(Hypertension. 1997;30:199-202.)
© 1997 American Heart Association, Inc.
Articles |
From the Department of Physiology and Molecular Medicine, Medical College of Ohio, Toledo.
Correspondence to Alan Y. Deng, PhD, Department of Physiology and Molecular Medicine, Medical College of Ohio, 3000 Arlington Ave, PO Box 10008, Toledo, OH 43699-0008. E-mail adeng{at}vortex.mco.edu
| Abstract |
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Key Words: hypertension, genetic rats, Dahl salt-sensitive rats, Milan rats, Wistar-Kyoto genetics
| Introduction |
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| Methods |
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Designation of Congenic Strains
Two congenic strains were produced in the current study. They
are designated as S.WKY-D2N35/neutral endopeptidase,
abbreviated S.WKY-D2N35/Nep, and S.MNS-alcohol
dehydrogenase/D2Mit5, abbreviated S.MNS-Adh/D2Mit5. In each
of the above designated congenic strains, the recipient strain, Dahl S,
goes first, followed by a period and the donor strain. After the
hyphen, two chromosome markers separated by a slash represent
the chromosome region of the recipient Dahl S strain being replaced by
that of the donor normotensive strain.
Construction of Congenic Strains
The basic breeding scheme in our studies was to put a specific
region of chromosome 2 of a normotensive strain (ie, WKY or MNS) onto
the genetic background of the Dahl S strain using a standard breeding
protocol.5 7 Dahl S rats were initially crossed to rats of
a normotensive strain. F1 progeny were backcrossed to Dahl
S rats to produce the first backcross generation (BC1). BC1
heterozygous for the chromosome region of interest were then
backcrossed again to Dahl S rats to produce BC2. This process was
repeated for a total of eight backcrosses, each time choosing rats
heterozygous throughout the region of interest as breeders for the next
backcross. At BC8, two heterozygotes were intercrossed to generate
homozygotes containing two alleles of the normotensive strain
throughout the entire chromosomal region of interest, thus fixing the
region of chromosome 2 from the normotensive strain on the background
of the S strain. In selecting rats on the basis of their
genotypes throughout the region of interest on chromosome 2,
the following markers were used: (1) for the S.WKY-D2N35/Nep
strain, markers D2N35, Camk, Nak
1,
Gca, and Nep; (2) for the
S.MNS-Adh/D2Mit5 strain, markers Adh, D2N35,
Camk, Nak
1, Gca, Nep,
At1b, and D2Mit5.
BP Measurement
Strain BP comparisons were done with pairs of strains and using
only male rats. For example, Dahl S and S.WKY-D2N35/Nep rats
were bred at the same time, and pups were weaned at 30 days of age.
Rats were caged four per cage, two Dahl S and two
S.WKY-D2N35/Nep rats. At 40 days of age, they were fed a 2%
NaCl diet (Teklad diet 94217, Harlan Teklad) with free access to water.
After 24 days on the diet, BP was taken once daily on each rat for 4
days by the tail-cuff method8 with rats in the conscious,
restrained state using equipment made by IITC Inc. Rats were warmed to
28°C for BP determination. At least three consistent BP
readings at a given daily session were obtained and averaged as the
reading for that session. The four BP sessions for each rat were
performed by two different operators, who did not know the identity of
the rats; each operator performed two sessions on each rat. The
readings from all four sessions were averaged as the BP of the rat.
Rats were killed by an overdose of sodium pentobarbital, and body
weight and heart weight were obtained.
Genotyping and Linkage Maps
Genotyping of genetic markers followed the same methods reported
previously.1 2 9 10 11 12 Simple sequence repeats for each
marker were amplified by the polymerase chain reaction (PCR) and
analyzed by agarose or polyacrylamide gel
electrophoresis. Linkage maps were constructed using the MAPMAKER/EXP
program.13 14
| Results |
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If there is truly a BP QTL present in the chromosome 2 regions in
question, the BP of the rats for both congenic strains should be
significantly lower than that of Dahl S rats.1 2 The
Table
shows pairwise strain comparisons of the congenic
rats with Dahl S rats. BPs of congenic S.WKY-D2N35/Nep rats
and congenic S.MNS-Adh/D2Mit5 rats were 44±4.6 and 29±
4.5 mm Hg lower than that of Dahl S rats, respectively. These
differences are highly statistically significant (P<.001).
