Multiple congenital anomalies-hypotonia-seizures syndrome 3 (MCAHS3) results from mutations in the phosphatidylinositol glycan biosynthesis class T (PIGT) gene leading to defects in glycosylphosphatidylinositol transamidase complex (GPI-TA) synthesis. Glycosylphosphatidylinositol serves as an anchor to more than 150 mammalian proteins for attachment on cell surfaces, enabling specific functional properties. Mutations in the PIGT gene result in disruption of this extremely important post-translational protein modification, yielding dysfunctional proteins leading to MCAHS3. An exhaustive literature search was conducted across various electronic databases to reveal only 41 reported cases of MCAHS3 worldwide, emphasizing the rarity of this condition. Multiple congenital anomalies-hypotonia-seizures syndrome 3 has been reported as secondary to 18 different known PIGT variants to date, manifesting as a varying spectrum of craniofacial dysmorphism, developmental delay with epilepsy, cardiac and renal malformations, and unique features in biochemical testing and neuroimaging. This review aims to highlight the constellation of clinical symptoms, diagnostic modalities, and management challenges associated with MCAHS3 cases. It would help determine optimal diagnostic and treatment strategies for newly identified cases and facilitate new research on this rare condition.

Biallelic PIGN variants have been described in Fryns syndrome, multiple congenital anomalies-hypotonia-seizure syndrome (MCAHS), and neurologic phenotypes. The full spectrum of clinical manifestations in relation to the genotypes is yet to be reported.
Genotype and phenotype data were collated and analyzed for 61 biallelic PIGN cases: 21 new and 40 previously published cases. Functional analysis was performed for 2 recurrent variants (c.2679C>G p.Ser893Arg and c.932T>G p.Leu311Trp).
Biallelic-truncating variants were detected in 16 patients-10 with Fryns syndrome, 1 with MCAHS1, 2 with Fryns syndrome/MCAHS1, and 3 with neurologic phenotype. There was an increased risk of prenatal or neonatal death within this group (6 deaths were in utero or within 2 months of life; 6 pregnancies were terminated). Incidence of polyhydramnios, congenital anomalies (eg, diaphragmatic hernia), and dysmorphism was significantly increased. Biallelic missense or mixed genotype were reported in the remaining 45 cases-32 showed a neurologic phenotype and 12 had MCAHS1. No cases of diaphragmatic hernia or abdominal wall defects were seen in this group except patient 1 in which we found the missense variant p.Ser893Arg to result in functionally null alleles, suggesting the possibility of an undescribed functionally important region in the final exon. For all genotypes, there was complete penetrance for developmental delay and near-complete penetrance for seizures and hypotonia in patients surviving the neonatal period.
We have expanded the described spectrum of phenotypes and natural history associated with biallelic PIGN variants. Our study shows that biallelic-truncating variants usually result in the more severe Fryns syndrome phenotype, but neurologic problems, such as developmental delay, seizures, and hypotonia, present across all genotypes. Functional analysis should be considered when the genotypes do not correlate with the predicted phenotype because there may be other functionally important regions in PIGN that are yet to be discovered.

Hyperphosphatasia with mental retardation syndrome 4 (HPMRS4) is a rare autosomal recessive disorder caused by glycosylphosphatidylinositol (GPI) deficiency. GPI deficiency results from a mutation in one of six known genes. Mutation in post-GPI attachment to protein phospholipase 3 gene (PGAP3) is linked to HPMRS4. Patients usually present with dysmorphic features, developmental delay, central hypotonia, and seizure. However, in our case, we report a novel homozygous missense mutation of PGAP3 gene in a female child who presented with megalocornea, which is an unusual clinical presentation for HPMRS4. Megalocornea, in her first days of life, led to a misdiagnosis of primary congenital glaucoma. Later, other common clinical features of HPMRS4 became apparent.

Inherited glycosylphosphatidylinositol (GPI) deficiencies are a group of clinically and genetically heterogeneous conditions belonging to the congenital disorders of glycosylation. PIGW is involved in GPI biosynthesis and modification, and biallelic pathogenic variants in this gene cause autosomal recessive GPI biosynthesis defect 11. Only five patients and two fetuses have been reported in the literature thus far. Here we describe a new patient with a novel homozygous missense variant in PIGW, who presented with hypotonia, severe intellectual disability, early-onset epileptic seizures, brain abnormalities, nystagmus, hand stereotypies, recurrent respiratory infections, distinctive facial features, and hyperphosphatasia. Our report expands the phenotype of GPI biosynthesis defect 11 to include stereotypies and recurrent respiratory infections. A detailed and long-term analysis of the electroclinical characteristics and review of the literature suggest that early-onset epileptic seizures are a key manifestation of GPI biosynthesis defect 11. West syndrome and focal-onset epileptic seizures are the most common seizure types, and the fronto-temporal regions may be the most frequently involved areas in these patients.

