Hepatic gene transfer of the catalytic subunit of the apolipoprotein B mRNA editing enzyme results in a reduction of plasma LDL levels in normal and watanabe heritable hyperlipidemic rabbits

A sense oligonucleotide to inducible nitric oxide synthase mRNA increases the survival rate of rats in septic shock

Natural antisense transcripts (asRNAs) that do not encode proteins are transcribed from rat, mouse, and human genes, encoding inducible nitric oxide synthase (iNOS), which catalyzes the production of the inflammatory mediator nitric oxide (NO). In septic shock, NO is excessively produced in hepatocytes and macrophages. The iNOS asRNA interacts with and stabilizes iNOS mRNA. We found that single-stranded 'sense' oligonucleotides corresponding to the iNOS mRNA sequence reduced iNOS mRNA levels by interfering with the mRNA-asRNA interactions in rat hepatocytes. The iNOS sense oligonucleotides that were substituted with phosphorothioate bonds and locked nucleic acids efficiently decreased the levels of iNOS mRNA and iNOS protein. In this study, the gene expression patterns in the livers of two endotoxemia model rats with acute liver failure were compared. Next, we optimized the sequence and modification of the iNOS sense oligonucleotides in interleukin 1β-treated rat hepatocytes. When a sense oligonucleotide was simultaneously administered with d-galactosamine and bacterial lipopolysaccharide (LPS) to rats, their survival rate significantly increased compared to the rats administered d-galactosamine and LPS alone. In the livers of the sense oligonucleotide-administered rats, apoptosis in the hepatocytes markedly decreased. These results suggest that natural antisense transcript-targeted regulation technology using iNOS sense oligonucleotides may be used to treat human inflammatory diseases, such as sepsis and septic shock.

Flow-cytometric visualization of C>U mRNA editing reveals the dynamics of the process in live cells

APOBEC1 is the catalytic subunit of the complex that edits ApolipoproteinB (ApoB) mRNA, which specifically deaminates cytidine 6666 to uracil in the human transcript. The editing leads to the generation of a stop codon, resulting in the synthesis of a truncated form of ApoB. We have developed a method to quantitatively assay ApoB RNA editing in live cells by using a double fluorescent mCherry-EGFP chimera containing a ∼300bp fragment encompassing the region of ApoB subject to RNA editing. Coexpression of APOBEC1 together with this chimera causes specific RNA editing of the ApoB fragment. The insertion of a stop codon between the mCherry and EGFP thus induces the loss of EGFP fluorescence. Using this method we analyze the dynamics of APOBEC1-dependent RNA editing under various conditions. Namely we show the interplay of APOBEC1 with known interactors (ACF, hnRNP-C1, GRY-RBP) in cells that are RNA editing-proficient (HuH-7) or -deficient (HEK-293T), and the effects of restricted cellular localization of APOBEC1 on the efficiency of the editing. Furthermore, our approach is effective in assaying the induction of RNA editing in Caco-2, a cellular model physiologically capable of ApoB RNA editing.

Induction of body weight loss through RNAi-knockdown of APOBEC1 gene expression in transgenic rabbits

In the search of new strategies to fight against obesity, we targeted a gene pathway involved in energy uptake. We have thus investigated the APOB mRNA editing protein (APOBEC1) gene pathway that is involved in fat absorption in the intestine. The APOB gene encodes two proteins, APOB100 and APOB48, via the editing of a single nucleotide in the APOB mRNA by the APOBEC1 enzyme. The APOB48 protein is mandatory for the synthesis of chylomicrons by intestinal cells to transport dietary lipids and cholesterol. We produced transgenic rabbits expressing permanently and ubiquitously a small hairpin RNA targeting the rabbit APOBEC1 mRNA. These rabbits exhibited a moderately but significantly reduced level of APOBEC1 gene expression in the intestine, a reduced level of editing of the APOB mRNA, a reduced level of synthesis of chylomicrons after a food challenge, a reduced total mass of body lipids and finally presented a sustained lean phenotype without any obvious physiological disorder. Interestingly, no compensatory mechanism opposed to the phenotype. These lean transgenic rabbits were crossed with transgenic rabbits expressing in the intestine the human APOBEC1 gene. Double transgenic animals did not present any lean phenotype, thus proving that the intestinal expression of the human APOBEC1 transgene was able to counterbalance the reduction of the rabbit APOBEC1 gene expression. Thus, a moderate reduction of the APOBEC1 dependent editing induces a lean phenotype at least in the rabbit species. This suggests that the APOBEC1 gene might be a novel target for obesity treatment.

