Induction of hepatic apolipoprotein B mRNA editing for reducing serum cholesterol levels: A breakthrough or a disaster

LEVELS: A BREAKTHROUGH OR A DISASTER? crons, which contain apo B-48 as the core protein. Unlike VLDL, chylomicrons are rapidly and completely cleared by Yamanaka S, Balestra ME, Ferrell LD, Fan J, Arnold KS, the liver after triglyceride hydrolysis, without giving rise to Taylor S, Taylor JM, et al. Apolipoprotein B mRNA-editing long-lived apo B–containing lipoproteins. protein induces hepatocellular carcinoma and dysplasia in The synthesis of apo B-48 is the result of mRNA editing, transgenic animals. Proc Natl Acad Sci U S A 1995;92:8483which consists of a specific base change from C to U in the 8487. apo B mRNA, creating a premature stop codon. The editing is mediated by the apo B–mRNA editing enzyme, which speABSTRACT cifically deaminates the cytidine residue at nucleotide 6666 during the processing of apo B pre-mRNA. The editing enApolipoprotein (apo-) B messenger RNA (mRNA) editzyme is a complex of several subunits. The catalytic subunit, ing is the deamination of cytidine that creates a new APOBEC-1 (apo B–mRNA editing enzyme catalytic polypeptermination codon and produces a truncated version of tide 1,) is a member of the cytidine deaminases gene family apo B (apo-B48). The cytidine deaminase catalytic suband contains a novel RNA binding motif, but requires addiunit (apo-B mRNA-editing enzyme catalytic polypeptide tional ‘‘auxiliary’’ factors for expressing the apo B–mRNA 1 (APOBEC-1)) of the multiprotein editing complex has editing reaction. Extracts from human, baboon, or rabbit been identified. We generated transgenic rabbits and liver that do not exhibit the editing activity can nonetheless mice expressing rabbit APOBEC-1 in their livers to desupplement APOBEC-1 to reconstitute apo B–mRNA editing termine whether hepatic expression would lower lowin vitro, suggesting that the livers of these species contain density lipoprotein cholesterol concentrations. The apothe additional ‘‘auxiliary’’ components of the apo B–mRNA B mRNA from the livers of the transgenic mice and editing enzyme complex. Almost 100% of the apo B mRNA rabbit was extensively edited, and the transgenic aniis edited in the intestine of all mammalian species that are mals had reduced concentrations of apo-B100 and lowstudied. APOBEC-1 is absent in rabbit, baboon, or human density lipoproteins compared with control animals. Unliver, and consequently, the liver of these species secrete expectedly, all of the transgenic mice and one transgenic VLDL containing the full-length apo B-100. rabbit had liver dysplasia, and many transgenic mice In horses, rats, mice, and dogs, however, hepatic apo B developed hepatocellular carcinomas. Many of the mRNA is also substantially edited. These species have low mouse livers were hyperplastic and filled with lipid. plasma LDL levels, probably because their livers secrete apo Other hepatic mRNAs with sequence motifs similar to B-48–containing VLDL, which are turned over much faster those of apo-B mRNA were examined for this type of than apo B-100–containing lipoproteins. In mice and rats editing (i.e., cytidine deamination). One of these, tyro(Greeve et al., manuscript in preparation), two different sine kinase, was edited in the livers of transgenic mice forms of APOBEC-1 are expressed in the liver and in the but not of controls. This result demonstrates that other intestine as a result of differential promoter usage and of mRNAs can be edited by the overexpressed, editing enalternative splicing. In these species, hepatic apo B–mRNA zyme, and suggests that aberrant editing of hepatic editing appears to be a genetic mechanism to regulate the mRNAs is involved in cell growth and that regulation is metabolism and to modulate the plasma concentrations of the cause of the tumorigenesis. Finally, these findings the atherogenic apo B-100–containing LDL. The mechanism compromise the potential use of APOBEC-1 for gene of the lack of APOBEC-1 expression in the liver of species, therapy to lower the plasma levels of low-density liposuch as in the livers of humans and rabbits, remains to be proteins. uncovered. By establishing APOBEC-1 transgenic rabbits, Yamanaka COMMENTS et al. have directly addressed the question of how the induction of apo B–mRNA editing in the liver of a species without Apolipoprotein (apo) B, the essential core protein of all liponatural hepatic editing influences plasma LDL levels. The proteins except the high-density lipoproteins, exists in two generation of transgenic rabbits required major refinements forms: the full-length protein, apo B-100 (512 kd), and the of the transgene technology. Simultaneously, Yamanaka et carboxy terminal truncated apo B-48 (248 kd). The human al. also generated transgenic mice that expressed active rabliver secretes triglyceride-rich), very-low-density lipoproteins bit APOBEC-1 or a mutant, enzymatically inactive form of (VLDL). Each VLDL particle contains one molecule of apo the protein. To achieve liver-specific expression of the APOB-100. After triglyceride hydrolysis by lipoprotein lipases, BEC-1 transgene, they used a vector containing a promoter, most of the VLDL-remnants are cleared within minutes by an intron, and a hepatic control-region of the human apolipothe liver, yet some VLDL particles are transformed by incomprotein E gene. Four independent transgenic mouse lines pletely understood processes into low-density lipoproteins were established from three transgenic mouse founders that (LDL). LDL are enriched in cholesterol-esters, have a expressed wild-type rabbit APOBEC-1 with estimated plasma half-life of about 1 day, and, if elevated, represent an important cause of development of atherosclerosis and transgene copy numbers of 3, 7, 10, and 17, respectively. In