Positional mapping of loci in the DiGeorge critical region at chromosome 22q11 using a new marker (D22S183)

A patient with DiGeorge syndrome with spina bifida and sacral myelomeningocele, who developed both hypocalcemia-induced seizure and epilepsy

DiGeorge syndrome - a component of the 22q11 deletion syndrome - causes a disturbance in cervical neural crest migration that results in parathyroid hypoplasia. Patients can develop hypocalcemia-induced seizures. Spina bifida is caused by failure of neurulation, including a disturbance in the adhesion processes at the neurula stage. Spina bifida has been reported as a risk factor for epilepsy. We report, for the first time, the case of a patient with DiGeorge syndrome with spina bifida and sacral myelomeningocele, who developed both hypocalcemia-induced seizures and epilepsy. The patient had spina bifida and sacral myelomeningocele at birth. At the age of 13 years, he experienced a seizure for the first time. At this time, the calcium concentration was normal. An electroencephalogram (EEG) proved that the seizure was due to epilepsy. Antiepileptic medications controlled the seizure. At the age of 29, the patient's calcium concentration began to reduce. At the age of 40, hypocalcemia-induced seizure occurred. At this time, the calcium concentration was 5.5mg/dL (reference range, 8.7-10.1mg/dL). The level of intact parathyroid hormone (PTH) was 6 pg/mL (reference range, 10-65 pg/mL). Chromosomal and genetic examinations revealed a deletion of TUP-like enhancer of split gene 1 (tuple1)-the diagnostic marker of DiGeorge syndrome. Many patients with DiGeorge syndrome have cardiac anomalies; however, our patient had none. We propose that the association among DiGeorge syndrome, spina bifida, epilepsy, cardiac anomaly, 22q11, tuple1, and microdeletion inheritance should be clarified for appropriate diagnosis and treatment.

Persistent fetal gamma-globin expression in adult transgenic mice following deletion of two silencer elements located 3' to the human Agamma-globin gene

Natural deletions of the human gamma-globin gene cluster lead to specific syndromes characterized by increased production of fetal hemoglobin in adult life and provide a useful model to delineate novel cis-acting elements involved in the developmental control of hemoglobin switching. A hypothesis accounting for these phenotypic features assumes that silencers located within the Agamma-to delta-gene region are deleted in hereditary persistence of fetal hemoglobin (HPFH) and deltabeta-thalassemias, leading to failure of switching. In the present study, we sought to clarify the in vivo role of two elements, termed Enh and F, located 3' to the Agamma-globin, in silencing the fetal genes. To this end, we generated three transgenic lines using cosmid constructs containing the full length of the globin locus control region (LCR) linked to the 3.3-kb Agamma-gene lacking both the Enh and F elements. The Enh/F deletion resulted in high levels of Agamma-globin gene expression in adult mice in all single copy lines, whereas, the LCR-Agamma single copy lines which retain the Enh and F elements exhibited complete normal switching of the fetal Agamma-gene. Our study documents directly for the first time the in vivo role of these two gene-proximal negative regulatory elements in silencing the fetal globin gene in the perinatal period, and thus these data may permit their eventual exploitation in therapeutic approaches for thalassemias.

Ubiquitous expression of the rtTA2S-M2 inducible system in transgenic mice driven by the human hnRNPA2B1/CBX3 CpG island

Background

A sensitive, ubiquitously expressed tetracycline inducible system would be a valuable tool in mouse transgenesis. However, this has been difficult to obtain due to position effects observed at different chromosomal sites of transgene integration, which negatively affect expression in many tissues. The aim of this study was to test the utility of a mammalian methylation-free CpG island to drive ubiquitous expression of the sensitive doxycycline (Dox) inducible rtTA2S-M2 Tet-transactivator in transgenic mice.

