cathepsin-K

Definition : Cathepsin K, abbreviated CTSK, is an enzyme that in humans is encoded by the CTSK gene. The protein encoded by this gene is a lysosomal cysteine protease involved in bone remodeling and resorption. This protein, which is a member of the peptidase C1 protein family, is expressed predominantly in osteoclasts.

Pathology

 Germline mutations cause sclerosing osteochondrodysplasia pycnodysostosis, a rare autosomal recessive skeletal dysplasia characterized by abnormal bone and tooth development.

IHC

 Positive staining - normal cells

• fibroblasts (skin, lung, J Am Acad Dermatol 2012;66:e89),
• osteoclasts
• Intervertebral disc (Arthritis Res Ther 2011;13:R140)
• Skin: stratum corneum, mature sebaceous cells, outer root sheath of hair follicles (J Eur Acad Dermatol Venereol 2013;27:e128)

 tumors

• alveolar soft parts sarcoma (Mod Pathol 2011;24:1313)
• basal cell carcinoma (J Eur Acad Dermatol Venereol 2013;27:e128)
• cholesteatoma (Mod Pathol 2001;14:1226), chordoma (Hum Pathol 2000;31:834)
• granular cell tumor,
• histiocytic lesions,
• melanoma (Am J Clin Pathol 2013;139:151)
• Kidney:
  • perivascular epithelioid cell tumor / PEComa (diffuse, Mod Pathol 2012;25:100), PRCC-TFE3 carcinomas (Mod Pathol 2011;24:1313)
  • high-grade oncocytic tumor / HOT

• Lung: lymphangioleiomyomatosis (Mod Pathol 2009;22:161)
• Osteoclastic giant cells
• Melanoma,
• osteoclastoma

 Negative staining

• Endothelium
• Most carcinomas (Am J Clin Pathol 2013;139:151)