Attempting to Solve the Pigmented Epithelioid Melanocytoma (PEM) Conundrum: PRKAR1A Inactivation Can Occur in Different Genetic Backgrounds (Common, Blue, and Spitz Subgroups) With Variation in Their Clinicopathologic Characteristics

PRKC Fusion Melanocytic Tumors, a Subgroup of Melanocytic Tumors More Closely Aligned to Blue Nevi Than to PRKAR1A-inactivated Pigmented Epithelioid Melanocytomas

Tumors morphologically classified as pigmented epithelioid melanocytomas (PEMs) are genomically diverse, with the 2 most common genomic subtypes being PRKC fusions or PRKAR1A inactivating mutations. PRKC fusions activate the Gαq/11 pathway similar to blue nevi. Conversely, inactivating mutations in PRKAR1A activate the Gαs pathway. We hypothesize that PRKC fusions have greater genomic overlap with blue nevi compared with PRKAR1A-inactivated PEMs. We characterized the clinical and morphologic features of 21 PRKC and PRKACB fusion melanocytic tumors and compared this to PRKAR1A mutated PEMs. To test our hypothesis regarding greater genomic overlap between PRKC fusions and blue nevi relative to PRKAR1A mutated PEMs, we performed a principal component analysis (PCA) using mRNA expression data. Lastly, we performed a meta-analysis focusing on the outcome data of PRKC fusions. PRKC fusions occur at a younger median age than PRKAR1A mutated PEMs (16 vs. 27). Histologically, PRKC fusions have solid aggregates of epithelioid melanocytes not typical of PRKAR1A mutated PEMs. The PCA plot showed no overlap between the PRKC fusion group and the PRKAR1A-mutated PEMs. There was a significant overlap between PRKC fusions and blue nevi. A meta-analysis of PRKC fusion cases in the literature suggests melanoma is uncommon, but the loss of BAP-1 nuclear expression may be associated with an adverse prognosis as in tumors from the blue nevus family. PRKC fusion melanocytic tumors have greater genomic overlap with blue nevi compared with PRKAR1A mutated PEMs. We recommend categorizing benign PRKC fusion melanocytic tumors as blue fusion nevi/tumors.

[Clinical, histological and genetic correlations in melanocytic tumours with chromosomal rearrangements]

In some tumoral subtypes chromosomal translocations lead to an oncogenic chimeric protein acting as a tumorigenesis driver event. The main fusion model combines the promoter swapping of an inactivated tumor suppressor gene and a functional kinase that evades its regulatory system. The range of described fusions keeps growing in the 2023 WHO classification of melanocytic tumours. It is not limited to the group of Spitz tumours as previously but now extends to blue tumours and dermal tumours with a melanocytic phenotype. Molecular pathology helps detect these anomalies using clinical and morphological features. This analysis is essential as this strongly conditions the adapted local treatment of such tumours who are often overtreated.

GRM1 Gene Fusions as an Alternative Molecular Driver in Blue Nevi and Related Melanomas

Activating mutations in GNAQ, GNA11, CYSLTR2, and PLCB4 genes are regarded as the main oncogenic drivers of blue nevi (BN) and blue malignant melanocytic tumors. Here we report 4 cases of blue melanocytic neoplasms devoid of these mutations but harboring GRM1 gene fusions. In this short series, there was no gender predominance (sex ratio, 1). The mean age at diagnosis was 40 years (range, 12-72). Tumors were located on the face (n = 2), forearm (n = 1), and dorsum of the foot (n = 1). Clinically, a plaque-like pre-existing BN was found in 2 cases, including a deep location; another case presented as an Ota nevus. Two cases were diagnosed as melanoma ex-BN, one as an atypical BN, and one as a plaque-like BN. Microscopic examination revealed a dermal proliferation of dendritic melanocytes in a sclerotic stroma. A dermal cellular nodule with atypia and mitotic activity was observed in 3 cases. Genetic investigation by whole exome RNA sequencing revealed MYO10::GRM1 (n = 2) and ZEB2::GRM1 (n = 1) fusions. A GRM1 rearrangement was identified by fluorescence in situ hybridization in the remaining case. SF3B1 comutations were present in the 2 melanomas, and both had a MYO10::GRM1 fusion. Array comparative genomic hybridization was feasible for 3 cases and displayed multiple copy number alterations in the 2 melanomas and limited copy number alterations in the atypical BN, all genomic profiles compatible with those of classical blue lesions. GRM1 was overexpressed in all cases compared with a control group of blue lesions with other typical mutations. Both melanomas rapidly developed visceral metastases following diagnosis, with a fatal outcome in one case and tumor progression under palliative care in the other. These data suggest that GRM1 gene fusions could represent an additional rare oncogenic driver in the setting of BN, mutually exclusive of classical canonical mutations, especially in plaque-type or Ota subtypes.

