Splicing misregulation of SCN5A contributes to cardiac-conduction delay and heart arrhythmia in myotonic dystrophy.

Myotonic dystrophy (DM) is caused by the expression of mutant RNAs containing expanded CUG repeats that sequester muscleblind-like (MBNL) proteins, leading to alternative splicing changes. Cardiac alterations, characterized by conduction delays and arrhythmia, are the second most common cause of death in DM. Using RNA sequencing, here we identify novel splicing alterations in DM heart samples, including a switch from adult exon 6B towards fetal exon 6A in the cardiac sodium channel, SCN5A. We find that MBNL1 regulates alternative splicing of SCN5A mRNA and that the splicing variant of SCN5A produced in DM presents a reduced excitability compared with the control adult isoform. Importantly, reproducing splicing alteration of Scn5a in mice is sufficient to promote heart arrhythmia and cardiac-conduction delay, two predominant features of myotonic dystrophy. In conclusion, misregulation of the alternative splicing of SCN5A may contribute to a subset of the cardiac dysfunctions observed in myotonic dystrophy.

The tumor suppressor Ikaros shapes the repertoire of notch target genes in T cells.

The Notch signaling pathway is activated in many cell types, but its effects are cell type- and stage-specific. In the immune system, Notch activity is required for the differentiation of T cell progenitors, but it is reduced in more mature thymocytes, in which Notch is oncogenic. Studies based on single-gene models have suggested that the tumor suppressor protein Ikaros plays an important role in repressing the transcription of Notch target genes. We used genome-wide analyses, including chromatin immunoprecipitation sequencing, to identify genes controlled by Notch and Ikaros in gain- and loss-of-function experiments. We found that Ikaros bound to and directly repressed the expression of most genes that are activated by Notch. Specific deletion of Ikaros in thymocytes led to the persistent expression of Notch target genes that are essential for T cell maturation, as well as the rapid development of T cell leukemias in mice. Expression of Notch target genes that are normally silent in T cells, but are activated by Notch in other cell types, occurred in T cells of mice genetically deficient in Ikaros. We propose that Ikaros shapes the timing and repertoire of the Notch transcriptional response in T cells through widespread targeting of elements adjacent to Notch regulatory sequences. These results provide a molecular framework for understanding the regulation of tissue-specific and tumor-related Notch responses.

Sequestration of DROSHA and DGCR8 by expanded CGG RNA repeats alters microRNA processing in fragile X-associated tremor/ataxia syndrome.

Fragile X-associated tremor/ataxia syndrome (FXTAS) is an inherited neurodegenerative disorder caused by the expansion of 55-200 CGG repeats in the 5' UTR of FMR1. These expanded CGG repeats are transcribed and accumulate in nuclear RNA aggregates that sequester one or more RNA-binding proteins, thus impairing their functions. Here, we have identified that the double-stranded RNA-binding protein DGCR8 binds to expanded CGG repeats, resulting in the partial sequestration of DGCR8 and its partner, DROSHA, within CGG RNA aggregates. Consequently, the processing of microRNAs (miRNAs) is reduced, resulting in decreased levels of mature miRNAs in neuronal cells expressing expanded CGG repeats and in brain tissue from patients with FXTAS. Finally, overexpression of DGCR8 rescues the neuronal cell death induced by expression of expanded CGG repeats. These results support a model in which a human neurodegenerative disease originates from the alteration, in trans, of the miRNA-processing machinery.

Dynamin 2 homozygous mutation in humans with a lethal congenital syndrome.

