PU.1 cooperates with IRF4 and IRF8 to suppress pre-B-cell leukemia.

The Ets family transcription factor PU.1 and the interferon regulatory factor (IRF)4 and IRF8 regulate gene expression by binding to composite DNA sequences known as Ets/interferon consensus elements. Although all three factors are expressed from the onset of B-cell development, single deficiency of these factors in B-cell progenitors only mildly impacts on bone marrow B lymphopoiesis. Here we tested whether PU.1 cooperates with IRF factors in regulating early B-cell development. Lack of PU.1 and IRF4 resulted in a partial block in development the pre-B-cell stage. The combined deletion of PU.1 and IRF8 reduced recirculating B-cell numbers. Strikingly, all PU.1/IRF4 and ~50% of PU.1/IRF8 double deficient mice developed pre-B-cell acute lymphoblastic leukemia (B-ALL) associated with reduced expression of the established B-lineage tumor suppressor genes, Ikaros and Spi-B. These genes are directly regulated by PU.1/IRF4/IRF8, and restoration of Ikaros or Spi-B expression inhibited leukemic cell growth. In summary, we demonstrate that PU.1, IRF4 and IRF8 cooperate to regulate early B-cell development and to prevent pre-B-ALL formation.

Activated Notch counteracts Ikaros tumor suppression in mouse and human T-cell acute lymphoblastic leukemia.

Activating NOTCH1 mutations occur in ~60% of human T-cell acute lymphoblastic leukemias (T-ALLs), and mutations disrupting the transcription factor IKZF1 (IKAROS) occur in ~5% of cases. To investigate the regulatory interplay between these driver genes, we have used a novel transgenic RNA interference mouse model to produce primary T-ALLs driven by reversible Ikaros knockdown. Restoring endogenous Ikaros expression in established T-ALL in vivo acutely represses Notch1 and its oncogenic target genes including Myc, and in multiple primary leukemias causes disease regression. In contrast, leukemias expressing high levels of endogenous or engineered forms of activated intracellular Notch1 (ICN1) resembling those found in human T-ALL rapidly relapse following Ikaros restoration, indicating that ICN1 functionally antagonizes Ikaros in established disease. Furthermore, we find that IKAROS mRNA expression is significantly reduced in a cohort of primary human T-ALL patient samples with activating NOTCH1/FBXW7 mutations, but is upregulated upon acute inhibition of aberrant NOTCH signaling across a panel of human T-ALL cell lines. These results demonstrate for the first time that aberrant NOTCH activity compromises IKAROS function in mouse and human T-ALL, and provide a potential explanation for the relative infrequency of IKAROS gene mutations in human T-ALL.

Distinct regulators control the expression of the mid-hindbrain organizer signal FGF8.

Local expression of FGF8 at the mid/hindbrain boundary (MHB) governs the development of multiple neurons and support cells. Here we show that the paired-domain protein Pax2 is necessary and sufficient for the induction of FGF8 in part by regulating the expression of Pax5&8. A network of transcription and secreted factors, including En1, Otx2, Gbx2, Grg4 and Wnt1&4, that is established independently of Pax2, further refines the expression domain and level of FGF8 at the MHB through opposing effects on Pax2 activity. Our results indicate that the expression of local organizing factors is controlled by combinatorial interaction between inductive and modulatory factors.

The transcriptional repressor CDP (Cutl1) is essential for epithelial cell differentiation of the lung and the hair follicle.

The mammalian Cutl1 gene codes for the CCAAT displacement protein (CDP), which has been implicated as a transcriptional repressor in diverse processes such as terminal differentiation, cell cycle progression, and the control of nuclear matrix attachment regions. To investigate the in vivo function of Cutl1, we have replaced the C-terminal Cut repeat 3 and homeodomain exons with an in-frame lacZ gene by targeted mutagenesis in the mouse. The CDP-lacZ fusion protein is retained in the cytoplasm and fails to repress gene transcription, indicating that the Cutl1(lacZ) allele corresponds to a null mutation. Cutl1 mutant mice on inbred genetic backgrounds are born at Mendelian frequency, but die shortly after birth because of retarded differentiation of the lung epithelia, which indicates an essential role of CDP in lung maturation. A less pronounced delay in lung development allows Cutl1 mutant mice on an outbred background to survive beyond birth. These mice are growth-retarded and develop an abnormal pelage because of disrupted hair follicle morphogenesis. The inner root sheath (IRS) is reduced, and the transcription of Sonic hedgehog and IRS-specific genes is deregulated in Cutl1 mutant hair follicles, consistent with the specific expression of Cutl1 in the progenitors and cell lineages of the IRS. These data implicate CDP in cell-lineage specification during hair follicle morphogenesis, which resembles the role of the related Cut protein in specifying cell fates during Drosophila development.

