MYB rearrangement and immunohistochemical expression in adenomyoepithelioma of the breast: a comparison with adenoid cystic carcinoma.

AIMS: Adenomyoepithelioma (AME) and adenoid cystic carcinoma (ACC) of the breast have been noted to occur simultaneously, raising the possibility that AME may represent a related or precursor lesion to ACC. ACC frequently harbours genetic rearrangement of the MYB gene. We sought to clarify the relationship between AME and ACC by comparing their rates of MYB expression by IHC and MYB rearrangement by FISH.

METHODS AND RESULTS: IHC and FISH for MYB rearrangement were performed on paraffin-embedded sections of 11 breast ACCs, 11 non-breast ACCs and 11 breast-AMEs. Using FISH, five of eight (63%) interpretable breast ACCs demonstrated MYB gene rearrangement. Nine of 11 (81%) breast ACCs demonstrated MYB expression (range = 20-95%). Of the three FISH-negative breast ACCs, two were solid variant and demonstrated strong MYB expression by IHC. Of the 10 interpretable non-breast ACCs, six showed MYB rearrangement, all of which were conventional type. Nine of these 11 (81%) cases showed MYB immunoexpression (range = 10-90%), including three solid-variant cases which were negative by FISH. No MYB rearrangements were detected by FISH in 10 interpretable AMEs. However, three of 11 cases (27%) showed weak to moderate MYB expression by IHC (range = 10-40%).

CONCLUSIONS: Our results indicate that AMEs do not harbour MYB gene rearrangement. IHC for MYB may be helpful in diagnosing FISH-negative cases of ACC, particularly the diagnostically more difficult solid variants. However, weak to moderate MYB expression in a subset of AMEs highlights not only a potential diagnostic pitfall, but also shared pathophysiology with ACC worth investigating further at the genomic level.