Rapid development of atherosclerosis in the world's smallest Microminipig fed a high-fat/high-cholesterol diet

AIM

Experimental studies of human atherogenesis require an appropriate animal model that mimics human physiology and pathology. Because swine physiology is similar to human physiology, we developed a hyperlipidemia-induced atherosclerosis model using the recently developed world's smallest Microminipig(TM).

METHODS

These animals weigh only 5kg at 3months of age, much smaller than any other miniature pig. We found that the administration of a high-fat/high-cholesterol diet containing at least 0.2% cholesterol without cholic acid for as little as eight weeks induces hypercholesterolemia and subsequent atherosclerosis in these animals.

RESULTS

The serum levels of low-density lipoprotein cholesterol(LDL-C) and the percent distribution of cholesterol in the LDL fractions were markedly increased. The hepatic expression of LDL receptor and hydroxymethylglutaryl-CoA reductase was coordinately decreased. The cholesteryl ester transfer protein activity, which plays a role in reverse cholesterol transport, was detected in the serum of the Microminipigs. Niemann-Pick C1-like 1 protein was expressed in both the liver and small intestine; however, hepatic apoB mRNA editing enzyme was not expressed. As in humans, and in contrast to that observed in mice, most of the hepatic lipase activity was localized in the liver. These results suggest that the hyperlipidemia-induced gene expression profile linked to cholesterol homeostasis and atherogenesis is similar in Microminipigs and humans.

CONCLUSION

We conclude that the characteristics of the Microminipig, including its easy handling size, make it an appropriate model for studies of atherosclerosis and related conditions.

The genetic basis for individual differences in mRNA splicing and APOBEC1 editing activity in murine macrophages

Alternative splicing and mRNA editing are known to contribute to transcriptome diversity. Although alternative splicing is pervasive and contributes to a variety of pathologies, including cancer, the genetic context for individual differences in isoform usage is still evolving. Similarly, although mRNA editing is ubiquitous and associated with important biological processes such as intracellular viral replication and cancer development, individual variations in mRNA editing and the genetic transmissibility of mRNA editing are equivocal. Here, we have used linkage analysis to show that both mRNA editing and alternative splicing are regulated by the macrophage genetic background and environmental cues. We show that distinct loci, potentially harboring variable splice factors, regulate the splicing of multiple transcripts. Additionally, we show that individual genetic variability at the Apobec1 locus results in differential rates of C-to-U(T) editing in murine macrophages; with mouse strains expressing mostly a truncated alternative transcript isoform of Apobec1 exhibiting lower rates of editing. As a proof of concept, we have used linkage analysis to identify 36 high-confidence novel edited sites. These results provide a novel and complementary method that can be used to identify C-to-U editing sites in individuals segregating at specific loci and show that, beyond DNA sequence and structural changes, differential isoform usage and mRNA editing can contribute to intra-species genomic and phenotypic diversity.

Recapture of hepatic apolipoprotein B mRNA editing may be a promising strategy to relieve nephrotic dyslipidemia

A high total plasma cholesterol concentration is the most common abnormality found in patients with kidney disease, which may be associated with the increased hepatic synthesis of apoB containing lipoproteins. ApoB mRNA editing plays an important physiological role in mammalian lipid metabolism by modifying the distribution of apoB-100 and apoB-48. However, it is regretful that apoB mRNA editing cannot be found in human liver because of the absence of apobec-1 expression. In this context, we hypothesize that the recapture of hepatic apoB mRNA editing may be a promising strategy to relieve nephrotic dyslipidemia. The data presented below focus on those which support this hypothesis with regards to evidence in vitro and in vivo. (1) Human wild-type apoB mRNA can be edited only when both apobec-1 and ACF proteins are presented simultaneously in vitro. (2) Adenoviral vectors can produce short-term expression of exogenous apobec-1 in the livers and lower plasma apoB-100 and LDL levels transiently. (3) Apobec-1 transgenic animals exhibit massive hepatic editing of apoB mRNA and fundamental decreased plasma levels of apoB-100 and LDL, but are exposed to high risk of liver dysplasia and hepatocellular carcinomas. In summary, taking into account the therapeutic security, we put forward that apobec-1 recombinant adenoviral vectors can be used for the recapture of hepatic apoB mRNA editing with a transient low-level manner and may achieve satisfactory lipid-lowing effect in nephropathic animals.

