Lipoprotein lipase. Molecular model based on the pancreatic lipase x-ray structure: consequences for heparin binding and catalysis

Lipoprotein lipase and pancreatic lipase have about 30% sequence identity, suggesting a similar tertiary fold. Three-dimensional models of lipoprotein lipase were constructed, based upon two recently determined x-ray crystal structures of pancreatic lipase, in which the active site was in an open and closed conformation, respectively. These models allow us to propose a few hypotheses on the structural determinants of lipoprotein lipase which are responsible for heparin binding, dimer formation, and phospholipase activity. The folding of the protein assembles a number of positive charge clusters at the back of the molecule, opposite the active site. These clusters probably form the heparin binding site, as confirmed by recent site-directed mutagenesis experiments. The active sites of lipoprotein lipase and pancreatic lipase look very similar, except for the lid (a surface loop covering the catalytic serine in the inactive state). A different open (active) conformation of the lid in both enzymes may be responsible for their differing substrate specificities. Predictions of the nature of the lipoprotein lipase dimer remain elusive, although our model enabled us to propose a few possibilities.

Tabulations and expectations regarding the genetics of human hypertension

Results from family studies suggest several possible single gene traits may be related to essential hypertension in humans. Results of segregation analysis are reviewed for six possibly related traits (high sodium-lithium countertransport, low urinary kallikrein excretion, high fasting plasma insulin level, a fat pattern index, dense LDL subfractions, and body mass index). Eight genetic loci (on chromosomes 1, 6, 8, 17 and 19) are possibly related to essential hypertension in humans and are also reviewed. Two of them (GRA and AGT) are well-established. Glucocorticoid remediable aldosteronism (GRA) is attributable to a well-defined mutation on chromosome 8q21 and leads to the production of high levels of abnormal adrenal steroid hormones with subsequent aldosteronism, early severe hypertension, and strokes. This form of hypertension is unresponsive to ordinary medications but very responsive to glucocorticoid hormone administration. The angiotensinogen (AGT) locus on chromosome 1q4 has been related to hypertension in sibship linkage studies, association studies, and studies of angiotensinogen levels by genotype in three different populations (Utah, France, Japan). This locus seems to be associated with more severe essential hypertension and also with preeclampsia. Genetic heterogeneity, imprecision in measuring specific phenotypes, and variability in sampling methods in populations studied may all contribute to weaknesses and inconsistencies between reported studies. However, growing evidence for several single gene traits promoting susceptibility to hypertension offers opportunities for identifying genetically profiled subsets of patients in whom specific environmental interventions or specific types of medications will achieve more focused prevention and treatment of hypertension and its complications.

Human tissue factor pathway inhibitor (TFPI) gene: complete genomic structure and localization on the genetic map of chromosome 2q

Tissue factor pathway inhibitor (TFPI), a protease inhibitor that circulates in association with plasma lipoproteins (VLDL, LDL and HDL), helps to regulate the extrinsic blood coagulation cascade. We have cloned a 125-kb genomic region containing the entire human TFPI gene on six overlapping cosmids and prepared a restriction map of this contig to clarify gene structure. More than half (45 kb) of the 85-kb gene is occupied with 5' noncoding elements: coding begins at exon 3. A HindIII RFLP identified with one cosmid was genotyped in the CEPH panel of 59 reference families. Linkage analysis using markers on human chromosome 2 located the TFPI gene on 2q, 36 cM proximal to D2S43(pYNZ15) and 13 cM distal to the crystalline gamma-polypeptide locus CRYGP1(p5G1).

Mutations in exon 3 of the lipoprotein lipase gene segregating in a family with hypertriglyceridemia, pancreatitis, and non-insulin-dependent diabetes

