Advertisement
J. Lipid Res.
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow An erratum has been published
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrowRequest Permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ko, C.
Right arrow Articles by Huang, L.-S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Ko, C.
Right arrow Articles by Huang, L.-S.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati  
What's this?

Journal of Lipid Research, Vol. 42, 844-855, May 2001
Copyright © 2001 by Lipid Research, Inc.

Original Article

Two novel quantitative trait loci on mouse chromosomes 6 and 4 independently and synergistically regulate plasma apoB levels

Carol Koa,c, Tsai-Ling Leea, Philip W. Laua, Jing Lib, Brandi T. Davisb, Emanuel Voyiaziakisa, David B. Allisonc,d, Streamson C. Chua, Jr.b,c,d, and Li-Shin Huanga,c
a Department of Medicine, College of Physicians and Surgeons, Columbia University, New York, NY 10032
b Department of Pediatrics, College of Physicians and Surgeons, Columbia University, New York, NY 10032
c Institute of Human Nutrition, College of Physicians and Surgeons, Columbia University, New York, NY 10032
d Obesity Research Center, St. Luke's Roosevelt Hospital, College of Physicians and Surgeons, Columbia University, New York, NY 10025

Correspondence to: Li-Shin Huang, at the Institute of Human Nutrition, College of Physicians and Surgeons, Columbia University, 630 W. 168th Street, PH 10-305, New York, NY 10032., lh99{at}columbia.edu (E-mail)


  ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES

An elevated plasma apolipoprotein B (apoB) level is a strong predictor of atherosclerosis and coronary heart disease. Epidemiologic and family linkage studies have suggested a genetic basis for the wide variations of plasma apoB levels in the general population. Using a human apoB transgenic (HuBTg) mouse model, we have previously shown that hepatic apoB-100 secretion is a major determinant of the high and low plasma human apoB levels in HuBTg mice of the C57BL/6 (B6) and 129/Sv (129) strains, respectively. In the present article, we present the identification of two novel quantitative trait loci (QTL) as major regulators of plasma human apoB levels in the F2 and N2 (backcrossed) offspring (n = 572) derived from crosses between the B6 and 129 mouse strains. These loci were designated ApoB regulator genes (Abrg), because the gene products are likely to be involved in the regulation of plasma apoB levels either directly or indirectly. The first locus, designated Abrg1, was mapped to chromosome 6 in 8-week-old male and female mice with a combined logarithm of odds ratio (LOD) score of 14 at the D6Mit55 marker (~45.9 cM). Abrg1 contributed approximately 35% of the genetic variance. The second locus, designated Abrg2, was mapped to chromosome 4 with an LOD score of 8.6 in 8-week-old male mice but an LOD score of only 2.0 in 8-week-old female mice at the D4Mit27 marker (~35 cM). Abrg2 contributed approximately 26% of the genetic variance. Epistasis between Abrg1 and Abrg2 was detected and accounted for approximately 12% of the genetic variance. The combination of these two QTL has major effects (>70%) on the regulation of plasma human apoB levels in the tested population.

In summary, we have identified two novel loci that have a major role in the regulation of plasma apoB levels and are likely to regulate the secretory pathway of apoB. The human orthologs for the Abrg loci are strong candidates for human disorders characterized by altered plasma apoB levels, such as FCHL and familial hypobetalipoproteinemia. — Ko, C., T-L. Lee, P. W. Lau, J. Li, B. T. Davis, E. Voyiaziakis, D. B. Allison, S. C. Chua, Jr., and L-S. Huang. Two novel quantitative trait loci on mouse chromosomes 6 and 4 independently and synergistically regulate plasma apoB levels. J. Lipid Res. 2001. 42: 844;–855.

Supplementary key words: genetics, mapping, apoB secretion, hypobetalipoproteinemia, familial combined hyperlipidemia, inbred mouse strains


  INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES

An elevated plasma apolipoprotein B (apoB) level is a strong predictor of atherosclerosis and coronary heart disease (1). ApoB is a key structural component of triglyceride-rich lipoproteins (2). ApoB exists in two forms, B-100 and B-48. Both are products of the same gene encoding a 14-kb mRNA and share the same amino-terminal 2,152 amino acids. ApoB-48 is derived by the introduction of a stop codon at residue 2153 of the B-100 message via an RNA-editing mechanism (3). In humans, apoB-100 and apoB-48 are required for the secretion of VLDL from the liver and chylomicrons from the intestine, respectively. ApoB-100 is also the ligand for the low density lipoprotein (LDL) receptor, which mediates cellular uptake of LDL particles, hydrolyzed products of very low density lipoproteins (VLDL). Plasma apoB levels, therefore, represent the balance between the secretion and clearance rates of apoB-containing lipoproteins. Mutations in the genes encoding either apoB or the LDL receptor result in altered plasma apoB and LDL cholesterol levels in patients with monogenic disorders such as familial hypobetalipoproteinemia (FHBL) (low plasma apoB), familial defective hyperapolipoproteinemia (high plasma apoB), and FH (high plasma apoB). These mutations, however, do not account for the wide variation of plasma apoB in the general population, nor do they explain other common lipoprotein disorders. Studies have shown that known mutations in the apoB gene account for only a small number of subjects with low plasma apoB levels (4). Kinetic studies have shown that decreased production of apoB-containing lipoproteins is responsible for the reduced plasma apoB levels in some subjects with non-apoB-linked hypobetalipoproteinemia (5). On the opposite end of the spectrum, only a few loci have been linked to either the elevation of plasma triglyceride and/or apoB levels in subjects with familial combined hyperlipidemia (FCHL), a disorder characterized by overproduction of apoB (6). This disorder is present in 1;–2% of the general population and in about 10;–20% of patients with coronary artery disease (CAD). The genetic basis of FCHL is heterogeneous, and mutations in the genes regulating hepatic apoB production represent plausible causes for the disorder.

Epidemiological and family linkage studies have suggested a genetic basis for the wide variations of plasma apoB levels in the general population (7) (8) (9) (10) (11). These studies also suggest that plasma apoB levels are controlled by major genes other than the apoB gene. Mouse genetics provides a powerful approach to identify quantitative trait loci (QTL) involved in complex diseases or traits. Using a human apoB transgenic (HuBTg) mouse model, we have identified three mouse strains suitable for genetic analysis of factors regulating hepatic apoB secretion and hence plasma apoB levels (12). By crossing various inbred mouse strains with a congenic HuBTg mouse strain of the C57BL/6 (B6) background, we have generated HuBTg mouse strains with varying plasma human apoB levels (60;–100 mg/dl), mimicking the variation of plasma apoB levels in human populations. Three mouse strains, 129/Sv, BALB/c, and C3H/HeJ, possess polymorphic alleles that, when crossed with the B6 HuBTg strain, lower plasma human apoB levels and apoB-100 secretion by 30;–40% in their F1 offspring. Our studies also suggested that differential hepatic apoB-100 secretion rates are likely to be due to post-transcriptional regulation because these mouse strains have similar hepatic human apoB mRNA levels and apoB RNA-editing activities (12). ApoB RNA-editing activity is active in the rodent liver (3), and differences in editing activity could alter the ratio of B-100 versus B-48 mRNA species and hence the synthesis and secretion of apoB-100-containing lipoproteins.

In the present article, we carried out genetic analyses in an attempt to identify factors that regulate plasma apoB levels via their effects on hepatic apoB-100 secretion rates. We chose the 129/Sv strain, a strain with low plasma apoB and hepatic apoB-100 secretion rates relative to the B6 strain, for genetic crosses with the congenic B6 HuBTg strain. From three independent crosses that generated more than 500 animals, we identified two QTL on chromosomes 6 and 4 that independently and synergistically regulate plasma apoB levels.


  MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES

Mouse crosses
A congenic HuBTg mouse strain of the C57BL/6J (B6) background was generated as described previously (12). The B6 HuBTg mouse strain has been maintained by continuously backcrossing with wild-type B6 mice purchased from the Jackson Laboratory (Bar Harbor, ME). To maintain hemizygosity of the human apoB transgene in all the animals used in this study, crosses were carried out between nontransgenic mice and hemizygous HuBTg mice. F1 HuBTg mice (129 x B6) were generated by crossing male B6 HuBTg mice with female 129/Sv (129) mice. HuBTg offspring were identified by polymerase chain reaction (PCR), using primers specific for the human apoB gene (13). The inbred 129 strain used has been maintained in our laboratory as described (12). For genetic analysis, F2 HuBTg mice (n = 173) were generated by intercrosses between transgenic and nontransgenic F1 mice. In addition, two types of backcrossed offspring were also generated for mapping analysis. By convention, offspring derived from backcrosses are designated N(I+1), where I denotes the number of backcrossed generations. The first backcross was carried out by crossing F1 HuBTg mice with the B6 parental strain. Offspring (n = 196) derived from this cross are designated N2/B6. The second backcross was carried out by crossing F1 HuBTg mice with the 129 parental strain. Offspring (n = 203) derived from this cross are designated N2/129. To generate congenic 129 HuBTg mice for phenotype analysis, 129 x B6 F1 HuBTg mice were continuously backcrossed to the 129 strain. Conventionally, N10 animals (derived after nine backcross generations) are considered congenic. We currently have N6 animals (derived after five backcross generations), which were phenotyped to compare plasma apoB levels among strains.

Localization and preservation of the human apoB transgene in HuBTg mice
The HuBTg mice were originally generated in the FVB/NJ (FVB) strain (13). The human apoB transgene is therefore carried on a piece of donor FVB chromosome, which was transferred onto the B6 background by backcrossing. In our initial genome scan of the F2 mice, we detected the presence of FVB alleles on chromosome 10. We subsequently localized the transgene by scanning two sets of animals, using chromosome 10 markers. The markers used were as follows: D10Mit206 (4.4 cM), D10Mit213 (6.6 cM), D10Mit282 (7.7 cM), D10Mit283 (8.7 cM), D10Mit17 (8.7 cM), D10Mit17 (12.0 cM), D10Mit214 (15.3 cM), D10Mit3 (16.4 cM), D10Mit284 (17.5 cM), D10Mit61 (24.0 cM), and D10Mit230 (47.0 cM). Nontransgenic (n = 10) and 129 x B6 F1 transgenic (n = 10) mice were scanned for chromosome 10 markers. HuBTg mice should have both FVB and B6 alleles, whereas nontransgenic mice should have both B6 and 129 alleles where the transgene is located. B6 congenic HuBTg mice (>N10, n = 110) were also scanned for these markers. These data showed an absolute linkage of the human apoB transgene with D10Mit214, D10Mit3, and D10Mit284 markers, which span approximately 2.2 cM of the genome. Subsequently, all transgenic animals were screened with D10Mit3 and D10Mit284 to ensure the preservation of the human apoB transgene in hemizygous animals.

Phenotype analysis
Mice were maintained on a 12-h light/dark cycle (light cycle: 7 AM;–7 PM), fed a rodent chow (PicoLab Rodent Chow, No. 5001; Purina Lab Chows, St. Louis, MO), and had free access to water. Mice were fasted for 4 h (10 AM;–2 PM) and retro-orbitally bled at 8 and 12 weeks of age. For plasma human apoB levels, an antibody specific to human apoB was used in immunoturbidimetric assays as described previously (12).

A genome-wide scan of selective male F2 animals with extreme apoB phenotypes
Twenty male F2 animals with plasma human apoB levels lower than 60 mg/dl (n = 10, the lowest 13%) or higher than 90 mg/dl (n = 10, the highest 13%) at both 8 and 12 weeks of age were selected from a total of 79 male F2 HuBTg mice. A total of 70 microsatellite markers at approximately 30-cM spacing was used to genotype these 20 animals with extreme phenotypes. The microsatellite markers used are shown in Table 1. The distance of the markers from the centromere of each chromosome is based on data posted by the Whitehead Institute (http://www.genome. wi.mit.edu/). For genotyping analysis, PCR primers were purchased from Research Genetics (Huntsville, AL) or custom synthesized by Life Technologies (Rockville, MD).


 
View this table:
[in this window]
[in a new window]
  		Table 1. Microsatellite markers used in the genome scan

Logarithm of odds (LOD) score and statistical analysis
Markers on chromosomes 6 and 4 were used to define the intervals containing QTL associated with plasma human apoB levels in F2 and N2 HuBTg mice. The markers used were as follows: D6Mit77, D6Mit9, D6Mit178, D6Mit64, D6Mit333, D6Mit339, D6Mit199, and D6Mit15 (chromosome 6); D4Mit99, D4Mit95, D4Mit108, D4Mit89, D4Mit275, D4Mit27, D4Mit145, D4Mit12, and D4Mit127 (chromosome 4). LOD scores between genetic markers and plasma human apoB levels were computed by Map Manager QT (version QTb28) (14). The association between plasma human apoB levels among genotypes in F2 and N2 animals was assessed by one-way ANOVA and Student's t-test, respectively. To estimate the contribution of each QTL to genetic variance, a mixed model of ANOVA was conducted with the statistical package SPSS (v 9.0; SPSS, Chicago, IL).

Riboprobe preparation and RNase protection assays
Total cellular RNA was isolated from the liver by the guanidinium thiocyanate method (15). For RNase protection assays, 20-µg samples of total cellular RNA from 12-week-old male B6 and 129 mice (n = 5 per group) were used. RNA probes for mouse Apobec-1 and Sec13r were generated by amplification of the target gene from liver RNA (male B6 mice) by RT-PCR. PCR primers used and the size of amplified products for each probe are listed below. Sec13r: sense, 5'-TGG TCG AGT GTT TAT TTG GA-3'; antisense, 5'-GAT GCA CAC CCA CTG TCC GTC-3' (191 bp); Apobec-1: sense, 5'-GGT ACT AGT TAC ACA CGA GGC CC-3'; antisense, 5'-AAG GTC TCA CTG TGT AAT TCA-3' (203 bp). PCR products were cloned into a PCRII vector, using a TA cloning kit obtained from InVitrogen (Carlsbad, CA). DNA sequences of each clone were verified by DNA sequencing, using an ABI 377 automatic DNA sequencer (PE Biosystems, Foster City, CA).

Antisense probes were synthesized with an in vitro transcription kit obtained from Promega (Madison, WI) and [{alpha}-32P]CTP (800 Ci/mmol). A HinfI fragment (100 bp) including the T3 promoter was isolated from a mouse ß-actin clone (Ambion, Austin, TX) and used as a reference RNA in RNase protection assays. RNase protection assays were carried out as described previously (16). Protected RNA fragments were separated in 5% polyacrylamide-7 M urea gels. Dried gels were exposed to X-ray films for 1;–2 days at -80°C. For quantitation, protected RNA fragments were cut and counted in a liquid scintillation counter.

