Genetic variants associated with protein C levels

BACKGROUND

Normal protein C (PC) plasma levels range widely in the general population. Factors influencing normal PC levels are thought to influence the risk of venous thrombosis. Little is known about the underlying genetic variants.

OBJECTIVES

We performed a genome scan of normal PC levels to identify genes that regulate normal PC levels.

PATIENTS/METHODS

We performed a variance components linkage analysis for normal PC levels in 275 individuals from a single, large family. We then sequenced candidate genes under the identified linkage peak in eight family members: four with high and four with low, but normal, PC levels. For variants showing a difference in carriers between those with high and low PC levels, we re-evaluated linkage in the 275 family members conditional on the measured genotype effect. Genotype-specific mean PC levels were determined using likelihood analysis. Findings were replicated in the Leiden Thrombophilia Study (LETS).

RESULTS

We identified a quantitative trait locus at chromosome 5q14.1 affecting normal PC plasma level variability. Next-generation sequencing of 113 candidate genes under the linkage peak revealed four SNPs in BHMT2, ACOT12, SSBP2 and XRCC4, which significantly increased PC levels in our thrombophilic family, but not in LETS.

CONCLUSIONS

We identified four genes at chromosome 5q14.1 that might influence normal PC levels. BHMT2 seems the most likely candidate to regulate PC levels via homocysteine, a competitive inhibitor to thrombin. Failure to replicate our findings in LETS might be due to differences between the studies in genetic background and linkage disequilibrium patterns.

Genome scan of clot lysis time and its association with thrombosis in a protein C-deficient kindred

BACKGROUND

 Previously, we found increased clot-lysis time (CLT), as measured with a plasma-based assay, to increase the risk of venous thrombosis in two population-based case-control studies. The genes influencing CLT are as yet unknown.

PATIENTS/METHODS

 We tested CLT as risk factor for venous thrombosis in Kindred Vermont II (n = 346), a pedigree suffering from a high thrombosis risk, partially attributable to a type I protein C deficiency. Furthermore, we tested for quantitative trait loci (QTLs) for CLT, using variance component linkage analysis.

RESULTS

 Protein C-deficient family members had shorter CLTs than non-deficient members (median CLT 67 min vs. 75 min). One standard deviation increase in CLT increased the risk of venous thrombosis 2.4-fold in non-deficient family members. Protein C deficiency without elevated CLT increased the risk 6.9-fold. Combining both risk factors yielded a 27.8-fold increased risk. The heritability of CLT was 42-52%. We found suggestive evidence of linkage on chromosome 11 (62 cM), partly explained by the prothrombin 20210A mutation, and on chromosome 13 (52 cM). Thrombin-activatable fibrinolysis inhibitor genotypes did not explain the variation in CLT.

CONCLUSION

Hypofibrinolysis appears to increase thrombosis risk in this family, especially in combination with protein C deficiency. Protein C deficiency is associated with short CLT. CLT is partly genetically regulated. Suggestive QTLs were found on chromosomes 11 and 13.

Type 2 diabetes susceptibility genes on chromosome 1q21-24

Type 2 diabetes (T2D) has been linked to chromosome 1q21-24 in multiple samples, including a Utah family sample. Variants in 13 of the numerous candidate genes in the 1q region were tested for association with T2D in a Utah case-control sample. The most promising, 19 variants in 6 candidates, were genotyped on the Utah family sample. Herein, we tested the 19 variants individually and in pairs for an effect on T2D risk in family members using a logistic regression model that accounted for gender, age, and BMI and attributed residual genetic effects to a polygenic component. Seven variants increased risk significantly through 5 pairs of interactions. The significant variant pairs were apolipoprotein A-II (APOA2) rs6413453 interacting with calsequestrin 1 (CASQ1) rs617698, dual specificity phosphatase 12 (DUSP12) rs1503814, and retinoid X receptor gamma (RXRG) rs10918169, a poly-T insertion-deletion polymorphism in liver pyruvate kinase (PKLR) interacting with APOA2 rs12143180, and DUSP12 rs1027702 interacting with RXRG rs10918169. Genotypes of these 5 variant pairs accounted for 25.8% of the genetic variance in T2D in these pedigrees.

A genetic basis for the interrelation of coagulation factors

BACKGROUND

Evidence found in the literature for a strong correlation between coagulation factors suggests that single genes might influence the plasma concentrations of multiple coagulation factors (i.e. pleiotropically acting genes).

OBJECTIVE

To determine whether there is a genetic basis for the correlation among coagulation factors by assessing the heritability of interrelated coagulation factors.

PATIENTS/METHODS

We performed principal components analysis, and subsequently variance components analysis, to estimate the heritability of principal components of coagulation factors in family members of a large French-Canadian kindred.

RESULTS

Four clusters were identified by principal components analysis in 200 family members who did not carry the protein C 3363C mutation. Cluster 1 consisted of prothrombin, factor VII (FVII), FIX, FX and protein S; cluster 2 consisted of FV, FIX, protein C and tissue factor pathway inhibitor; cluster 3 consisted of FVIII and von Willebrand factor; and cluster 4 consisted of antithrombin, protein C and FVII. The heritability of the principal components estimated by variance components analysis was, respectively, 37%, 100%, 37%, and 37%.

CONCLUSION

Our findings support the hypothesis that genes can influence plasma levels of interrelated coagulation factors.

Transcription factor 7-like 2 polymorphisms and type 2 diabetes, glucose homeostasis traits and gene expression in US participants of European and African descent

AIMS/HYPOTHESIS

We sought to determine: (1) the role of previously described transcription factor 7-like 2 (TCF7L2) variants in type 2 diabetes in African American individuals and in participants of European ancestry; (2) the physiological impact of these variants on glucose homeostasis; and (3) whether the non-coding variants altered TCF7L2 expression in adipocytes and transformed lymphocytes.

METHODS

Association studies were conducted by genotyping 932 Europid and African American diabetic and control participants. Family studies were conducted in 673 members of 68 Europid families ascertained for at least two diabetic siblings. Metabolic studies were conducted in 585 non-diabetic individuals who had undergone frequently sampled intravenous glucose tolerance tests to determine insulin sensitivity and insulin secretion. Gene expression studies were conducted in 74 adipose samples and 64 muscle samples from non-diabetic individuals with known genotypes and also in 55 lymphoblastoid cell lines.

RESULTS

TCF7L2 variants were associated with type 2 diabetes in a Europid case-control population and in families, but not in African Americans. Risk alleles increased the 60 min post-challenge glucose value in Europid families and reduced insulin sensitivity by 45% in Europids, but did not alter insulin secretion. TCF7L2 expression was not altered by genotype and did not correlate with insulin sensitivity or BMI.

CONCLUSIONS/INTERPRETATION

We confirmed TCF7L2 as a risk factor in a population of European descent, where it reduced glucose tolerance and insulin sensitivity, but not insulin secretion. We found no role in African Americans and could not explain the association by altered adipocyte or muscle gene expression.

Genome scan of venous thrombosis in a pedigree with protein C deficiency

Kindred Vermont II has a high frequency of venous thrombosis, occurring primarily in pedigree members with type I protein C deficiency due to a 3363 inserted (Ins) C mutation in exon 6 of the protein C gene. However, only a subset of 3363 InsC carriers have suffered thrombotic episodes, suggesting that the increased risk of thrombosis results upon the co-occurrence of 3363 InsC with a second, unknown, thrombophilic mutation that segregates independently within the pedigree. To test this hypothesis and to localize the co-occurring gene, we performed a genome scan of venous thrombosis in Kindred Vermont II. Non-parametric linkage statistics identified three potential gene locations, on chromosomes 11q23 (nominal P < 0.0001), 18p11.2-q11.2 (P < 0.0007), and 10p12 (P < 0.0003), supporting the presence of at least one additional thrombophilic mutation in the pedigree. Identification of the unknown mutation(s) promises to reveal a new genetic risk factor for thrombophilia, contribute to our understanding of the blood clotting mechanism, and expand our knowledge of the diversity of oligogenic disease.