A proband with chylomicronemia, pancreatitis, and non-insulin-dependent diabetes (NIDDM) bears two different mutations in exon 3 of the lipoprotein lipase (LPL) gene: a missense mutation, 75Arg-->Ser, inherited through the paternal line and a truncation, 73Tyr-->Ter, through the maternal line. NIDDM appeared to be independently segregating. The R75S mutant was studied in extracts and media from transfected COS-1 cells. Detectable amounts of catalytically competent R75S LPL suggested destabilization of the active homodimer as with exon 5 mutants (Hata et al. 1992. J. Biol. Chem. 267:20132-20139). Hydrolysis of a short-chain fatty acid ester indicated that R75S does not directly affect activation of LPL by apoC-II. Subjects with NIDDM and wild-type LPL, and nondiabetic middle-aged carriers of the 73Tyr-->Ter truncation had moderate hypertriglyceridemia (260-521 mg/dl) and reduced high density lipoprotein cholesterol. A maternal aunt with NIDDM carried the truncation. Her phenotype (triglycerides of 5,300 mg/dl, eruptive xanthomatosis, and recurrent pancreatitis) was as severe as that in homozygotes or compound heterozygotes. We conclude: (a) diabetic carriers of dysfunctional LPL alleles are at risk for severe lipemia; and (b) the physiologic defects in NIDDM may be additive or synergistic with heterozygous LPL deficiency.

A genetic linkage map with 29 loci spanning human chromosome 13q

A genetic linkage map for the long arm of human chromosome 13 contains 29 loci derived from 38 probe and enzyme combinations and two protein polymorphisms. Thirteen loci form a continuous linkage map of 106 cM in males and 230 cM in females; each was placed on the map with support of at least 1000:1 against alternative orders. On a sex-combined basis, the mean distance between markers is less than 13 cM. The order of loci on the genetic map agrees with physical localization data that show that together these 13 loci cover 13q13 to 13q34. This map was used to regionally localize the 16 remaining loci. The linkage maps reported here should prove to be useful to investigators mapping disease genes and other genetic markers on human chromosome 13.

An extended genetic linkage map and an "index" map for human chromosome 17

Our previous genetic map for chromosome 17 has been expanded to include 72 loci defined by 90 RFLP markers and four microsatellite markers assayed by the polymerase chain reaction. Forty-one of these loci were ordered with odds greater than 1000:1 against local inversion, and the other 31 were ordered within 95% confidence limits. From the set of 41 unambiguously mapped loci, 14 well-spaced "index markers" can be extracted for efficient genetic studies. The complete map spans 173 cM (136 cM in males and 214 cM in females); average spacing between markers is 4.2 cM.

Familial dyslipidaemic hypertension and other multiple metabolic syndromes

Data from several different studies are reviewed suggesting that a subset of hypertension is associated with metabolic abnormalities involving lipids, insulin, and often obesity, all aggregating strongly in families. Persons with 'familial dyslipidaemic hypertension (FDH)' have an especially high risk of early coronary disease. The clinical and biochemical features of FDH are compared with Reaven's Syndrome X, familial combined hyperlipidaemia, dense LDL subfractions, diabetes, impaired glucose tolerance, central and general obesity, pre-diabetes, pre-hypertension, and heterozygous lipoprotein lipase deficiency. Some contribution from major gene effects is suggested in specific subsets reported in several different genetic studies reviewed in this report. It seems likely that multiple metabolic abnormalities are genetically heterogeneous. The data also suggest significant contributions from environmental factors such as diet and physical activity.

Molecular basis of human hypertension: role of angiotensinogen

Essential hypertension is a common human disease believed to result from the interplay of multiple genetic and environmental determinants. In genetic studies of two large panels of hypertensive sibships from widely separated geographical areas, we obtained evidence of genetic linkage between the angiotensinogen gene (AGT) and hypertension, demonstrated association of AGT molecular variants with the disease, and found significant differences in plasma concentrations of angiotensinogen among hypertensive subjects with different AGT genotypes. The corroboration and replication afforded by these results support the interpretation that molecular variants of AGT constitute inherited predispositions to essential hypertension in humans.

