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Original Studies |
Department of Endocrinology, Radcliffe Infirmary (P.J.H., J.A.H.W.), Oxford, United Kingdom OX2 6HE; Oxford Transplant Center, Churchill Hospital (S.E.M., K.I.W.), Oxford, United Kingdom OX3 7LJ; Division of Clinical Sciences, Northern General Hospital (A.P.W.), Sheffield, United Kingdom S5 7AU; and Department of Medicine, John Radcliffe Hospital (J.I.B.), Oxford, United Kingdom OX3 9DU
Address all correspondence and requests for reprints to: Dr. P. J. Hunt, Department of Endocrinology, Riverside Block, Christchurch Hospital, Private Bag 4710, Christchurch 8001, New Zealand.
| Abstract |
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Polymorphisms in the genes for interleukin-1
(IL-1
), IL-1ß,
IL-1 receptor antagonist, IL-1 receptor 1, IL-4, IL-4 receptor, IL-6,
IL-10, and transforming growth factor-ß were investigated. Genotyping
was performed using the PCR and sequence-specific primers. Analysis
showed a reduced frequency of the variant t allele in the IL-4 promoter
polymorphism (position -590) in patients with GD and in the entire
patient group (GD and AIH) compared with the control group [corrected
P (Pc) = 0.00004 and
Pc < 0.00001 for GD and all patients,
respectively]. This was reflected in a reduction in the heterozygote
genotype in the patient groups compared to the controls [c/t
heterozygotes GD, 12%; Pc = 0.06, odds ratio, 0.4
(95% confidence interval, 0.20.7); all patients, 11%;
Pc = 0.008; odds ratio, 0.4 (95% confidence
interval, 0.20.7); control subjects, 23%]. There were no
significant differences between the study groups for the other
polymorphisms examined, and subgroup analysis revealed no association
with clinical parameters of disease.
These results suggest that an IL-4 variant or a closely linked gene has a modest protective effect against the development of autoimmune thyroid disease, particularly GD. This variation in the IL-4 gene may provide further clues to the pathogenesis of autoimmune thyroid disease and other organ-specific autoimmune diseases. Furthermore, these results suggest that subtle variation in immunoregulatory genes may be associated with autoimmune disease states.
| Introduction |
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Cytokines participate in the induction and effector phases of the
immune and inflammatory response and are therefore likely to play a
critical role in the development of autoimmune thyroid disease.
Intrathyroidal inflammatory cells and thyroid follicular cells have
been shown to produce a variety of cytokines, including
interleukin-1
(IL-1
), IL-1ß, IL-2, IL-4, IL-6, IL-8, IL-10,
IL-12, IL-13, IL-14, tumor necrosis factor-
, and interferon-
(IFN
) (10, 11). The cytokine network is complex, with cytokines
having both diverse and overlapping functions, including effects that
are promoted or inhibited by other cytokines. Cytokine secretion
profiles can be considered as either pro- or antiinflammatory or,
alternatively, on the basis of the animal model as either T helper cell
type 1 (Th1) responses promoting cell-mediated immunity (IL-2 and
IFN
) or T helper cell type 2 (Th2) responses promoting humoral
immunity (IL-4, IL-5, IL-6 IL-10, and IL-13) (12). Despite attempts to
classify autoimmune thyroid disease as a classical Th1- or Th2-mediated
disease, no clear conclusions can be drawn, and a mixed Th1/Th2
response is seen in both GD and AIH (10). Cytokines in the thyroid
gland also have a role in regulating antigen presentation and
lymphocyte trafficking by enhancing the expression of HLA class II and
adhesion molecules on thyroid follicular cells (10). In
thyroid-associated ophthalmopathy, cytokines (IL-1
, tumor necrosis
factor-
, and IFN
) are pathogenic, promoting inflammation and
fibroblast proliferation, leading to the accumulation of
glycosaminoglycans (13). Polymorphisms in genes encoding these crucial
immunomodulatory molecules may result in an altered level of expression
and hence must be considered important candidate genes for autoimmune
disease susceptibility and severity. As cytokines interact
functionally, this study examines the contributions of several
candidate genes with potential immunoregulatory roles in autoimmune
thyroid disease. The candidate genes were selected for investigation if
their gene product was likely to be important in the regulation of the
cellular or humoral immune response and if a single nucleotide
polymorphism within the gene was amenable to genotyping.
