Mutations and Copy Number Increase of HRAS in Spitz Nevi with Distinctive Histopathological Features

Abstract

Spitz nevus is a benign melanocytic neoplasm that can be difficult or impossible to histologically distinguish from melanoma. We have recently described copy number increases of chromosome 11p in a subset of Spitz nevi. To study the molecular and histological features of this group, we studied 102 Spitz nevi for 11p copy number increases using fluorescence in situ hybridization (FISH) on tissue arrays. Copy number increases of at least threefold were found in 12 cases (11.8%) and involved the HRAS gene on chromosome 11p. Sequence analysis of HRAS showed frequent oncogenic mutations in cases with copy number increase (8/12 or 67%), contrasting with rare HRAS mutations in cases with normal HRAS copy numbers (1/21 or 5%, P < 0.0001). Tumors with 11p copy number increases were larger, predominantly intradermal, had marked desmoplasia, characteristic cytological features, and had an infiltrating growth pattern. Proliferation rates in the majority of these cases were low to absent. HRAS activation by either mutation or copy number increase alone could explain several of the histological features that overlap with those of melanoma. We speculate that HRAS activation in the absence of co-operating additional genetic alterations drives the partially transformed melanocytes of these Spitz nevi into senescence or a stable growth arrest. Although there is no data suggesting that Spitz nevi with HRAS activation are at risk for progression to melanoma, future studies are warranted to assess their biological behavior more accurately. (Am J Pathol 2000, 157:967-972) Spitz nevi are benign melanocytic neoplasms that can resemble melanoma lesions on histopathological examination. Sophie Spitz first described them as "juvenile melanoma" in 1948 and initially regarded them as a subset of childhood melanoma, in which there was usually a benign course.1 Spitz nevi account for about 1% of surgically removed melanocytic nevi.2 Although current histological criteria reliably distinguish melanoma from Spitz nevus in most cases, there is a significant overlap of criteria in a subset of cases. It is a current matter of debate whether Spitz nevus and melanoma reside at the opposing ends of a biological spectrum 3 or represent two separate entities. 4 The diagnostic uncertainty of cases with overlapping histological criteria is expressed in terms like atypical Spitz nevus, malignant Spitz nevus, and spitzoid melanoma. The lack of diagnostic selectivity of current histological criteria as well as the lack of consensus among pathologists is well documented, 5-7 and misdiagnosis of Spitz nevus as melanoma and vice versa is repeatedly reported. -10Previously we reported that chromosomal changes can assist with distinguishing the two entities.11 The majority of Spitz nevi has a normal karyotype when analyzed by comparative genomic hybridization, whereas increased copies of chromosome 11p are a recurrent finding in a minority of Spitz nevi.11,12 The aim of this study was to identify additional examples of Spitz nevi with copy number increase of chr...

The Spitzoid lesion: rethinking Spitz tumors, atypical variants, ‘Spitzoid melanoma' and risk assessment

“…27,34,40-43 S100 protein and Mart-1 show diffuse expression throughout both Comparative genomic hybridization and fluorescence in situ hybridization The majority of Spitz tumors studied by comparative genomic hybridization (CGH) have had no chromosomal abnormalities. 44,45 On the other hand, amplification of chromosome 11p has been observed in a subset of cases and was found to correlate with Spitz tumors that were often larger in size, dermal-based, desmoplastic, had vesicular nuclei in melanocytes, and exhibited dermal infiltrating features. Further study of Spitz tumors with gains of 11p has shown an increase in copy number of and mutations in the HRAS gene by fluorescence in situ hybridization.…”

“…Other conclusions from the latter studies were that Spitz tumors are probably clonal proliferations, the majority of melanocytes are diploid with some large nuclei being polyploid, and Spitz tumors may be cytogenetically distinct from melanoma. 45 Loss of heterozygosity Two independent studies have recently demonstrated loss of heterozygosity on chromosome 9p with DNA polymorphic markers in two of 27 and five of five 'Spitz nevi'. 46,47 The latter findings provide additional evidence for the close relationship between Spitz tumors and melanoma.…”

A diagnostic algorithm for atypical spitzoid tumors: guidelines for immunohistochemical and molecular assessment

“…HRAS activation found in a subset of Spitz nevi, either by amplification of chromosome 11p or by HRAS activation mutation, 6 could give rise to incompletely transformed melanocytes that share several histological features with melanoma cells but have limited proliferative activity. Atypical spitzoid tumors display both Spitz nevus and spitzoid melanoma features.…”

“…The number of specific florescent colored dots per Initial 4-probe FISH assay for diagnosis of melanoma. The initial set of FISH probes for melanoma was derived from a combinatorial analysis of the array-based comparative genomic hybridization data set of Bastian et al [4][5][6] showing the best discriminatory difference between melanomas and nevi. Fourteen FISH probes were assembled from 13 regions on eight different chromosomes (1, 6, 7, 9, 10, 11, 17, and 20) from the array-based comparative genomic hybridization database and one additional probe targeting KIT on chromosome 4 as a potential therapeutic target.…”

“…2,3 To address these difficulties, appropriate molecular and immunohistochemical tests have been developed using a panel of selected immunomarkers and genes that contribute the most to melanoma pathogenesis. [4][5][6][7] In particular, recent application of fluorescence in situ hybridization (FISH) and arraybased comparative genomic hybridization assays has uncovered an increasing spectrum of potential chromosomal aberrations implicated in atypical spitzoid tumors. Using a combination of immunohistochemical and cytogenetic/molecular tests, the diagnostic algorithm proposed here aims to assess the malignant potential of atypical spitzoid tumors.…”