Low Expression of the Apolipoprotein B mRNA–Editing Transgene in Mice Reduces LDL Levels but Does Not Cause Liver Dysplasia or Tumors

Abstract

Abstract-Hepatic expression of apolipoprotein (apo) B mRNA-editing enzyme catalytic polypeptide 1 (APOBEC-1) has been proposed as a gene therapy approach for lowering plasma low density lipoprotein (LDL) levels. However, high-level expression of APOBEC-1 in transgenic mouse and rabbit livers causes liver dysplasia and hepatocellular carcinoma. To determine the physiological and pathological effects of low-level hepatic expression of APOBEC-1, we used a 52-kb rat APOBEC-1 genomic clone (RE4) to generate transgenic mice expressing low levels of APOBEC-1 (2 to 5 times those in nontransgenic mice). Liver function, liver histology, editing of apoB mRNA at the normal editing site (C 6666 ), and abnormal editing at multiple sites (hyperediting) in these mice were compared with those in transgenic mice expressing intermediate (I-20) or high (I-28) levels of APOBEC-1 in the liver. Hyperediting of mRNA coding for the novel APOBEC-1 target 1 (NAT1) was also examined. In the high-expressing I-28 line, 50% of the mice had palpable tumors at 15 weeks of age, whereas in the intermediate-expressing I-20 line, 50% of the mice had evidence of liver tumors after 1 year. In contrast, low-expressing RE4 mice had normal liver function and histology and did not develop liver tumors when examined at 3 to 17 months of age. Moreover, hyperediting of apoB and NAT1 mRNA in the liver was robust in the I-20 mice but barely detectable in the RE4 mice. The low-level expression resulted in sufficient APOBEC-1 to edit essentially all apoB mRNA at the normal editing site, virtually eliminating apoB-100 and LDL in the plasma of RE4 mice. When RE4 mice were crossed with human apoB transgenic mice, which possess high plasma LDL concentrations, plasma LDL levels in the offspring were reduced to very low levels. These results indicates that long-term hepatic expression of APOBEC-1 at low levels sufficient to eliminate LDL does not cause apparent liver damage or liver tumors in transgenic mice. RE4 APOBEC-1 transgenic mice should prove valuable for studying the roles of apoB-containing lipoproteins in lipid metabolism and atherosclerosis. 1 ApoB-100, the full-length protein, is synthesized in the liver and is essential for the assembly and secretion of VLDLs, 2,3 the precursors of LDLs. LDLs transport about two thirds of the cholesterol in the plasma and are the major atherogenic lipoproteins in humans. ApoB-100 is the major protein component of LDL and mediates their uptake by the LDL receptor. ApoB-100 is also a component of other atherogenic lipoproteins, such as Lp(a) and ␤-VLDL. ApoB-48, the amino-terminal 48% of apoB-100, is synthesized in the small intestine of all mammalian species and is essential for the assembly and secretion of chylomicrons. ApoB-48 is formed by apoB mRNA editing of the same nuclear mRNA that encodes apoB-100. 4 The apoB mRNAediting catalytic polypeptide 1 (APOBEC-1) deaminates the cytidine in codon 2153 (C 6666 AA) to form a uridine. The resulting in-frame stop codon (U 6666 AA) causes premature translation termination an...

Intestine-specific expression of Apobec-1 rescues apolipoprotein B RNA editing and alters chylomicron production in Apobec1 mice

“…5E , fractions 24-28) compared with WT mice, where apoB48 appeared in broad population of LDL-sized particles ( Fig. 5C , fractions [18][19][20][21][22][23][24][25][26][27][28][29]. Triglyceride distribution was indistinguishable between the genotypes, almost exclusively within chylomicron and VLDL fractions, as expected ( Fig.…”

“…Int/O mice since earlier work indicated a dose response for hyperediting in apobec-1 transgenic liver ( 21,22 ). Our fi ndings demonstrate that low-expressor Apobec1…”

Quantitative trait loci that determine lipoprotein cholesterol levels in DBA/2J and CAST/Ei inbred mice,

“…A second study did not report the effect of transgene insertion in mice but reported that HDL increased in one transgenic rabbit (68). A third study of Apobec1 on an Ldlr knockout background indicated no change in HDL cholesterol (69), whereas a fourth study using a mixed genetic background indicated a decrease in HDL cholesterol (70). On balance, it appears that Apobec1 has little effect on HDL cholesterol levels, at least when mice are fed chow.…”

Hypermutation induced by APOBEC-1 overexpression can be eliminated

“…Several reports demonstrated that adenoviral delivery of rat APOBEC-1 to the liver of rabbits and mice or liver-specific overexpression of APOBEC-1 in apoB transgenic mice effectively decreased LDL cholesterol levels (Teng et al 1994;Yamanaka et al 1995;Hughes et al 1996;Kozarsky et al 1996;Teng et al 1997;Qian et al 1998). However, high constitutive APOBEC-1 expression in mouse liver resulted in liver dysplasia and hepatocellular carcinomas (Yamanaka et al 1995).…”