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Division of Endocrinology, Diabetes, and Bone Diseases (B.G., L.L., Y.B., E.C., Y.T.), Department of Medicine, Mount Sinai School of Medicine, New York, New York 10029; and Division of Statistical Genetics (D.A.G.), Columbia University, New York, New York 10032
Address all correspondence and requests for reprints to: Yaron Tomer, M.D., Division of Endocrinology, Diabetes, and Bone Diseases, Box 1055, Mount Sinai Medical Center, One Gustave L. Levy Place, New York, New York 10029. E-mail: Yaron.Tomer{at}mssm.edu.
| Abstract |
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Objective: Our objective was to identify the joint susceptibility genes for T1D and AITD.
Design: We conducted a family-based linkage and association study.
Setting: The study took place at an academic medical center.
Participants: Participants included 55 multiplex families (290 individuals) in which T1D and AITD clustered (T1D-AITD families).
Main Outcome Measures: We conducted tests for linkage and family-based associations (transmission disequilibrium test) with four candidate genes: human leukocyte antigen (HLA), cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), insulin variable number of tandem repeats (VNTR), and thyroglobulin.
Results: Linkage evidence to HLA appeared when subjects with either T1D or AITD were considered affected [maximum LOD score (MLS), 2.2]. The major HLA haplotype contributing to the shared susceptibility was DR3-DQB1*0201, with DR3 conferring most of the shared risk. The CTLA-4 gene showed evidence for linkage only when individuals with both T1D and AITD were considered affected (MLS, 1.7), and the insulin VNTR showed evidence for linkage when individuals with either T1D or AITD were considered affected (MLS, 1.9); i.e. it may contribute to the familial aggregation of T1D and AITD.
Conclusions: The HLA class II locus contributes to the shared risk for T1D and AITD, and the major HLA haplotype contributing to this association is DR3-DQB1*0201. Additional non-HLA loci contribute to the joint susceptibility to T1D and AITD, and two potential candidates include the CTLA-4 and insulin VNTR loci.
| Introduction |
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| Subjects and Methods |
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The project was approved by the institutional review board. Fifty-five Caucasian families (290 individuals) were analyzed. Families were ascertained through a patient with T1D who had at least one first-degree relative with T1D and at least one additional first-degree relative with AITD. T1D was diagnosed based on the American Diabetes Association criteria (16) with age at diagnosis under 15 yr. AITD includes Graves disease and Hashimotos thyroiditis, both diagnosed as previously described (17).
Genotyping of microsatellite markers in the major histocompatibility complex (MHC) region
The HLA gene locus is located on chromosome 6p21(3439 cM). We analyzed six microsatellite markers that span the MHC region, including one marker that is located very close to the DR locus on its telomeric side (D6S273) and another close to the DQ locus also telomeric to DR [TNF
microsatellite (TNF
-ms)]. This enabled us to test for linkage around the MHC region as well as inside the MHC class II gene region. Primers for marker amplification were purchased from ABI (Foster City, CA), and genotyping was performed as previously described (18).
HLA typing
Molecular typing of HLA-DR and HLA-DQB1 was carried out according to the requirements of the American Society for Histocompatibility (19). The major alleles of HLA-DR and HLA-DQB1 were typed using the technique of group-specific PCR amplification, followed by restriction enzyme digestion, as previously described (20, 21).
Genotyping the insulin VNTR locus
The insulin-VNTR polymorphism, located 5' to the insulin gene, is a tandem repetition of 14- to 15-bp oligonucleotides. It has two main alleles, the shorter class I alleles (2844 repeats) and the longer class III alleles (138159 repeats). Family members were typed for the insulin-VNTR class I and class III alleles using the 23 HphI restriction fragment length polymorphism, as previously described (22, 23). The 23 HphI restriction fragment length polymorphism was shown to be in extremely tight linkage disequilibrium (>99.7% concordance) with the VNTR in Caucasians.
Genotyping the CTLA-4 locus
Linkage analysis for the CTLA-4 locus was performed using the highly informative microsatellite marker D2S325 (24), located 1 cM downstream from the CTLA-4 genes (24).
Genotyping the Tg locus
We used the microsatellite marker Tgms2 located inside intron 27 of the Tg gene to test for linkage to the Tg locus (25).
