© 1999 American Heart Association, Inc.
Atherosclerosis and Lipoproteins |
Linkage of a Candidate Gene Locus to Familial Combined Hyperlipidemia
Lecithin:Cholesterol Acyltransferase on 16q
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Abstract |
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Abstract—Familial combined hyperlipidemia (FCHL) is a common lipid disorder characterized by elevated levels of plasma cholesterol and triglycerides that is present in 10% to 20% of patients with premature coronary artery disease. To study the pathophysiological basis and genetics of FCHL, we previously reported recruitment of 18 large families. We now report linkage studies of 14 candidate genes selected for their potential involvement in the aspects of lipid and lipoprotein metabolism that are altered in FCHL. We used highly polymorphic markers linked to the candidate genes, and these markers were analyzed using several complementary, nonparametric statistical allele-sharing linkage methodologies. This current sample has been extended over the one in which we identified an association with the apolipoprotein (apo) AI-CIII-AIV gene cluster. We observed evidence for linkage of this region and FCHL (P<0.001), providing additional support for its involvement in FCHL. We also identified a new locus showing significant evidence of linkage to the disorder: the lecithin:cholesterol acyltransferase (LCAT) locus (P<0.0006) on chromosome 16. In addition, analysis of the manganese superoxide dismutase locus on chromosome 6 revealed a suggestive linkage result in this sample (P<0.006). Quantitative traits related to FCHL also provided some evidence of linkage to these regions. No evidence of linkage to the lipoprotein lipase gene, the microsomal triglyceride transfer protein gene, or several other genes involved in lipid metabolism was observed. The data suggest that the lecithin:cholesterol acyltransferase and apolipoprotein AI-CIII-AIV loci may act as modifying genes contributing to the expression of FCHL.
Key Words: familial combined hyperlipidemia • lipid metabolism • genetics • lecithin:cholesterol acyltransferase
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Familial combined hyperlipidemia (FCHL) is the most common genetic lipid disorder observed in patients with coronary artery disease and their relatives, with a frequency of
1% in the US white and European populations. FCHL occurs in 10%
to 20% of patients with premature coronary artery
disease.1 2 3 4 5 6 7 8 9 The disorder is characterized by elevated
concentrations of plasma triglycerides and/or plasma
cholesterol in probands and their affected relatives.
Moreover, affected relatives exhibit multiple lipoprotein
phenotypes, a predisposition to premature coronary
artery disease, and a vertical pattern of transmission of
hyperlipidemia.1 Early studies suggested
that the familial aggregation of FCHL was consistent with a
dominant, mendelian trait,1 8 9 but subsequent studies
have suggested a more complex mode of
inheritance.5 7 9 10 11 12 One characteristic of FCHL is the
variability in lipoprotein profile, referred to as multiple lipoprotein
phenotype, seen in affected relatives; thus, affected
individuals may exhibit a Fredrickson type IIa, IIb, or IV
hyperlipidemia.1 13 The fully expressed
FCHL phenotype involves elevated levels of LDL, IDL, and VLDL.