Heritability of plasma concentrations of clotting factors and measures of a prethrombotic state in a protein C-deficient family

BACKGROUND

Earlier studies found strong support for a genetic basis for regulation of coagulation factor levels and measures of a prethrombotic state (d-dimer, prothrombin fragment 1.2).

OBJECTIVES

Estimation of how much of the variation in the levels of coagulation factors and measures of a prethrombotic state, including measures of protein C activation and inactivation, could be attributed to heritability and household effect.

PATIENTS AND METHODS

Blood samples were collected from 330 members of a large kindred of French-Canadian origin with type I protein C deficiency. Heritability and common household effect were estimated for plasma concentrations of prothrombin, factor (F)V, factor VIII, factor (F)IX, fibrinogen, von Willebrand factor (VWF), antithrombin, protein C, protein S, protein Z, protein Z-dependent protease inhibitor (ZPI), fibrinopeptide A (FPA), protein C activation peptide (PCP), activated protein C-protein C inhibitor complex (APC-PCI), activated protein C-alpha1-antitrypsin complex (APC-alpha1AT), prothrombin fragment 1.2 (F1.2) and d-dimer, using the variance component method in sequential oligo-genic linkage analysis routines (SOLAR).

RESULTS

The highest heritability was found for measures of thrombin activity (PCP and FPA). High estimates were also found for prothrombin, FV, FIX, protein C, protein Z, ZPI, APC-PCI and APC-alpha1AT. An important influence of shared household effect on phenotypic variation was found for VWF, antithrombin, protein S and F1.2.

CONCLUSIONS

We found strong evidence for the heritability of single coagulation factors and measures of a prethrombotic state. Hemostatic markers with statistically significant heritability constitute potential targets for the identification of novel genes involved in the control of quantitative trait loci.

Characterization of the human prostaglandin H synthase 1 gene (PTGS1): exclusion by genetic linkage analysis as a second modifier gene in familial thrombosis

Genetic evidence from a large Vermont kindred indicates that an unknown gene promotes thrombosis when inherited in conjunction with type I protein C deficiency. Cyclooxygenase-1 [prostaglandin H synthase 1 gene (PTGS1)] was tested as a plausible candidate for the unknown gene because of its role in primary hemostasis. The complete sequence of PTGS1 (25 638 nucleotides) was determined from a 37 kb human genomic cosmid clone to characterize intronic regions and subsequently to allow the search for mutations by direct sequencing of genomic DNA. Northern blot analysis confirms usage of a newly described distal poly-adenylation signal. Short tandem repeat (STR) sequences found in intron 2 and the 3' flanking region were developed as new genetic markers for PTGS1. The position of PTGS1 was refined on the CHLC chromosome 9 linkage map using the new markers scored in four Centre d'Etude du Polymorphisme Humain families and multipoint linkage analysis. Direct sequencing of DNA from members of the Vermont kindred led to the discovery of two new single nucleotide polymorphisms (SNPs) that give rise to non-conservative amino acid changes in the signal peptide (Arg(8) to Trp and Pro(17) to Leu) of cyclooxygenase-1. Linkage analysis of the SNP and STR markers indicated that PTGS1 is not the interacting gene associated with an increased incidence of thrombosis in the Vermont kindred.

Evaluation of apolipoprotein A-II as a positional candidate gene for familial Type II diabetes, altered lipid concentrations, and insulin resistance

AIMS/HYPOTHESIS

We hypothesized that apolipoprotein A-II sequence variation was responsible for the observed linkage of Type II (non-insulin-dependent) diabetes mellitus to the apolipoprotein A-II region in Northern European families ascertained for multiple diabetic siblings, and might also influence insulin sensitivity and secretion, non-esterified fatty acids, and lipids.

METHODS

We recruited 698 members of 63 families for pedigree studies and additional unrelated people providing 117 diabetic and 130 control subjects. We screened the apolipoprotein A-II gene by single strand conformation polymorphism analysis and fluorescent sequence analysis. Variants were typed by oligonucleotide ligation assay, restriction digest of amplification products, or radioactive fragment analysis for the microsatellite polymorphism. Association of each variant with Type II diabetes was tested in the case-control population by chi-square analysis, or using transmission disequilibrium test in families. Haplotypes were established in families using SIMWALK and tested for association with diabetes and quantitative traits.

RESULTS

No detected variant altered the coding sequence of the gene. Three single nucleotide polymorphisms showed modest evidence for an association, but no variant or haplotype was associated with diabetes in families. Similarly, we found no association with non-esterified fatty acid concentrations, HDL concentrations, or fasting insulin. In contrast, we found evidence for an association of some haplotypes and individual variants with 2-h post-challenge glucose and measures of insulin secretion.

CONCLUSION/INTERPRETATION

Apolipoprotein A-II is not likely to explain the observed linkage of Type II diabetes, but variation in this gene could alter insulin secretion and post-challenge glucose.

Quantitative trait linkage analysis of lipid-related traits in familial type 2 diabetes: evidence for linkage of triglyceride levels to chromosome 19q

Macrovascular disease is a major complication of type 2 diabetes. Epidemiological data suggest that the risk of macrovascular complications may predate the onset of hyperglycemia. Hypertriglyceridemia, low levels of HDL cholesterol, and an atherogenic profile characterize the insulin resistance/metabolic syndrome that is also prevalent among nondiabetic members of familial type 2 diabetic kindreds. To identify the genes for lipid-related traits, we first performed a 10-cM genome scan using 440 markers in 379 members of 19 multiplex families ascertained for two diabetic siblings (screening study). We then extended findings for three regions with initial logarithm of odds (LOD) scores >1.5 to an additional 23 families, for a total of 576 genotyped individuals (extended study). We found heritabilities for all lipid measures in the range of 0.31 to 0.52, similar to those reported by others in unselected families. However, we found the strongest evidence for linkage of triglyceride levels to chromosome 19q13.2, very close to the ApoC2/ApoE/ApoC1/ApoC4 gene cluster (LOD 2.56) in the screening study; the LOD increased to 3.16 in the extended study. Triglyceride-to-HDL cholesterol ratios showed slightly lower LOD scores (2.73, extended family) in this same location. Other regions with LOD scores >2.0 included HDL linkage to chromosome 1q21-q23, where susceptibility loci for both familial type 2 diabetes and familial combined hyperlipidemia have been mapped, and to chromosome 2q in the region of the NIDDM1 locus. Neither region showed stronger evidence for linkage in the extended studies, however. Our results suggest that genes in or near the ApoE/ApoC2/ApoC1/ApoC4 cluster on 19q13.2 may contribute to the commonly observed hypertriglyceridemia and low HDL seen in diabetic family members and their offspring, and thus may be a candidate locus for the insulin resistance syndrome.

