BAP1-deficient and VE1-negative atypical Spitz tumor

Beyond typical histology of BAP1-inactivated melanocytoma

BAP1-inactivated melanocytoma (BIM) is a novel subgroup of melanocytic neoplasm listed in the 5th edition of WHO classification of skin tumor. BIM is characterized by two molecular alterations, including a mitogenic driver mutation (usually BRAF gene) and the loss of function of BAP1, a tumor suppressor gene located on chromosome 3p21, which encodes for BRCA1-associated protein (BAP1). The latter represents a nuclear-localized deubiquitinase involved in several cellular processes including cell cycle regulation, chromatin remodeling, DNA damage response, differentiation, senescence and cell death. BIMs are histologically characterized by a population of large epithelioid melanocytes with well-demarcated cytoplasmic borders and copious eosinophilic cytoplasm, demonstrating loss of BAP1 nuclear expression by immunohistochemistry. Recently, we have published a series of 50 cases, extending the morphological spectrum of the neoplasm and highlighting some new microscopic features. In the current article, we focus on some new histological features, attempting to explain and link them to certain mechanisms of tumor development, including senescence, endoreplication, endocycling, asymmetric cytokinesis, entosis and others. In light of the morphological and molecular findings observed in BIM, we postulated that this entity unmasks a fine mechanism of tumor in which both clonal/stochastic and hierarchical model can be unified.

RAF1 Gene Fusions as a Possible Driver Mechanism in Rare BAP1-Inactivated Melanocytic Tumors: A Report of 2 Cases

BRCA1-associated protein (BAP1)-inactivated melanocytic tumor (BIMT) is a group of epithelioid melanocytic neoplasms characterized by the loss of function of BAP1, a tumor suppressor gene located on chromosome 3p21. They occur sporadically or in the setting of an autosomal-dominant cancer susceptibility syndrome that predisposes to the development of different internal malignancies. Most of these cutaneous lesions are associated with a BRAF-mutated melanocytic nevus and therefore are included in the group of combined nevi in the last WHO classification of skin tumors. Apart from a BRAF mutation, an NRAS mutation has been reported in rare cases, whereas in some lesions no driver mutation has been detected. Here, we report 2 cases of BIMTs with a BAP1 mutation and a RAF1 fusion. Both lesions proved to be BRAF and NRAS wild type and were associated with a conventional melanocytic nevus with dysplastic junctional features. We suggest that RAF1 fusions can represent an underlying driver genetic event in these cases. Our study extends the morphological and molecular spectrum in BIMTs.

BAP1-inactivated melanocytic tumor with preserved BAP1 expression? Morphology to the rescue!

BAP1-inactivated melanocytic tumors typically present with distinctive histopathological changes and loss of nuclear BAP1 protein expression. Rare cases exhibit the typical morphology but with preserved expression of BAP1. In the current issue of Journal of Cutaneous Pathology, Linos et al. describe such a case and provide a comprehensive molecular-genetic exploration to explain such a phenomenon.

New and evolving concepts of melanocytic nevi and melanocytomas

In daily clinical practice melanocytic nevi are commonly encountered. Traditionally, both benign and malignant melanocytic tumors have been sub-classified by their histopathologic characteristics with differing criteria for malignancy applied to each group. Recently, many of the mutations that initiate nevus formation have been identified and specific sets of mutations are found in different subtypes of nevi. Whereas a single mutation appears sufficient to initiate a nevus, but is not enough to result in melanoma, specific combinations of mutations have been identified in some melanocytic tumors that are regarded to be of low biologic potential. The term "melanocytoma" has recently been proposed by the World Health Organization to describe those tumors that demonstrate genetic progression beyond the single mutations that are found in nevi but are not frankly malignant. Melanocytomas occupy intermediate genetic stages between nevus and melanoma and likely have an increased risk of malignant transformation as compared to nevi. This review provides an update on the broad spectrum of melanocytic nevi and melanocytomas and outlines their key histopathologic and genetic features.

BRCA1-associated protein (BAP1)-inactivated melanocytic tumors

Although discussed using variable terminology, cutaneous BRCA1-associated protein (BAP1)-inactivated melanocytic tumor (BIMT) has been considered a discrete diagnostic entity since 2011. Here, we review the initial genomic studies that identified these distinct melanocytic tumors and the clinical and histopathological features that define these tumors. These epithelioid, predominantly dermal, and melanocytic tumors present as erythematous nodules and histopathologically have features that may overlap with Spitz nevi and nevoid melanoma. There is no sex predilection, and cutaneous BIMTs can appear at any age; however, in most familial (germline mutant) cases patients have multiple cutaneous tumors with a first diagnosis in the second or third decade of life; ocular melanoma and other tumors are increasingly identified in these kindreds with germline BAP1 mutation. These tumors have been described with a myriad of terms including: Wiesner nevus, nevoid melanoma-like melanocytic proliferation (NEMMP), BAP1 mutant Spitz nevus, BAP1 mutant nevoid melanoma, cutaneous BAPoma, and most recently cutaneous BIMT.

