Identification of genetic variants in the human indoleamine 2,3-dioxygenase (IDO1) gene, which have altered enzyme activity.

Objectives: Indoleamine 2,3-dioxygenase (IDO), a rate-limiting enzyme in tryptophan catabolism, is a key regulator of immune tolerance. We identified genetic variations in the IDO1 gene and evaluated their functional activities using in-vitro transfection studies.

Methods: We resequenced the exons and the intron/exon borders of the IDO1 gene in 96 samples from the Coriell DNA Repository. To determine the functional effects of the coding variations that were predicted to have functional consequences, we expressed three of the variant cDNAs in COS-7 and HEK293 cells and determined their enzyme activity.

Results: Seventeen variants were identified; three were nonsynonymous single nucleotide polymorphisms (Ala4Thr, Arg77His, Leu197Ile) and one was a 9 bp deletion in exon 7. Compared with the wild-type protein, the Arg77His and the 9 bp deletion resulted in significantly reduced protein expression and in nearly complete loss of enzyme activity. The allelic frequencies of these two functional variants were approximately 1% and were exclusively observed in the African-American samples.

Conclusion: We conclude that there are naturally occurring polymorphisms that render the human IDO1 gene nonfunctional and should result in reduced IDO activity in affected individuals.