Role of common sequence variants in insulin secretion in familial type 2 diabetic kindreds: the sulfonylurea receptor, glucokinase, and hepatocyte nuclear factor 1alpha genes

OBJECTIVE

We have demonstrated high heritability of insulin secretion measured as acute insulin response to glucose times insulin sensitivity (disposition index). Furthermore, we showed that obese normoglycemic family members of a type 2 diabetic proband failed to compensate for the insulin resistance of obesity by increasing insulin secretion. In this study, we tested the primary hypotheses that previously described variants in the pancreatic sulfonylurea receptor gene (SUR1 or ABCC8), glucokinase (GCK) gene, or hepatocyte nuclear factor 1alpha (TCF1 or HNF1alpha) gene contribute to the inherited deficiencies of insulin secretion and beta-cell compensation to insulin resistance, as well as the secondary hypotheses that these variants altered insulin sensitivity.

RESEARCH DESIGN AND METHODS

We typed 124 nondiabetic members of 26 familial type 2 diabetic kindreds who had undergone tolbutamide-modified intravenous glucose tolerance tests for two variants of the ABCC8 (sulfonylurea) gene, two variants of the GCK gene, and one common amino acid variant in the TCF1 (HNF1alpha) gene. All family members were classified as normal or having impaired glucose tolerance based on oral glucose tolerance testing. We used minimal model analysis to calculate the insulin sensitivity index (S1) and glucose effectiveness (SG), and acute insulin response to glucose was calculated as the mean insulin excursion above baseline during the first 10 min after the glucose bolus. Disposition index (DI), a measure of beta-cell compensation for insulin sensitivity, was calculated as insulin sensitivity times acute insulin response. Effects of polymorphisms were determined using mixed effects models that incorporated family membership and by a likelihood analysis that accounted for family structure through polygenic inheritance.

RESULTS

An intronic variant of the ABCC8 gene just upstream of exon 16 was a significant determinant of both DI and an analogous index based on acute insulin response to tolbutamide. Surprisingly, heterozygous individuals showed the lowest indexes, whereas the DI in the two homozygous states did not differ significantly. Neither the exon 18 variant nor the variants in the GCK and TCF1 genes were significant in this model. However, combined genotypes of ABCC8 exon 16 and 18 variants again significantly predicted both indexes of glucose and tolbutamide-stimulated insulin secretion. Unexpectedly, a variant in the 3' untranslated region of the GCK gene interacted significantly with BMI to predict insulin sensitivity.

CONCLUSIONS

The exon 16 variant of the ABCC8 gene reduced beta-cell compensation to the decreased insulin sensitivity in the heterozygous state. This may explain the observation from several groups of an association of the ABCC8 variants in diabetes and is consistent with other studies showing a role of ABCC8 variants in pancreatic beta-cell function. However, our study focused on individuals from relatively few families. Ascertainment bias, family structure, and other interacting genes might have influenced our unexpected result. Additional studies are needed to replicate our observed deficit in beta-cell compensation in individuals heterozygous for ABCC8 variants. Likewise, the role of the GCK 3' variant in the reduced insulin sensitivity of obesity will require further study.

Effect of the peroxisome proliferator-activated receptor-gamma 2 pro(12)ala variant on obesity, glucose homeostasis, and blood pressure in members of familial type 2 diabetic kindreds

The Pro(12)Ala (P12A) variant of exon B of the peroxisome proliferator-activated receptor gamma(2) (PPAR gamma) been variably associated with obesity, insulin sensitivity, diabetes, and dyslipidemia, but its role in insulin resistance-associated traits remains uncertain. We tested the hypothesis that this variant is associated with the insulin resistance syndrome by genotyping 619 members of 52 familial type 2 diabetes kindreds. A subset of 124 family members underwent iv glucose tolerance tests and minimal model determination of insulin sensitivity. We estimated the frequency of the A12 allele as 0.12, within the range observed in random Caucasian samples. We were unable to demonstrate any effect on direct measures of insulin sensitivity, and no trait was linked to markers near PPAR gamma on chromosome 3q. However, body mass index, serum total cholesterol levels, triglyceride levels, systolic and diastolic blood pressures, and glucose concentration showed at least a trend to association (P < 0.1) when tested separately for a family-based association. When these 6 traits were included in a multivariate analysis, body mass index, systolic and diastolic blood pressures, triglyceride levels, and glucose concentration remained significantly associated with the P12A variant (P < 0.05), whereas the effect of P12A on liability for diabetes was not significant. The predicted means for each trait and each genotype suggested that the P12A variant acted most like a recessive mutation, with the major effect among homozygous individuals who comprise only 1--2% of the population. We confirm an association of the P12A variant in traits commonly ascribed to the insulin resistance syndrome, but not with direct measures of insulin sensitivity. The tendency for this variant to act in a recessive manner with effects on multiple traits may explain the inconsistent associations noted in previous studies.

Genetic screening of candidate genes for a prothrombotic interaction with type I protein C deficiency in a large kindred

The incomplete penetrance of thrombosis in familial protein C deficiency suggests disease occurs when this deficit is combined with additional abnormalities in the hemostatic system. The pattern of inherited thrombophilia in the Vermont II kindred, which is affected by a clinically dominant type I protein C deficiency, provides strong evidence for a second unidentified gene that segregates independently of protein C deficiency and increases susceptibility to thrombosis. To test the second gene hypothesis, thirty-four candidate genes for proteins involved in hemostasis or inflammation were tested as the unknown defect, using highly polymorphic short tandem repeat (STR) markers in an informative subset (n = 31) of the kindred. The genes considered are; alpha-fibrinogen, beta-fibrinogen, gamma-fibrinogen, prothrombin, tissue factor, factor V, protein S, complement component 4 binding protein, factor XI, factor XII, factor XIIIa, factor XIIIb, histidine rich glycoprotein, high molecular weight kininogen, kallikrein, von Willebrands factor, platelet factor 4, thrombospondin, antithrombin III, alpha-1-antitrypsin, thrombomodulin, plasminogen, tissue plasminogen activator, urokinase plasminogen activator, plasminogen activator inhibitor-1, plasminogen activator inhibitor-2, protein C inhibitor, alpha-2-plasmin inhibitor, kallistatin, lipoprotein a, interleukin 6, interleukin 1, cystathionine-beta-synthase, and methylenetetrahydrofolate reductase. Mutations in many of these genes have been previously established as independent risk factors for thrombosis. However, linkage analysis provided no evidence to implicate any of the candidate genes as the second inherited factor that promotes thrombophilia in this kindred.

The G20210A prothrombin polymorphism is not associated with increased thromboembolic risk in a large protein C deficient kindred

Likelihood analysis was used to test the effect of the G20210A prothrombin mutation and the His107Pro protein C mutation (resulting from a C insertion) on thrombosis status and prothrombin level in a large kindred of French Canadian descent with type I protein C deficiency. Genotypes were available on 279 pedigree members or their spouses. Of this total, 36 pedigree members were heterozygous for the G20210A variant and one pedigree member was homozygous for G20210A, while 64 were heterozygous for the His107Pro protein C mutation. The factor V Leiden mutation (Arg506Gln) was observed in only one of 181 tested family members. Objectively verified thrombosis was present in 26 of the 279 pedigree members. Thrombosis was suspected in an additional 19 pedigree members. The transmission disequilibrium test of Spielman, 1996, as extended to pedigrees, was used to test for excess transmission of G20210A or His107Pro to thrombosis cases, with transmission of 0.5 specifying no effect. Although the His107Pro mutation was over transmitted (0.837 +/- 0.075 p <0.001) to thrombosis cases in this pedigree, the G20210A variant was not (0.491 +/- 0.130 NS). Measured genotype analysis was used to examine a total of 184 individuals for the relationship between prothrombin level and both the G20210A variant and thrombosis. The G20210A variant increased prothrombin level from 97 +/- 2% to 124 +/- 4% (p <0.0001), but thrombosis status was not associated with any additional increase in prothrombin level. Thus, in a large thrombophilic, protein C deficient kindred, with the G20210A variant present in a proportion (13%) far higher than the general Caucasian population (approximately 2%), neither the presence of the variant nor the plasma concentration of prothrombin were associated with increased risk for thrombosis. These findings contrast with those of others who have established the G20210A variant as a thrombophilic risk factor; and emphasize the complex nature of the multigenic pathogenesis of thrombophilia.

