Clear Cell Sarcoma of the Kidney
Clear cell sarcoma of the kidney is not a Wilms tumor variant, but it is an important primary renal tumor associated with a significantly higher rate of relapse and death than favorable-histology Wilms tumor. In addition to pulmonary metastases, clear cell sarcoma also spreads to bone, brain, and soft tissue. The classic pattern of clear cell sarcoma of the kidney is defined by nests or cords of cells separated by regularly spaced fibrovascular septa. Previously, relapses have occurred in long intervals after the completion of chemotherapy (up to 10 years), however with current therapy relapses after 3 years are uncommon. The brain is a frequent site of recurrent disease.[14,15]
While little is known about the biology of clear cell sarcoma of the kidney, the t(10;17)(q22;p13) translocation has been reported in clear cell sarcoma of the kidney. As a result of the translocation, the YWHAE-FAM22 fusion transcript is formed; this transcript was detected in 12% of clear cell sarcoma of the kidney cases in one series.
Rhabdoid Tumors of the Kidney
Rhabdoid tumors are extremely aggressive malignancies that generally occur in infants and young children. The most common locations are the kidney and central nervous system (CNS) (atypical teratoid/rhabdoid tumor), although rhabdoid tumors can also arise in most soft tissue sites. Initially they were thought to be a rhabdomyosarcomatoid variant of Wilms tumor when they occurred in the kidney.
Histologically, the most distinctive features of rhabdoid tumors of the kidney are rather large cells with large vesicular nuclei, a prominent single nucleolus, and in some cells, the presence of globular eosinophilic cytoplasmic inclusions. A distinct clinical presentation with fever, hematuria, young age (mean age 11 months), and high tumor stage at presentation suggests a diagnosis of rhabdoid tumor of the kidney. Approximately two-thirds of patients will present with advanced stage. Bilateral cases have been reported. Rhabdoid tumors of the kidney tend to metastasize to the lungs and the brain. As many as 10% to 15% of patients with rhabdoid tumors of the kidney also have CNS lesions. Relapses occur early (median time from diagnosis is 8 months).[18,20]
Rhabdoid tumors in all anatomical locations have a common genetic abnormality—the mutation and/or deletion of the SMARCB1 (also called hSNF5 or INI1) gene located at chromosome 22q11. This gene encodes a component of the SWI/SNF chromatin remodeling complex that has an important role in transcriptional regulation.[21,22] Based on gene expression analysis in rhabdoid tumors, it is hypothesized that rhabdoid tumors arise within early progenitor cells during a critical developmental window in which loss of SMARCB1 directly results in repression of neural development, loss of cyclin-dependent kinase inhibition, and trithorax/polycomb dysregulation. Identical mutations may give rise to a brain or kidney tumor. Germline mutations of SMARCB1 have been documented for patients with one or more primary tumors of the brain and/or kidney, consistent with a genetic predisposition to the development of rhabdoid tumors.[24,25] Approximately 35% of patients with rhabdoid tumors have germline SMARCB1 alterations. In most cases, the mutations are de novo, and not inherited from a parent. Germline mosaicism has been suggested for several families with multiple affected siblings. It appears that those patients with germline mutations may have the worst prognosis.