DNA sequence analysis of Sec13r cDNA
Mouse Sec13r cDNAs (953 bp) were amplified from both B6 and 129 liver RNAs, using the Access RT-PCR system kit (Promega). PCR primers used were as follows: sense, 5'-ATG GTG TCA GTA ATG AAC ACT GTG GAC A-3'; and antisense, 5'-TGG CCC TCT GTG ATG GAG GCT GAC ACA GAA C-3'. PCR products were directly sequenced with internal sequences for the Sec13r cDNA, which were obtained from the mouse Expressed Sequence Tag (EST) database (accession numbers AA870418 and AA108956). PCR products were also cloned into a PCRII vector, using a TA cloning kit obtained from InVitrogen. DNA sequences of each clone were also sequenced with both universal primers and Sec13r-specific primers. Sequence analyses were carried out with an ABI 377 automatic DNA sequencer.


  RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES

Genetic control of apoB in two inbred mouse strains
To identify genetic factors regulating plasma apoB levels, F2 mice hemizygous for the human apoB transgene were generated from intercrosses between 129 x B6 F1 HuBTg and nontransgenic F1 littermates. A total of 159 F2 HuBTg mice were generated and their plasma human apoB levels were measured at 8 and 12 weeks of age. In addition, plasma human apoB levels of congenic B6, 129 x B6 F1 and congenic 129 HuBTg mice were assessed. Plasma human apoB levels of 8-week-old animals are shown in Fig 1. Mean plasma apoB levels of both 8- and 12-week-old animals from each cross are shown in Table 2.



View larger version (16K):
[in this window]
[in a new window]
  		Figure 1. Distribution of plasma human apoB levels in 8-week-old HuBTg mouse strains. A: Male HuBTg mice. B: Female HuBTg mice. Animals were bled at 8 weeks of age after a 4-h fast. B6 HuBTg mice (filled squares) were the parental strain with high plasma human apoB levels. F1 mice (open squares) were generated from crosses between male B6 HuBTg mice and females of the wild-type 129 inbred strain. 129 HuBTg mice (filled inverted triangles) were generated by backcrossing F1 HuBTg mice to wild-type 129 inbred mice. F2 mice (filled triangles) were generated from crosses between F1 transgenic and nontransgenic animals. Mean plasma apoB levels in each group of animals are shown as horizontal lines in each column and their values are shown in Table 2.


 
View this table:
[in this window]
[in a new window]
  		Table 2. Mean plasma apoB levels in HuBTg mice from various crosses

In 8-week-old male mice, mean plasma human apoB levels (66 ± 10 mg/dl) of the F1 HuBTg mice were significantly lower than that of the B6 parental strain (95 ± 16 mg/dl, P < 0.0001). They were not significantly different from that of the 129 parental strain (72 ± 9 mg/dl, P = 0.07). Mean plasma human apoB levels of male F2 HuBTg mice (72 ± 17 mg/dl) were comparable to that of the 129 parental strain, but significantly lower (P < 0.0001) than that of B6 HuBTg mice. Plasma apoB levels of the F2 HuBTg mice were slightly higher (P = 0.01) than that of the F1 HuBTg mice. Similar trends were observed in female mice as well as 12-week-old animals of both genders (Table 2). We also noted that mean plasma apoB levels in male mice decreased as these animals aged (8 vs. 12 weeks: B6, P < 0.0001; F1, P = 0.001; 129, P = 0.002). The same phenomenon, however, was not observed in female mice. Overall, these data show that plasma human apoB levels in these animals appear to be controlled by a major gene or genes with a dominant mode of inheritance.

Two chromosomal regions associated with apoB phenotype
To favor the identification of genes with major effects on plasma human apoB levels, a selective genotyping strategy was applied to animals with extreme phenotypes. From a total of 79 male F2 HuBTg mice, animals with plasma human apoB levels in the highest (>90 mg/dl, n = 10) and the lowest (<60 mg/dl, n = 10) 13% of the F2 sample were subjected to a genome-wide scan, using 70 microsatellite markers spanning the mouse genome. These markers have an average spacing of 30 cM in each chromosome. The genome scan revealed possible linkages of the apoB phenotype with markers on chromosomes 6 and 4. As shown in Table 3, a suggestion of linkage ({chi}2 = 8.8, P = 0.01) was detected between the apoB phenotype and the D6Mit55 marker (~45.9 cM) on chromosome 6 (Table 3). The genome scan also revealed a possible linkage of plasma apoB phenotype with D4Mit204 (~61.2 cM) on chromosome 4 (Table 3). Neither marker was linked to the human apoB transgene on chromosome 10.


 
View this table:
[in this window]
[in a new window]
  		Table 3. {chi}2 analysis of chromosome 6 and 4 markers in selected male F2 HuBTg mice

To establish linkage between plasma human apoB levels and the D6Mit55 and D4Mit204 markers, the remaining male F2 and female F2 samples were also genotyped. Linkage was assessed by likelihood ratio statistics, using Map Manager QT (14). The results are shown in Table 4. The LOD scores for D6Mit55 were 2.6 and 3.0 for 8-week-old male and female mice, respectively. The combined LOD score was 5.6, indicating a significant linkage between D6Mit55 and plasma human apoB levels in 8-week-old F2 mice. Linkage between D6Mit55 and plasma human apoB levels (combined LOD = 3.7) was also significant in 12-week-old male and female F2 mice. Table 4 also shows a suggestive linkage (LOD = 2.6) of the D4Mit204 marker with plasma human apoB levels in 8-week-old male F2 mice. There was no apparent linkage of the D4Mit204 marker to plasma human apoB levels in female F2 mice.


 
View this table:
[in this window]
[in a new window]
  		Table 4. LOD score analysis of chromosome 6 and 4 markers with plasma human apoB in animals from three independent crosses

Confirmation of the linkages between plasma human apoB levels and chromosomes 6 and 4 markers in two backcrosses
To independently confirm the linkages between chromosomes 6 and 4 markers with plasma human apoB levels, two additional crosses were generated. Male F1 HuBTg mice were backcrossed to female mice of the B6 or 129 strain to generate N2/B6 and N2/129 mice, respectively. Plasma human apoB levels from 8-week-old N2 mice are shown in Fig 2. Mean plasma apoB levels from both 8- and 12-week-old N2 animals are shown in Table 2. As with the F2 cross, the distribution of plasma apoB levels from N2 mice also suggested a dominant mode of inheritance of gene(s) involved in the regulation of plasma apoB levels.



View larger version (18K):
[in this window]
[in a new window]
  		Figure 2. Distribution of plasma human apoB levels in 8-week-old backcrossed animals. A: Male HuBTg mice. B: Female HuBTg mice. Animals were bled at 8 weeks of age after a 4-h fast. N2/B6 mice (solid circles) were generated by crossing male F1 HuBTg mice to female wild-type B6 mice. N2/129 mice (open circles) were generated by crossing male F1 HuBTg mice to female wild-type 129 mice. Mean plasma apoB levels in each group of animals are shown as vertical lines in each column and their values are shown in Table 2. The position of arrows indicates the mean plasma apoB levels for B6, 129, or F1 animals.

All N2 mice were genotyped with D6Mit55 and D4Mit204. Linkages between these two markers and plasma human apoB levels were assessed. The results are shown in Table 4. In male N2/B6 mice, D6Mit55 was significantly linked to plasma human apoB levels in 12-week-old animals (LOD = 4.0) but not in 8-week-old animals (LOD = 1.4). This age effect was similarly observed in female N2/B6 mice. In both male and female N2/129 mice, significant (or highly suggestive) linkage was observed in both age groups. Table 4 also shows that gender, age, and genetic background all have an effect on the degree of linkage between D4Mit204 and plasma human apoB levels in N2 mice. As observed for the female F2 mice, D4Mit204 failed to show a linkage in either female N2/B6 or female N2/129 mice. In male mice, linkage was suggestive in 12-week-old N2/B6 mice (LOD = 2.3) and in 8-week-old male N2/129 mice (LOD = 2.0).