A genome-wide search for type 2 diabetes susceptibility genes in Utah Caucasians

Considerable evidence supports a major inherited component of type 2 diabetes. We initially conducted a genome-wide scan with 440 microsatellite markers at 10-cM intervals in 19 multigenerational families of Northern European ancestry with at least two diabetic siblings. Initial two-point analyses of these families directed marker typing of 23 additional families. Subsequently, all available marker data on the total of 42 families were analyzed using both parametric and nonparametric multipoint methods to test for linkage to type 2 diabetes. One locus on chromosome 1q21-1q23 met genome-wide criteria for significant linkage under a model of recessive inheritance with a common diabetes allele (logarithm of odds [LOD] = 4.295). Both pedigree-based nonparametric linkage (NPL) analysis and affected sib pair (MAPMAKER/SIBS) nonparametric methods also showed the highest genome-wide scores at this region, near markers CRP and APOA2, but failed to meet levels of genome-wide significance. The risk of type 2 diabetes to siblings of a diabetic person when compared with the population (lambdaS) was estimated from MAPMAKER/SIBS to be 2.8 in these 42 families. Simulation studies using study data confirmed a genome-wide significance level of P<0.05 (95% CI 0.005-0.0466). However, analysis of 20 similarly ascertained but smaller families failed to confirm this linkage. The LOD score with 50% heterogeneity for all 62 families considered together was only 2.25, with an estimated lambdaS of 1.87. Our data suggest a novel diabetes susceptibility locus near APOA2 on chromosome 1 in a region with many transcribed genes.

Heritability of pancreatic beta-cell function among nondiabetic members of Caucasian familial type 2 diabetic kindreds

Both defective insulin secretion and insulin resistance have been reported in relatives of type 2 diabetic subjects. We tested 120 members of 26 families with a type 2 diabetic sibling pair with a tolbutamide-modified, frequently sampled i.v. glucose tolerance test to determine the insulin sensitivity index (S(I)) and acute insulin response to glucose (AIRglucose). A measure of beta-cell compensation for insulin sensitivity was calculated as the product S(I) x AIRglucose, based on the demonstrated hyperbolic relationship between insulin sensitivity and insulin secretion. A percentile score for this compensation was assigned based on published values. Of the 120 family members, 26 had previously diagnosed impaired glucose tolerance on oral testing, and 94 had normal glucose tolerance tests. As a group, family members showed a significantly lower S(I) x AIRglucose than a similar, previously reported, control population, even when impaired glucose tolerance members were excluded. We performed a multivariate analysis of diabetes status, S(I), AIRglucose and to estimate the heritability of each trait and the genetic and environmental correlations between traits. We estimated the heritability of S(I) x AIRglucose to be 67 +/- 3% when all members were included and 70 +/- 4% when only normal glucose tolerance members were considered. Both AIRglucose and S(I) were also familial, albeit with lower heritabilities (38 +/- 1% and 38 +/- 2%, respectively, for all family members). Both S(I) x AIRglucose and S(I) showed strong negative genetic correlations with diabetes (-85 +/- 3% and -87 +/- 2%, respectively, all family members), whereas AIRglucose did not correlate with diabetes. We conclude that insulin secretion, as measured by S(I) x AIRglucose, is decreased in nondiabetic members of familial type 2 diabetic kindreds, that S(I) x AIRglucose in these high risk families is highly heritable, and that the same polygenes may determine diabetes status and a low S(I) x AIRglucose. Our data suggest that insulin secretion, when expressed as an index normalized for insulin sensitivity, is more familial than either insulin sensitivity or first phase insulin secretion alone and may be a very useful trait for identifying genetic predisposition to type 2 diabetes.

Is there a link between African iron overload and the described mutations of the hereditary haemochromatosis gene?

Over 80%, of Caucasian patients with hereditary haemochromatosis are homozygotes for a C282Y mutation in the HFE gene on chromosome 6. Recent evidence suggests that a genetic factor may also be involved in the pathogenesis of African iron overload, although the locus has not been described. PCR analysis of DNA from 25 southern Africans, identified by segregation analysis as having a high probability of carrying the putative African iron-loading gene, failed to identify any subjects with the C282Y mutation. The possible genetic defect in African iron overload appears to be different from that described in most cases of hereditary haemochromatosis in Caucasians.

An unknown genetic defect increases venous thrombosis risk, through interaction with protein C deficiency

We used two-locus segregation analysis to test whether an unknown genetic defect interacts with protein C deficiency to increase susceptibility to venous thromboembolic disease in a single large pedigree. Sixty-seven pedigree members carry a His107Pro mutation in the protein C gene, which reduces protein C levels to a mean of 46% of normal. Twenty-one carriers of the mutation and five other pedigree members had verified thromboembolic disease. We inferred the presence in this pedigree of a thrombosis-susceptibility gene interacting with protein C deficiency, by rejecting the hypothesis that the cases of thromboembolic disease resulted from protein C deficiency alone and by not rejecting Mendelian transmission of the interacting gene. When coinherited with protein C deficiency, the interacting gene conferred a probability of a thrombotic episode of approximately 79% for men and approximately 99% for women, before age 60 years.

Evidence of genetic transmission in African iron overload

Iron overload in Africa was previously regarded as purely due to excessive iron in traditional beer, but we recently found evidence that transferrin saturation and unsaturated iron binding capacity may be influenced by an interaction between dietary iron content and a gene distinct from any HLA-linked locus. To determine if serum ferritin follows a genetic pattern and to confirm our previous observations, we studied an additional 351 Zimbabweans and South Africans from 45 families ranging in size from two to 54 members. Iron status was characterized with repeated morning measurements of serum ferritin, transferrin saturation, and unsaturated iron binding capacity after supplementation with vitamin C. For each measure of iron status, segregation analysis was consistent with an interaction between a postulated iron-loading gene and dietary iron content (P < .01). In the most likely model, transferrin saturation is 75% and serum ferritin is 985 micrograms/L in a 40-year-old male heterozygote with an estimated beer consumption of 10,000 L, whereas the saturation is 36% and serum ferritin is 233 micrograms/L in an unaffected individual with identical age, sex, and beer consumption. This segregation analysis provides further evidence for a genetic influence on iron overload in Africans.

Recessive inheritance of obesity in familial non-insulin-dependent diabetes mellitus, and lack of linkage to nine candidate genes

Segregation analysis of body-mass index (BMI) supported recessive inheritance of obesity, in pedigrees ascertained through siblings with non-insulin dependent diabetes mellitus (NIDDM). BMI was estimated as 39 kg/m2 for those subjects homozygous at the inferred locus. Two-locus segregation analysis provided weak support for a second recessive locus, with BMI estimated as 32 kg/m2 for homozygotes. NIDDM prevalence was increased among those subjects presumed to be homozygous at either locus. Using both parametric and nonparametric methods, we found no evidence of linkage of obesity to any of nine candidate genes/regions, including the Prader-Willi chromosomal region (PWS), the human homologue of the mouse agouti gene (ASP), and the genes for leptin (OB), the leptin receptor (OBR/DB), the beta3-adrenergic receptor (ADRB3), lipoprotein lipase (LPL), hepatic lipase (LIPC), glycogen synthase (GYS), and tumor necrosis factor alpha (TNFA).