Alanine-glyoxylate aminotransferase-deficient mice, a model for primary hyperoxaluria that responds to adenoviral gene transfer

Mutations in the alanine-glyoxylate amino transferase gene (AGXT) are responsible for primary hyperoxaluria type I, a rare disease characterized by excessive hepatic oxalate production that leads to renal failure. We generated a null mutant mouse by targeted mutagenesis of the homologous gene, Agxt, in embryonic stem cells. Mutant mice developed normally, and they exhibited hyperoxaluria and crystalluria. Approximately half of the male mice in mixed genetic background developed calcium oxalate urinary stones. Severe nephrocalcinosis and renal failure developed after enhancement of oxalate production by ethylene glycol administration. Hepatic expression of human AGT1, the protein encoded by AGXT, by adenoviral vector-mediated gene transfer in Agxt(-/-) mice normalized urinary oxalate excretion and prevented oxalate crystalluria. Subcellular fractionation and immunofluorescence studies revealed that, as in the human liver, the expressed wild-type human AGT1 was predominantly localized in mouse hepatocellular peroxisomes, whereas the most common mutant form of AGT1 (G170R) was localized predominantly in the mitochondria.

Simultaneous expression of apolipoprotein B mRNA editing enzyme and scavenger receptor BI mediated by a therapeutic gene expression system

Cardiovascular diseases are often accompanied by elevated LDL particles and endothelial dysfunction. We have examined the possibility of concurrently reducing LDL levels and modulating endothelial function using a single helper-dependent adenovirus vector system to simultaneously express the apolipoprotein B mRNA editing enzyme (Apobec1) and the scavenger receptor, class B, type I (SR-BI) genes under the control of separate promoters (designated HD-C2). Apobec1 edits apoB mRNA at nucleotide C-6666 to produce truncated apoB48 and is normally expressed in small intestine only. SR-BI is a receptor for multiple ligands with distinct tissue-specific functions. Expression of Apobec1 in HepG2 cells resulted in apoB mRNA editing, leading to decreased apoB100 abundance (to 6% of control) and the appearance of apoB48. Editing of apoB mRNA in HepG2 cells resulted in decline in apoB mRNA levels of 50%. This was probably the result of nonsense-mediated decay of edited message, since over-expression of Apobec1 increased neither Apobec1 complementary factor (ACF) mRNA nor protein abundance. Over-expression of SR-BI in human endothelial cells activated endothelial nitric oxide synthase (eNOS) activity by phosphorylation of eNOS at residue Ser-1177 in the presence of HDL, leading to increased production of the anti-atherogenic molecule nitric oxide (NO). Taken together, this study demonstrates that using one vector delivery system to express two genes in two different cell types results in the cell-specific beneficial effects of decreasing apoB100 production and increasing eNOS activities. This combined gene expression approach may provide an improved therapeutic strategy by targeting multiple sites in the mechanism of cardiovascular injury.

The cytidine deaminases AID and APOBEC-1 exhibit distinct functional properties in a novel yeast selectable system

Activation-induced cytidine deaminase (AID) is indispensable for immunoglobulin maturation by somatic hypermutations and class switch recombination and is supposed to deaminate cytidines in DNA, while its homolog APOBEC-1 edits apolipoprotein (apo) B mRNA by cytidine deamination. We studied the editing activity of APOBEC-1 and AID in yeast using the selectable marker Gal4 linked to its specific inhibitor protein Gal80 via an apo B cassette (Gal4-C) or via the variable region of a mouse immunoglobulin heavy chain gene (Gal4-VH). Expression of APOBEC-1 induced C to U editing in up to 15% of the Gal4-C transcripts, while AID was inactive in this reaction even in the presence of the APOBEC-1 complementation factor. After expression of APOBEC-1 as well as AID approximately 10(-3) of yeast cells survived low stringency selection and expressed beta-galactosidase. Neither AID nor APOBEC-1 mutated the VH sequence of Gal4-VH, and consequently the yeast colonies did not escape high stringent selection. AID, however, induced frequent plasmid recombinations that were only rarely observed with APOBEC-1. In conclusion, AID cannot substitute APOBEC-1 to edit the apo B mRNA, and the expression of AID in yeast is not sufficient for the generation of point mutations in a highly transcribed Gal4-VH sequence. Cofactors for AID induced somatic hypermutations of immunoglobulin variable regions, that are present in B cells and a variety of non-B cells, appear to be missing in yeast. In contrast to APOBEC-1, AID alone does not exhibit an intrinsic specificity for its target sequences.

