Sulfatide (3-O-sulfogalactosylceramide, SM4) is a glycosphingolipid, highly multifunctional and particularly enriched in the myelin sheath of neurons. The role of sulfatide has been implicated in various biological fields such as the nervous system, immune system, host-pathogen recognition and infection, beta cell function and haemostasis/thrombosis. Thus, alterations in sulfatide metabolism and production are associated with several human diseases such as neurological and immunological disorders and cancers. The unique lipid-rich composition of myelin reflects the importance of lipids in this specific membrane structure. Sulfatide has been shown to be involved in the regulation of oligodendrocyte differentiation and in the maintenance of the myelin sheath by influencing membrane dynamics involving sorting and lateral assembly of myelin proteins as well as ion channels. Sulfatide is furthermore essential for proper formation of the axo-glial junctions at the paranode together with axonal glycosphingolipids. Alterations in sulfatide metabolism are suggested to contribute to myelin deterioration as well as synaptic dysfunction, neurological decline and inflammation observed in different conditions associated with myelin pathology (mouse models and human disorders). Body fluid biomarkers are of importance for clinical diagnostics as well as for patient stratification in clinical trials and treatment monitoring. Cerebrospinal fluid (CSF) is commonly used as an indirect measure of brain metabolism and analysis of CSF sulfatide might provide information regarding whether the lipid disruption observed in neurodegenerative disorders is reflected in this body fluid. In this review, we evaluate the diagnostic utility of CSF sulfatide as a biomarker for neurodegenerative disorders associated with dysmyelination/demyelination by summarising the current literature on this topic. We can conclude that neither CSF sulfatide levels nor individual sulfatide species consistently reflect the lipid disruption observed in many of the demyelinating disorders. One exception is the lysosomal storage disorder metachromatic leukodystrophy, possibly due to the genetically determined accumulation of non-metabolised sulfatide. We also discuss possible explanations as to why myelin pathology in brain tissue is poorly reflected by the CSF sulfatide concentration. The previous suggestion that CSF sulfatide is a marker of myelin damage has thereby been challenged by more recent studies using more sophisticated laboratory techniques for sulfatide analysis as well as improved sample selection criteria due to increased knowledge on disease pathology.

β-Mannosidosis is a lysosomal storage disorder characterized by accumulation of disaccharides due to deficiency of the lysosomal enzyme β-mannosidase. The disease is caused by mutations in MANBA and is extremely rare in humans. Although the clinical presentation is heterogeneous, common symptoms include various degrees of developmental delay, behavioral disturbances, hearing loss, and frequent infections. We report a 15-yr-old girl presenting with mild intellectual disability, sensorineural hearing loss, severe behavioral disturbances, dysmorphic traits, and evolving angiokeratomas. Copy-number variation analysis of next-generation sequencing (NGS) data indicated increased coverage in exons 8–11 of MANBA. Low β-mannosidase activity (1 µkatal/kg protein, refv 25–40) established the diagnosis of β-mannosidosis. Whole-genome sequencing (WGS) and cDNA analysis revealed a novel homozygous intragenic inverted duplication in MANBA, where a 13.1-kb region between introns 7 and 11 was duplicated and inserted in an inverted orientation, creating a 67-base nonduplicated gap at the insertion point. Both junctions showed microhomology regions. The inverted duplication resulted in exon skipping of exons 8–9 or 8–10. Our report highlights the importance of copy-number variation analysis of data from NGS and in particular the power of WGS in the identification and characterization of copy-number variants.

Lysosomal storage diseases are a group of disorders where accumulation of catabolites is manifested in the lysosomes of different cell types. In metachromatic leukodystrophy (Arylsulfatase A [EC.3.1.6.8] deficiency) storage of the glycosphingolipid sulfatide in the brain leads to demyelination, resulting in neuromotor co-ordination deficits and regression. In a mouse model for metachromatic leukodystrophy, the ASA null mutant mouse, the accumulation of sulfatide in correlation to phenotype has been thoroughly investigated. Another lipid species reported to accumulate in patients with metachromatic leukodystrophy is the sulfatide related lipid lysosulfatide. Lysosulfatide was shown to be a cytotoxic compound in cell culture experiments and thus suggested to be involved in the pathology of metachromatic leukodystrophy. In this study, we further investigated the developmental profile of lysosulfatide in the brain of ASA null mutant mice by using high performance liquid chromatography. Lysosulfatide could be detected in the brain of normal mice (ASA +/+) from 1.8 months up to 23.1 months of age. From the age of 8.8 months the lysosulfatide levels remained constant at 1 pmol/mg wet tissue. The developmental change (< 20 months) of brain lysosulfatide showed an accumulation in ASA null mutant mice at ages above one month compared to its normal counterpart (ASA +/+). Thus, the ASA null mutant mouse might be a suitable model to further investigate the role of lysosulfatide in the pathogenesis of metachromatic leukodystrophy.

