SLC39A8-congenital disorder of glycosylation (SLC39A8-CDG) is a rare autosomal recessive metabolic disease of manganese transport, leading to defective glycosylation and mitochondrial dysfunction. An eight-month-old male infant with severe hypotonia, developmental delay, and dystonic episodes was initially misdiagnosed as epilepsy. Genetic testing identified a homozygous pathogenic variant in the SLC39A8 gene, and biochemical analysis confirmed low manganese levels. Upon initiation of oral manganese sulfate therapy, the patient demonstrated significant clinical improvement, including the achievement of new motor milestones. To our knowledge, this is the first documented case in Bulgaria. This case underscores the importance of early genetic diagnosis and targeted metabolic treatment in altering the clinical trajectory of SLC39A8-CDG. Timely recognition allows for intervention in a disorder that, despite its rarity, has a modifiable course and potential for meaningful developmental gains. SLC39A8-CDG is characterized by mild-to-profound developmental delay, intellectual disability, hypotonia, feeding difficulties with poor weight gain and growth deficiency, dystonia, spasticity, epilepsy, ophthalmologic manifestations including cortical blindness and strabismus, and sensorineural hearing impairment. The diagnosis of SLC39A8-CDG is established in a proband with characteristic clinical features and suggestive laboratory findings (decreased whole blood manganese, elevated xanthine on urinary purines/pyrimidines, and evidence of altered glycosylation) by identification of biallelic pathogenic variants in SLC39A8 on molecular genetic testing. Targeted therapy: Manganese supplementation with titration to identify adequate manganese dose prior to adding galactose supplementation. Supportive care: Developmental and educational support; standard treatments for spasticity, seizures, feeding issues, ophthalmologic involvement, hearing impairment, and musculoskeletal complications; family support and care coordination as needed. Surveillance: Assess for abnormal glycosylation using mass spectrometry, and measure blood manganese levels as needed to titrate manganese dose; brain MRI every one to two years; assess developmental progress, educational needs, seizures, changes in tone, movement disorders, growth, nutrition, feeding, mobility, self-help skills, clinical evidence of scoliosis, and family needs at each visit; assess visual acuity and for strabismus with frequency per ophthalmologist; annual aspartate transaminase, alanine transaminase, and albumin in those with evidence of liver disease; assess for osteopenia/osteoporosis every one to two years or as needed; DXA scan every three to five years starting in adolescence. Agents/circumstances to avoid: Fever, hepatotoxic drugs, and drugs contraindicated in mitochondriopathies. Evaluation of relatives at risk: All at-risk sibs of any age should have molecular genetic testing for the familial SLC39A8 pathogenic variants in order to identify as early as possible those who would benefit from prompt initiation of manganese and galactose supplementation. SLC39A8-CDG is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for an SLC39A8 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 being unaffected and not a carrier. Once the SLC39A8 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal and preimplantation genetic testing are possible.