IL-1
and IL-1ß are pleiotropic cytokines with primarily
proinflammatory effects, including stimulation of IL-2 and IL-6. Both
IL-1
and IL-1ß as well as the naturally occurring IL-1 receptor
antagonist (IL-1RA) act via the IL-1 receptor 1. Hence, disruption of
the balance among IL-1
, IL-1ß, IL-1RA, and their receptors may
result in disease. Polymorphisms of IL-1
(position -889) and
IL-1ß (exon 5, +3962) (14) have been associated with juvenile
rheumatoid arthritis (15, 16) and insulin-dependent diabetes mellitus
(14, 17), respectively. Variation in the IL-1RA gene (exon 2) has been
associated with a variety of autoimmune disorders (18, 19, 20, 21), including
Graves disease (22), although this finding has not been replicated
(23).
IL-4 mediates the humoral immune response, and polymorphisms in IL-4 (position -590) and its receptor gene (nucleotide 1902) have been associated with atopy (24, 25). IL-6 is a key inflammatory cytokine, and elevated systemic levels are seen in many conditions, including autoimmune thyroid disease (10). Promotor region variation of the IL-6 gene has been associated with juvenile rheumatoid arthritis (26). IL-10 enhances B cell proliferation and an IL-10 polymorphism (position -1082) has been associated with systemic lupus erythematosus (27) and rheumatoid arthritis (28). Transforming growth factor-ß (TGFß) is another cytokine with important modulatory functions, including an inhibitory role in B cell maturation.
To investigate whether variability in these immunoregulatory genes may influence disease susceptibility or severity, polymorphisms were assessed in a case control association study using a unified method of genotyping.
| Subjects and Methods |
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The patient cohort comprised 215 Caucasoid patients with autoimmune thyroid disease [135 GD (111 women) and 77 AIH (67 women)] recruited from the endocrinology clinics in Oxford and Sheffield. GD was defined by the presence of hyperthyroidism and diffuse goiter, supported by the presence of either thyroid antibodies (peroxidase and/or thyroglobulin) or thyroid eye disease. AIH was diagnosed by the presence of primary hypothyroidism and positive thyroid antibodies with or without goiter. Information was also obtained on age at diagnosis, size of goiter, presence of other autoimmune disease, and, for GD, severity of ophthalmopathy and relapse rates. The study was approved by the respective local ethics committees, and all subjects gave written informed consent.
The control population comprised 101 Causcasoid cadaveric renal allograft donors. The representative nature of this control population of the general Caucasoid population has previously been shown in HLA genotyping reports (29).
Genotyping methodology
All genotyping was performed using PCR-sequence specific primers
(PCR-SSP). DNA was extracted from 10 mL ethylenediamine tetraacetate
blood using the Puregene kit (Gentra Systems, Minneapolis, MN). PCR
primers were designed with allele specificity determined by the
terminal 3'-nucleotide. For detection of two or more closely related
polymorphisms within the same gene, forward and reverse
allele-specific primers were used (PCR-haplotyping) (30), thus
minimizing the number of PCR reactions and formally identifying the
cis/trans orientation of the alleles. Primers and
concentrations used for IL-1
(-889t/c), IL-1ß (+3962 t/c), IL-4
(-590c/t), IL-6 (+3247a/g), IL-10 (-1082a/g, -819c/t, -592 c/a),
and TGFß (-880g/a, -509c/t, aa10L/P, aa263T/I) have been previously
described (31). Primers for IL-1ß (-511 c/t), IL-1RA, IL-1 receptor
1, and IL-4 receptor are listed in Table 1
. To confirm adequate DNA amplification,
all reaction mixes also contained control primers. Further details of
the PCR-SSP methodology, including PCR amplification and gel
electrophoresis, have been published previously (32). An example is
shown in Fig. 1
.