Results

An 8 kb genomic fragment from the methylation-free CpG island of the human hnRNPA2B1-CBX3 housekeeping gene locus was tested. In a number of transgenic mouse lines obtained, rtTA2S-M2 expression was detected in many tissues examined. Characterisation of the highest expressing rtTA2S-M2 transgenic mouse line demonstrated Dox-inducible GFP transgene expression in many tissues. Using this line we also show highly sensitive quantitative induction with low doses of Dox of an assayable plasma protein transgene under the control of a Tet Responsive Element (TRE). The utility of this rtTA2S-M2 line for inducible expression in mouse embryos was also demonstrated using a GATA-6 Tet-inducible transgene to show specific phenotypes in the embryonic lung, as well as broader effects resulting from the inducible widespread overexpression of the transgene.

Conclusion

The ubiquitously expressing rtTA2S-M2 transgenic mouse line described here provides a very useful tool for studying the effects of the widespread, inducible overexpression of genes during embryonic development and in adult mice.

Mucormycosis in hematologic patients

BACKGROUND AND OBJECTIVES

To evaluate the clinical characteristics of patients affected by hematologic malignancies who developed mucormycosis and to ascertain the factors which influenced the outcome following mycotic infection.

DESIGN AND METHODS

This was a retrospective study conducted over a 15-year period (1987-2001). The study included 59 patients with hematologic malignancies with a proven or probable mucormycosis admitted in 18 Hematology Divisions in tertiary care or university hospitals.

RESULTS

The most frequent sites of infection were lung (64%) and orbito-sinus-facial (24%); cerebral involvement observed in 19% of cases was always associated with other sites of infection. Antifungal treatment was empirically administered in 49 patients (83%); 7 patients underwent radical surgical debridement (12%). Therapy was successful for only 18 patients (37%). Forty-seven patients died within 3 months of the diagnosis of fungal infection: the cause of death was mucormycosis in 41 patients (87%) and progression of hematologic disease in 6 patients (13%). At univariate analysis, the factors that correlated with a positive outcome from infection were the following: male sex, amphotericin B treatment, neutrophil recovery from post-chemotherapy aplasia. At multivariate analysis, the only factor that significantly correlated with recovery from infection was the liposomal amphotericin B treatment.

INTERPRETATION AND CONCLUSIONS

Mucormycosis is a rare filamentous fungal infection that occurs most frequently in neutropenic patients with acute leukemia. It does not seem to have increased in recent years. Although a reduction of mortality has been observed recently, the mortality rate still remains high. Extensive and aggressive diagnostic and therapeutic procedures are essential in order to improve the prognosis in these patients.

Multiple interactions between regulatory regions are required to stabilize an active chromatin hub

The human beta-globin locus control region (LCR) is required for the maintenance of an open chromatin configuration of the locus. It interacts with the genes and the hypersensitive regions flanking the locus to form an active chromatin hub (ACH) transcribing the genes. Proper developmental control of globin genes is largely determined by gene proximal regulatory sequences. Here, we provide the first functional evidence of the role of the most active sites of the LCR and the promoter of the beta-globin gene in the maintenance of the ACH. When the human beta-globin gene promoter is deleted in the context of a full LCR, the ACH is maintained with the beta-globin gene remaining in proximity. Additional deletion of hypersensitive site HS3 or HS2 of the LCR shows that HS3, but not HS2, in combination with the beta-globin promoter is crucial for the maintenance of the ACH at the definitive stage. We conclude that multiple interactions between the LCR and the beta-globin gene are required to maintain the appropriate spatial configuration in vivo.

Persistent gamma-globin expression in adult transgenic mice is mediated by HPFH-2, HPFH-3, and HPFH-6 breakpoint sequences

Deletions at the 3' end of the human beta-globin locus are associated with the hereditary persistence of fetal hemoglobin (HPFH) in adults, potentially through the juxtaposition of enhancer elements in the vicinity of the fetal gamma-globin genes. We have tested how sequences at the HPFH-2, HPFH-3, and HPFH-6 breakpoints, which act as enhancers in vitro, affect the silencing of a locus control region A gamma (LCRA gamma) transgene in the adult stage of mice. We found persistent A gamma expression in the adult blood of most of the multicopy HPFH-2, HPFH-3, or HPFH-6 lines, in contrast to the control LCRA gamma lines which were silenced. Cre-mediated generation of single copy lines showed persistent gamma gene expression maintained in some of the HPFH-2 and HPFH-6 lines, but not in any of the HPFH-3 or LCRA gamma lines. In the HPFH-2 and HPFH-6 lines, persistent gamma gene expression correlated with euchromatic transgene integrations. Thus, our observations provide support for a model whereby HPFH conditions arise from the juxtaposition of enhancers as well as permissive chromatin subdomains in the vicinity of the gamma-globin genes.