SLC39A8, a gene located on chromosome 4q24, encodes for the manganese (Mn) transporter ZIP8 and its detrimental variants cause a type 2 congenital disorder of glycosylation (CDG). The common SLC39A8 missense variant A391T is associated with increased risk for multiple neurological and systemic disorders and with decreased serum Mn. Patients with SLC39A8-CDG present with different clinical and neuroradiological features linked to variable transferrin glycosylation profile. Galactose and Mn supplementation therapy results in the biochemical and clinical amelioration of treated patients. Here, we report clinical manifestations, neuroradiological features and glycophenotypes associated with novel SLC39A8 variants (c.1048G > A; p.Gly350Arg and c.131C > G; p.Ser44Trp) in two siblings of the same Italian family. Furthermore, we describe a third patient with overlapping clinical features harbouring the homozygous missense variant A391T. The clinical phenotype of the three patients was characterized by severe developmental disability, dystonic postural pattern and dyskinesia with a more severe progression of the disease in the two affected siblings. Neuroimaging showed a Leigh syndrome-like pattern involving the basal ganglia, thalami and white matter. In the two siblings, atrophic cerebral and cerebellum changes consistent with SLC39A8-CDG were detected as well. Serum transferrin isoelectric focusing (IEF) yielded variable results with slight increase of trisialotransferrin isoforms or even normal pattern. MALDI-MS showed the presence of hypogalactosylated transferrin N-glycans, spontaneously decreasing during the disease course, only in one affected sibling. Total serum N-glycome depicted a distinct pattern for the three patients, with increased levels of undergalactosylated and undersialylated precursors of fully sialylated biantennary glycans, including the monosialo-monogalacto-biantennary species A2G1S1. Clinical, MRI and glycosylation features of patients are consistent with SLC39A8-CDG. We document two novel variants associated with Leigh syndrome-like disease presentation of SLC39A8-CDG. We show, for the first time, a severe neurological phenotype overlapping with that described for SLC39A8-CDG in association with the homozygous A391T missense variant. We observed a spontaneous amelioration of transferrin N-glycome, highlighting the efficacy of MS-based serum glycomics as auxiliary tool for the diagnosis and clinical management of therapy response in patients with SLC39A8-CDG. Further studies are needed to analyse more in depth the influence of SLC39A8 variants, including the common missense variant, on the expression and function of ZIP8 protein, and their impact on clinical, biochemical and neuroradiological features.

Congenital disorders of glycosylation (CDG) are a growing group of inborn metabolic disorders with multiorgan presentation. SLC39A8-CDG is a severe subtype caused by biallelic mutations in the manganese transporter SLC39A8, reducing levels of this essential cofactor for many enzymes including glycosyltransferases. The current diagnostic standard for disorders of N-glycosylation is the analysis of serum transferrin. Exome and Sanger sequencing were performed in two patients with severe neurodevelopmental phenotypes suggestive of CDG. Transferrin glycosylation was analyzed by high-performance liquid chromatography (HPLC) and isoelectric focusing in addition to comprehensive N-glycome analysis using matrix-assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry (MS). Atomic absorption spectroscopy was used to quantify whole blood manganese levels. Both patients presented with a severe, multisystem disorder, and a complex neurological phenotype. Magnetic resonance imaging (MRI) revealed a Leigh-like syndrome with bilateral T2 hyperintensities of the basal ganglia. In patient 1, exome sequencing identified the previously undescribed homozygous variant c.608T>C [p.F203S] in SLC39A8. Patient 2 was found to be homozygous for c.112G>C [p.G38R]. Both individuals showed a reduction of whole blood manganese, though transferrin glycosylation was normal. N-glycome using MALDI-TOF MS identified an increase of the asialo-agalactosylated precursor N-glycan A2G1S1 and a decrease in bisected structures. In addition, analysis of heterozygous CDG-allele carriers identified similar but less severe glycosylation changes. Despite its reliance as a clinical gold standard, analysis of transferrin glycosylation cannot be categorically used to rule out SLC39A8-CDG. These results emphasize that SLC39A8-CDG presents as a spectrum of dysregulated glycosylation, and MS is an important tool for identifying deficiencies not detected by conventional methods.

A common missense variant in SLC39A8 is convincingly associated with schizophrenia and several additional phenotypes. Homozygous loss-of-function mutations in SLC39A8 result in undetectable serum manganese (Mn) and a Congenital Disorder of Glycosylation (CDG) due to the exquisite sensitivity of glycosyltransferases to Mn concentration. Here, we identified several Mn-related changes in human carriers of the common SLC39A8 missense allele. Analysis of structural brain MRI scans showed a dose-dependent change in the ratio of T2w to T1w signal in several regions. Comprehensive trace element analysis confirmed a specific reduction of only serum Mn, and plasma protein N-glycome profiling revealed reduced complexity and branching. N-glycome profiling from two individuals with SLC39A8-CDG showed similar but more severe alterations in branching that improved with Mn supplementation, suggesting that the common variant exists on a spectrum of hypofunction with potential for reversibility. Characterizing the functional impact of this variant will enhance our understanding of schizophrenia pathogenesis and identify novel therapeutic targets and biomarkers.