Pax5/BSAP maintains the identity of B cells in late B lymphopoiesis.

The B lineage commitment factor Pax5 (BSAP) is expressed throughout B cell development. To investigate its late function, we generated a mouse strain carrying a floxed Pax5 allele that was conditionally inactivated by CD19-cre or Mx-cre expression. Pax5 deletion resulted in the preferential loss of mature B cells, inefficient lymphoblast formation, and reduced serum IgG levels. Mature B cells radically changed their gene expression pattern in response to Pax5 inactivation. Most B cell antigens were downregulated on the cell surface, and the transcription of B cell-specific genes was decreased, whereas the expression of non-B lymphoid genes was activated in Pax5-deficient B cells. These data demonstrate that Pax5 is essential for maintaining the identity and function of B cells during late B lymphopoiesis.

Pax5 determines the identity of B cells from the beginning to the end of B-lymphopoiesis.

Despite being one of the most intensively studied cell types, the molecular basis of B cell specification is largely unknown. The Pax5 gene encoding the transcription factor BSAP is required for progression of B-lymphopoiesis beyond the pro-B cell stage. Pax5-deficient pro-B cells are, however, not yet committed to the B-lymphoid lineage, but instead have a broad lymphomyeloid developmental potential. Pax5 appears to mediate B-lineage commitment by repressing the transcription of non-B-lymphoid genes and by simultaneously activating the expression of B-lineage-specific genes. Pax5 thus functions both as a transcriptional repressor and activator, depending on its interactions with corepressors of the Groucho protein family or with positive regulators such as the TATA-binding protein. Once committed to the B-lineage, B cells require Pax5 function to maintain their B-lymphoid identity throughout B cell development.

Fidelity and infidelity in commitment to B-lymphocyte lineage development.