Linkage analyses
Two-point linkage analyses. Two-point LOD scores for the different markers studied were computed using LIPED software (26) assuming both dominant and recessive models (27). Twins studies have reported the concordance rates for monozygotic twins to be 3050% for T1D (28, 29, 30) and 3080% for AITD (31, 32), suggesting a penetrance of approximately 3050% for T1D and AITD. Therefore, and to choose a conservative estimate, all linkage analyses were performed at an assumed 30% penetrance. In addition, the nonparametric LOD (NPL) scores were also computed using the GeneHunter program.
Heterogeneity testing and multipoint linkage analysis. Multipoint LOD scores were computed by the Allegro program (33) using all the markers spanning the MHC locus. Multipoint linkage analysis yields the maximum information for each family for the area of interest. Using Allegro, we set the inheritance parameters identical to those that gave the maximum LOD scores in the two-point analyses. Marker placement and distances for the multipoint analysis were obtained from the Genethon maps (34). The order of the markers and recombination fractions in the Genethon maps were verified on our data set. In addition, we tested for heterogeneity in our data set with multipoint heterogeneity LOD scores (HLODs), computed by the Allegro program (33) using all the markers spanning the MHC locus.
Affectedness and disease models used in the linkage analyses. Some individuals in our families had T1D, some had AITD, and some had both T1D and AITD (APS variant). Therefore, we analyzed the data using four models. 1) For model 1, T1D (n = 55 families), only individuals with T1D were considered as affected, whether they had T1D alone or with AITD. This model tests for linkage to T1D. 2) For model 2, AITD (n = 55 families), only individuals with AITD were considered as affected, whether they had AITD alone or with T1D. This model tests for linkage to AITD. 3) For model 3, T1D or AITD (n = 55 families), individuals with T1D or AITD (or both) were considered as affected; a locus showing linkage under this model contributes to the clustering of T1D and AITD within the same family. 4) For model 4, T1D and AITD (APS variant) (n = 31 families), only individuals with both T1D and AITD were considered as affected. Loci identified using this model contribute only to the combined phenotypes of T1D and AITD in the same individual (considered to be an APS variant) (11) but do not necessarily contribute to the clustering of T1D and AITD in families.
Association analyses
Family-based association analyses were performed using the transmission disequilibrium test (TDT). The TDT analysis was performed using Genehunter version 2.0 (35). The TDT compares the rate of transmission of parental alleles to affected offspring with the rate expected if there is no preferential transmission (36). We performed the TDT analyses for offspring affected by T1D alone, offspring affected by AITD alone, and offspring affected by both T1D and AITD (APS variant). A significantly increased transmission of a certain allele to affected offspring indicates association of that allele with the disease phenotype. Conversely, a significantly decreased transmission of a certain allele to affected offspring indicates a negative association of that allele with the disease phenotype, i.e. a protective effect.
| Results |
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We analyzed 55 families; all families were multiplex for T1D (i.e. more than one affected) and had at least one additional family member with AITD, and 21 families (38%) were multiplex for AITD. In 31 families (56%), at least one family member had both T1D and AITD (APS variant) (3). On average, the families had 5.3 members. We had a total of 290 individuals. Of these, 148 individuals were affected; 68 had T1D alone, 45 had AITD alone, and 35 had both T1D and AITD (APS variant) (Table 3
). Of the 80 AITD patients (with or without T1D), 62 had Hashimotos thyroiditis and 18 had Graves disease.
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Two-point linkage analysis and NPL analysis.
Six markers spanning the HLA locus were analyzed for linkage with T1D and AITD. When we analyzed for model 1 (T1D) the two-point maximum LOD score (MLS) was 7.43 at marker D6S273 for the recessive model, at 30% penetrance and a
(
indicates the recombination fraction at which the MLS was obtained) of 0.05. Analysis for model 2 (AITD) gave a two-point MLS of 0.95 at marker TNF
-ms (recessive model, 30% penetrance,
= 0.1). When we analyzed for model 3 (all T1D or AITD patients considered affected) the two-point MLS was 2.21 at marker D6S273 (recessive model, 30% penetrance,
= 0.2), and considering only individuals that had both T1D and AITD as affected (APS variant, model 4) gave an MLS of 1.4 at marker TNF
-ms (recessive model, 30% penetrance,
= 0.01). Similar results were obtained by the NPL analysis. The maximum NPL score for model 1 (T1D) was 4.8, obtained at marker TNF
-ms, the maximum NPL score for model 2 (AITD) was 0.6 (at marker TNF
-ms), the maximum NPL score for model 3 (T1D or AITD) was 3.6, (at marker TNF
-ms), and the maximum NPL score for model 4 (T1D and AITD) was 0.8 between markers TNF
-ms and D6S273. Thus, both the parametric and the nonparametric (NPL) linkage analyses showed positive LOD scores for model 3 (T1D or AITD), suggesting that the HLA locus contributes to the familial clustering of T1D and AITD. However, the LOD scores were significantly higher for T1D (model 1), reflecting the well-known major influence of HLA on the etiology of T1D (37).
