TRIM28 haploinsufficiency predisposes to Wilms tumor

Two percent of patients with Wilms tumors have a positive family history. In many of these cases the genetic cause remains unresolved. By applying germline exome sequencing in two families with two affected individuals with Wilms tumors, we identified truncating mutations in TRIM28. Subsequent mutational screening of germline and tumor DNA of 269 children affected by Wilms tumor was performed, and revealed seven additional individuals with germline truncating mutations, and one individual with a somatic truncating mutation in TRIM28. TRIM28 encodes a complex scaffold protein involved in many different processes, including gene silencing, DNA repair and maintenance of genomic integrity. Expression studies on mRNA and protein level showed reduction of TRIM28, confirming a loss-of-function effect of the mutations identified. The tumors showed an epithelial-type histology that stained negative for TRIM28 by immunohistochemistry. The tumors were bilateral in six patients, and 10/11 tumors are accompanied by perilobar nephrogenic rests. Exome sequencing on eight tumor DNA samples from six individuals showed loss-of-heterozygosity (LOH) of the TRIM28-locus by mitotic recombination in seven tumors, suggesting that TRIM28 functions as a tumor suppressor gene in Wilms tumor development. Additionally, the tumors showed very few mutations in known Wilms tumor driver genes, suggesting that loss of TRIM28 is the main driver of tumorigenesis. In conclusion, we identified heterozygous germline truncating mutations in TRIM28 in 11 children with mainly epithelial-type Wilms tumors, which become homozygous in tumor tissue. These data establish TRIM28 as a novel Wilms tumor predisposition gene, acting as a tumor suppressor gene by LOH.

Wnt signaling inhibits Forkhead box O3a-induced transcription and apoptosis through up-regulation of serum- and glucocorticoid-inducible kinase 1

In human cancers, mutations in components of the Wnt signaling pathway lead to beta-catenin stabilization and result in augmented gene transcription. HCT116 colon cancer cells carry stabilizing mutations in beta-catenin and exhibit an elevated activation of Wnt signaling. To clarify the role of an overactive Wnt signaling, we used DNA microarray analysis to search for genes whose expression is up-regulated after knockdown of the wild type adenomatous polyposis coli (APC) tumor suppressor in HCT116 cells, which further enhances Wnt signaling activation. Serum and glucocorticoid-inducible kinase 1 (SGK1) was among the most up-regulated genes following APC knockdown through small interfering RNA. Up-regulation of SGK1 in response to small interfering RNA against APC was inhibited by concomitant knockdown of beta-catenin. Quantitative real time reverse transcription-PCR, Western blot, and chromatin immunoprecipitation analyses confirmed that SGK1 is a direct beta-catenin target gene. SGK1 negatively regulates the pro-apoptotic transcription factor Forkhead box O3a (FoxO3a) via phosphorylation and exclusion from the nucleus. We show that Wnt signaling activation results in FoxO3a exclusion from the nucleus and inhibits expression of FoxO3a target genes. Importantly, FoxO3a mutants that fail to be phosphorylated and therefore are regulated by SGK1 are not influenced by activation of Wnt signaling. In line, knockdown of SGK1 relieves the effects of Wnt signaling on FoxO3a localization and FoxO3a-dependent transcription. Finally, we show that induction of Wnt signaling inhibits FoxO3a-induced apoptosis. Collectively our results indicate that evasion of apoptosis is another feature employed by an overactive Wnt signaling.

Restricted expression of mouse GCMa/Gcm1 in kidney and thymus

Mouse Glial Cells Missing a (mGCMa) belongs to a small family of transcription factors named after the prototypical GCM from Drosophila. mGCMa was expected to regulate gliogenesis, but instead found to be primarily expressed during development in trophoblasts of chorion and labyrinth. Its deletion causes abnormal placental labyrinth formation and results in embryonic lethality. Here we identify kidney and thymus as sites of mGCMa expression. In thymus, mGCMa is restricted to few small clusters of cells, in kidney to the S3 segment of proximal tubules. mGCMa expression is primarily postnatal, arguing for a role in organ function rather than organ development.