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Allele frequencies were compared between the patient groups and the control group using the binomial distribution probability. Genotype frequencies were also compared between the entire patient group and the control group and between the subgroups of GD and AIH compared to the control group using the genotype relative risk method of Lathrop (33). P values were corrected for the number of independent comparisons of haplotype or allele variants that were made (n = 12) and a corrected P (Pc) value of less than 0.05 was considered significant.
| Results |
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The major finding was a striking reduction in the frequency of the IL-4
(-590) variant t allele in the patients with GD and the overall
patient group (GD and AIH) compared to the control group (GD:
Pexact = 0.00003; Pc =
0.0004; GD and AIH: Pexact<0.00001;
Pc < 0.00001; Table 2
).
For the patients with AIH, a similar trend was seen, but this was not
significant when corrected for the number of comparisons made
(Pexact = 0.006; Pc =
0.07). Genotype analysis showed that the reduction in t allele
frequency resulted in fewer IL-4 heterozygote genotypes in the patients
with GD compared to controls [P = 0.005;
Pc = 0.06; odds ratio (OR), 0.4; 95% confidence
interval, 0.20.7] which was also seen when all patients were
compared with controls (P = 0.0007; Pc
= 0.008; OR, 0.4; 95% confidence interval, 0.20.7; Table 3
). Again, a similar tendency was seen
for the patients with AIH (P = 0.01; Pc
= 0.11), but this was not significant when corrected for the number of
observations. Allele and genotype frequencies for the other gene
polymorphisms did not differ significantly between the patient and
control groups (data not shown, but available from the authors).
Further subgroup analysis was performed, and no significant
associations of polymorphisms (including the IL-4 -590 polymorphism)
were detected with age of onset of disease, size of goiter, recurrent
GD, titer of thyroid peroxidase or thyroglobulin antibodies, thyroid
eye disease, or presence of other autoimmune disease (data not
shown).
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| Discussion |
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IL-4 is a key cytokine in immune regulation. It is produced by T cells, mast cells, and eosinophils and causes proliferation of IgE- and IgG-secreting B cells. It also stimulates the expression of HLA class II antigens via STAT6 (signal transducer and activator of transcription-6) (34) and opposes the Th1 cell inflammatory response. Indeed, IL-4 is considered the pivotal cytokine polarizing the immune response toward a Th2 cell response, and although the initial trigger for IL-4 production remains unknown, genetic variation is likely to play a role (35). The IL-4 polymorphism investigated in this study is a c to t base change in the promotor region of IL-4 at position -590 (36). This promotor polymorphism appears to be functional, with increased transcriptional activity attributed to the variant allele (24). Association of this polymorphism (24, 37, 38) and an IL-4 receptor gain of function polymorphism (25) with atopy provides evidence that IL-4 is an important mediator of allergic disease.
The mechanism by which IL-4 is involved in the development of autoimmune thyroid disease, particularly GD, requires explanation. T cell activation is believed to be the key event in the initiation of autoimmune thyroid disease (39), and thus, cytokines are likely to be intimately involved in the process. The patients with autoimmune thyroid disease in our study exhibited a lower prevalence of the variant t allele than the controls, which may reflect a overall lower activity of IL-4. This would favor an inflammatory immune response mediated by Th1 cells. In humans, Th1 cytokines stimulate the production of an IgG1 isotype response (40), which is the predominant pathogenic TSH receptor autoantibody seen in GD (and sometimes in AIH) (41, 42). IgG1 isotypes are also prevalent among thyroglobulin and peroxidase antibodies (43, 44). Hence, in autoimmune thyroid disease, lower IL-4 activity may result in a propensity to develop IgG1 autoantibodies along with polarizing the immune response toward cell-mediated immunity. Supporting this mechanism, IL-4 has been shown to inhibit organ-specific autoimmune disease in animals (45, 46).