Intermediate inheritance of Tourette syndrome, assuming assortative mating

Segregation analysis incorporating assortative mating was used to test for major locus inheritance of Tourette syndrome in a single large pedigree containing 182 members. The analysis provided evidence of a major locus with an intermediate inheritance pattern for which the penetrance was estimated from the data as 28% in heterozygotes and 98%-99% in homozygotes. A significant assortative mating correlation was estimated from the data as 70%-79%. In contrast, when assortative mating was not included in the model, intermediate inheritance was not inferred. If, in addition, constancy of the allele frequencies across generations was not assumed, Mendelian transmission was rejected. Each subject, affected or unaffected, was assigned a score reflecting the presence and severity of symptoms. Higher means scores in affected homozygotes than in affected heterozygotes suggested greater severity in homozygotes: genotype information was obtained from genotype probabilities computed assuming intermediate inheritance.

Phenotypic assortative mating in segregation analysis

A model of phenotypic assortative mating was developed for application in segregation analysis. The model assumed a constant spouse correlation across the range of a quantitative trait or the liability to a discrete trait. Four traits were analyzed to evaluate: 1) the feasibility of applying likelihood analysis to pedigree data in order to distinguish between assortative mating and shared environmental effects as the source of spouse correlation; and 2) the impact on segregation analysis of the failure to account for either assortative mating or shared environmental effects, as appropriate. Height ratio (the ratio of sitting to standing height) and eye color comprised the traits for which the observed spouse correlation reflected assortative mating; serum cholesterol and peptic ulcers (with genotypes defined by the ABO blood group) comprised the traits for which the observed spouse correlation reflected shared environmental effects. For all four traits the test statistics agreed with the known cause of spouse correlation; however, significance was not attained for height ratio or serum cholesterol. The ability to distinguish between the causes of spouse correlation in pedigree data presumably depends on trait and sample characteristics which remain to be delineated. Despite significant spouse correlation, its omission from the segregation analysis model did not undermine the inference of major locus inheritance for any of the four traits. However, the lack of an impact for these traits does not preclude an impact for other traits of ignoring the appropriate spouse correlation in segregation analysis.

HLA-linked rheumatoid arthritis

Twenty-eight pedigrees were ascertained through pairs of first-degree relatives diagnosed with rheumatoid arthritis (RA). RA was confirmed in 77 pedigree members including probands; the absence of disease was verified in an additional 261 pedigree members. Pedigree members were serologically typed for HLA. We used likelihood analysis to statistically characterize the HLA-linked RA susceptibility locus. The genetic model assumed tight linkage to HLA. The analysis supported the existence of an HLA-linked RA susceptibility locus, estimated the susceptibility allele frequency as 2.16%, and estimated the lifetime penetrance as 41% in male homozygotes and as 48% in female homozygotes. Inheritance was recessive in males and was nearly recessive in females. In addition, the analysis attributed 78% of the variance within genotypes to genetic or environmental effects shared by siblings. The genetic model inferred in this analysis is consistent with previous association, linkage, and familial aggregation studies of RA. The inferred HLA-linked RA susceptibility locus accounts for approximately one-half of familial RA, although it accounts for only approximately one-fifth of the RA in the population. Although other genes may account for the remaining familial RA, a large portion of RA cases may occur sporadically.

Evidence for multiple genes determining sodium transport

Sodium transport comprises a set of interacting systems. Consequently, a defective sodium transport gene affects multiple sodium transport systems, and a sodium transport variable measured on a sample of individuals reflects genetic variation from a number of different genes, complicating the task of identifying the effect of a single gene. To test for genes which affect sodium transport, we first applied principal components analysis to 14 variables related to sodium transport, thereby defining uncorrelated sources of variation in the variables. The sample consisted of 1,218 members of 68 pedigrees ascertained through probands with early-onset stroke, hypertension, or coronary heart disease. Segregation analysis of the 14 principal components scores provided evidence for 8 genetic variants which alter sodium transport. One of the 8 variants is recessive, has homozygous genotype frequency estimated as 8.8% of the population, and increases sodium-lithium countertransport, the passive sodium leak, body mass index, and triglyceride; the genetic variant may coincide with an insulin resistance gene. A second of the 8 variants is also recessive, has homozygous genotype frequency estimated as 7.4% of the population, and increases intraerythrocytic sodium and the passive sodium leak while decreasing sodium pump number; the genetic variant may reduce pump number. Two of the 8 variants substantially increase sodium-lithium countertransport; frequency estimates for heterozygotes for the dominant variant and homozygotes for the recessive variant equal 1.8% and 3.1%, respectively. Another of the 8 variants is recessive, has homozygous genotype frequency estimated as 1.9%, and increases body mass index. Each of the 3 remaining variants is rare and expressed in less than 1% of the sample.

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.

Genetic basis of familial dyslipidemia and hypertension: 15-year results from Utah

The genetic and environmental determinants of hypertension, lipid abnormalities, and coronary artery disease (CAD) have been studied for 15 years in Utah in population-based multigenerational pedigrees (2500 subjects among 98 pedigrees), twin pairs (74 monozygous and 78 dizygous), hypertensive siblings (131 sibships), siblings with CAD before age 55 (45 sibships), and anecdotally ascertained pedigrees with type II diabetes (271 subjects among 16 pedigrees), lipoprotein lipase deficiency (106 subjects in a single pedigree), and familial hypercholesterolemia (502 heterozygotes among 50 pedigrees). Estimates of heritability ranged from 20 to 75% for blood pressures and blood lipids. A strong positive family history predicts a future occurrence of hypertension (relative risk [RR] = 3.8) and CAD (RR = 12.7). Segregating single-gene effects were found for several 'intermediate phenotypes' associated with hypertension (erythrocyte sodium-lithium countertransport, intraerythrocytic sodium, a relative fat pattern, total urinary kallikrein excretion, and fasting insulin levels). Strong single-gene effects in segregation analysis were also found for low-density lipoprotein (LDL) cholesterol, lipoprotein (a) (Lp[a]), low high-density lipoprotein (HDL) cholesterol, and high apolipoprotein (apo) B. Deoxyribonucleic acid (DNA) markers of lipid abnormalities or hypertension have included LDL-receptor defects, lipoprotein lipase deficiency, high Lp(a), familial defective apo B, decreased quantitative levels of apo B, apo E phenotype, angiotensinogen, and 'glucocorticoid remediable aldosteronism (GRA) hypertension.' Also tested in Utah studies, but not found to be DNA markers for hypertension, were the genetic loci for the structural genes for renin and angiotensin-converting enzyme, and the sodium antiport system. In addition, important gene-gene interactions (LDL receptor with apo E2) and gene-environment interactions (kallikrein with potassium intake) were found.(ABSTRACT TRUNCATED AT 250 WORDS)

A gene-environment interaction between inferred kallikrein genotype and potassium

