Efficient 3'-end formation of human beta-globin mRNA in vivo requires sequences within the last intron but occurs independently of the splicing reaction

Glutamate-cysteine ligase modifier subunit: mouse Gclm gene structure and regulation by agents that cause oxidative stress

Glutamate-cysteine ligase is a heterodimer comprising a modifier (GCLM) and a catalytic (GCLC) subunit. In mouse Hepa-1c1c7 hepatoma cell cultures, we found that tert-butylhydroquinone (tBHQ; 50 microM) induces the GCLM and GCLC mRNAs approximately 10- and approximately 2-fold, respectively, and that these increases primarily reflect de novo transcription. We determined that the mouse Gclm gene has seven exons, spanning 22.3 kb; all exons, intron-exon junctions, and 4.7 kb of 5'-flanking region were sequenced. By RNase protection analysis, we identified two major and several minor transcription start-site clusters over a 300-bp region. The Gclm 5'-flanking region is GC-rich and lacks a canonical TATA box. Transient and stable transfection studies, using luciferase reporter constructs containing incremental Gclm 5'-flanking deletions (4.7-0.5 kb), showed high basal activity but only modest ( approximately 2-fold) inducibility by tBHQ. The only candidate motif for oxidative stress regulation (in the 4.7-kb region we sequenced) is a putative inverted electrophile response element (EPRE) 9 bp upstream from the 5'-most transcription start-site. Site-directed mutagenesis of this -9 EPRE demonstrated minimal (30-40%) decreases in tBHQ induction and no effect on basal activity-suggesting that this EPRE might be necessary but not sufficient. The nuclear erythroid factor-2 (NEF2)-related factor-2 (NRF2) is known to transactivate via EPRE motifs. In the presence of co-transfected NRF cDNA expression vector, however, no increase in Gclm promoter activity was observed. Thus, the endogenous Gclm gene shows robust transcriptional activation by tBHQ in the intact Hepa-1 cell, but reporter constructs containing up to 4.7 kb of promoter (having only the one EPRE at -9) demonstrate a disappointing response, indicating that the major tBHQ-responsive regulatory element of the mouse Gclm gene must exist either further 5'- or 3'-ward of the 4.7-kb region studied.

A high-capacity, capsid-modified hybrid adenovirus/adeno-associated virus vector for stable transduction of human hematopoietic cells

To achieve stable gene transfer into human hematopoietic cells, we constructed a new vector, DeltaAd5/35.AAV. This vector has a chimeric capsid containing adenovirus type 35 fibers, which conferred efficient infection of human hematopoietic cells. The DeltaAd5/35.AAV vector genome is deleted for all viral genes, allowing for infection without virus-associated toxicity. To generate high-capacity DeltaAd5/35.AAV vectors, we employed a new technique based on recombination between two first-generation adenovirus vectors. The resultant vector genome contained an 11.6-kb expression cassette including the human gamma-globin gene and the HS2 and HS3 elements of the beta-globin locus control region. The expression cassette was flanked by adeno-associated virus (AAV) inverted terminal repeats (ITRs). Infection with DeltaAd5/35.AAV allowed for stable transgene expression in a hematopoietic cell line after integration into the host genome through the AAV ITR(s). This new vector exhibits advantages over existing integrating vectors, including an increased insert capacity and tropism for hematopoietic cells. It has the potential for stable ex vivo transduction of hematopoietic stem cells in order to treat sickle cell disease.

Dietary regulation of expression of glucose-6-phosphate dehydrogenase

The family of enzymes involved in lipogenesis is a model system for understanding how a cell adapts to dietary energy in the form of carbohydrate versus energy in the form of triacylglycerol. Glucose-6-phosphate dehydrogenase (G6PD) is unique in this group of enzymes in that it participates in multiple metabolic pathways: reductive biosynthesis, including lipogenesis; protection from oxidative stress; and cellular growth. G6PD activity is enhanced by dietary carbohydrates and is inhibited by dietary polyunsaturated fats. These changes in G6PD activity are a consequence of changes in the expression of the G6PD gene. Nutrients can regulate the expression of genes at both transcriptional and posttranscriptional steps. Most lipogenic enzymes undergo large changes in the rate of gene transcription in response to dietary changes; however, G6PD is regulated at a step subsequent to transcription. This step is involved in the rate of synthesis of the mature mRNA in the nucleus, specifically regulation of the efficiency of splicing of the nascent G6PD transcript. Understanding the mechanisms by which nutrients alter nuclear posttranscriptional events will help uncover new information on the breadth of mechanisms involved in gene regulation.