No justification of routine screening for 22q11 deletions in patients with overt cleft palate

The velo-cardio-facial syndrome (VCFS), caused by a submicroscopic deletion of chromosome 22q11, is the most common syndrome that has palatal anomalies as a major feature. A possible strategy for early detection of VCFS is routine screening for 22q11 deletions in all infants with cleft palate (CP). The purpose of this study was to evaluate whether this strategy is preferable to testing on clinical suspicion. At the Nijmegen Cleft Palate Craniofacial Center, 58 new patients with overt CP were routinely tested, using fluorescence in situ hybridization (FISH), for a 22q11 deletion. One deletion was identified in a newborn girl with an overt CP who was clinically not suspected of having VCFS. Based on this study (n = 45) and the literature (n = 54), the prevalence of 22q11 deletions among children with CP, but without any other symptoms of VCFS, is estimated to be one in 99. We take the view that this figure is rather low and that early discovery will rarely have significant clinical or genetic consequences. Because CP patients remain under medical attention, almost all of the infants with isolated CP and VCFS will be recognized as having the syndrome at a later age when additional features have developed. Therefore, we conclude that routine FISH testing for 22q11 deletions in infants with overt CP is not indicated, provided clinical follow-up is guaranteed.

Functional and comparative analysis of globin loci in pufferfish and humans

To further our understanding of the regulation of vertebrate globin loci, we have isolated cosmids containing alpha- and beta-globin genes from the pufferfish Fugu rubripes. By DNA fluorescence in situ hybridization (FISH) analysis, we show that Fugu contains 2 distinct hemoglobin loci situated on separate chromosomes. One locus contains only alpha-globin genes (alpha-locus), whereas the other also contains a beta-globin gene (alpha beta-locus). This is the first poikilothermic species analyzed in which the physical linkage of the alpha- and beta-globin genes has been uncoupled, supporting a model in which the separation of the alpha- and beta-globin loci has occurred through duplication of a locus containing both types of genes. Surveys for transcription factor binding sites and DNaseI hypersensitive site mapping of the Fugu alpha beta-locus suggest that a strong distal locus control region regulating the activity of the globin genes, as found in mammalian beta-globin clusters, may not be present in the Fugu alpha beta-locus. Searching the human and mouse genome databases with the genes surrounding the pufferfish hemoglobin loci reveals that homologues of some of these genes are proximal to cytoglobin, a recently described novel member of the globin family. This provides evidence that duplication of the globin loci has occurred several times during evolution, resulting in the 5 human globin loci known to date, each encoding proteins with specific functions in specific cell types.

Velo-cardio-facial syndrome: clinical report of a 70-year-old woman

A clinical report is presented of a 70-year-old female in whom, after more than 40 years residential psychiatric care, the diagnosis of velo-cardio-facial syndrome (VCFS) was ultimately established; the patient has a 46, XX.ish del (22)(q11.2q11.2)(D(22)S(75)-) karyotype. It is advocated that a rather specific psychopathological profile is present in patients with VCFS, for which the term psychopathological phenotype is introduced, that should include data from genetics, neuropathology, development, psychology, and psychiatry.

