Long‐term clinical trajectory of microvillus inclusion disease associated with <i>STXBP2</i> ‐related familial hemophagocytic lymphohistiocytosis type 5: A case report

Familial hemophagocytic lymphohistiocytosis type 5 is caused by biallelic pathogenic variants in STXBP2, which encodes syntaxin-binding protein, a key regulator of vesicle trafficking. In addition to immune dysregulation, patients with familial hemophagocytic lymphohistiocytosis type 5 may present with severe, persistent diarrhea associated with microvillus inclusion disease. However, the long-term clinical course of microvillus inclusion disease in this context remains poorly characterized due to limited survival beyond early childhood. We report a 22-year-old male with familial hemophagocytic lymphohistiocytosis type 5 who developed chronic diarrhea in infancy that persisted despite following bone marrow transplantation. The diagnosis of microvillus inclusion disease was established 16 years after transplantation based on characteristic histopathological findings. The patient also developed Fanconi syndrome, a rare complication in this setting, which improved following intensification of parenteral nutrition. Long-term follow-up revealed that the patient was dependent on parenteral nutrition not only for caloric intake but also for the correction of electrolyte imbalance. Growth retardation and multiple endocrine abnormalities improved with nutritional optimization, highlighting the systemic impact of chronic undernutrition. This patient expands the long-term extrahematopoietic phenotype of STXBP2-related familial hemophagocytic lymphohistiocytosis type 5 and underscores the importance of individualized nutritional management.