High-level erythroid-specific gene expression in primary human and murine hematopoietic cells with self-inactivating lentiviral vectors

Use of oncoretroviral vectors in gene therapy for hemoglobinopathies has been impeded by low titer vectors, genetic instability, and poor expression. Fifteen self- inactivating (SIN) lentiviral vectors using 4 erythroid promoters in combination with 4 erythroid enhancers with or without the woodchuck hepatitis virus postregulatory element (WPRE) were generated using the enhanced green fluorescent protein as a reporter gene. Vectors with high erythroid-specific expression in cell lines were tested in primary human CD34(+) cells and in vivo in the murine bone marrow (BM) transplantation model. Vectors containing the ankyrin-1 promoter showed high-level expression and stable proviral transmission. Two vectors containing the ankyrin-1 promoter and 2 erythroid enhancers (HS-40 plus GATA-1 or HS-40 plus 5-aminolevulinate synthase intron 8 [I8] enhancers) and WPRE expressed at levels higher than the HS2/beta-promoter vector in bulk unilineage erythroid cultures and individual erythroid blast-forming units derived from human BM CD34(+) cells. Sca1(+)/lineage(-) Ly5.1 mouse hematopoietic cells, transduced with these 2 ankyrin-1 promoter vectors, were injected into lethally irradiated Ly5.2 recipients. Eleven weeks after transplantation, high-level expression was seen from both vectors in blood (63%-89% of red blood cells) and erythroid cells in BM (70%-86% engraftment), compared with negligible expression in myeloid and lymphoid lineages in blood, BM, spleen, and thymus (0%-4%). The I8/HS-40-containing vector encoding a hybrid human beta/gamma-globin gene led to 43% to 113% human gamma-globin expression/copy of the mouse alpha-globin gene. Thus, modular use of erythroid-specific enhancers/promoters and WPRE in SIN-lentiviral vectors led to identification of high-titer, stably transmitted vectors with high-level erythroid-specific expression for gene therapy of red cell diseases.

A novel delta beta fusion gene expresses hemoglobin A (HbA) not Hb Lepore: Senegalese delta(0)beta(+) thalassemia

This study identified and characterized a novel delta beta fusion gene in which the delta-globin gene promoter is linked to intact beta-globin coding sequences in a Senegalese family. It results from a 7.4-kb deletion that removes the delta-globin coding sequences, the delta beta intergenic region as well as the beta-globin gene promoter and causes delta(0)beta(+) thalassemia with hemoglobin A expressed at the 11% to 15% range. The phenotype of this naturally occurring delta beta hybrid gene not only clarifies, in an in vivo context, the respective strength of delta- and beta-globin gene promoters, but also emphasizes the importance of beta-globin intragenic sequences in the expression of beta-globin chains. (Blood. 2001;98:1261-1263)

Quality control of gene expression in the nucleus

Proteins are responsible for most cellular and extra-cellular functions. If altered, proteins can loose their normal activity and/or gain new properties. Either way the consequences may be deleterious for the cell and lead to disease at the organism level. Not surprisingly, eukaryotes have evolved mechanisms to recognize abnormal messenger RNAs and prevent them from producing faulty proteins. Protein-encoding genes are transcribed in the nucleus by RNA polymerase II as precursor RNAs that undergo extensive processing before being translocated to the cytoplasm for translation by the ribosomes. This spatial and temporal separation between RNA and protein synthesis offers an immense opportunity for control and regulation. Here we review recent studies that are beginning to unravel how the coupling between transcription, processing and transport of mRNAs contributes to control the quality of gene expression in the nucleus.