These particles have a precursor-product relationship, as the liver
secretes VLDL, which then undergoes lipolysis to IDL and subsequently
to LDL. The principal structural protein of these particles is
apolipoprotein B (apoB), an integral membrane protein required for VLDL
packaging. Elevated levels of apoB are a useful indicator of
FCHL.13 Other traits frequently associated with FCHL
include small, dense LDL particles, low HDL levels, insulin resistance,
and elevated postprandial levels of free fatty
acids.3 12 14 15 16 17 The molecular mechanisms contributing to FCHL are largely unknown. Because hypertriglyceridemia is often the earliest manifestation of FCHL, it has been proposed that the primary defect(s) involves triglyceride metabolism, with secondary effects on cholesterol metabolism.1 18 Physiological studies have suggested that patients with FCHL have increased secretion of VLDL triglycerides and associated apoB,18 19 20 21 22 23 raising the possibility that this is the primary metabolic defect. There is also substantial evidence that the FCHL phenotype results in part from defects in lipoprotein catabolism. In particular, about one third of FCHL patients exhibit decreased levels of lipoprotein lipase (LPL), the enzyme primarily responsible for lipolysis of chylomicrons and VLDL,4 24 25 although only a small fraction of this decrease in LPL levels appears to be due to mutations of the LPL gene.26 27 28 29 30 Furthermore, there is impaired removal of chylomicron remnants after an oral fat load, and this is associated with increased plasma apoCIII concentrations.5 31 32 We and others have observed significant evidence for linkage (and association) between specific alleles of the apoAI-CIII-AIV gene cluster and FCHL.32 33 34 A compelling hypothesis for FCHL is that it involves 1 or more primary gene defects affecting VLDL synthesis. These defects then act in combination with various secondary variations, most likely those affecting lipoprotein catabolism, such as the decrease in LPL enzyme activity or variations at the apoAI-CIII-AIV gene cluster.13 32 35 This can explain the variable expression of the FCHL phenotype among family members and the complex mode of inheritance.
A relatively small number of candidate genes for FCHL, such as apoB, LPL, and the apoAI-CIII-AIV cluster, have been previously examined, mostly by association studies. The results suggest that apoB gene variations do not contribute significantly to the trait36 37 and that the LPL and apoAI-CIII-AIV genes are likely to play a role as secondary or modifier genes.28 29 30 32 33 36 38 To further identify potential primary and secondary defects contributing to the expression of FCHL, we studied 14 separate candidate gene loci in 18 well-characterized FCHL families. Linkage results reveal a new locus that appears to contribute to the expression of this disorder, the lecithin:cholesterol acyltransferase (LCAT) gene locus on chromosome 16.
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Methods |
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Ascertainment of FCHL Study Population
Eighteen unrelated FCHL probands were recruited through the Lipid Clinic of the Utrecht University Hospital. The probands met the following minimum criteria: (1) a primary hyperlipidemia with varying phenotypic expression: plasma cholesterol >250 mg/dL (>6.5 mmol/L) and/or triglycerides >200 mg/dL (>2.3 mmol/L), in addition to apoB concentrations exceeding the mean +2 SDs for age-adjusted levels; (2) at least 1 first-degree relative with a different hyperlipidemic phenotype; (3) a positive family history of premature coronary heart disease defined as myocardial infarction or cardiovascular disease before 60 years of age; and (4) absence of xanthomas. Exclusion criteria included diabetes, obesity, familial hypercholesterolemia (absence of tendon xanthomas), or type III hyperlipidemia (absence of apoE2/E2). All subjects gave informed consent, and the Human Investigation Review Committee of the University Hospital, Utrecht, approved the study protocol. Families were ascertained as previously described.5 31 32 33 The clinical and biochemical characteristics of the study population (n=481), which comprised 18 probands, their hyperlipidemic and normolipidemic relatives, and spouses, are summarized in Table 1
. Over 95% of the relatives and spouses
of probands were collected without regard to their FCHL status. In the
present study, DNA samples from 481 subjects of the original data
set (n=582)32 33 were available for molecular and linkage
analysis. Hyperlipidemic family members (n=151)
were assigned the FCHL phenotype based on plasma
cholesterol >250 mg/dL and/or plasma
triglycerides >200 mg/dL and/or apoB >75th percentile for
age-matched controls.15 Plasma apoB concentrations were
used as an additional marker to identify affected FCHL subjects,
complementing the original definition by Goldstein et al,1
who used 90th percentile cholesterol and 90th
percentile triglyceride as cutoff points. Plasma apoB
concentrations reflect the combined effects of high hepatic apoB
secretion22 23 24 25 and impaired removal of apoB-containing
lipoproteins in FCHL.17 Normolipidemic family members
(n=176) exhibited plasma cholesterol <250 mg/dL, plasma
triglycerides <200 mg/dL, and apoB levels <75th
percentile matched for age and sex. Additionally, normolipidemic
relatives were younger than hyperlipidemic relatives
and spouses. It is possible that a subset of these normolipidemic
individuals have yet to express the FCHL phenotype. As
previously reported, hyperlipidemic family members had
higher plasma cholesterol, triglycerides, LDL
cholesterol, and apoB concentrations than did
normolipidemic relatives and spouses.
