Atherosclerosis and Lipoproteins |
From Cardiovascular Genetics (S.C.H., P.N.H.), Department of Medicine, University of Utah, Salt Lake City; Myriad Genetics Inc, (K.B., M.T.M., T.L.T., B.B.W., D.G.B., M.H.S., M.E.S.), Salt Lake City, Utah; and Novartis Institute for Biomedical Research (B.R.B.), Summit, NJ.
Correspondence to Steven C. Hunt, PhD, University of Utah, Cardiovascular Genetics, 410 Chipeta Way, Room 167, Salt Lake City, UT 84108. E-mail steve{at}ucvg.med.utah.edu
| Abstract |
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17 cM interval between polymorphic markers D1S2130
and D1S1596. This region appears to overlap the region found linked to
severe hypercholesterolemia in French and
Spanish families. The identification of the gene in this region may
provide important pathophysiological insights into
new mechanisms that may lead to highly elevated LDL
cholesterol and other associated dyslipidemic
phenotypes.
Key Words: familial hypercholesterolemia genetics LDL cholesterol linkage analysis
| Introduction |
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There are >600 mutations described for the LDL receptor.7 Despite this large number, mutations have not been identified in up to 30% of all persons with FH. In our recent study, we were able to find specific mutations in only 19 of 47 probands with clinical FH.8 Linkage to the LDL receptor was found in an additional 4 of 5 families large enough to test for linkage. Linkage analysis in 1 pedigree (kindred 1173) clearly excluded linkage to either the LDL receptor or the apo B gene.
FH and FDB are currently underdiagnosed throughout the world, and many of those who have been diagnosed with either of these 2 disorders are not adequately treated.9 10 The statin drugs have greatly increased physicians ability to control high levels of LDL cholesterol, yet not all persons respond equally to treatment. The existence of another locus that causes elevated LDL cholesterol similar to FH and FDB may elucidate other disease mechanisms for which new, effective drugs could be developed. Such drugs could not only help to target the defects of this new locus, but also might be used as therapy for other genetic cholesterol disorders such as FH, FDB, familial combined hyperlipidemia, and polygenic hypercholesterolemia. To investigate the possibility of another locus for elevated LDL cholesterol (referred to as FH38 ), we performed a genome-wide linkage analysis on kindred 1173.
| Methods |
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Blood samples were collected at the University of Utah Cardiovascular Genetics Clinic in the morning after an overnight 12- to 16-hour fast. Measured LDL cholesterol was obtained by using a microscale procedure on a Roche FARA II automated analyzer, as described by Wu et al12 in our laboratory, which participates in the Centers for Disease Control and Prevention Standardization Program. For those persons who were currently on lipid-lowering medications, the highest historically measured or calculated LDL cholesterol value was used to define affected status for analysis. Three persons currently on statin therapy did not have available historical measured lipid levels and were classified as being affected. DNA was extracted from frozen buffy coats by using the Lipid Research Clinics guidelines.13 Written, informed consent was obtained from all subjects, and the project was approved by the University of Utah Institutional Review Board.
Genotyping
We genotyped the 31 family members for 583 anonymous
autosomal markers with an average spacing of 5.7 cM for the genome
search. Most of the markers are trinucleotide or
tetranucleotide repeats and are described in public
databases such as Marshfield (http://www.marshmed.org/genetics),
Genethon (http://www.genethon.fr), and the Human Genome Database (GDB;
http://gdbwww.gdb.org). Polymerase chain reaction products were
analyzed at Myriad Genetics, Inc, on an ABI 377
fluorescent sequencing machines with the use of standard
methods. Allele numbers were internally generated and do not refer
to allele numbers defined in public databases. Inheritance of all
alleles was verified by using the PEDCHECK
program.14 In addition, 2 markers were added at the apo B
locus and 3 markers at the LDL receptor locus to verify absence of
linkage to these 2 genes. The genetic map used for all analyses
and tables was generated internally by Myriad Genetics, Inc, using the
CRI-MAP program15 on 3916 meioses
and closely corresponds to the Marshfield map. However, the Marshfield
map is used in Figure 4
to permit comparisons of recombinant
boundaries between our study and another study.