The combined data from all three crosses showed a significant linkage of plasma human apoB levels to D6Mit55 in both male (LOD = 7.2;–9.5, depending on age) and female mice (LOD = 6.6;–6.8). These data also showed a significant linkage to D4Mit204 in male mice (LOD = 5.0;–5.4), but not in female mice (LOD = 0.8;–1.8). Overall, these data confirmed that in addition to the F2 cross, both D6Mit55 and D4Mit204 markers were linked to plasma human apoB levels in the N2 crosses.

Definition of the chromosome 6 and 4 QTL intervals
To define the intervals on chromosomes 6 and 4 that were associated with plasma human apoB levels, markers flanking either D6Mit55 or D4Mit204 were used to genotype male animals from F2 and N2 crosses. LOD scores were computed for each marker on chromosomes 6 and 4 and plotted in Fig 3 and Fig 4, respectively. As shown in Fig 3C, the combined data from all three crosses showed that the LOD score peaked at D6Mit55. These data suggested that the QTL is likely to be near D6Mit55 and within an interval of ~28 cM (40.4;–57.9 cM), spanning D6Mit64 and D6Mit199 on chromosome 6. However, when each cross was considered separately ( Fig 3A and Fig B), a smaller interval of ~9 cM (40.4;–49.2 cM) encompassing D6Mit64 and D6Mit333 could be defined.



View larger version (17K):
[in this window]
[in a new window]
  		Figure 3. LOD score analysis for the Abrg1 locus on chromosome 6. Male HuBTg mice from F2, N2/B6, and N2/129 crosses were genotyped for chromosome 6 microsatellite markers. The map distance (centimorgans) to the centromere for each marker is plotted on the x axis and is indicated in parentheses after each marker. The LOD score for each marker is shown on the y axis. A: Plasma apoB levels from 8-week-old male animals were used for LOD score analysis. B: Plasma apoB levels from 12-week-old male animals were used for LOD score analysis. C: LOD scores from all three crosses of the same age group (either 8 or 12 weeks) were combined.



View larger version (19K):
[in this window]
[in a new window]
  		Figure 4. LOD score analysis for the Abrg2 locus on chromosome 4. Male HuBTg mice from F2, N2/B6, and N2/129 crosses were genotyped for chromosome 4 microsatellite markers. The map distance (centimorgans) to the centromere for each marker is plotted on the x axis and is indicated in parentheses after each marker. The LOD score for each marker is shown on the y axis. A: Plasma apoB levels from 8-week-old male animals were used for LOD score analysis. B: Plasma apoB levels from 12-week-old male animals were used for LOD score analysis. C: LOD scores from all three crosses of the same age group (either 8 or 12 weeks) were combined.

In Fig 4C, the combined data showed that the LOD score peaked around D4Mit275 (30.6 cM) and D4Mit27 (35 cM). Because previous analyses (Table 4) failed to show linkage of D4Mit204 with plasma apoB levels in females, we genotyped female HuBTg mice from all three crosses (n = 260) with the D4Mit27 marker. The results showed a suggestive linkage between D4Mit27 and plasma human apoB levels in female mice (8 weeks old, LOD = 2.0; 12 weeks old, LOD = 2.6) (data not plotted). Overall, this second QTL likely exists near the D4Mit275 and D4Mit27 markers and within an interval of ~30 cM (30.6;–61.2 cM) spanning the D4Mit275 and D4Mit204 markers. However, it did not appear to be as important, if important at all, in female mice.

The products of the genes residing in the aforementioned intervals likely regulate plasma apoB levels. We suggest the provisional designation, ApoB regulator genes, for these newly identified loci. The QTL at the chromosome 6 interval will be designated ApoB regulator gene 1 (Abrg1), and the QTL at the chromosome 4 interval will be designated ApoB regulator gene 2 (Abrg2). The LOD score for Abrg1 is 14 in 8-week-old animals, using D6Mit55 as the marker. Using D4Mit27 as the marker, the LOD score for Abrg2 is 8.6 in 8-week-old male mice and 2.0 for 8-week-old female mice.

Relationship of plasma human apoB levels with Abrg1 and Abrg2 loci
To assess the relationship between Abrg alleles and plasma human apoB levels, animals were grouped on the basis of genotypes, using D6Mit55 and D4Mit27 for Abrg1 and Abrg2, respectively. The results for Abrg1 and Abrg2 are shown in Table 5 and Table 6, respectively. As shown at the top of Table 5, male 8-week-old F2 mice heterozygous for Abrg1 had plasma human apoB levels (70 ± 15 mg/dl) between those of homozygotes for either the B6 (84 ± 12 mg/dl) or the 129 (65 ± 22 mg/dl) allele. Similar trends were observed in 12-week-old male and female F2 mice as well as in N2/B6 and N2/129 mice of both genders. These data suggest a codominant mode of inheritance for the B6 and the 129 alleles of the Abrg1 QTL.


 
View this table:
[in this window]
[in a new window]
  		Table 5. Association of Abrg1 with plasma human apoB levels


 
View this table:
[in this window]
[in a new window]
  		Table 6. Association of Abrg2 with plasma human apoB levels

The effect of Abrg2 on plasma apoB levels varied dependent on age, genetic background, and gender (Table 6). In male N2/B6 mice, heterozygotes had significantly lower plasma apoB levels than did homozygotes for the B6 allele. In male N2/129 mice, heterozygotes had significantly higher plasma apoB levels compared with homozygotes for the 129 allele. These results suggest that the B6 and the 129 alleles of Abrg2 are likely to be codominantly expressed in male N2 mice. However, in male F2 mice, heterozygotes for Abrg2 had similar plasma apoB levels compared with those of homozygotes for the 129 allele. These data suggested that the dominance of these alleles varied by genetic background.

Our previous data analysis suggested a possible linkage (LOD = 2.0;–2.6) of Abrg2 to plasma human apoB levels in female mice. Table 6 confirms that there was a significant association between the two in female N2/B6 mice. Female N2/B6 mice heterozygous for Abrg2 had significantly lower plasma human apoB levels than homozygotes for the B6 allele. However, female N2/129 mice heterozygous for Abrg2 had plasma human apoB levels similar to those of animals homozygous for the 129 allele mice. These data suggest that, in female mice, the B6 allele of the Abrg2 QTL may be recessive relative to the 129 allele, and its effect on plasma human apoB levels was realized only in the context of the B6 background.

Epistasis between the Abrg1 and Abrg2 loci
To assess potential interactions between Abrg1 and Abrg2, animals were grouped according to genotype, using D6Mit55 and D4Mit27 as markers for the Abrg1 and Abrg2 QTL, respectively. Results from 8-week data and 12-week data were similar. Only 8-week data are shown in Table 7. In male F2 mice (Table 7, top), plasma human apoB levels were lower when animals had at least one 129 allele in both loci. For example, animals doubly heterozygous for both loci had plasma human apoB levels (67 ± 13 mg/dl) that were significantly lower than those in heterozygotes for the Abrg1 locus (83 ± 13 mg/dl) or heterozygotes for the Abrg2 locus (83 ± 10 mg/dl). These results demonstrated the existence of epistasis between Abrg1 and Abrg2 in the determination of plasma human apoB levels. Similar interactions were observed in male N2 mice of either the B6 (Table 7, middle) or the 129 (Table 7, bottom) background. An interaction between the two loci was also observed in female mice (Table 7). However, unlike in male mice, homozygosity of the 129 allele for Abrg2 had no additive effect on lowering plasma human apoB levels in female F2 and N2/129 mice. This was likely due to the apparent dominant nature of the 129 allele in female mice as described earlier (Table 6).


