IRF4 haploinsufficiency in a multiplex family with Whipple's disease.

Tropheryma whipplei is the bacterial agent responsible for Whipple's disease (WD). However, WD occurs only very rarely in Tw-infected individuals. We investigated the cause of disease in two relatives with WD. We studied a son and his mother, both presenting an articular form of WD, at the ages of 38 and 60 years, respectively, these episodes occurring five years apart. Both were otherwise healthy. We performed whole-exome sequencing, characterized a candidate variant biochemically, and performed an immunological analysis on the patients' leukocytes. Both patients were heterozygous for a rare missense variant of the gene encoding the transcription factor IRF4 (p.R25S), for which haploinsufficiency was previously reported to underlie WD in a large multiplex family. This variant was hypomorphic for DNA binding and transcription induction. It did not exert negative dominance. Immunity was otherwise normal in these two patients. Haploinsufficiency for IRF4 can thus underlie WD in T. whipplei-infected individuals from at least two unrelated families.