Glucose Phosphate Isomerase Deficiency In 2 Patients With Novel Mutations Presenting As Severe Neurologic Abnormalities and Transfusion Dependent Hemolytic Anemia

Abstract

▪ IntroductionGlucose phosphate isomerase (GPI) deficiency is the third most common red cell enzymopathy. GPI is an enzyme that reversibly catalyzes the conversion of glucose-6-phosphate into fructose 6-phosphate in the second step of glycolysis. Patients afflicted by GPI deficiency have chronic hemolysis and may also suffer from acute hemolytic crises. There are 184 known mutations of the GPI gene and to date, a neurological deficit is found in only five patients and only two of these have been characterized at a molecular level. We report 2 patients with previously unknown mutations of the GPI gene associated with, severe neurologic abnormalities and hemolytic anemia. Case 1He was born at 38 weeks gestation; marked pallor and hepatosplenomegaly were noted at birth. The bilirubin was elevated at birth (indirect 7.5 mg/dl and direct 2.2mg/dl) requiring phototherapy. He has transfusion dependent anemia since birth. Enzymes studied were performed which showed GPI levels of 2.02 EU/gm hb ( normal range 16.3-24.7 ) and elevated glucose 6 phosphate dehydrogenase , pyruvate kinase and hexokinase. The pyrimidine 5'-nucleotidase screen was normal. In his subsequent course, he started to have seizures at 6 months of age, refractory to anticonvulsant therapy. He has severe hypotonia and global developmental delay. Magnetic resonance imaging of the brain showed generalized cerebral atrophy with no evidence of kernicterus. Case 2He was noted to have anemia and marked hepatosplenomegaly at birth. He required exchange transfusions and phototherapy in the neonatal period. He has subsequently suffered from lifelong transfusion-dependent hemolytic anemia. He also suffers from developmental delay, ataxia, spasticity, and seizures. Of note, MRI did not exhibit evidence of kernicterus.Table. Mutations of the GPI gene in both the patientsMutation : Change in amino acid and locationLocation of amino acid in GPI crystal structureCarrier parentCASE 1Histidine to arginine at codon 191 (H191R)located in alpha helix tenFatherGlycine to tryptophan at codon 381 (G381W)located in beta sheet fourMotherCASE 2Arginine to glutamine at codon 104 (R104Q)boundary of alpha helix sevenMotherLeucine to phenylalanine at codon 297 (L297F).located in alpha helix fifteenFatherThe mutations were predicted to be pathogenic (probably damaging) by PolyPhen. Both these mutations were in a highly conserved residue. Genetic probe for preimplantation diagnosis is being used for selection of an embryo which is does not have GPI deficiency and is also a potential HLA match with the hope of undergoing hematopoietic stem cell transplantation to avoid the complications of chronic transfusions and iron overload. DiscussionGPI has many functions. In dimeric form, it exhibits its catalytic function. In monomeric form, it acts as a neurotrophic growth factor, neuroleukin, which in vitro promotes survival of neurons. Abnormalities in neuroleukin have been found in motor neuron disease and in patients with central nervous system abnormalities in patients with acquired immunodeficiency disease. These effects of GPI/neuroleukin and relative deficiency in brain and neurons of this protein may explain the neurologic presentation. Decreased phosphatide phosphatase 1 activity, a lipogeneic enzyme due to mTOR activation by accumulated glucose-6-phosphate has been suggested to contribute to the neurologic symptoms. Why some of these mutation are associated with neurologic deficits while most others are not is not known. It has been speculated that the mutations which affect the folding may cause altered structure and function causing neurologic symptoms as well as hemolytic anemia while mutations affecting the catalytic site presents only as hemolytic anemia without neurologic symptoms. Disclosures:No relevant conflicts of interest to declare.