During B-lymphocyte development in mouse fetal liver and bone marrow, a pre-B I cell stage is reached in which the cells express B-lineage-specific genes, such as CD19, Ig alpha and Igbeta and VpreB and lambda5, which encode the surrogate light (SL) chain. In these pre-B I cells both alleles of the immunoglobulin heavy (IgH) chain locus are D(H)J(H) rearranged. Transplantation of pre-B I cells from wild-type (e.g. C57Bl/6) mice in histocompatible RAG-deficient hosts leads to long-term reconstitution of some of the mature B-cell compartments and to the establishment of normal IgM levels, a third of the normal serum IgA levels, and IgG levels below the detection limit. Neither T-lineage nor myeloid cells of donor origin can be detected in the transplanted hosts, indicating that the pre-B I cells are committed to B-lineage differentiation. Consequently, the B-cell-reconstituted hosts respond to T-cell-independent antigens but not to T-cell-dependent antigens. Responses to T-cell-dependent antigens can be restored in the pre-B I-cell-transplanted, RAG-deficient hosts by the concomitant transplantation of mature CD4+ T cells. The transplanted wild-type pre-B I cells do not home back to the bone marrow and become undetectable shortly after transplantation. B-lymphocyte development in Pax-5-deficient mice becomes arrested at the transition of pre-B I to pre-B II cells i.e. at the stage when V(H) to D(H)J(H) rearrangements occur and when the pre-B-cell receptor, complete with muH chains and SL chains, is normally formed. T-lineage and myeloid cell development in these mice is normal. Pre-B I cells of Pax-5-deficient mice have a wild-type pre-B I-cell-like phenotype: while they do not express Pax-5-controlled CD19 gene, and express Ig alpha to a lesser extent, they express Igbeta, VpreB and lambda5, and proliferate normally in vitro on stromal cells in the presence of interleukin (IL)-7. Clones of these pre-B I cells carry characteristic D(H)J(H) rearrangements on both IgH chain alleles. However, removal of IL-7 from the tissue cultures, unlike wild-type pre-B I cells, does not induce B-cell differentiation to surface IgM-expressing B cells, but induces macrophage differentiation. This differentiation into macrophages requires either the presence of stromal cells or addition of macrophage colony-stimulating factor (M-CSF). Addition of M-CSF followed by granulocyte-macrophage colony-stimulating factor induces the differentiation to MHC class II-expressing, antigen-presenting dendritic cells. In vitro differentiation to granulocytes and osteoclasts can also be observed in the presence of the appropriate cytokines. Moreover, transplantation of Pax-5-deficient pre-B I clones into RAG-deficient hosts, while not allowing B-cell differentiation, leads to the full reconstitution of the thymus with all stages of CD4-CD8- and CD4+CD8+ thymocytes, to normal positive and negative selection of thymocytes in the thymus, and to the development of normal, reactive mature CD4+ and CD8+ T-cell compartments in the peripheral lymphoid tissues, all carrying the clone-specific D(H)J(H) rearrangements. On the other hand, Ig alpha, Igbeta, VpreB and lambda5 are turned off in the thymocytes, demonstrating that the expression of these genes does not commit cells irreversibly to the B lineage. Further more, Pax-5-deficient pre-B I cells are long-term reconstituting cells. They home back to the bone marrow of the RAG-deficient host, can be reisolated and regrown in tissue culture, and can be retransplanted into a secondary RAG-deficient host. This again develops thymocytes and mature T cells and allows the transplanted clonal pre-B I cells to home to the bone marrow.

Functional equivalence of the transcription factors Pax2 and Pax5 in mouse development.

Pax2 and Pax5 arose by gene duplication at the onset of vertebrate evolution and have since diverged in their developmental expression patterns. They are expressed in different organs of the mouse embryo except for their coexpression at the midbrain-hindbrain boundary (MHB), which functions as an organizing center to control midbrain and cerebellum development. During MHB development, Pax2 expression is initiated prior to Pax5 transcription, and Pax2(-/-) embryos fail to generate the posterior midbrain and cerebellum, whereas Pax5(-/-) mice exhibit only minor patterning defects in the same brain regions. To investigate whether these contrasting phenotypes are caused by differences in the temporal expression or biochemical activity of these two transcription factors, we have generated a knock-in (ki) mouse, which expresses a Pax5 minigene under the control of the Pax2 locus. Midbrain and cerebellum development was entirely rescued in Pax2(5ki/5ki) embryos. Pax5 could furthermore completely substitute for the Pax2 function during morphogenesis of the inner ear and genital tracts, despite the fact that the Pax5 transcript of the Pax2(5ki )allele was expressed only at a fivefold lower level than the wild-type Pax2 mRNA. As a consequence, the Pax2(5ki )allele was able to rescue most but not all Pax2 mutant defects in the developing eye and kidney, both of which are known to be highly sensitive to Pax2 protein dosage. Together these data demonstrate that the transcription factors Pax2 and Pax5 have maintained equivalent biochemical functions since their divergence early in vertebrate evolution.

Transcriptional repression by Pax5 (BSAP) through interaction with corepressors of the Groucho family.

Pax5 (BSAP) functions as both a transcriptional activator and repressor during midbrain patterning, B-cell development and lymphomagenesis. Here we demonstrate that Pax5 exerts its repression function by recruiting members of the Groucho corepressor family. In a yeast two-hybrid screen, the groucho-related gene product Grg4 was identified as a Pax5 partner protein. Both proteins interact cooperatively via two separate domains: the N-terminal Q and central SP regions of Grg4, and the octapeptide motif and C-terminal transactivation domain of Pax5. The phosphorylation state of Grg4 is altered in vivo upon Pax5 binding. Moreover, Grg4 efficiently represses the transcriptional activity of Pax5 in an octapeptide-dependent manner. Similar protein interactions resulting in transcriptional repression were also observed between distantly related members of both the Pax2/5/8 and Groucho protein families. In agreement with this evolutionary conservation, the octapeptide motif of Pax proteins functions as a Groucho-dependent repression domain in Drosophila embryos. These data indicate that Pax proteins can be converted from transcriptional activators to repressors through interaction with corepressors of the Groucho protein family.

