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J E Hixson

Publications and source records attributed to J E Hixson.

At least 55 records · Page 3Linked to original sources
PubMed1996

Effects of a major gene for apolipoprotein A-I concentration are thyroid hormone dependent in Mexican Americans.

Apolipoprotein A-I (apoA-I) is the principal protein component of HDL cholesterol. The thyroid hormone triiodothryonine (T3) is known to be a potent mediator of expression of the apoA-I structural gene (APOA1). Using complex segregation analysis, we detected a major gene influencing plasma concentration of apoA-I and examined its interaction with T3 serum level in Mexican Americans participating in the San Antonio Family Heart Study. Strong evidence for a major locus with two alleles (A and a) determining apoA-I level was obtained when interaction with T3 was allowed. The major gene appears not to be linked to the APOA1 structural locus. Genotypes differed significantly in their relationships to T3 level. The AA and Aa genotypes showed a positive relationship with T3 level, while the rarer aa homozygote showed a strong negative relationship with T3. The relative variance in apoA-I concentration due to this major gene varied from 56% to 18%, depending on T3 level. On average, the major gene accounts for 30% of apoA-I variation, and shared-household effects account for an additional 11%. These findings suggest that thyroid hormone has an important role in the genetic control of lipoprotein metabolism.

Adult↗
PubMed1996

Identification of promoter sequences in the 5' untranslated region of the baboon apolipoprotein[a] gene.

Like humans, baboons possess apolipoprotein[a] (apo[a]), a unique protein component of the atherogenic lipoprotein [a] (Lp[a]) particle. Baboon apo[a] also exhibits extensive variation with respect to size and serum levels. In this report, we have cloned the 5' flanking region of the baboon apo[a] gene (I isoform) and performed promoter mapping studies to identify sequences that control apo[a] transcription. The sequence of the baboon apo[a] 5' flanking region is similar to the human gene, and contains two Alu repeats that distinguish the apo[a] gene from plasminogen and other apo[a]-like genes. The transcription start site for the baboon apo[a]gene is located 85 bp upstream from the major start site for the human apo[a] gene. For promoter mapping studies, we constructed two sets of deletion clones (5' to 3' and 3' to 5') in luciferase reporter plasmids for transfection of hepatic cell lines (HepG2 and HUh7). These experiments showed that the 5' untranslated region (5' UTR) contains a positive promoter element with 85% identity to the consensus binding site for hepatocyte nuclear factor 1 alpha (HNF-1 alpha), and a negative element that is functional in HepG2 cells, but not Huh7 cells. Transfection assays with HeLa cells showed that the positive promoter element acts in an hepatocyte-specific manner. We also cloned the 5' flanking region from a baboon carrying a null allele that produced no detectable hepatic transcripts or serum isoforms in vivo. Surprisingly, the 5' flanking regions of the null allele possessed a promoter that was functional in transfection assays. We conclude that the baboon apo[a] gene 5'UTR contains hepatocyte-specific promoter elements, but that other unknown sequences must influence apo[a] expression in vivo.

Alleles↗
PubMed1995

Discovery of a brain promoter from the human transferrin gene and its utilization for development of transgenic mice that express human apolipoprotein E alleles.

Transgenic mice carrying heterologous genes directed by a 670-bp segment of the regulatory sequence from the human transferrin (TF) gene demonstrated high expression in brain. Mice carrying the chimeric 0.67kbTF-CAT gene expressed TF-CAT in neurons and glial cells of the nucleus basalis, the cerebrum, corpus callosum, cerebellum, and hippocampus. In brains from two independent TF-CAT transgenic founder lines, copy number of TF-CAT mRNA exceeded the number of mRNA transcripts encoding either mouse endogenous transferrin or mouse endogenous amyloid precursor protein. In two transgenic founder lines, the chloramphenicol acetyltransferase (CAT) protein synthesized from the TF-CAT mRNA was estimated to be 0.10-0.15% of the total soluble proteins of the brain. High expression observed in brain indicates that the 0.67kbTF promoter is a promising director of brain expression of heterologous genes. Therefore, the promoter has been used to express the three common human apolipoprotein E (apoE) alleles in transgenic mouse brains. The apoE alleles have been implicated in the expression of Alzheimer disease, and the human apoE isoforms are reported to interact with different affinities to the brain beta-amyloid and tau protein in vitro. Results of this study demonstrate high expression and production of human apoE proteins in transgenic mouse brains. The model may be used to characterize the interaction of human apoE isoforms with other brain proteins and provide information helpful in designing therapeutic strategies for Alzheimer disease.

