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Department of Molecular Cell Biology, Leiden University Medical Center (L.M.t H., J.A.M.), 2333 AL Leiden; the Department of Epidemiology and Biostatistics, Erasmus University (R.P.S., D.E.G.), 3000 DR Rotterdam; the Julius Center for Patient Orientated Research, Utrecht University (R.P.S., D.E.G.), 3508 GA Utrecht; and the Institute for Research in Extramural Medicine, Vrije Universiteit Amsterdam (J.M.D., G.N., R.J.H.), 1081 BT Amsterdam, The Netherlands
Address all correspondence and requests for reprints to: Dr. J. Antonie Maassen, Department of Molecular Cell Biology, Leiden University Medical Center, Wassenaarseweg 72, 2333 AL Leiden, The Netherlands. E-mail: maassen{at}rullf2.medfac.leidenuniv.nl
| Abstract |
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No association was found for variants in the genes for amylin, IRS-1, and coagulation factor V, nor was there any evidence for epi-static interactions between these gene variants. A significant difference in the frequency of the Arg972 allele of the IRS-1 gene was observed between control subjects from Hoorn and Rotterdam (9.4% vs. 18.6%; P < 0.05). The insulin receptor Met985 variant was found at frequencies of 4.4% and 1.8%, respectively, in type 2 diabetic (n = 433) and normoglycemic patients (n = 799; P < 0.02). Inclusion of data from two other studies yielded a summarized odds ratio of 1.87 (95% confidence interval, 1.063.29; P = 0.03).
We conclude that the association between the Met985 variant in the insulin receptor gene and type 2 diabetes, which we previously reported in the Rotterdam study, is supported by the joint analysis with a second population-based study and other studies. The large regional differences in allele frequency of the Arg972 allele of IRS-1 gene makes genetic association studies of this gene less reliable.
| Introduction |
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Several genes have been proposed to contribute to the pathogenesis of common late-onset type 2 DM (2, 3). Remarkably, associations found in a particular population were often not confirmed in other populations, suggesting regional differences in the contribution of certain diabetes susceptibility genes or an artifactual association (4, 5). The replication of an association in an independent second population strengthens the significance of such an association. The possibility should also be considered that multiple gene variants in combination with epistatic effects and environmental factors contribute to the pathogenesis of diabetes. The observation that transgenic animals with combinations of heterozygous null alleles of the insulin receptor, insulin receptor substrate-1 (IRS-1), and/or the glucokinase genes develop manifest diabetes with a high frequency corroborates this possibility (6, 7).
We previously reported the association of a Met985 variant in the insulin receptor gene with type 2 DM (8). We have now extended our previous study in a second population-based study in The Netherlands, the Hoorn study (9). We estimated the common odds ratio for four published studies on the Met985 variant. In addition, we have tested whether known variants in other genes, previously reported to be associated with type 2 DM in other populations, also associated with type 2 diabetes in The Netherlands (10, 11, 12), and the occurrence of epistatic effects between gene variants was analyzed.
| Subjects and Methods |
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Subjects participating in this study were randomly selected from
two population-based studies in The Netherlands, the Rotterdam study
and the Hoorn study. The Rotterdam study is a population-based cohort
study of determinants for chronic disabling diseases in the elderly
(age, >55 yr; n = 7983) (13). The Hoorn study, also a
population-based study, examines potential determinants for glucose
intolerance and related disorders (9). The initial cohort of 2484
subjects, aged 5075 yr, was extended with additional subjects of the
same age category with known type 2 DM to obtain sufficient diabetic
participants for this study. They were recruited via general
practitioners. The characteristics of both study populations are shown
in Table 1
. All subjects participating in
the study gave informed consent, and the studies were approved by the
appropriate medical ethics committees. We used randomly selected
samples of subjects from both studies.
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DNA was extracted from peripheral blood samples according to standard procedures and was stored at -30 C. We previously described a detection method for the Met985 mutation in the insulin receptor gene; however, to facilitate detection of the Met985 variant in larger numbers of DNA samples we used the method described by Hansen et al. (8, 14).
The prevalences of the gene variants in the amylin (Ser20-Gly), coagulation factor V (Leiden mutation, Arg506-Gln), and IRS-1 (Gly972-Arg) genes were examined by PCR-restriction fragment length polymorphism-based methods, as described previously (10, 11, 12).
Meta analysis of the association of the insulin receptor Met985 variant
Other published studies describing the prevalence of the Met985 variant have been reviewed for their suitability to be included in a meta analysis (8, 14, 15, 16, 17). Two studies were excluded because of the absence of the mutation in the diabetic group studied (16) or the small sample size (15).
Statistical analysis
Genotypic or allelic frequencies were compared between diabetic and control subjects. Results were analyzed by Fishers exact test. Association of gene variants with diabetes-related parameters was tested by t test or Mann-Whitney U test when appropriate. Common odds ratios for the studies were estimated after testing the homogeneity of the population-specific odds ratios revealed no significant differences using the Mantel-Haenszel test. The statistical software packages SPSS 7.0 and StatXact3.1 were used (SPSS, Inc., Chicago, IL).
| Results |
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Insulin receptor Val985-Met mutation
The prevalence of the Met985 allele was tested in
diabetic and control groups from the Hoorn study. Carrier frequencies
in both groups were 3.7% and 2.7%, respectively (Table 2
; by exact test, P =
0.34). The previously reported carrier frequencies in the Rotterdam
study (5.6% and 1.3%, respectively) are also shown in Table 2
(8).
The Zelen statistic for homogeneity of the two studies was 0.07
(P > 0.1). The Mantel-Haenszel common odds ratio was
subsequently estimated to be 2.34 [P < 0.02; 95%
confidence interval (CI), 1.154.74]. No associations were found with
diabetes-related parameters, such as fasting, and postload glucose and
insulin levels, age of onset, lipids, hypertension, BMI, and waist/hip
ratio.
