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Departments of Neurology (C.-W.L., T.-K.L., S.-D.C., Y.-C.C.), Radiology (H.H.W.), and Internal Medicine (S.-W.W., P.-W.W.), Chang Gung Memorial Hospital, Kaohsiung Medical Center, Chang Gung University College of Medicine, Kaohsiung, Taiwan 83305; Department of Biochemistry and Center for Cellular and Molecular Biology (C.-F.L., T.-L.C., Y.-H.W.), National Yang-Ming University, Taipei, Taiwan 112
Address all correspondence and requests for reprints to: Pei-Wen Wang, M.D., Department of Internal Medicine, Chang Gung Memorial Hospital, Kaohsiung, 123, Ta-Pei Road, Niao Sung Hsiang, Kaohsiung, Taiwan 83305. E-mail: cwliou{at}ms22.hinet.net.
| Abstract |
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Methods: Our study examined mtDNA from 462 subjects with type 2 diabetes (T2DM) and 592 normoglycemic controls (non-DM). Each participants body mass index (BMI), fasting plasma glucose, fasting insulin concentration, insulin resistance index, and ß-cell function were measured. Sequencing for mtDNA, focusing on exploration of the hypervariable polycytosine tract within the control region, was also conducted in all subjects.
Results: Prevalence of the mtDNA 16189 variant was significantly different between DM and non-DM subjects (39.2% vs. 30.7% respectively; P = 0.004). Increased incidence of DM was noted in those harboring the 16189 variant compared with those lacking the variant (multivariate odds ratio, 1.38; 95% confidence interval, 1.071.80). Moreover, increased BMI was identified as an aggravating factor for development of DM in subjects harboring the variant. Odds ratio determinations yielded 2.14 in overweight and 4.63 in obese subjects harboring the variant in comparison with subjects without (1.83 in overweight and 2.16 in obese subjects). This is consistent with a progressively increased prevalence of the mtDNA 16189 variant in the non-DM groups with higher fasting insulin concentration, insulin resistance index, and ß-cell function (all Ptrend < 0.005).
Conclusion: The mtDNA 16189 variant can influence development of T2DM. The demonstrated dynamic between the 16189 variant and increased BMI exemplify an additive effect of genetic and environmental factors on the pathogenesis of T2DM.
| Introduction |
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| Subjects and Methods |
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A total of 1054 unrelated Taiwanese of ethnic Chinese backgrounds were enrolled in this study. The subjects were divided into two groups according to their personal history and presence of DM. Group 1 (DM group) consisted of 462 subjects with known past histories of type 2 DM who were receiving regular follow-up care in our hospital. Individuals who had received supplementary insulin therapy for DM because of long-term decline of pancreatic function and who had diabetes onset before the age of 30 were not included in our study. Of the DM group, 5.6% were on diet control, 6.5% were treated with insulin sensitizer, and 87.9% were taking insulin secretogogues. Group 2 (non-DM group) comprised 592 nondiabetic subjects randomly selected from the health screening center or outpatient service. The nondiabetic status was determined by the patients history and fasting plasma glucose (FPG) less than 110 mg/dl. Informed prior consent was given by all subjects. The studies were conducted according to the guidelines of the Declaration of Helsinki, and the study protocols were accepted by the Ethics Committee of the Chang Gung Memorial Hospital.
Participants were all older than 41 yr, with the oldest being 80 yr old. In each group, subjects were further divided into four different subgroups according to their age at the time of the study. The age subgroups were 4150, 5160, 6170, and older than 71 yr. Each subgroup contained variable case numbers ranging from 61 to 216. The uneven number of diabetic subjects in the four age subgroups results from the random selection of subjects from the out-patient service. Table 1
summarizes the demographic data of these subjects.
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27 kg/m2), overweight (24 kg/m2
BMI < 27 kg/m2), and normal (BMI < 24 kg/m2). This standard is generally used and is widely accepted in the assessment of Taiwanese people (13). Patients were allocated into the weight subgroups (Table 1Insulin resistance (HOMA-IR) and pancreatic ß-cell function (HOMA-BCF) were calculated using the homeostasis model assessment (HOMA) via the quantitative determination of fasting state plasma glucose and insulin (14). The equations were simplified as: HOMA-IR = [fasting insulin concentration (FIC) x FPG]/22.5 and HOMA-BCF (%) = (20 x FIC)/(FPG 3.5). FIC is fasting serum insulin concentration (µU/ml), and FPG is fasting plasma glucose (mmol/liter).