Genetic linkage of the human gene for phenylethanolamine N-methyltransferase (PNMT), the adrenaline-synthesizing enzyme, to DNA markers on chromosome 17q21-q22

We have determined the genetic location of the human gene encoding phenylethanolamine N-methyltransferase (PNMT), the terminal enzyme of the catecholamine pathway catalyzing the synthesis of epinephrine (adrenaline) from norepinephrine. This gene is linked to DNA markers on the long arm of chromosome 17, q21-q22, most closely to the DNA markers MFD15 (D17S250) (Zmax = 15.0, theta = 0.065) and fLB17.1 (Zmax = 14.6, theta = 0.045). Multipoint linkage analysis placed the PNMT locus in the interval fLB17.1-CMM86 (D17S74), at 4 centiMorgans (cM) distal to fLB17.1, and at 17 cM proximal to CMM86. Mapping of the PNMT gene will provide the basis for genetic linkage studies in families with disease which might pathogenetically involve this enzyme. The human chromosomal region 17q21-22 identified here to harbour the PNMT gene may be syntenic to the chromosomal region in the stroke-prone spontaneously hypertensive rat (SHR-SP) recently linked to blood-pressure regulation. As an increase of PNMT activity has been associated with the development of hypertension in SHR-SP, it will be of interest to perform comparative mapping of the PNMT gene.

Two apolipoprotein B gene defects in a kindred with hypobetalipoproteinemia, one of which results in a truncated variant, apoB-61, in VLDL and LDL

We report the presence of two distinct defects of the gene for apolipoprotein B, one resulting in a new truncated variant, apoB-61, in a kindred with familial hypobetalipoproteinemia (FHB). The proband (age 33) and a sister (age 36) are both compound heterozygotes with total cholesterol levels of 39 mg/dl and 50 mg/dl, and apoB levels of 1 mg/dl and 2 mg/dl in plasma, respectively. Both appear to be asymptomatic. The apoB-61 mutation, present in a total of five individuals and inherited from the proband's father, is a 37 bp deletion in exon 26 starting with nucleotide 8525. This results in an apoB of 2784 amino acids with 12 novel carboxy-terminal residues. The apoB-61 is present to a considerable degree, relative to apoB-100, in the proband's very low (VLDL) and low density (LDL) lipoprotein fractions. Both lipoprotein fractions have abnormal particle size distribution by electron microscopy. The LDL contain cuboidal particles. Total cholesterol, LDL cholesterol, and apoB levels in the family display three phenotypic patterns: normal, low, and extremely low. ApoB haplotyping indicates the presence of another defective apoB allele in a total of seven individuals. This allele leads to low levels of apoB-100. The second apoB gene-linked defect occurring together with the apoB-61 mutation explains the 3-phenotype pattern. The severe hypocholesterolemia seen in the proband and a sister result from the genetic compound state involving both alleles. This study shows that severe hypolipidemia in an individual heterozygous for a truncation in apoB is likely to involve a second genomic defect.

Absence of linkage between the angiotensin converting enzyme locus and human essential hypertension

The angiotensin converting enzyme (ACE) is a key component of the renin angiotensin system that contributes to the regulation of blood pressure (BP). Recent demonstration of linkage between the ACE locus and elevated BP in a rat model of hypertension has further emphasized ACE as a candidate gene in human hypertension. We report the localization of the ACE gene on the genetic map of chromosome 17, and identify an extremely polymorphic marker at the human growth hormone (hGH) locus which shows no recombination with ACE. We have found no evidence to support linkage between the ACE locus and hypertension, which suggests that mutations at the ACE locus do not commonly contribute to the pathogenesis of hypertension in our test population.

Evidence of genetic heterogeneity in five kindreds with familial hypertrophic cardiomyopathy

BACKGROUND

Recently, two families with hypertrophic cardiomyopathy have been shown to have mutations in the cardiac beta-myosin heavy chain gene (beta-MHC) located on the long arm of chromosome 14.

METHODS AND RESULTS

We have performed linkage analysis of five newly ascertained pedigrees with more than 50 chromosomal markers detecting polymorphisms. Our findings confirm the linkage to beta-MHC gene locus on chromosome 14 in one family (LOD score, 4.50) and suggest linkage to the same gene in another kindred. Chromosome 14 markers were not linked to the disease gene in the other three kindreds, however, and a test for genetic heterogeneity was statistically significant. Moreover, markers for the beta-MHC gene identified affected individuals who were recombinants with respect to this gene and the disease phenotype in these three kindreds.

CONCLUSIONS

These results provide conclusive evidence that hypertrophic cardiomyopathy in separate families is caused by mutations in disease genes at two or more locations in the genome.