For GD, the reduced frequency of the variant t allele was highly significant and associated with small confidence intervals (OR, 0.4; 95% confidence interval, 0.20.7). Although not directly comparable, these results are in keeping with previously identified candidate genes for GD, such as HLA DR3 and CTLA-4 (relative risks of 25 and 23, respectively) (2). It is of interest that a similar trend for an association of the IL-4 polymorphism was seen in the group with AIH compared to the control group. There is some evidence suggesting that the two disorders are related and may represent two ends of a spectrum of autoimmune thyroid disease phenotype. Thus, the two diseases commonly cluster within the same family (47), monozygotic twins are described where one has GD and the other has Hashimotos thyroiditis (48), and clinically an individual may fluctuate between GD and AIH. Aspects of the pathogenesis of the two disorders are also related; both are associated with antibodies to thyroid tissue and exhibit some similarities in immune function (10). The lack of a significant association after Bonferroni correction for the group with AIH may be because fewer individuals with AIH were genotyped than those with GD (type 1 error).
Although the above explanation supports a pathophysiological mechanism for the association of this IL-4 variant with autoimmune thyroid disease, it is also possible that this association is not due to the IL-4 gene, but to another gene in linkage disequilibrium. The genes for IL-3, IL-5, IL-13, and granulocyte-macrophage colony-stimulating factor are all in close proximity. Of these, IL-13 deserves further attention because of the major role it plays in the cytokine network (34).
The other cytokine gene polymorphisms evaluated in this study did not reveal any significant associations with autoimmune thyroid disease. Although these negative results exclude any major genetic effect, identifying genes exerting very minor effects or influencing clinical phenotype requires extremely large study cohorts, particularly when several candidate genes are evaluated, and adjustments for multiple comparisons are necessary.
This study has demonstrated a significant association of IL-4 genotype with susceptibility to autoimmune thyroid disease. This suggests that variation in amplification and regulation of immune responses within the thyroid determine clinical disease. However, variation in IL-4 accounts for only part of the abnormal immunoregulatory response seen in autoimmune thyroid disease, and as in other complex diseases, there are likely to be many polymorphic genes, each exerting only a minor effect. This finding is not only of interest to autoimmune thyroid disease, but may also be of relevance to other organ-specific autoimmune diseases. Further examination of polymorphisms in cytokine genes should enhance our understanding of the cytokine network and provide clues to the pathogenesis of particular diseases.
| Acknowledgments |
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| Footnotes |
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Received July 22, 1999.
Revised December 29, 1999.
Accepted January 14, 2000.
| References |
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polymorphism. Arthritis Rheum. 38:221228.[Medline]
and interleukin-1 receptor
antagonist genes and Graves disease in a North American Caucasian
population. J Clin Endcrinol Metab. 81:44764478.[Abstract]
subunit of the interleukin-4 receptor. N Engl J Med. 337:17201725.This article has been cited by other articles:
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K. K. L. Chong, S. W. Y. Chiang, G. W. K. Wong, P. O. S. Tam, T.-K. Ng, Y.-J. Hu, G. H. F. Yam, D. S. C. Lam, and C.-P. Pang Association of CTLA-4 and IL-13 Gene Polymorphisms with Graves' Disease and Ophthalmopathy in Chinese Children Invest. Ophthalmol. Vis. Sci., June 1, 2008; 49(6): 2409 - 2415. [Abstract] [Full Text] [PDF] |
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T. Kleinrath, C. Gassner, P. Lackner, M. Thurnher, and R. Ramoner Interleukin-4 Promoter Polymorphisms: A Genetic Prognostic Factor for Survival in Metastatic Renal Cell Carcinoma J. Clin. Oncol., March 1, 2007; 25(7): 845 - 851. [Abstract] [Full Text] [PDF] |
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J. Oden and I. M. Cheifetz Neonatal Thyrotoxicosis and Persistent Pulmonary Hypertension Necessitating Extracorporeal Life Support Pediatrics, January 1, 2005; 115(1): e105 - e108. [Abstract] [Full Text] [PDF] |
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J. M. Heward, R. Nithiyananthan, A. Allahabadia, S. Gibson, J. A. Franklyn, and S. C. L. Gough No Association of an Interleukin 4 Gene Promoter Polymorphism with Graves' Disease in the United Kingdom J. Clin. Endocrinol. Metab., August 1, 2001; 86(8): 3861 - 3863. [Abstract] [Full Text] [PDF] |
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