DNA analyses
Total DNA was extracted from peripheral leukocytes and amplified using the PCR techniques as described previously (15). Two primers were used to amplify the mtDNA sequence of interest. The forward primer consisted of nt 1597115990 of mtDNA and the reverse primer spanned nt 1647116452 of the mtDNA. The presence of 16189 mtDNA variant was determined by using a combination of PCR-restriction fragment length polymorphism analysis with the enzyme MnlI. PCR products were digested with 1 U of the enzyme for at least 1 h at 37 C and subjected to electrophoresis with both positive and negative controls on a 2% agarose gel for 45 min at 80 V. DNA bands were visualized under high-intensity UV illumination after ethidium bromide staining. Moreover, for verification of the presence of this polycytosine tract within the mtDNA control region, we also performed DNA sequencing using an ABI 377 automated sequencer (Applied Biosystems, Warrington, UK) in all subjects. A polycytosine tract was defined as having eight or more continuous cytosines between the bp region 1618416193. Table 2
lists the different sequencing patterns and their prevalence in the available data for 1038 patients. Data from 16 patients were deemed to be unusable because of incomplete sequencing.
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Statistical analysis was performed using the Statistical Package for Social Science program (SPSS for Windows, version 11.5; SPSS, Chicago, IL), and data were calculated using Excel software (Microsoft, Seattle, WA). Continuous variables were expressed as means ± SD values and compared by performing the Students t test. For comparison of data regarding the proportion of mtDNA variant, the
2 test was performed. Stepwise logistic regression analysis was used to identify the most accurate linear combinations of distinguishing variables (patient age categories, patient sex, BMI categories, and the 16189 mtDNA variant). Baseline variables with inclusion threshold of P < 0.05 and exclusion threshold of P > 0.1 were entered as predictor variables to control for confounding effects. Results of the logistic regression model were presented as the odds ratio (OR) and 95% confidence interval (CI).
| Results |
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Distribution of our randomly selected diabetic patients showed the highest proportion of subjects in the age group 6170 (38.7%, 179 of 462) followed by 7180 (24.7%, 114 of 462) and 5160 (23.4%, 108 of 462) (Table 1
). This distribution is compatible with the reported prevalence of DM in present-day Taiwan (16). Comparisons between the corresponding age subgroups within the two major groups noted higher 16189 variant rates in all the diabetic age subgroups (Table 1
). The rate difference was particularly significant in the 6170 subgroup. However, rate differences were insignificant following the multivariate adjustments for sex and BMI in all age subgroups (Table 1
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Comparisons of average BMI between subjects with and without the mtDNA 16189 variant within the overall group, within the DM and non-DM groups, within separate gender groups and four different age subgroups are shown in Table 3
. The BMI comparisons exhibited no statistical differences between subjects with or without the variant within all groups and subgroups. There is evident interrelation between both increased BMI and/or the presence of the mtDNA 16189 variant, and the development of DM but there is no direct evidence pointing to the 16189 variant causing increased BMI. The age groups 5160 and 6170 have a higher percentage of DM patients and are notable for their association with an increased prevalence of the mtDNA 16189 variant as well as increased BMI, which implicates a combined effect of these factors on the development of DM in these age groups. Accordingly we further analyzed the combined influence of the mtDNA variant and increased BMI on development of DM and rendered a crescendo increase of OR from 1 to 4.63 for development of DM (Table 4
). The results show the effect of independent genetic and environmental factors combining to accentuate the generation of DM.