A chimaeric 11 beta-hydroxylase/aldosterone synthase gene causes glucocorticoid-remediable aldosteronism and human hypertension

Glucocorticoid-remediable aldosteronism (GRA), an autosomal dominant disorder, is characterized by hypertension with variable hyperaldosteronism and by high levels of the abnormal adrenal steroids 18-oxocortisol and 18-hydroxycortisol, which are all under control of adrenocorticotropic hormone and suppressible by glucocorticoids. These abnormalities could result from ectopic expression of aldosterone synthase, which is normally expressed only in adrenal glomerulosa, in the adrenal fasciculata. Genes encoding aldosterone synthase and steroid 11 beta-hydroxylase (expressed in both adrenal fasciculata and glomerulosa), which are 95% identical and lie on chromosome 8q (refs 7, 10), are therefore candidate genes for GRA. Here we demonstrate complete linkage of GRA in a large kindred to a gene duplication arising from unequal crossing over, fusing the 5' regulatory region of 11 beta-hydroxylase to the coding sequences of aldosterone synthase (maximum lod score 5.23 for complete linkage, odds ratio of 170,000:1). This mutation can account for all the physiological abnormalities of GRA. Our result represents the demonstration of a mutation causing hypertension in otherwise phenotypically normal animals or humans.

Genetic markers in human hypertension

A rapidly increasing abundance of genetic markers which can be characterized on a large scale makes an application of the genetic linkage strategy, so successful with Mendelian diseases, ever more attractive for genetic investigations of common diseases such as essential hypertension. Variability in expression, lack of phenotypic specificity and multiple causation create unique challenges when approaches originally designed for single gene inheritance are applied to disorders with complex determination.

MicroVNTR (CTTT)n at the apo C-III locus

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Type III dyslipoproteinemia in patients heterozygous for familial hypercholesterolemia and apolipoprotein E2. Evidence for a gene-gene interaction

In four large pedigrees with heterozygous familial hypercholesterolemia (FH) genetically linked to the low density lipoprotein receptor locus, we have observed a strong interaction between the presence of FH and a single apolipoprotein (apo) E2 allele, resulting in a markedly increased prevalence of type III dyslipoproteinemia (DLPIII). DLPIII was defined by chemical criteria. None of the patients with DLPIII had tuberous or palmar xanthomas characteristic of classically defined type III hyperlipoproteinemia. After exclusion of four persons with apo E 2-2 phenotype, there were 89 FH patients and 110 non-FH subjects. Definite DLPIII (defined as a very low density lipoprotein [VLDL] cholesterol to plasma triglyceride ratio greater than 0.30 with plasma triglycerides greater than or equal to 150 mg/dl) was present in 26% of 43 FH patients with a single E2 allele compared with only 3.4% of 29 non-FH subjects with an E2 allele (p = 0.003). To further characterize this interaction we performed a two-way analysis of covariance, after adjustment for age, sex, and body mass index, to test for any interaction between FH and the apo E loci. There was a statistically significant interaction between FH and the presence of a single E2 allele for the ratio of VLDL cholesterol to plasma triglycerides and for a newly derived estimate of beta-VLDL cholesterol concentration. Estimated beta-VLDL cholesterol level was strongly correlated with age in the subgroup with FH and an E2 allele but not in other subgroups. There was no difference in estimated beta-VLDL cholesterol between sexes. Correlation between estimated beta-VLDL cholesterol level and body mass index in persons older than 18 years was of only marginal significance and of similar magnitude in persons with or without an apo E2 allele. Present knowledge suggests that beta-VLDLs are highly atherogenic; if so, then a sizable subset of FH patients having at least one apo E2 allele and DLPIII may be at increased risk for premature coronary heart disease.

