[SMARCB1(INI1)-deficient renal cell carcinoma: medullary and beyond : Evolving concepts]

During the last decades, the SWI/SNF chromatin-remodeling complex has received enormous recognition as a major player in the molecular pathogenesis of diverse neoplasms. Accordingly, SWI/SNF defects affecting different subunits of the complex became defining genetic features in the nosology of different neoplastic entities. In the kidney, loss of SMARCB1(INI1) as a major component of the SWI/SNF complex has emerged as the defining genetic marker for renal medullary carcinoma and pediatric malignant rhabdoid tumor. Diagnosis of these two rare entities is based on a set of defined demographic, clinicopathological, immunophenotypic, and genetic (SMARCB1 loss) criteria. Moreover, the sickle cell trait is considered a prerequisite for renal medullary carcinoma. Current knowledge illustrates that SMARCB1 loss is encountered in three major tumor categories in the kidney: (1) histologically defined neoplasms that are primarily driven by de novo SMARCB1 loss (renal medullary carcinoma and malignant rhabdoid tumor); (2) SMRACB1-deficient renal cell carcinoma (RCC) with variable non-specific histology ranging from collecting duct-like, papillary high-grade (papillary type 2), or medullary-like (lacking sickle cell trait), to fully undifferentiated; and (3) biphasic (dedifferentiated) RCC showing a variable SMARCB1-deficient undifferentiated component. The latter variant most frequently originates from pre-existing clear cell RCC but may rarely superimpose on papillary or chromophobe RCC. This review summarizes the major defining features of the emerging SMARCB1-deficient renal neoplasms. All SMARCB1-deficient carcinomas have a poor prognosis in common. Therefore, exact diagnosis of these tumors is a prerequisite for studies investigating new therapies.