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| Discussion |
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Presently we have demonstrated that the prevalence of the 16189 variant is higher in type 2 DM patients than in the age- and sex-matched non-DM subjects. The OR for the presence of the variant in the diabetic patients is 1.38. A multivariate logistic regression analysis identified this variant as an independent risk factor for development of DM. These results are in agreement with two previous investigations of different ethnic origin and are also consistent with the findings of higher levels of fasting insulin, insulin resistance, and ß-cell function in our subjects who harbored the 16189 variant (8, 11). The latter findings are often suggested as being indicative of a pre-diabetic state. Although this study did not offer haplogroup analysis for determination of racial-specific characteristics, similar reports from two Chinese institutions located in two geographically disparate regions may weaken the possibility of a founder effect in this mtDNA variant (11, 24) and strengthen the possibility that its occurrence is independent. This study also demonstrates that increased BMI additionally contributes to the development of DM in subjects harboring the 16189 variant, compared with those lacking the variant in Taiwans population. This association is particularly significant with an increased OR (from 2.16 to 4.63) in the presence of DM compared with obese subjects not harboring the mtDNA variant. Odds ratios also increased from 1.83 to 2.14 in the overweight subgroup harboring the variant. These results are consistent with the suggestion that both the presence of the mtDNA variant and BMI change contributed to the development of DM. Furthermore, being an environmental factor, increased BMI may be superimposed on the mtDNA genetic factors aggravating the development of DM. Our study offer substantial support for the recent suggestion that genetic as well as environmental factors are all important determinants for type 2 DM (25, 26).
Prevalence of the mtDNA 16189 variant is higher in all DM age subgroups compared with the corresponding non-DM age subgroups. The rate difference is particularly significant in the 6170 subgroup. However, the difference shows no statistical significance following a multivariate adjustment for BMI. These findings are consistent with the idea that age differences might not be implicated in the pathological mechanism of development of DM in people who harbor the 16189 variant, but rather people in this age category have another factor involved in the determination. This suggestion is reflected in the higher BMIs observed in those aged 5170 than in those younger than 50 and older than 70 yr. The discrepancies in BMI increases in certain age subgroups are associated with the economic and nutritional improvements that have occurred in Taiwan in the years after World War II (13, 22). All these observations may be helpful in proving the positive interrelation between the 16189 variant, increased BMI, and development of DM. However, more reproducible data in other Asian nations sharing similar social development is required.
In the present study, we did not unearth a correlation between the presence of the mtDNA 16189 variant and a higher average BMI over all subjects in either the DM or the non-DM group. Similarly, associations were absent in various age subgroups of the DM and non-DM groups. These observations are at variance with a previous study from Korea, in which a higher average BMI was associated with the presence of the 16189 variant in normal non-diabetic subjects (17). Earlier presumptions that the 16189 variant is possibly related to body weight are not strengthened by our study results (17, 27, 28). Some other genetic factors or effects of climatic differences between temperate northern countries and more tropical regions may be a determining factor for BMI discrepancies but requires more comparative study (29). It is hypothesized that a T-to-C transition at nt 16189 in the non-coding nt position of mtDNA creates an unstable polycytosine tract, which may interfere with normal mtDNA replication and transcription (7). Moreover, it is also possible that an increased BMI, acting as an additional environmental factor, aggravates the oxidative stress status, exerting compensatory pressure on mitochondria that cannot adequately respond in subjects harboring a functionally subnormal 16189 mtDNA variant (30, 31).
| Acknowledgments |
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| Footnotes |
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Disclosure Statement: The authors have nothing to disclose.
First Published Online October 10, 2006
Abbreviations: BMI, Body mass index; CI, confidence interval; DM, diabetes mellitus; FIC, fasting insulin concentration; FPG, fasting plasma glucose; HOMA, homeostasis model assessment; mtDNA, mitochondrial DNA; nt, nucleotide; OR, odds ratio.
Received March 24, 2006.
Accepted September 29, 2006.
| References |
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C transition) with metabolic syndrome in Chinese adults. J Clin Endocrinol Metab 90:50375040This article has been cited by other articles:
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S. Das, A. J. Bennett, U. Sovio, A. Ruokonen, H. Martikainen, A. Pouta, A.-L. Hartikainen, S. Franks, P. Elliott, J. Poulton, et al. Detailed Analysis of Variation at and around Mitochondrial Position 16189 in a Large Finnish Cohort Reveals No Significant Associations with Early Growth or Metabolic Phenotypes at Age 31 Years J. Clin. Endocrinol. Metab., August 1, 2007; 92(8): 3219 - 3223. [Abstract] [Full Text] [PDF] |
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