A syndrome involving intrauterine growth retardation, microcephaly, cerebellar hypoplasia, B lymphocyte deficiency, and progressive pancytopenia.

We report a new complex syndrome involving profound failure to thrive with severe intrauterine growth retardation, cerebellar abnormalities, microcephaly, a complete lack of B lymphocyte development, and secondary, progressive marrow aplasia. B cell differentiation was found to be blocked at the pro-B cell stage. Although not strictly proven, a genetic origin is likely, according to similar cases reported in the literature. Three candidate genes, PAX5, encoding B cell-specific activator protein, a factor involved in B cell lineage commitment, stromal cell-derived factor 1, and CXCR4, encoding a chemokine and its receptor, respectively, were thought to be responsible for this disease, given the similarity between the phenotype of the corresponding knock-out mice and the clinical features of the patient. However, the genomic DNA sequences of these 3 genes were normal, and normal amounts of stromal cell-derived factor 1 and CXCR4 were present. These data strongly suggest that another molecule is involved in early B cell differentiation, hematopoiesis, and cerebellar development in humans.

Lineage commitment in lymphopoiesis.

The mechanisms controlling the commitment of hematopoietic progenitor cells to the lymphoid lineages are still mostly unknown. Recent findings indicate that the earliest phase of B cell development may proceed in two steps. At the onset of B-lymphopoiesis, the transcription factors E2A and EBF coordinately activate the B-cell-specific gene expression program. Subsequently, Pax5 appears to repress the promiscuous transcription of lineage-inappropriate genes and thus commits progenitor cells to the B-lymphoid pathway by suppressing alternative cell fates. B-lineage commitment by Pax5 seems to occur in a stochastic manner in the bone marrow, as indicated by the random activation of only one of the two Pax5 alleles in early pro-B cells. In contrast, loss- and gain-of-function analyses have implicated the Notch1 receptor in the specification of the T cell fate, which may thus be controlled by instructive signals in the thymus.

Pax2 and homeodomain proteins cooperatively regulate a 435 bp enhancer of the mouse Pax5 gene at the midbrain-hindbrain boundary.

Pax and homeodomain transcription factors are essential for the formation of an organizing center at the midbrain-hindbrain boundary (mhb) which controls the genesis of the midbrain and cerebellum in the vertebrate embryo. Pax2 and Pax5 are sequentially activated in this brain region, with Pax2 expression preceding that of Pax5. Using a transgenic reporter assay, we have now identified a conserved 435 bp enhancer in the 5' flanking region of mammalian Pax5 genes which directs lacZ expression in the correct temporal and spatial pattern at the mhb. This minimal enhancer is composed of two distinct elements, as shown by protein-binding assays with mhb-specific extracts. The proximal element contains overlapping consensus binding sites for members of the Pax2/5/8 and POU protein families, whereas a distal element is bound by homeodomain and zinc finger transcription factors. Expression analysis of transgenes carrying specific mutations in these recognition motifs identified the Pax- and homeodomain-binding sites as functional elements which cooperatively control the activity of the mhb enhancer. lacZ genes under the control of either the minimal enhancer or the endogenous Pax5 locus were normally expressed at the mhb in Pax5 mutant embryos, indicating that this enhancer does not depend on autoregulation by Pax5. In Pax2 mutant embryos, expression of the endogenous Pax5 gene was, however, delayed and severely reduced in lateral aspects of the neural plate which, on neural tube closure, becomes the dorsal mhb region. This cross-regulation by Pax2 is mediated by the Pax-binding site of the minimal enhancer which, upon specific mutation, resulted in severely reduced transgene expression in the dorsal part of the mhb. Together these data indicate that Pax2 and homeodomain proteins directly bind to and cooperatively regulate the mhb enhancer of Pax5.

Commitment to the B-lymphoid lineage depends on the transcription factor Pax5.

