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Department of Medicine, University of Birmingham, Queen Elizabeth Hospital (J.M.H., A.A., J.D., J.C.-S., A.D., M.C.S., A.H.B., J.A.F., S.C.L.G.), Birmingham, United Kingdom; Royal Bournemouth Hospital (M.A.), Bournemouth, United Kingdom; and Birmingham Heartlands Hospital (P.M.D., A.H.B., S.C.L.G.), Birmingham, United Kingdom B9 5SS
Address all correspondence and requests for reprints to: Dr. Stephen C. L. Gough, Department of Medicine, University of Birmingham, Birmingham Heartlands Hospital, Bordesley Green East, Birmingham, United Kingdom B9 5SS. E-mail: s.c.gough{at}bham.ac.uk
| Abstract |
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2 = 11.95; 1 d.f.;
P = 0.0005). Lack of preferential transmission to
unaffected siblings (53%;
2 = 0.19; 1 d.f.;
P = NS) excluded segregation distortion. These
results show that linkage disequilibrium between GD and the HLA class
II region is due to the extended haplotype DRB1*0304-DQB1*02-DQA1*0501. | Introduction |
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s = 3, the ratio of the expected
proportion of affected sibling pairs sharing zero alleles identical by
descent, 0.25, and the observed proportion) (2), it seems likely that
it will play a similar role in GD. This region is an obvious candidate
for a role in the genetic susceptibility to GD, as there is aberrant
expression of the major histocompatibility complex (MHC) class II
antigens on follicular cells (the target cell in the autoimmune
process) and on activated lymphocytes in patients with disease. Although classical linkage analysis has been successfully used to find major genes, its ability to detect genes of modest effect has been limited (9). This may explain why linkage analysis of the HLA region in GD in family-based studies (10, 11, 12) has failed to replicate case control data (13, 14, 15, 16, 17, 18).
We have, therefore, examined the MHC HLA class II region in patients with GD using the alternative approach of linkage disequilibrium analysis in two independent United Kingdom datasets. The Transmission Disequilibrium Test (TDT) (19) was used in a family-based study to replicate the findings of a population-based case control study.
| Subjects and Methods |
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The study was approved by the respective local ethics committees, and all subjects gave informed, written consent.
Genotyping of datasets
DNA was prepared from 10 mL whole blood using the Nucleon Bacc II kit from Nucleon Biosciences (UK). The HLA-DRB1, DQB1, and DQA1 regions were amplified using the phototyping method of the PCR, as previously published (21). Primers were obtained from Oswel (UK) and R&D Systems (UK). Results were visualized on a 1% ethidium bromide-stained agarose gel under ultraviolet light.
Statistical analysis
Analysis of the case control data was performed using the
2 test with 95% confidence limits (CL). All
P values were corrected for the number of comparisons made,
and P < 0.05 was considered significant. Odds ratios
(ORs) were calculated by the method of Woolf with Haldanes
modification for small numbers (22).
The TDT (19) was used to assess linkage disequilibrium between the HLA susceptibility haplotype and disease in the family dataset. A significant excess of transmission frequency of the associated haplotype DRB110304-DQB1102-DQA110501 from parents heterozygous for that haplotype to affected offspring was taken as evidence of linkage disequilibrium. Comparisons of transmissions to unaffected offspring using the 2 x 2 test of heterogeneity were performed to exclude segregation distortion.
| Results |
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Two hundred and twenty-eight Graves patients and 364 healthy control subjects were available to be genotyped at the DRB1, DQB1, and DQA1 loci. All patients and control subjects were successfully genotyped at all 3 loci.
The distribution of DRB1 alleles among patients and control subjects is
summarized in Table 1
. A significant
increase in the frequency of the DRB110304 allele was seen in patients
compared to control subjects (47% vs. 24%, respectively;
OR = 2.72; pc < 1.4 x
10-5).
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2 = 0.62; P
= NS). Linkage disequilibrium between DRB110304 and DQA110501 was
confirmed by the increased frequency of the DQA110501 allele in
DRB110304-positive (69%) compared to DRB110304-negative (31%)
Graves patients (
2 = 23.4; P < 1
x 10-6).