Effects of three genetic loci in a pedigree with multiple lipoprotein phenotypes

In the course of familial investigations of coronary artery disease, we identified an extended kinship in which several members were affected with type IIa hyperlipoproteinemia (HLPIIa), type III dyslipoproteinemia (DLPIII), or hypobetalipoproteinemia (HBLP). To study the genetic defects responsible for plasma lipoprotein abnormalities in this pedigree and to investigate the phenotypic effect of different genotypic combinations, we used molecular markers for apolipoprotein (apo) B, apo E, and the low density lipoprotein (LDL) receptor to characterize segregation at each locus. Linkage analysis showed that elevated LDL cholesterol levels and the HBLP phenotype were due to defects at the LDL receptor and the apo B loci, respectively. One pedigree member, who inherited both an LDL receptor allele linked with elevated LDL cholesterol levels and an apo B allele linked with HBLP, had a normal lipoprotein phenotype. Seven patients who simultaneously inherited the defective LDL receptor allele and one or two apo E2 alleles manifested DLPIII. The E2 alleles in this pedigree were shown by DNA sequence analysis to be the common E2 158 (arginine----cysteine) allele. These findings suggested a possible interaction between the abnormal LDL receptor and apo E2 alleles, resulting in the expression of DLPIII in the presence of a single copy of ago E2.

The effect of genetic determinants of low density lipoprotein levels on lipoprotein (a)

Levels of Lipoprotein(a) [Lp(a)] correlate directly with atherosclerosis risk. The Lp(a) particle is physically and chemically similar to low density lipoprotein (LDL), the main difference being the presence of apolipoprotein(a) [apo(a)] bonded to the apoB-100 moiety of LDL. Genetic variation of apo(a) primarily determines Lp(a) phenotype. However, other genetic factors may also have a role in determining Lp(a) levels. Large families provide a unique opportunity to evaluate the contribution of genetic factors to disease. In several large Utah kindreds with various genetic abnormalities of lipoprotein metabolism we determined that: 1) Lp(a) levels are associated with defects at the apoB gene; 2) Lp(a) levels are not associated with defects at the LDL-receptor gene; 3) high density lipoprotein (HDL) levels are associated with genetic variation at the apo(a) locus; and 4) the DNA sequence of the apoB-100 binding domain does not vary between siblings with high and low Lp(a) levels.

The CEPH consortium linkage map of human chromosome 1

This paper describes the Centre d'Etude du Polymorphisme Humain (CEPH) consortium linkage map of human chromosome 1. The map contains 101 loci defined by genotypes generated from CEPH family DNAs with 146 different contributions from 11 laboratories. A total of 58 loci are uniquely placed on the map with likelihood support of at least 1000:1. The map extends from loci in the terminal bands of both chromosome arms (locus D1Z2 in 1p36.3 and D1S68 in 1q44) and is anchored at the centromere by the D1Z5 alpha-satellite polymorphism. With the exception of a single locus, the remaining loci are arrayed on the fixed map in short intervals and their possible locations are indicated. Multipoint linkage analyses provided estimates that the male, female, and sex-averaged maps extend for 308, 478, and 390 cM, respectively. The sex-averaged map contains only four intervals greater than 15 cM, and the mean genetic distance between the 58 uniquely placed loci is 6.7 cM.

Exclusion of the Na(+)-H+ antiporter as a candidate gene in human essential hypertension

The primary abnormalities that contribute to the pathogenesis of human essential hypertension are unknown. The known genetic contribution to this disorder suggests the possible use of genetic linkage analysis to test whether specific candidate genes contribute to the pathogenesis of either essential hypertension or intermediate phenotypes. Among such phenotypes, elevated erythrocyte Na(+)-Li+ countertransport (SLC) is the best known, supporting major gene inheritance by pedigree analysis. Striking similarities between SLC and Na(+)-H+ exchange suggest that mutations at the Na(+)-H+ antiporter gene locus (APNH) might result in elevated SLC and contribute to the subsequent pathogenesis of hypertension. We have tested these hypotheses by genetic linkage analysis, with APNH as a candidate gene. By determining genotypes at APNH and flanking loci in pedigrees that support major gene segregation of elevated SLC, we have excluded linkage of APNH and the major SLC locus with a LOD score of -5.91, an odds ratio of almost 1,000,000:1 against linkage. In the analysis of 93 hypertensive sibling pairs, we have further demonstrated that APNH explains none of the variance in SLC in hypertensive individuals (r2 = 6 x 10(-7), p greater than 0.99). Finally, we have directly tested for linkage of APNH to genes predisposing toward hypertension by linkage in hypertensive sibling pairs. Mean allele sharing at APNH is not greater than expected from random assortment in hypertensive siblings (0.92 versus 1.0, p greater than 0.80), and the upper 95% confidence limit of this value (1.04) indicates that mutations at APNH rarely if ever contribute to the pathogenesis of hypertension in this population.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic linkage and heterogeneity in type I Charcot-Marie-Tooth disease (hereditary motor and sensory neuropathy type I)