Heterozygous mutations in dynamin 2 (DNM2) have been linked to dominant Charcot-Marie-Tooth neuropathy and centronuclear myopathy. We report the first homozygous mutation in the DNM2 protein p.Phe379Val, in three consanguineous patients with a lethal congenital syndrome associating akinesia, joint contractures, hypotonia, skeletal abnormalities, and brain and retinal hemorrhages. In vitro membrane tubulation, trafficking and GTPase assays are consistent with an impact of the DNM2p.Phe379Val mutation on endocytosis. Although DNM2 has been previously implicated in axonal and muscle maintenance, the clinical manifestation in our patients taken together with our expression analysis profile during mouse embryogenesis and knockdown approaches in zebrafish resulting in defects in muscle organization and angiogenesis support a pleiotropic role for DNM2 during fetal development in vertebrates and humans.

Transcriptome analysis for Notch3 target genes identifies Grip2 as a novel regulator of myogenic response in the cerebrovasculature.

OBJECTIVE: Notch3 is critically important for the structure and myogenic response of distal arteries, particularly of cerebral arteries. However, signaling pathways acting downstream of Notch3 remain largely unknown. METHODS AND RESULTS: Transcriptome analysis using tail arteries of Notch3-null mice identified a core set of 17 novel Notch3-regulated genes confirmed in tail or brain arteries. Postnatal deletion of RBP-Jκ in smooth muscle cells recapitulated the structural, functional, and molecular defects of brain arteries induced by Notch3 deficiency. Transient in vivo blockade of the Notch pathway with γ-secretase inhibitors uncovered, in addition to Notch3, 6 immediate responders, including the voltage-gated potassium channel Kv1.5, which opposes to myogenic constriction of brain arteries, and the glutamate receptor-interacting protein 2 (Grip2) with no previously established role in the cerebrovasculature. We identified a vascular smooth muscle cell isoform of Grip2. We showed that Notch3-RBP-Jκ specifically regulates this isoform. Finally, we found that cerebral arteries of Grip2 mutant mice, which express an N-terminally truncated Grip2 protein, exhibited selective attenuation of pressure-induced contraction. CONCLUSIONS: Our data provide insight into how Notch3 signals in the brain arteries, establishing the postnatal requirement of smooth muscle RBP-Jκ in this context. Notch3-regulated transcriptome provides potential for modulating myogenic response in the cerebrovasculature.

Profiling target genes of FGF18 in the postnatal mouse lung: possible relevance for alveolar development.

Better understanding alveolarization mechanisms could help improve prevention and treatment of diseases characterized by reduced alveolar number. Although signaling through fibroblast growth factor (FGF) receptors is essential for alveolarization, involved ligands are unidentified. FGF18, the expression of which peaks coincidentally with alveolar septation, is likely to be involved. Herein, a mouse model with inducible, lung-targeted FGF18 transgene was used to advance the onset of FGF18 expression peak, and genome-wide expression changes were determined by comparison with littermate controls. Quantitative RT-PCR was used to confirm expression changes of selected up- and downregulated genes and to determine their expression profiles in the course of lung postnatal development. This allowed identifying so-far unknown target genes of the factor, among which a number are known to be involved in alveolarization. The major target was adrenomedullin, a promoter of lung angiogenesis and alveolar development, whose transcript was increased 6.9-fold. Other genes involved in angiogenesis presented marked expression increases, including Wnt2 and cullin2. Although it appeared to favor cell migration notably through enhanced expression of Snai1/2, FGF18 also induced various changes consistent with prevention of epithelial-mesenchymal transition. Together with antifibrotic effects driven by induction of E prostanoid receptor 2 and repression of numerous myofibroblast markers, this could prevent alveolar septation-driving mechanisms from becoming excessive and deleterious. Last, FGF18 up- or downregulated genes of extracellular matrix components and epithelial cell markers previously shown to be up- or downregulated during alveolarization. These findings therefore argue for an involvement of FGF18 in the control of various developmental events during the alveolar stage.

Tripartite motif 24 (Trim24/Tif1α) tumor suppressor protein is a novel negative regulator of interferon (IFN)/signal transducers and activators of transcription (STAT) signaling pathway acting through retinoic acid receptor α (Rarα) inhibition.

