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Laboratoire de Génétique Moléculaire Humaine (H.H.K., S.M., H.A.), Faculté de Médecine de Sfax; Centre de Biotechnologie de Sfax (A.R.); and Service dEndocrinologie (N.K., M.A.), CHU Hédi Chaker de Sfax, 3029 Sfax, Tunisia
Address all correspondence and requests for reprints to: Prof. Hammadi Ayadi, Faculté de Médecine, Laboratoire de Génétique Moléculaire Humaine, Av. Majida Boulila, 3029 Sfax, Tunisia. E-mail: hammadi.ayadi{at}fmsf.rnu.tn.
| Abstract |
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| Introduction |
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Candidate gene studies have proven very effective in detecting susceptibility genes for many diseases as well as genes important for disease progression and severity. Population-based, case-control studies have shown a consistent association of AITDs with many candidate genes involved in immune response or thyroid physiology. Immunoglobulin heavy chain (Gm) (15), IL-1 receptor antagonist (20), T-cell antigen receptor ß (21), and some MHC haplotypes have been found to be associated with GD in some populations but not in others (13, 14, 22, 23, 24, 25). Many thyroid-restricted genes have been tested, such as the TSH receptor gene (TSHR) (26), thyroid peroxidase (27), and thyroglobulin (Tg) (28). Association of GD to TSHR gene has been found in some populations, but this has been difficult to reproduce in others (26, 29). Only the Tg gene has been found to be linked and associated with AITD (28).
The PDS gene (7q31), responsible for Pendred syndrome (congenital sensorineural hearing loss and goiter) (OMIM 274600), was identified in 1997 (30). The PDS gene encodes a transmembrane protein known as pendrin, which is expressed in the thyroid, kidney, and fetal cochlea (30). Pendrin functions as a transporter of iodide and chloride (31). Iodide is of known importance in the thyroid, thereby emphasizing a potential critical role for pendrin in thyroid physiology. In vitro and in vivo findings showed that iodine organification was altered in thyroid cells obtained from a patient with Pendred syndrome (32). Interestingly, significantly greater amounts of pendrin were encountered in thyroid tissue from patients with GD (33). In addition, concurrence of Pendred syndrome, autoimmune thyroiditis, and simple goiter in one family has been described (34). These findings prompted us to analyze the possible contribution of the PDS gene in the genetic susceptibility of AITDs.
In this study, we analyzed polymorphic microsatellite markers around the PDS gene, using case-control and families-based designs, to evaluate the role of PDS in the genetic control of AITDs in a sample from Tunisia. Our findings suggest PDS may be a susceptibility gene to AITDs.
| Subjects and Methods |
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One hundred forty-one unrelated patients (30 men and 111 women) with GD, 54 unrelated patients (50 women and 4 men) with HT, and 38 (31 women and 7 men) unrelated patients with PIM were included in this study. Data gathered from the patient group were compared with those obtained from a control population of 154 unrelated healthy subjects (75 men and 79 women) originating from the same area with no clinical evidence or family history of AITD and inflammatory joint disease.
Families
Fifteen families (104 individuals containing 46 patients) were analyzed in the current study. Three families were simplex and the others contained more than two patients. Six families were mixed containing both GD and HT patients and the remaining families contained only one pathology (seven with GD and two with HT). On the average, the families had 6.9 members.
Clinical assessment
GD was diagnosed on the basis of clinical and laboratory evidence of hyperthyroidism requiring treatment, palpable diffuse goiter, high thyroid hormonal rates, positive anti-TSHR, antithyroid peroxidase, and/or antithyroglobulin antibodies. HT was diagnosed by documented clinical and biochemical hypothyroidism requiring thyroid hormone replacement and presence of autoantibodies to thyroid peroxidase, with or without antibodies to thyroglobulin. PIM was diagnosed by the presence of hypothyroidism requiring T3 or T4 replacement. Patients with PIM have an atrophic gland.