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Genetic Markers for Candidate Genes
Genetic markers appropriate for testing specific candidate loci
have been previously reported for a number of the genes studied
herein.39 40 41 42 43 44 45 46 47 These include the apoAI-CIII-AIV gene
cluster, apoAII, apoB, apoE, carboxyl ester lipase (CEL), cholesteryl
ester transfer protein (CETP), hepatic lipase (HL), LCAT, LDL receptor
(LDLR), LPL, manganese superoxide dismutase (MnSOD), microsomal
triglyceride transfer protein (MTP), peroxisome
proliferator-activated receptor
(PPAR), and the VLDL
receptor (VLDLR) (Table 2). Four
candidate loci (CEL, CETP, HL, and LCAT) had not previously been mapped
in detail; in these cases, polymorphic genetic markers were
selected by employing a radiation hybrid mapping
strategy.42 48 49 51 The genetic markers were then
genotyped in 481 individuals of the 18 FCHL kindreds. DNA was
isolated from EDTA-augmented blood by following standard procedures,
and polymerase chain reaction amplification was conducted according to
recommended protocols.41 ApoE genotypes were
determined as described previously.17
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Linkage Methods
To assess the cosegregation of the FCHL phenotypes with
genetic markers at the 14 candidate loci, we used several
nonparametric linkage tests, ie, tests for which the mode
of inheritance of the disease does not have to be
hypothesized.52 53 At each candidate gene locus, the
markers were tested for linkage individually. Initially, this was done
using only affected sibling pairs. Positive linkage was followed by a
linkage analysis of the data from all available sibling pairs
in the sample. When significant evidence of linkage was observed for
the individual marker, multiple markers in the same chromosomal region
were typed and combined into haplotypes to maximize the linkage
information.
The advantage of analyzing only affected sibling pairs for a disease with complex inheritance, such as FCHL, is that each analyzed individual is assumed to have 1 or more of the disease genes, and the results are not confounded by reduced penetrance. However, because multiple genes may contribute to a complex disorder such as FCHL, different combinations of such genes may lead to clinical disease. This reduces the power of any linkage analysis, since in such a case not all affected pairs will demonstrate allele sharing at any given disease gene locus. Nevertheless, if any given locus contributes to the disease in a large proportion of the cases, affected sib pairs will share that gene in a greater proportion than expected by chance. Any 2 random sib pairs are expected to share a given allele 50% of the time. If there is linkage of the disease to a specific locus, then affected sib pairs are expected to exhibit a mean allele sharing significantly >0.5. We calculated the mean allele sharing at each locus of interest and tested each for a significant difference from the expected value of 0.5 by using the SIBPAL subprogram of the SAGE package.54
When a candidate gene showed evidence for linkage, we extended the analyses to include 2 additional groups of sib pairs, 1 in which both sibs were unaffected (clinically concordant unaffected sibling pairs) and another in which 1 was affected and the other was not (clinically discordant sibling pairs). At linked markers, both clinically concordant affected and clinically concordant unaffected sib pairs are expected to demonstrate an increased sharing of marker alleles identical-by-descent, while clinically discordant pairs should exhibit decreased sharing below the expected value of 0.50.55 Clinically concordant unaffected and clinically discordant sib pairs may provide weaker evidence for linkage than affected sib pairs, as some unaffected siblings may carry the disease gene but not express it owing to the lack of other genetic and/or environmental factors that contribute to the development of the disease (ie, reduced penetrance).
