Solitary mucosal neuroma in the maxillary incisor gingival papilla without multiple endocrine neoplasia type 2B (MEN 2B): a case report and literature review

Mucosal neuroma (MN) is a benign neural tumor of peripheral nerves histologically characterized by irregular tortuous bundles of nerve cells with prominent perineurium that lie scattered throughout the submucosa. The tumor is usually associated with the multiple endocrine neoplasia type 2B (MEN 2B) but rarely occurs without the other components of MEN 2B. We present a case of solitary MN without MEN 2B that occurs in the maxillary incisor gingival papilla that has not been reported yet and review the literature.


Background
In 1975, Khairi et al. [1] proposed that the condition of combined medullary thyroid carcinoma (MTC), pheochromocytomas and mucosal neuroma (MN) should be referred to as multiple endocrine neoplasia (MEN) type 3, also known as multiple endocrine neoplasia type 2B (MEN 2B) now. MEN 2B is an autosomal dominant syndrome associated with RET gene mutations and characterized by an aggressive form of MTC and bilateral pheochromocytomas (PCC) [2,3]. MN of MEN 2B is always observed on the mucosal surfaces of the lips, tongue, eyelids, and intestines of patients [4], but rarely occur without the other abnormality about MEN 2B. MN without MEN 2B have been reported in the rectosigmoid colon [5], bronchi [6,7], conjunctiva [8], laryngeal [9,10,15], tongue [12][13][14][15], lip [13]and hard palate [16], but there has been no reported case in the gingiva to our knowledge.
The solitary MN without MEN that occurs in the gingival papilla is easy to be confused with epulis and other oral neural tumors (ONTs) such as solitary circumscribed neuromas (SCN) in clinical practice. Epulis is an in ammatory reactive tumor-like proliferator with limited growth on the gingiva, especially on the gingival papilla. It is derived from the periodontal ligament and connective tissue of the gingiva. Because it has no tumor biological characteristics and structure, it is a non-genuine tumor, but it is easy to relapse after resection [18]. Solitary circumscribed neuromas (SCN), also known as Palisaded encapsulated neuroma (PEN), was rst reported in 1972 by Reed et al. [19], The oral cavity is the second most frequent location for SCN after the skin, and it most often appears as painless super cial nodule same as MN [20].
Clinically, MN usually presents as a rm, slowly enlarging, small, circumscribed mass, which is painless in most cases [3,7,13,16], although it can frequently cause symptoms such as pain and paresthesia [5,6,9,10,15]. In the vast majority of cases, complete surgical excision is curative; however, recurrences have been reported in some cases [14,17]. Histologically, MN is characterized by irregular tortuous bundles of nerve cells with prominent perineurium that lie scattered throughout the submucosa [11,32].
Herein, we present a case of solitary MN without MEN 2B occurs in the gingival papilla that has not been reported yet.

Case Presentation
A 29-year-old Asian female patient presented to the outpatient clinic of Department of Periodontology, Shanghai Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine (Shanghai, China) with a 2-year history of an upper anterior gingiva painless mass accompanied with bleeding on brushing teeth occasionally for several years. She brushes her teeth twice a day for 3 ~ 5 minutes, with the use of dental oss. Her past medical history was noncontributory. Intraoral examination revealed a small, pinkcolored, well-de ned mass which is tough and basal wide in the labial gingival papilla between maxillary central incisor (teeth nos. 8 and 9) (Fig. 1). At baseline, the patient had poor oral hygiene and gingiva was slightly red and swollen, plaque and calculus deposition, and a small amount of pigmentation was found on the tooth surface; A means probing depth (PD) of 2.83 ± 1.02 mm, means clinical attachment loss (CAL) of 1.18 ± 0.68 mm and 73% of bleeding on probing (BOP). The mandibular left second molar (tooth no. 18) had a full crown restoration with Class II tooth mobility, and the lingual root furcation can be probed horizontally (Class II furcation involvement).
The patient underwent Oral Panoramic Radiography and digital periapical lm exam, and the results showed that there were no obvious abnormalities in the imaging performance of the remaining teeth except for tooth no. 18 ( Fig. 2a and b). The primary differential diagnoses considered for the gingival mass included peripheral ossifying broma ( brous epulis), broma, and other oral benign tumors.