Genotyping
DNA from patients and controls were obtained from peripheral blood as previously described (35). DNAs from families were genotyped with four microsatellite markers of CA repeat (order of the markers: D7S501-D7S496-D7S2459-D7S692) localized in the PDS chromosomal region with one intragenic marker (D7S2459). The map position and order of markers were published by Everett et al. (30). The distance between the 5' end (D7S501) and the 3' end (D7S692) of the PDS region is 1.7 cM. DNAs from unrelated patients and controls were genotyped with only three markers (D7S496, D7S2459, and D7S692).
Genotyping was undertaken using a radioactive detection system. Microsatellite markers were amplified in 96-well Falcon flexi plates by the PCR using Techne thermocyclers. PCRs were performed in a final volume of 50 ml, containing 60 ng genomic DNA, 50 pmol of each primer, 1.25 mM deoxynucleotide triphosphate, 10 mM Tris pH 9, 50 mM KCl, 1.5 mM MgCl2, 0.1% triton X-100, 0.01% gelatin, and 1 U Taq DNA polymerase.
Each PCR was performed using a hot-start procedure, and amplification was carried out using 35 cycles of denaturation at 94 C for 40 sec, with annealing at 55 C for 40 sec, and elongation at 72 C for 40 sec, followed by a final elongation at 72 C for 2 min. Five milliliters of each sample were denatured at 96 C for 2 min and resolved on 57% denaturing polyacrylamide gel with 8.3 M urea. The separated bands were transferred onto nylon membrane (Hybond N+) and visualized by autoradiography, after exposition to XAR-5 film (Kodak, Rochester, NY) for 248 h at -70 C.
Disease models
Until now it was not clear whether GD and HT are two distinct disorders with different or common etiologies. To evaluate the contribution of the PDS gene in each pathology, we analyzed the data under six models (Table 1
): 1, all AITD patients were considered affected; 2, just GD patients were considered affected (under this model HT patients were considered as unaffected even if they had relatives with GD); 3, just HT patients were considered affected (under this model GD patients were considered as unaffected even if they had relatives with HT); 4, only mixed families were analyzed (AITD patients were considered affected); and 5, families with only GD were analyzed; and 6, families with only HT were analyzed).
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Case-control association study.
The distribution of the alleles in unrelated patients with AITDs vs. controls were compared using the
2 test. The association was evaluated using relative predispositional effect methods (23). Statistical significance was reached when P was less than 0.05, and Fishers exact test was used when necessary. Odds ratios (ORs) and their corresponding 95% confidence intervals (CIs) were calculated according to Woolfs formula (36).
Family-based association tests.
We used the family-based association test (FBAT) package (37) to test for association in the presence of linkage (38). FBAT provides a
2 test statistic that tests the composite null hypothesis of no linkage or no disequilibrium without concerns about confounding because of admixture. FBAT has the advantage over the conventional transmission disequilibrium test (TDT) of being able to use data from all family members, not just case-parent trios.
We also used the TDT approach (39) as implemented in GeneHunter (version 1.3, http://waldo.wi.mit.edu/ftp/distribution/software/genehunter/gh2/) to test for association. In this implementation cases in which one parent is missing are used only when the genotyped parent and the proband are both distinct heterozygotes (40). This program also allows the use of multilocus TDT to test for transmission disequilibrium of haplotypes for two to four markers.
Linkage
We performed multipoint nonparametric linkage analysis using Merlin (41). Merlin implements a new algorithm using sparse gene flow trees, which perform exact and approximate likelihood calculations for single-point and multipoint linkage analysis. Merlin allows time saving in computation comparatively to GeneHunter (42) and Allegro (43) for some family settings in complex pedigrees. Merlin outputs both NPLall (Z) scores and their corresponding allele sharing LOD scores obtained by the Kong and Cox
method under exponential model (44) to test for allele sharing among affected individuals with their asymptotic P values. It also gives the maximum possible Kong and Cox LOD scores (LODmax) one might get if fully informative markers linked to an unobserved disease locus of maximal effect were available, conditional on observed phenotypes and family structure.
Single-point and multipoint parametric LOD score linkage analysis was also performed with GeneHunter (42) under both dominant and recessive genetic models, assuming 50% penetrance, no phenocopy, and 5% gene frequencies as suggested in the MMLS approach of Greenberg et al. (45). An admixture test was also used to assess heterogeneity among families.
