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Cutaneous neuroendocrine (Merkel cell) carcinoma most often arises de novo in the background of a clonally integrated virus, the Merkel cell polyomavirus, and is notable for positive expression of retinoblastoma 1 (RB1) protein and low expression of p53 compared with the rare Merkel cell polyomavirus-negative Merkel cell carcinomas. Combined squamous and Merkel cell tumors are consistently negative for Merkel cell polyomavirus. Little is known about their immunophenotypic or molecular profile. Herein, we studied 10 combined cutaneous squamous cell and neuroendocrine carcinomas for immunohistochemical expression of p53, retinoblastoma 1 protein, neurofilament, p63, and cytokeratin 20 (CK20). We compared mutation profiles of five combined Merkel cell carcinomas and seven ‘pure’ Merkel cell carcinomas using targeted next-generation sequencing. Combined tumors were from the head, trunk, and leg of Caucasian males and one female aged 52–89. All cases were highly p53- and p63-positive and neurofilament-negative in the squamous component, whereas RB1-negative in both components. Eight out of 10 were p53-positive, 3/10 p63-positive, and 3/10 focally neurofilament-positive in the neuroendocrine component. Six out of 10 were CK20-positive in any part. By next-generation sequencing, combined tumors were highly mutated, with an average of 48 mutations per megabase compared with pure tumors, which showed 1.25 mutations per megabase. RB1 and p53 mutations were identified in all five combined tumors. Combined tumors represent an immunophenotypically and genetically distinct variant of primary cutaneous neuroendocrine carcinomas, notable for a highly mutated genetic profile, significant p53 expression and/or mutation, absent RB1 expression in the context of increased RB1 mutation, and minimal neurofilament expression.

Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine malignancy of the skin. The cell of origin of MCC is thus far unknown and proposed cells of origin include Merkel cells, pro-/pre- or pre-B cells, epithelial stem cells, and dermal stem cells. In this study, we aimed to shed further light on the possibility that a subset of MCC tumors arise from epithelial stem cells of the skin by examining the expression of hair follicle and epidermal stem cell markers in MCC and normal human skin. We also aimed to elucidate any correlation between the expression of these markers and tumor Merkel cell polyomavirus (MCPyV) status or other clinicopathological characteristics or patient survival. Expression of CK19, SOX9, LGR5, and LRIG1 in MCC and normal human skin was studied by immunohistochemistry, and the staining patterns or intensities were statistically correlated with patient, tumor, MCPyV, and survival parameters. In a cohort of 137 cases of MCC, we observed dot-like immunoexpression of CK19 in 30 cases (22.1%) and homogeneous expression in 103 cases (75.7%). We also observed positive immunoexpression of SOX9 in 21 cases (15.3%), LGR5 in 118 cases (86.1%), and LRIG1 in 117 cases (86.0%). Immunoexpression of LRIG1 was found to correlate with better overall and MCC-specific survival. We observed frequent immunoexpression of several hair follicle and epidermal stem cell markers in MCC and found LRIG1 to be a positive prognostic marker in MCC.