Recent genetic studies in mice have established that the nuclear receptor coregulator Trim24/Tif1α suppresses hepatocarcinogenesis by inhibiting retinoic acid receptor α (Rara)-dependent transcription and cell proliferation. However, Rara targets regulated by Trim24 remain unknown. We report that the loss of Trim24 resulted in interferon (IFN)/STAT pathway overactivation soon after birth (week 5). Despite a transient attenuation of this pathway by the induction of several IFN/STAT pathway repressors later in the disease, this phenomenon became more pronounced in tumors. Remarkably, Rara haplodeficiency, which suppresses tumorigenesis in Trim24(-/-) mice, prevented IFN/STAT overactivation. Moreover, together with Rara, Trim24 bound to the retinoic acid-responsive element of the Stat1 promoter and repressed its retinoic acid-induced transcription. Altogether, these results identify Trim24 as a novel negative regulator of the IFN/STAT pathway and suggest that this repression through Rara inhibition may prevent liver cancer.

Misregulated alternative splicing of BIN1 is associated with T tubule alterations and muscle weakness in myotonic dystrophy.

Myotonic dystrophy is the most common muscular dystrophy in adults and the first recognized example of an RNA-mediated disease. Congenital myotonic dystrophy (CDM1) and myotonic dystrophy of type 1 (DM1) or of type 2 (DM2) are caused by the expression of mutant RNAs containing expanded CUG or CCUG repeats, respectively. These mutant RNAs sequester the splicing regulator Muscleblind-like-1 (MBNL1), resulting in specific misregulation of the alternative splicing of other pre-mRNAs. We found that alternative splicing of the bridging integrator-1 (BIN1) pre-mRNA is altered in skeletal muscle samples of people with CDM1, DM1 and DM2. BIN1 is involved in tubular invaginations of membranes and is required for the biogenesis of muscle T tubules, which are specialized skeletal muscle membrane structures essential for excitation-contraction coupling. Mutations in the BIN1 gene cause centronuclear myopathy, which shares some histopathological features with myotonic dystrophy. We found that MBNL1 binds the BIN1 pre-mRNA and regulates its alternative splicing. BIN1 missplicing results in expression of an inactive form of BIN1 lacking phosphatidylinositol 5-phosphate-binding and membrane-tubulating activities. Consistent with a defect of BIN1, muscle T tubules are altered in people with myotonic dystrophy, and membrane structures are restored upon expression of the normal splicing form of BIN1 in muscle cells of such individuals. Finally, reproducing BIN1 splicing alteration in mice is sufficient to promote T tubule alterations and muscle weakness, a predominant feature of myotonic dystrophy.

SOX2 is an oncogene activated by recurrent 3q26.3 amplifications in human lung squamous cell carcinomas.

Squamous cell carcinoma (SCC) of the lung is a frequent and aggressive cancer type. Gene amplifications, a known activating mechanism of oncogenes, target the 3q26-qter region as one of the most frequently gained/amplified genomic sites in SCC of various types. Here, we used array comparative genomic hybridization to delineate the consensus region of 3q26.3 amplifications in lung SCC. Recurrent amplifications occur in 20% of lung SCC (136 tumors in total) and map to a core region of 2 Mb (Megabases) that encompasses SOX2, a transcription factor gene. Intense SOX2 immunostaining is frequent in nuclei of lung SCC, indicating potential active transcriptional regulation by SOX2. Analyses of the transcriptome of lung SCC, SOX2-overexpressing lung epithelial cells and embryonic stem cells (ESCs) reveal that SOX2 contributes to activate ESC-like phenotypes and provide clues pertaining to the deregulated genes involved in the malignant phenotype. In cell culture experiments, overexpression of SOX2 stimulates cellular migration and anchorage-independent growth while SOX2 knockdown impairs cell growth. Finally, SOX2 over-expression in non-tumorigenic human lung bronchial epithelial cells is tumorigenic in immunocompromised mice. These results indicate that the SOX2 transcription factor, a major regulator of stem cell function, is also an oncogene and a driver gene for the recurrent 3q26.33 amplifications in lung SCC.