To evaluate evidence for linkage of the marker region tested, we used pointwise P values to calculate regionwise P values by applying the formula of Lander and Kruglyak (46) with C = 1 (one chromosome under test) and G = 0.017 (length in morgans of the region tested). Note that genome-wide criteria (given in Ref. 46) should not be used here because we were investigating a new candidate region without any prior information or evidence for linkage (e.g. from a previous genome scan).
The marker mapping information was obtained from the Genethon map (47). The last update of this map (http://www.gdb.org/hugo/chr7/geneticmaps.html) was used. When available, we used in linkage analysis the marker allele frequencies estimated on an independent sample of 154 unaffected individuals drawn from the general population (except for marker D7S501).
| Results |
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Case-control studies
The D7S2459 microsatellite marker polymorphism analysis revealed nine alleles with sizes ranging from 132 to 152 bp (Table 2
). When the allelic and genotypic frequencies corresponding to the D7S2459 in GD patients and controls were compared, no significant differences were found (P > 0.05). However, analysis of the allelic frequency distribution in HT and PIM patients showed a significant difference from control data (HT:
2 = 132.03, 8 degrees of freedom, P = 1.07 10-24; PIM:
2 = 35.64, 8 degrees of freedom, P = 2 10-5; HT and PIM:
2 = 87.58, 8 degrees of freedom, P = 1.4 10-15). In addition, the comparison between HT and PIM allele distributions showed a significant difference (P = 1.6 10-13), reflecting a very important allelic heterogeneity between types of AITD. To evaluate the risk of each allele in determining genetic susceptibility of HT and PIM, ORs are shown in Table 2
. The highest OR was found with the 144-bp and 132-bp alleles respectively (HT: OR = 7.81; 95% CI, 3.96<OR<15.52, P = 2 10-12; PIM: OR = 9.27, 95% CI, 1.29<OR<103.59, Fishers exact test: P = 0.01). To reveal the relative effects (predisposing, protective, or neutral) of the D7S2459 alleles, we applied the relative predispositional effects method (23). After removing the 144-bp and 132-bp alleles from both patient (HT and PIM, respectively) and control data, and repeating comparisons, the significance of overall
2 was preserved (HT:
2 = 100.09, 7 degrees of freedom, P = 1.03 10-18; PIM:
2 = 27.16, 7 degrees of freedom, P = 3.1 10-4). Indeed, more than one allele contributes in this significant association with predisposing or protective effects (Table 2
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The D7S496 microsatellite marker revealed 13 alleles with size ranging from 121 to 145 bp (Table 3
). Analysis of this polymorphism in GD patients showed a significant difference between patient and control allele frequencies distributions (GD:
2 = 32.99, 12 degrees of freedom, P = 10-3). The highest OR was found with the 145-bp allele (OR = 3.92, 95% CI, 1.21<OR<16.53; P = 0.01) and the smallest significant OR was found with the 131-bp allele (OR = 0.20, 95% CI, 0.05<OR<0.62; P = 0.001). To evaluate the effect of each allele, we applied the relative predispositional effect method. After removing the 145-bp and 131-bp alleles (separately) from patient and control data, and repeating comparisons, the significance of overall
2 was preserved in both comparisons (P = 0.0055 and P = 0.0174, respectively). Analysis of the D7S496 polymorphism in the HT patient showed no significant association between marker and disease (P > 0.05).
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Results of allelic tests in FBAT (Table 4
) showed particularly highly significant association with the combined AITD phenotype (P = 0.0089) and at a lower degree with the GD only model (P = 0.0253) for allele 121 bp of D7S496. Significant association at the 5% level was also found for AITD and mixed disease models with allele 173 bp of D7S501.
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Linkage analyses
Results of linkage analysis are reported in Table 5
. We notice that significant evidence for linkage was found with marker D7S496 in the GD only model (Z = 2.12, LOD = 0.81, P = 0.026, regionwise P = 0.033 < 0.05). It is interesting to note that for all cases studied, and for each marker interval, maximum LOD values were reached on marker positions and the highest marker information was found for marker D7S496 (data not shown); this marker has the maximum number of typed individuals and the highest reported LOD score for almost all models (Table 5
).
