Non-Clear Cell Renal Cell Carcinomas: From Shadow to Light.

Non-clear cell renal cell carcinomas (RCCs) account for up to 25% of kidney cancers and encompass distinct diseases with distinct pathologic features, different molecular alterations, and various patterns of response to systemic therapies. Recent advances in molecular biology and large collaborative efforts helped to better define the oncogenic mechanisms at play in papillary, chromophobe, collecting duct, medullary, translocation, and sarcomatoid RCCs. Papillary RCCs are divided into several subsets of tumors characterized by distinct gene expression profiles, chromatin remodeling genes, cell cycle changes, and alterations of the MET pathway. Chromophobe RCC genomic analysis revealed mostly metabolic pathway alterations with mitochondrial dysfunctions. Translocation RCCs are characterized by MITF fusions and wide genomic reprogramming. Collecting duct carcinomas are distinct entities from upper tract urothelial carcinomas associated with high T-cell infiltration and metabolic alterations. Medullary RCCs present alterations of the INI1 gene and rhabdoid features at pathologic analysis. Finally, sarcomatoid RCCs represent sarcomatoid differentiation for any subsets of RCCs with specific alterations associated with mesenchymal dedifferentiation. From the standpoint of systemic therapy, more than a decade of using VEGF and mTOR inhibitors showed that they generally had limited efficacy in non-clear cell RCCs compared with clear cell RCCs. MET inhibitors are actively being developed for papillary RCC with a specific focus on MET-driven tumors. Other strategies under investigation include CDK4/6 inhibitors in tumors with cell cycle alterations and EZH2 inhibitors in RCCs with INI1 loss. The emergence of immune checkpoint inhibitors and combination strategies enlarges the spectrum of investigational treatments. Better understanding of driver and passenger alterations and better patient stratification along with dedicated clinical networks will be key to improving the management of these rare tumors.