Spitz Nevus and Other Spitzoid Tumors in Children. Part 2: Cytogenetic and Molecular Features. Prognosis and Treatment

Melanocytic neoplasms with spitzoid morphology (Spitz nevi, atypical Spitz tumors, and spitzoid melanomas) may be benign or malignant. Because the malignant potential of atypical Spitz tumors is uncertain, the proper therapeutic approach has been much debated over the years. Promising new techniques for molecular analysis have enabled better predictions of the biological behavior of these tumors. We review their cytogenetic features and prognosis and also provide an update of the most recent recommendations for management.

BAP1-deficient tumor/nevus with germline aberration: A potential pitfall in assessing melanocytic neoplasms with single nucleotide polymorphism array

BRCA1-associated protein 1 (BAP1) is a tumor suppressor gene, located on chromosome 3p21, encoding BAP1 nuclear protein, which is associated with a subset of melanocytic tumors with distinct cytologic features. Single nucleotide polymorphism array (SNP-array) is a molecular karyotyping technique that can detect copy number variations and loss of heterozygosity in various fresh and formalin-fixed paraffin-embedded tissues. Herein we present a 56-year-old female, who presented with a lesion on her left nose/cheek that was growing in size and changing in color. Histopathology was characteristic of a BAP1-deficient melanocytic neoplasm, with a biphasic population of cytologically bland conventional nevomelanocytes and a proliferation of large epithelioid melanocytes with abundant eosinophilic cytoplasm. Immunohistochemistry for BAP1 showed loss of nuclear labeling in the epithelioid melanocytes. SNP-array revealed a chromosome 21q22.1 monoaberration with no chromosome 3 abnormalities. The detection of this aberration prompted a discussion as to whether the lesion was best designated as a nevus or tumor. SNP-array on the patient's blood showed the same monoaberration of chromosome 21q22.1. This case emphasizes the importance of interpreting microarray results in the context of morphology, as germline aberrations can be a pitfall when assessing the genomic stability of a melanocytic proliferation by SNP array.

Immunoexpression of BAP1, ROS1, and ALK in Spitzoid Melanocytic Tumors

BACKGROUND

Spitzoid tumors are a heterogeneous group of melanocytic neoplasms that frequently imposes diagnostic difficulties. Lately, several advances in molecular biology afforded significant discoveries on the pathogenesis of these tumors. BAP1 (BRCA-1 associated protein-1) inactivation and anomalous expression of kinase translocation-related proteins are among the main criteria launched by new classification proposals. Our aim was to systematically assess the immunoexpression of BAP1, ROS1 (receptor tyrosine kinase c-Ros oncogene 1), and ALK (anaplastic lymphoma receptor tyrosine kinase) proteins in an unpublished series of spitzoid tumors.

METHODS

Retrospective study based on 47 formalin-fixed paraffin-embedded tissue samples from 3 different institutions. BAP1, ROS1, and ALK immunostains were performed in all cases. We included 27 Spitz tumors without significant abnormality, 15 atypical spitzoid tumors, and 5 spitzoid melanomas.

RESULTS

We observed loss of BAP1 nuclear immunolabeling in 4.3% of evaluable cases (2/46), both of them atypical spitzoid tumors. The proportional frequency of BAP1-inactivated cases among atypical spitzoid tumors was 14.2% (2/14). No immunoexpression of ROS1 or ALK was found.

CONCLUSIONS

Our study revealed 2 additional BAP1-inactived cases and described its respective frequency. The absence of anomalous expression of translocation-related proteins ALK and ROS1 in this series, composed predominantly of low-grade/low-risk tumors, indicates that translocated spitzoid lesions may not be as prevalent as initially suggested, at least in some populations. Furthermore, our findings encourage additional investigation on unequal occurrence of such immunomarkers among different diagnostic categories of spitzoid neoplasms.

The complex management of atypical Spitz tumours

In recent years, advances in molecular genetic characterisation have revealed that atypical Spitz tumours (ASTs) are basically heterogeneous diseases, although the clinical relevance of these findings is yet to be determined. Evidence of molecularly-defined diverse groups of lesions continues to accumulate; however, conflicting, confusing, and overlapping terminology has fostered ambiguity and lack of clarity in the field in general. The lack of fundamental diagnostic (morphological) unambiguous classification framework results in a number of challenges in the interpretation of the molecular genetic data. In this review, we discuss the main difficulties for pathologists and clinicians in the complex management of ASTs, with particular emphasis on the different genetic and biological features of recently-described entities, and offer our view of what could be medically reasonable to guide a rational approach in light of current data.