An association between the gene NUDT15 and the risk of developing leukopenia - low white blood cell count - in individuals with Crohn’s disease, following treatment with a common class of immunosuppressants, is reported in a paper published this week in Nature Genetics. The findings may help doctors identify patients who are a high risk for this adverse reaction.

Thiopurine-induced leukopenia is a side effect of treatment with thiopurine drugs, which are commonly used in patients with diseases such as cancer or inflammatory bowel disease. The incidence of this reaction in individuals with Crohn’s disease is much higher in those of Asian descent (over 30%), in comparison to those of European descent (approximately 5%).

Kyuyoung Song and colleagues analysed genetic data from 978 Korean individuals with Crohn’s disease, a subset of which had experienced thiopurine-induced leukopenia. They found a strong association between leukopenia and a genetic variant in NUDT15, which encodes an enzyme that can degrade certain types of DNA building blocks that have been damaged by oxidation. The variant of NUDT15 associated with the risk of leukopenia is common in the Asian population, occurring in up to 13% of individuals.