Heterogeneity testing and multipoint analysis.
As for the two-point analysis, multipoint linkage analyses using Allegro also showed positive LOD scores for models 1 (T1D) and 3 (T1D or AITD) (Fig. 1
). For affectedness T1D (model 1) the maximum multipoint LOD score when allowing for heterogeneity (HLOD) was 7.57 between markers TNF
-ms and D6S273 (Fig. 1
). For affectedness AITD (model 2) the HLOD was 1.01.5 throughout the HLA region. Multipoint linkage analysis for affectedness T1D or AITD (model 3) gave a maximum multipoint HLOD score of 2.2 between markers D6S273 and D6S439 (Fig. 1
), and the maximum multipoint HLOD score for affectedness T1D and AITD (model 4) was 1.6 between markers D6S464 and TNF
-ms. These results support our previous data (12) showing that the clustering of AITD with T1D in families is partially determined by the HLA locus. However, the LOD score for HLA was highest for T1D alone, and when we added the AITD-affected individuals to the analysis, the LOD score decreased from 7.57 to 2.2. This decrease in LOD score when individuals with AITD were added to the analysis shows that HLA contributes significantly less to the inherited susceptibility to AITD in T1D families. Therefore, other non-HLA genes must contribute to the strong familial clustering of T1D and AITD.
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Linkage analyses of non-HLA candidate genes
CTLA-4 locus.
Linkage analysis for the CTLA-4 locus was performed using the microsatellite marker D2S325. Analysis for model 1 (T1D) and for model 2 (AITD) gave low positive LOD scores (0.3 and 0.9, respectively) (Table 5
), whereas analysis for model 3 (T1D or AITD) gave negative two-point LOD scores (MLS = 0 at
= 0.5; Table 5
). This suggested that CTLA-4 does not contribute to the familial clustering of T1D and AITD in our data set. However, when considering only individuals that had both T1D and AITD as affected (APS variant, model 4), the MLS was 1.7 (recessive model, 30% penetrance,
= 0.05), suggesting evidence for linkage (Table 5
). Because AITD includes both Graves disease and Hashimotos thyroiditis and most of the patients with T1D and AITD in our families had T1D and Hashimotos thyroiditis, we reanalyzed the data considering individuals that had both T1D and Hashimotos thyroiditis as affected. The MLS when considering individuals that had both T1D and Hashimotos thyroiditis as affected was 2.1 (recessive model, 30% penetrance,
= 0.05), suggesting that T1D plus Hashimotos thyroiditis is the distinct phenotype that is linked with CTLA-4 in our T1D-AITD families. This suggests that patients with T1D and AITD/Hashimotos thyroiditis (APS variant) may be a genetically distinct subgroup of diabetics influenced by CTLA-4. Therefore, these data may help explain previous inconsistent studies of CTLA-4 in T1D with some reporting significant linkage/association of CTLA-4 to T1D (38, 39) and others reporting no (40, 41) or very weak (42) associations. These inconsistent results of previous studies could be explained by noting that the T1D data sets were likely collected without regard to the AITD status of the probands. Because approximately 1020% of T1D patients have AITD (1), the presence or absence of AITD in such patients can represent a significant source of variation from study to study. Our results showed that in the subset of T1D patients who also have AITD (APS variant), there is strong linkage to CTLA-4.
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= 0.01), showing evidence for linkage (Table 5
Tg locus.
We used the microsatellite Tgms2, located inside intron 27 of Tg, to test for linkage to the Tg locus. Analysis for all models gave low LOD scores (< 0.5) (Table 5
). These low LOD scores cannot show or exclude evidence for linkage at the Tg locus in our 55 T1D-AITD families.
| Discussion |
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-ms (data not shown), supporting our conclusion that in the T1D-AITD families, the HLA locus is linked with AITD. Thus, it is possible that the AITD phenotype that is seen in T1D families has a different genetic etiology, and potentially a different pathogenesis, than the AITD phenotype seen in families in which only AITD clusters.