The ratios of heart weight to body weight of congenic rats were also
highly significantly lower than those of Dahl S rats (Table
).
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In our other linkage studies, an additional BP QTL was also detected in F2 SxWKY and F2 SxMNS populations. This QTL is located in a region on rat chromosome 10.1 12 To verify that this region on chromosome 10 does not contain any residual genome from WKY or MNS in the two congenic strains developed for chromosome 2, chromosome markers located in the chromosome 10 region of interest were genotyped for the rats of the S.WKY-D2N35/Nep and S.MNS-Adh/D2Mit5 congenic strains. The results showed that the entire chromosome 10 region in question in each congenic strain is homozygous SS. The chromosome 10 QTL is the only other QTL we know about in crosses of Dahl S with MNS or WKY and is the only other region tested.
| Discussion |
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BP differences between congenic and Dahl S rats were corroborated by
differences in heart weightbody weight ratios. It is noteworthy that
the only valid comparisons are within experiments, not between
experiments. Thus although, for example, BP was consistent in
the Dahl S strain between experiments (205.8±3.91 versus
209.8±4.11 mm Hg), heart weightbody weight ratio (3.80±0.047
versus 4.23±0.054) was not (Table
). The latter is attributed to
different people dissecting the hearts between experiments. In our
studies, we use heart weightbody weight ratio as an index for
corroborating chronic BP differences. In all our linkage and congenic
studies so far carried out, heart weight and BP correlate very well
(Reference 1717 ; Table
, current study; and unpublished data, 1996). It is
also likely, however, that heart weight is influenced by loci that act
independently of BP QTL.18 19 20 21
Although there are differences in the precise protocol used for
genotyping, BP effects of congenic strains in the present study are
consistent with those of cosegregation studies using
F2 populations. In both F2 WKYxS and
F2 MNSxS populations, the rats homozygous for the WKY or
MNS QTL allele had BP lower than the rats homozygous for the Dahl S
QTL allele.2 Likewise, the BPs of both congenic
strains were also lower than that of Dahl S rats (Table
).
Theoretically, there could be more than one BP QTL located in the chromosome 2 region in question. Consequently, the BP effect observed in each congenic strain could come from the effects of these separate QTL. This possibility of having multiple QTL in the congenic region will be resolved by constructing congenic substrains, each one containing a smaller substituted segment within the broad chromosome 2 region under study.
The BP reduction of the congenic S.WKY-D2N35/Nep strain is
about 15 mm Hg more than that of the congenic
S.MNS-Adh/D2Mit5 strain (Table
). This difference could arise
if the QTL allele of the WKY rat was different from that of the MNS
rat; differing QTL alleles could have distinctive BP effects.
Another possibility is that the WKY and MNS rats could have the same
QTL allele in the D2N35/Nep region but that the larger
substitution in the congenic S.MNS-Adh/D2Mit5 strain might
contain other genes modifying its effect. The construction of
S.MNS-Adh/D2Mit5 rats with a larger region of chromosome 2
was done because the putative QTL-containing region was poorly defined
(larger) in the linkage analysis of the F2 SxMNS
population.2 Also, there was evidence that a second QTL
around At1b might exist,2 11 22 although
At1b itself as the QTL was not supported by DNA sequence
determinations of coding regions.11 On the basis of
previous data, one would expect the longer segment in
S.MNS-Adh/D2Mit5 to lower BP more than the shorter segment
of S.WKY-D2N35/Nep. The opposite was the case. Thus, the
difference in BP between the congenic strains is unexplained, although
each strain obviously had a major effect on BP.
The region between D2N35 and Nep on chromosome 2 harbors
several genes that could be construed as candidates for the BP QTL.
These include Camk, Nak
1, Gca, and
Nep (Figure
). Because the substituted region is large (41
cM), speculation as to the identity of the actual QTL is premature.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received October 11, 1996; first decision November 5, 1996; accepted January 7, 1997.
| References |
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