Alleles↗
PubMed1995

Baboon lipoprotein lipase: cDNA sequence and variable tissue-specific expression of two transcripts.

We have isolated two lipoprotein lipase (LPL)-encoding cDNA (LPL) clones from a baboon cardiac cDNA library, one of which spans a region from nucleotide (nt) 705 of the coding sequence to the poly(A) tail (2.8 kb). We used reverse transcription followed by PCR (RT-PCR), and anchor-ligated rapid amplification of cDNA ends (RACE) to amplify the remaining 5' region of the LPL transcript. Sequence comparisons reveal that the baboon nt sequence is 95% identical to the human cDNA sequence (ranging from 97.5 to 92.7% in the coding and noncoding regions, respectively). Less than 2% of nt substitutions cause changes between baboon and human amino acid (aa) sequences. The aa in the catalytic triad residues, the heparin-binding site in exon 6, as well as aa in positions where missense mutations cause LPL deficiency, are identical in baboons and humans. Characterization of the tissue-specific expression of LPL using Northern blots of total RNA showed that spinal cord expressed the most LPL transcripts of all baboon tissues examined. Like humans, baboons have two transcript sizes of approx. 3.6 and 3.4 kb in most tissues that express LPL, and sequencing of the 3' untranslated region (UTR) shows this is due to two polyadenylation sites. In contrast, only the larger 3.6-kb transcript is detected in RNA isolated from central nervous system (CNS) tissues. We used RT-PCR to show that the polyadenylation signal that produces the 3.4-kb message is present in CNS LPL transcripts, but is not utilized.

Amino Acid Sequence↗
PubMed1995

Linkage mapping in Papio baboons: conservation of a syntenic group of six markers on human chromosome 1.

We have established multipoint genetic linkage among six loci in baboons (Papio hamadryas). Published PCR primers designed to amplify five human microsatellite loci were used to amplify homologous loci in 229 pedigreed baboons. Southern blotting was used to type two RFLPs in a functional gene (anti-thrombin III) in a subset of those animals. All six loci are known to map to human chromosome 1q, a region of the genome predicted by karyotype studies to be conserved in baboons. Pairwise recombination frequencies and lod scores indicate that the six loci are also linked in baboons. Recombination distances among the loci are similar to those reported for humans. Like humans, the baboons exhibit higher rates of recombination in females than in males. This study demonstrates that (1) microsatellite loci first described and characterized in the human genome can be effectively used for genetic linkage mapping in non-human primates, (2) a group of genetic loci known to be linked on human chromosome 1q are also linked in the baboon genome, and (3) sex differences in recombination frequencies among loci on human chromosome 1q are also observed in the genome of this Old World monkey. This constitutes the first reported multipoint linkage map in any nonhuman primate.

Animals↗
PubMed1995

Major gene with sex-specific effects influences fat mass in Mexican Americans.

