GM3 synthase deficiency (GM3SD) is an autosomal recessive disorder resulting from mutations in the ST3GAL5 gene. It is characterized by intellectual disability, microcephaly, psychomotor and developmental delay, hearing and visual impairments, and changes in skin pigmentation. This study aims to broaden the genetic mutation spectrum of GM3SD through the report of a de novo mutation and a comprehensive summary of GM3SD phenotype to aid in genetic counseling and prenatal diagnosis. Compound heterozygous variants in ST3GAL5 (NM_003896.4: c.1000delC, p.Arg334Glufs*15 and c.207-1G > T, p.Cys70Glufs*81) were identified via trio-whole exome sequencing (trio-WES) and confirmed pathogenic through functional experiments. Notably, c.207-1G > T was a newly discovered variant. Additionally, previously reported GM3SD mutations were classified into R288X and non-R288X, revealing that R288X mutations were more likely to manifest developmental, emotional abnormalities, and severe feeding difficulties. This study reveals a novel mutation in ST3GAL5 and provides a comprehensive overview of GM3SD phenotype, aiding in the diagnosis and genetic counseling of GM3SD in clinical practice.

GM3 Synthase Deficiency (GM3SD) is a neurodevelopmental disorder resulting from pathogenic variants in the ST3GAL5 gene, which encodes GM3 synthase, a glycosphingolipid (GSL)-specific sialyltransferase. This enzyme adds a sialic acid to the terminal galactose of lactosylceramide (LacCer) to produce the monosialylated ganglioside GM3. In turn, GM3 is extended by other glycosyltransferases to generate nearly all the complex gangliosides enriched in neural tissue. Pathogenic mechanisms underlying the neural phenotypes associated with GM3SD are unknown. To explore how loss of GM3 impacts neural-specific glycolipid glycosylation and cell signaling, GM3SD patient fibroblasts bearing one of two different ST3GAL5 variants were reprogrammed to induced pluripotent stem cells (iPSCs) and then differentiated to neural crest cells (NCCs). GM3 and GM3-derived gangliosides were undetectable in cells carrying either variant, while LacCer precursor levels were elevated compared to wildtype (WT). NCCs of both variants synthesized elevated levels of neutral lacto- and globo-series, as well as minor alternatively sialylated GSLs compared to WT. Ceramide profiles were also shifted in GM3SD variant cells. Altered GSL profiles in GM3SD cells were accompanied by dynamic changes in the cell surface proteome, protein O-GlcNAcylation, and receptor tyrosine kinase abundance. GM3SD cells also exhibited increased apoptosis and sensitivity to erlotinib-induced inhibition of epidermal growth factor receptor signaling. Pharmacologic inhibition of O-GlcNAcase rescued baseline and erlotinib-induced apoptosis. Collectively, these findings indicate aberrant cell signaling during differentiation of GM3SD iPSCs and also underscore the challenge of distinguishing between variant effect and genetic background effect on specific phenotypic consequences. Early clinical features of GM3 synthase deficiency include infantile onset of severe irritability with feeding difficulties, early and intractable seizures, growth failure with acquired microcephaly, sensorineural hearing impairment, hypotonia, and poor visual function. Over time, affected individuals experience severe-to-profound developmental delay and intellectual disability, can develop dystonia with hyperkinetic movements, and may develop pigmentary skin changes of the hands and feet. Affected individuals often have frequent ear infections and pneumonia without evidence of immune dysfunction. The diagnosis of GM3 synthase deficiency is established in a proband with suggestive findings and biallelic pathogenic variants in ST3GAL5 identified by molecular genetic testing. Treatment of manifestations: Treatment for new-onset or worsening irritability is based on identification of inciting factors; for example, motility agents for constipation, antibiotic treatment of infections, and standard therapies for GERD, such as proton pump inhibitors. Seizures are treated with anti-seizure medication (ASM), although the majority of electrographic seizures in affected children are clinically silent. No ASM has been demonstrated effective for this condition specifically, ASM treatment may be only partially effective, and multiple ASMs may be required. Feeding therapy may be useful, with consideration of gastrostomy tube placement for persistent poor feeding / growth failure. Hearing aids may be helpful on a case-by-case basis. There is no specific treatment for optic atrophy or cortical blindness, although referral to low vision services is recommended. Dystonia, developmental delay / intellectual disability, sleep disturbances, infectious illnesses, and scoliosis are treated per standard methods. Surveillance: At each visit: measure growth parameters, evaluate status and safety of oral intake, monitor for new manifestations (seizures, changes in tone, movement disorders), monitor developmental progress, assess for behavioral changes (new-onset or worsening irritability; aggressive or self-injurious behaviors), monitor for constipation/GERD/emesis, monitor for signs/symptoms of pneumonia, and obtain a non-invasive SpO2. Annually: physical exam for signs/symptoms of scoliosis. As clinically indicated: Audiologic evaluation, ophthalmologic evaluation, and assessment of mobility, physical medicine, and OT/PT needs. Therapies under investigation: Oral supplementation with GM3 gangliosides did not alter disease course and demonstrated only modest short-term benefit to affected individuals. GM3 synthase deficiency is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for an ST3GAL5 pathogenic variant, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of inheriting neither of the familial pathogenic variants. Once the ST3GAL5 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal and preimplantation genetic testing are possible.

