(Hypertension. 1999;33:949-953.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the Section on Clinical Pharmacology, Division of Medicine, Imperial College of Science, Technology, and Medicine, Hammersmith Hospital, London, UK, and the Department of Genetics and Biometry (C.S.H.), Roslin Institute (Edinburgh), Roslin, Scotland, UK.
| Abstract |
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Key Words: genetics rats, inbred strains cardiac mass heart hypertrophy chromosome genes
| Introduction |
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With the advent of highly polymorphic markers spread across the entire rat genome, it has become feasible to conduct combined segregation and linkage studies to define quantitative trait loci (QTL) that contribute to strain differences in cardiac mass. In most cases, these have involved the analysis of cosegregation in crosses between hypertensive strains or between a hypertensive and a normotensive strain.3 4 5 6 7 8 9 Although some QTL affecting cardiac mass seem to segregate independently of those controlling blood pressure (BP), it is not clear whether these loci also play a role in determining interstrain differences in cardiac mass when the BP of both parental strains is not elevated. To minimize the confounding effects of BP, we have therefore conducted a segregation and genetic linkage analysis using the F2 progeny of a cross between 2 inbred normotensive rat strains, Wistar- Kyoto (WKY) and Fischer 344 (F344), which exhibit a substantial difference in adult cardiac mass.
| Methods |
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All animals were given unlimited access to standard laboratory rat diet (Special Diet Services) and drinking water. All procedures were conducted in accordance with Imperial College of Science, Technology and Medicine guidelines.
BP Measurement
Twelve-week-old rats were anesthetized with an
intraperitoneal injection of sodium pentobarbitone,
60 mg/kg body wt, and systemic arterial pressure was
measured by means of a cannula inserted into the left carotid artery.
The cannula was exteriorized through the back of the neck and attached
to a pressure transducer (Cobe Laboratories Inc) connected to a MacLab
4 integrated data acquisition system (AD Instruments). Once an animal
had regained consciousness and its BP had stabilized, continuous
pressure recordings were made for 20 minutes and the average of
these readings was used to estimate mean arterial
pressure.
Total Heart Weight, Left Ventricle Weight, and Right Ventricle
Weight Determination
At 12 weeks of age, each animal was weighed and then given a
lethal overdose of pentobarbitone. The thorax was opened and the heart
was removed from the great vessels, blotted with tissue paper to remove
as much blood as possible, and weighed to give total heart weight
(THW). Both atria were then removed, and the right ventricle was
dissected free and weighed. The left ventricle and septum were weighed
together (LVW).
Genomic DNA Preparation
The liver and kidneys were removed from F2
animals, snap-frozen in liquid nitrogen, and stored at -80°C.
Genomic DNA was extracted from these tissues with the use of a Nucleon
genomic DNA extraction kit (Scotlab).
Genotyping
Polymerase chain reaction (PCR) amplification was used to
genotype the F2 animals at microsatellite
loci that are polymorphic in the WKYxF344 cross. PCR primer pairs
for the microsatellite markers were obtained from Research Genetics or
Genosys Biotechnologies. The reagents for PCR were 50 ng of genomic
DNA, 300 nmol/L of each primer, 200 µmol/L
deoxyribonucleotide triphosphates, 1.5 mmol/L
MgCl2, 50 mmol/L KCl, 10 mmol/L
Tris-HCl (pH 9.0), 0.1% Triton X-100, and 0.25 U of Taq DNA
polymerase (Promega), in a final reaction volume of 10 µL. PCR was
performed for 35 cycles with an Omnigene thermocycler (Hybaid) and the
program 1 minute at 94°C, 1 minute at 55°C, and 30 seconds at
72°C, followed by a final elongation step of 9 minutes at 72°C. The
amplification products were resolved by electrophoresis through a
4% agarose gel (NuSieve 3:1 agarose, FMC Bioproducts) and
visualized by staining with ethidium bromide. Only microsatellite
markers that amplified reliably and distinguished the 2 parental
alleles were used for further analysis. For some markers,
it was necessary to alter the cycling parameters and
MgCl2 concentration to minimize the presence of
nonspecific bands.
