先天性グリコシルホスファチジルイノシトール（ＧＰＩ）欠損症

Glycosylphosphatidylinositol (GPI) serves as an anchor protein for human cells, and genetic defects in this protein can lead to inherited GPI deficiency (IGD). Intellectual disability, distinctive facial features, epilepsy, hyperphosphatasia, and multiple organ anomalies characterize IGD, with severity varying based on the affected genes. A 53-day-old male, born to nonconsanguineous parents, presented with poor feeding and fever. The patient exhibited unique facial characteristics, hyperphosphatasia, and reduced complement levels. During his 2.5-month admission, he exhibited recurrent fever and infection. Whole-genome sequencing revealed compound heterozygous variants in the PIGW gene, c.[173T>C];[617_620del], p.[Leu58Pro];[Val206Glyfs*3]. Flow cytometry indicated a marked decrease in CD16 expression on granulocytes, leading to the diagnosis of IGD in the patient. This patient exhibited persistent low complement levels (CH50, C3, and C4) alongside elevated immune complexes, which may reflect complement activation secondary to infections. Transient complement abnormalities of this kind may occasionally accompany recurrent infections in patients with GPI biosynthesis defect 11. This case broadens the phenotypic spectrum of PIGW-related IGD and highlights the importance of further research to clarify the relationship between infection, complement activation, and GPI biosynthesis.

Thirty genes are involved in the biosynthesis and modification of glycosylphosphatidylinositol (GPI)-anchored proteins. Defects in these genes cause inherited GPI deficiency, whose main clinical features include intellectual disability, developmental delay, and intractable seizures, and sometimes hyperphosphatasia and multiple congenital anomalies. The mechanisms of its systemic, especially neurological, manifestations largely remain unclear, and no fundamental therapy has yet been established. In this study, we report the development of effective adeno-associated virus (AAV)-mediated gene therapy using a mouse model of inherited GPI deficiency caused by phosphatidylinositol glycan anchor biosynthesis class O (PIGO) mutations, with the aim of developing gene therapy applicable to human patients. By optimizing AAV vectors with the most effective and safest promoter, we achieved significant amelioration of neuronal phenotype and growth impairment, with no cases of liver cancer observed. We further determined the optimal administration route and therapeutic dose required for effective systemic delivery. These results provide proof of concept for the therapeutic efficacy of AAV-based gene replacement therapy for inherited GPI deficiency.

OBJECTIVES: Inherited glycosylphosphatidylinositol (GPI) deficiencies (IGDs) impair the expression of GPI-anchored proteins and produce diverse clinical phenotypes that complicate diagnosis. Since we had previously identified CD16b as a useful diagnostic biomarker, we aimed to establish a standardized flow cytometric assay suitable for routine clinical application. METHODS: We analyzed granulocyte expression of CD16b in 29 IGD patients and 101 controls (21 non-IGD and 80 healthy adults). Granulocyte CD16b expression was quantified as geometric mean fluorescence intensity (gMFI) by flow cytometry in our laboratory and a commercial testing company. Receiver operating characteristic (ROC) analysis was used to define diagnostic cut-offs. RESULTS: CD16b on granulocytes showed the largest decrease (approximately 85%) among GPI-anchored proteins in IGD patients. Mean gMFI was 41,660 ± 36,286 in IGD versus 88,370 ± 24,856 in healthy controls (p < 0.001). ROC analysis yielded an optimal cut-off of 53,431 (sensitivity 0.73, specificity 0.96, area under the curve (AUC) 0.84, Youden index 0.69). We defined 40,000 as the definitive diagnostic threshold and 60,000 as the borderline value. These cut-offs were validated with the commercial assay, which provides results within 2-3 days and is reimbursed by Japan's national health insurance. CONCLUSIONS: Measurement of granulocyte CD16b provides a reliable and rapid screening method for IGD that complements genetic testing. Implementation of this assay by a commercial provider, together with validated cutoffs, enables nationwide, insurance-covered diagnostic testing and functional confirmation of GPI pathway gene variants, representing a significant advance in IGD diagnosis.

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are attached to the cell surface via a glycolipid anchor, GPI, whose conserved core is synthesized from phosphatidylinositol (PI) in the endoplasmic reticulum through a series of enzymatic reactions. Most PI species in mammalian cells contain diacylglycerol, whereas GPI-APs predominantly possess 1-alkyl-2-acylglycerol. The basis for this characteristic lipid structure has remained unclear. Lipidomic analysis revealed that 1-alkyl-2-acyl PIs, although minor components of cellular PI, are preferentially used by GPI-N-acetylglucosaminyltransferase, which catalyzes the first step of GPI biosynthesis. GPI intermediates containing 1-alkyl-2-acylglycerol were further enriched in subsequent biosynthetic steps, resulting in mature GPIs primarily harboring this lipid species. We demonstrate that a 1-alkyl-containing precursor lipid derived from peroxisomes, likely 1-alkyl-glyceronephosphate, contributes to the formation of 1-alkyl-2-acyl PIs. Disruption of glyceronephosphate O-acyltransferase (GNPAT) or alkylglycerone phosphate synthase (AGPS), the first two enzymes of the peroxisomal ether-lipid pathway, abolished 1-alkyl-2-acyl PI, yielding GPI-APs containing only diacylglycerol. Lipidomic profiling of GPI biosynthetic intermediates in GPI-defective cells revealed accumulation of defective-step-specific intermediates, enabling the use of this approach for diagnosing inherited GPI deficiency (IGD).

Cefiderocol, a novel β-lactam antibiotic, exhibits potent activity against carbapenemase-producing Enterobacterales (CPE). While its clinical efficacy has been reported for infections caused by KPC-type CPE and Stenotrophomonas maltophilia, evidence regarding its effectiveness against IMP-type CPE remains primarily derived from in vitro studies, with limited clinical data available. This is the first pediatric case successfully treated with cefiderocol for IMP-type CPE bacteremia. He was a 6-year-old boy with inherited glycosylphosphatidylinositol deficiency and acute lymphoblastic leukemia undergoing chemotherapy. He developed bacteremia that blood culture multiplex PCR identified Klebsiella pneumoniae with IMP gene. The combination therapy of cefiderocol 60mg/kg/dose every 8 hours and gentamicin 5mg/kg/dose once daily sterilized blood culture, and subsequent monotherapy with cefiderocol was continued for a total of 14 days. Additional molecular test in the strain detected IMP-1 carbapenemase, SHV extended spectrum β-lactamase and EBC-type AmpC β-lactamase. Cefiderocol was susceptible at minimum inhibitory concentration 0.5μg/mL. Further study is needed for cefiderocol treatment for IMP-type CPE infection in children.