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| Discussion |
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This study provided strong evidence that the PDS gene is an important susceptibility gene for AITD in our population. Association analysis using case-control study showed a significant association between D7S496 and GD. Direct analysis of the PDS gene using one intragenic microsatellite marker (D7S2459) showed a strong association with HT. Results of FBAT showed particularly highly significant association with AITD and at a lower degree with GD only model for allele 121bp of D7S496. Results obtained by TDT were in good agreement with those of FBAT, and evidence for linkage and association of allele 121 bp of D7S496 with AITD is confirmed. Multipoint nonparametric linkage analysis using Merlin showed an interesting evidence for linkage with marker D7S496 in families with GD only. These results strongly suggest that the PDS gene contributes to AITD emergence.
Absence of linkage between HT and the PDS gene could be explained by the reduced number of patients in the studied sample, by the weak contribution of the PDS gene in HT development and/or the presence of an additional candidate gene to GD in the PDS region. Indeed, this region contains other nearby candidate genes for autoimmune diseases, including B-cell receptor-associated protein (BAP29). Physical mapping (http://www.genome.ucsc.edu) showed that D7S496 microsatellite marker was very close to the BAP29 gene. Thus, this gene could thereby be another susceptibility gene to GD in the PDS gene region. Moreover, this chromosomal region (7q2231.1) showed a suggestive linkage to other autoimmune diseases, such as type 1 diabetes (48), multiple sclerosis (49), and inflammatory bowel disease (Crohns disease and ulcerative colitis) (50), thereby suggesting that this region may include a common susceptibility gene for these autoimmune diseases. However, the causal gene may just be linked to PDS.
The PDS gene may predispose to AITD by two global mechanisms: level of gene expression and/or protein activity in the thyroid tissue. An important part of both mechanisms are dependent of the genomic sequence of the gene. Indeed, sequence change in or near the PDS gene may alter its expression or pendrin interaction with other molecules. In fact, many studies report the involvement of intronic microsatellites repeat in many gene expressions by the splicing enhancement and/or the tissue-specific alternative splicing (51, 52, 53, 54). The D7S2459 microsatellite marker could play a similar role in PDS gene, and further work is required to evaluate the effect of AC repeat length in gene expression and mRNA stability. In addition, a recent work examining the regulatory network controlling the pendrin synthesis, using a cultured rat thyroid cell line (FRTL-5), showed that PDS expression was found to be significantly induced by low concentration of Tg, and the increase in PDS gene expression was transcriptional (33, 55). However, this regulation is not exclusive for the PDS gene. Indeed, Tg is a transcriptional regulator of many other thyroid-restricted genes (55).
Recently a study using 102 families (264 patients) showed strong evidence for linkage of AITD to the Tg region (8q24) (28). Direct analysis using Tg intragenic microsatellites markers demonstrated that the Tg gene was both linked and associated with AITD. This gene is considered the first thyroid-specific gene to be found linked and associated with AITD (28). But a full genome screening in a large Tunisian family affected with AITDs did not show any linkage with 8q24 chromosome (12). However, direct analysis of Tg gene using intragenic markers is necessary to exclude or involve this gene.
The pendrin and Tg play a dynamic role in iodide flux and thyroid hormone formation (55); therefore, it is easy to imagine the involvement of pendrin in thyroid gland stability, but we need more investigation to explain how pendrin can create an autoimmune process. Therefore, the same problem may exist for each thyroid-specific gene found to be associated or linked with AITD. Furthermore, understanding the interaction of the PDS gene and its product with the whole of molecules expressed in the thyroid tissue is central to elucidate the role of this protein in AITD physiopathology.
In conclusion, we found evidence in these Tunisian samples that PDS may be a susceptibility gene for AITD.
| Acknowledgments |
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| Footnotes |
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Abbreviations: AITD, Autoimmune thyroid disease; CI, confidence interval; FBAT, family-based association test; GD, Graves disease; HT, Hashimoto thyroiditis; MHC, major histocompatibility complex; OR, odds ratio; PIM, primary idiopathic myxedema; TDT, transmission disequilibrium test; Tg, thyroglobulin; TSHR, TSH receptor.
Received September 18, 2002.
Accepted February 19, 2003.
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