Brief Genetics Report IL12B Polymorphism and Type 1 Diabetes in the Italian Population A Case-Control Study

Type 1 diabetes is caused by the immune-mediated destruction of the insulin-secreting pancreatic -cells, which is under polygenic control. Several putative diabetes loci have been sorted by whole genome–linkage analyses (1–2) or by directly testing an a priori hypothesis on selected genes or genomic regions (3–4). Linkage with chromosome 5q33–34 was recently detected in 249 sib-pairs from 187 British and Australian multiplex families (5). Two noncoding polymorphisms of the interleukin 12B (IL12B) gene, mapping in the linked region, were strongly associated (P 10 ) with the disease in the subset of siblings sharing two 5q33 identicalby-descent alleles. The polymorphisms were an AT repeat in intron 4 and an A-C substitution in the 3 untranslated region (UTR). The preferential transmission of the A allele (allele 1 in the original study) of the 3 UTR variation was replicated in an independent sample of 235 Australian type 1 diabetic simplex families (P 10 ). Moreover, differences in the IL12B mRNA levels were related to the 3 UTR genotype (5). Given the relevance of this finding for prediction and preventive measures, it is important to test whether these results can be replicated in other populations. We performed a large case-control study in the continental Italian population. The A-C 3 UTR variation was typed in 470 unrelated type 1 diabetic patients and 544 unrelated control subjects. Allele, phenotype, and genotype frequencies did not significantly differ between type 1 diabetic patients and control subjects (Table 1). A slight, not significant, increase of the C allele ( 1.9%) was observed in patients compared with unaffected subjects. This result goes in the opposite direction of that reported by Morahan et al. (5), who showed a preferential transmission to diabetic patients of the more frequent A allele. No difference was detected in the frequencies of the 3 UTR alleles when the patients were stratified for each of the HLA high-risk (DR3/DR4, DR4/DR4, DR3/DR3, and DR4/DRX) and lowrisk (DR3/DRX and DRX/DRX) genotypes (Table 1). Also, sex or age at onset (before or after 15 years) did not affect the IL12B frequencies (data not shown). To rule out the possible influence of population admixture, we typed the parents of 121 diabetic patients from the case-control panel for the 3 UTR poymorphism and inferred the affected family–based control subjects (AFBACs). AFBAC allele frequencies (A 71.2% and C 28.8%) did not significantly differ from those of population control subjects. In addition, to assign intragenic haplotypic combinations, we typed the intron 4 microsatellite in 16 diabetic families (64 chromosomes). We detected two alleles (frequencies 73.4 and 26.6%), which were combined in only two haplotypes with the 3 UTR locus. This corresponds to the linkage disequilibrium (LD) pattern previously described (5). Therefore, the lack of association in the Italian population was not due to a population-specific pattern of LD in the region of interest. It is unlikely that our failure to detect an association was the result of a lack of statistical power of the test because the size of our diabetic and control subject panel had 80% probability to detect a significant (P 0.05) association, with a relative risk of 1.4 for the A/A or the A/C genotypes. This risk is lower than the genotypic relative risk ( 1.9) that can be calculated from the genotype From the Institute of Cell Biology, CNR, Campus Buzzati-Traverso, Monterotondo, Rome, Italy; the Department of Clinical Science, Division of Endocrinology, University of Rome “La Sapienza”, Rome, Italy; the Department of Medical Science, Eastern Piedmont University, Novara, Italy; the Department of Internal Medicine, University of Tor Vergata, Rome, Italy; and Libera Università Campo Biomedico, Rome, Italy. Address correspondence and reprint requests to Lorenza Nisticò, Institute of Cell Biology, CNR, Campus Buzzati-Traverso, Via E. Ramarini 32, Monterotondo 00016, Rome, Italy. E-mail: [email protected]. Received for publication 9 January 2002 and accepted in revised form 11 February 2002. AFBAC, affected family–based control subject; LD, linkage disequilibrium; UTR, untranslated region.