TDT and S-TDT analyses revealed that both markers within the CTLA4 gene were significantly associated with IDDM in a Russian population. Both TDT and combined tests showed a clear association of the Thr17Ala dimorphism in the gene (Table ). For the second polymorphic marker tested, the dinucleotide microsatellite, a significant difference in the transmission of the three alleles was observed, even after correction for multiple alleles (Table ). This suggests that the (AT)n repeat polymorphism of the CTLA4 gene is associated with IDDM in this Russian population.
Transmission disequilibrium analysis in discordant sibling pairs showed that the Ala17 variant of the CTLA4 gene and allele 17 at the intragenic dinucleotide microsatellite were associated with a high risk of IDDM. The maximum LOD scores were also sufficient to indicate that the CTLA-4 region of chromosome 2q33 is linked to, and associated with, IDDM in a Russian population.
Thus, our findings are consistent with those of earlier family studies showing a positive association between the CTLA4 gene and IDDM in most populations tested and role of the Ala17 allele is predisposing the individual to the disorder [3
]. Linkage analysis with a CTLA4 dinucleotide marker in 48 Italian IDDM families gave a ML of 3.22 which is consistent with our results [11
]. Our findings suggest that allele 17 is a common 'predisposition' allele of the dinucleotide marker. Our data are consistent with those of Marron et al. [12
] who reported a significantly higher frequency of allele 17 to affected siblings in pooled diabetic pedigrees from multiple ethnic groups than of other alleles of the dinucleotide marker. Allele 17 was in strong linkage disequilibrium with the Ala allele in the Russian population and in other populations tested (data not shown) [12
The CTLA4 gene (IDDM12) is located in the vicinity of two other IDDM susceptibility regions, IDDM7 (2q31) [21
] and IDDM13 (2q34) [22
], and the genes encoding CD28, IA-2 and islet tyrosine phosphatase, which may be considered to be candidate IDDM susceptibility genes [23
]. This closeness to other IDDM susceptibility regions and putative susceptibility genes may account for the weakness or absence of association of the CTLA4 gene with IDDM in some populations.
A functional significance of the CTLA4 codon 17 dimorphism is unknown. The amino acid substitution thought unlikely to affect the function of the CTLA-4 molecule. The CTLA4 microsatellite (AT)n
repeat may affect RNA stability, for the AT-rich sequences in 3' untranslated regions of mRNA molecules for example [24
]. Linkage between the two CTLA4 polymorphic markers has been shown [26
]. This may indirectly affect CTLA4 signal transduction or intracellular sorting [27
]. The Ala17 allele is probably correlated with rapid-onset insulin deficiency, and functional studies with Ala/Ala homozygous patients may elucidate the functional significance of codon 17 dimorphism in T-cell regulation [28
CTLA4 is known to be involved in the fine tuning of the immune response, the disruption of which may result in autoimmunity. Recent animal model-based studies have shown that CTLA4Ig injection can prevent relapse of the autoimmune process in BB rats [28
]. This suggests that CTLA4 may be a suitable target for the specific prevention of β-cell autoimmunity [27
Our data, showing the linkage and association of the CTLA4 gene with IDDM, together with many other population-based fundings, suggest that the chromosome 2q33 region (IDDM12) is a true IDDM susceptibility locus. The CTLA4 region may function as a common locus for HLA.genes and genes conferring susceptibility to autoimmunity in general. A number of findings, including our studies of association between CTLA4 codon 17 and Graves' disease [29
], support this view.