Exp. Anim. 57(2), 135–138, 2008
An OLETF Allele of Hyperglycemic QTL
Nidd3/of Is Dominant
Hiroyuki KOSE1), Takahisa YAMADA2), and Kozo MATSUMOTO1)
1)Division for Animal Research Resources, Institute of Health Biosciences, The University of Tokushima
Graduate School, Tokushima, 770-8503 and 2)Laboratory of Animal Breeding and Genetics,
Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
Abstract: The OLETF rat is a well-established model for the study of type 2 diabetes
associated with obesity and has been shown to possess multiple hyperglycemic alleles in its
genome. Here we focused on and carefully characterized one of the previously reported
congenic strains, F.O-Nidd3/of that carries the OLETF allele of the Nidd3/of locus (also known
as Niddm21 in the Rat Genome Database) in the normoglycemic F344 genetic background.
A prominent finding was that the F1 progeny between the congenic and the F344 stain, whose
genotype is heterozygote at the Nidd3/of locus, showed mild hyperglycemia equal to the
parental congenic rat, suggesting that the OLETF allele is dominant. To our knowledge, this
is the first study in which a diabetic QTL has been directly demonstrated to be dominant by
using congenic strains.
Key words: congenic, dominant, QTL
(Received 25 July 2007 / Accepted 10 October 2007)
Address corresponding: K. Matsumoto, Division of Animal Research Resources, Institute of Health Biosciences, The University of Tokushima
Graduate School, Kuramoto-cho, Tokushima 770-8503, Japan
Given that a recent estimate of diabetes patients
throughout the world is over one hundred millions, un-
derstanding its basis of molecular genetics is urgently
required [7, 22]. From the medical standpoint, the es-
tablishment of animal resources of disease models is
essential in order to take full advantage of rapidly de-
veloping genome pharmacological approaches . The
OLETF rat genetically mimics a human condition in
which individuals are susceptible to type 2 diabetes [9,
16, 17]. Prominently, there are multiple genomic com-
ponents that are linked to the expression of a hypergly-
cemic phenotype in this rat strain [14, 20, 21]. Our ul-
timate goal is to articulate how each of these
disease-causing polymorphisms leads to form the condi-
tion which makes the individual rat vulnerable to the
diabetes-causing external stimuli. To achieve this, we
have generated a series of congenic rat strains, each of
which harbors a single hyperglycemic QTL or quantita-
tive trait locus of an average of approximately 30 cM in
the genetic background of the wild-type strain . An
earlier study demonstrated that most, if not all, of the
congenic rats were mildly hyperglycemic, as predicted
by the fact that each QTL contributes only small ge-
netic variance. Here we conducted an extensive analy-
sis on one of the strains, F.O-Nidd3/of, with an emphasis
on testing a hypothesis, suggested by an initial whole-
genome scan, that this locus exerts a heterosis effect;
that is to say, the heterozygote genotype shows higher
blood glucose levels than the homozygote of either one
of the parental strains . We attempted to address the
H. KOSE, T. YAMADA, AND K. MATSUMOTO
issue directly by making comparisons among the F.O-
Nidd3/of strain, the F344 normoglycemic strain and the
F1 progeny between F.O-Nidd3/of and the F344 rat
([F344 × F.O-Nidd3/of]F1). If the OLETF allele of
Nidd3/of is indeed heterotic, then the blood glucose lev-
els of the [F344 × F.O-Nidd3/of]F1 rats would be the
highest among the strains.
The F.O-Nidd3/of strain possesses a 35-cM segment
of OLETF-derived chromosome 8 demarcated by
D8Rat58 and D8Mgh17 . Nidd3/of locus is also
known as Niddm21 in the RGD (http://rgd.mcw.edu/).
The F1 progenies were produced by intercrossing F.O-
Nidd3/of males and F344 females (F344/NSlc). The
control F344 6-week-old males (F344/NSlc) were pur-
chased from Japan SLC, Inc. (Hamamatsu, Japan). All
rats were kept under specific pathogen-free conditions.
The temperature (21 ± 2°C), humidity (55 ± 10%), and
ventilation were all controlled. Rats had free access to
tap water and standard laboratory chow (MF; Oriental
Yeast Co., Japan) and were maintained on a 12-h light
and dark cycle (7:00/19:00). Animal procedures used in
this study were approved by the University of Tokushi-
ma Animal Experimentation Committee.