Position and sequence requirements for poly(A) length regulation by the poly(A) limiting element

The poly(A)-limiting element (PLE) is a cis-acting sequence that acts to limit poly(A) tail length on pre-mRNA to <20 nt. Functional PLEs are present in a number of genes, underscoring the generality of this control mechanism. The current study sought to define further the position requirements for poly(A) length regulation and the core sequence that comprises a PLE. Increasing the spacing between the PLE and the upstream 3' splice site or between the PLE and the downstream AAUAAA had no effect on poly(A) length control. However, moving the PLE from the terminal exon to either an upstream exon or intron eliminated poly(A) length control. Poly(A) length control was further evaluated using a battery of constructs in which the PLE was maintained in the terminal exon, but where upstream introns were either deleted, modified, or replaced with a polypyrimidine tract. Poly(A) length control was retained in all cases, indicating that the key feature is the presence of the PLE in the terminal exon. A battery of mutations demonstrated the importance of the 5' pyrimidine-rich portion of the element. Finally, UV crosslinking experiments identified an approximately 62-kDa protein in Hela nuclear extract that binds to a wild-type 23-nt PLE RNA oligonucleotides but not to a mutated nonfunctional form of the element.

Regulation of the processing of glucose-6-phosphate dehydrogenase mRNA by nutritional status

Expression of glucose-6-phosphate dehydrogenase (G6PD) gene during starvation and refeeding is regulated by a posttranscriptional mechanism occurring in the nucleus. The amount of G6PD mRNA at different stages of processing was measured in RNA isolated from the nuclear matrix fraction of mouse liver. This nuclear fraction contains nascent transcripts and RNA undergoing processing. Using a ribonuclease protection assay with probes that cross an exon-intron boundary in the G6PD transcript, the abundance of mRNAs that contain the intron (unspliced) and without the intron (spliced) was measured. Refeeding resulted in 6- and 8-fold increases in abundance of G6PD unspliced and spliced RNA, respectively, in the nuclear matrix fraction. However, the amount of G6PD unspliced RNA was at most 15% of the amount of spliced RNA. During refeeding, G6PD spliced RNA accumulated at a rate significantly greater than unspliced RNA. Further, the amount of partially spliced RNA exceeded the amount of unspliced RNA indicating that the enhanced accumulation occurs early in processing. Starvation and refeeding did not regulate either the rate of polyadenylation or the length of the poly(A) tail. Thus, the G6PD gene is regulated during refeeding by enhanced efficiency of splicing of its RNA, and this processing protects the mRNA from decay, a novel mechanism for nutritional regulation of gene expression.

Split gene origin and periodic introns

The introns-early view has been challenged for several genes; prominent instances are triose phosphate isomerase (TPI), aldolase, pyruvate kinase (PK), alcohol dehydrogenase (ADH), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and myosin heavy chain. While some of their introns appear to be phylogenetically ancient and/or to delineate exons corresponding to protein modules, a considerable number seemingly do not. But it is argued here that many of these anomalous introns are periodic, that is, relics of internal sequence repetitions within the ancestral gene. Some of these periodic-intron patterns are shared between related genes, as in the alphabeta -barrels of TPI, aldolase and PK, or the Rossmann nucleotide-binding domain common to PK, ADH and GAPDH. This is further evidence for the ancestral status of these introns. The myosin heavy chain C-terminal rod region is paradoxical in that its sequence is clearly periodic but its intron placements are not; however, they exhibit a remarkable coherence of intron translational phases, suggesting that these introns may also have originally had a periodic arrangement now obscured by intron slipping.