Urinary kallikrein excretion has been shown statistically to be partially determined by a major gene in large Utah pedigrees with the use of segregation analysis. A previous twin analysis of environmental factors influencing urinary kallikrein level showed that urinary potassium twin differences were strongly related to differences in urinary kallikrein. The present study uses 769 individuals in 58 Utah pedigrees to analyze the association of urinary potassium with urinary kallikrein within statistically inferred kallikrein genotypes. Fitting genotype-specific curves relating urinary kallikrein level to 12-hour urinary potassium amount within a major gene, polygene, and common environment model, we showed a significant statistical urinary potassium interaction with the inferred major gene for kallikrein (P = .0002). The heterozygotes (with a frequency of 50%) had a significant association between urinary kallikrein and potassium (slope, 0.51 +/- 0.04 SD), whereas there was no association with potassium in the low homozygotes, suggesting a genetic defect involving the kallikrein response to potassium. The model predicted that an increase in urinary potassium excretion of 0.8 SD above the mean in these pedigrees would be associated with high kallikrein levels in the heterozygotes similar to the high homozygotes. A decrease of 1.3 SD in urinary potassium excretion in heterozygous individuals was associated with kallikrein levels similar to the homozygous individuals with low kallikrein. Because in the steady state urinary potassium represents dietary potassium intake, this study suggests that an increase in dietary potassium intake in 50% of these pedigree members, estimated to be heterozygous at the kallikrein locus, would be associated with an increase in an underlying genetically determined low kallikrein level.(ABSTRACT TRUNCATED AT 250 WORDS)

Variance components/major locus likelihood approximation for quantitative, polychotomous, and multivariate data

Pearson [Philos Trans R Soc Lond [A] 200:1-66, 1903], Mendell and Elston [Biometrics 30:41-57, 1974], and Rice et al. [Biometrics 35:451-459, 1979] approximated the likelihood of the multifactorial model on a dichotomous phenotype by a procedure of successive univariate computation and conditioning. Hasstedt [Pap: Pedigree Analysis Package, Rev. 3. 1989] and Demenais [Am J Hum Genet 49:773-785, 1991] extended the algorithm to include a major locus. Here I extend the algorithm to polychotomous, quantitative, and multivariate phenotypes, add a major locus to the model, and describe and evaluate the accuracy of an approximation of the resulting variance components/major locus model.

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.

Prevention of familial cardiovascular disease by screening for family history and lipids in youths

We analyzed medical family history information from 51,053 families of high school students in Utah and Texas and cholesterol measurements from 853 youths and 1618 adults in Utah families with cardiovascular disease (CVD) to assess the utility of different approaches to risk-factor evaluation for youths. The major question addressed was in which youths should blood cholesterol be tested? Applying National Cholesterol Education Program recommendations suggested that 36% in Utah and 38% in Texas be tested. Heterozygous familial hypercholesterolemia (hFH) is the best documented and most serious cholesterol disorder readily diagnosed in youths. In Utah families ascertained for CVD in adults, blood cholesterol levels among youths were significantly bimodal with hFH present in 84% of youths in the upper cholesterol mode. Blood cholesterol levels in adults from the same families were less bimodal with hFH present in 38% of adults in the upper mode. More overlap existed between high and normal modes in adults than in youths. Data from this study suggest that family histories and cholesterol concentrations obtained from high school students may meet the needs of cholesterol screening, education, and follow-up in a controlled and cost-effective setting.

Major gene effect for insulin levels in familial NIDDM pedigrees

Insulin resistance and hyperinsulinemia are familial traits that may precede and predict the onset of non-insulin-dependent diabetes mellitus (NIDDM). In some populations, the distribution of fasting insulin levels and measures of in vivo insulin action suggest the effects of a single major gene. We previously noted hyperinsulinemia among unaffected members of 16 large white pedigrees ascertained through two or more NIDDM siblings. To examine the hypothesis that insulin levels are determined by a single major genetic locus, we used segregation analysis to examine fasting insulin levels in 206 family members and 65 spouses who had normal glucose tolerance tests by World Health Organization criteria. Segregation analysis supported a major locus determining fasting insulin levels and segregating as an autosomal recessive allele with a frequency of 0.25. Thus, homozygotes represented 6.25% of the population, and homozygosity for the hyperinsulinemia allele elevated the mean fasting insulin level from 70.3 to 211.1 pM (11.7-35.2 microU/ml). The analysis apportioned the variance in fasting insulin as 33.1% due to the major autosomal locus, 11.4% due to polygenic inheritance, and 55.5% due to unmeasured effects. Homozygotes for the recessive allele had higher 1-h insulin levels than all others (911.7 vs. 427.2 pM [152.0 vs. 71.2 microU/ml]). We also found evidence for a major locus determining 1-h-stimulated insulin levels, with codominant inheritance as the most likely pattern in inheritance. The causal relationship between these findings and NIDDM has not been determined, and segregation of direct measures of insulin action remains to be demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

Iron overload in Africa. Interaction between a gene and dietary iron content

BACKGROUND AND METHODS

In contrast to hemochromatosis, which in white populations is inherited through a gene linked to the HLA locus, iron overload in sub-Saharan Africa is believed to result solely from increased dietary iron derived from traditional home-brewed beer. To examine the hypothesis that African iron overload also involves a genetic factor, we used likelihood analysis to test for an interaction between a gene (the hypothesized iron-loading locus) and an environmental factor (increased dietary iron) that determines transferrin saturation and unsaturated iron-binding capacity. We studied 236 members of 36 African families chosen because they contained index subjects with iron overload. Linkage to the HLA region was tested with use of lod scores.

RESULTS

In the index subjects, increased iron was present in both hepatocytes and cells of the mononuclear-phagocyte system. Among family members with increased dietary iron due to the consumption of traditional beer, transferrin saturation in serum was distributed bimodally, with 56 normal values (less than 60 percent saturation) and 44 elevated values; the mean serum ferritin concentration was five times higher in the subjects with elevated transferrin saturation (P less than 0.005). The pedigree analysis provided evidence of both a genetic effect (P less than 0.005) and an effect of increased dietary iron (P less than 0.005) on transferrin saturation and unsaturated iron-binding capacity. In the most likely model, increased dietary iron raised the mean transferrin saturation from 30 to 81 percent and lowered the mean unsaturated iron-binding capacity from 38 to 13 mumol per liter in subjects heterozygous for the iron-loading locus. The hypothesis of tight linkage to HLA was rejected.

CONCLUSIONS

Iron overload in Africa may be caused by an interaction between the amount of dietary iron and a gene distinct from any HLA-linked gene.

Complex segregation analysis of autism

A complex segregation analysis of autism in 185 Utah families was carried out using the mixed model. The 209 affected individuals in these families represent nearly complete ascertainment of the autistic cases born in Utah between 1965 and 1984. The sibling recurrence risk for autism was 4.5% (95% confidence limits 2.8%-6.2%). Likelihoods were maximized for major-gene models, a polygenic model, a sibling-effect model, and a mixed model consisting of major-gene and shared-sibling effects. The analysis provided no evidence for major-locus inheritance of autism. Subdivision of the sample according to the probands' IQ levels showed that sibling recurrence risk did not vary consistently with IQ level. A segregation analysis of families in which the proband had an IQ less than 50 also failed to provide evidence for a major locus. However, because of the etiologic heterogeneity of this disorder, genetic analysis of other meaningful subsets of families could prove informative.

Are there interactions and relations between genetic and environmental factors predisposing to high blood pressure?