Haplotype information can improve the significance level of a true linkage result by increasing the informativeness over that of single markers. Often, haplotyping allows the number of alleles shared identical-by-descent at a given marker to be assessed unambiguously. Because the result for LCAT was new and significant by our criterion, we typed an additional 3 markers and tested the haplotype for linkage to FCHL. To incorporate information from all pairs, we used the Haseman-Elston52 linear regression method as programmed in the SIBPAL subroutine of SAGE. For each sibling pair, the squared difference in the disease status (affected pairs are coded as 1 and unaffected pairs are coded as zero) is regressed against their estimated proportion of haplotypes shared identical-by-descent. A significant negative regression, representing a greater proportion of shared haplotypes for those who are clinically concordant, is taken as evidence for linkage.
In addition to FCHL, 4 related quantitative traits were assessed for linkage to the 14 candidate genes. The squared trait differences between sib pairs for levels of total plasma triglyceride, cholesterol, apoB, and apoCIII were regressed against the number of marker alleles that they share identical-by-descent for each locus.52 53 54 A significantly negative regression line was taken as evidence for linkage. We performed all quantitative analyses initially on the untransformed data; logarithmically transformed data were analyzed when the quantitative trait distribution was nonnormal. Because only candidate genes were tested in these analyses, a significance level of 0.001 was chosen as the criterion for indicating evidence for linkage.
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Results |
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Fourteen candidate loci were tested for linkage to FCHL in 18 FCHL families consisting of 481 individuals. The genes selected have been implicated in VLDL synthesis and secretion (apoB, MTP), lipolysis (CEL, HL, LPL), lipoprotein processing and remodeling (apoAI-CIII-AIV, CETP, LPL, HL, LCAT), HDL metabolism (apoAII, HL, LCAT, CETP, LPL), lipoprotein removal (apoE, LDLR, VLDLR), and cellular metabolism (PPAR
, MnSOD).4 25 56 57 58 59 60 61 62 63 64 Highly
informative genetic markers (Table 2
) either at or near the
candidate loci were chosen on the basis of previous studies or after
physical mapping of the candidate genes by radiation hybrid
analysis (see Methods). By using a nonparametric
sib-pair linkage methodology, 11 of the candidate gene loci did not
exhibit significant evidence of linkage to the qualitative FCHL trait
(P>0.05, Table 3). Although
not meeting the formal statistical threshold of P<0.001,
the MnSOD locus on chromosome 6 did yield suggestive evidence of
linkage (P<0.006), with a mean allele sharing of 0.57
(data not shown). Two candidate loci, the apoAI-CIII-AIV locus
(chromosome 11) and the LCAT locus (chromosome 16), exhibited
significant evidence of linkage (Table 3). Mean allele
sharing in affected sibling pairs at the markers linked to apoCIII and
LCAT was 0.62 and 0.59, respectively, significantly different from the
expected value of 0.50 when there is no linkage to the region.
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We then tested the FCHL-associated traits of triglycerides, total cholesterol, apoB, and apoCIII levels for linkage to the LCAT, apoAI-CIII-AIV, and MnSOD chromosomal regions. Two of these markers demonstrated possible linkage to the constellation of FCHL-related traits: the LCAT locus on chromosome 16 was linked to the traits of cholesterol (P<0.03) and apoCIII levels (P<0.03), and the MnSOD locus on chromosome 6 was linked to the traits of cholesterol (P<0.02), apoB levels (P<0.02), and apoCIII levels (P<0.03).
To further characterize the LCAT region, 3 additional markers were
typed, D16S514, D16S400, and D16S408, spanning, in addition to
the original marker, a 12-cM distance. These markers were combined into
a haplotype, thus allowing assessment of their inheritance as a group.
