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- Presentation
Three Dermatopathology Cases: BAP-Inactivated Tumor, Light Chain Deposition Disease, and Jumping Cholla Cactus Injury
Description
The speaker presented three dermatopathology cases. The first was a finger lesion in a 41-year-old that looked histologically like a BAP1-inactivated melanocytic tumor, with epithelioid cells and features suggestive of BAP loss; however, BAP1 immunohistochemistry was retained, and next-generation sequencing confirmed BAP1 inactivation, showing that retained staining does not exclude this diagnosis. The second case involved a 46-year-old woman with progressive acral nodules, joint destruction, and a monoclonal gammopathy; biopsy showed eosinophilic dermal deposits resembling amyloid, but Congo red was negative, immunostains and immunofluorescence showed IgG kappa restriction, and electron microscopy demonstrated parallel microtubules rather than amyloid fibrils, leading to a diagnosis of light chain deposition disease. The final case was a 65-year-old woman with a finger rash and ankle lesions after hiking in Joshua Tree; biopsy showed chronic inflammation, granulomatous reaction, and crystalline foreign material under polarized light, and further history revealed exposure to jumping cholla cactus, confirming a foreign body reaction from cactus spines.
View moreConclusions
- BAP1-inactivated melanocytic tumors can show retained BAP1 immunostaining, so molecular confirmation may be necessary when morphology suggests the diagnosis.
- A retained BAP1 IHC result does not exclude a BAP1-inactivated tumor if sequencing demonstrates an inactivating mutation.
- Cutaneous paraprotein deposition can present with striking skin and soft tissue lesions in patients with monoclonal gammopathy.
- SPEP alone is not sensitive enough to rule out paraproteinemia, and immunofixation is needed for more reliable diagnosis.
- Not all light chain deposition disease forms amyloid, so pathology must distinguish true amyloid from non-amyloid light chain deposits using stains and electron microscopy.
- Electron microscopy can help separate amyloid fibrils from other organized immunoglobulin deposits by their different ultrastructural patterns.
- Granulomatous foreign-body reactions in skin can be caused by tiny cactus spines, especially from jumping cholla exposure.
- Careful clinicopathologic correlation and history taking are essential for diagnosing unusual dermatopathology cases.
- Beyond the Stain: BAP1-Inactivated Melanocytic Tumors With Retained BAP1 Nuclear Expression due to Missense Mutations in the Catalytic Domain. Journal of Cutaneous Pathology. First published 11 March 2026. DOI link.
- BAP1-inactivated melanocytic tumor
- Paraproteinemia & Skin
- Diagnosing paraproteinemia
- AL Amyloid
- Light chain deposition disease