An overview of published observations suggests that both genetic predisposition and environment work together to produce hypertension in most persons. High blood pressure before age 55 occurs 3.8 times more often among persons with a strong positive family history of high blood pressure. Much of the total variability in blood pressure in modern populations seems attributable to genetic factors. Estimates of the proportion of the variance attributable to all genetic factors (heritability [H2]) vary from 25% in pedigree studies to 65% in twin studies for sitting diastolic blood pressure. Several biochemical traits associated with high blood pressure are highly genetic (H2, 78-84%) and may help elucidate the pathophysiology of high blood pressure. While pertinent environmental factors such as salt intake, alcohol use, and amount of exercise also correlate significantly among relatives, only 7% of the total variance of diastolic blood pressure seems attributable to all shared environmental factors in family households. Thus most familial aggregation of high blood pressure appears to be due to genes rather than shared family environment. Practical benefit may result from identifying familial predisposition in multiple siblings with high blood pressure before age 55 and lipid abnormalities (labeled "familial dyslipidemic hypertension"). About 12% of high blood pressure patients have familial dyslipidemic hypertension and also have high risk of early coronary heart disease. Hyperinsulinemia and central obesity as well as high triglycerides and low high density lipoprotein cholesterol are prominent features of familial dyslipidemic hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic traits related to hypertension and electrolyte metabolism

The genetic and cultural heritability and intercorrelation of traits related to hypertension have been carried out in 98 Utah pedigrees (2,500 person) and 58 sibships with two or more hypertensive persons (131 hypertensive persons). Although none of these traits has been established as a marker for "sodium-sensitive hypertension," many of them are related at least indirectly to both electrolyte metabolism and risk of hypertension. Significant recessive monogenic effects and high total heritability (52-84%) were found for urinary kallikrein, high fat pattern index, intraerythrocytic sodium, Na-Li countertransport, and ouabain binding sites. Familial correlations more strongly attributable to shared environment than to genetic effects were found for Na,K-ATPase pump activity, intraerythrocytic magnesium, plasma digoxin-like factor, plasma renin activity, and plasma sodium concentration. All anthropometric variables tested showed highly significant genetic heritability with low and insignificant shared family environmental effects. Several of the genetically determined cellular cation tests also correlated with other genetic traits including plasma lipids, anthropometric measurements, and other cellular cation tests. Among hypertensive individuals with familial dyslipidemic hypertension, plasma insulin levels correlated with obesity and lipid abnormalities and with several cellular cation flux tests associated with hypertension.

A prospective study of sodium-lithium countertransport and hypertension in Utah

A 7-year prospective study of a cohort of 1,458 normotensive adults from Utah pedigrees, screened from 1980 to 1985, was done to determine whether baseline levels of sodium-lithium countertransport were associated with an increased risk of future hypertension. Subsequent new hypertension (n = 39) was ascertained in 1989 from detailed follow-up medical questionnaires (67% response). Previous segregation analyses on a subset of these pedigree members who responded (n = 342) using family relationships in addition to countertransport levels have shown statistically inferred major gene segregation of sodium-lithium countertransport levels. In the normotensive adults inferred by segregation analysis to carry the recessive major gene for high sodium-lithium countertransport, new-onset hypertension occurred in 18.8% (3 of 16) compared with 3.7% (12 of 326) in the low sodium-lithium countertransport genotype group (relative risk, 4.6 [1.6, 13.9]; p = 0.03). However, an elevated baseline sodium-lithium countertransport level without genotype information from segregation analysis did not increase the risk of future hypertension in the complete cohort of adult pedigree members (relative risk, 1.02 [0.85, 1.22]). Adjustment for other risk factors reduced the relative risk to 0.90 (0.72, 1.11). We conclude that the presence of a major gene for sodium-lithium countertransport or another closely linked gene, rather than the actual level of sodium-lithium countertransport, may increase the risk of hypertension onset. High sodium-lithium countertransport levels do not increase the risk of future hypertension for individuals in whom only polygenic and environmental effects determine sodium-lithium countertransport level.(ABSTRACT TRUNCATED AT 250 WORDS)

A variance components/major locus likelihood approximation on quantitative data

The variance components/major locus model encompasses a major locus, a polygenic component, and shared environmental effects. The model attributes familial correlations to polygenic and shared environmental effects when testing for major locus inheritance or accounts for the major locus when estimating variance components. Because exact computation of the likelihood of the variance components/major locus model on quantitative data requires excessive computer time, I developed an approximation. The approximation retained the general shape of the likelihood surface. Accuracy of the approximation did not vary consistently with allele frequency or the size of the major locus effect.

Multigenic human hypertension: evidence for subtypes and hope for haplotypes

Hypertension that occurs before the age of 60 years is strongly aggregated in families, mostly due to genetic factors with weaker contributions from a shared family environment. Hypertension is probably a heterogeneous collection of overlapping subsets of pathophysiological mechanisms, such as dyslipidemia, obesity, hyperinsulinemia and cation metabolism. Highly heritable traits such as sodium-lithium countertransport, urinary kallikrein excretion and a body fat pattern index show evidence of major gene segregation in families with hypertension. They are thought to be intermediate phenotypes in the chain of pathophysiological events leading from specific genes to the distant phenotype of hypertension. They provide evidence of measurable contributions from single gene traits to the susceptibility to hypertension. Genetic linkage studies have suggested that other specific loci (e.g. histocompatibility leukocyte antigen, blood group MN and the haptoglobin protein) contribute to the susceptibility to hypertension. DNA sequencing has shown a point mutation for lipoprotein lipase that conveys susceptibility to lipid abnormalities, and possibly also hypertension, as seen in families with dyslipidemic hypertension. Further application of these approaches, especially in families that include multiple siblings with hypertension, shows promise of a true understanding of how the combined effects of a few specific genes, the polygenic background and selected environmental factors can lead to essential hypertension. This understanding should foster better tailored and more effective approaches to the prevention, diagnosis and treatment of hypertension.

Population-based frequency of dyslipidemia syndromes in coronary-prone families in Utah

The frequency of familial dyslipidemia syndromes was determined from blood tests in 33 objectively ascertained families with early coronary heart disease (CHD) (two or more siblings with CHD by the age of 55 years). Three fourths of persons with early CHD in these families had 90th percentile lipid abnormalities (cholesterol level at or above the 90th percentile, triglyceride level at or above the 90th percentile, and/or high-density lipoprotein cholesterol (HDL-C) level at or less than the 10th percentile). The HDL-C and triglyceride abnormalities were twice as common as low-density lipoprotein-cholesterol abnormalities. The most common syndromes found were familial combined hyperlipidemia (36% to 48% of families with CHD), familial dyslipidemic hypertension (21% to 54% of families with CHD), and isolated low levels of HDL-C (15%), with overlapping familial dyslipidemic hypertension with familial combined hyperlipidemia and low-level HDL-C. Well-defined monogenic syndromes were uncommon: familial hypercholesterolemia being 3% and familial type III hyperlipidemia, 3%. Another 15% of families with CHD had no lipid abnormalities at the 90th percentile. Physicians should learn to recognize and treat these common familial syndromes before the onset of CHD by evaluating family history and all three standard blood lipid determinations. Failure to recognize and treat them leaves affected family members at high risk of premature CHD.

Genetics of hypertension: what we know and don't know

Human arterial hypertension is likely a multifactorial trait resulting from multiple measurable monogenes, blended polygenes, shared family environment, and individual environment. Familial aggregation of hypertension and familial correlation of blood pressure appears to be more due to genes than to shared family environment. Total genetic heritability of 80% with some recessive major gene effects have been found for several traits associated with hypertension including urinary kallikrein excretion, intraerythrocytic sodium, and sodium-lithium countertransport. Other interesting factors regarding hypertension genetics include: non-modulation of the renin angiotensin system, intralymphocytic sodium, ionized calcium, and several genetic markers such as haptoglobin, HLA, and MNS blood type. Probably the most clinically useful information regarding the genetics of hypertension is evolving in several studies reporting a strong association of hypertension with dyslipidemia, diabetes, and obesity.

