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Original Studies |
The Research Institute for Children (Q.-Y.C., D.N., X.-Y.Z., A.K., Y.-J.H., A.V., R.G., S.C., M.L., N.K.M.), Childrens Hospital, Harahan, Louisiana 70123; Departments of Pediatrics (Q.-Y.C., A.K., A.V., R.G., S.C., M.S.L., N.K.M.) and Medicine (F.S.), LSUMS, New Orleans, Louisiana 70112; Institute for the Treatment of Diabetes and Metabolism (J.W.), Methodist Hospital, New Orleans, Louisiana 70127; Department of Medicine (R.R., K.E.F.), Tulane University Medical Center, New Orleans, Louisiana 70112; and Division of Endocrinology and Metabolism (Y.T.), Mount Sinai School of Medicine, New York, New York 10029
Address correspondence and requests for reprints to: Noel K. Maclaren, M.D., Director, Research Institute for Children, 520 Elmwood Park Boulevard, Suite 160, Harahan, Louisiana 70123. E-mail: nkmaclaren{at}aol.com
| Abstract |
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| Introduction |
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Though Graves disease is also common in African Americans, few studies have addressed the associations between HLA class II genes and Graves disease in this ethnic group. In two recent studies, one group found DQ3 to be associated with Graves disease (7), whereas another group found no significant associations between Graves disease and the HLA class II genes they tested, which included the major alleles of the DRB1, DQA1, and DQB1 loci (8). It has not yet been reported that African American patients with Graves disease have distinct HLA genotypes, with respect to their ages at onset, nor that the disease is associated with DRB3 or CTLA-4, both of which have been reported to be associated with Graves disease in Caucasians (2, 4, 9, 10, 11, 12, 13). We therefore studied a group of African American patients, with mostly adult onset Graves disease, and racially matched controls, seeking HLA and CTLA-4 associations. We studied the major alleles of DRB1, DQB1, DQA1, the four subtypes of DRB3, presence of DRB4, and CTLA-4 gene polymorphisms. We report here that DRB3 and DQA1*0501 are positively associated with Graves disease in this group of patients, with highest odds ratio (OR) for the HLA-haplotype of DRB3/DQA1*0501, especially when the haplotype is DRB3*0202/DQA1*0501.
| Subjects and Methods |
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The subjects comprised 39 [34 female (F), 6 male (M)] African American patients with Graves disease and 47 racially matched controls (33 F, 14 M) from the metropolitan area of New Orleans, Louisiana. An additional 10 (8 F, 2 M) African American patients with Graves disease were from the Mount Sinai Medical Center in New York. The diagnosis of Graves disease was established according to a classical clinical presentation, typical histories, and physical findings (diffusely enlarged goiter and degrees of proptosis and/or exopthalmos), as well as definitive laboratory findings of hyperthyroidism and antibodies to TSH receptors by a radioimmunoprecipitation assay (Kronus Co, CA). The HLA allelic frequencies were similar for patients from New Orleans and New York (data not shown). Thus, the two groups were pooled for this study, albeit they were also analyzed separately. The mean age at onset for the 49 patients was 49 ± 12 yr, with a range of 1773 yr. The 47 controls were from healthy laboratory staff, hospital volunteers, and 9 patient controls with multinodular goiter with no exopthalmos, no family history of Graves disease, and no antibodies to TSH receptors. The mean age for the 47 controls was 47.2 ± 12.3 yr. The HLA frequencies in these controls were similar to those of African Americans with type II diabetes whom we studied (data not shown). Patients and controls were studied after written informed consent was obtained, as approved by the respective Institute Review Boards.
HLA DR, DQ typing
Molecular typing of HLA DR and DQ was carried out according to the requirements of The American Society for Histocompatibility. Samples of genomic DNA were prepared from peripheral blood mononuclear cells by proteinase K digestion, followed by phenolchloroform extraction, as previously described (14). The primer pairs for HLA typing were designed according to published sequences (14, 15). The major types of HLA-DRB1, DQB1, DRB3, DRB4, and four subtypes of DRB3 were typed using the technique of sequence-specific priming (6, 14). Briefly, PCR amplifications were performed on 50100-ng genomic DNA in 25-µL reaction vols containing 50 mmol/L KCL, 10 mmol/L Tris-HCL (pH 8.3), 1.5 mmol/L MgCl2, 60 mmol/L each deoxynucleotide triphosphates. The PCR reactions were then subjected to 35 cycles of 30 sec at 94 C for denaturing, 30 sec at 65.5 C for annealing, and 30 sec at 72 C for extension, using an automated thermal cycler (9600; Perkin-Elmer Cetus, California). The amplified genomic DNA products were separated in 2.5% agarose gel, stained with ethidium bromide, and visualized under ultraviolet illumination.