When we considered both T1D and AITD as affected, the MLS was 2.2, which is significantly lower than the MLS obtained when considering only T1D as affected (MLS = 7.57) (Fig. 1
). One possible explanation for this observation is that in our T1D-AITD families, T1D may show strong evidence of linkage to the HLA locus, although AITD does not. Thus, considering AITD-affected individuals as affected introduces genetic heterogeneity into the data because another non-HLA locus contributes to the familial clustering of T1D and AITD at least in some of the families.
Our TDT analysis revealed preferential transmission of HLA haplotypes DR3-DQB1*0201 and DR4-DQB1*0302 to offspring affected with T1D alone. The same was true for offspring affected with both T1D and AITD (APS variant). However, when looking at offspring affected with AITD alone, only DR3-DQB1*0201 was preferentially transmitted, whereas DR4-DQB1*0302 was preferentially not transmitted. These data may suggest that DR3-DQB1*0201 haplotype confers susceptibility to both diseases, whereas the haplotype DR4-DQB1*0302 is specific to T1D. Indeed, it is well known that the DR4-DQB1*0302 haplotype shows the strongest association with T1D (48, 49, 50). It is also likely that in offspring with both T1D and AITD (APS offspring), the major haplotype is DR3-DQB1*0201, whereas the preferential transmission of the DR4-DQB1*0302 haplotype reflects the strong influence of this haplotype on the T1D component.
To look for differential effects of HLA-DR and HLA-DQB1 alleles on susceptibility to T1D and AITD, we compared the haplotypes DR3-DQB1*0201 and DR4-DQB1*0302 to other haplotypes containing one, but not both, of their constituent alleles (Table 4
). This analysis has suggested that within the DR3-DQB1*0201 haplotype, DR3 was the primary allele conferring most of the risk to both T1D and AITD, whereas DQB1*0201, in linkage disequilibrium with DR3, may have a secondary role. Similarly, our data suggested that DQB1*0302 was the primary allele conferring susceptibility to T1D, whereas DR4, in linkage disequilibrium, may have a secondary role. However, we cannot exclude the possibility that these associations may reflect the effects of another nearby gene or genes that may be the causative gene and is in linkage disequilibrium with the specific HLA alleles that we found to be associated with T1D and/or AITD.
One potential weakness of our study is the relatively small number of families used in TDT analysis, particularly when analyzing family members affected by AITD only. Therefore, additional studies in independent data sets are needed to confirm these data. However, despite the small numbers, we were able to identify haplotypes that were associated with T1D and/or AITD in our data set.
Results similar to our own have been reported in two previous studies looking at HLA associations in Caucasian families with both T1D and AITD. Santamaria et al. (51) compared 39 subjects with both T1D and AITD to 17 AITD-only affected siblings of T1D probands. They showed that individuals with both T1D and AITD were more likely to have alleles DQB1*0201 and DQB1*0302, whereas individuals with AITD only were more likely to have DQB1*0201 but not DQB1*0302. Dorman et al. (52) studied 25 T1D families in which at least one parent and one offspring had Hashimotos thyroiditis and found a 2-fold increase in the prevalence of DQA1*0501-DQB1*0201 among family members with Hashimotos compared with those without. No difference in the prevalence of DQA1*0301-DQB1*0302 among these two groups was observed. Similarly, our data showed that the haplotype DR3-DQB1*0201 contributed to the shared susceptibility to T1D and AITD, whereas the haplotype DR4-DQB1*0302 was T1D specific. We have extended these observations and present data that may suggest that DR3 and not DQB1*0201 is most likely the primary allele contributing to this joint susceptibility to T1D and AITD.
Our study also demonstrated that the haplotypes DR3-DQB1*0201 and DR4-DQB1*0302 predispose to the combined phenotype of T1D and AITD in the same individual, considered a variant of APS (3). Two other studies investigated HLA associations in Caucasian patients with variants of APS. Huang et al. (53) HLA-typed 31 unrelated patients with APS2 (autoimmune adrenalitis plus at least one other autoimmune disorder) and divided the patients into two subgroups: 17 patients whose autoimmune phenotype included T1D and/or islet-cell or glutamic acid decarboxylase antibodies and 14 patients without such evidence of ß-cell autoimmunity. In the former group, the haplotypes DR3-DQB1*0201 and DR4-DQB1*0302 were more frequent compared with controls. However, in the APS2 patients lacking ß-cell autoimmunity, only the haplotype DR3-DQB1*0201 was increased, lending further evidence to the notion that DR3-DQB1*0201 is associated with multiple endocrine organ autoimmunity. Wallaschofski et al. (11) reported slightly different findings. In this study, 112 unrelated APS patients were divided into 29 patients with APS2 (as defined above) and 83 patients with APS3 (one autoimmune endocrinopathy other than autoimmune adrenalitis plus at least one other autoimmune disorder). Of note, 21 (25%) of the APS3 patients had T1D and 82 (99%) had AITD. The haplotypes DR3-DQB1*0201 and DR4-DQB1*0302 were both increased in the APS2 patients compared with controls, but only DR4-DQB1*0302 was increased in the APS3 patients. The authors concluded that the extended haplotype DR4-DQA1*0301-DQB1*0302 is associated with APS2 and APS3. These results are different from ours, which showed that both haplotypes DR3-DQB1*0201 and DR4-DQB1*0302 contributed to the APS variant consisting of T1D and AITD. However, Wallaschofski et al. did not present subgroup analysis looking specifically at APS patients with the T1D and AITD phenotype.