Increased adiposity has repeatedly been identified as a major risk factor for a variety of chronic diseases. However, the question still remains whether the amount of adipose tissue itself is genetically mediated. To address this question, a segregation analysis, using maximum likelihood techniques as implemented in the computer program Pedigree Analysis Package (PAP), was performed on fat mass (kilograms of body fat) in a large sample of extended Mexican American families residing in San Antonio, TX. The only model not rejected was a Mendelian mixed model for fat mass, incorporating genotype x sex interaction. In males the major gene accounted for 37% of the total variance compared with 43% in females. In both sexes homozygous recessive individuals have a fat mass more than double that of individuals of the other two genotypes. It was possible to reject linkage of the anonymous major gene for fat mass with several candidate loci for obesity. However, tentative evidence of linkage was detected with markers on both chromosomes 2 and 11, thereby providing hypotheses for future testing.

Adolescent↗
PubMed1995

Detection and characterization of new mutations in the human angiotensinogen gene (AGT).

As part of the multicenter project entitled "Pathobiological Determinants of Atherosclerosis in Youth (PDAY)," we are testing polymorphisms in candidate genes of atherosclerosis and hypertension for associations with arterial lesions in autopsied young persons. In this study, we used temperature gradient gel electrophoresis (TGGE) to type the Met235-->Thr polymorphism in exon 2 of the angiotensinogen gene (AGT) that is associated with essential hypertension in some human populations. In addition to Met235-->Thr, we detected and sequenced four other TGGE variants in exon 2 of AGT. These included two new amino acid substitutions (Thr209-->Ile and Leu211-->Arg) that were found only among black PDAY cases. The frequency of the Ile209 mutation was 0.002 and the frequency of the Arg211 was 0.006 in 260 black PDAY cases. The other two TGGE variants were Tyr248-->Cys and a T-->C substitution at nucleotide position 171 that had been identified in previous studies. We also developed restriction isotyping for rapid typing of each AGT variant using PCR amplification and digestion with diagnostic restriction enzymes.

Angiotensinogen↗
PubMed1995

A major locus influencing plasma high-density lipoprotein cholesterol levels in the San Antonio Family Heart Study. Segregation and linkage analyses.

To detect and measure the effects of a single locus on quantitative variation in plasma concentrations of HDL cholesterol (HDL-C), we conducted statistical genetic analyses on data from 526 Mexican American individuals in 25 randomly ascertained pedigrees. By using maximum-likelihood complex segregation analysis, we found evidence for a major locus with a codominant mixture model that included the phenotypic means, standard deviations, relative frequency of a low HDL-C allele, and heritability for plasma HDL-C levels, plus the effects of sex (genotype specific), age-by-sex, age2-by-sex, plasma concentrations of apolipoprotein (apo)AI and triglycerides (genotype specific), exogenous sex hormone use, and menopausal status under an unrestricted general model. Inclusion of the four covariates (in addition to the sex and age-by-sex effects) accounted for nearly 79% of the variance in total plasma HDL-C levels. Of the remaining 21% of the variance, the detected major locus accounted for approximately 55% in men and 21% in women; the total genetic contributions to the variance by genes were approximately 82% in men and 69% in women. Linkage analyses with penetrance parameter estimates from the segregation analysis excluded tight linkage between the detected major locus and markers for the following candidate loci: the apoAI/apoCIII genomic region (P < .05), apoB (P < .01), hepatic lipase (P < .001), lipoprotein lipase (P < .001), and the LDL receptor (P < .001). While not excluding the apoE locus (LOD = -0.348, P < .21), the analysis provided no support for tight linkage between it and the detected major locus.

Adolescent↗
PubMed1995

Evidence for a major gene affecting postchallenge insulin levels in Mexican-Americans.

Hyperinsulinemia, which is considered a hallmark of insulin resistance, precedes the development of non-insulin-dependent diabetes mellitus (NIDDM). Results of family and twin studies have shown that heredity influences insulin resistance and insulin levels. In Caucasian families ascertained through two or more NIDDM siblings, it has been reported that single genes with large effects, i.e., major genes, influence both fasting and 1-h postchallenge insulin levels. To determine whether a major gene affects 2-h postchallenge insulin levels in Mexican-Americans, we conducted segregation analyses using data collected on 527 pedigreed individuals from 27 families in San Antonio, TX. Probands for the families were randomly ascertained and all first-, second-, and third-degree relatives aged 16 years and older were invited to participate. Subjects received a 2-h oral glucose tolerance test, and diabetes was diagnosed according to World Health Organization criteria. We found that an autosomal dominant major gene best described the inheritance of 2-h insulin levels (ln-transformed) in these 27 families. Of the individuals in the population, 17% were homozygous for the 2-h low-insulin allele (back-transformed mean = 125 pmol/l) and 83% were heterozygous or homozygous for the 2-h high-insulin allele (back-transformed mean = 406 pmol/l). This major gene accounted for 31% of the variance in ln(2-h insulin levels) in this population. Using quantitative trait linkage analyses, we excluded tight linkage between this gene affecting 2-h insulin levels and three candidate loci for insulin levels: the insulin receptor gene, the low-density lipoprotein receptor gene, and the glucokinase gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Molecular basis for "null" lipoprotein(a) phenotypes and the influence of apolipoprotein(a) size on plasma lipoprotein(a) level in the baboon.

High plasma levels of lipoprotein(a) (Lp(a)) and its unique apolipoprotein, apo(a), are an independent risk factor for cardiovascular disease. Plasma Lp(a) levels vary over a 1000-fold range and are determined by the apo(a) locus, which has at least 34 alleles expressing apo(a) isoforms with molecular weights from < 300,000 to > 800,000. In addition, "null" apo(a) alleles produce no detectable plasma apo(a). We used primary cultures of baboon hepatocytes to investigate the molecular basis for null apo(a) phenotypes. Immunoprecipitation of apo(a) after radiolabeling of hepatocytes revealed that some null alleles gave rise to intracellular protein products that were not secreted. Pulse-chase analysis and endoglycosidase digests demonstrated that these proteins were retained in the endoplasmic reticulum. We also examined the molecular basis for the documented inverse correlation between apo(a) size and plasma Lp(a) concentration. Steady-state labeling and pulse-chase analysis of hepatocytes from animals expressing two isoforms of apo(a) revealed that the endoplasmic reticulum residence time of secreted apo(a) isoforms was determined by their size. This accounted for the inverse relationship between isoform size and level of secretion. We conclude that the efficiency of post-translational processing of apo(a) is a major determinant of plasma Lp(a) concentration.

Animals↗
PubMed1994

Intracellular processing of apo(a) in primary baboon hepatocytes.

We have developed a serum-free medium for the long-term culture of highly differentiated primary baboon hepatocytes. Hepatocytes isolated from animals with defined plasma Lp(a) levels and apo(a) glycoprotein phenotypes were used to study the assembly of Lp(a). A combination of steady-state and pulse-chase labeling studies and endoglycosidase digests demonstrated that apo(a) was synthesized as a lower molecular weight precursor. After a prolonged period of time in the endoplasmic reticulum, apo(a) was converted to a mature form and secreted. A proportion of mature apo(a) also had a prolonged residence time in the trans Golgi apparatus. In all experiments, apoB co-immunoprecipitated with apo(a) from the culture medium but not from the cell lysates, supporting an extracellular association of the proteins for the formation of Lp(a). Analysis of hepatic RNA from 29 'null' Lp(a) phenotype baboons revealed that one-third of the animals had detectable apo(a) transcripts, whereas the remainder had no detectable apo(a) mRNA. The baboon hepatocyte system therefore represents a valuable model to examine the effect of allelic variation at the apo(a) locus on Lp(a) assembly.

Alleles↗
PubMed1993

Baboon apolipoprotein A-IV. Identification of Lys76-->Glu that distinguishes two common isoforms and detection of length polymorphisms at the carboxyl terminus.