At least 150 human proteins are glycosylphosphatidylinositol-anchored proteins (GPI-APs). The protein moiety of GPI-APs lacking transmembrane domains is anchored to the plasma membrane with GPI covalently attached to the C-terminus. The GPI consists of the conserved core glycan, phosphatidylinositol and glycan side chains. The entire GPI-AP is anchored to the outer leaflet of the lipid bilayer by insertion of fatty chains of phosphatidylinositol. Because of GPI-dependent membrane anchoring, GPI-APs have some unique characteristics. The most prominent feature of GPI-APs is their association with membrane microdomains or membrane rafts. In the polarized cells such as epithelial cells, many GPI-APs are exclusively expressed in the apical surfaces, whereas some GPI-APs are preferentially expressed in the basolateral surfaces. Several GPI-APs act as transcytotic transporters carrying their ligands from one compartment to another. Some GPI-APs are shed from the membrane after cleavage within the GPI by a GPI-specific phospholipase or a glycosidase. In this review, I will summarize the current understanding of GPI-AP biosynthesis in mammalian cells and discuss examples of GPI-dependent functions of mammalian GPI-APs.

Defects in the human glycosylphosphatidylinositol anchor biosynthetic pathway are associated with inherited glycosylphosphatidylinositol (GPI)-deficiencies characterized by a broad range of clinical phenotypes including multiple congenital anomalies, dysmorphic faces, developmental delay, hypotonia, and epilepsy. Biallelic variants in PIGN, encoding phosphatidylinositol-glycan biosynthesis class N have been recently associated with multiple congenital anomalies hypotonia seizure syndrome.
Our patient is a 2 year old male with hypotonia, global developmental delay, and focal epilepsy. Trio whole-exome sequencing revealed heterozygous variants in PIGN, c.181G > T (p.Glu61*) and c.284G > A (p.Arg95Gln). Analysis of FLAER and anti-CD59 by flow-cytometry demonstrated a shift in this patient\'s granulocytes, confirming a glycosylphosphatidylinositol-biosynthesis defect, consistent with PIGN-related disease.
To date, a total of 18 patients have been reported, all but 2 of whom have congenital anomalies and/or obvious dysmorphic features. Our patient has no significant dysmorphic features or multiple congenital anomalies, which is consistent with recent reports linking non-truncating variants with a milder phenotype, highlighting the importance of functional studies in interpreting sequence variants.

Glycosylphosphatidylinositol (GPI) is a glycolipid that tethers more than 150 different proteins to the cell surface. Aberrations in biosynthesis of GPI anchors cause congenital disorders of glycosylation with clinical features including intellectual disability (ID), seizures, and facial dysmorphism. Here, we present two siblings with ID, cerebellar hypoplasia, cerebellar ataxia, early-onset seizures, and minor facial dysmorphology. Using exome sequencing, we identified a homozygous nonsense variant (NM_001127178.1:c.1640G>A, p.Trp547*) in the gene Phosphatidylinositol Glycan Anchor Biosynthesis, Class G (PIGG) in both the patients. Variants in several other GPI anchor synthesis genes lead to a reduced expression of GPI-anchored proteins (GPI-APs) that can be measured by flow cytometry. No significant differences in GPI-APs could be detected in patient granulocytes, consistent with recent findings. However, fibroblasts showed a reduced global level of GPI anchors and of specific GPI-linked markers. These findings suggest that fibroblasts might be more sensitive to pathogenic variants in GPI synthesis pathway and are well suited to screen for GPI-anchor deficiencies. Based on genetic and functional evidence, we confirm that pathogenic variants in PIGG cause an ID syndrome, and we find that loss of function of PIGG is associated with GPI deficiency.

Glucose-6-phosphate isomerase (GPI) deficiency cause hereditary nonspherocytic hemolytic anemia (HNSHA) of variable severity in individuals homozygous or compound heterozygous for mutations in GPI gene. This work presents clinical features and genotypic results of two patients of Portuguese origin with GPI deficiency. The patients suffer from a mild hemolytic anemia (Hb levels ranging from 10 to 12.7g/mL) associated with macrocytosis, reticulocytosis, hyperbilirubinemia, hyperferritinemia and slight splenomegaly. Genomic DNA sequencing revealed in one patient homozygosity for a new missense mutation in exon 3, c.260G>C (p.Gly87Ala), and in the second patient compound heterozygosity for the same missense mutation (p.Gly87Ala), along with a frameshift mutation resulting from a single nucleotide deletion in exon 14, c.1238delA (p.Gln413Arg fs*24). Mutation p.Gln413Arg fs*24 is the first frameshift null mutation to be described in GPI deficiency. Molecular modeling suggests that the structural change induced by the p.Gly87Ala pathogenic variant has direct impact in the structural arrangement of the region close to the active site of the enzyme.

Hemolytic anemia due to GPI deficiency can be severe and life threatening during fetal life. When parents decline invasive testing, ultrasound monitoring of fetuses at risk is feasible. Intrauterine transfusion can be effective for the treatment of severe fetal anemia due to GPI deficiency.