Inhibition of the synthesis of apolipoprotein B-containing lipoproteins

Increased serum concentrations of low density lipoproteins represent a major cardiovascular risk factor. Low-density lipoproteins are derived from very low density lipoproteins secreted by the liver. Apolipoprotein (apo)B that constitutes the essential structural protein of these lipoproteins exists in two forms, the full length form apoB-100 and the carboxy-terminal truncated apoB-48. The generation of apoB-48 is due to editing of the apoB mRNA which generates a premature stop translation codon. The editing of apoB mRNA is an important regulatory event because apoB-48-containing lipoproteins cannot be converted into the atherogenic low density lipoproteins. The apoB gene is constitutively expressed in liver and intestine, and the rate of apoB secretion is regulated post-transcriptionally. The translocation of apoB into the endoplasmic reticulum is complicated by the hydrophobicity of the nascent polypeptide. The assembly and secretion of apoB-containing lipoproteins within the endoplasmic reticulum is strictly dependent on the microsomal tricylceride transfer protein which shuttles triglycerides onto the nascent lipoprotein particle. The overall synthesis of apoB lipoproteins is regulated by proteosomal and nonproteosomal degradation and is dependent on triglyceride availability. Noninsulin dependent diabetes mellitus, obesity and the metabolic syndrome are characterized by an increased hepatic synthesis of apoB-containing lipoproteins. Interventions aimed to reduce the hepatic secretion of apoB-containing lipoproteins are therefore of great clinical importance. Lead targets in these pathways are discussed.

Reduced expression and increased CpG dinucleotide methylation of the rat APOBEC-1 promoter in transgenic rabbits

Editing of apolipoprotein (apo) B mRNA in liver limits the plasma LDL levels in horses, dogs, rats or mice. Species such as man or rabbit do not edit the hepatic apo B mRNA and are therefore susceptible to atherosclerosis and coronary artery disease due to elevated plasma LDL levels. The catalytic subunit APOBEC-1 is the only missing component of the apo B mRNA editing enzyme complex in the human or rabbit liver. Here we describe the generation of transgenic rabbits in which APOBEC-1 expression is mediated by the proximal promoter of the rat APOBEC-1 gene. These transgenic rabbits are healthy and fertile, and rat APOBEC-1 mRNA is expressed in liver, intestine, kidney, lung, brain and muscle. The transgenic APOBEC-1 expression is low and not sufficient to induce editing in rabbit liver. In rat, the proximal APOBEC-1 promoter demonstrates a progressive loss of CpG dinucleotide methylation towards the core promoter region that is entirely unmethylated. In the transgenic rabbits, this distinct pattern of CpG methylation is lost, and throughout the entire rat APOBEC-1 promoter, >90% of the CpGs are methylated. Thus, the weak proximal rat APOBEC-1 promoter appears to be down-regulated in the rabbit and may be species-specific.

Reconstitution of mRNA editing in yeast using a Gal4-apoB-Gal80 fusion transcript as the selectable marker

We describe a fusion transcript of Gal4 linked to its specific inhibitor protein Gal80 by 276 nucleotides of apolipoprotein (apo) B sequence as a selectable marker for mRNA editing. Editing of apoB mRNA is catalyzed by an editing enzyme complex that introduces a stop codon by deamination of C to U. The catalytic subunit APOBEC-1 is a cytidine deaminase and requires a second essential component recently cloned and termed APOBEC-1 complementing factor (ACF) or APOBEC-1-stimulating protein (ASP). The aim of this study was to demonstrate that APOBEC-1 plus ACF/ASP comprise all that is required for editing of apoB mRNA in vivo. Expression of APOBEC-1 and Gal4 fused to its inhibitor Gal80 by an intervening unedited apoB sequence (Gal4-apoB(C)-Gal80) did not result in the Gal4-dependent expression of HIS3 and beta-galactosidase in the yeast strain CG1945. Co-expression of APOBEC-1 and ACF/ASP induced editing of the apoB site in up to 13% of the Gal4-apoB(C)-Gal80 transcripts and enabled selection of yeast cells for robust expression of HIS3 and beta-galactosidase. Additional expression of the alternative splicing regulatory protein KSRP increased the editing of the apoB site by APOBEC-1 and ACF/ASP to 21%. Thus, APOBEC-1 and ACF/ASP represent the core apoB mRNA editing enzyme in vivo. This study demonstrates for the first time the successful use of a selectable marker for mRNA editing. The Gal4-Gal80 system is analogous to the two-hybrid assay and may have broader applications for the study of other mRNA processing reactions.