 
View this table:
[in this window]
[in a new window]
  		Table 7. Epistasis between the Abrg1 and Abrg2 loci

We also assessed these data from 8-week-old animals by using a mixed model ANOVA. These analyses showed that approximately 30% of the total variance of human apoB levels in 8-week-old F2 mice was contributed by genetic variance. The Abrg1 and Abrg2 loci contributed approximately 35% and 26%, respectively, to genetic variance. The interaction between the two loci accounted for approximately 12% of genetic variance. Thus, Abrg1, Abrg2, and the epistasis between the two loci contributed to a majority (73%) of the genetic variation seen in plasma human apoB levels in crosses derived from the B6 and 129 strains.

Analysis of positional candidate genes
Possible positional candidates for Abrg1 and Abrg2 were considered, using the Mouse Genome Informatics database (http://www.informatics.jax.org/). On the basis of the known functions of the genes, we identified three positional candidates on chromosome 6 that could potentially regulate apoB secretion. These genes, Sec13r, Pparg, and Apobec1, are localized between D6Mit55 and D6Mit333. Both the human homologs SEC13R (3p25-24) and PPARG (3p25) were mapped to chromosome 3, whereas the human ortholog APOBEC-1 was localized to chromosome 12p31 (17) (18) (19). Segments of human chromosomes 9 and 1 were found to be syntenic to the chromosome 4 interval containing Abrg2. However, no apparent positional candidates were detected. Detailed information on syntenic human chromosome segments can be found at the National Center of Biotechnology Information (http://www.ncbi. nlm.nih.gov/Homology/).

We compared the expression levels of the Sec13r and Apobec1 genes between B6 and 129 mice by RNase protection assays. Fig 5A shows that there was no difference in hepatic Sec13r mRNA levels between B6 and 129 mice (100 ± 24% vs. 100 ± 11%). Fig 5B shows that there was no differences in the hepatic Apobec-1 mRNA levels between B6 and 129 mice (100 ± 22% vs. 110 ± 18%). We also attempted to determine potential structural variations of the Sec13r gene between the two strains. Sec13r cDNAs were amplified from liver RNA samples and sequenced. These analyses, however, did not reveal any variation in the coding regions of the gene between the two strains.



View larger version (67K):
[in this window]
[in a new window]
  		Figure 5. Hepatic expression of positional candidate genes in B6 and 129 mice. Total liver cellular RNAs from B6 (lanes 1;–5) and 129 (lanes 6;–10) mice were subjected to RNase protection assays. Autoradiograms obtained from the assays are shown. Sec13r (A) and Apobec-1 (B) riboprobes were used along with a ß-actin riboprobe as an internal control. Protected fragments were cut and counted. After normalization, no differences in hepatic Sec13r (100 ± 24% vs. 100 ± 11%) and Apobec-1 (B6 vs. 129 = 100 ± 22% vs. 110 ± 18%) mRNA levels were found between the two mouse strains.


  DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES

Plasma apoB levels are regulated by the secretion and clearance of apoB-containing lipoproteins. We have previously demonstrated the suitability of specific HuBTg mouse strains for the identification of genetic factors involved in the regulation of plasma apoB levels. In these strains, plasma human apoB levels can be used as a surrogate marker for hepatic apoB-100 secretion (12). In this article, we present the genetic analysis of F2 HuBTg animals and animals backcrossed to either the B6 (N2/B6) or the 129 (N2/129) strain. Genetic variance accounted for approximately 30;–40% of the total plasma apoB variance in mature animals at 8 and 12 weeks of age. Our results revealed two novel loci, designated ApoB regulator gene (Abrg), which were associated with plasma human apoB levels in the HuBTg mice. The first locus, Abrg1, was mapped to chromosome 6 in both male and female mice with a combined LOD score of 14;–16 (depending on age) at the D6Mit55 marker (~45.9 cM). Abrg1 contributed to approximately 35% of genetic variance. The second locus, Abrg2, was mapped to chromosome 4 with an LOD score of 6.3;–8.6 in male mice but an LOD score of only 2.0;–2.6 in female mice at the D4Mit27 marker (~35 cM). Abrg2 contributed to approximately 26% of genetic variance. Epistasis between Abrg1 and Abrg2 was detected and accounted for approximately 12% of genetic variance. We have, therefore, identified two QTL, which together have major effects (> 70%) on the regulation of plasma human apoB levels in the tested population.

Using a HuBTg mouse model, we have, for the first time, successfully identified two novel loci that are involved in the regulation of plasma apoB levels. These two loci were not linked to the endogenous mouse apoB gene (chromosome 12) (20) or the human apoB transgene (chromosome 10). As described in Materials and Methods, the human apoB transgene (with its native promoter) was inserted into the proximal end (15.3;–17.5 cM) of chromosome 10 and was maintained in hemizygosity in tested animals by intercrossing hemizygous transgenic animals and nontransgenic animals in all crosses used in this article. We previously noted that strain differences in plasma apoB-100 levels between B6 and 129 are parallel between mouse and human apoB-100 proteins and that the plasma human apoB level is a suitable marker for the hepatic apoB-100 secretion rate in these animals (12). These data implied that the cellular machinery regulating mouse apoB secretion is also effective on the transgenic human apoB proteins.

In this article, we showed that in 8-week-old male mice, plasma human apoB levels of the congenic B6 HuBTg mouse strain (~95 mg/dl) were significantly higher than those of the congenic 129 HuBTg mouse strain (~72 mg/dl). Male F1 HuBTg mice had slightly lower, but not statistically significant, plasma apoB levels (~66 mg/dl) compared with the 129 HuBTg mice. The two Abrg loci and their interaction may explain a majority of the differences in plasma apoB levels between the two mouse strains. The B6 and 129 alleles of the Abrg1 locus appeared to affect plasma apoB levels codominantly in both male and female mice. Whereas both the B6 and 129 alleles of the Abrg2 locus affected plasma human apoB levels codominantly in male mice, the 129 allele appeared to be dominant in female mice. An additive effect on plasma apoB levels was also observed when animals were heterozygous for both alleles. Despite the codominant nature of the Abrg loci in male mice, mean plasma apoB levels in the F1 HuBTg mice were similar to those of 129 HuBTg mice. These data suggest the involvement of other, unidentified Abrg loci in the regulation of plasma apoB levels in these mice. This is in agreement with the fact that approximately 30% of genetic variance of apoB was not accounted for by the Abrg1 and Abrg2 loci.

Using male animals from F2 and N2 crosses, we defined the intervals for the two Abrg loci. The chromosome 4 interval for Abrg2 was defined by two markers spanning approximately 30 cM (D4Mit275;–D4Mit204: 30.6;–61.2 cM). Abrg2 is likely to be located near the D4Mit27 marker (35 cM), because this marker had the highest LOD score. No apparent positional candidates for Abrg2 were detected in or near the defined interval. The Abrg1 locus is likely to be near or within an interval of approximately 9 cM defined by D6Mit64 (40.4 cM) and D6Mit333 (49.2 cM). The LOD score of the interval peaked at the D6Mit55 marker (45.9 cM). Three positional candidates were detected that could potentially regulate the assembly and secretion of apoB lipoproteins. These genes are Sec13r, Pparg, and Apobec1, which mapped between the D6Mit55 (45.9 cM) and the D6Mit333 (49.2 cM) markers. Both the human homologs SEC13R (3p25-24) and PPARG (3p25) were mapped to chromosome 3, whereas the human ortholog APOBEC-1 was localized to chromosome 12p31 (17) (18) (19).

Mouse Sec13r (17) encodes a protein homologous to yeast secretory pathway protein 13 (Sec13). Sec13 participates in vesicular transport between the endoplasmic reticulum and the Golgi apparatus (21). Although specialized vesicles have not been directly implicated in intracellular apoB transport, they are plausible regulators of the apoB secretory pathway (22) (23). Our data showed that hepatic Sec13r mRNA levels were similar between B6 and 129 strains. No structural variations in the coding region of the gene were found between the two strains. However, the candidacy of Sec13r for Abrg1 remains open because the possibility that variations in the 5' and/or 3' untranslated regions of the Sec13r mRNA exists, which may affect the efficiency of protein translation, resulting in variations in protein levels and/or functions. The second candidate, Pparg, encodes a protein, peroxisome proliferator-activated receptor {gamma} (PPAR{gamma}), which is predominantly expressed in adipose tissue and plays an important role in adipogenesis (24). Genetic variations in human PPARG have been associated with body mass index, insulin sensitivity, and severe obesity (25) (26) (27) (28). Strain differences in PPAR{gamma} may lead to differential adiposity which, in turn, may affect plasma free fatty acid levels and alter the output of apoB-containing lipoproteins from the liver. However, mice heterozygous for PPAR{gamma} deficiency have no significant differences in body weights or basal plasma free fatty acid levels compared with wild-type mice (29). Comparison studies to determine the candidacy of Pparg for Abrg1 are currently underway. Finally, the third candidate, Apobec1, encodes an enzyme involved in the editing of apoB mRNA (30). Strain differences in RNA-editing activity could alter plasma apoB-100 levels by altering the ratio of apoB-100 and apoB-48 mRNAs. However, we found no differences in the Apobec-1 mRNA between B6 and 129 mice. In a previous study, we tested the activity of apoB RNA editing in both the B6 and 129 x B6 F1 mouse strains (12). Similar amounts of mouse and human apoB mRNA were shown to be edited in the two strains. Taken together, these data make Apobec1 a less likely candidate for the Abrg1 gene.

The identification of novel loci that regulate plasma apoB levels may allow for the identification of candidate genes involved in human disease in which plasma apoB levels are altered. Family linkage studies have shown that the apoB gene is not linked to the low apoB phenotype in certain forms of FHBL (31) (32). A study has localized a novel FHBL susceptibility locus to chromosome 3p21.1-22 in a 40-member FHBL kindred (33). The secretion rates of apoB-containing lipoproteins were also shown to be reduced in these FHBL subjects (5). The chromosome segment (3p21.1-22) containing the human FHBL susceptibility locus is in close proximity to the human chromosome 3 segment (3p25-24) syntenic to the chromosome 6 interval containing the mouse Abrg1 locus. It is therefore tempting to speculate that the mouse Abrg1 locus is a possible mouse ortholog for the human FHBL susceptibility locus.

Mouse Abrg loci may also provide candidate genes for the form of FH that is not linked to the LDL receptor, apoB, or apoE genes (34) (35) (36). In addition, they are potential candidate genes for FCHL, which is characterized by the overproduction of apoB (37) (38) (39) (40) (41). Genome scan studies have identified several chromosome segments associated with altered plasma LDL cholesterol levels in FH (35) (36) and FCHL (42) (43) (44) (45). Neither Abrg1 nor Abrg2 mapped to any of the chromosome intervals linked to the disorders mentioned above. Therefore, the human orthologs for the newly identified mouse Abrg loci likely represent novel loci regulating plasma human apoB levels.

We have previously shown that the main determinant for plasma apoB levels in the two mouse strains (B6 and 129) is the hepatic apoB-100 secretion rate via posttranscriptional mechanisms (12). Therefore, mouse Abrg1 and Abrg2 are likely involved in the regulation of the assembly and secretion of apoB-containing lipoproteins. Posttranscriptional and particularly posttranslational regulation has been amply documented in cultured cells to play a crucial role in the regulation of apoB secretion (22) (46). Our studies have shown that Abrg1 and Abrg2 interact with each other to regulate plasma apoB levels in vivo. The identification of Abrg1 and Abrg2 may reveal new pathways involved in the regulation of apoB secretion, potentially providing novel sites for pharmacological therapy (47) in subjects with elevated plasma apoB and cholesterol levels and hence with a high risk for atherosclerosis and CAD.

Manuscript received November 27, 2000; and in revised form February 5, 2001 Abbreviations: Abrg, apolipoprotein B regulator gene; FHBL, familial hypobetalipoproteinemia; FCHL, familial hypobetalipoproteinemia; LOD, logarithm of odds ratio


  REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES

  1. Sniderman, A., Shapiro, S., Marpole, D., Skinner, B., Teng, B., Kwiterovich, P. O., Jr. 1980. Association of coronary atherosclerosis with hyperapobetalipoproteinemia. Proc. Natl. Acad. Sci. USA. 77:604-608. [increased protein but normal cholesterol levels in human plasma low density (beta) lipoproteins][Abstract/Free Full Text].

  2. Kane, J. P., and R. J. Havel. 1989. Disorders of the biogenesis and secretion of lipoproteins containing the B apolipoproteins. In The Metabolic Bases of Inherited Disease. C. R. Scriver, A. R. Beaudet, W. S. Sly, D. Valle, J. B. Stanbury, J. B. Wyngaarden, and D. S. Frederickson, editors. McGraw-Hill, New York. 1139;–1164.

  3. Davidson, N. O., Anant, S., MacGinnitie, A. J. 1995. Apolipoprotein B messenger RNA editing: insights into the molecular regulation of post-transcriptional cytidine deamination. Curr. Opin. Lipidol. 6:70-74[Medline].

  4. Wu, J., Kim, J., Li, Q., Kwok, P. Y., Cole, T. G., Cefalu, B., Averna, M., Schonfeld, G. 1999. Known mutations of apoB account for only a small minority of hypobetalipoproteinemia. J. Lipid Res. 40:955-959[Abstract/Free Full Text].

  5. Elias, N., Patterson, B. W., Schonfeld, G. 2000. In vivo metabolism of ApoB, ApoA-I, and VLDL triglycerides in a form of hypobetalipoproteinemia not linked to the ApoB gene. Arterioscler. Thromb. Vasc. Biol. 20:1309-1315[Abstract/Free Full Text].

  6. Aouizerat, B. E., Allayee, H., Bodnar, J., Krass, K. L., Peltonen, L., de Bruin, T. W., Rotter, J. I., Lusis, A. J. 1999. Novel genes for familial combined hyperlipidemia. Curr. Opin. Lipidol. 10:113-122[Medline].

  7. Beaty, T. H., Kwiterovich, P. O., Jr., Khoury, M. J., White, S., Bachorik, P. S., Smith, H. H., Teng, B., Sniderman, A. 1986. Genetic analysis of plasma sitosterol, apoprotein B, and lipoproteins in a large Amish pedigree with sitosterolemia. Am. J. Hum. Genet. 38:492-504[Medline].

  8. Hasstedt, S. J., Wu, L., Williams, R. R. 1987. Major locus inheritance of apolipoprotein B in Utah pedigrees. Genet. Epidemiol. 4:67-76[Medline].

  9. Amos, C. I., Elston, R. C., Srinivasan, S. R., Wilson, A. F., Cresanta, J. L., Ward, L. J., Berenson, G. S. 1987. Linkage and segregation analyses of apolipoproteins A1 and B, and lipoprotein cholesterol levels in a large pedigree with excess coronary heart disease: the Bogalusa Heart Study. Genet. Epidemiol. 4:115-128[Medline].

