Insulinoma-associated 1: A sensitive marker of neuroendocrine differentiation in cutaneous and metastatic neuroendocrine tumors

Understanding Merkel Cell Carcinoma: Pathogenic Signaling, Extracellular Matrix Dynamics, and Novel Treatment Approaches

Merkel cell carcinoma (MCC) is a rare but aggressive neuroendocrine skin cancer, driven by either Merkel cell polyomavirus (MCPyV) integration or ultraviolet (UV)-induced mutations. In MCPyV-positive tumors, viral T antigens inactivate tumor suppressors pRb and p53, while virus-negative MCCs harbor UV-induced mutations that activate similar oncogenic pathways. Key signaling cascades, including PI3K/AKT/mTOR and MAPK, support tumor proliferation, survival, and resistance to apoptosis. Histologically, MCC consists of small round blue cells with neuroendocrine features, high mitotic rate, and necrosis. The tumor microenvironment (TME) plays a central role in disease progression and immune escape. It comprises a mix of tumor-associated macrophages, regulatory and cytotoxic T cells, and elevated expression of immune checkpoint molecules such as PD-L1, contributing to an immunosuppressive niche. The extracellular matrix (ECM) within the TME is rich in proteoglycans, collagens, and matrix metalloproteinases (MMPs), facilitating tumor cell adhesion, invasion, and interaction with stromal and immune cells. ECM remodeling and integrin-mediated signaling further promote immune evasion and therapy resistance. Although immune checkpoint inhibitors targeting PD-1/PD-L1 have shown promise in treating MCC, resistance remains a major hurdle. Therapeutic strategies that concurrently target the TME-through inhibition of ECM components, MMPs, or integrin signaling-may enhance immune responses and improve clinical outcomes.

Evaluation of immunohistochemical expression of novel neuroendocrine marker INSM1 and histological tumor growth pattern in well-differentiated neuroendocrine tumors of the appendix: 15-year single tertiary center experience

INTRODUCTION

Well-differentiated neuroendocrine tumors of the appendix (A-WDNET) are the most common neoplasms of the appendix but are rare and incidental. INSM1 is a novel transcription factor marker with high sensitivity and specificity for neuroendocrine differentiation. It may display architecturally diverse tumor growth patterns including solid, nest, insular trabecular, and acinar.

AIMS

The aim of this study was to determine the staining expression of INSM1 in A-WDNETs and detail the morphological tumor growth patterns.

MATERIALS AND METHODS

INSM1 immunohistochemistry was performed on 35 A-WDNET patients. Tumors were histologically classified according to their growth patterns.

RESULTS

Thirty-five patients, 60% of whom were male, had a mean age of 30 years at diagnosis and a mean tumor size of 0.9 cm. Eight percent of the tumors invaded the mucosa/submucosa, 34.3% showed invasion into the muscularis propria, 57.1% showed invasion into the subserosa or mesoappendix, LN metastasis was observed in two patients, lymph-vascular invasion in two patients, and perineural invasion in five patients. Ninety-four percent of the tumors were grade 1, the mitotic rate was >2% in two cases, and Ki-67 PI was >3% in two cases. INSM1 was positive in all cases (100%), 1+ 8.6%, 2+ 5.7%, 3+ 17.1%, 4+ 68.6%, weak staining in 11.4%, moderate staining in 22.9%, and strong staining in 67.7%. Type A histological growth pattern was observed in 54.3%, type B in 31.4%, and type C in 14.3% architecturally.

CONCLUSIONS

INSM1 was positive in all A-WDNET morphological patterns and was 100% sensitive. INSM1 IHC can be used as an alternative to traditional neuroendocrine markers or in combination with the diagnosis of neuroendocrine tumors.

Hiding in Plain Sight: An Atypical Presentation of the Uncommon Merkel Cell Carcinoma

Merkel cell carcinoma (MCC) is a cutaneous neoplasm that is challenging to diagnose secondary to its rarity. We report a case involving a 76-year-old Caucasian female with a seemingly benign skin nodule on her right forearm. Histopathological analysis revealed characteristics of MCC, including uniform round cells with minimal cytoplasm and fine granular chromatin. Immunohistochemical staining confirmed insulinoma-associated protein 1 (INSM1) positivity, a marker with high sensitivity and specificity in localized MCC diagnosis. The subsequent treatment plan involved wide local excision, sentinel lymph node evaluation, and radiation therapy, aligning with therapeutic standards for MCC. Negative positron emission tomography (PET) scans and follow-up for one year have demonstrated no evidence of recurrence or additional lesions. This case demonstrates the challenges in diagnosing MCC and the need for histopathological and immunohistochemical assessments for an accurate diagnosis. Diagnostic markers, INSM1, are important distinguishing factors between MCC and other skin cancers. In conclusion, our case contributes to the literature in diagnosing MCC and successful treatment, while emphasizing the need for immunohistochemical markers for accurate diagnosis and guiding therapeutic decisions.

Cutaneous Neuroendocrine Mucinous Carcinomas Are Low-grade But May Be Associated With Other Cancers

Primary cutaneous mucinous sweat gland carcinoma is said to be prognostically stratifiable by neuroendocrine differentiation, however, this assertion is based on historical data and older staining techniques. We aimed to evaluate the percentage of mucinous and nonmucinous adnexal tumors expressing the newer, more sensitive neuroendocrine marker insulinoma-associated protein 1 (INSM1), and to assess clinicopathologic features in patients cohorted by this phenotype. Of 12 available adnexal/cutaneous adenocarcinomas, 9 were mucinous, 3/9 of which were INSM1-negative, and 2/3 with nodal metastases. Of 3 nonmucinous cases, all were INSM1-negative, 1/3 with nodal metastasis, and 2/3 with lymphovascular invasion. In contrast, of 6 mucinous INSM1-positive cases, no cases had LVI or metastasis, however, 3 patients died during follow-up, 2 from breast or urothelial cancer. A fourth patient developed breast carcinoma. INSM1-positive tumors, from cheek (3), scalp (2), and chin (1) were estrogen receptor and progesterone receptor positive. No cases of apocrine adenoma or hidrocystoma, basal cell, or sebaceous carcinoma labeled with INSM1. While most primary cutaneous mucinous carcinomas are of the neuroendocrine type, our study confirms the presence of occasional non-neuroendocrine mucinous carcinomas. We validate the association of such tumors and nonmucinous non-neuroendocrine adnexal carcinoma with intermediate-grade behavior, including lymph node metastases, but not death. Conversely, neuroendocrine expressing primary cutaneous mucinous carcinoma may represent the well-differentiated neuroendocrine neoplasm/neuroendocrine tumor primary to skin, with low-grade behavior, but attendant risk of germline susceptibility to other aggressive extracutaneous tumors. Routine assessment of cutaneous adnexal carcinoma with INSM1 and longer term follow-up and cancer screening of patients with positive staining is recommended.

[Whipple's triad with high and low insulin levels]

A 69-year-old female patient and a 70-year-old male patient were admitted to hospital with recurrent, severe hypoglycemic episodes and a typical manifestation of Whipple's triad. In the female, elevated levels of insulin, C‑peptide and pro-insulin together with pathological findings during a fasting test proved the presence of an insulinoma, which could be detected by Ga-68-DOTATOC-PET-CT in the pancreas. There was a very rare co-existence of a neuroendocrine Merkel cell carcinoma. In the male, levels of insulin and C‑peptide were suppressed and a diagnosis of paraneoplastic hypoglycemia by IGF‑2 secretion was made with increased glucose disposal in skeletal muscle proven by ¹⁸F‑FDG-PET-CT.