Our data showed that the CTLA-4 gene also contributed to the development of both T1D and AITD in the same individual (APS variant). Three other studies have examined the CTLA-4 gene in APS patients. Kemp et al. (54) found an association of CTLA-4 with an APS variant in which the main components were vitiligo with AITD or T1D. Another study from Japan found an association between the G allele of the CTLA-4 A/G49 single-nucleotide polymorphism and younger T1D patients with AITD (55). In contrast, Donner et al. (56) found no association between CTLA-4 and an APS variant consisting of Addisons disease and AITD or T1D. Therefore, it is likely that the CTLA-4 gene contributes to the expression of only certain APS variants. Because, as noted above, distinct HLA class 2 haplotypes have also been shown to be associated with certain APS variants, we hypothesize that specific combinations of HLA and CTLA-4 alleles, as well as alleles of other genes, predispose to specific APS phenotypes.
Our linkage analysis showed that the insulin VNTR locus may contribute to the clustering of T1D and AITD within families, a finding of some surprise given that this locus has so far been associated only with T1D. These results may imply that the insulin VNTR locus may harbor a gene that contributes to the familial clustering of T1D and AITD. A recent study has shown no association of the VNTR polymorphism with Graves disease (57). Therefore, if our data of linkage of T1D and AITD to the insulin VNTR locus are confirmed, they may suggest that another gene in this locus and not the VNTR polymorphism itself is the autoimmunity locus in this region.
In conclusion, our data have shown the HLA haplotype DR3-DQB1*0201 contributes to the genetic susceptibility to T1D and AITD, whereas DR4-DQB1*0302 is specific for T1D. Within these haplotypes, the DR3 and DQB1*0302 alleles may play the primary roles, respectively. In addition, the insulin VNTR locus may contribute to the clustering of T1D and AITD in families, and the CTLA-4 locus may play a role in the development of T1D and AITD in the same individual (a variant of APS). Thus, several genes are involved in joint susceptibility to T1D and AITD. However, we do not know whether these genes interact in conferring risk for T1D and AITD. We have previously shown an additive effect on the odds ratio for HLA-DR3 and the G allele of the CTLA-4 A/G49 single-nucleotide polymorphism suggestive of an interaction between the HLA-DR genes and the CTLA-4 gene in predisposing to Graves disease (58). Another study suggesting interaction between HLA and CTLA-4 was also recently reported (59). Thus, it is possible that the autoimmunity genes contributing to the joint susceptibility to T1D and AITD interact and that their interactions may influence disease phenotype and severity. The molecular basis for the interactions between susceptibility genes in complex diseases is unknown. These interactions could represent the cumulative effect of increased statistical risk, or alternatively, there may be molecular interactions between the susceptibility genes or their products, which ultimately determine disease phenotype.
| Acknowledgments |
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| Footnotes |
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This work was first presented as an abstract at the 86th Annual Meeting of The Endocrine Society, New Orleans, LA, June 2004 (Abstract OR 29-4).
First Published Online May 31, 2005
Abbreviations: AITD, Autoimmune thyroid diseases; APS, autoimmune polyglandular syndrome; CTLA-4, cytotoxic T lymphocyte-associated antigen 4; HLA, human leukocyte antigen; HLOD, heterogeneity logarithm of odds; MHC, major histocompatibility complex; MLS, maximum LOD score; T1D, type 1 diabetes; TDT, transmission disequilibrium test; Tg, thyroglobulin; TNF
-ms, TNF
microsatellite; VNTR, variable number of tandem repeats.
Received November 15, 2004.
Accepted May 23, 2005.
| References |
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |