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The hSNF5 subunit of human SWI/SNF ATP-dependent chromatin remodeling complexes is a tumor suppressor that is inactivated in malignant rhabdoid tumors (MRTs). Here, we report that loss of hSNF5 function in MRT-derived cells leads to polyploidization and chromosomal instability. Re-expression of hSNF5 restored the coupling between cell cycle progression and ploidy checkpoints. In contrast, cancer-associated hSNF5 mutants harboring specific single amino acid substitutions exacerbated poly-and aneuploidization, due to abrogated chromosome segregation. We found that hSNF5 activates the mitotic checkpoint through the p16 INK4a-cyclinD/CDK4-pRb-E2F pathway. These results establish that poly-and aneuploidy of tumor cells can result from mutations in a chromatin remodeler. Received January 12, 2005; revised version accepted January 27, 2005. ATP-dependent chromatin remodeling factors are critical components of the elaborate machinery that controls gene expression in eukaryotic cells (Becker and Horz 2002). The multisubunit SWI/SNF complex is the prototypical chromatin remodeling factor, present in all eukaryotes (Mohrmann and Verrijzer 2005). Human SNF5 (hSNF5, also known as Ini1, Baf47, or SmarcB1) encodes for a universal SWI/SNF subunit and tumor suppressor that is mutated in malignant rhabdoid tumors (MRTs) (Versteege et al. 1998;Klochendler-Yeivin et al. 2002;Roberts and Orkin 2004). MRTs are rare but highly aggressive pediatric cancers with a high mortality rate.Carriers of germline mutations are predisposed to various cancers and, consistent with a classic tumor suppressor phenotype, the wild-type allele is either lost or deleted in a large proportion of tumors (Biegel et al. 1999; Sevenet et al. 1999a,b;Taylor et al. 2000). hSNF5 mutations are also associated with a number of neoplasms other than MRTs (Grand et al. 1999; Sevenet et al. 1999a,b;Roberts and Orkin 2004). SNF5 inactivation studies in mice established its requirement during early embryogenesis and its role as a tumor suppressor (Klochendler-Yeivin et al. 2000;Roberts et al. 2000;Guidi et al. 2001;Roberts et al. 2002).Several studies found that re-expression of hSNF5 in MRT-derived cell lines caused an accumulation in G0/ G1, cellular senescence, and apoptosis (Ae et al. 2002;Betz et al. 2002;Versteege et al. 2002;Zhang et al. 2002;Oruetxebarria et al. 2004). These effects are largely the result of direct transcriptional activation of the tumor suppressor p16INK4a by hSNF5, which appears to be both necessary and sufficient for reduced cell proliferation and induction of cellular senescence and apoptosis (Oruetxebarria et al. 2004). p16INK4a controls the activity of pRb via inhibition of the cyclin D1-CDK4 kinase, which phosphorylates pRb (Lowe and Sherr 2003). Tumor suppressor pRb is a corepressor that is tethered to a broad range of genes by the E2F transcription factors. Hyperphosphorylation of pRb causes its dissociation from E2F, and relieves its antiproliferative activities. In addition to genes required for cell cycle progression from G1 to...

Polymicrogyria is a malformation of the developing cerebral cortex caused by abnormal organization and characterized by many small gyri and fusion of the outer molecular layer. We have identified autosomal-recessive mutations in RTTN, encoding Rotatin, in individuals with bilateral diffuse polymicrogyria from two separate families. Rotatin determines early embryonic axial rotation, as well as anteroposterior and dorsoventral patterning in the mouse. Human Rotatin has recently been identified as a centrosome-associated protein. The Drosophila melanogaster homolog of Rotatin, Ana3, is needed for structural integrity of centrioles and basal bodies and maintenance of sensory neurons. We show that Rotatin colocalizes with the basal bodies at the primary cilium. Cultured fibroblasts from affected individuals have structural abnormalities of the cilia and exhibit downregulation of BMP4, WNT5A, and WNT2B, which are key regulators of cortical patterning and are expressed at the cortical hem, the cortex-organizing center that gives rise to Cajal-Retzius (CR) neurons. Interestingly, we have shown that in mouse embryos, Rotatin colocalizes with CR neurons at the subpial marginal zone. Knockdown experiments in human fibroblasts and neural stem cells confirm a role for RTTN in cilia structure and function. RTTN mutations therefore link aberrant ciliary function to abnormal development and organization of the cortex in human individuals.

We describe a syndrome of primary microcephaly with simplified gyral pattern in combination with severe infantile epileptic encephalopathy and early-onset permanent diabetes in two unrelated consanguineous families with at least three affected children. Linkage analysis revealed a region on chromosome 18 with a significant LOD score of 4.3. In this area, two homozygous nonconserved missense mutations in immediate early response 3 interacting protein 1 (IER3IP1) were found in patients from both families. IER3IP1 is highly expressed in the fetal brain cortex and fetal pancreas and is thought to be involved in endoplasmic reticulum stress response. We reported one of these families previously in a paper on Wolcott-Rallison syndrome (WRS). WRS is characterized by increased apoptotic cell death as part of an uncontrolled unfolded protein response. Increased apoptosis has been shown to be a cause of microcephaly in animal models. An autopsy specimen from one patient showed increased apoptosis in the cerebral cortex and pancreas beta cells, implicating premature cell death as the pathogenetic mechanism. Both patient fibroblasts and control fibroblasts treated with siRNA specific for IER3IP1 showed an increased susceptibility to apoptotic cell death under stress conditions in comparison to controls. This directly implicates IER3IP1 in the regulation of cell survival. Identification of IER3IP1 mutations sheds light on the mechanisms of brain development and on the pathogenesis of infantile epilepsy and early-onset permanent diabetes.