ST3GAL5 encodes GM3 synthase (ST3 beta-galactoside alpha-2,3-sialyltransferase 5; ST3GAL5), which synthesizes GM3 by transferring sialic acid to lactosylceramide. GM3, a sialic acid-containing glycosphingolipid known as ganglioside, is a precursor to the biosynthesis of various more complex gangliosides that are active in the brain. Biallelic variants in ST3GAL5 cause GM3 synthase deficiency (GM3SD), a rare congenital disorder of glycosylation. GM3SD was first identified in the Amish population in 2004. We report two siblings diagnosed with GM3SD due to novel compound heterozygous ST3GAL5 variants. The novel ST3GAL5 variants, detected by whole-exome sequencing in the patients, were confirmed to be pathogenic by GM3 synthase assay. The clinical courses of these patients, which began in infancy with irritability and growth failure, followed by developmental delay and hearing loss, were consistent with previous case reports of GM3SD. The older sibling underwent deep brain stimulation for severe involuntary movements at the age of 9 years. The younger sibling suffered from acute encephalopathy at the age of 9 months and subsequently developed refractory epilepsy. Reports of GM3SD outside the Amish population are rare, and whole-exome sequencing may be required to diagnose GM3SD in non-Amish patients. Since an effective treatment for GM3SD has not yet been established, we might select deep brain stimulation as a symptomatic treatment for involuntary movements in GM3SD.

Mast cells are a significant source of cytokines and chemokines that play a role in pathological processes. Gangliosides, which are complex lipids with a sugar chain, are present in all eukaryotic cell membranes and comprise lipid rafts. Ganglioside GM3, the first ganglioside in the synthetic pathway, is a common precursor of the specifying derivatives and is well known for its various functions in biosystems. Mast cells contain high levels of gangliosides; however, the involvement of GM3 in mast cell sensitivity is unclear. Therefore, in this study, we elucidated the role of ganglioside GM3 in mast cells and skin inflammation. GM3 synthase (GM3S)-deficient mast cells showed cytosolic granule topological changes and hyperactivation upon IgE-DNP stimulation without affecting proliferation and differentiation. Additionally, inflammatory cytokine levels increased in GM3S-deficient bone marrow-derived mast cells (BMMC). Furthermore, GM3S-KO mice and GM3S-KO BMMC transplantation showed increased skin allergic reactions. Besides mast cell hypersensitivity caused by GM3S deficiency, membrane integrity decreased and GM3 supplementation rescued this loss of membrane integrity. Additionally, GM3S deficiency increased the phosphorylation of p38 mitogen-activated protein kinase. These results suggest that GM3 increases membrane integrity, leading to the suppression of the p38 signalling pathway in BMMC and contributing to skin allergic reaction.

GM3 synthase deficiency (GM3SD) is an infantile-onset epileptic encephalopathy syndrome caused by biallelic loss-of-function mutations in ST3GAL5. Loss of ST3GAL5 activity in humans results in systemic ganglioside deficiency and severe neurological impairment. No disease-modifying treatment is currently available. Certain recombinant adeno-associated viruses (rAAVs) can cross the blood-brain barrier to induce widespread, long-term gene expression in the CNS and represent a promising therapeutic strategy. Here, we show that a first-generation rAAV-ST3GAL5 replacement vector using a ubiquitous promoter restored tissue ST3GAL5 expression and normalized cerebral gangliosides in patient-derived induced pluripotent stem cell neurons and brain tissue from St3gal5-KO mice but caused fatal hepatotoxicity when administered systemically. In contrast, a second-generation vector optimized for CNS-restricted ST3GAL5 expression, administered by either the intracerebroventricular or i.v. route at P1, allowed for safe and effective rescue of lethality and behavior impairment in symptomatic GM3SD mice up to a year. These results support further clinical development of ST3GAL5 gene therapy.