Normal production rate of apolipoprotein B in LDL receptor-deficient mice

The low density lipoprotein (LDL) receptor is well known for its role in mediating the removal of apolipoprotein B (apoB)-containing lipoproteins from plasma. Results from in vitro studies in primary mouse hepatocytes suggest that the LDL receptor may also have a role in the regulation of very low density lipoprotein (VLDL) production. We conducted in vivo experiments using LDLR-/-, LDLR+/-, and wild-type mice (LDLR indicates LDL receptor gene) in which the production rate of VLDL was measured after the injection of [35S]methionine and the lipase inhibitor Triton WR1339. Despite the fact that LDLR-/- mice had a 3.7-fold higher total cholesterol level and a 2.1-fold higher triglyceride level than those of the wild-type mice, there was no difference in the production rate of VLDL triglyceride or VLDL apoB between these groups of animals. Experiments were also conducted in apobec1-/- mice, which make only apoB-100, the form of apoB that binds to the LDL receptor. Interestingly, the apobec1-/- mice had a significantly higher production rate of apoB than did the wild-type mice. However, despite significant differences in total cholesterol and triglyceride levels, there was no difference in the production rate of total or VLDL triglyceride or VLDL apoB between LDLR-/- and LDLR+/- mice on an apobec1-/- background. These results indicate that the LDL receptor has no effect on the production rate of VLDL triglyceride or apoB in vivo in mice.

Hammerhead ribozyme as a therapeutic agent for hyperlipidemia: production of truncated apolipoprotein B and hypolipidemic effects in a dyslipidemia murine model

In humans, overproduction of apolipoprotein B (apoB) is positively associated with premature coronary artery diseases. To reduce the levels of apoB mRNA, we used adenovirus-mediated vector to target hammerhead ribozyme at GUA(6679) downward arrow of apoB mRNA (designated AvRB15) in the liver of a dyslipidemic mouse model that is deficient in apoB mRNA editing enzyme and overexpresses human apoB100. In this study, we delivered approximately 4 x 10(11) virus particles of AvRB15 (active ribozyme) or AvRB15-mutant (inactive ribozyme) to the animals. Using Southern blot analysis, we readily detected RB15 DNA in the mouse liver as long as day 35 after injection. This result was correlated with the RNA expression of RB15 by RNase protection assay. Using reverse ligation-mediated polymerase chain reaction, the 3' cleavage product of apoB mRNA was detected, and the exact cleavage site was confirmed by sequencing. Importantly, the levels of human and mouse apoB mRNA decreased approximately 80% after AvRB15 transduction. There was a marked decrease in plasma cholesterol, triglyceride, and human apoB of 42, 51, and 62%, respectively, when compared with the inactive ribozyme-treated group. Moreover, ribozyme cleavage of apoB mRNA generated a truncated protein of the expected size (apoB48.1), which was associated with lipoprotein particles in the very low density, low density, and high density lipoprotein fractions. Taken together, these results indicate that apoB mRNA-specific hammerhead ribozyme can be used as a potential therapeutic agent to modulate apoB gene expression and to treat hyperlipidemia.

RNA editing: cytidine to uridine conversion in apolipoprotein B mRNA

RNA editing is a post-transcriptional process that changes the informational capacity within the RNA. These processes include alterations made by nucleotide deletion, insertion and base conversion. A to I and C to U conversion occurs in mammals and these editing events are catalysed by RNA binding deaminases. C to U editing of apoB mRNA was the first mammalian editing event to be identified. The minimal protein complex necessary for apoB mRNA editing has been determined and consists of APOBEC-1 and ACF. Overexpression of APOBEC-1 in transgenic animals caused liver dysplasia and APOBEC-1 has been identified in neurofibromatosis type 1 tumours, suggesting that RNA editing may be another mechanism for tumourigenesis. Several APOBEC-1-like proteins have been identified, including a family of APOBEC-1-related proteins with unknown function on chromosome 22. This review summarises the different types of RNA editing and discusses the current status of C to U apoB mRNA editing. This knowledge is very important in understanding the structure and function of these related proteins and their role in biology.