In vivo transposition of Minos, a Drosophila mobile element, in mammalian tissues

Transposable elements have been used widely in the past 20 years for gene transfer and insertional mutagenesis in Drosophila. Transposon-based technology for gene manipulation and genomic analysis currently is being adopted for vertebrates. We tested the ability of Minos, a DNA transposon from Drosophila hydei, to transpose in mouse tissues. Two transgenic mouse lines were crossed, one expressing Minos transposase in lymphocytes under the control of the CD2 promoter/locus control region and another carrying a nonautonomous Minos transposon. Only mice containing both transgenes show excision of the transposon and transposition into new chromosomal sites in thymus and spleen cells. In addition, expression of Minos transposase in embryonic fibroblast cell lines derived from a transposon-carrying transgenic mouse resulted in excision of the transposon. These results are a first step toward a reversible insertional mutagenesis system in the mouse, opening the way to develop powerful technologies for functional genomic analysis in mammals.

Deletions at chromosome regions 7q11.23 and 7q36 in a patient with Williams syndrome

We report on a patient with Williams syndrome and a complex de novo chromosome rearrangement, including microdeletions at 7q11.23 and 7q36 and additional chromosomal material at 7q36. The nature of this additional material was elucidated by spectral karyotyping and first assigned to chromosome 22. Subsequent fluorescence in situ hybridization (FISH) experiments showed that it consisted of satellite material only. Refinement of the 7q36 breakpoint was performed with several FISH probes, showing a deletion distal to the triphalangeal thumb (TPT) region. The phenotype of the patient principally results from the microdeletion of the 7q11.23; the small deletion at 7qter and the extra satellite material may not be of clinical significance.

Baculovirus infection of nondividing mammalian cells: mechanisms of entry and nuclear transport of capsids

We have studied the infection pathway of Autographa californica multinuclear polyhedrosis virus (baculovirus) in mammalian cells. By titration with a baculovirus containing a green fluorescent protein cassette, we found that several, but not all, mammalian cell types can be infected efficiently. In contrast to previous suggestions, our data show that the asialoglycoprotein receptor is not required for efficient infection. We demonstrate for the first time that this baculovirus can infect nondividing mammalian cells, which implies that the baculovirus is able to transport its genome across the nuclear membrane of mammalian cells. Our data further show that the virus enters via endocytosis, followed by an acid-induced fusion event, which releases the nucleocapsid into the cytoplasm. Cytochalasin D strongly reduces the infection efficiency but not the delivery of nucleocapsids to the cytoplasm, suggesting involvement of actin filaments in cytoplasmic transport of the capsids. Electron microscopic analysis shows the cigar-shaped nucleocapsids located at nuclear pores of nondividing cells. Under these conditions, we observed the viral genome, major capsid protein, and electron-dense capsids inside the nucleus. This suggests that the nucleocapsid is transported through the nuclear pore. This mode of transport seems different from viruses with large spherical capsids, such as herpes simplex virus and adenovirus, which are disassembled before nuclear transport of the genome. The implications for the application of baculovirus or its capsid proteins in gene therapy are discussed.

Characterisation of transcriptionally active and inactive chromatin domains in neurons

The tandemly organised ribosomal DNA (rDNA) repeats are transcribed by a dedicated RNA polymerase in a specialised nuclear compartment, the nucleolus. There appears to be an intimate link between the maintenance of nucleolar structure and the presence of heterochromatic chromatin domains. This is particularly evident in many large neurons, where a single nucleolus is present, which is separated from the remainder of the nucleus by a characteristic shell of heterochromatin. Using a combined fluorescence in situ hybridisation and immunocytochemistry approach, we have analysed the molecular composition of this highly organised neuronal chromatin, to investigate its functional significance. We find that clusters of inactive, methylated rDNA repeats are present inside large neuronal nucleoli, which are often attached to the shell of heterochromatic DNA. Surprisingly, the methylated DNA-binding protein MeCP2, which is abundantly present in the centromeric and perinucleolar heterochromatin, does not associate significantly with the methylated rDNA repeats, whereas histone H1 does overlap partially with these clusters. Histone H1 also defines other, centromere-associated chromatin subdomains, together with the mammalian Polycomb group factor Eed. These data indicate that neuronal, perinucleolar heterochromatin consists of several classes of inactive DNA, that are linked to a fraction of the inactive rDNA repeats. These distinct chromatin domains may serve to regulate RNA transcription and processing efficiently and to protect rDNA repeats against unwanted silencing and/or homologous recombination events.

