JAK inhibitor ameliorates inflammatory bowel disease in a patient with IKZF1 haploinsufficiency.

IKAROS, encoded by IKZF1, is a crucial transcription factor regulating hematopoiesis and B cell development. While IKZF1 haploinsufficiency variants are associated with various immunological disorders, inflammatory bowel disease (IBD) has been rarely reported. We report a case of IKZF1 haploinsufficiency presenting with an atypical IBD phenotype and its response to filgotinib. The patient was previously diagnosed with IKZF1 haploinsufficiency and presented with chronic diarrhea, fatigue and anemia. Laboratory findings indicated folate deficiency-induced megaloblastic anemia and malabsorption syndrome. Endoscopic examination showed inflammation with erythema in the colon and extensive villous blunting of the small intestine. Immunohistochemical analysis revealed increased pSTAT3/5 in the colon. Considering the clinical features and increased JAK-STAT cascade, treatment with filgotinib was initiated. At 10 weeks post-treatment, we observed improvement in endoscopic findings and suppression of pSTAT3/5. This case extends the clinical spectrum of IKZF1 haploinsufficiency. A JAK1 inhibitor is considered to be useful for IKZF1 haploinsufficiency-associated IBD.