Current knowledge regarding the genetics of human hypertension

Observations over 11 years from the University of Utah Cardiovascular Genetics Research Clinic and published data from other studies are reviewed to illustrate research approaches, developing results and prospects for future studies. Strong associations with hypertension have been found for several biochemical tests that show substantial genetic determination. Suggestions of recessive major gene effects and significant polygenic background determinations have been found for several variables, including urinary kallikrein excretion, intracellular sodium concentration, sodium-lithium countertransport and sodium-potassium cotransport. Each of these variables is related in some way to sodium or potassium metabolism, or both, and may help to improve the understanding of a possibly inherited susceptibility to hypertension that is related to dietary electrolyte intake. A second major group of factors involving familial predisposition to hypertension include lipid abnormalities (increased very-low- and low-density lipoprotein cholesterol and decreased high-density lipoprotein cholesterol); increased fasting insulin levels or insulin resistance, or both; obesity (especially central or upper body obesity); and multiple environmental factors influencing these metabolic systems, including dietary fat, carbohydrate and calorie intake; physical exercise; and certain antihypertensive medications that adversely affect lipid metabolism and glucose tolerance. Some studies even suggest a possible link between these two large groups of factors (electrolyte metabolism and lipid-insulin metabolism). Hypertriglyceridaemia and hyperinsulinaemia are both significantly correlated with increased levels of several cation-flux tests. It is recommended that studies of human hypertension apply these biochemical profiles to study sibships with two or more hypertensive siblings as a cost-effective initial approach.(ABSTRACT TRUNCATED AT 250 WORDS)

Recessive inheritance of a relative fat pattern

We defined a relative-fat-pattern index (RFPI) as the ratio of subscapular skinfold thickness to the sum of subscapular and suprailiac skinfold thicknesses and computed RFPI for 774 adults (age greater than or equal to 25 years) in 59 pedigrees ascertained through cases of cardiovascular disease. Likelihood analysis of RFPI supported recessive inheritance of an allele with a frequency of 46%, which elevated mean RFPI from .412 to .533 when homozygous. The analysis apportioned the variance in RFPI as 42.3% due to the major locus, 9.5% due to polygenic inheritance, and 48.2% due to random environmental effects. Homozygotes for the recessive allele tended to have small suprailiac skinfold thicknesses rather than large subscapular skinfold thicknesses. Homozygotes were more frequent in younger than in older cases of obesity, coronary heart disease, essential hypertension, and diabetes mellitus; the increase was significant for all but diabetes.

Recessive inheritance of a high number of sodium pump sites

The number of sodium pump sites on erythrocytes was measured on 1,847 individuals in 80 Utah kindreds ascertained through probands with cardiovascular disease. Likelihood analysis supported recessive inheritance of high pump number. The major locus explained 14.0% of the variance in pump number; polygenic inheritance explained another 63.4%. Homozygotes for the recessive allele occurred with a frequency of 1.74% and had a mean pump number estimated as 566.0 sites/red blood cell (RBC) versus a mean of 312.2 sites/RBC for the other genotypes. Young individuals with the high pump number genotype were leaner, and older adults with the high pump number genotype were heavier. Diabetes and early hypertension were more prevalent in women with the high pump number genotype. Although not significant, obesity in adults of both sexes and early coronary heart disease in men were more prevalent in individuals with the high pump number genotype.

Genetic heritability and common environmental components of resting and stressed blood pressures, lipids, and body mass index in Utah pedigrees and twins

The relative contributions of genes and shared environment to cardiovascular risk factors were studied in twins and pedigrees in 1983-1985. Sitting, standing, isometric hand grip, bicycling, and mentally stressed (serial subtraction) blood pressures were obtained from 146 male monozygous twins, 162 male dizygous twins, and 1,102 healthy adults in 67 Utah pedigrees. Fasting total plasma cholesterol, triglycerides, high density lipoprotein cholesterol (HDL), and body mass index were also measured. Heritability was estimated before and after adjusting for 12 environmental variables (measures of socioeconomic status; personality types; exercise levels; use of tobacco, alcohol, coffee, etc.) by using age-adjusted twin intraclass correlations. These heritabilities were compared with those obtained from a variance components analysis of the pedigree data separating genetic and common household effects. Sitting and standing blood pressure heritability estimates were much higher from twin than from pedigree data (39-63% in twins vs. 16-22% in pedigrees), as were those for cholesterol and triglycerides (65 and 75% from twins vs. 42 and 37% from pedigrees) and body mass index (51 vs. 21%). Estimates were similar for heritability of HDL cholesterol (51 vs. 45%). Most of the stressed blood pressure heritabilities were similar to sitting blood pressure estimates. No common household effect (except for adjusted HDL cholesterol (24%), p less than 0.01) was statistically significant for the lipids, blood pressures, or body mass index. Environmental variables correlated much better in monozygous twins and spouses than in dizygous twins, brothers, or sisters. Spouse correlations for lipids, blood pressures, and body mass index were low, with a maximum of 0.12 (p less than 0.05) for HDL cholesterol. We conclude that genes contribute much more than shared environment to the well-recognized familial correlation of blood pressures, lipids, and body mass index.

A gene for high urinary kallikrein may protect against hypertension in Utah kindreds

The inheritance of 12-hour overnight total urinary kallikrein excretion and its association with family history of essential hypertension were studied in 405 normotensive adults and 391 youths in 57 Utah pedigrees. Total urinary kallikrein excretion was highly familial with 51% of the total variance attributable to a dominant allele for high total urinary kallikrein excretion and 27% attributable to the combined effects of polygenes and shared family environment. An estimated 28% of the population has one or two copies of the dominant allele for high total urinary kallikrein excretion (2.3 SD units higher than the low homozygotes). About 83% of the population could be assigned to one of the two genotypic populations. Individuals with the high total urinary kallikrein excretion genotype were significantly less likely to have one or two hypertensive parents (relative odds = 0.56, p = 0.042). We conclude that a dominant allele expressed as high total urinary kallikrein excretion may be associated with decreased risk of essential hypertension. Further studies should be performed to confirm this finding and to test for interactions between this apparently protective gene and other genetic and environmental determinants of essential hypertension.

Estimation of genetic model parameters: variables correlated with a quantitative phenotype exhibiting major locus inheritance

A major locus that is detected through its effect on one phenotype (a primary trait) may also affect other quantitative phenotypes or qualitative disease endpoints (secondary traits). The pattern of effects of the major locus on a set of primary and secondary traits suggests candidate defects for the mutant allele. The effects are directly estimable when "measured genotypes" or a tightly linked marker allow unambiguous assignment of major locus genotypes. When genotypes assignments are ambiguous for a major locus detected through its effect on a quantitative primary trait, we propose estimators using genotypic probabilities. Making certain reasonable assumptions, we demonstrate asymptotic unbiasedness of these genotypic probability estimators of the genotypic means and variances for either the quantitative primary or secondary traits, of the covariances between quantitative primary and secondary traits, and of prevalences for the secondary qualitative traits. An important application of genotypic probability estimators is to define an effect of a major locus that cannot be detected upon analysis of the variable; for example, major locus effects may be defined for hypertension or blood pressure as secondary traits, but not detected as primary traits.