Presence of mitochondrial dysfunction in a case of Fanconi syndrome with normoglycemic MODY1.

Maturity-onset diabetes of the young 1 (MODY1) is characterized by macrosomia and transient hypoglycemia in neonates, in addition to diabetes mellitus (DM). Only patients with MODY1 harboring a pathogenic variant (c.187C > T; p.R63W) in HNF4A are sure to develop Fanconi syndrome (FS). Here we report the successful diagnosis of MODY1 in a patient harboring p.R63W before confirmation of DM-related hyperglycemia after being alerted to the presence of abnormal mitochondria in a kidney-biopsy specimen revealed by electron microscopy. The patient was born at 39 weeks of gestation with macrosomia, elevated levels of liver enzymes, and transient hypoglycemia. At three years of age, proteinuria was found by chance, and further laboratory investigations revealed metabolic acidosis, mild renal dysfunction, hypouricemia, proteinuria, aminoaciduria, and glycosuria. On this basis, we diagnosed the patient as having FS and performed percutaneous renal biopsy. Light microscopy revealed no evidence of proximal tubule disorder, but electron microscopy demonstrated mitochondria with disordered cristae in glomerular podocytes and giant mitochondria in proximal tubules. Mitochondrial nephropathy was suspected, and skin fibroblasts from the patient grown on galactose medium showed decreased oxygen consumption suggestive of mitochondrial dysfunction. Therefore, genetic testing was performed and a pathogenic variant (c.187C > T; p.R63W) in HNF4A was detected. Mitochondrial dysfunction in a Drosophila and murine model of patients with both MODY1 and FS has already been reported, and we detected it in this human MODY1/FS patient on the basis of functional tests and imaging. We believe mitochondrial dysfunction may be involved in the pathogenesis of MODY1/FS.