Inhibition of histone deacetylases in rats self-administering cocaine regulates lissencephaly gene-1 and reelin gene expression, as revealed by microarray technique.

Injection of the histone deacetylase inhibitor trichostatin A (TsA) to rats has been shown to decrease their motivation to self-administer cocaine. In the present study, we investigated alterations in gene expression patterns in the anterior cingulate cortex and nucleus accumbens of rats self-administering cocaine and treated with TsA. Using oligonucleotide microarrays, we identified 722 probe sets in the cortex and 136 probe sets in the nucleus accumbens that were differentially expressed between vehicle and TsA-treated rats that self-administered cocaine. Microarray data were validated by real-time PCR for seven genes. Using immunohistochemistry, we further investigated the expression of Lis1 and reelin genes, because (i) they were similarly regulated by TsA at the mRNA level; (ii) they belong to the same signal transduction pathway; (iii) mutations within both genes cause lissencephaly. Cocaine self-injection was sufficient to activate the two genes at both the mRNA and protein levels. TsA treatment was found to up-regulate both Lis1 and reelin protein expression in the cortex and to down-regulate it in the nucleus accumbens of rats self-administering cocaine. The data suggest that the two proteins contribute to establish neurobiological mechanisms underlying brain plasticity whereby TsA lowers the motivation for cocaine.

ADCK3, an ancestral kinase, is mutated in a form of recessive ataxia associated with coenzyme Q10 deficiency.

Muscle coenzyme Q(10) (CoQ(10) or ubiquinone) deficiency has been identified in more than 20 patients with presumed autosomal-recessive ataxia. However, mutations in genes required for CoQ(10) biosynthetic pathway have been identified only in patients with infantile-onset multisystemic diseases or isolated nephropathy. Our SNP-based genome-wide scan in a large consanguineous family revealed a locus for autosomal-recessive ataxia at chromosome 1q41. The causative mutation is a homozygous splice-site mutation in the aarF-domain-containing kinase 3 gene (ADCK3). Five additional mutations in ADCK3 were found in three patients with sporadic ataxia, including one known to have CoQ(10) deficiency in muscle. All of the patients have childhood-onset cerebellar ataxia with slow progression, and three of six have mildly elevated lactate levels. ADCK3 is a mitochondrial protein homologous to the yeast COQ8 and the bacterial UbiB proteins, which are required for CoQ biosynthesis. Three out of four patients tested showed a low endogenous pool of CoQ(10) in their fibroblasts or lymphoblasts, and two out of three patients showed impaired ubiquinone synthesis, strongly suggesting that ADCK3 is also involved in CoQ(10) biosynthesis. The deleterious nature of the three identified missense changes was confirmed by the introduction of them at the corresponding positions of the yeast COQ8 gene. Finally, a phylogenetic analysis shows that ADCK3 belongs to the family of atypical kinases, which includes phosphoinositide and choline kinases, suggesting that ADCK3 plays an indirect regulatory role in ubiquinone biosynthesis possibly as part of a feedback loop that regulates ATP production.

Novel insights into the relationships between dendritic cell subsets in human and mouse revealed by genome-wide expression profiling.