Hepatic lipid accumulation, altered very low density lipoprotein formation and apolipoprotein E deposition in apolipoprotein E3-Leiden transgenic mice

BACKGROUND/AIM

Apolipoprotein (apo) E-deficiency leads to hepatic steatosis and impaired Very Low Density Lipoprotein (VLDL)-triglyceride production rates in mice. A mutant apoE isoform, apoE3-Leiden, is associated with a dominantly inherited form of dysbetalipoproteinemia in humans. The aim of this study was to evaluate the effects of APOE*3-Leiden expression on hepatic lipid content, VLDL formation and liver morphology in mice.

METHODS

Comparison of lipid parameters and liver morphology in mouse strains with different expression of the APOE*3-Leiden transgene with and without co-expression of human APOCI.

RESULTS

Hepatic triglyceride content was increased to maximally 233% of control values, depending on hepatic APOE*3-Leiden expression. Hepatic secretion of VLDL-associated triglycerides was impaired (-20%) in high-expressing transgenics, with a concomitant increase from 1.6 to 8.1 of the apoB48/ apoB100 ratio in newly-formed VLDL. Hepatocytes of the transgenic mice contained characteristic inclusions, up to 20 microm in diameter, in numbers dependent on APOE*3-Leiden expression and independent of APOCI expression. These inclusions contained material positively reacting with antihuman apoE antibodies. Immunogold-labeling confirmed the presence of apoE3-Leiden within these inclusions and also revealed the presence of the mutant protein on sinusoidal membranes, in multivesicular bodies and in peroxisomes, i.e., a distribution pattern similar to that of endogenous apoE in rodents. Nascent VLDL particles associated with the Golgi apparatus were also labeled.

CONCLUSION

This study has demonstrated that introduction of human apoE3-Leiden in mice, in addition to its reported effects on lipolysis and lipoprotein clearance, leads to hepatic deposition of the mutant apolipoprotein, development of fatty liver and to altered hepatic VLDL secretion. The latter findings are consistent with a role of apoE in the regulation of intrahepatic lipid metabolism.

Purification and molecular cloning of a novel essential component of the apolipoprotein B mRNA editing enzyme-complex

Editing of apolipoprotein B (apoB) mRNA requires the catalytic component APOBEC-1 together with "auxiliary" proteins that have not been conclusively characterized so far. Here we report the purification of these additional components of the apoB mRNA editing enzyme-complex from rat liver and the cDNA cloning of the novel APOBEC-1-stimulating protein (ASP). Two proteins copurified into the final active fraction and were characterized by peptide sequencing and mass spectrometry: KSRP, a 75-kDa protein originally described as a splicing regulating factor, and ASP, a hitherto unknown 65-kDa protein. Separation of these two proteins resulted in a reduction of APOBEC-1-stimulating activity. ASP represents a novel type of RNA-binding protein and contains three single-stranded RNA-binding domains in the amino-terminal half and a putative double-stranded RNA-binding domain at the carboxyl terminus. Purified recombinant glutathione S-transferase (GST)-ASP, but not recombinant GST-KSRP, stimulated recombinant GST-APOBEC-1 to edit apoB RNA in vitro. These data demonstrate that ASP is the second essential component of the apoB mRNA editing enzyme-complex. In rat liver, ASP is apparently associated with KSRP, which may confer stability to the editing enzyme-complex with its substrate apoB RNA serving as an additional auxiliary component.

Liver-directed gene therapy: promises, problems and prospects at the turn of the century

Although liver-directed gene therapy arrived later than gene therapy directed at bone marrow cells, intrinsic advantages of the liver as a target organ make it likely that gene therapy for liver diseases will be among the first therapeutically relevant applications of this treatment modality at the onset of the 21st century. Vectorology for gene transfer to the liver is advancing rapidly, and it is safe to predict that gene therapy vehicles that will be in clinical use a decade from now, have not yet been developed. None of the currently available modes of gene transfer to the liver is optimal for all types of applications. Nonetheless, the concerted effort of many investigators has provided a wide choice of non-viral and viral vectors for gene transfer to the liver for use in specific situations. Original strategies for liver-directed gene therapy included substitution of missing gene products, overexpression of intrinsic or extrinsic genes and inhibition of expression of specific genes. To the list is now added the possibility of site-specific correction or generation of mutations within specific genes in somatic cells of living adult animals. Thus, despite some initial faux pas, liver-directed gene therapy is poised to make an important impact on health care in the year 2000 and beyond.

