[461] Spitz Nevi Frequently Show DNA Copy Number Abnormalities by FISH Using Probes for RREB1 (6p25), MYB (6q23), CEP6 (6 Centromere), and CCND1 (11q13)

Kenneth J Craddock, Ziad Hindi, Caroline Shiau, Ayman Al-Habeeb, Danny Ghazarian. University Health Network, Toronto, ON, Canada

Background: Malignant melanoma can be difficult to distinguish from a benign melanocytic lesion by histology. A fluorescence in situ hybridization (FISH) probe set targeting abnormalities commonly seen in malignant melanoma but not found in benign nevi has shown encouraging results in previous studies. However, there is a paucity of data describing the findings of this probe set in Spitz nevi, a benign melanocytic lesion that can be difficult to distinguish from melanoma.
Design: Multicolour FISH was performed using a commercially available probeset (Abbott Laboratories, Abbott Park, IL), on formalin-fixed, paraffin-embedded tissue samples from 20 Spitz nevi, diagnosed by histology, all lacking atypical features. Fluorescent signals for each probe were enumerated by 2 observers in 30 cells each per lesion. An algorithm using signal counts from a combination of 4 probes targeting chromosome 6p25 (containing RREB1 gene), 6 centromere (CEP6), 6q23 (containing MYB gene), and 11q13 (containing CCND1 gene) was used as suggested by the manufacturer. The following criteria were used: (1) the average CCND1 or MYB signals per nuclei is greater than or equal to 2.5 or (2) percent loss of MYB relative to CEP6 is greater than or equal to 31% or (3) the percentage of abnormal nuclei for RREB1 is greater than or equal to 63%. If at least one of the three criteria were met, the specimen was designated FISH positive. If none of the criteria were met, the specimen was designated as FISH negative.
Results: FISH results were obtained for all 20 Spitz nevi. Seven (35%) tested positive: 3 showed copy number gain at CCND1, 3 showed gain at MYB (2 showed gain at both CCND1 and MYB), and 3 others had loss of MYB.
Conclusions: In contrast to usual benign melanocytic nevi, we found that abnormal copy number changes at CCND1 and MYB are not uncommonly seen in Spitz nevi; therefore their detection does not aid in the distinction between benign Spitz nevi and malignant melanoma. However, we note a difference in RREB1 findings between Spitz and melanoma. While we did detect copy number gains at RREB1 in a subset of Spitz cases, none reached the relatively high threshold required for an abnormal result by the manufacturers algorithm; in contrast, we have previously reported RREB1 to be significantly gained in 65 % of melanomas. If this FISH probe set is to be used to aid in the diagnosis of atypical melanocytic lesions, one must be aware that abnormalities may be detected in Spitz nevi, and that the pattern of abnormalities may differ.
Category: Dermatopathology

Tuesday, March 5, 2013 9:30 AM

Poster Session III # 45, Tuesday Morning

 

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