BACKGROUND: Dendritic cells (DCs) are a complex group of cells that play a critical role in vertebrate immunity. Lymph-node resident DCs (LN-DCs) are subdivided into conventional DC (cDC) subsets (CD11b and CD8alpha in mouse; BDCA1 and BDCA3 in human) and plasmacytoid DCs (pDCs). It is currently unclear if these various DC populations belong to a unique hematopoietic lineage and if the subsets identified in the mouse and human systems are evolutionary homologs. To gain novel insights into these questions, we sought conserved genetic signatures for LN-DCs and in vitro derived granulocyte-macrophage colony stimulating factor (GM-CSF) DCs through the analysis of a compendium of genome-wide expression profiles of mouse or human leukocytes. RESULTS: We show through clustering analysis that all LN-DC subsets form a distinct branch within the leukocyte family tree, and reveal a transcriptomal signature evolutionarily conserved in all LN-DC subsets. Moreover, we identify a large gene expression program shared between mouse and human pDCs, and smaller conserved profiles shared between mouse and human LN-cDC subsets. Importantly, most of these genes have not been previously associated with DC function and many have unknown functions. Finally, we use compendium analysis to re-evaluate the classification of interferon-producing killer DCs, lin-CD16+HLA-DR+ cells and in vitro derived GM-CSF DCs, and show that these cells are more closely linked to natural killer and myeloid cells, respectively. CONCLUSION: Our study provides a unique database resource for future investigation of the evolutionarily conserved molecular pathways governing the ontogeny and functions of leukocyte subsets, especially DCs.

Loss of Trim24 (Tif1alpha) gene function confers oncogenic activity to retinoic acid receptor alpha.

Hepatocellular carcinoma (HCC) is a major cause of death worldwide. Here, we provide evidence that the ligand-dependent nuclear receptor co-regulator Trim24 (also known as Tif1alpha) functions in mice as a liver-specific tumor suppressor. In Trim24-null mice, hepatocytes fail to execute proper cell cycle withdrawal during the neonatal-to-adult transition and continue to cycle in adult livers, becoming prone to a continuum of cellular alterations that progress toward metastatic HCC. Using pharmacological approaches, we show that inhibition of retinoic acid signaling markedly reduces hepatocyte proliferation in Trim24-/- mice. We further show that deletion of a single retinoic acid receptor alpha (Rara) allele in a Trim24-null background suppresses HCC development and restores wild-type expression of retinoic acid-responsive genes in the liver, thus demonstrating that in this genetic background Rara expresses an oncogenic activity correlating with a dysregulation of the retinoic acid signaling pathway. Our results not only provide genetic evidence that Trim24 and Rara co-regulate hepatocarcinogenesis in an antagonistic manner but also suggest that aberrant activation of Rara is deleterious to liver homeostasis.

Gene expression profiling in lung fibroblasts reveals new players in alveolarization.

Little is known about the molecular basis of lung alveolarization. We used a microarray profiling strategy to identify novel genes that may regulate the secondary septation process. Rat lung fibroblasts were extemporaneously isolated on postnatal days 2, 7, and 21, i.e., before, during, and after septation, respectively. Total RNA was extracted, and cRNAs were hybridized to Affymetrix rat genome 230 2.0 microarrays. Expression levels of a selection of genes were confirmed by real-time PCR. In addition to genes already known to be upregulated during alveolarization including drebrin, midkine, Fgfr3, and Fgfr4, the study allowed us to identify two remarkable groups of genes with opposite profiles, i.e., gathering genes either transiently up- or downregulated on day 7. The former group includes the transcription factors retinoic acid receptor (RXR)-gamma and homeobox (Hox) a2, a4, and a5 and genes involved in Wnt signaling (Wnt5a, Fzd1, and Ndp); the latter group includes the extracellular matrix components Comp and Opn and the signal molecule Slfn4. Profiling in whole lung from fetal life to adulthood confirmed that changes were specific for alveolarization. Two treatments that arrest septation, hyperoxia and dexamethasone, inhibited the expression of genes that are upregulated during alveolarization and conversely enhanced that of genes weakly expressed during alveolarization and upregulated thereafter. The possible roles of these genes in secondary septation are discussed. Gene expression profiling analysis on freshly isolated cells represents a powerful approach to provide new information about differential regulation of genes during alveolarization and pathways potentially involved in the pathogenesis of bronchopulmonary dysplasia.

