Kyle Furge

Using large-scale exome sequencing, Andrew Futreal and colleagues have identified a second frequently mutated gene (after VHL) in clear cell renal cell carcinomas, the most frequent type of kidney cancer. PBRM1, a member of the SWI/SNF complex involved in transcriptional regulation, is mutated in about 40% of cases and is shown to function as a tumour suppressor gene. PBRM1 was independently found as a putative cancer gene involved in pancreatic cancer in a mouse transposon screen. These results — together with the fact that other components of the same complex are known cancer genes — unambiguously identify PBRM1 as a major cancer gene. Using large-scale exome sequencing, this study identifies a second (after VHL) frequently mutated gene in clear cell renal cell carcinomas, the most frequent type of kidney cancer. PBRM1, a member of the SWI/SNF complex involved in transcriptional regulation, is mutated in about 40% of cases and shown to function as tumour suppressor gene. PBRM1 was independently found as a putative cancer gene involved in pancreatic cancer in a mouse transposon screen. The genetics of renal cancer is dominated by inactivation of the VHL tumour suppressor gene in clear cell carcinoma (ccRCC), the commonest histological subtype. A recent large-scale screen of ∼3,500 genes by PCR-based exon re-sequencing identified several new cancer genes in ccRCC including UTX (also known as KDM6A)1, JARID1C (also known as KDM5C) and SETD2 (ref. 2). These genes encode enzymes that demethylate (UTX, JARID1C) or methylate (SETD2) key lysine residues of histone H3. Modification of the methylation state of these lysine residues of histone H3 regulates chromatin structure and is implicated in transcriptional control3. However, together these mutations are present in fewer than 15% of ccRCC, suggesting the existence of additional, currently unidentified cancer genes. Here, we have sequenced the protein coding exome in a series of primary ccRCC and report the identification of the SWI/SNF chromatin remodelling complex gene PBRM1 (ref. 4) as a second major ccRCC cancer gene, with truncating mutations in 41% (92/227) of cases. These data further elucidate the somatic genetic architecture of ccRCC and emphasize the marked contribution of aberrant chromatin biology.

Tyrosine kinase inhibitors (TKI) targeting angiogenesis via inhibition of the vascular endothelial growth factor pathway have changed the medical management of metastatic renal cell carcinoma. Although treatment with TKIs has shown clinical benefit, these drugs will eventually fail patients. The potential mechanisms of resistance to TKIs are poorly understood. To address this question, we obtained an excisional biopsy of a skin metastasis from a patient with clear cell renal carcinoma who initially had a response to sunitinib and eventually progressed with therapy. Tumor pieces were grafted s.c. in athymic nude mice. Established xenografts were treated with sunitinib. Tumor size, microvascular density, and pericyte coverage were determined. Plasma as well as tissue levels for sunitinib were assessed. A tumor-derived cell line was established and assessed in vitro for potential direct antitumor effects of sunitinib. To our surprise, xenografts from the patient who progressed on sunitinib regained sensitivity to the drug. At a dose of 40 mg/kg, sunitinib caused regression of the subcutaneous tumors. Histology showed a marked reduction in microvascular density and pericyte dysfunction. More interestingly, histologic examination of the original skin metastasis revealed evidence of epithelial to mesenchymal transition, whereas the xenografts showed reversion to the clear cell phenotype. In vitro studies showed no inhibitory effect on tumor cell growth at pharmacologically relevant concentrations. In conclusion, the histologic examination in this xenograft study suggests that reversible epithelial to mesenchymal transition may be associated with acquired tumor resistance to TKIs in patients with clear cell renal carcinoma.

The Birt-Hogg-Dubé (BHD) disease is a genetic cancer syndrome. The responsible gene, BHD, has been identified by positional cloning and thought to be a novel tumor suppressor gene. BHD mutations cause many types of diseases including renal cell carcinomas, fibrofolliculomas, spontaneous pneumothorax, lung cysts, and colonic polyps/cancers. By combining Gateway Technology with the Ksp-Cre gene knockout system, we have developed a kidney-specific BHD knockout mouse model. BHD(flox/flox)/Ksp-Cre mice developed enlarged kidneys characterized by polycystic kidneys, hyperplasia, and cystic renal cell carcinoma. The affected BHD(flox/flox)/Ksp-Cre mice died of renal failure at approximate three weeks of age, having blood urea nitrogen levels over tenfold higher than those of BHD (flox/+)/Ksp-Cre and wild-type littermate controls. We further demonstrated that these phenotypes were caused by inactivation of BHD and subsequent activation of the mTOR pathway. Application of rapamycin, which inhibits mTOR activity, to the affected mice led to extended survival and inhibited further progression of cystogenesis. These results provide a correlation of kidney-targeted gene inactivation with renal carcinoma, and they suggest that the BHD product FLCN, functioning as a cyst and tumor suppressor, like other hamartoma syndrome-related proteins such as PTEN, LKB1, and TSC1/2, is a component of the mTOR pathway, constituting a novel FLCN-mTOR signaling branch that regulates cell growth/proliferation.