高チロシン血症２型

4-Hydroxylphenylpyruvate dioxygenase (HPPD) is a crucial enzyme in the tyrosine catabolic pathway, catalyzing the conversion of 4-hydroxylphenylpyruvate (HPP) to homogentisate (HG). Missense substitutions in HPPD are associated with type III tyrosinemia and hawkinsinuria. This study investigated disease-related variants in terms of their roles in HPPD structure stability and function. Our whole-cell assay showed a loss of soluble protein expression for G154S, Y160C, and I267F variants, suggesting that the three locations at the domain interface can be critical for proper protein folding. The A33T, A268V, and I335M variants exhibited low soluble protein expression and reduced bioactivity, indicating the three locations at their specific structural motifs affect protein folding but can be less effective. The biochemical analysis found that the N241S variant underwent an uncoupled reaction, forming an oxepinone intermediate that reacts with cysteine and forms the hawkinsin adduct. The A33T and V212M variants, which produced no intermediate product, exhibited similar substrate binding affinity as WT enzymes, but they had decreased structural stability and HG production (aligned with bioassay findings). The reduced structural stability and HG production, and the loss of substrate binding with HPPD-Co(II) complex for the A268V variant suggested that its location is related to the stable conformation of the metal binding motif. The reduced substrate binding affinity and catalytic efficiency for V340L variant suggested the effect at the active site entrance. This study showed the molecular underpinnings of how disease-related substitutions at specific structural locations affect the structural stability and function of HPPD.

Hereditary tyrosinemia type 1 (HT1) stems from the loss of fumarylacetoacetate hydrolase (FAH) activity, causing severe liver-kidney disease. Nitisinone does not restore FAH function and carries metabolic and dietary burdens. Here, we used an integrated workflow guided by X-ray structures of human FAH to obtain small-molecule pharmacological chaperones that bind with low-μM affinity and stabilize FAH. Hits were validated by NMR and isothermal titration calorimetry. Protein stabilization was assessed by DOSY-NMR and circular dichroism; functional effects were tested in FAH activity assays, a CRISPR-engineered cellular model, and testing in an animal model of HT1. Compounds shifted the G337S pathological variant toward the active dimer and slowed unfolding/aggregation, resulting in dose-dependent enhancement of FAH activity and partial rescue of FAH homeostasis in cells and the liver tissue of a mouse model of HT1. These molecules support a therapeutic approach that could complement nitisinone in HT1.

OBJECTIVES: Tyrosinemia type III is an extremely rare autosomal recessive disorder of tyrosine metabolism caused by mutations in the HPD gene, which encodes 4-hydroxyphenylpyruvate dioxygenase (HPPD). Wolff-Parkinson-White (WPW) syndrome is a congenital cardiac conduction disorder characterized by the presence of an accessory atrioventricular pathway. While each condition is rare in isolation, their coexistence has not been previously reported. CASE PRESENTATION: We present a unique case of a 6-year-old boy with known WPW syndrome who was admitted with ketotic hypoglycemia after prolonged fasting and omission of propranolol doses. Metabolic work-up revealed persistently elevated plasma tyrosine levels. Genetic testing confirmed tyrosinemia type III due to a novel homozygous HPD variant [c.559A>G (p.Asn187Asp)]. The persistence of the WPW pattern despite decreased plasma tyrosine levels suggests that there is no direct causal relationship. He was also diagnosed with attention-deficit/hyperactivity disorder, specific learning disorder, and borderline intellectual functioning. CONCLUSIONS: This case highlights the importance of metabolic evaluation in pediatric patients presenting with unexplained hypoglycemia, particularly in the presence of pre-existing cardiac disorders.

Hepatocyte transplantation (HTx) offers a safer, less invasive alternative to orthotopic liver transplantation for inherited metabolic liver diseases, especially in high-risk pediatric patients. Combining HTx with ex vivo gene editing is a promising autologous therapeutic strategy using the patient's cells. We investigated the feasibility of this approach by applying CRISPR-Cas9 gene knock-out to neonatal mouse hepatocytes and comparing their engraftment potential with that of mature adult cells in the Fah-/- mouse model of hereditary tyrosinemia type I (HT1). Electroporation-mediated gene editing did not significantly impair the ability of neonatal hepatocytes to engraft in vivo. Quantitative histological analysis revealed comparable liver repopulation levels between recipients of gene-edited neonatal cells and adult cells after hepatoxicity-mediated selection, providing a benchmark for electroporation-mediated gene editing in neonatal hepatocytes, and supporting the development of genetically corrected neonatal hepatocyte products as a crucial long-term or bridge-to-transplant therapeutic strategy for pediatric liver disease.

BACKGROUND: Tyrosinemia type I is a rare autosomal recessive inborn error of metabolism caused by deficiency of fumarylacetoacetate hydrolase (FAH), an enzyme essential for the final breakdown of tyrosine. The enzyme defect leads to accumulation of toxic metabolites, primarily affecting the liver, kidneys, and brain. Typically, diagnosed in infancy due to acute or chronic liver dysfunction, refractory rickets is an uncommon presentation. We report a child with tyrosinemia type I presenting with refractory rickets and hepatosplenomegaly but with preserved liver function. CASE PRESENTATION: A three-year-old child presented with bilateral limb deformities unresponsive to multiple courses of vitamin D and calcium. The child also exhibited floppiness, polyuria, and polydipsia. Evaluation revealed hepatomegaly and hypophosphatemic rickets secondary to Fanconi syndrome, later confirmed to be due to tyrosinemia type I. The child was treated with phosphate supplementation, nitisinone, and a tyrosine- and phenylalanine-restricted diet, with favorable follow-up. CONCLUSION: Tyrosinemia type I may present beyond infancy with renal-predominant manifestations such as refractory rickets and preserved hepatic function. Early diagnosis and treatment are essential to prevent progression and mortality.