Transcriptomic analysis of the sulfate starvation response of Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an opportunistic pathogen that causes a number of infections in humans, but is best known for its association with cystic fibrosis. It is able to use a wide range of sulfur compounds as sources of sulfur for growth. Gene expression in response to changes in sulfur supply was studied in P. aeruginosa E601, a cystic fibrosis isolate that displays mucin sulfatase activity, and in P. aeruginosa PAO1. A large family of genes was found to be upregulated by sulfate limitation in both isolates, encoding sulfatases and sulfonatases, transport systems, oxidative stress proteins, and a sulfate-regulated TonB/ExbBD complex. These genes were localized in five distinct islands on the genome and encoded proteins with a significantly reduced content of cysteine and methionine. Growth of P. aeruginosa E601 with mucin as the sulfur source led not only to a sulfate starvation response but also to induction of genes involved with type III secretion systems.

Loss of alpha6 integrins in keratinocytes leads to an increase in TGFbeta and AP1 signaling and in expression of differentiation genes.

Mice lacking the alpha6 integrin chain die at birth with severe skin blistering. To further study the function of alpha6 integrin in skin, we generated conditionally immortalized cell lines from the epidermis of wild-type and alpha6 deficient mouse embryos. Mutant cells presented a decreased adhesion on laminin 5, the major component of the basement membrane in the skin, and on laminins 10/11 and 2. A DNA array analysis revealed alterations in the expression of extracellular matrix (ECM) components including laminin 5, cytoskeletal elements, but also membrane receptors like the hemidesmosomal components integrin beta4 and collagen XVII, or growth factors and signaling molecules of the TGFbeta, EGF, and Wnt pathways. Finally, an increase of several epidermal differentiation markers was observed in cells and tissue at the protein level. Further examination of the mutant tissue revealed alterations in the filaggrin signal. These differences may be linked to an upregulation of the TGFbeta and the Jun/Fos pathways in mutant keratinocytes. These results are in favor of a role for integrin alpha6beta4 in the maintenance of basal keratinocyte properties and epidermal homeostasis.

Identification of a novel BBS gene (BBS12) highlights the major role of a vertebrate-specific branch of chaperonin-related proteins in Bardet-Biedl syndrome.

Bardet-Biedl syndrome (BBS) is primarily an autosomal recessive ciliopathy characterized by progressive retinal degeneration, obesity, cognitive impairment, polydactyly, and kidney anomalies. The disorder is genetically heterogeneous, with 11 BBS genes identified to date, which account for ~70% of affected families. We have combined single-nucleotide-polymorphism array homozygosity mapping with in silico analysis to identify a new BBS gene, BBS12. Patients from two Gypsy families were homozygous and haploidentical in a 6-Mb region of chromosome 4q27. FLJ35630 was selected as a candidate gene, because it was predicted to encode a protein with similarity to members of the type II chaperonin superfamily, which includes BBS6 and BBS10. We found pathogenic mutations in both Gypsy families, as well as in 14 other families of various ethnic backgrounds, indicating that BBS12 accounts for approximately 5% of all BBS cases. BBS12 is vertebrate specific and, together with BBS6 and BBS10, defines a novel branch of the type II chaperonin superfamily. These three genes are characterized by unusually rapid evolution and are likely to perform ciliary functions specific to vertebrates that are important in the pathophysiology of the syndrome, and together they account for about one-third of the total BBS mutational load. Consistent with this notion, suppression of each family member in zebrafish yielded gastrulation-movement defects characteristic of other BBS morphants, whereas simultaneous suppression of all three members resulted in severely affected embryos, possibly hinting at partial functional redundancy within this protein family.

Pitfalls of homozygosity mapping: an extended consanguineous Bardet-Biedl syndrome family with two mutant genes (BBS2, BBS10), three mutations, but no triallelism.