As shown in the Figure, the allele
sharing for the haplotype exhibited a pattern of excess sharing for
both affected and unaffected concordant pairs as well as reduced
sharing in clinically discordant pairs, providing additional support
for linkage of the LCAT gene region to FCHL. Discordant disease status
(1 affected and 1 unaffected) in the sib pair should result in
decreased allele sharing, while the same disease status (either
both affected or both unaffected) should result in increased sharing.
The proportion of haplotypes shared identical-by-descent by affected
pairs was 0.60, and the linear regression linkage analysis
yielded a P value of 0.002.
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) for the 16q haplotype and with a minus
(-) for no linkage (the null hypothesis). |
Discussion |
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The molecular and genetic mechanisms contributing to the pathogenesis and expression of FCHL have remained largely unknown since the original delineation of the disorder by Goldstein and colleagues.1 The analyses presented herein have revealed a new locus showing significant evidence of linkage to FCHL: the LCAT locus on chromosome 16. In addition, the previously reported linkage of the apoAI-CIII-AIV gene cluster with FCHL was supported in an extended sample by using a highly polymorphic genetic marker. The MnSOD locus on chromosome 6 gave possible evidence for linkage to FCHL as well. Several analyses of the data provide linkage evidence for the LCAT locus: first, an increased allele sharing in affected sib pairs; second, a pattern of decreased sharing in discordant sib pairs and increased sharing in unaffected sib pairs expected with linkage; and third, additional support when additional markers and sibling pairs were included. In addition, consistent with the findings of several other studies, our data provide no support for linkage to 5 distinct loci, the apoAII, apoB, HL, LDLR, and LPL loci on chromosomes 1, 2, 15, 19, and 8, respectively. Furthermore, the MTP, VLDLR, and CETP loci on chromosomes 4, 9, and 16, respectively, do not appear to play an important role in the expression of FCHL in this population.
Recently, Porkaa et al13 reported that the 90th percentile cutoff values in the Finnish population for cholesterol and/or triglycerides combined represented the optimal criteria for the diagnosis of FCHL. This finding is in agreement with the original analysis presented by Goldstein et al1 and with our own approach to FCHL phenotyping in Dutch whites.32 We recognize, however, that the current approach to diagnosis should be considered as interim, pending identification of the predisposing variations in the actual susceptibility genes.
Lecithin:Cholesterol Acyltransferase
This enzyme is synthesized in the liver and circulates in the
plasma as a component of HDL particles. The LCAT/CETP locus (the 2
genes are roughly 12 cM apart on chromosome 16) was shown previously to
exhibit significant linkage with LDL particle
size.15 58 65 Our data suggest that LCAT rather than CETP
is the primary locus responsible for the observed linkage, based on the
markers spanning the region. In addition, there is biological evidence
supporting such a conclusion. Subjects with severe combined
hyperlipidemia show significantly increased
cholesterol ester mass transfer from HDL to VLDL but have
reduced or even absent net mass cholesterol ester transfer
from HDL to LDL.65 This phenomenon, which potentially
contributes to small, dense LDL particle formation, is caused by the
low affinity of CETP protein for the LDL from combined
hyperlipidemia subjects as well as increased
concentrations of the most active cholesterol ester
acceptor, VLDL. Guerin et al66 concluded that the
biochemical characteristics of HDL and VLDL lipoproteins in
combined-hyperlipidemia subjects were sufficient
explanation for the altered composition rather than the plasma protein
concentration of CETP itself. Several lines of biological evidence
support this linkage as well. Transgenic rabbits overexpressing LCAT
exhibit elevated levels of VLDL and LDL, similar to the
phenotype in FCHL.67 In addition, fish-eye
disease, a familial syndrome of LCAT deficiency, results in increased
VLDL levels and hypertriglyceridemia, among
other phenotypic abnormalities.49 Thus, LCAT may act as a
modifier of the FCHL phenotype. Alternatively, a gene closely
linked to LCAT must be considered. Whether the LCAT gene is in fact
responsible for the linkage requires further testing in population or
family-based association (linkage disequilibrium) studies.