An intrinsic but cell-nonautonomous defect in GATA-1-overexpressing mouse erythroid cells

GATA-1 is a tissue-specific transcription factor that is essential for the production of red blood cells. Here we show that overexpression of GATA-1 in erythroid cells inhibits their differentiation, leading to a lethal anaemia. Using chromosome-X-inactivation of a GATA-1 transgene and chimaeric animals, we show that this defect is intrinsic to erythroid cells, but nevertheless cell nonautonomous. Usually, cell nonautonomy is thought to reflect aberrant gene function in cells other than those that exhibit the phenotype. On the basis of our data, we propose an alternative mechanism in which a signal originating from wild-type erythroid cells restores normal differentiation to cells overexpressing GATA-1 in vivo. The existence of such a signalling mechanism indicates that previous interpretations of cell-nonautonomous defects may be erroneous in some cases and may in fact assign gene function to incorrect cell types.

Surface expression of glycoprotein Ibα is dependent on glycoprotein Ibβ: evidence from a novel mutation causing Bernard-Soulier syndrome

Bernard-Soulier syndrome is a rare bleeding disorder caused by a quantitative or qualitative defect in the platelet glycoprotein (GP) Ib-IX-V complex. The complex, which serves as a platelet receptor for von Willebrand factor, is composed of 4 subunits: GPIb, GPIbβ, GPIX, and GPV. We here describe the molecular basis of a novel form of Bernard-Soulier syndrome in a patient in whom the components of the GPIb-IX-V complex were undetectable on the platelet surface. Although confocal imaging confirmed that GPIb was not present on the platelet surface, GPIb was readily detectable in the patient's platelets. Moreover, immunoprecipitation of plasma with specific monoclonal antibodies identified circulating, soluble GPIb. DNA-sequence analysis revealed normal sequences for GPIb and GPIX. There was a G to A substitution at position 159 of the gene encoding GPIbβ, resulting in a premature termination of translation at amino acid 21. Studies of transient coexpression of this mutant, W21stop-GPIbβ, together with wild-type GPIb and GPIX, demonstrated a failure of GPIX expression on the surface of HEK 293T cells. Similar results were obtained with Chinese hamster ovary  IX cells, a stable cell line expressing GPIb that retains the capacity to re-express GPIX. Thus, we found that GPIbβ affects the surface expression of the GPIb-IX complex by failing to support the insertion of GPIb and GPIX into the platelet membrane.

Surface expression of glycoprotein Ibα is dependent on glycoprotein Ibβ: evidence from a novel mutation causing Bernard-Soulier syndrome

Bernard-Soulier syndrome is a rare bleeding disorder caused by a quantitative or qualitative defect in the platelet glycoprotein (GP) Ib-IX-V complex. The complex, which serves as a platelet receptor for von Willebrand factor, is composed of 4 subunits: GPIb, GPIbβ, GPIX, and GPV. We here describe the molecular basis of a novel form of Bernard-Soulier syndrome in a patient in whom the components of the GPIb-IX-V complex were undetectable on the platelet surface. Although confocal imaging confirmed that GPIb was not present on the platelet surface, GPIb was readily detectable in the patient's platelets. Moreover, immunoprecipitation of plasma with specific monoclonal antibodies identified circulating, soluble GPIb. DNA-sequence analysis revealed normal sequences for GPIb and GPIX. There was a G to A substitution at position 159 of the gene encoding GPIbβ, resulting in a premature termination of translation at amino acid 21. Studies of transient coexpression of this mutant, W21stop-GPIbβ, together with wild-type GPIb and GPIX, demonstrated a failure of GPIX expression on the surface of HEK 293T cells. Similar results were obtained with Chinese hamster ovary  IX cells, a stable cell line expressing GPIb that retains the capacity to re-express GPIX. Thus, we found that GPIbβ affects the surface expression of the GPIb-IX complex by failing to support the insertion of GPIb and GPIX into the platelet membrane.