Cardiovascular gene therapy

Vascular gene transfer potentially offers new treatments for cardiovascular diseases. It can be used to overexpress therapeutically important proteins and correct genetic defects, and to test experimentally the effects of various genes in a local vascular compartment. Vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF) gene transfers have improved blood flow and collateral development in ischaemic limb and myocardium. Promising therapeutic effects have been obtained in animal models of restenosis or vein-graft thickening with the transfer of genes coding for VEGF, nitric-oxide synthase, thymidine kinase, retinoblastoma, growth arrest homoeobox, tissue inhibitor of metalloproteinases, cyclin or cyclin-dependent kinase inhibitors, fas ligand and hirudin, and antisense oligonucleotides against transcription factors or cell-cycle regulatory proteins. First experiences of VEGF gene transfer and decoy oligonucleotides in human beings have been reported. However, further developments in gene-transfer vectors, gene-delivery techniques and identification of effective treatment genes will be required before the full therapeutic potential of gene therapy in cardiovascular disease can be assessed.

Absence of APOBEC-1 mediated mRNA editing in human carcinomas

The transgene expression of the catalytic subunit APOBEC-1 of the apo B mRNA editing enzyme-complex can cause hepatocellular carcinoma in mice and rabbits. It has been proposed that aberrant editing of mRNA may represent a novel oncogenic principle. This investigation aimed to define whether such aberrant hyperediting mediated by APOBEC-1 occurs in human carcinomas. Editing and hyperediting of apo B, NAT1 or NF1 mRNA was not identified in any of 28 resected tumor specimens, including hepatocellular, bile duct, gastric, colorectal, pancreatic adeno- and neuroendocrine, lung adeno-, medullary thyroid and breast carcinoma, soft tissue sarcoma and neuroblastoma. In most types of carcinoma, significant levels for full-length APOBEC-1 mRNA could not be detected. Low level expression of APOBEC-1 was found in colorectal and gastric carcinoma where most of the APOBEC-1 mRNA is inactivated by alternate splicing. The 'auxiliary' components of the apo B mRNA editing enzyme-complex are missing in many tumors including colorectal and gastric carcinoma, but are highly expressed in hepatocellular, lung adeno- and breast carcinoma all of which lack APOBEC-1. Taken together, either APOBEC-1 or the 'auxiliary' components of the apo B mRNA editing enzyme-complex or both are missing in human carcinomas resulting in the absence of mRNA editing. Currently, there is no evidence that aberrant editing mediated by APOBEC-1 contributes to the tumorigenesis of natural human carcinomas.

Gene therapy for atherosclerosis and atherosclerosis-related diseases

Gene therapy for atherosclerosis-related disorders of lipoprotein metabolism is primarily directed to liver and aims at long-lasting correction of familial hypercholesterolemia, lipoprotein / hepatic lipase deficiency, and Apolipoprotein A, B, or E -related diseases. Treatment of complications of atherosclerosis (eg, restenosis, ischemia) requires local gene transfer to arterial wall or ischemic muscle with transient gene expression. Catheter-mediated approach or direct injections have been used in clinical trials for the treatment of restenosis and for the induction of angiogenesis in ischaemic limb and myocardium. Other possible applications of local gene transfer include antithrombotic treatment and stabilization of vulnerable plaques.

Normal perinatal rise in serum cholesterol is inhibited by hepatic delivery of adenoviral vector expressing apolipoprotein B mRNA editing enzyme (Apobec1) in rabbits

BACKGROUND

Prenatal or neonatal hepatic gene delivery may result in more effective therapy for inborn errors of metabolism due to the immature immune system of the perinatal animal, and the ability to intervene prior to any significant cellular damage. Newborn New Zealand White rabbits have low serum levels of cholesterol at birth, with a significant and sustained rise of cholesterol while they are nursing. We used this physiologic hypercholesterolemia model to study the effect of adenovirus-mediated hepatic gene transfer of rat apolipoprotein B mRNA editing enzyme (Apobec1) on modulation of plasma cholesterol levels.

METHODS AND RESULTS

Transcutaneous injection of recombinant adenovirus expressing Apobec1 (AvApobec1) into the liver of newborn rabbits in vivo resulted in efficient Apobec1 expression until Day 50, as detected by PCR-Southern blot analysis. By in vitro editing assay, liver extracts of AvApobec1-treated rabbits were found to have apoB mRNA editing activities of approximately 12, 15, and 15%, on Days 2, 10, and 20 after AvApobec1 administration, compared with 0% editing activity in AvLacZ control vector-injected animals. This physiological level of Apobec1 expression was associated with the production of apoB-48-containing lipoprotein particles from rabbit liver, with a concomitant 30% reduction in total plasma cholesterol compared to AvLacZ-treated or untreated control animals.

