1. ¹Department of Hematology, Oncology and Tumor Immunology, University Medical Center Charité, Campus Berlin-Buch, Berlin, Germany
2. ²Department of Urology, University Medical Center Charité, Campus Berlin-Mitte, Berlin, Germany
3. ³Institute of Human Genetics, University Hospital Schleswig-Holstein, Campus Kiel, Germany

Correspondence: PT Daniel, Clinical and Molecular Oncology, Charité – Campus Berlin-Buch, Humboldt-University, Lindenberger Weg 80, Berlin-Buch 13125, Germany. Tel.: +49-30-9417-1642 or -1644; Fax: +49-30-9417-1644 or -1641; E-mail: pdaniel@mdc-berlin.de

Received 8 June 2005; Revised 1 August 2005; Accepted 11 August 2005; Published online 2 December 2005.

Abstract

We report for the first time inactivation of a tissue-specific Bcl-2 homology domain 3 (BH3)-only protein as a common aspect in human cancer. In detail, we show that loss of the BH3-only protein natural born killer (Nbk)/Bcl-2-interacting killer (Bik) is a common feature of clear-cell renal cell carcinoma (RCC). While strong Nbk expression is found in the renal tubuli and the epithelial lining of the glomerula, a consistent loss of Nbk expression was observed in primary RCC tissue and RCC cell lines. Mutation of Nbk is, however, rare, whereas deletion of the Nbk gene at 22q13.2 is frequent. In addition to loss of heterozygosity (LOH), DNA methylation mediates transcriptional silencing of the Nbk gene. The conditional restoration of Nbk/Bik expression led to apoptotic death of RCC but not of nonmalignant renal epithelia. A broader expression analysis of RCC cell lines for BH3-only proteins revealed that loss of Nbk coincides with failure to express Bim, whereas Puma, Bid and BNIP3 are readily detectable and, in case of Puma, inducible by p53. These data delineate a role for defects in BH3-only proteins as tumor suppressors in RCC and may explain at the same time the impressive clinical apoptosis resistance of RCC.