Material And Methods
The patient was informed that periodontal treatment was initiated via oral hygiene instructions and professional supragingival bio lm control, including supragingival and subgingival scaling, 4-6 weeks after periodontal initial therapy, the patient should be re-evaluation, including the mass size and periodontal clinical indexes.
After periodontal initial therapy, excisional biopsy was performed of the mass. The tissue sections were xed in formalin, embedded in para n, and then were stained with hematoxylin-eosin (HE) and immunohistochemistry (IHC). Hematoxylin-eosin staining was performed as follows: The specimen was xed in 4% paraformaldehyde at 4˚C for 24 h. The specimen was then dehydrated as follows: 70% ethanol (60 min), 80% ethanol (40 min), 95% ethanol (30 min), 100% ethanol (25 min) at room temperature, and embedded in para n. Following embedding, the specimen was sliced into 5-μm thick slices. The slides were depara nized in xylene, rehydrated in 100% ethanol, 95% ethanol, 80% ethanol and 70% ethanol for 2 min each, and then stained with hematoxylin and eosin for 1 min, all at room temperature. Following heatinduced epitope retrieval, slides were incubated with antibodies speci c for S-100 protein, epithelial membrane antigen (EMA), neuro lament protein (NFP), and neuron speci c enolase (NSE). To rule out the possibility of MEN 2B, we also did endocrinological examinations and ultrasonography of the thyroid gland.
The literature review was performed using the PubMed electronic database to identify relevant publications before December 2019 using the following search term: "mucosal neuroma without MEN type 2B". From the publications obtained in this search, those pertaining to cases of NM without MEN type 2B were included in the review. Additionally, a manual search was conducted by cross-referencing the retrieved manuscripts. All available data were reviewed, including epidemiology, clinical presentation, histopathologic examination ndings, and surgical intervention (Table 1).

Case presentation
Six weeks after SRP, the re-evaluation clinical data showed a reduction in mean PD (2.67 ± 0.81 mm) and a percentage of BOP (20%). There was no signi cant change in the size of the mass in the buccal gingival papilla between teeth nos. 8 and 9 ( Fig. 2c and d).
Then, we performed an excisional biopsy of the mass. Examination of the biopsy specimen showed a 0.6 cm*0.6 cm*0.5 cm, gray-white colored, well de ned mucosal tissue. Histologically, nerve bundles in various sizes surrounded by normal connective tissue in the submucosa can be seen under HE staining, and nerve bundles are wrapped by thick perineurium (Fig. 3a and b).
Immunohistochemically, the mass showed strongly positive staining of S-100 protein, neuron speci c enolase (NSE), neuro lament protein (NFP) and weakly positive of epithelial membrane antigen (EMA). (Fig. 4a-h). Based on these features, the pathological diagnosis was MN of the maxillary incisor gingiva. 3 months later, the operation area heals well (Fig. 3c).
To rule out the possibility of MEN 2B, we did endocrinological examinations. The serum concentrations of carcinoembryonic antigen, epinephrine, norepinephrine, dopamine, and calcitonin were all within normal limits. Ultrasonography of the thyroid gland was normal. These examinations showed no signs of MEN 2B. Finally, the lesion was clinically diagnosed as a solitary gingival mucosal neuroma. In the rst 6 months of follow-up, there was no evidence of recurrence and other components of MEN 2B. The patient was asymptomatic and she is still being followed up every 6 months.
According to the data reviewed, the patients were between 4 and 73 years of age and the lesion had no signi cant sex predilection. MN without MEN 2B in the oral cavity usually presents as a painless subcutaneous mass. MN in the oropharynx (corner of the mouth, lips, tongue, palate, buccal mucosa and throat) was seen in 12 of 18 cases (66.7%) and a less common location was the bronchia (2/18, 11.9%), conjunctival fornix (1/18, 5.56%), rectosigmoid colon (1/18, 5.56%) and larynx (1/18, 5.56%). Location unknown in one case. No cases of MN without MEN 2B in the gingiva have been reported.
The preferred surgical treatment in the majority of cases was local excision, and 2 cases of recurrence have been reported in previous studies.