CTLA-4 polymorphisms
The polymorphism of CTLA-4 at the amino acid position 17 in exon 1 was examined by restriction enzyme (BbvI) digestion of the PCR products produced using the primer pair 5'-gct cta ctt cct gaa gac ct-3 and 5'-agt ctc act cac ctt tgc ag-3'. PCR reactions were performed as for HLA typing, except that the annealing temperature was 62 C. Some 20 µL of the amplified product was digested with 5 U BbvI (Pharmegen) at a final vol of 30 µL at 37 C for 2 h. The digested DNA was then separated with 2.5% agarose gel and visualized as for HLA typing. The presence of G allele at the nucleotide of 49 gives rise to a cleavage site for restriction enzyme BbvI.
Statistical analyses
For the calculation of allelic frequencies of patients, compared with controls, subjects with heterozygous alleles were counted for the presence of each allele of interest, whereas subjects with homozygous alleles were counted only once. The numbers of the subjects with a specific allele were then divided by the total numbers of the subjects in the group and multiplied by 100 to express frequencies as percentages. Chi-square tests with Yates correction were used, and P-values of less than 0.05 were considered to be statistically significant. The OR were calculated using Haldanes modification of Woolfs method.
| Results |
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The frequencies of DR and DQ alleles were compared between the 49
patients and the 47 controls (Tables 1
and 2
). DRB1*03, which is associated with
Graves disease in Caucasians, was similar between the disease and the
control groups. However, the patients with Graves disease had a
higher frequency of DRB1*11 (DR5) than controls, although the
difference was not significant after Yates correction.
Similarly, there were no significant differences for the frequency of
DQB1 alleles between the patients and controls, whereas the frequency
of DQB1*0302 was nonsignificantly decreased after Yates correction.
DQB1*07, which is reported to be negatively associated with Graves
disease in Caucasians, was not significantly different between the
patients and the controls in this study. However, the frequency of
DQA1*0501 was significantly higher in the Graves patients than in the
controls [49% (24 of 49) vs. 25.5% (12 of 47),
P = 0.018, X2 =
5.63].
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The frequency of DRB3 was increased significantly in patients with
Graves disease, over that in controls [75.5% (37 of 49)
vs. 57.4% (27 of 47), P = 0.006,
X2 = 3.52]. There was no difference for the
frequency of DRB4 (in linkage with DRB1*07) between the disease and the
controls (data not shown). For the four subtypes of DRB3, the frequency
of DRB3*0202 alone was increased significantly in the patients, whereas
the remaining three subtypes had frequencies similar to controls (Table 3
).
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The allelic frequencies in the patients and controls were also
analyzed for the haplotype of DRB3/DQA1*0501 (Table 3
). The frequency
of the haplotype of DRB3/DQA1*0501 was significantly higher in the
patients than in the controls [46.9% (23 of 49) vs. 21.3%
(10 of 47), P = 0.008, X2 =
7.0]. Further, the most significant association was found for the
haplotype of DRB3*0202/DQA1*0501 [40.8% (20 of 49 vs.
10.6% (5 of 47), P = 0.0008, X2
= 11.34].
Polymorphism of CTLA-4
The nucleotide A/G polymorphisms at the amino acid position of 17
located at the exon 1 of CTLA-4 gene were used to calculate the
difference between the Graves group and the controls. No differences
were found in the distributions of the A or G alleles, nor did the AA,
AG, or GG genotypes differ between patient and the controls (Table 4
). Similar results were observed when we
excluded the 10 subjects from the New York area and compared the
allelic frequencies of the tested HLA DR, DQ, or CTLA4 polymorphisms of
the remaining 39 subjects with Graves disease from New Orleans with
those of 47 controls (data not shown).