Identification of a novel gene encoding a PrP-like protein expressed as chimeric transcripts fused to PrP exon 1/2 in ataxic mouse line with a disrupted PrP gene

1. Mouse lines lacking prion protein (PrP(C)) have a puzzling phenotypic discrepancy. Some, but not all, developed late-onset ataxia due to Purkinje cell degeneration. 2. Here, we identified aberrant mRNA species in the brain of Ngsk Prnp0/0 ataxic, but not in nonataxic Zrch Prnp0/0 mouse line. These mRNAs were chimeric between the noncoding exons 1 and 2 of the PrP gene (Prnp) and the novel sequence encoding PrP-like protein (PrPLP), a putative membrane glycoprotein with 23% identity to PrP(C) in the primary amino acid structure. The chimeric mRNAs were generated from the disrupted Prnp locus of Ngsk Prnp0/0 mice lacking a part of the Prnp intron 2 and its splice acceptor signal. 3. In the brain of wild-type and Zrch Prnp0/0 mice, PrPLP mRNA was barely detectable. In contrast, in the brain of Ngsk Prnp0/0 mice, PrP/PrPLP chimeric mRNAs were expressed in neurons, at a particularly high level in hippocampus pyramidal cells and Purkinje cells under the control of the Prnp promoter. 4. In addition to the functional loss of PrP(C), ectopic PrPLP expression from the chimeric mRNAs could also be involved in the Purkinje cell degeneration in Ngsk Prnp0/0 mice.

Length increase of the human alpha -globin 3'-untranslated region disrupts stability of the pre-mRNA but not that of the mature mRNA

Polyadenylation increases the stability of mRNA molecules. By studying the effect of the length of 3'-untranslated region (UTR) on mRNA levels, we have found that alpha-globin pre-mRNA is stabilized by a mechanism that does not modulate the half-life of mature mRNA. The insertion of DNA fragments of various unrelated sequences into the 3'-UTR of the human alpha-globin gene strongly reduces mRNA abundance upon transfection into choriocarcinoma JEG-3 cells. We found an inverse relationship between mRNA levels and the length of the introduced fragments. In fact, mRNA levels as low as 1% were observed after inserting a 477-nucleotide (nt) fragment, whereas inserting a fragment of 86 nt at the same position had no effect on mRNA accumulation. DNA insertion induced no change in transcription rate or in half-life of mature mRNA. Semi-quantitative reverse transcription-polymerase chain reaction revealed that inserting a 477-nt fragment in the 3'-UTR resulted in decreased levels of nuclear pre-mRNA in proportion to that observed for mature mRNA. In contrast, the insertion of the 477-nt exogenous DNA in the last intron had no effect on mRNA levels despite the presence of intronic sequences in the pre-mRNA. This shows that the reduction of pre-mRNA level was not due to the insertion of putative ribonuclease cleavage sites or the insertion of a segment DNA that reduces the elongation efficiency. Taken together, our results strongly support the existence of a pre-mRNA stabilizing mechanism that can be disrupted by increasing the length of the 3'-UTR. The fact that the half-life of mature mRNA is not affected by DNA insertion is compatible with a pre-mRNA-specific stabilizing mechanism that acts specifically before polyadenylation.

Gene regulation and deregulation: a beta globin perspective

The study of the beta globin gene has provided great insights into the mechanisms of gene regulation and expression. In this review, we consider the normal regulation and expression of the beta globin gene and illustrate how the various steps may be affected, providing a basis for understanding the molecular pathophysiology of beta thalassemia. Mutations causing beta thalassemia can be classified as beta0 or B+ according to whether they abolish or reduce the production of beta globin chains. The vast majority of beta thalassemia is caused by point mutations, mostly single base substitutions, within the gene or its immediate flanking sequences. Rarely, beta thalassemia is caused by major deletions of the beta globin cluster. All these mutations behave as alleles of the beta locus but in several families the beta thalassemia phenotype segregates independently of the beta globin complex, and are likely to be caused by mutations in trans-acting regulatory factors.

Locus control region activity by 5′HS3 requires a functional interaction with β-globin gene regulatory elements: expression of novel β/γ-globin hybrid transgenes