Discussion
Oral neurogenic tumors (ONTs) originate from cells associated with components of the peripheral nervous system. As an ONT, mucosal neuroma (MN) can be distinguished histologically from other ONTs such as neuro broma, neurilemmoma and Palisaded encapsulated neuroma (PEN). Brie y, the microscopic examination of the MN shows nerve bundles in various sizes surrounded by normal connective tissue, and nerve bundles are wrapped by thick nerve bundle membrane, which is not usually seen in PENs [11], and the MN is not encapsulated and does not have palisading nuclei whereas neurilemomas and PENs are encapsulated [21].
Neuro bromas are not encapsulated and have no palisading, but neurilemmal cells and possibly broblasts participate in their formation, resulting in irregular combinations of these elements [22]. However, plexiform neuro bromas (PN), atypical types of neuro bromas, may be confused with mucosal neuromas, because both tumors have similar microscopic ndings [23]. Epithelial membrane antigen (EMA) is the most useful marker in differentiating MN from plexiform neuro broma immunohistochemically. The perineurial cells of the former show the marker and tumor cells of the latter do not [23]. Although the immunohistochemistry of our case showed weakly positive of EMA, almost all plexiform neuro bromas occurred in patients with neuro bromatosis type 1 (NF1). Furthermore, the microscopic examination of the plexiform neuro broma usually shows enlarged nerve bundles and the interstitium mostly mucoid, so the histological features of our case still resembled MN.
Some studies have shown that the proliferating nerve bundles express EMA, suggesting that the nerve bundles have differentiated to the perineurial cells [33]. While other studies have shown that there is no EMA-positive nerve bundle membrane, the author speculates that it may be due to the poor immunohistochemical technique or insu cient differentiation of the perineurial cells [9]. In our case, the structure of the nerve bundle membrane is obvious under the microscope, and EMA is positively expressed in the epithelial area, so we infer that the weakly positive reaction may be caused by insu cient differentiation of the perineurial cells.
MNs occur in the oral cavity, especially in the gingival papilla, which is also easy to be confused with other tumor-like lesions such as epulis [31]. As a most common type of epulis, peripheral ossifying bromas often show pink-colored, tough, and well-de ned mass the same as MNs. Peripheral ossifying bromas can occur in all age groups but are more common in 10 to 40 years old. Histologically, peripheral ossifying bromas usually show a stromal broblastic proliferation with intermixed hypermineralized bony tissue under HE staining, so we can differentiate MNs from peripheral ossifying bromas.
MNs usually occur in the oral cavity as a component of MEN 2B, which is a syndrome of multiple MNs, medullary thyroid carcinoma (MTC), pheochromocytoma (PCC), bumpy lips, and marfanoid habitus, although not always concurrently. Oral MNs are the most common component among these components, presenting usually at early infancy, and are most often found in the lips, tongue and buccal mucosa, other less common sites being the palate and gingiva [4,24]. When the lips are involved, the lips are diffusely enlarged (bumpy lips). When the tongue and buccal mucosa are affected, they appear as semicircular nodules or pimples. Our patient showed no abnormalities in the skeletal structure, lip shape, sonographic examination of the thyroid, or in any of the endocrine examinations, and there were no tumors elsewhere in the oral cavity or the ocular region.
MN without MEN 2B is very rare, a total of 19 cases have been reported including the case reported in this article. A solitary MN is present in the absence of other diagnostic signs, so it is necessary to combine the clinical examination, histopathological evaluation, IHC, radiography, and biochemical studies. Our patient was diagnosed as a solitary gingival mucosal neuroma without MEN 2B. Nevertheless, some investigators have suggested that MTC and PCC appear later [25,30]. Thus, follow-up studies including radiography, endocrine examinations are necessary.

Conclusions
This study showed a rare mucosal neuroma in the gingiva papilla without MEN, the histopathological evaluation and immunoreaction of S-100 protein, EMA, NSE and NFP staining can be helpful in the differential diagnosis of MN. It is hoped that a greater understanding of MN without MEN in the oral cavity will avoid potential misdiagnosis, and contribute to determining the correct management, which appears to be complete surgical excision with close follow-up for recurrence and other components of MEN surveillance.    Figure 1 Intraoral photograph. At baseline, intraoral examination revealed a small, pink-colored, well-de ned mass in the patient's labial gingival papilla between maxillary central incisor (teeth nos. 8 and 9).  Histological hematoxylin and eosin photomicrographs. Histologically, irregular tortuous bundles of nerve cells (asterisks) with prominent perineurium (arrows) that lie scattered throughout the submucosa can be seen under HE staining (a to c). Magni cation: ×50 (a), ×100 (b), ×200 (c). 3 months after operation, the operation area heals well (d).