Angiomyolipoma has a unique immunophenotype with co-expression of muscle-specific actin and melanocytic markers such as HMB-45 and
Melan-A. The most recently developed melanocytic markers,
microphthalmia transcription factor and
tyrosinase, have not been studied in the diagnosis of
angiomyolipoma. We tested 29 renal
angiomyolipomas (21 classic histology, 4 epithelioid variants, 2 lipomatous variants, and 2 leiomyomatous variants) with an immunohistochemical panel, including
microphthalmia transcription factor,
tyrosinase, HMB-45,
Melan-A, and muscle-specific actin. Results were compared with 15
renal cell carcinomas (9 conventional types, 6 with sarcomatoid change), 2
leiomyosarcomas, 5
liposarcomas, and 1 unclassified high-grade
sarcoma.
Microphthalmia transcription factor expression was seen in 22 of 29
angiomyolipomas, one
renal cell carcinoma, and one
well-differentiated liposarcoma (that is, 2 of 23 non-
angiomyolipomas; sensitivity 75%, specificity 91%).
Tyrosinase expression was seen in 4 of 29
angiomyolipomas and 0 of 23 non-
angiomyolipomas (sensitivity 14%, specificity 100%). HMB-45 was positive in 24 of 29
angiomyolipomas and 0 of 23 non-
angiomyolipomas (sensitivity 83%, specificity 100%).
Melan-A was expressed by 25 of 29
angiomyolipomas and 0 of 23 non-
angiomyolipomas (sensitivity 86%, specificity 100%). Muscle-specific actin was expressed by 29 of 29
angiomyolipomas and 2 of 23 non-
angiomyolipomas (both
leiomyosarcomas; sensitivity 100%, specificity 91% [100% excluding
leiomyosarcomas]).
Microphthalmia transcription factor showed the most widespread staining in
angiomyolipoma (50% of cases staining more than half of the
tumor cells) followed by
Melan-A (24% of cases staining more than 50%). Only three cases showed positivity for all four melanocytic markers, while in one case each only
microphthalmia transcription factor and
Melan-A were positive. We conclude that
microphthalmia transcription factor, but not
tyrosinase immunostaining, has a sensitivity and specificity that rivals those of the established markers, HMB-45 and
Melan-A, in the diagnosis of
angiomyolipoma. Our data supports the use of a panel in difficult cases that includes
antibodies to
microphthalmia transcription factor, either
Melan-A or HMB-45, and muscle-specific actin to provide the best mix of high sensitivity, high specificity, nuclear and cytoplasmic immunolocalization, and widespread staining of cells within a given
tumor.