Linkage and Statistical Analysis
The Mapmaker/EXP 3.0 and Mapmaker/QTL 1.1 computer
packages were used to construct genetic linkage maps and to localize
QTL relative to the position of the microsatellite
markers.10 11 At each 1-cM interval along the genetic map,
assuming either a free, additive, dominant, or recessive mode of
inheritance, the program computed the maximum
log10 likelihood (LOD) to support the presence of
a QTL at that position. A graphical representation of LOD
versus chromosome position was then generated for each of the genetic
models. In addition, it was possible to obtain an estimate of the
fraction of the total variance explained by a particular QTL. The
effect on THW and LVW of alleles at marker loci identified within a
1-LOD support interval of a QTL was evaluated by 1-way ANOVA. Depending
on the genetic model used during the Mapmaker/QTL analysis, the
LOD score thresholds for significant linkage were set as follows: free
genetics model, 4.3; dominant and recessive models, 3.4; codominant
model, 3.3. These stringent criteria, originally proposed by Lander and
Kruglyak,12 for genome-wide QTL mapping involving
experimental intercrosses are designed to reduce type 1 errors. The
statistical significance of an interstrain difference in the phenotypic
mean or variance was determined with the Student's t test
or the F test, with correction for multiple testing. In these cases,
the significance level
was set at 0.05.
| Results |
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There was a small but statistically significant (P<0.05) difference between mean systemic BP of the 2 parental strains (WKY BP=80.4±3.6 mm Hg [n=9]; F344 BP=86.0±6.0 mm Hg [n=9]) (Table 1). Despite their lower systemic pressures, the WKY rats had considerably larger hearts than F344 rats. No significant correlation was found between BP and THW, LVW, or RVW phenotypes in either strain (data not shown).
Segregation Analysis
The average phenotypic variances of the 3 nonsegregating
generations (WKY, F344, and F1) were used to estimate the environmental
variance of THW and LVW. The proportion of the total
F2 variance that was due to genetic variation,
expressed as heritability, was calculated according to the formula
([total F2 variance-environmental
variance]x100/total F2 variance) and was found
to be 56% for THW and 55% for LVW.
Linkage Analysis
A genetic linkage map for each of the 20 autosomes was constructed
with genotypic data from polymorphic microsatellite markers (the X
and Y chromosomes were not screened) (Table 2). The best map order for the markers
was determined by multipoint linkage analysis using
Mapmaker/EXP 3.0. Genetic distances between markers were calculated
with the Haldane mapping function.
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One hundred sixty-one microsatellite markers with a difference in
allele size
5% between WKY and F344 parental strains were used
in a whole genome search for QTL affecting THW, LVW, or the ratio of
LVW to body weight. In a primary screen, 65 F2
animals comprising the top and bottom quartiles of the cumulative
frequency distribution of LVW phenotype were typed across all
the autosomes. The remaining 61 F2 animals were
then genotyped with the use of markers for regions showing
evidence of linkage (LOD>1.5) in the primary screen.
A genetic linkage map for chromosome 3 was generated with data from 11
polymorphic microsatellite markers for all 126
F2 progeny (Figure 2). Using the Mapmaker/QTL program, we
found a significant QTL on chromosome 3 affecting both THW and LVW,
supported by peak LOD scores of 4.4 for THW and 4.8 for LVW, that
accounted for 15.1% and 16.5% of the total variance in THW and LVW,
respectively. This region containing the QTL was centered around the 3
markers D3Rat29, D3Rat34, and D3Rat38, which span a genetic distance of
9.5 cM. The peak LOD scores for both THW and LVW were associated with
the marker D3Rat29. The 1-LOD support interval for the position of the
QTL was 16 cM. Under the dominant genetic model, linkage with TWH and
LVW was supported by LOD scores of 3.0 and 2.3, respectively, compared
with LOD scores of 4.8 and 4.4 in the free genetic model. Thus, the
likelihood ratio against the dominant model was 125:1 for THW and 63:1
for LVW. When tested as a likelihood ratio test against
2 distribution with 1 degree of freedom, the
dominant model is significantly less likely (P=0.01 to
0.001) for both traits. The results of 1-way ANOVA of the cosegregation
of THW, LVW, or LVW/body weight, with the QTL characterized by these 3
markers, are shown in Table 3. The locus
marked by D3Rat29 seemed to affect both THW and LVW in a codominant
fashion, inasmuch as the F2 animals homozygous
for the WKY (WW) or F344 (FF) allele had the largest and smallest
mean THW and LVW, respectively, and the mean values for the
heterozygotes (WF) fell between the phenotypic means of the 2
homozygous groups. Rats homozygous for the F344 allele at the
marker D3Rat29 have LVW and THW
62 mg (P<0.05) and 80 mg
(P<0.05) lower than those homozygous for the WKY
allele.