Various protein isoforms have been identified for human apolipoprotein A-IV (apoA-IV). However, investigations of their physiological effects have been limited because of low frequencies for many of the apoA-IV variants. Recent discovery of extensive variation in baboon apoA-IV using isoelectric focusing (IEF) makes this primate species an excellent model for genetic studies of apoA-IV. In this study, the molecular basis for net charge differences between two common apoA-IV isoforms (I and E) was determined by cloning and sequencing of intestinal cDNAs from homozygous baboons. An A-->G substitution was found in the third amphipathic repeat of the E isoform. This substitution causes a Lys-->Glu substitution at amino acid position 76 (Lys76-->Glu), adding two negative charges to the E isoform compared to the I isoform, consistent with their relative mobilities on IEF gels. Restriction isotyping was used to identify the substitution in leukocyte DNA from 15 baboons that had been typed by IEF, thus verifying Lys76-->Glu as the basis for the charge differences between the I and E isoforms. Physiological effects of the Lys76-->Glu substitution on high density lipoprotein-C levels were investigated in 431 baboons carrying the E and I isoforms. These studies revealed that the I isoform was associated with higher levels of high density lipoprotein-C on a high cholesterol, saturated fat diet (p = 0.04). The cDNA sequences showed that the carboxyl terminus of baboon apoA-IV contains a region of hydrophilic repeats (Glu-Gln-X-Gln) that is the largest yet found in any species (nine repeats compared to three to five repeats in human, mouse, and rat). A common length polymorphism was identified that inserts a single amino acid to form a five amino acid repeat. This is the first report of this type of length variation (insertion of a single amino acid rather than insertion of an entire repeat) in this region. In addition, a rare variant was found that inserts an entire four-amino-acid repeat, similar to the human apoA-IV-0 isoform.

Amino Acid Sequence↗
PubMed1993

Baboon lecithin cholesterol acyltransferase (LCAT): cDNA sequences of two alleles, evolution, and gene expression.

Lecithin cholesterol acyltransferase (LCAT) is a key enzyme of cholesterol metabolism that catalyzes esterification of cholesterol for packaging in high-density lipoprotein (HDL) particles. In this study, we cloned and sequenced LCAT cDNA from baboon, a nonhuman primate model of atherosclerosis. LCAT sequences have been highly conserved over approximately 25 million years since the divergence of the baboon and human lineages. The baboon and human sequences are 97% identical at the nucleotide (nt) level and 98% identical at the amino acid (aa) level. Only 18% of the nt substitutions change the aa sequence (nonsynonymous substitutions). The substitutions between baboon and human LCAT do not alter key functional sites including the interfacial substrate active site, asparagine-linked glycosylation sites, or sites at which rare mutations cause human familial LCAT deficiencies. We also sequenced LCAT cDNA for a less common allele that is associated with higher LCAT activities and altered lipoprotein phenotypes. There were no sequence differences between the two alleles, which suggests that genotypic effects are most likely due to allelic differences in gene expression. The tissue specificity of LCAT expression was investigated using an RNase protection assay calibrated with known amounts of synthetic human LCAT RNA. In a survey of baboon tissues, the highest levels of LCAT mRNA were found in the cerebellum and liver and trace amounts in the ileum, spleen and cerebral cortex.

Alleles↗
PubMed1993

A DNA polymorphism for lecithin:cholesterol acyltransferase (LCAT) is associated with high density lipoprotein cholesterol concentrations in baboons.

We investigated the effects of a polymorphic PvuII site in the gene for lecithin:cholesterol acyltransferase (LCAT) on serum high density lipoprotein cholesterol (HDL-C) and apolipoprotein A-I (apo A-I) concentrations in a population of 750 pedigreed baboons. We also tested for genotype by diet interactions using data on HDL-C and apo A-I concentrations on two diets (chow and high-cholesterol, saturated fat). A significant (P < 0.001) association between the LCAT genotypes and HDL-C levels was observed. On both diets, animals homozygous for the less common allele had HDL-C levels that averaged 18-19% lower than animals homozygous for the more common allele. HDL-C levels of the heterozygotes were intermediate. The LCAT RFLP accounted for approximately 5% of the variation in HDL-C levels on the two diets. We observed no strong evidence for an LCAT genotype by diet interaction effect.