Apolipoprotein AI-CIII-AIV
Previous association and linkage studies have implicated the
apoAI-CIII-AIV gene cluster in FCHL.32 33 34 Recently, we
carried out an association study confirming the involvement of the
apoAI-CIII-AIV gene cluster in FCHL.32 Three restriction
fragment length polymorphisms revealed that a minor allele was
associated with elevated plasma cholesterol,
triglycerides, LDL cholesterol, apoB, and
apoCIII levels in FCHL probands and hyperlipidemic
relatives versus controls.33 In addition,
nonparametric sib-pair linkage analysis revealed
significant evidence of linkage between these restriction fragment
length polymorphisms in the gene cluster and the FCHL
phenotype.32 Moreover, demonstration of 2
different susceptibility haplotypes in the gene cluster revealed a
paradigm of complex genetic contribution to FCHL.32 A
similar pattern was observed in a recent study of Finnish FCHL
families, although the results did not attain statistical significance
in that sample.68 The data reported here continue to
support the previous linkage finding in our sample by using a more
informative genetic marker (a tetranucleotide repeat within
the apoCIII gene). Although there are fewer sib pairs available for
analysis in this study compared with our previous
study,32 the stronger linkage result with the gene cluster
is likely due to the dramatic increase in the heterozygosity index of
the microsatellite marker compared with the restriction fragment length
polymorphism haplotypes (0.95 and 0.41, respectively) described
previously.15 This finding is complemented by biochemical
studies. Expression of the apoCIII gene has been shown to be a
determinant of plasma VLDL triglyceride levels in
mice.69 In humans, the apoCIII gene affects plasma apoCIII
concentrations, and the elevated apoCIII concentrations in FCHL
subjects predict impaired postprandial lipemia.31 Thus,
the involvement of the apoCIII gene in FCHL expression is not only
evident from the linkage results but biologically plausible as
well.15
Manganese Superoxide Dismutase
Although the results of our candidate gene analysis with
markers linked to the MnSOD gene did not reach the criterion set for
linkage, positive linkage results with related phenotypes in
other samples suggest that this gene locus may have a role in FCHL.
Previously, we reported significant linkage of the MnSOD locus with LDL
particle size in a set of families enriched for coronary artery
disease but without marked
hyperlipidemia.58 We have also observed
linkage of LDL particle size to this locus in FCHL
families15 and in a sample of subjects undergoing dietary
intervention.65 In the present study, the MnSOD gene
was examined because the small, dense LDL trait is commonly associated
with FCHL, and this gene has been correspondingly linked to LDL
particle size.12 14 15 The mechanisms by which MnSOD, a
mitochondrial enzyme responsible for the dismutation of superoxides
generated as a by-product of cellular respiration, may influence
lipid metabolism are yet unclear. Genetic inactivation of
the mitochondrial form of superoxide dismutase in mice results in
abundant hepatic lipid accumulation, dilated
cardiomyopathy, and early neonatal
death.70 MnSOD has been shown to influence cellular
responses to tumor necrosis factor-, which is a potent modulator of
LPL expression.64 Reduced LPL activity is a frequent
finding in FCHL patients.4 71 It is also conceivable that
an increased incorporation of oxidized lipids, which accumulate
intracellularly as a consequence of reduced clearance of superoxides
by MnSOD, may affect the catabolism of VLDL.
