We have studied the association of CD, T1D and HLA DRB1-DQA1-DQB1 haplotypes in a large Bedouin kindred. Prevalences of celiac disease and of CD/TgAA+ in this extended family are higher than those reported in studies of European or European-American families but consistent with those reported in Saharawi Arabs [31
], who are more similar to Bedouins in their cultural history and genetic background. It has been suggested that the high prevalence of celiac disease in nomadic Arabs may reflect a selective advantage related to a reduced risk for intestinal infections in individuals with celiac enteropathy at a time when the negative effects of the disease were minimized by low gluten intake in the traditional diet [31
Ours is the first study to examine the association between celiac and islet autoimmunity in an Arab community with a high prevalence of CD/TgAA+. To our knowledge, this is also the first study to examine the co-inheritance of celiac and islet autoimmunity and associations with inbreeding in any population, as these analyses are dependent on data from extended families. Our analyses of mean kinship coefficients confirmed a stronger contribution of genetic factors to the risk for CD/TgAA+ compared with T1D by demonstrating that relatives with CD/TgAA+ relatives are more closely related (mean kinship coefficient 0.166) than are pairs of relatives with T1D/Ab+ (mean kinship coefficient 0.128). We also found that pairs of relatives with different autoimmunity phenotypes are more closely related (mean kinship coefficient 0.144) than are pairs of relatives without celiac or islet autoimmunity (0.056) and that both phenotypes are strongly associated with consanguinity. The prevalence of CD/TgAA+ is also higher in family members with T1D/Ab+ (16.7%) compared with family members without T1D/Ab+ (7.6%), but the difference is not statistically significant. Williams and colleagues [32
] found similar evidence for shared genetic susceptibility factors but with limited phenotypic overlap between islet and celiac autoimmunity in unaffected first degree relatives of patients with T1D. Overall, our findings suggest that the comorbidity of islet and celiac disease is mediated primarily through a common association with DRB1*
0201, while other HLA haplotypes and the genetic background may have more disease-specific effects.
The HLA haplotypes associated with celiac autoimmunity in this family are similar to those found in other Bedouin families and European or European-derived populations [1
]. We do not see a difference in the distribution of HLA haplotypes between CD patients and TgAA+ individuals without CD, but we may not have sufficient power to distinguish the two groups. Consistent with other studies, the high-risk DRB1*
0201 haplotype is a primary determinant of celiac disease risk: there is a significantly higher frequency of genotypes with DRB1*
0201 among CD/TgAA+-affected individuals. In fact, all affected family members have DRB1*
0201, which encodes the DQ(α1*
02) molecule in cis
. There was no evidence for a disease association with any other DRB1-DQA1-DQB1 haplotype, including DRB1*
0302, which, although present in combination with DRB1*
0201 in one affected relative, was in very low frequency in the kindred as a whole (1 of 43 founder haplotypes).
Our data support other studies indicating DQB1*
0201 homozygotes are at increased risk for celiac autoimmunity. Although the sample size is too small to determine if their higher risk reflects a dosage effect of DQ(α1*
02), the fact that DRB1*
02 heterozygotes contribute disportionately to the increased risk supports this hypothesis. In contrast, none of seven relatives with the DRB1*
02 genotype have developed celiac autoimmunity, which conflicts with consistent evidence favoring an effect of this genotype in which the DQ(α1*
02) molecule is encoded in trans
. While this genotype is a high-risk genotype in southern European populations [34
] and among Ashkenazi Jews [35
], it did not appear to be a common high-risk genotype in a study of ten Bedouin families [33
], or in Saharawi Arabs [37
]. Thus, it is not clear if the apparent absence of an effect of this genotype in the Bedouin kindred reflects our small sample size or population-specific effects as reported in other studies [34
Several studies have reported the DQB1*
0201 genotype is associated with celiac disease progression, severity, and/or complications [6
]. Although we confirmed the diagnosis of CD in all affected relatives by serological testing and in some cases by biopsy, additional clinical data for these relatives are limited. However, it is noteworthy that one of the two CD-affected relatives who developed intestinal lymphoma is homozygous for DQB1*
0201 while the other is not ().
To our knowledge, our whole genome screen of this Bedouin kindred is the first linkage analysis of celiac disease in a non-European-derived population. We do not find any evidence of linkage - even suggestive evidence - at locations corresponding to any previously reported peaks, including the MHC region at 6p21.3. The only suggestive evidence for linkage (p=0.01) occurred at 12p12-13 (p=0.0098), a region spanning approximately 9.5 megabases with over 100 known or predicted genes. This region has never before been linked with celiac disease, and obviously replication is necessary. The apparent absence of linkage with the MHC may be explained by low marker density, ambiguities in distinguishing DRB1*0301-DQA1*0501-DQB1*0201 haplotypes that are identical-by-descent from HLA-identical haplotypes that are identical-by-state, or both. In particular, we have shown that the linkage results are critically dependent on the identity-by-descent status of two DRB1*0301-DQA1*0501-DQB1*0201 haplotypes that have identical alleles at four microsatellite markers extending 6-7 cM on the centromeric side of DQB1. Due to the difficulty in distinguishing the ancestral relationships of these haplotypes, it is not possible to determine how much of the increased risk for celiac autoimmunity among close relatives is explained by identity-by-descent haplotype sharing for the MHC among affected relatives. However, based on the diversity of haplotypes found in combination with DRB1*0301-DQA1*0501-DQB1*0201 in affected relatives, it is unlikely that autozygosity for this haplotype contributes to the increased risk for celiac autoimmunity in the offspring of consanguineous matings.
Sheffield and colleagues have discussed the power of linkage and association studies in extended Bedouin kindreds in the context of rare disorders that display extensive heterogeneity [39
]. Bardet-Biedl syndrome is a specific example in which a rare mutation at any one of at least ten different loci causes obesity, learning disabilities, retinopathy, polydactyly and other disease phenotypes. Several of these loci were localized by linkage analyses in individual Bedouin families. Given the extensive locus and allelic heterogeneity, it is unlikely any of these loci would have been identified if the investigators had pooled data from different families to increase sample size. Although it is reasonable to suppose that the genetic homogeneity of extended Bedouin kindreds will have the same advantages in reducing the complexity of multigenic disorders, novel analyses using computer simulations are likely to be required to account for increased complexities of the family structures, examples of which are described herein. Other limitations are the reduced informativeness of genetic markers, reduced power of family-based linkage and association analyses, and difficulties in distinguishing disease-causing mutations from polymorphisms that are unique to the kindred under study [39
]. The current and previous studies [23
] of a Bedouin kindred with a high prevalence and celiac autoimmunity are the first steps toward determining if the benefit of studies of complex phenotypes in isolated inbred populations will be sufficient to overcome the inherent limitations.