Alleles↗
PubMed1993

The human apolipoprotein B 3' hypervariable region: detection of eight new alleles and comparisons of allele frequencies in blacks and whites.

We investigated common length polymorphisms in the hypervariable region located 3' to the human gene encoding apolipoprotein B (APOB 3' HVR) as part of the "Pathobiological Determinants of Atherosclerosis in Youth (PDAY)" study. PDAY is a multicenter study of young persons who died of external causes (accident, homicide, and suicide). The APOB 3' HVR contains multiple copies of AT-rich tandem repeats (15bp) called hypervariable elements (HVE). Using polymerase chain reaction (PCR) to amplify APOB sequences in hepatic DNA samples, we identified 22 different HVR alleles among 232 PDAY cases. In addition to 14 previously identified alleles, we detected 8 new alleles that had not been observed in population surveys. Of these new alleles, 7 were present only in black cases. We also examined distributions of HVR allele frequencies for blacks and whites. The frequency distributions for whites did not differ from those from previous studies of French populations (P = 0.3811) and Austrian populations (P = 0.1885). In contrast, the allele frequency distribution for blacks differed from whites (P < 0.001). Blacks had higher frequencies of smaller alleles (< or = 33 repeats) and larger alleles (> or = 37 repeats) than whites. We also sequenced specific HVR alleles to identify differences responsible for size variation. The most frequent alleles were identical in sequence to HVR alleles described in previous studies. However, one allele was not identical in sequence to an equivalent-sized allele from a previous study. In all likelihood, detection of sequence substitutions in the APOB 3' HVR would result in an even greater amount of allelic variability than detected by size differences alone.

Adolescent↗
PubMed1992

Baboon apolipoprotein C-I: cDNA and gene structure and evolution.

We have isolated and characterized cDNA and genomic clones for apolipoprotein C-I (apoC-I) encoded by the APOC1 locus in baboons. Baboon apoC-I cDNA is only 410 bp in size, but the gene spans 4.5 kb including four small exons and three introns containing a large number of Alu repeats. The coding sequences of apoC-I cDNA and genomic clones are identical, indicating that this genomic clone contains the functional gene for apoC-I rather than a pseudogene like human APOC1'. We also detected a second gene in Southern blots of baboon genomic DNA that may correspond to the human APOC1' pseudogene. Two start sites for baboon APOC1 transcription were mapped to nucleotides that are 7 and 9 bp downstream from the predominant start site for human APOC1 transcription. Alignment of Alu repeats showed that the 5' region of the baboon APOC1 gene is more similar to that of the human pseudogene APOC1', and the 3' region and coding sequences are more similar to those of human APOC1. These regions are separated by an Alu repeat that is present only in the baboon gene, perhaps reflecting its role in gene rearrangement or conversion. Sequence comparisons from baboon, human, dog, and rat showed extensive differences in apoC-I amino acid sequences, which are less conserved than nucleotide sequences. However, comparisons of hydrophilicity profiles show significant conservation of protein domains that may be important for apoC-I function.

Amino Acid Sequence↗
PubMed1992

Linkage heterogeneity between the C3 and LDLR and the APOA4 and APOA1 loci in baboons.

We detected linkage in baboons between loci for the third component of complement (C3) and the low-density lipoprotein receptor (LDLR), lod score = 5.53, and loci for apolipoprotein A1 (APOA1) and apolipoprotein AIV (APOA4), lod score = 14.59. We also found evidence for linkage heterogeneity among the baboon pedigrees between LDLR and C3 (P = 0.04) and between APOA1 and APOA4 (P = 0.01).

Animals↗