The transcription factor GATA6 is essential for early extraembryonic development

The gene coding for the murine transcription factor GATA6 was inactivated by insertion of a beta-galactosidase marker gene. The analysis of heterozygote GATA6/lacZ mice shows two inductions of GATA6 expression early in development. It is first expressed at the blastocyst stage in part of the inner mass and in the trophectoderm. The second wave of expression is in parietal endoderm (Reichert's membrane) and the mesoderm and endoderm that form the heart and gut. Inactivation leads to a lethality shortly after implantation (5.5 days postcoitum). Chimeric experiments show this to be caused by an indirect effect on the epiblast due to a defect in an extraembryonic tissue.

Murine MHC class II locus control region drives expression of human beta-glucocerebrosidase in antigen presenting cells of transgenic mice

Gaucher disease is the most prevalent lysosomal storage disorder in humans, resulting from an inherited deficiency of the enzyme glucocerebrosidase. Although the enzyme is ubiquitously expressed, cells of the reticuloendothelial system are particularly affected since they accumulate the undigested glucosylceramide substrate through their role in scavenging and breaking down cell debris. Gaucher disease is an attractive target for somatic gene therapy. To test the ability to express the enzyme in the affected cell types we have generated transgenic mice expressing human glucocerebrosidase under the control of the murine major histocompatibility complex (MHC) class II Ead locus control region (LCR). The four transgenic lines express the human enzyme in a copy number-dependent manner, independent of the integration site of the transgene. Over-expression of the human enzyme in mice did not result in any abnormal phenotype or pathology during the period of observation (> 2 years). The enzyme is expressed in B cells, monocytes, dendritic cells, thymic epithelial cells, and macrophages in various tissues: the peritoneal cavity, bone marrow, spleen, kidney, gastrointestinal tract, Kupffer cells in the liver and alveolar macrophages in lungs. Expression in the brain was limited to perivascular macrophages and was not seen in microglial cells. Therefore, the MHC class II LCR could potentially be of use in somatic gene therapy for type 1 Gaucher disease.

The transcription factor GATA6 is essential for early extraembryonic development

The gene coding for the murine transcription factor GATA6 was inactivated by insertion of a beta-galactosidase marker gene. The analysis of heterozygote GATA6/lacZ mice shows two inductions of GATA6 expression early in development. It is first expressed at the blastocyst stage in part of the inner cell mass and in the trophectoderm. The second wave of expression is in parietal endoderm (Reichert's membrane) and the mesoderm and endoderm that form the heart and gut. Inactivation leads to a lethality shortly after implantation (5.5 days postcoitum). Chimeric experiments show this to be caused by an indirect effect on the epiblast due to a defect in an extraembryonic tissue.

Deletion of a region that is a candidate for the difference between the deletion forms of hereditary persistence of fetal hemoglobin and deltabeta-thalassemia affects beta- but not gamma-globin gene expression

The analysis of a number of cases of beta-globin thalassemia and hereditary persistence of fetal hemoglobin (HPFH) due to large deletions in the beta-globin locus has led to the identification of several DNA elements that have been implicated in the switch from human fetal gamma- to adult beta-globin gene expression. We have tested this hypothesis for an element that covers the minimal distance between the thalassemia and HPFH deletions and is thought to be responsible for the difference between a deletion HPFH and deltabeta-thalassemia, located 5' of the delta-globin gene. This element has been deleted from a yeast artificial chromosome (YAC) containing the complete human beta-globin locus. Analysis of this modified YAC in transgenic mice shows that early embryonic expression is unaffected, but in the fetal liver it is subject to position effects. In addition, the efficiency of transcription of the beta-globin gene is decreased, but the developmental silencing of the gamma-globin genes is unaffected by the deletion. These results show that the deleted element is involved in the activation of the beta-globin gene perhaps through the loss of a structural function required for gene activation by long-range interactions.