Sialic acid storage diseases are neurodegenerative disorders characterized by accumulation of sialic acid in the lysosome. These disorders are caused by mutations in SLC17A5, the gene encoding sialin, a sialic acid transporter located in the lysosomal membrane. The most common form of sialic acid storage disease is the slowly progressive Salla disease, presenting with hypotonia, ataxia, epilepsy, nystagmus and findings of cerebral and cerebellar atrophy. Hypomyelination and corpus callosum hypoplasia are typical as well. We report a 16 year-old boy with an atypically mild clinical phenotype of sialic acid storage disease characterized by psychomotor retardation and a mixture of spasticity and rigidity but no ataxia, and only weak features of hypomyelination and thinning of corpus callosum on MRI of the brain. The thiobarbituric acid method showed elevated levels of free sialic acid in urine and fibroblasts, indicating sialic acid storage disease. Initial Sanger sequencing of SLC17A5 coding regions did not show any pathogenic variants, although exon 9 could not be sequenced. Whole exome sequencing followed by RNA and genomic DNA analysis identified a homozygous 6040 bp insertion in intron 9 of SLC17A5 corresponding to a long interspersed element-1 retrotransposon (KF425758.1). This insertion adds two splice sites, both resulting in a frameshift which in turn creates a premature stop codon 4 bp into intron 9. This study describes a novel pathogenic variant in SLC17A5, namely an intronic transposal insertion, in a patient with mild biochemical and clinical phenotypes. The presence of a small fraction of normal transcript may explain the mild phenotype. This case illustrates the importance of including lysosomal sialic acid storage disease in the differential diagnosis of developmental delay with postnatal onset and hypomyelination, as well as intronic regions in the genetic investigation of inborn errors of metabolism.

Sulfatide (3′‐sulfogalactosyl‐ceramide) is a glycosphingolipid mainly located in the nervous system, but has also been found in the islets of Langerhans. Previous studies have suggested that sulfatide is involved in insulin processing and secretion. In this study, sulfatide expression and metabolism in pancreas and isolated islets of the type II diabetes models, ob/ob‐ and db/db mouse, was investigated using TLC‐ELISA, metabolic labelling and electron microscopy. As in non‐diabetic Lewis rat and human pancreas, sulfatide was located in secretory granules of the beta cells. However, the type II diabetic animal models and their background strains had an altered sulfatide expression, involving the lack of the C16:0 sulfatide fatty acid isoform, compared to non‐diabetic Lewis rat, BALB/c mouse and human pancreatic tissue, in which the two dominating pancreatic sulfatide isoforms C16:0 and C24:0 are expressed. Correspondingly, in isolated ob/ob islets, sulfatide synthesis excluded the production of C16:0 sulfatide. Insulin administration to ob/ob mouse, which lowers beta cell activity, resulted in significantly increased sulfatide expression in pancreas (p=0.0003), but still no expression of the C16:0 sulfatide isoform. In vitro, the C16:0 sulfatide was shown to be the isomer involved in the preservation of insulin crystals. Thus, it is hypothesized that the selection of sulfatide isomers in pancreas might be a genetic factor contributing to disease development in type II diabetic animal models.

The peroxisome biogenesis disorders, which are caused by mutations in any of 13 different PEX genes, include the Zellweger spectrum disorders. Severe defects in one of these PEX genes result in the absence of functional peroxisomes which is seen in classical Zellweger syndrome. These patients present with hypotonia and seizures shortly after birth. Other typical symptoms are dysmorphic features, liver disease, retinal degeneration, sensorineural deafness, polycystic kidneys, and the patient does not reach any developmental milestones. We report a case of a patient with Zellweger spectrum disorder due to a novel mutation in the PEX10 gene, presenting with a mild late-onset neurological phenotype. The patient, an Assyrian girl originating from Iraq, presented with sensorineural hearing impairment at the age of 5 followed by sensorimotor polyneuropathy, cognitive delay, impaired gross and fine motor skills, and tremor and muscle weakness in her teens. Analyses of biochemical markers for peroxisomal disease suggested a mild peroxisomal defect and functional studies in fibroblasts confirmed the existence of a peroxisome biogenesis disorder. Diagnosis was confirmed by next generation sequencing analysis, which showed a novel homozygous mutation (c.530 T > G (p.Leu177Arg) (NM_153818.1)) in the PEX10 gene predicted to be pathogenic. This case highlights the importance of performing biochemical, functional, and genetic peroxisomal screening in patients with clinical presentations milder than those usually observed in Zellweger spectrum disorders.