Backrgound Merkel cell carcinoma (MCC) is a rare, aggressive primary cutaneous neuroendocrine carcinoma, frequently associated with clonal Merkel cell polyomavirus integration. MCC can pose significant diagnostic challenges due to its diverse clinical presentation and its broad histological differential diagnosis. Histologically, MCC presents as a small-round-blue cell neoplasm, where the differential diagnosis includes basal cell carcinoma, melanoma, hematologic malignancies, round cell sarcoma and metastatic small cell carcinoma of any site. We here report strong aberrant immunoreactivity for BCOR in MCC, a marker commonly used to identify round cell sarcomas and other neoplasms with BCOR alterations. Methods Based on strong BCOR expression in three index cases of MCC, clinically mistaken as sarcoma, a retrospective analysis of three patient cohorts, comprising 31 MCC, 19 small cell lung carcinoma (SCLC) and 5 cases of neoplasms with molecularly confirmed BCOR alteration was conducted. Immunohistochemical staining intensity and localization for BCOR was semi-quantitatively analyzed. Results Three cases, clinically and radiologically mimicking a sarcoma were analyzed in our soft tissue and bone pathology service. Histologically, the cases showed sheets of a small round blue cell neoplasm. A broad panel of immunohistochemistry was used for lineage classification. Positivity for synaptophysin, CK20 and Merkel cell polyoma virus large T-antigen lead to the diagnosis of a MCC. Interestingly, all cases showed strong positive nuclear staining for BCOR, which was included for the initial work-up with the clinical differential of a round cell sarcoma. We analyzed a larger retrospective MCC cohort and found aberrant weak to strong BCOR positivity (nuclear and/or cytoplasmic) in up to 90% of the cases. As a positive control, we compared the expression to a small group of BCOR -altered neoplasms. Furthermore, we investigated a cohort of SCLC as another neuroendocrine neoplasm and found in all cases a diffuse moderate to strong BCOR positivity. Conclusions This study demonstrates that neuroendocrine neoplasms, such as MCC and SCLC can express strong aberrant BCOR. This might represent a diagnostic challenge or pitfall, in particular when MCC is clinically mistaken as a soft tissue or a bone sarcoma.

Merkel cell carcinoma is a primary cutaneous neuroendocrine carcinoma, which once metastatic is difficult to treat. Recent mutation analyses of Merkel cell carcinoma revealed a low number of mutations in Merkel cell polyomavirus-associated tumors, and a high number of mutations in virus-negative combined squamous cell and neuroendocrine carcinomas of chronically sun-damaged skin. We speculated that the paucity of mutations in virus-positive Merkel cell carcinoma may reflect a pathomechanism that depends on derangements of chromatin without alterations in the DNA sequence (epigenetic dysregulation). One central epigenetic regulator is the Polycomb repressive complex 2 (PRC2), which silences genomic regions by trimethylating (me3) lysine (K) 27 of histone H3, and thereby establishes the histone mark H3K27me3. Recent experimental research data demonstrated that PRC2 loss in mice skin results in the formation of Merkel cells. Prompted by these findings, we explored a possible contribution of PRC2 loss in human Merkel cell carcinoma. We examined the immunohistochemical expression of H3K27me3 in 35 Merkel cell carcinomas with pure histological features (22 primary and 13 metastatic lesions) and in 5 combined squamous and neuroendocrine carcinomas of the skin. We found a strong reduction of H3K27me3 staining in tumors with pure histologic features and virus-positive Merkel cell carcinomas. Combined neuroendocrine carcinomas had no or only minimal loss of H3K27me3 labeling. Our findings suggest that a PRC2-mediated epigenetic deregulation may play a role in the pathogenesis of virus-positive Merkel cell carcinomas and in tumors with pure histologic features.

Merkel cell carcinoma is a highly aggressive cutaneous neuroendocrine tumor that has been associated with Merkel cell polyomavirus in up to 80% of cases. Merkel cell polyomavirus is believed to influence pathogenesis, at least in part, through expression of the large T antigen, which includes a retinoblastoma protein-binding domain. However, there appears to be significant clinical and morphological overlap between polyomavirus-positive and polyomavirus-negative Merkel cell carcinoma cases. Although much of the recent focus of Merkel cell carcinoma pathogenesis has been on polyomavirus, the pathogenesis of polyomavirus-negative cases is still poorly understood. We hypothesized that there are underlying human somatic mutations that unify Merkel cell carcinoma pathogenesis across polyomavirus status, and to investigate we performed whole exome sequencing on five polyomavirus-positive cases and three polyomavirus-negative cases. We found that there were no significant differences in the overall number of single-nucleotide variations, copy number variations, insertion/deletions, and chromosomal rearrangements when comparing polyomavirus-positive to polyomavirus-negative cases. However, we did find that the retinoblastoma pathway genes harbored a high number of mutations in Merkel cell carcinoma. Furthermore, the retinoblastoma gene (RB1) was found to have nonsense truncating protein mutations in all three polyomavirus-negative cases; no such mutations were found in the polyomavirus-positive cases. In all eight cases, the retinoblastoma pathway dysregulation was confirmed by immunohistochemistry. Although polyomavirus-positive Merkel cell carcinoma is believed to undergo retinoblastoma dysregulation through viral large T antigen expression, our findings demonstrate that somatic mutations in polyomavirus-negative Merkel cell carcinoma lead to retinoblastoma dysregulation through an alternative pathway. This novel finding suggests that the retinoblastoma pathway dysregulation leads to an overlapping Merkel cell carcinoma phenotype and that oncogenesis occurs through either a polyomavirus-dependent (viral large T antigen expression) or polyomavirus-independent (host somatic mutation) mechanism.