The human β-globin locus control region (LCR) contains chromatin opening and transcriptional enhancement activities that are important to include in β-globin gene therapy vectors. We previously used single-copy transgenic mice to map chromatin opening activity to the 5′HS3 LCR element. Here, we test novel hybrid globin genes to identify β-globin gene sequences that functionally interact with 5′HS3. First, we show that an 850-base pair (bp) 5′HS3 element activates high-level β-globin gene expression in fetal livers of 17 of 17 transgenic mice, including 3 single-copy animals, but fails to reproducibly activate Aγ-globin transgenes. To identify the β-globin gene sequences required for LCR activity by 5′HS3, we linked the 815-bp β-globin promoter to Aγ-globin coding sequences (BGT34), together with either the β-globin intron 2 (BGT35), the β-globin 3′ enhancer (BGT54), or both intron 2 and the 3′ enhancer (BGT50). Of these transgenes, only BGT50 reproducibly expresses Aγ-globin RNA (including 7 of 7 single-copy animals, averaging 71% per copy). Modifications to BGT50 show that LCR activity is detected after replacing the β-globin promoter with the 700-bp Aγ-globin promoter, but is abrogated when an AT-rich region is deleted from β-globin intron 2. We conclude that LCR activity by 5′HS3 on globin promoters requires the simultaneous presence of β-globin intron 2 sequences and the 260-bp 3′ β-globin enhancer. The BGT50 construct extends the utility of the 5′HS3 element to include erythroid expression of nonadult β-globin coding sequences in transgenic animals and its ability to express antisickling γ-globin coding sequences at single copy are ideal characteristics for a gene therapy cassette.

Locus control region activity by 5′HS3 requires a functional interaction with β-globin gene regulatory elements: expression of novel β/γ-globin hybrid transgenes

The human β-globin locus control region (LCR) contains chromatin opening and transcriptional enhancement activities that are important to include in β-globin gene therapy vectors. We previously used single-copy transgenic mice to map chromatin opening activity to the 5′HS3 LCR element. Here, we test novel hybrid globin genes to identify β-globin gene sequences that functionally interact with 5′HS3. First, we show that an 850-base pair (bp) 5′HS3 element activates high-level β-globin gene expression in fetal livers of 17 of 17 transgenic mice, including 3 single-copy animals, but fails to reproducibly activate Aγ-globin transgenes. To identify the β-globin gene sequences required for LCR activity by 5′HS3, we linked the 815-bp β-globin promoter to Aγ-globin coding sequences (BGT34), together with either the β-globin intron 2 (BGT35), the β-globin 3′ enhancer (BGT54), or both intron 2 and the 3′ enhancer (BGT50). Of these transgenes, only BGT50 reproducibly expresses Aγ-globin RNA (including 7 of 7 single-copy animals, averaging 71% per copy). Modifications to BGT50 show that LCR activity is detected after replacing the β-globin promoter with the 700-bp Aγ-globin promoter, but is abrogated when an AT-rich region is deleted from β-globin intron 2. We conclude that LCR activity by 5′HS3 on globin promoters requires the simultaneous presence of β-globin intron 2 sequences and the 260-bp 3′ β-globin enhancer. The BGT50 construct extends the utility of the 5′HS3 element to include erythroid expression of nonadult β-globin coding sequences in transgenic animals and its ability to express antisickling γ-globin coding sequences at single copy are ideal characteristics for a gene therapy cassette.

Position-dependent inhibition of the cleavage step of pre-mRNA 3'-end processing by U1 snRNP

The 3' ends of most eukaryotic pre-mRNAs are generated by 3' endonucleolytic cleavage and subsequent polyadenylation. 3'-end formation can be influenced positively or negatively by various factors. In particular, U1 snRNP acts as an inhibitor when bound to a 5' splice site located either upstream of the 3'-end formation signals of bovine papilloma virus (BPV) late transcripts or downstream of the 3'-end processing signals in the 5' LTR of the HIV-1 provirus. Previous work showed that in BPV it is not the first step, 3' cleavage, that is affected by U1 snRNP, but rather the second step, polyadenylation, that is inhibited. Since in HIV-1 the biological requirement is to produce transcripts that read through the 5' LTR cleavage site rather than being cleaved there, this mechanism seemed unlikely to apply. The obvious difference between the two examples was the relative orientation of the 3'-end formation signals and the U1 snRNP-binding site. In vitro assays were therefore used to assess the effect of U1 snRNP bound at various locations relative to a cleavage/polyadenylation site on the 3' cleavage reaction. U1 snRNP was found to inhibit cleavage when bound to a 5' splice site downstream of the cleavage/polyadenylation site, as in the HIV-1 LTR. U1 snRNP binding at this location was shown not to affect the recruitment of multiple cleavage/polyadenylation factors to the cleavage substrate, indicating that inhibition is unlikely to be due to steric hindrance. Interactions between U1A, U1 70K, and poly(A) polymerase, which mediate the effect of U1 snRNP on polyadenylation of other pre-mRNAs, were shown not to be required for cleavage inhibition. Therefore, U1 snRNP bound to a 5' splice site can inhibit cleavage and polyadenylation in two mechanistically different ways depending on whether the 5' splice site is located upstream or downstream of the cleavage site.