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| Discussion |
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To our knowledge, this is the first report to test the association between DRB3 and Graves disease in African American patients. DRB3 and one of the subtype DRB3*0101 were reported to be associated with Graves disease in Caucasians (2, 4). However, studies also suggested that the association between DRB3 with Graves disease in Caucasians could be secondary to its linkage disequilibrium with other susceptible genes, such as DRB1*0301 (3, 5, 18). This is also in concordance with our previous study on Caucasian patients with juvenile onset, but not adult onset, Graves disease (6). Therefore, the association of DRB3*0101 (if any) with Graves disease could be attributable to its linkage disequilibrium with DRB1*0301 (DR3). However, we found DRB3*0202 to be positively associated with adult onset Graves disease in Caucasians, whereas DRB1*03 without DRB3*0202 was not (6). This is similar to the results found in the current group of African American patients with adult onset of Graves disease. These data indicate that DRB3*0202 could be a common susceptible allele for adult onset Graves disease in these two ethnic groups. Alternatively, the presence of susceptible HLA genes in the current group of African American patients could be attributable to its genetic admixture with Caucasian Americans. The frequency of DRB1*11, in fact, in this group of patients is similar to our previously reported data for Caucasian patients (6). Our patients, however, had reduced frequencies of the DRB1*04/DQB1*0302 haplotype, as expected (given that it is a characteristic haplotype of Caucasians).
Accumulated evidence indicates that African Americans share some genes, including HLA from American Caucasians. For example, DQA1 allelic frequencies are similar between Caucasian and African Americans living in Florida (19). The frequencies of HLA DRB1 in African Americans are intermediate between those of the two founding populations, native Africans and Europeans (20). However, HLA allelic frequencies and haplotype distributions through genetic admixture may vary between groups of African Americans residing in separate areas of the United States (20, 21). Future studies should involve larger numbers of patients and controls than we have here, to avoid potential racial admixture errors.
The frequency of DQA1*0501 was increased significantly in the patients with adult onset of Graves disease reported here, but this was not the case for a group of African American patients with Graves disease reported in another study (8). It is noted that the frequency of DQA1*0501 in African American control subjects varies, dependent on their residing areas (8, 22). Accordingly, the inconsistent association between DQA1*0501 and Graves disease could again be attributable to different genetic admixtures within the studied subjects. However, controversial results for DQA1*0501 have also been reported for Caucasians patients with Graves disease (23, 24, 25). Apart from racial heterogeneities of the studied subjects, factors like linkage disequilibria, ages at onset, and gender could have also contributed to the inconsistency of the results. For example, the association of DQA1*0501 with Graves disease in juvenile Caucasian patients was found to be attributable to its linkage disequilbrium with DRB1*0301 (5), although DQA1*0501 was still found in association with Graves disease reported in an earlier study, despite the exclusion of subjects with DR3 from analysis (3). It is noted that no study in the literature has been carried out to analyze for association of DQA1*0501 with adult onset Graves disease after stratification for DR3. Similarly, the increased frequency of the haplotype of DRB3/DQA1*0501, especially the haplotype DRB3*0202/DQA1*0501 found in the present study, is expected on the basis of a linkage disequilbrium between DRB3 and DQA1*0501. It is possible, however, that the combination of the susceptible HLA alleles DRB3*0202 and DQA1*0501 could confer additional risks for Graves disease.
The codon 17 polymorphism of CTLA-4 gene conferred no susceptibility to Graves disease in this group of African Americans. However, CTLA-4 gene has been fairly consistently reported to be associated with Graves disease in Caucasian patients (9, 10, 11, 12, 13). Again, the factors discussed above may play roles leading to our results, which seemingly contradict those in Caucasian patients. The CTLA-4 molecule plays an important role in T cell regulation, being expressed in activated T cells, leading to their eventual apoptosis and arrest of the response. Thus, further studies should be carried out in association with Graves disease in African Americans, using a larger numbers of subjects, before concluding that the CTLA-4 gene is not involved.
In conclusion, when we combine this data with that in Caucasian patients, our results suggest that the primary disease susceptible locus in Graves disease is DRB3 and its *0202 subtype, and/or DQA1*0501 but not DRB1. HLA class II genes may play roles in the pathogenesis of Graves disease, especially in young Caucasian patients, whereas the environment replete with high iodine consumption or putative thyroidal viruses may be more important to the Graves disease seen in African American adults than in Caucasians. If the primary association between Graves disease and DRB3 (*0202) is confirmed by others, then studies of binding affinities of TSHR peptides to DRB3 molecules would seem to be indicated.
| Acknowledgments |
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| Footnotes |
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Received September 3, 1999.
Revised November 8, 1999.
Accepted December 14, 1999.
| References |
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