Merkel cell carcinoma of the skin is a malignant neuroendocrine tumour, whose prognostic criteria are a matter of dispute. Specifically, no predictor is presently available in stage I–II tumours. We collected clinical and follow-up data from 70 Merkel cell carcinomas of the skin. The same cases were studied for p63 expression by immunohistochemistry, by reverse-transcription PCR (RT-PCR) and TP63 gene status by FISH and for presence of Merkel cell polyomavirus by PCR. Stage emerged as a significant prognostic parameter ( P =0.008). p63 expression, detected in 61% (43/70) of cases by immunohistochemistry, was associated with both decreased overall survival ( P <0.0001) and disease-free survival ( P <0.0001). Variable expression patterns of the different p63 isoforms were found only in cases immunoreactive for p63. In these latter lesions, at least one of the N-terminal p63 isoforms was detected and TAp63 α was the most frequently expressed isoform. TP63 gene amplification was observed by FISH in only one case. Presence of Merkel cell polyomavirus DNA sequences was detected in 86% (60/70) of Merkel cell carcinomas and did not emerge as a significant prognostic parameter. Merkel cell carcinoma cases at low stage (stage I-II) represented over half (40/70 cases, 57%) of cases, and the clinical course was uneventful in 25 of 40 cases while 15 cases died of tumour (10/40 cases) within 34 months or were alive with disease (5/40 cases) within 20 months. Interestingly, a very strict correlation was found between evolution and p63 expression ( P <0.0001). The present data indicate that p63 expression is associated with a worse prognosis in patients with Merkel cell carcinoma, and in localised tumours it represents the single independent predictor of clinical evolution.

Sentinel lymph node biopsy is used to stage Merkel cell carcinoma, but its prognostic value has been questioned. Furthermore, predictors of outcome in sentinel lymph node positive Merkel cell carcinoma patients are poorly defined. In breast carcinoma, isolated immunohistochemically positive tumor cells have no impact, but in melanoma they are considered significant. The significance of sentinel lymph node metastasis tumor burden (including isolated tumor cells) and pattern of involvement in Merkel cell carcinoma are unknown. In this study, 64 Merkel cell carcinomas involving sentinel lymph nodes and corresponding immunohistochemical stains were reviewed and clinicopathological predictors of outcome were sought. Five metastatic patterns were identified: (1) sheet-like ( n =38, 59%); (2) non-solid parafollicular ( n =4, 6%); (3) sinusoidal, ( n =11, 17%); (4) perivascular hilar ( n =1, 2%); and (5) rare scattered parenchymal cells ( n =10, 16%). At the time of follow-up, 30/63 (48%) patients had died with 21 (33%) attributable to Merkel cell carcinoma. Patients with pattern 1 metastases had poorer overall survival compared with patients with patterns 2–5 metastases ( P =0.03), with 22/30 (73%) deaths occurring in pattern 1 patients. Three (10%) deaths occurred in patients showing pattern 5, all of whom were immunosuppressed. Four (13%) deaths occurred in pattern 3 patients and 1 (3%) death occurred in a pattern 2 patient. In multivariable analysis, the number of positive sentinel lymph nodes (1 or 2 versus >2, P <0.0001), age (<70 versus ≥70, P =0.01), sentinel lymph node metastasis pattern (patterns 2–5 versus 1, P =0.02), and immune status (immunocompetent versus suppressed, P =0.03) were independent predictors of outcome, and could be used to stratify Stage III patients into three groups with markedly different outcomes. In Merkel cell carcinoma, the pattern of sentinel lymph node involvement provides important prognostic information and utilizing this data with other clinicopathological features facilitates risk stratification of Merkel cell carcinoma patients who may have management implications.