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Linkage Analysis
Linkage analysis was performed after coding LDL
cholesterol as a dichotomous phenotype as described
above.11 There was clear bimodality of LDL
cholesterol in the pedigree, so all persons could be easily
assigned as affected or unaffected.8 The 7 members of the
pedigree who were not sampled were coded as unknowns. For all 2-point
linkage analyses, 2 general parametric models (dominant
and recessive) were run by using the
FASTLINK16 17 modifications of the
MLINK program.18 19 For the parametric
analysis, we used a gene frequency of 0.001 for the dominant
model and of 0.0447 for the recessive model. These gene frequency
estimates yield a prevalence of affected persons of 0.2% for both
models if the gene is fully penetrant. This is the estimated population
frequency of FH and is the maximum expected frequency for this new
locus. Power for linkage generally increases as the gene frequency
decreases, thereby making this assumption conservative. Penetrances for
a single liability class were set at 0.0 for the unaffected
genotype (no sporadic cases) and 0.95 for the affected
genotype (5% have the gene but do not express the
phenotype). Even though transmission of the phenotype
clearly appears to follow a dominant mode of inheritance in this
pedigree, a recessive model was also used to help rule out other
genetic regions that might contribute in a recessive manner to LDL
cholesterol levels in the family.
Multipoint linkage analysis was performed with Myriad Genetics MCLINK software program.20 MCLINK generates multipoint marker haplotypes by using a blocked Gibbs sampling approach; convergence was verified with multiple runs. Generated haplotypes were compared with and agreed with visually assigned haplotypes. Multipoint plots of the best linkage region are produced showing maximum log of the odds (LOD) scores calculated at each marker across the chromosome.
| Results |
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Figure 1
shows the distribution of the
2-point LOD scores for the dominant and recessive models for the 583
markers. The 2-point LOD scores presented are the maximum
values across all recombination fractions. Table 2
shows all markers under either the
dominant or recessive models that had a 2-point LOD score >1.0. Only
16 markers (2.7%) had LOD scores >1.0 for the dominant model, with 8
of those markers occurring in the same region of chromosome 1. The
maximum LOD score of 5.7 occurred for marker D1S2134. Three other
chromosome 1 markers were all >3.0. Chromosome 17 was the only other
chromosome with >1 marker with a LOD score >1.0. Other than
chromosomes 1 and 17, no other chromosome had a marker with a LOD score
>2.0.
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The recessive model resulted in 14 markers with 2-point LOD scores
>1.0 (2.4%), with 6 of them occurring on chromosome 3, though grouped
in 2 different unlinked locations (Table 2
). None of the LOD
scores were >2.2. Table 2
also shows the multipoint LOD scores
from MCLINK. Chromosome 1 had a highly significant LOD
score of 6.8 (0.0 recombination) at markers D1S2134 and D1S1661. Only
chromosomes 1 and 17 show multipoint LOD scores >2.0, with chromosome
17 having a maximum multipoint LOD score of 2.6. Table 2
also
shows how the LOD scores changed from the 2-point to multipoint
analyses. Under the multipoint dominant model, all of the
chromosome 1 LOD scores increased. The largest LOD score for chromosome
17 decreased from 2.72 to 2.58, although the information from this
marker helped to increase the LOD scores for surrounding markers. The
multipoint LOD score for the chromosome 22 marker was also increased.
However, this marker was the most telomeric marker tested, and the
multipoint LOD score for the marker next to it was 0, suggesting that
there is no linkage in this region. The multipoint LOD scores for the
markers on chromosomes 2, 14, and 18 decreased.
The recessive multipoint LOD scores were all <1.0 except for
chromosomes 1, 3, and 8. The chromosome 1 LOD score was 1.58 and
overlapped with the region found for the dominant model. The multipoint
LOD scores for chromosomes 3 and 8 were 2.46 and 1.33, respectively,
with only chromosome 3 showing both suggestive linkage and surrounding
marker support for linkage. Two-point LOD scores at markers near the
LDL receptor locus (D19S865, D19S1165, and D19S221) were all <
-2.0 at
=0.05. The 2 markers near the apo B gene (D2S220 and
D2S305) also had negative LOD scores (-1.9 and -1.7 at
=0.01 and
both <-4.0 at
=0), excluding linkage.
