The perivascular epithelioid cell (PEC) is a unique cell type coexpressing
contractile proteins (mainly α-smooth muscle actin), melanocytic markers, including
microphthalmia-associated transcription factor (MITF), and
estrogen and
progesterone receptors. It is constantly present in a group of
tumors called
PEComas. Renal
PEComas include the common
angiomyolipoma as well as less common lesions such as microscopic
angiomyolipoma, intraglomerular lesions,
angiomyolipoma with epithelial
cysts, epithelioid
angiomyolipoma,
oncocytoma-like
angiomyolipoma and
lymphangioleiomyomatosis of the renal sinus. It has been demonstrated that most of these lesions are determined by mutations affecting genes of the
tuberous sclerosis complex,
tuberous sclerosis 1 (TSC1) and
tuberous sclerosis 2 (TSC2), with eventual deregulation of the RHEB/MTOR/RPS6KB2 pathway, and it has been observed that some
PEComas regressed during
sirolimus therapy, an MTOR inhibitor. Recently, overexpression of MITF has been related to the expression of the
papain-like
cysteine protease cathepsin K in osteoclasts where it has inhibited MTOR. The aim of this study is to evaluate
cathepsin K immunohistochemically in the entire spectrum of
PEComa lesions in the kidney. The study population consisted of 84 renal
PEComa lesions, including 5 composed predominantly of fat (
lipoma-like
angiomyolipoma), 15 almost exclusively composed of spindle-shaped smooth muscle cells (
leiomyoma-like
angiomyolipoma) and 31 common
angiomyolipomas composed of a mixture of fat, spindle and epithelioid smooth muscle cells, and abnormal thick-walled blood vessels, 15 microscopic
angiomyolipomas, 5 intraglomerular lesions, 2
oncocytoma-like
angiomyolipomas, 8 epithelioid
angiomyolipomas, 2
angiomyolipomas with epithelial
cysts and 1 example of
lymphangioleiomyomatosis of the renal sinus. In all of the renal
PEComas,
cathepsin K was found to be constantly and strongly expressed and seems to be a more powerful marker than other commonly used markers for their identification, especially to confirm the diagnosis on needle biopsies.