itself (Fig. 1A–D). While the optic chiasm and cavernous sinuses were not displaced, the mass size had increased in comparison to an MRI taken 1 year earlier (14 mm × 16 mm × 14 mm, not shown). As a result, the patient underwent a trans-sphenoidal resection of the mass.

Figure 1.

(A–D) MRI showing a sellar mass measuring 16 mm × 19 mm × 15 mm. The sellar mass appears as mostly bright on T1 images, which is suggestive of hemorrhage or proteinaceous components. The mass abuts the optic chiasm with a midline pituitary infundibulum. There is no midline shift, extra-axial fluid collection, or abnormal enhancement in the brain parenchyma. Arrows point to the mass. (A, T1 coronal view; B, T1 sagittal view; C, T2 coronal view; D, T2 axial view). (E–G) Photomicrographs of neoplastic cells stained with hematoxylin and eosin (200–400× original magnification) showing papillary architecture and focal areas of hemorrhage (marked with asterisks). (H) Representative image showing approximately 4% of neoplastic cells positive for Ki-67 (200× original magnification). (I, J) Immunohistochemical images of sellar mass showing strong positive immunostaining for (I) CK7 and (J) nuclear TTF-1 (400× original magnification).

Histologically the mass displayed prominent papillary architecture that was more complex than that of normal choroid plexus. The papillary epithelium showed both focal crowding and some pseudostratification (Fig. 1E–G). Blood accumulation including some hemosiderin-laden macrophages was present in the vicinity of the mass (Fig. 1E, F). No mitotic activity was identified and the Ki67 proliferation index of the papilloma determined by digital analysis was 4% (Fig. 1H). Immunohistochemistry (IHC) of the tumor revealed strong CK7 positivity in the epithelial cells, a finding that supports an epithelial papillary lesion (Fig. 1I). Further, the nuclei showed strong positive staining for TTF-1 (Fig. 1J), which raised the consideration of thyroid, lung, pituicyte and choroid plexus as possible origins of this mass (5, 7). No abnormal nuclear staining for beta-catenin was present, EMA staining was patchy, and IHC staining for chromogranin, synaptophysin, LH, TSH, FSH, prolactin, GH, ACTH, BRAF V600E, CK20, Napsin, S-100, PAX8, and CEA were all negative (Table). The negative IHC results decreased the likelihood that the tumor was a pituicytoma, ependymoma, neuroendocrine tumor, pituitary adenoma, metastatic papillary thyroid carcinoma, craniopharyngioma (adamantinomatous or papillary), or metastatic gastric or pulmonary carcinomas (Table). The tumor’s histologic appearance and immunohistochemical staining profile supported consideration of the tumor as the World Health Organization described choroid plexus of ectopic location over neoplasms of thyroid, lung, or posterior pituitary gland origin. Although the lesion was considered compatible with an eCPP of the sella, only approximately 5% of CPPs show positively for TTF-1. Therefore, we entertained the possibility that this TTF-1-positive papillary tumor was the novel CNS tumor entity PPETS recently proposed by Roncaroli et al. To address this diagnostic uncertainty, we performed DNA methylation profile analysis.

TABLE.

Immunohistochemistry Phenotype of the Sellar Mass and their Corresponding Diagnostic Interpretations

IHC Marker	Result	Interpretation
CK7	+	Supports epithelial papillary lesion
TTF-1	+	Supports origin from thyroid, lung, pituicyte, or choroid plexus
S-100	−	Argues against neural origin
GFAP	−	Argues against pituicytoma or ependymoma
Chromogranin and synaptophysin	−	Argues against neuroendocrine tumor
Pituitary hormones	−	Argues against pituitary adenoma
(LH, TSH, FSH, GH, ACTH, Prolactin)
PAX8 and thyroglobulin	−	Argues against thyroid carcinoma
BRAF V600E	−	Argues against metastatic papillary thyroid carcinoma or papillary craniopharyngioma
Beta-catenin (nuclear)	−	Rules out adamantinomatous craniopharyngioma
CK20 and CEA	−	Argues against GI metastasis
Napsin	−	Rules out metastic lung adenocarcinoma

Neoplastic cells evolve through aberrant gene editing and epigenetic modifications, including DNA methylation, histone modifications, nucleosome remodeling, and expression of microRNAs (12). Recently, DNA methylation arrays have been shown to improve diagnostic precision of CNS tumor classification by identification of DNA methylation profiles characteristic of the majority of formally recognized CNS neoplasms. This approach has contributed to the reliability of histology-based pathology diagnoses of known CNS tumor entities and also assisted in identifying novel CNS tumor entities (13). In this case of a possible novel CNS tumor entity, DNA methylation profiling was performed using genomic DNA extracted from formalin-fixed paraffin-embedded tissue sections with the Infinium Methylation EPIC kit. Methylation-based tumor classification analysis was performed using the Tumor Classifier software pipeline that was developed using a DNA methylation-based CNS tumor classification method through machine learning algorithms (14, 15). This CNS methylation profiling approach was especially helpful in tumor classification since histologic appearance and IHC analysis of this tumor supported a primary CNS neoplasm over one of extracranial origins. The completed methylation profile analysis of this sellar tumor did not match any previously characterized CNS tumors with a high methylation profile confidence score (>0.85), including but not limited to choroid plexus tumors, craniopharyngiomas, or pituitary adenomas. Recently, it had been proposed that TTF-1-positive papillary epithelial neoplasms represent epithelial variants of pituicytomas. However, the methylation profile confidence score for pituicytoma was very low (0.3), therefore not diagnostic of a tumor of definite pituicyte origin (16).

In summary, given the histologic similarity that this tumor shares with the 4 primary tumors documented by Roncaroli et al and the methylation profile results of the neoplasm described here, this tumor most closely matches a PPETS, a primary sellar tumor entity with distinctive papillary architecture, robust nuclear TTF-1 expression, and diffuse cytokeratin positivity. While the cell of origin of PPETS is not determined, we propose that PPETS represents a novel CNS tumor entity that is distinct from previously classified CNS neoplasms.

CONFLICT OF INTEREST

The authors have no duality or conflicts of interest to declare.

DATA AVAILABILITY

The datasets generated and/or analyzed during the current study are not publicly available due to being protected patient information but are available from the corresponding author on reasonable request.