All analyses were performed on 30-week-old males.
The oral glucose tolerance test (OGTT) and fat tissue
measurements were performed as previously reported
[12, 15]. Serum insulin levels were determined with an
ELISA kit from Morinaga, Japan. The serum levels of
total cholesterol, triglycerides, and non-esterified fatty
acids were determined with reagents from Wako, Japan.
The statistical significance of differences was evaluated
using ANOVA, followed by post hoc analyses with
Shown in Fig. 1 is the result of OGTT analysis. Con-
sistent with our previous study, the F.O-Nidd3/of rat
showed mild hyperglycemia . However, contrary to
our expectation, the postprandial plasma glucose levels
of F1 progeny, which is a heterozygote at the Nidd3/of
locus, were not any higher than those of the F.O-Nidd3/
of rat. There was no statistically significant difference
between the congenic and F1 rats, suggesting that the
mode of inheritance of the locus is dominant at least in
this particular genetic setting. The discrepancy can per-
haps most readily be explained by the fact that there is
a high degree of heterogeneity in an F2 population in a
whole genome study. On the other hand the genomic
milieu is essentially homogeneous in the congenic strain.
In theory, greater than 99.9% of the genome outside the
introgressed segment is identical between the congenic
and its host strain . Therefore, given that the ob-
served heterosis was not some form of artifact, the re-
sultant hypothesis would predict the epistatic interaction,
that is, yet another genetic component(s) that empowers
specifically the heterozygote genotype at the Nidd3/of
locus to exert greater influence on glucose metabolism.
Indeed, we recently demonstrated that epistatic interac-
tion among OLETF alleles of hyperglycemic QTLs does
exist . Though our data did not support heterosis,
finding that a QTL exerts the dominant mode of inheri-
tance is quite novel. Previously, we tested the mode of
inheritance of Nidd1/of and Nidd2/of and found OLETF
alleles at both loci to be recessive . Recently, Doung
et al. examined 10 hypertensive QTLs of the DSS (Dahl
salt sensitive) rat, similarly using the congenic strains,
and found that only one of the hypertensive alleles was
To gain insight into the mechanisms of dominant in-
heritance, we measured various biochemical parameters
for the fasting state (Table 1). As indicated in Fig. 1,
fasting glucose levels of F.O-Nidd3/of are higher than
those of both the control F344 rat and the F1 rat. This
is rather difficult to interpret since apparently the OLETF
allele in the transheterozygote with the corresponding
Fig. 1. OGTT result for F.O-Nidd3/of congenic (n=20, diamond),
[F344 × F.O-Nidd3/of]F1 (n=14, triangle) and the F344
rat (n=9, circle). Bars represent the mean ± SE. *P<0.05,
**P<0.01 vs F344, ‡‡P<0.01 vs F.O-Nidd3/of rats.
DOMINANT QTL ALLELE IN THE OLETF RAT
F344 allele tended to lower the plasma glucose, yet in
the homozygote it exerted the opposite effect. In our
previous report, statistical significance was not demon-
strated between the congenic and the control for the
fasting condition . In contrast, postprandial hyper-
glycemia was reproduced in the current study, suggesting
that it is possible that the genetic variance, if any, of the
Nidd3/of locus is highly vulnerable to some unknown
external conditions. Therefore, we think it is too specu-
lative to discuss how the OLETF allele at this locus is
involved in the fasting glucose levels.
Consistent with the OGTT data, AUC or area under
curve was higher for both the congenic and the F1 rats.
Concerning other parameters, none of the factors exam-
ined showed any differences among the strains, except
epididymal fat mass and its adiposity index. Only the
F1 rats increased the fat mass, and this might reflect some
aspect of heterosis we initially anticipated. However,
increased fat mass is unlikely to be associated with
Nidd3/of-caused hyperglycemia because the fat mass is
normal for F.O-Nidd3/of congenic rats. Our independent
whole genome scan searching for obesity QTL identified
a locus, Obs3 in the chromosome 8 . However, Obs3
barely overlaps with Nidd3/of and this locus influences
mesenteric rather than epididymal fat, making it an un-
likely candidate for explaining the observation. Although
central (visceral) obesity is known to be more closely
associated with glucose metabolism than peripheral
(subcutaneous) obesity, it is unknown whether the in-
creased epididymal fat mass in the F1 rats has any major
physiological effect .