Enhanced expression of transgenes from adeno-associated virus vectors with the woodchuck hepatitis virus posttranscriptional regulatory element: implications for gene therapy

The woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) evolved to stimulate the expression of intronless viral messages. To determine whether this ability to enhance expression could be useful in nonviral and heterologous viral gene delivery systems, we analyzed the ability of the WPRE to elevate the expression of a cDNA encoding the green fluorescent protein (GFP) in these contexts. We find that the WPRE can stimulate the expression of GFP when the gene is delivered by transfection or transduction with recombinant adeno-associated virus (AAV). Enhancement occurred both during transient expression and when the gene is stably incorporated into the genome of target cells. This enhancement required that the WPRE be located in cis within the GFP message, and was observed in both transformed cell lines and primary human fibroblasts. These results demonstrate that the WPRE will be an effective tool for increasing the long-term expression of transgenes in gene therapy.

Formation of mRNA 3' ends in eukaryotes: mechanism, regulation, and interrelationships with other steps in mRNA synthesis

Formation of mRNA 3' ends in eukaryotes requires the interaction of transacting factors with cis-acting signal elements on the RNA precursor by two distinct mechanisms, one for the cleavage of most replication-dependent histone transcripts and the other for cleavage and polyadenylation of the majority of eukaryotic mRNAs. Most of the basic factors have now been identified, as well as some of the key protein-protein and RNA-protein interactions. This processing can be regulated by changing the levels or activity of basic factors or by using activators and repressors, many of which are components of the splicing machinery. These regulatory mechanisms act during differentiation, progression through the cell cycle, or viral infections. Recent findings suggest that the association of cleavage/polyadenylation factors with the transcriptional complex via the carboxyl-terminal domain of the RNA polymerase II (Pol II) large subunit is the means by which the cell restricts polyadenylation to Pol II transcripts. The processing of 3' ends is also important for transcription termination downstream of cleavage sites and for assembly of an export-competent mRNA. The progress of the last few years points to a remarkable coordination and cooperativity in the steps leading to the appearance of translatable mRNA in the cytoplasm.

Formation of mRNA 3' ends in eukaryotes: mechanism, regulation, and interrelationships with other steps in mRNA synthesis

Formation of mRNA 3' ends in eukaryotes requires the interaction of transacting factors with cis-acting signal elements on the RNA precursor by two distinct mechanisms, one for the cleavage of most replication-dependent histone transcripts and the other for cleavage and polyadenylation of the majority of eukaryotic mRNAs. Most of the basic factors have now been identified, as well as some of the key protein-protein and RNA-protein interactions. This processing can be regulated by changing the levels or activity of basic factors or by using activators and repressors, many of which are components of the splicing machinery. These regulatory mechanisms act during differentiation, progression through the cell cycle, or viral infections. Recent findings suggest that the association of cleavage/polyadenylation factors with the transcriptional complex via the carboxyl-terminal domain of the RNA polymerase II (Pol II) large subunit is the means by which the cell restricts polyadenylation to Pol II transcripts. The processing of 3' ends is also important for transcription termination downstream of cleavage sites and for assembly of an export-competent mRNA. The progress of the last few years points to a remarkable coordination and cooperativity in the steps leading to the appearance of translatable mRNA in the cytoplasm.