The murine CYLN2 gene: genomic organization, chromosome localization, and comparison to the human gene that is located within the 7q11.23 Williams syndrome critical region

Cytoplasmic linker proteins (CLIPs) have been proposed to mediate the interaction between specific membranous organelles and microtubules. We have recently characterized a novel member of this family, called CLIP-115. This protein is most abundantly expressed in the brain and was found to associate both with microtubules and with an organelle called the dendritic lamellar body. CLIP-115 is highly homologous to CLIP-170, or restin, which is a protein involved in the binding of endosomes to microtubules. Using the rat cDNA as a probe we have isolated overlapping cosmids containing the complete murine and part of the human CYLN2 (cytoplasmic linker-2) genes, which encode CLIP-115. The murine gene spans 60 kb and consists of 17 exons, and its promoter is embedded in a CpG island. Murine CYLN2 maps to the telomeric end of mouse chromosome 5. The human CYLN2 gene is localized to a syntenic region on chromosome 7q11.23, which is commonly deleted in Williams syndrome. It spans at least 140 kb at the 3' end of the deletion. Human CYLN2 is very likely identical to the previously characterized, incomplete WSCR4 and WSCR3 transcription units.

Intergenic transcription and transinduction of the human beta-globin locus

We have identified novel nuclear transcripts in the human beta-globin locus using nuclear run-on analysis in erythroid cell lines and in situ hybridization analysis of erythroid tissue. These transcripts extend across the LCR and intergenic regions but are undetectable in nonerythroid cells. Surprisingly, transient transfection of a beta-globin gene (epsilon, gamma, or beta) induces transcription of the LCR and intergenic regions from the chromosomal beta-globin locus in nonerythroid cell lines. The beta-globin genes themselves, however, remain transcriptionally silent. Induction is dependent on transcription of the globin gene in the transfected plasmid but does not require protein expression. Using in situ hybridization analysis, we show that the plasmid colocalizes with the endogenous beta-globin locus providing insight into the mechanism of transinduction.

The murine homologue of HIRA, a DiGeorge syndrome candidate gene, is expressed in embryonic structures affected in human CATCH22 patients

A wide spectrum of birth defects is caused by deletions of the DiGeorge syndrome chromosomal region at 22q11. Characteristic features include cranio-facial, cardiac and thymic malformations, which are thought to arise form disturbances in the interactions between hindbrain neural crest cells and the endoderm of the pharyngeal pouches. Several genes have been identified in the shortest region of deletion overlap at 22q11, but nothing is known about the expression of these genes in mammalian embryos. We report here the isolation of several murine embryonic cDNAs of the DiGeorge syndrome candidate gene HIRA. We identified several alternatively spliced transcripts. Sequence analysis reveals that Hira bears homology to the p60 subunit of the human Chromatin Assembly Factor I and yeast hir1p and Hir2p, suggesting that Hira might have some role in chromatin assembly and/or histone regulation. Whole mount in situ hybridization of mouse embryos at various stages of development show that Hira is ubiquitously expressed. However, higher levels of transcripts are detected in the cranial neural folds, frontonasal mass, first two pharyngeal arches, circumpharyngeal neural crest and the limb buds. Since many of the structures affected in DiGeorge syndrome derive from these Hira expressing cell populations we propose that haploinsufficiency of HIRA contributes to at least some of the features of the DiGeorge phenotype.

Heterochromatin effects on the frequency and duration of LCR-mediated gene transcription

Locus control regions (LCRs) are responsible for initiating and maintaining a stable tissue-specific open chromatin structure of a locus. In transgenic mice, LCRs confer high level expression on linked genes independent of position in the mouse genome. Here we show that an incomplete LCR loses this property when integrated into heterochromatic regions. Two disruption mechanisms were observed. One is classical position-effect variegation, resulting in continuous transcription in a clonal subpopulation of cells. The other is a novel mechanism resulting in intermittent gene transcription in all cells. We conclude that only a complete LCR fully overcomes heterochromatin silencing and that it controls the level of transcription by ensuring activity in all cells at all times rather than directly controlling the rate of transcription.