Most of the Nidd3/of locus corresponds to 11q21–q25
of the human chromosome 11. It is intriguing that sev-
eral studies have identified either obesity or diabetic loci
in this region [2, 3, 5, 6]. Indeed, among the 14 hyper-
glycemic QTLs we identified, the Nidd3/of is one of the
most profound loci in terms of numbers of diabetes-re-
lated QTLs reported in human studies. According to the
NCBI database, approximately 130 genes or ESTs are
annotated in this region. Unfortunately, none of the
genes closely linked to D8Rat49, localized to the LOD
score peak, is considered to be a candidate from their
predicted or known gene functions. However, a series
of new discoveries, such as non-coding RNA or epi-
genetics, has challenged the traditional central dogma
of the last several years. Therefore, we think that ar-
Table 1. Comparison of metabolic parameters
[F344 × F.O-Nidd3/of]F1
80.0 ± 3.1 88.1 ± 1.8*
1.46 × 104 ± 354***
4.08 ± 0.35
59.7 ± 2.8
167.8 ± 15.3
0.85 ± 0.052
77.8 ± 1.8‡‡
1.43 × 104 ± 471**
3.99 ± 0.44
63.7 ± 2.8
140.2 ± 15.4
0.71 ± 0.055
1.22 × 104 ± 174
3.01 ± 0.40
52.4 ± 5.7
122.4 ± 25.3
0.64 ± 0.058
Fat weight (g)
8.9 ± 0.6
10.3 ± 0.7
10.2 ± 0.9
9.7 ± 0.4
10.9 ± 0.3
10.6 ± 0.3
10.4 ± 0.5
11.1 ± 0.4
13.3 ± 0.4** ‡‡‡
Adiposity index (%)¶
Body weight (g)
2.33 ± 0.19
2.65 ± 0.12
2.62 ± 0.22
387.4 ± 10.3
2.35 ± 0.08
2.66 ± 0.07
2.57 ± 0.05
409.9 ± 5.3
2.48 ± 0.11
2.64 ± 0.10
3.16 ± 0.08*
419.3 ± 5.2** ‡‡‡
Thirty-one-week-old fasted males were used for all measurements except glucose, insulin and body
weight, which were measured during OGTT analysis at 30 weeks of age. Data are shown as means
± SE. TCHO, total cholesterol; TG, triglycerides; NEFA, non-esterified fatty acids. *P<0.05;
**P<0.01; ***P<0.001; vs F344 rats, ‡‡P<0.01; ‡‡‡P<0.001; vs F.O-Nidd3/of rats. ¶: Adiposity
index was determined using each fat pad and body weight (percentage of fat weight/body
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H. KOSE, T. YAMADA, AND K. MATSUMOTO
ticulate genetic analyses using refined polygenic models,
such as QTL congenic strains, will become important in
the understanding of the intricate genomic network of
In conclusion, we provide evidence that the Nidd3/of
is inherited in a dominant fashion. We should, however,
keep in mind that the genetic homogeneity of the con-
genic strain underlies the definitive revelation of the
Mendelian inheritance. From the characterization of
monogenic traits, it is believed that the dominant trait is
the result of 1) haploinsufficiency ; 2) ectopic func-
tion of a protein product, more commonly known as the
dominant negative mutant ; and 3) ectopic expression
of a gene, the most famous example of which is a homeo-
tic mutation in Drosophila . It is yet to be elucidated
whether or not these paradigms apply to the polygenic
trait. We expect that the findings of this study will aid
identification of the causative gene as well as hint at
molecular networks which might lead to the understand-
ing of the hidden heterosis initially implied.
This study was supported in part by a grant from the
National Bio Resource Project (NBRP) for the Rat in
Japan (K.M.) and from the Ministry of Education,
Culture, Sports, Science and Technology of Japan (H.K.
and K.M.). We thank Takako Koizumi, Daisuke Sano
for technical assistance of breeding congenic strains.
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