The Pax5 gene encoding the B-cell-specific activator protein (BSAP) is expressed within the haematopoietic system exclusively in the B-lymphoid lineage, where it is required in vivo for progression beyond the pro-B-cell stage. However, Pax5 is not essential for in vitro propagation of pro-B cells in the presence of interleukin-7 and stromal cells. Here we show that pro-B cells lacking Pax5 are also incapable of in vitro B-cell differentiation unless Pax5 expression is restored by retroviral transduction. Pax5-/- pro-B cells are not restricted in their lineage fate, as stimulation with appropriate cytokines induces them to differentiate into functional macrophages, osteoclasts, dendritic cells, granulocytes and natural killer cells. As expected for a clonogenic haematopoietic progenitor with lymphomyeloid developmental potential, the Pax5-/- pro-B cell expresses genes of different lineage-affiliated programmes, and restoration of Pax5 activity represses this lineage-promiscuous transcription. Pax5 therefore plays an essential role in B-lineage commitment by suppressing alternative lineage choices.

Long-term in vivo reconstitution of T-cell development by Pax5-deficient B-cell progenitors.

The mechanisms controlling the commitment of haematopoietic progenitors to the B-lymphoid lineage are poorly understood. The observations that mice deficient in E2A and EBF lack B-lineage cells have implicated these two transcription factors in the commitment process. Moreover, the expression of genes encoding components of the rearrangement machinery (RAG1, RAG2, TdT) or pre-B-cell receptor (lambda5, VpreB, Igalpha, Igbeta) has been considered to indicate B-lineage commitment. All these genes including E2A and EBF are expressed in pro-B cells lacking the transcription factor Pax5. Here we show that cloned Pax5-deficient pro-B cells transferred into RAG2-deficient mice provide long-term reconstitution of the thymus and give rise to mature T cells expressing alpha/beta-T-cell receptors. The bone marrow of these mice contains a population of cells of Pax5-/- origin with the same phenotype as the donor pro-B cells. When transferred into secondary recipients, these pro-B cells again home to the bone marrow and reconstitute the thymus. Hence, B-lineage commitment is determined neither by immunoglobulin DJ rearrangement nor by the expression of E2A, EBF, lambda5, VpreB, Igalpha and Igbeta. Instead, our data implicate Pax5 in the control of B-lineage commitment.

Monoallelic expression of Pax5: a paradigm for the haploinsufficiency of mammalian Pax genes?

It is generally assumed that most mammalian genes are transcribed from both alleles. Hence, the diploid state of the genome offers the advantage that a loss-of-function mutation in one allele can be compensated for by the remaining wild-type allele of the same gene. Indeed, the vast majority of human disease syndromes and engineered mutations in the mouse genome are recessive, indicating that recessiveness is the 'default' state. However, a minority of genes are semi-dominant, as heterozygous loss-of-function mutation in these genes leads to phenotypic abnormalities. This condition, known as haploinsufficiency, has been described for five of the nine mammalian Pax genes, which are associated with mouse developmental mutants and human disease syndromes. Recently we have reported that the Pax5 gene is subject to allele-specific regulation during B cell development. Pax5 is predominantly transcribed from only one of its two alleles in early B-lymphoid progenitors and mature B cells, while it transiently switches to a biallelic mode of transcription in pre-B and immature B cells. As a consequence, B-lymphoid tissues are mosaic with regard to the transcribed allele, and heterozygous mutation of Pax5 therefore results in deletion of B lymphocytes expressing only the mutant allele. The allele-specific regulation of Pax5 raises the intriguing possibility that monoallelic expression may also be the mechanism causing the haploinsufficiency of other Pax genes. In this review, we discuss different models accounting for the haploinsufficiency of mammalian Pax genes, provide further evidence in support of the allele-specific regulation of Pax5 and discuss the implication of these findings in the context of the recent literature describing the stochastic and monoallelic activation of other hematopoietic genes.

Pax2/5 and Pax6 subdivide the early neural tube into three domains.