Inefficient processing impairs release of RNA from the site of transcription

We describe here for the first time the site of retention within the nucleus of pre-mRNA processing mutants unable to be exported to the cytoplasm. Fluorescence in situ hybridization was used to detect transcripts from human beta-globin genes that are either normal or defective in splicing or 3' end formation. Nuclear transcripts of both wild-type and mutant RNAs are detected only as intranuclear foci that colocalize with the template gene locus. The kinetics of transcript release from the site of transcription was assessed by treatment of cells with the transcriptional inhibitors actinomycin D, alpha-amanitin and DRB. These drugs induce the rapid disappearance of nuclear foci corresponding to wild-type human beta-globin RNA. In contrast, pre-mRNA mutants defective in either splicing or 3' end formation and which fail to be transported to the cytoplasm, are retained at the site of transcription. Therefore, 3' end processing and splicing appear to be rate limiting for release of mRNA from the site of transcription.

Analysis of gamma-globin expression cassettes in retrovirus vectors

With the goal of optimizing retrovirus vectors for human gamma-globin, we studied the effect of several globin gene expression elements on vector titer, stability, and expression. We found that all combinations tested were genetically stable, but that vectors with therapeutic titers (0.5 to 2 x 10(6) colony-forming units/ml) could be achieved only by either partially or fully deleting the second intron of the Agamma-globin gene. Efficient transfer and high-level expression was achieved only when an optimized beta-globin promoter was linked to an Agamma-globin cassette containing an intact intron 1 and a 714-bp internal deletion of intron 2. When flanked by two copies of the HS-40 enhancer core from the alpha-globin locus, this cassette expressed gamma-globin mRNA at 46 +/- 19% per copy of mouse alpha-globin in the murine erythroleukemia cell line MEL585. Complete deletion of the first or second intron diminished expression to < or = 2.0%, and deletion of the HS-40 enhancer diminished expression to 7 +/- 8%. High-level, uniform expression of gamma-globin protein was confirmed in MEL585 clones (n = 12) transduced with the optimized vector. Efficient but variable expression of the optimized vector was also observed in erythroid progenitor colonies (n = 6) grown from transduced mouse bone marrow. Taken together, these studies demonstrate the role of intronic, promoter, and enhancer sequences on retrovirus vectors for human gamma-globin, and the development of an optimized vector capable of efficient expression in a murine erythroid cell line and primary cultures.

Woodchuck hepatitis virus posttranscriptional regulatory element enhances expression of transgenes delivered by retroviral vectors

The expression of genes delivered by retroviral vectors is often inefficient, a potential obstacle for their widespread use in human gene therapy. Here, we explored the possibility that the posttranscriptional regulatory element of woodchuck hepatitis virus (WPRE) might help resolve this problem. Insertion of the WPRE in the 3' untranslated region of coding sequences carried by either oncoretroviral or lentiviral vectors substantially increased their levels of expression in a transgene-, promoter- and vector-independent manner. The WPRE thus increased either luciferase or green fluorescent protein production five- to eightfold, and effects of a comparable magnitude were observed with either the immediate-early cytomegalovirus or the herpesvirus thymidine kinase promoter and with both human immunodeficiency virus- and murine leukemia virus-based vectors. The WPRE exerted this influence only when placed in the sense orientation, consistent with its predicted posttranscriptional mechanism of action. These results demonstrate that the WPRE significantly improves the performance of retroviral vectors and emphasize that posttranscriptional regulation of gene expression should be taken into account in the design of gene delivery systems.

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.

Terminal exon definition occurs cotranscriptionally and promotes termination of RNA polymerase II

Analysis of nascent transcription from the human epsilon- and beta-globin genes shows that transcriptional termination occurs within 1.5 kb of the poly(A) site and is dependent on the presence of functional poly(A) signals. Even so, transcripts that have not been cleaved at the poly(A) site are detected up to the termination region, suggesting that there is a kinetic lag between transcription over the poly(A) signal and its effect on transcriptional termination. Surprisingly, mutation of the splice acceptor (SA) of the beta-globin IVS2 also abolishes transcriptional termination. Our results emphasize the interconnection of transcription and RNA processing by showing that the enhancement of 3' end processing by the terminal splice acceptor occurs cotranscriptionally.