A NOVEL HETEROZYGOUS SI GENE MUTATİON (C.1783G>T; P.G595X) İN A CHİLD WİTH CLİNİCAL FEATURES OF CONGENİTAL SUCRASE-ISOMALTASE DEFİCİENCY: A CASE REPORT

ABSTRACT Congenital sucrase–isomaltase deficiency (CSID) is increasingly recognized across a broad clinical spectrum, including milder presentations associated with heterozygous SI variants. Case: In this report, a child with chronic abdominal pain, bloating, frequent loose stools and recurrent perianal dermatitis is presented. Genetic testing identified a novel heterozygous nonsense SI variant, c.1783G>T (p.Gly595X). Cascade testing revealed the same variant in his father, who had intermittent loose stools, postprandial bloating, recurrent oral aphthae and IBS-like symptoms, suggesting a mild phenotype. Initiation of oral sacrosidase resulted in marked improvement in stool frequency, abdominal symptoms and perianal dermatitis. Conclusion: This case illustrates the clinical relevance of heterozygous SI variants and expands the mutational spectrum of CSID. The findings underscore the importance of considering SI-related carbohydrate malabsorption in children with chronic loose stools and functional gastrointestinal symptoms when routine investigations are unrevealing. KEYWORDS; Sucrase-isomaltase deficiency, SI gene, Carbohydrate malabsorption, Chronic diarrhea, Nonsense mutation, Sacrosidase therapy