Melanocytic naevi, melanocytomas and emerging concepts

With the elucidation of the genetics of melanocytic tumours, new concepts have emerged. An important one is the identification of 'intermediate' melanocytic tumours, those with genetic progression events beyond those of melanocytic naevi but that are not fully malignant. Thus, melanocytic tumours exist on a genetic spectrum that likely corresponds to biological behaviour. There are multiple pathways to melanoma development with different initiating events and characteristic benign melanocytic neoplasms and the precise placement of tumours on these pathways remains to be established and the corresponding risks of progression quantified. In this review, I discuss the classification of melanocytic naevi based on clinical, histopathological and genetic features, as well as the concept of melanocytomas with discussion of specific recognisable subtypes.

Blue naevi and the blue tumour spectrum

Blue naevi (BN) form a wide group of benign dermal melanocytic proliferations. They are genetically distinct from common and Spitz naevi with frequent hotspot mutations occurring in Gαq genes. Clinically, BN display a female predominance, elective sites of emergence and a great variety of subtypes related to specific regions of the skin linked to early embryological genetic events. Histologically, most BN are located in the dermis with small, bland, spindled and dendritic pigmented melanocytes within a fibrous background. Variation in tumour volume, fibrosis, and melanin pigment load can be broad. A growth in size and cellularity can occur within a subset of tumours as they acquire the morphological features of cellular blue naevi, with a biphasic architecture associating a dendritic blue naevus morphology near the surface, and deep vertical cellular expansions of medium-sized, bland melanocytes often reaching the subcutis. Sclerosing and myxoid variants can be observed either as individual or combined modifications that can add complexity to an otherwise straightforward diagnosis. Malignant progression of a cellular blue naevus is exceptional with an intermediate stage named atypical cellular blue naevus. Malignant blue melanomas are fast growing, large, pigmented tumours with most often obvious features of malignancy. However, they are difficult to separate from other malignant dermal melanocytic proliferations. Herein, we will extensively detail and illustrate the clinical, histological and genetic features of the vast spectrum of blue naevi and related entities in the skin.

Modern Concepts in Melanocytic Tumors

The advent of molecular pathology has fueled unprecedented advances in the diagnosis and understanding of melanocytic tumors. These advances, however, have also generated concepts that may be difficult to grasp for clinical practitioners, who are not always conversant with the array of genetic techniques employed in the laboratory. These same practitioners, however, are being increasingly called on to provide treatments that are often based on the latest molecular findings for melanocytic tumors. We review the most recent concepts in the pathway classification of melanocytic tumors, including intermediate lesions known as melanocytomas. We examine the genetic and molecular techniques used to study these tumors, look at where they overlap, and discuss their limitations and some of the most difficult-to-interpret results.

The Spectrum of Spitz Melanocytic Lesions: From Morphologic Diagnosis to Molecular Classification

Spitz tumors represent a distinct subtype of melanocytic lesions with characteristic histopathologic features, some of which are overlapping with melanoma. More common in the pediatric and younger population, they can be clinically suspected by recognizing specific patterns on dermatoscopic examination, and several subtypes have been described. We now classify these lesions into benign Spitz nevi, intermediate lesions identified as “atypical Spitz tumors” (or Spitz melanocytoma) and malignant Spitz melanoma. More recently a large body of work has uncovered the molecular underpinning of Spitz tumors, including mutations in the HRAS gene and several gene fusions involving several protein kinases. Here we present an overarching view of our current knowledge and understanding of Spitz tumors, detailing clinical, histopathological and molecular features characteristic of these lesions.