Primary neuroendocrine carcinoma of the skin, or Merkel cell carcinoma, is the most aggressive cutaneous neoplasm. In spite of its similarities to small cell carcinomas from other locations, Merkel cell carcinoma shows many peculiarities probably related to its epidermal origin and the etiologic role of UV radiation. We have immunohistochemically investigated 43 markers on a tissue microarray in which 31 surgically resected Merkel cell carcinomas were represented. Of these, 15 patients remained free of disease after removal, whereas 16 developed metastases. Immunoreactivity was scored according to staining intensity and the percentage of positive cells. We found statistically significant correlations between metastatic tumor spread and overexpression of matrix metalloproteinase (MMP) 7, MMP10/2, tissue inhibitor of metalloproteinase 3, vascular endothelial growth factor (VEGF), P38, stromal NF-kappaB, and synaptophysin. Also detected were statistically significant correlations between the expression levels of MMP7 and VEGF, MMP7 and P21, MMP7 and P38, MMP10/2 and VEGF, P38 and synaptophysin, P38 and P53, and P21 and stromal NF-kappaB. These findings may be helpful in predicting the clinical course of Merkel cell carcinoma and are potentially useful for the development of targeted therapies.

Objective. —To study the expression of thyroid transcription factor 1 (TTF-1) and cytokeratin 20 (CK20) in pulmonary small cell carcinomas, extrapulmonary small cell carcinomas, and Merkel cell carcinomas, and thereby determine whether these markers are helpful in distinguishing these 3 groups of small cell neuroendocrine carcinomas. Materials and Methods. —Immunostaining for TTF-1 and CK20 was performed in 102 cases of small cell carcinoma (pulmonary, 52; extrapulmonary, 50) and 23 cases of Merkel cell carcinoma. The results for the 3 groups were compared. Results. —Thyroid transcription factor 1 was expressed in 82.7% of pulmonary small cell carcinomas, 42.0% of extrapulmonary small cell carcinomas (range, 33.3–53.3% for the various sites), and 0% of Merkel cell carcinomas. Cytokeratin 20 staining was consistently negative in pulmonary small cell carcinomas, and positive in 4.0% of extrapulmonary small cell carcinomas and 100% of Merkel cell carcinomas. Conclusions. —Immunostaining for TTF-1, especially when combined with immunostaining for CK20, can aid in the distinction between Merkel cell carcinoma and small cell carcinoma (both pulmonary and extrapulmonary). However, in individual cases, these markers cannot be used to distinguish between pulmonary and extrapulmonary small cell carcinomas due to the extensive overlap in immunophenotypes.

Dear Editor, We herein report an unusual case of a cytokeratin-20 positive/cytokeratin-7 negative (CK20+/CK7–) Merkel cell carcinoma (MCC) that also showed immunoreactivity to two different thyroid transcription factor-1 (TTF-1) clones. Merkel cell carcinoma with cytokeratin 20-negative and thyroid transcription factor-1-positive immunostaining admixed with squamous cell carcinoma. Sierakowski A, Al-Janabi K, Dam Hv, Sood M. Metastatic Merkel cell carcinoma with positive expression of thyroid transcription factor-1 – a case report.