Figure 2
shows the shape of the
significant multipoint linkage tracing for the dominant model for
chromosome 1. The 1-LOD support region is from 66 to 78 cM, a 12 cM
region. Figure 3
shows the pedigree
members with their estimated haplotypes and recombinants. Recombinants
defining the minimal genetic region occur in individuals 7 and 50
between markers D1S2130 and D1S1596, resulting in an
17-cM region
(Figure 4
). All persons with the disease
haplotype (18/18) expressed the abnormal phenotype, and all
phenotypically affected persons (18/18) had the disease haplotype. Two
affected pedigree members were children, ages 4 and 9, when lipids were
first measured.
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| Discussion |
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The linkage to chromosome 1p32 overlaps with a region recently found linked to high LDL cholesterol levels in a large French family and other smaller Spanish families.21 The European families had a maximum heterogeneity LOD score of 2.88 for all clinical FH-like pedigrees (27% estimated to be linked), and the LOD score for the single large pedigree was 3.13. The large size of the Utah pedigree greatly increases the power to detect linkage, while the high LOD score of 6.76 validates the previously published lower French LOD score.
Our linkage region is defined by 7 recombination events within the
pedigree, giving a maximum potential region between D1S2130 and
D1S1596. The addition of markers in this region could reduce its size
substantially, since D1S1661 and D1S405 are not informative in
recombinant meioses. It should be noted that none of our
genotyped markers were the same as those used in the French
study. The integration of the markers and recombinants from both
studies is shown in Figure 4
, which uses the Marshfield map
order and distances, rather than the Myriad or French sample map
distances. Varret et al21 found 4 recombinants, 2 in
affected persons and 2 in unaffected persons. Using unaffected persons
for recombinant boundaries necessitates the assumption of nearly
complete penetrance. One of their pedigree members was a haplotype
carrier but not affected, while another was affected but not a
haplotype carrier, suggesting that penetrance in their pedigree is
incomplete. Nevertheless, using only the 2 affected persons delimits
nearly the same recombinant region.
Our recombinants move their telomeric boundary (D1S472) toward the centromere, whereas their centromeric boundary (D1S211) is within our region. In our laboratory, the D1S211 marker is difficult to score and is no longer used. We are hesitant to use this marker to define a recombinant boundary. Using the D1S197 boundary instead of the D1S211 boundary yields a 5.7-cM region from D1S2130 to D1S197 compared with a 1.4-cM region if the D1S211 boundary is valid. Note that the order in the Marshfield map for markers D1S472 and D1S255, which are 1 cM apart, is switched from the order estimated directly from the family data in the study by Varret et al. This switch has the effect of increasing their reported linked interval by a small amount over that reported in their study, but after recombinants from the 2 studies are combined, it has no effect.
There are no obvious candidate genes in the minimal linkage region, and the region appears to be a gene-poor region. Therefore, to validate further the recombinant boundaries and to narrow the linkage region so that cloning of the responsible gene may proceed, we are currently adding more markers to the region and expanding the family to find additional recombinants. Additional clinically defined FH families are also being studied to determine whether any might be linked to the FH3 locus. We are also investigating whether other phenotypes might be linked to this region, suggesting a pleiotropic effect of the gene.
Our findings, then, substantiate and extend previous work8 21 that identified a third genetic locus responsible for FH in humans. Other known FH loci are represented by the LDL receptor and by the ligand for it (apo B). The genetic lesion that underlies FH3 may be another component of the same pathway, or it may represent a novel mechanism that contributes to human cholesterol metabolism. Questions of whether a substantial fraction of clinically defined FH families are linked to FH3 and whether other non-FH phenotypes are associated with FH3 can be addressed in future work.
| Acknowledgments |
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Received January 6, 2000; accepted February 18, 2000.
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