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| Discussion |
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We performed an initial segregation analysis of the cardiac
mass phenotypes that suggested that 1 to 2 genes were involved
in determining the interstrain difference. From the graphs published by
Lander and Botstein,10 our study has 90% power to detect
2 QTL by the use of the traditional method of genotyping all progeny.
We then used polymorphic markers for the WKYxF344 cross that
permitted us to perform a comprehensive screen of >99% of the rat
genome for QTL (based on a recent reevaluation of the size of the rat
genome).14 This search identified a significant QTL for
LVW and THW on chromosome 3 that accounts for
16% of the total
variance of these phenotypes. Even though the maximal distance
between markers was never greater than 31 cM, it is possible that we
may have missed genetic factors with an autosomal recessive or
sex-linked pattern of inheritance or autosomal dominant loci of small
effect. We analyzed only male rats because at the time of the
breeding program, there was no evidence of sex-dependent effects on
cardiac mass. Since then, there have been 2 studies describing
sex-dependent effects in crosses of hypertensive with nonhypertensive
rat strains (not F344 or WKY).6 8
Our segregation data provide evidence for a significant genetic contribution to the total variance of cardiac mass and confirm the work of Tanase and colleagues2 who studied the segregation of cardiac mass in 23 inbred rat strains. They concluded that the effect of genetic factors on cardiac mass was greater than that of BP. Several studies have reported QTL contributing to the regulation of adult cardiac mass independent of the influence of BP on chromosomes 2,9 8,7 10,4 12,5 6 and 17.3 In crosses with hypertensive strains however, it is unclear whether differences in cardiac mass are truly independent of arterial pressure or whether they reflect technical artifact caused by the use of mechanical pressure transducers that may be less sensitive to cumulative pressure increments than myocardial sensors.
In a cross between Dahl salt-sensitive (SS/Jr) and
salt-resistant (SR/Jr) rats, Cicila and
colleagues15 found that the endothelin-3 locus
(Edn3) on chromosome 3, a region at least 70 cM from our
QTL, cosegregated with BP and relative heart weight in an SS/JrxF1
(SS/JrxSR/Jr) population. In a subsequent study, reported only in
abstract form,16 this group has introgressed the Dahl
salt-resistant Edn3 allele into the
salt-sensitive strain. Their data indicate the presence of 2 QTL on
chromosome 3 controlling BP and relative heart weight: one is
associated with marker Pck1 (closely linked to
Edn3), and the other is associated with marker D3Wox3,
100 cM away. Our QTL lies at least 70 cM away from the
Edn3 locus, and it is possible that the markers D3Wox3 and
D3Rat29 are defining the same locus in these crosses. The improved
linkage map for chromosome 3 reported recently by Dene and
colleagues17 does not clarify this issue. In any
case, it may be necessary to produce congenic strains to establish
conclusively whether 1 or 2 QTLs for heart weight are present in
chromosome 3.
We have searched the rat genome databases and syntenic regions in mouse
and human databases to identify potential candidate genes on chromosome
3. Four genes have been assigned to a region within the 1-LOD support
interval of the QTL: the glucagon gene (Gsg), the gene for
the
-polypeptide of the type II voltage-gated sodium channel locus
(Scn2a1), the prostaglandin synthase 1 gene
(Ptgs1), and the adenylate kinase gene
(Ak1) (Figure 2). The relevance of these genes to the
phenotypic variance in cardiac mass is not clear, but the
prostaglandin synthetase gene is an interesting candidate
because it has prostaglandin
endoperoxidase/cyclooxygenase activity and
catalyzes the conversion of arachidonic acid to
prostaglandin H2, from which various
vasoactive prostaglandins (PGE2,
PGF2
, prostacyclin, thromboxane)
are synthesized.18 Although there are no previous
reports of cosegregation of the Ptgs1 locus with cardiac
mass phenotype, a role for PGF2
in
cardiac myocyte hypertrophy has been suggested by the
ability of various prostaglandins to induce
hypertrophy of ventricular myocytes in culture
to an extent comparable to that induced by several well-known
hypertrophic factors, such as phenylephrine, and
angiotensin II.19
In conclusion, we report the results of a genome scanning study to search for chromosomal regions that control the difference in cardiac mass between the normotensive WKY and F344 inbred rat strains. We have detected a locus on chromosome 3 that includes the prostaglandin synthase 1 gene.
| Acknowledgments |
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| Footnotes |
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Received April 30, 1998; first decision May 19, 1998; accepted December 9, 1998.
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