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OBJECTIVE--Zinc transporter eight (SLC30A8) is a major target of autoimmunity in human type 1A diabetes and is implicated in type 2 diabetes in genome-wide association studies. The type 2 diabetes nonsynonymous single nucleotide polymorphism (SNP) affecting aa[sub 325] lies within the region of highest ZnT8 autoantibody (ZnT8A) binding, prompting an investigation of its relationship to type 1 diabetes. RESEARCH DESIGN AND METHODS--ZnT8A radioimmuno-precipitation assays were performed in 421 new-onset type 1 diabetic Caucasians using COOH-terminal constructs incorporating the known human aa[sub 325] variants (Trp, Arg, and Gln). Genotypes were determined by PCR-based SNP analysis. RESULTS--Sera from 224 subjects (53%) were reactive to Arg[sub 325] probes, from 185 (44%) to Trp[sub 325] probes, and from 142 (34%) to Gln[sub 325] probes. Sixty subjects reacted only with Arg[sub 325] constructs, 31 with Trp[sub 325] only, and 1 with Gln[sub 325] only. The restriction to either Arg[sub 325] or Trp[sub 325] corresponded with inheritance of the respective C- or T-alleles. A strong gene dosage effect was also evident because both Arg- and Trp-restricted ZnT8As were less prevalent in heterozygous than homozygous individuals. The SLC30A8 SNP allele frequency (75% C and 25% T) varied little with age of type 1 diabetes onset or the presence of other autoantibodies. CONCLUSIONS--The finding that diabetes autoimmunity can be defined by a single polymorphic residue has not previously been documented. It argues against ZnT8 autoimmunity arising from molecular mimicry and suggests a mechanistic link between the two major forms of diabetes. It has implications for antigen-based therapeutic interventions because the response to ZnT8 administration could be protective or immunogenic depending on an individual's genotype. Diabetes 57:2693-2697, 2008 [ABSTRACT FROM AUTHOR]