  10. Pairitz, G., Davignon, J., Mailloux, H., Sing, C. F. 1988. Sources of interindividual variation in the quantitative levels of apolipoprotein B in pedigrees ascertained through a lipid clinic. Am. J. Hum. Genet. 43:311-321[Medline].

  11. Tiret, L., Steinmetz, J., Herbeth, B., Visvikis, S., Rakotovao, R., Ducimetiere, P., Cambien, F. 1990. Familial resemblance of plasma apolipoprotein B: the Nancy Study. Genet. Epidemiol. 7:187-197[Medline].

  12. Voyiaziakis, E., Ko, C., O'Rourke, S. M., Huang, L. S. 1999. Genetic control of hepatic apoB-100 secretion in human apoB transgenic mouse strains. J. Lipid Res. 40:2004-2012[Abstract/Free Full Text].

  13. Callow, M. J., Stoltzfus, L. J., Lawn, R. M., Rubin, E. M. 1994. Expression of human apolipoprotein B and assembly of lipoprotein(a) in transgenic mice. Proc. Natl. Acad. Sci. USA. 91:2130-2134[Abstract/Free Full Text].

  14. Manly, K. F., Olson, J. M. 1999. Overview of QTL mapping software and introduction to map manager QT. Mamm. Genome. 10:327-334[Medline].

  15. Chomczynski, P., Sacchi, N. 1987. Single-step method of RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction. Anal. Biochem. 162:156-159[Medline].

  16. Jiang, X. C., Masucci-Magoulas, L., Mar, J., Lin, M., Walsh, A., Breslow, J. L., Tall, A. 1993. Down-regulation of mRNA for the low density lipoprotein receptor in transgenic mice containing the gene for human cholesteryl ester transfer protein. Mechanism to explain accumulation of lipoprotein B particles. J. Biol. Chem. 268:27406-27412[Abstract/Free Full Text].

  17. Swaroop, A., Yang-Feng, T. L., Liu, W., Gieser, L., Barrow, L. L., Chen, K. C., Agarwal, N., Meisler, M. H., Smith, D. I. 1994. Molecular characterization of a novel human gene, SEC13R, related to the yeast secretory pathway gene SEC13, and mapping to a conserved linkage group on human chromosome 3p24-p25 and mouse chromosome 6. Hum. Mol. Genet. 3:1281-1286[Abstract/Free Full Text].

  18. Greene, M. E., Blumberg, B., McBride, O. W., Yi, H. F., Kronquist, K., Kwan, K., Hsieh, L., Greene, G., Nimer, S. D. 1995. Isolation of the human peroxisome proliferator activated receptor gamma cDNA: expression in hematopoietic cells and chromosomal mapping. Gene Expr. 4:281-299[Medline].

  19. Espinosa, R., III, Funahashi, T., Hadjiagapiou, C., Le Beau, M. M., Davidson, N. O. 1994. Assignment of the gene encoding the human apolipoprotein B mRNA editing enzyme (APOBEC1) to chromosome 12p13.1. Genomics. 24:414-415[Medline].

  20. Lusis, A. J., Taylor, B. A., Quon, D., Zollman, S., LeBoeuf, R. C. 1987. Genetic factors controlling structure and expression of apolipoproteins B and E in mice. J. Biol. Chem. 262:7594-7604[Abstract/Free Full Text].

  21. Barlowe, C., Orci, L., Yeung, T., Hosobuchi, M., Hamamoto, S., Salama, N., Rexach, M. F., Ravazzola, M., Amherdt, M., Schekman, R. 1994. COPII: a membrane coat formed by Sec proteins that drive vesicle budding from the endoplasmic reticulum. Cell. 77:895-907[Medline].

  22. Yao, Z., Tran, K., McLeod, R. S. 1997. Intracellular degradation of newly synthesized apolipoprotein B. J. Lipid Res. 38:1937-1953[Abstract].

  23. Asp, L., Claesson, C., Boren, J., Olofsson, S. O. 2000. ADP-ribosylation factor 1 and its activation of phospholipase D are important for the assembly of very low density lipoproteins. J. Biol. Chem. 275:26285-26292[Abstract/Free Full Text].

  24. Brun, R. P., Kim, J. B., Hu, E., Spiegelman, B. M. 1997. Peroxisome proliferator-activated receptor gamma and the control of adipogenesis. Curr. Opin. Lipidol. 8:212-218[Medline].

  25. Yen, C. J., Beamer, B. A., Negri, C., Silver, K., Brown, K. A., Yarnall, D. P., Burns, D. K., Roth, J., Shuldiner, A. R. 1997. Molecular scanning of the human peroxisome proliferator activated receptor gamma (hPPAR gamma) gene in diabetic Caucasians: identification of a Pro12Ala PPAR gamma 2 missense mutation. Biochem. Biophys. Res. Commun. 241:270-274[Medline].

  26. Deeb, S. S., Fajas, L., Nemoto, M., Pihlajamaki, J., Mykkanen, L., Kuusisto, J., Laakso, M., Fujimoto, W., Auwerx, J. 1998. A Pro12Ala substitution in PPARgamma2 associated with decreased receptor activity, lower body mass index and improved insulin sensitivity. Nat. Genet. 20:284-287[Medline].

  27. Barroso, I., Gurnell, M., Crowley, V. E., Agostini, M., Schwabe, J. W., Soos, M. A., Maslen, G. L., Williams, T. D., Lewis, H., Schafer, A. J., Chatterjee, V. K., O'Rahilly, S. 1999. Dominant negative mutations in human PPARgamma associated with severe insulin resistance, diabetes mellitus and hypertension. Nature. 402:880-883. [see comments][Medline].

  28. Ristow, M., Muller-Wieland, D., Pfeiffer, A., Krone, W., Kahn, C. R. 1998. Obesity associated with a mutation in a genetic regulator of adipocyte differentiation. N. Engl. J. Med. 339:953-959[Abstract/Free Full Text].

  29. Miles, P. D., Barak, Y., He, W., Evans, R. M., Olefsky, J. M. 2000. Improved insulin-sensitivity in mice heterozygous for PPAR-gamma deficiency. J. Clin. Invest. 105:287-292[Medline].

  30. Davidson, N. O. 1993. Apolipoprotein B mRNA editing: a key controlling element targeting fats to proper tissue. Ann. Med. 25:539-543[Medline].

  31. Fazio, S., Sidoli, A., Vivenzio, A., Maietta, A., Giampaoli, S., Menotti, A., Antonini, R., Urbinati, G., Baralle, F. E., Ricci, G. 1991. A form of familial hypobetalipoproteinaemia not due to a mutation in the apolipoprotein B gene. J. Intern. Med. 229:41-47[Medline].

  32. Pulai, J. I., Neuman, R. J., Groenewegen, A. W., Wu, J., Schonfeld, G. 1998. Genetic heterogeneity in familial hypobetalipoproteinemia: linkage and non-linkage to the apoB gene in Caucasian families. Am. J. Med. Genet. 76:79-86[Medline].

  33. Yuan, B., Neuman, R., Duan, S. H., Weber, J. L., Kwok, P. Y., Saccone, N. L., Wu, J. S., Liu, K. Y., Schonfeld, G. 2000. Linkage of a gene for familial hypobetalipoproteinemia to chromosome 3p21.1-22. Am. J. Hum. Genet. 66:1699-1704[Medline].

  34. Haddad, L., Day, I. N., Hunt, S., Williams, R. R., Humphries, S. E., Hopkins, P. N. 1999. Evidence for a third genetic locus causing familial hypercholesterolemia. A non-LDLR, non-APOB kindred. J. Lipid Res. 40:1113-1122[Abstract/Free Full Text].

  35. Varret, M., Rabes, J. P., Saint-Jore, B., Cenarro, A., Marinoni, J. C., Civeira, F., Devillers, M., Krempf, M., Coulon, M., Thiart, R., Kotze, M. J., Schmidt, H., Buzzi, J. C., Kostner, G. M., Bertolini, S., Pocovi, M., Rosa, A., Farnier, M., Martinez, M., Junien, C., Boileau, C. 1999. A third major locus for autosomal dominant hypercholesterolemia maps to 1p34.1-p32. Am. J. Hum. Genet. 64:1378-1387[Medline].

  36. Hunt, S. C., Hopkins, P. N., Bulka, K., McDermott, M. T., Thorne, T. L., Wardell, B. B., Bowen, B. R., Ballinger, D. G., Skolnick, M. H., Samuels, M. E. 2000. Genetic localization to chromosome 1p32 of the third locus for familial hypercholesterolemia in a Utah kindred. Arterioscler. Thromb. Vasc. Biol. 20:1089-1093[Abstract/Free Full Text].

  37. Chait, A., Albers, J. J., Brunzell, J. D. 1980. Very low density lipoprotein overproduction in genetic forms of hypertriglyceridaemia. Eur. J. Clin. Invest. 10:17-22[Medline].

  38. Janus, E. D., Nicoll, A. M., Turner, P. R., Magill, P., Lewis, B. 1980. Kinetic bases of the primary hyperlipidaemias: studies of apolipoprotein B turnover in genetically defined subjects. Eur. J. Clin. Invest. 10:161-172[Medline].

  39. Arad, Y., Ramakrishnan, R., Ginsberg, H. N. 1990. Lovastatin therapy reduces low density lipoprotein apoB levels in subjects with combined hyperlipidemia by reducing the production of apoB-containing lipoproteins: implications for the pathophysiology of apoB production. J. Lipid Res. 31:567-582[Abstract].

  40. Cortner, J. A., Coates, P. M., Bennett, M. J., Cryer, D. R., Le, N. A. 1991. Familial combined hyperlipidaemia: use of stable isotopes to demonstrate overproduction of very low-density lipoprotein apolipoprotein B by the liver. J. Inherit. Metab. Dis. 14:915-922[Medline].

  41. Teng, B., Sniderman, A. D., Soutar, A. K., Thompson, G. R. 1986. Metabolic basis of hyperapobetalipoproteinemia. Turnover of apolipoprotein B in low density lipoprotein and its precursors and subfractions compared with normal and familial hypercholesterolemia. J. Clin. Invest. 77:663-672.

  42. Pajukanta, P., Nuotio, I., Terwilliger, J. D., Porkka, K. V., Ylitalo, K., Pihlajamaki, J., Suomalainen, A. J., Syvanen, A. C., Lehtimaki, T., Viikari, J. S., Laakso, M., Taskinen, M. R., Ehnholm, C., Peltonen, L. 1998. Linkage of familial combined hyperlipidaemia to chromosome 1q21-q23. Nat. Genet. 18:369-373[Medline].

  43. Castellani, L. W., Weinreb, A., Bodnar, J., Goto, A. M., Doolittle, M., Mehrabian, M., Demant, P., Lusis, A. J. 1998. Mapping a gene for combined hyperlipidaemia in a mutant mouse strain. Nat. Genet. 18:374-377[Medline].

  44. Aouizerat, B. E., Allayee, H., Cantor, R. M., Davis, R. C., Lanning, C. D., Wen, P. Z., Dallinga-Thie, G. M., de Bruin, T. W., Rotter, J. I., Lusis, A. J. 1999. A genome scan for familial combined hyperlipidemia reveals evidence of linkage with a locus on chromosome 11. Am. J. Hum. Genet. 65:397-412[Medline].

  45. Pajukanta, P., Terwilliger, J. D., Perola, M., Hiekkalinna, T., Nuotio, I., Ellonen, P., Parkkonen, M., Hartiala, J., Ylitalo, K., Pihlajamaki, J., Porkka, K., Laakso, M., Viikari, J., Ehnholm, C., Taskinen, M. R., Peltonen, L. 1999. Genomewide scan for familial combined hyperlipidemia genes in Finnish families, suggesting multiple susceptibility loci influencing triglyceride, cholesterol, and apolipoprotein B levels. Am. J. Hum. Genet. 64:1453-1463[Medline].

  46. Dixon, J. L., Ginsberg, H. N. 1993. Regulation of hepatic secretion of apolipoprotein B-containing lipoproteins: information obtained from cultured liver cells. J. Lipid Res. 34:167-179[Abstract].

  47. Tikkanen, M. J. 1996. Statins: within-group comparisons, statin escape and combination therapy. Curr. Opin. Lipidol. 7:385-388[Medline].


Add to CiteULike CiteULike   Add to Complore Complore   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Reddit Reddit   Add to Technorati Technorati    What's this?


This article has been cited by other articles:


Home page
 Physiol. GenomicsHome page
C. L. Welch, S. Bretschger, P.-Z. Wen, M. Mehrabian, N. Latib, J. Fruchart-Najib, J. C. Fruchart, C. Myrick, and A. J. Lusis
Novel QTLs for HDL levels identified in mice by controlling for Apoa2 allelic effects: confirmation of a chromosome 6 locus in a congenic strain
Physiol Genomics, March 12, 2004; 17(1): 48 - 59.
[Abstract] [Full Text] [PDF]

Home page
 J. Lipid Res.Home page
C. Ko, S. M. O'Rourke, and L.-S. Huang
A fish oil diet produces different degrees of suppression of apoB and triglyceride secretion in human apoB transgenic mouse strains
J. Lipid Res., October 1, 2003; 44(10): 1946 - 1955.
[Abstract] [Full Text] [PDF]

Home page
 J. Lipid Res.Home page
E. Sehayek, E. M. Duncan, H. J. Yu, L. Petukhova, and J. L. Breslow
Loci controlling plasma non-HDL and HDL cholesterol levels in a C57BL /6J x CASA /Rk intercross
J. Lipid Res., September 1, 2003; 44(9): 1744 - 1750.
[Abstract] [Full Text] [PDF]

Home page
 Hum Mol GenetHome page
U. Ribbhammar, L. Flornes, L. Backdahl, H. Luthman, S. Fossum, and J. C. Lorentzen
High resolution mapping of an arthritis susceptibility locus on rat chromosome 4, and characterization of regulated phenotypes
Hum. Mol. Genet., September 1, 2003; 12(17): 2087 - 2096.
[Abstract] [Full Text] [PDF]

Home page
 J. Lipid Res.Home page
R. J. Neuman, B. Yuan, D. S. Gerhard, K.-Y. Liu, P. Yue, S. Duan, M. Averna, and G. Schonfeld
Replication of linkage of familial hypobetalipoproteinemia to chromosome 3p in six kindreds
J. Lipid Res., March 1, 2002; 43(3): 407 - 415.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow An erratum has been published
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrowRequest Permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ko, C.
Right arrow Articles by Huang, L.-S.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Ko, C.
Right arrow Articles by Huang, L.-S.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati  
What's this?

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
 All ASBMB Journals   Journal of Biological Chemistry 
 Molecular and Cellular Proteomics   ASBMB Today 
Advertisement
spacer
Advertisement
Advertisement