The extensive genetic heterogeneity of Bardet-Biedl syndrome (BBS) is documented by the identification, by classical linkage analysis complemented recently by comparative genomic approaches, of nine genes (BBS1-9) that account cumulatively for about 50% of patients. The BBS genes appear implicated in cilia and basal body assembly or function. In order to find new BBS genes, we performed SNP homozygosity mapping analysis in an extended consanguineous family living in a small Lebanese village. This uncovered an unexpectedly complex pattern of mutations, and led us to identify a novel BBS gene (BBS10). In one sibship of the pedigree, a BBS2 homozygous mutation was identified, while in three other sibships, a homozygous missense mutation was identified in a gene encoding a vertebrate-specific chaperonine-like protein (BBS10). The single patient in the last sibship was a compound heterozygote for the above BBS10 mutation and another one in the same gene. Although triallelism (three deleterious alleles in the same patient) has been described in some BBS families, we have to date no evidence that this is the case in the present family. The analysis of this family challenged linkage analysis based on the expectation of a single locus and mutation. The very high informativeness of SNP arrays was instrumental in elucidating this case, which illustrates possible pitfalls of homozygosity mapping in extended families, and that can be explained by the rather high prevalence of heterozygous carriers of BBS mutations (estimated at one in 50 in Europeans).

Polyglutamine expansion causes neurodegeneration by altering the neuronal differentiation program.

Huntington's disease (HD) and spinocerebellar ataxia type 7 (SCA7) belong to a group of inherited neurodegenerative diseases caused by polyglutamine (polyQ) expansion in corresponding proteins. Transcriptional alteration is a unifying feature of polyQ disorders; however, the relationship between polyQ-induced gene expression deregulation and degenerative processes remains unclear. R6/2 and R7E mouse models of HD and SCA7, respectively, present a comparable retinal degeneration characterized by progressive reduction of electroretinograph activity and important morphological changes of rod photoreceptors. The retina, which is a simple central nervous system tissue, allows correlating functional, morphological and molecular defects. Taking advantage of comparing polyQ-induced degeneration in two retina models, we combined gene expression profiling and molecular biology techniques to decipher the molecular pathways underlying polyQ expansion toxicity. We show that R7E and R6/2 retinal phenotype strongly correlates with loss of expression of a large cohort of genes specifically involved in phototransduction function and morphogenesis of differentiated rod photoreceptors. Accordingly, three key transcription factors (Nrl, Crx and Nr2e3) controlling rod differentiation genes, hence expression of photoreceptor specific traits, are down-regulated. Interestingly, other transcription factors known to cause inhibitory effects on photoreceptor differentiation when mis-expressed, such as Stat3, are aberrantly re-activated. Thus, our results suggest that independently from the protein context, polyQ expansion overrides the control of neuronal differentiation and maintenance, thereby causing dysfunction and degeneration.

Notch activation is an early and critical event during T-Cell leukemogenesis in Ikaros-deficient mice.

The Ikaros transcription factor is both a key regulator of lymphocyte differentiation and a tumor suppressor in T lymphocytes. Mice carrying a hypomorphic mutation (Ik(L/L)) in the Ikaros gene all develop thymic lymphomas. Ik(L/L) tumors always exhibit strong activation of the Notch pathway, which is required for tumor cell proliferation in vitro. Notch activation occurs early in tumorigenesis and may precede transformation, as ectopic expression of the Notch targets Hes-1 and Deltex-1 is detected in thymocytes from young Ik(L/L) mice with no overt signs of transformation. Notch activation is further amplified by secondary mutations that lead to C-terminal truncations of Notch 1. Strikingly, restoration of Ikaros activity in tumor cells leads to a rapid and specific downregulation of Notch target gene expression and proliferation arrest. Furthermore, Ikaros binds to the Notch-responsive element in the Hes-1 promoter and represses Notch-dependent transcription from this promoter. Thus, Ikaros-mediated repression of Notch target gene expression may play a critical role in defining the tumor suppressor function of this factor.