The Tumor Biomarker Series: INI1

This is the last in my tumor biomarker series -- at least until future significant biomarkers are established. I conclude this series with a short description of INI1, a marker for atypical teratoid/rhabdoid tumor (AT/RT). A clinically aggressive embryonal tumor of infancy, AT/RT is characterized by mutations in SMARCB1/INI1 (HSNF5). Immunohistochemical evaluation of AT/RT for the INI1 protein using the BAF47 antibody shows a loss of labelling in tumor cell nuclei, with retention of staining in internal positive control cells such as endothelial cells. Since AT/RT has morphologic overlap with medulloblastoma, CNS PNET, choroid plexus carcinoma, GBM, and other malignant tumors of childhood, INI1 immunohistochemistry is extremely useful in arriving at a diagnosis of AT/RT. A diagnosis of AT/RT carries implications for genetic counseling as this tumor -- in about a one-third of cases -- is a component of the rhabdoid tumor predisposition syndrome (RTPS) wherein there is a germline mutation of SMARCB1/INI1. Because of the risk associated with RTPS, the germline status of SMARCB/INI1 is typically assessed for each new case of AT/RT.