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Transmission of the haplotype DRB110304-DQB1102-DQA110501 from
heterozygous parents to affected and unaffected siblings was analyzed
using a standard TDT, McNemars test (19). Of the 98 families
available for study, 54 were informative for this haplotype. A
significant increase in transmission (T) compared to nontransmission
(NT) of the DRB110304-DQB1102-DQA110501 haplotype was seen in affected
offspring [44 T (72%) vs. 17 NT (28%);
2 =
11.95; 1 d.f.; P = 0.0005; Table 5
]. There was no preferential
transmission of this haplotype to unaffected offspring (53%). The
2 x 2 test of heterogeneity comparing transmission of the
DRB110304-DQB1102-DQA110501 haplotype to affected and unaffected
offspring (
2 4.1; 1 d.f.; P = 0.04)
confirms that the significant excess of transmissions to affected
offspring was not the result of segregation distortion.
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| Discussion |
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Due to the increased frequencies of the alleles DRB110304, DQB1102, and DQA110501, the strong linkage disequilibrium between them, and their lack of independent association with disease, we investigated the frequency of the haplotype DRB110304-DQB1102-DQA110501 in our cohort of Graves patients and control subjects. An increase in the frequency of this haplotype was seen in patients compared to control subjects and was consistent with the allele frequency data (OR = 2.72; confidence limits = 1.913.87; pc = 1.8 x 10-5). This result suggests that this haplotype is conferring genetic susceptibility to Graves disease in our population.
Although the case control approach has been frequently used to test candidate genes (28), it is widely accepted that differences in allele frequencies between a diseased population and normal control subjects may arise for reasons other than linkage disequilibrium between alleles of the marker and alleles of the disease locus. False positives can occur as the result of random chance events, a common occurrence in small datasets, or to population stratification (29). To minimize random chance events, it is important to use adequately sized datasets; to eliminate population stratification, it is crucial for susceptibility genes to be examined in family-based studies.
Classical linkage analysis has identified major genes in complex diseases, although its ability to detect genes of modest effect has proved limited (9). The TDT is a more powerful approach for detecting susceptibility loci that may be missed by classical linkage analysis (9). The detection of such susceptibility loci is important because, although likely to have small effects, the magnitude of their attributable risk may be large, as they are frequent in the general population. Moreover, by using single affected individuals and parents, the TDT does not require families with multiple affected siblings. This approach is particularly valuable, therefore, in diseases of late onset, such as autoimmune thyroid disease.
In the present study we used the TDT, which has been considered to be a test of linkage in the presence of linkage disequilibrium (19). In an independent family dataset, we examined the frequency of transmission of DRB110304-DQB1102-DQA110501 from parents heterozygous for the haplotype. We found a significant preferential transmission of the DRB110304-DQB1102-DQA110501 haplotype to affected Graves index cases (72%), providing evidence of linkage disequilibrium.
To overcome the confounding problem of subclinical or undiagnosed autoimmune thyroid disease in unaffected siblings, all had thyroid function and thyroid antibody status measured. Eight of 100 unaffected siblings (8%) were found to have evidence of subclinical autoimmune hypothyroidism, including a raised TSH receptor level and positive thyroid antibodies, and were excluded from our analysis. The transmission of the disease haplotype DRB110304-DQB1102-DQA110501 to unaffected siblings (25 transmissions, 53%) was reassuring and demonstrated that well characterized unaffected siblings can be used as controls for TDT analysis in GD. Inclusion of siblings with subclinical undiagnosed autoimmune thyroid disease as unaffected subjects, however, had no significant effect on the family results (data not shown). The 2 x 2 test of heterogeneity, performed between affected and unaffected offspring, ruled out segregation distortion as an alternative explanation to linkage disequilibrium of DRB110304-DQB1102-DQA110501 with GD.
In conclusion, we have shown for the first time in a family-based study that the MHC HLA class II region on chromosome 6p is in linkage disequilibrium with GD in Caucasians in the United Kingdom. The TDT in the family study provides strong confirmatory evidence for linkage disequilibrium between the haplotype DRB110304-DQB1102-DQA110501 and disease found in the case control study. No independent association of DQA110501 was seen over and above linkage disequilibrium with DR alleles, suggesting that genetic susceptibility to GD cannot be attributed to any one allele.
| Acknowledgments |
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| Footnotes |
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2 Smith and Nephew Medical Research Fellow. ![]()
Received March 13, 1998.
Revised April 21, 1998.
Accepted June 15, 1998.
| References |
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