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Amylin Ser20-Gly mutation
This mutation was not detected in 141 individuals with type 2 DM
from the Rotterdam study (Table 2
).
Coagulation factor V, Leiden mutation
We tested normoglycemic and diabetic subjects from the Rotterdam
study for the prevalence of the Gln506 mutation. Equal
allele frequencies of this variant were detected in the Dutch control
and type 2 DM groups (Table 2
). Homozygotic carriers were not
detected.
IRS-1 Gly972-Arg mutation
We tested the prevalence of this mutation in type 2 diabetic and
normoglycemic subjects from the Hoorn and Rotterdam studies. The
genotypic frequencies are presented in Table 2
. The two studies showed
opposite trends in the prevalence of the Arg972 allele in
the control groups, but none of them showed a significant difference
from their corresponding diabetic group. Diabetic cohorts had equal
prevalences of the Arg972 allele. However, the difference
between the two control groups was significant (P <
0.05). We also tested whether the Arg972 variant associates
with obesity in our cohorts. This association was not observed (Table 3
). To examine the possible association
of the Arg972 variant with insulin resistance, we examined
the frequency of the Arg972 mutation in a cohort of 142
normoglycemic, hyperinsulinemic individuals from the Rotterdam study. A
carrier frequency (11%) similar to that in the type 2 DM group was
found (P > 0.3), but it was significantly lower than
that in the control group (P < 0.05). No significant
differences were seen in fasting and postload insulin levels between
carriers and noncarriers in both the control and diabetic groups, nor
was an association seen with other diabetes-related parameters, such as
age of onset, BMI, fasting and postload glucose levels, or waist/hip
ratio.
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We tested whether certain combinations of gene variants show
epistasis in the association with type 2 DM or related parameters. For
this part of the study we also included two single nucleotide
polymorphisms in the sulfonylurea receptor gene: 1) the intron
16 -3t allele, which is associated with type 2 DM in The
Netherlands (allele frequency, 48% and 41% in type 2 DM and controls
groups, respectively; P < 0.02; t Hart et
al., submitted) and secondly the silent exon 18 Thr759
variant (C
T; allele frequency, 5% in both groups) not associated
with type 2 DM in The Netherlands, but which shows association in other
populations (18). All combinations were found in both type 2 diabetic
and normoglycemic subjects at similar frequencies (data not shown). An
analysis of diabetes-related parameters, such as fasting and 2-h
glucose and insulin levels or BMI revealed no obvious association with
any of the combinations. Because of the low frequencies of most
combinations, a reliable statistical evaluation was not always
possible.
| Discussion |
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The mutation in the Amylin gene, reported by Sakagashira et al. to occur at high frequency in Japanese individuals with type 2 DM, was not detected in our cohorts. Recently, another study in Caucasians with type 2 DM also failed to detect this mutation (20). Also, data obtained in a Taiwanese population did not show an association of this mutation with type 2 diabetes (21). Thus, it is unlikely that this mutation plays a significant role in the pathogenesis of type 2 DM.
A high prevalence of thrombosis and other cardiovascular complications occurs in type 2 diabetic patients. This finding triggered a search for the association of thrombogenic factors with diabetes. It was previously shown that the Gln506 variant in the coagulation factor V gene associates with thrombosis in different ethnic populations (22, 23). A study among Italians showed an association of the same mutation with type 2 DM (12). We were unable to demonstrate such an association with type 2 diabetes in The Netherlands. Other studies also found no evidence for the association of this mutation with diabetes (24, 25). Taken together, these results suggests that the association of this mutation with diabetes might be regional or artifactual.
Several reports have indicated an association of the 972 variant in the IRS-1 gene with type 2 DM, although the associations with diabetes generally are weak or absent (26, 27). In addition, a higher prevalence of this mutation in obese subjects was seen (26, 28). Furthermore, it was shown that this mutation was associated with insulin resistance in some populations (27, 29). Transfection studies showed clear changes in the signal transduction properties of the mutant IRS-1 protein (30). Our data do not show an association of the IRS-1 mutant with diabetes, BMI, or hyperinsulinemia. We do see a significant difference in the prevalence of the Arg972 allele between the control cohorts in Rotterdam and Hoorn, indicating that regional differences in the distribution of this genotype occurs. Such a situation complicates the interpretation of association studies of this variant. There was no evidence for a selection bias in the control group of the Rotterdam study or any other group in our study. Also, the equal distribution of several other (nondiabetic) variants in the Rotterdam and Hoorn studies argues against a selection bias or genetic admixture in our groups (data not shown).
We also searched for possible epistatic effects between the gene variants. We additionally included in the analysis variants in the sulfonylurea receptor gene, which have been shown to be associated with type 2 DM and reduced ß-cell function in several populations (18, 31, 32) (t Hart et al., submitted). In cases where combinations occurred with a frequency to allow statistical testing, no evidence for epistatic effects was detected. All combinations were found at equal frequencies in subjects with NGT or type 2 diabetes mellitus, arguing against a contribution of epistatic effects between those gene variants. Furthermore, the other combinations occurring at low frequencies also did not suggest an enhanced association with type 2 diabetes.
We conclude that the Met985 variant in the insulin receptor gene is significantly associated with type 2 DM and represents a risk factor for the development of type 2 DM. Variants in the genes for amylin, IRS-1, and coagulation factor V do not seem to be associated with type 2 DM in The Netherlands, either alone or by epistatic interaction with each other.
| Acknowledgments |
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| Footnotes |
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Received October 8, 1998.
Revised December 9, 1998.
Accepted December 14, 1998.
| References |
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