Spitz Melanoma of Childhood With A Novel Promoter Hijacking Anaplastic Lymphoma Kinase (C2orf42-ALK) Rearrangement

A Clinical, Morphologic, and Molecular Comparison of Bonafide Spitz Melanomas and ASTs in the Pediatric Population

Pediatric Spitz melanoma (SM) with bonafide metastatic disease is rare. In this study, we assembled the largest cohort to date of pediatric SM with a verified Spitz-associated genomic driver and clinical follow-up demonstrating bonafide metastasis. We compared the clinical, morphologic, and molecular features of these SMs to a control cohort of 57 pediatric atypical Spitz tumors (ASTs). Pediatric SM patients were significantly older than AST patients (12 vs 8 years of age). While not statistically significant, SMs were more likely to be heavily pigmented (5/7 SMs vs 11/57 ASTs), to have a sheet-like growth pattern (3/7 SMs vs 8/57 ASTs), and have severe nuclear atypia (6/7 SMs vs 20/57 ASTs). SMs had significantly greater mitotic activity (avg of 4.3/mm 2 in SMs and 2.7/mm 2 in ASTs, P =0.008) and more frequent larger cell size ( P =0.006). However, none of these features were specific and could also be seen in ASTs. The presence of homozygous deletions of 9p21 in conjunction with TERT promoter hot spot mutations or PTEN deletions (n=2), as well as MYC overexpression or amplification (n=2) were only seen in the SMs and none of the ASTs. These findings were mutually exclusive in the SM group and mutually exclusive with the presence of complex chromosomal copy number aberrations, which were seen in the remaining 3 pediatric SMs. This study demonstrates that there are multiple pathways to malignancy for pediatric SMs and none of our commonly used biomarkers have a particularly high sensitivity. Hence, the optimal distinction of pediatric SM from ASTs will continue to require the integration of clinical, histologic, and molecular data.

Spitz Melanocytic Tumors: A Fascinating 75-Year Journey

Over the last 75 years, our understanding of Spitz lesions has undergone substantial evolution. Initially considered a specific type of melanoma, the perception has shifted towards recognizing Spitz lesions as a spectrum comprising Spitz nevi, Spitz melanocytomas, and Spitz melanomas. Spitz lesions are known for posing a significant diagnostic challenge regarding the distinction between benign neoplasms displaying atypical traits and melanomas. A comprehensive understanding of their molecular basis and genomic aberrations has significantly improved precision in classifying and diagnosing these challenging lesions. The primary aim of this review is to encapsulate the current understanding of the molecular pathogenesis and distinct clinicopathologic characteristics defining this intriguing set of tumors.

An update on genomic aberrations in Spitz naevi and tumours

Spitz neoplasms continue to be a diagnostic challenge for dermatopathologists and are defined by distinctive morphological and genetic features. With the recent advancements in genomic sequencing, the classification, diagnosis, and prognostication of these tumours have greatly improved. Several subtypes of Spitz neoplasms have been identified based on their specific genomic aberrations, which often correlate with distinctive morphologies and biological behaviour. These genetic driver events can be classified into four major groups, including: (1) mutations [HRAS mutations (with or without 11p amplification) and 6q23 deletions]; (2) tyrosine kinase fusions (ROS1, ALK, NTRK1-3, MET and RET); (3) serine/threonine kinase fusions and mutations (BRAF, MAP3K8, and MAP2K1); and (4) other rare genomic aberrations. These driver genomic events are hypothesised to enable the initial proliferation of melanocytes and are often accompanied by additional genomic aberrations that affect biological behaviour. The discovery of theses genomic fusions has allowed for a more objective definition of a Spitz neoplasm. Further studies have shown that the majority of morphologically Spitzoid appearing melanocytic neoplasms with aggressive behaviour are in fact BRAF or NRAS mutated tumours mimicking Spitz. Truly malignant fusion driven Spitz neoplasms may occur but are relatively uncommon, and biomarkers such as homozygous 9p21 (CDKN2A) deletions or TERT-p mutations can have some prognostic value in such cases. In this review, we discuss the importance and various methods of identifying Spitz associated genomic fusions to help provide more definitive classification. We also discuss characteristic features of the various fusion subtypes as well as prognostic biomarkers.