Reduced guanidinoacetate in plasma of patients with autosomal dominant Fanconi syndrome due to heterozygous <scp>P341L <i>GATM</i></scp> variant and study of organoids towards treatment

Autosomal dominant Fanconi syndrome due to a <i>GATM</i> variant (GATM-FS), causes accumulation of misfolded arginine-glycine amidinotransferase (AGAT) in proximal renal tubules leading to cellular injury. GATM-FS presents during childhood and progresses to end-stage kidney disease (ESKD) in adults. We study creatine metabolism in two individuals of unrelated families with a known <i>GATM</i> variant and the effect of creatine supplementation in kidney organoids. Plasma and urine metabolites were measured by mass spectrometry. Brain creatine was assessed by magnetic resonance spectroscopy (MRS). Guanidinoacetate (GAA) synthesis by the AGAT mutant was measured in patient-derived immortalized lymphocytes using stable isotopes of arginine and glycine. The effect of creatine on <i>GATM</i> expression was assessed in human kidney cells and organoids. Several family members from two unrelated families were diagnosed with Fanconi syndrome and had the c.1022C>T (p. P341L) variant in <i>GATM</i>. Two affected individuals in both families had moderately reduced plasma GAA levels. In comparison to wild-type cells, GAA synthesis by patient-derived <i>GATM</i> ^P341L+/- lymphoblastoid cell lines (LCL) was reduced, but not absent as in <i>GATM</i> cells from a patient with creatine deficiency syndrome. In vitro studies on human kidney organoids revealed reduced AGAT expression after treatment with creatine. Finally, we showed in one patient that creatine supplementation (5 g daily) substantially increased plasma creatine levels. We report low plasma and urine GAA in patients with autosomal dominant GATM-FS and show that creatine downregulates AGAT in human kidney cells.