The nested expression patterns of the paired-box containing transcription factors Pax2/5 and Pax6 demarcate the midbrain and forebrain primordium at the neural plate stage. We demonstrate that, in Pax2/5 deficient mice, the mesencephalon/metencephalon primordium is completely missing, resulting in a fusion of the forebrain to the hindbrain. Morphologically, in the alar plate the deletion is characterized by the substitution of the tectum (dorsal midbrain) and cerebellum (dorsal metencephalon) by the caudal diencephalon and in the basal plate by the replacement of the midbrain tegmentum by the ventral metencephalon (pons). Molecularly, the loss of the tectum is demonstrated by an expanded expression of Pax6, (the molecular determinant of posterior commissure), and a rostral shift of the territory of expression of Gbx2 and Otp (markers for the pons), towards the caudal diencephalon. Our results suggest that an intact territory of expression of Pax2/5 in the neural plate, nested between the rostral and caudal territories of expression of Pax6, is necessary for defining the midbrain vesicle.

twin of eyeless, a second Pax-6 gene of Drosophila, acts upstream of eyeless in the control of eye development.

The Drosophila Pax-6 gene eyeless (ey) plays a key role in eye development. Here we show tht Drosophila contains a second Pax-6 gene, twin of eyeless (toy), due to a duplication during insect evolution. Toy is more similar to vertebrate Pax-6 proteins than Ey with regard to overall sequence conservation, DNA-binding function, and early expression in the embryo, toy and ey share a similar expression pattern in the developing visual system, and targeted expression of Toy, like Ey, induces the formation of ectopic eyes. Genetic and biochemical evidence indicates, however, that Toy functions upstream of ey by directly regulating the eye-specific enhancer of ey. Toy is therefore required for initiation of ey expression in the embryo and acts through Ey to activate the eye developmental program.

The partial homeodomain of the transcription factor Pax-5 (BSAP) is an interaction motif for the retinoblastoma and TATA-binding proteins.

Pax-5 codes for the transcription factor BSAP, which plays an important role in midbrain patterning, B cell development, and lymphoma formation. Pax-5 is known to control gene expression by recognizing its target genes via the NH2-terminal paired domain and by regulating transcription through a COOH-terminal regulatory module consisting of activating and inhibitory sequences. The central region of Pax-5 contains a sequence with significant homology to the first alpha-helix of the paired-type homeodomain. This partial homeodomain has been highly conserved throughout vertebrate evolution because it is found not only in Pax-5 but also in the related Pax-2 and Pax-8 members of the same Pax subfamily. Here we report that the partial homeodomain binds the TATA-binding protein (TBP) and retinoblastoma (Rb) gene product. Both TBP and Rb were shown by coimmunoprecipitation experiments to directly associate with Pax-5 in vivo. The conserved core domain of TBP and the pocket region as well as COOH-terminal sequences of Rb are required for interaction with the partial homeodomain of Pax-5 in in vitro binding assays. Furthermore, Pax-5 was specifically bound only by the underphosphorylated form of Rb. These data indicate that Pax-5 is able to contact the basal transcription machinery through the TBP-containing initiation factor TFIID, and that its activity can be controlled by the cell cycle-regulated association with Rb.

Independent regulation of the two Pax5 alleles during B-cell development.

The developmental control genes of the Pax family are frequently associated with mouse mutants and human disease syndromes. The function of these transcription factors is sensitive to gene dosage, as mutation of one allele or a modest increase in gene number results in phenotypic abnormalities. Pax5 has an important role in B-cell and midbrain development. By following the expression of individual Pax5 alleles at the single-cell level, we demonstrate here that Pax5 is subject to allele-specific regulation during B-lymphopoiesis. Pax5 is predominantly transcribed from only one allele in early progenitors and mature B cells, whereas it switches to a biallelic transcription mode in immature B cells. The allele-specific regulation of Pax5 is stochastic, reversible, independent of parental origin and correlates with synchronous replication, in contrast with imprinted and other monoallelically expressed genes. As a consequence, B-lymphoid tissues are mosaics with respect to the transcribed Pax5 allele, and thus mutation of one allele in heterozygous mice results in deletion of the cell population expressing the mutant allele due to loss of Pax5 function at the single-cell level. Similar allele-specific regulation may be a common mechanism causing the haploinsufficiency and frequent association of other Pax genes with human disease.