The segregation patterns of DNA markers from the pericentromeric regions of chromosomes 1 and 17 were studied in seven pedigrees segregating an autosomal dominant gene for Charcot-Marie-Tooth neuropathy type I (CMT I; hereditary motor and sensory neuropathy I). A multilocus analysis with four markers (pMCR-3, pMUC10, FY, and pMLAJ1) spanning the pericentromeric region of chromosome 1 excluded the CMT I gene from this region in six pedigrees but gave some evidence for linkage to the region of Duffy in one pedigree. Linkage of the CMT I gene to markers in the pericentromeric region of chromosome 17 (markers pA10-41, pEW301, p3.6, and pTH17.19) was established; however, in these seven pedigrees homogeneity analysis with chromosome 17 markers detected significant genetic heterogeneity. This analysis suggested that three of the seven pedigrees are not linked to this same region. Overall, two of the seven CMT I pedigrees were not linked to markers tested from chromosomes 1 or 17. These results confirm genetic heterogeneity in CMT I and implicate the existence of a third autosomal locus, in addition to a locus on chromosome 17, and a probable locus on chromosome 1. This evidence of etiological heterogeneity, supported by statistical tests, will have to be taken into consideration when fine-structure genetic maps of the regions around CMT I are constructed.

Compound heterozygote for lipoprotein lipase deficiency: Ser----Thr244 and transition in 3' splice site of intron 2 (AG----AA) in the lipoprotein lipase gene

Cloning and sequencing of translated exons and intron-exon boundaries of the lipoprotein lipase gene in a patient of French descent who has the chylomicronemia syndrome revealed that he was a compound heterozygote for two nucleotide substitutions. One (TCC----ACC) leads to an amino acid substitution (Ser----Thr244), while the other alters the 3' splice site of intron 2 (AG----AA). The functional significance of the Thr244 amino acid substitution was established by in vitro expression in cultured mammalian cells.

Direct detection and automated sequencing of individual alleles after electrophoretic strand separation: identification of a common nonsense mutation in exon 9 of the human lipoprotein lipase gene

Large-scale screening by direct sequencing of DNA to detect molecular variants remains a laborious endeavor whose difficulty is compounded by heterozygosity. We show that mobility shifts of single-stranded DNA electrophoresed under nondenaturing conditions can be used not only to detect variants (Orita,M. et al., 1989, Genomics, 5, 874-879), but also to separate and sequence directly individual alleles. In this manner, we have identified a common variant of human lipoprotein lipase resulting from a nonsense mutation in exon 9 of the gene. Whether this variant is of functional significance remains to be determined.

Phenotypic expression of heterozygous lipoprotein lipase deficiency in the extended pedigree of a proband homozygous for a missense mutation

Familial lipoprotein lipase (LPL) deficiency is a rare genetic disorder accompanied by well-characterized manifestations. The phenotypic expression of heterozygous LPL deficiency has not been so clearly defined. We studied the pedigree of a proband known to be homozygous for a mutation resulting in nonfunctional LPL. Hybridization of DNA from 126 members with allele-specific probes detected 29 carriers of the mutant allele. Adipose tissue LPL activity, measured previously, was reduced by 50% in carriers, but did not reliably distinguish them from noncarriers. Carriers were prone to the expression of a form of familial hypertriglyceridemia characterized by increased plasma triglyceride, VLDL cholesterol and apolipoprotein B, and decreased LDL and HDL cholesterol concentrations. These manifestations were age modulated, with conspicuous differences between carriers and noncarriers observed only after age 40. Several noncarriers exhibited similar lipid abnormalities, but without the inverse relationship between VLDL cholesterol and LDL cholesterol noted among carriers. In addition to age and carrier status, the potentially reversible conditions, obesity, hyperinsulinemia and lipid-raising drug use were contributory. Thus heterozygous lipoprotein lipase deficiency, together with age-related influences, may account for a form of familial hypertriglyceridemia.