CONCLUSION

Neonatal intrahepatic delivery of a first-generation adenoviral vector results in efficient gene transfer with little immune response, suggesting that repeated administration may be possible in the neonatal period.

Inhibition of the apolipoprotein B mRNA editing enzyme-complex by hnRNP C1 protein and 40S hnRNP complexes

The apolipoprotein (apo) B mRNA can be modified by a posttranscriptional base change from cytidine to uridine at nucleotide position 6666. This editing of apo B mRNA is mediated by a specific enzyme-complex of which only the catalytic subunit APOBEC-1 (apo B mRNA editing enzyme component 1) has been cloned and extensively characterized. In this study, two-hybrid selection in yeast identified hnRNP C1 protein to interact with APOBEC-1. Recombinant hnRNP C1 protein inhibited partially purified apo B mRNA editing activity from rat small intestine and bound specifically to apo B sense RNA around the editing site. The inhibition of apo B mRNA editing by hnRNP C1 protein was not due to masking of the RNA substrate as the mutant protein M104 spanning the RNA-binding domain of hnRNP C1 protein bound strongly to the apo B RNA, but did not inhibit the editing reaction. The apo B mRNA editing enzyme-complex of rat liver nuclear extracts sedimented in sucrose density gradients around 22-27S, but did not contain hnRNP C1 protein that was found exclusively within 40S hnRNP complexes. The removal of 40S hnRNP complexes increased the activity of the 22-27S editing enzyme-complex. Adding back 40S hnRNP complexes with hnRNP C1 protein resulted in an inhibition of the 22-27S apo B mRNA editing enzyme-complex, while addition of 18S fractions had no effect. In conclusion, hnRNP C1 protein identified by two-hybrid selection in yeast is a potent inhibitor of the apo B mRNA editing enzyme-complex. The abundant hnRNP C1 protein, which is contiguously deposited on nascent pre-mRNA during transcription and is involved in spliceosome assembly and mRNA splicing, is a likely regulator of the editing of apo B mRNA which restricts the activity of APOBEC-1 to limited and specific editing events.

Distinct promoters induce APOBEC-1 expression in rat liver and intestine

The expression of apolipoprotein (apo) B can be modulated by mRNA editing, a unique posttranscriptional base change in the apo B mRNA. Apo B-48, the translation product of edited apo B mRNA, is not a precursor of the atherogenic low density lipoproteins and lipoprotein(a). In humans and various other mammals, the apo B mRNA is edited in the intestine but not in the liver, which exclusively secretes apo B-100-containing lipoproteins as precursors for low density lipoprotein formation. In species such as the rat, mouse, dog, and horse, apo B mRNA is also edited in the liver, resulting in low plasma levels of low density lipoprotein. Editing of the apo B mRNA is mediated by the apo B mRNA-editing enzyme complex, of which the catalytic subunit APOBEC-1 is not expressed in the liver of species without hepatic editing. To understand the molecular basis for liver-specific expression of APOBEC-1 and the editing of hepatic apo B mRNA, the expression pattern and genomic organization of the rat APOBEC-1 gene have been characterized. The rat APOBEC-1 gene contains 6 exons and 2 promoters with distinct activities. The expression of APOBEC-1 in the rat liver is the result of a promoter located upstream, with tissue-specific exon use and alternate splicing within the 5'-untranslated region of APOBEC-1 mRNA encoded by exon 2. In addition to the liver, this promoter also induces APOBEC-1 expression in the spleen, lung, kidney, heart, and skeletal muscle. The promoter located downstream belongs to a new class of TATA-less promoters and is responsible for the abundant expression of APOBEC-1 in the intestine. Mapping of the transcriptional start sites and deletion analysis of the promoter regions by using luciferase as the reporter gene have defined the regulatory elements of both promoters. The downstream, intestine-specific promoter contains a negative regulatory element between -1100 and -500, which appears to restrict its activity to the intestine. The upstream, liver-specific promoter of the rat APOBEC-1 gene induces APOBEC-1 expression and editing of apo B mRNA in human hepatoma HuH-7 and Hep G2 cells. Understanding the molecular basis for the liver-specific expression of APOBEC-1 in the rat promises new strategies to induce APOBEC-1 expression in the human liver for the reduction of atherogenic lipoprotein levels by hepatic apo B mRNA editing.