Evidence for an Oligogenic Model
Our current understanding of FCHL is that it represents a
complex metabolic syndrome, characterized by hepatic and
possibly intestinal hypersecretion of lipoproteins, which in turn
generates increased flux through the lipolytic cascade and
receptor-mediated uptake routes. Therefore, each of the pathways
involved is vulnerable to gene modifications that reduce the maximal
functional capacity of the pathway under conditions of high lipoprotein
flux caused by hepatic hypersecretion. In our view, major gene effects
are expected to influence the regulation of hepatic lipoprotein
secretion or the level of substrate supply to lipoprotein-secreting
organs. Such major gene effects are expected to be present in all
or the vast majority of FCHL subjects. Modifier gene effects can, at
least in theory, be expected at the level of lipoprotein secretion,
lipolysis, processing, remodeling, or cellular uptake, provided that
the gene(s) involved plays a functional role in lipoprotein
metabolism in FCHL. It is unlikely that LCAT is the primary
determinant of FCHL because it would not be expected to influence VLDL
assembly or secretion. Several kinetic studies have revealed a 2-fold
increase in VLDL production, implicating a defect in VLDL
synthesis as the primary cause.18 19 20 21 22 23 In addition,
modifier loci, which may not significantly influence plasma lipid
levels under normal circumstances, are likely to affect lipid levels
under conditions of increased flux of VLDL in FCHL. The apoAI-CIII-AIV
gene cluster and, less frequently, the LPL locus appear to be 2 such
modifier loci, as identified previously.30 32
ApoAI-CIII-AIV and LCAT are presently considered response modifier
genes. Although the present study has been performed on 1 of the
largest collections of affected FCHL individuals, our ability to detect
modifier gene effects was limited to those with at least an estimated
15% contribution (see below). Modifier genes are expected to be common
in FCHL families, and combinations of modifier genes may prove to be
frequent as well. Various combinations of modifier genes could have an
impact on the expression of the multiple type
hyperlipidemia, characteristic of FCHL.
Evidence from these analyses indicates that multiple
genes contribute to the expression of FCHL. The FCHL genes, which can
be detected by these linkage analyses, can be characterized by
reference to the parameter known as lambda.72
Lambda represents the FCHL risk to the siblings of an affected
individual compared with that of the general population. For FCHL,
lambda may be as large as 25, calculated by dividing the nearly 50%
risk to sibs by the 1% to 2% risk to the general population. Hauser
et al73 have provided tables that can be used to estimate
the minimum lambda of genes that can be identified in an affected
sib-pair linkage analysis wherein the numbers of nuclear
families and degrees of parental genotyping vary. Using
P<0.001 as the level of significance (a log of the odds
score of 2.0) and this sample size of 100 affected sib pairs, Table 2 of their article indicates we should be able to identify an
FCHL gene with a lambda of 2.0 or larger 80% of the time. By using a
multiplicative model for the genetic contribution to FCHL, such a gene
could have a contribution of as little as 15% to this disorder.
In conclusion, we provide evidence herein, by linkage analysis in FCHL families, that 2 distinct loci, the apoAI-CIII-AIV and LCAT loci on chromosomes 11 and 16, respectively, may contribute to the expression of FCHL. These results illustrate the genetic complexity of this disorder and contribute to our understanding of this most common dyslipidemia predisposing to premature coronary artery disease.
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Acknowledgments |
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This study was supported by National Institutes of Health grant HL-28481 (to A.J.L. and J.I.R.), by a grant from the Dutch Heart Foundation (D 91.101) (to T.W.A.d.B.), and by the Cedars-Sinai Board of Governors’ Chair in Medical Genetics (to J.I.R.). The results of the SIBPAL analysis were obtained by using the program package SAGE and were supported by US Public Health Services Resource grant P41 RR03655 from the Division of Resources. T.W.A.d.B. is a recipient of the PIONEER research grant of the Dutch Organization of Scientific Research (NWO). We thank Dr Richard Gregg, Bristol-Myers Squibb Co, for supplying the polymorphic genetic marker for MTP. We are grateful to Dr John Kane, University of California, San Francisco, for discussions and for suggesting PPAR
as a candidate gene. We thank Margreet van Linde-Sibenius for excellent
technical assistance. In addition, we would also like to thank the
patients, relatives, and spouses for participating in this
study. Received November 19, 1998; accepted March 24, 1999.
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