Longitudinal Characterization of the Clinical Course of Intermediate-Severe Salla Disease.

BACKGROUND: Biallelic pathogenic variants in SLC17A5 cause three forms of free sialic acid storage disease categorized based on severity from least to most severe: Salla disease, intermediate-severe Salla disease, and infantile free sialic acid storage disease. Intermediate-severe Salla disease is the most recently described form. Here, we report a longitudinal characterization of intermediate-severe Salla disease progression in two sisters carrying the following biallelic variants in SLC17A5: c.406A>G (p.Lys136Glu) and c.819+1G>A. METHODS: A retrospective review of medical records was performed. A developmental questionnaire was completed to obtain further clinical information. For functional characterization of the predicted splice site variant, RNA was extracted from patient blood samples and sequenced. RESULTS: Disease onset occurred within the first six months of life in both patients. Early childhood development was delayed with achievement of some milestones followed by a developmental plateau in late childhood. After this, both patients began a slow and progressive neurological regression in adolescence. Functional studies confirmed the pathogenicity of the c.819+1G>A variant, resulting in a frameshift and deletion of exon 6. CONCLUSIONS: We present a detailed study describing the clinical course of intermediate-severe Salla disease with over 15 to 20 years of evolution and demonstrate the pathogenicity of the c.819+1G>A splice site variant.

The molecular mechanism of sialic acid transport mediated by Sialin.

Malfunction of the sialic acid transporter caused by various genetic mutations in the SLC17A5 gene encoding Sialin leads to a spectrum of neurodegenerative conditions called free sialic acid storage disorders. Unfortunately, how Sialin transports sialic acid/proton (H+) and how pathogenic mutations impair its function are poorly defined. Here, we present the structure of human Sialin in an inward-facing partially open conformation determined by cryo-electron microscopy, representing the first high-resolution structure of any human SLC17 member. Our analysis reveals two unique features in Sialin: (i) The H+ coupling/sensing requires two highly conserved Glu residues (E171 and E175) instead of one (E175) as implied in previous studies; and (ii) the normal function of Sialin requires the stabilization of a cytosolic helix, which has not been noticed in the literature. By mapping known pathogenic mutations, we provide mechanistic explanations for corresponding functional defects. We propose a structure-based mechanism for sialic acid transport mediated by Sialin.

Psychiatric symptoms in Salla disease.

Salla disease (SD) is a rare lysosomal storage disorder characterised by intellectual disability ataxia, athetosis, nystagmus, and central nervous system demyelination. Although the neurological spectrum of SD's clinical phenotype is well defined, psychotic symptoms in SD remain unreported. We reviewed the presence of psychiatric symptoms in patients diagnosed with SD. Medical records of all SD patients at Oulu University Hospital during the years 1982-2015 were systematically reviewed to evaluate the presence of psychiatric symptoms. Psychiatric symptoms were frequently associated with SD (10/24, 42%), and two patients were described as developing psychosis as adolescents. We reported their clinical characteristics in detail and assessed the prevalence of psychiatric symptoms in a cohort of 24 patients. Other psychiatric factors associated with SD were sleeping disorders (8/24, 32%), aggressive behaviour disorders or restlessness (6/24, 25%), and off-label antipsychotic medication (4/24, 17%). This report expands the knowledge of the phenotypic spectrum of SD and demonstrates the importance of recognising the possibility of psychiatric symptoms, including psychosis, in persons with SD.

Free urinary sialic acid levels may be elevated in patients with pneumococcal sepsis.

OBJECTIVES: Urine free sialic acid (UFSA) is an important diagnostic biomarker for sialuria (GNE variants) and infantile sialic acid storage disease/Salla disease (SLC17A5 variants). Traditionally, UFSA has been measured using specific single-plex methodology in relatively small cohorts of patients with clinical symptoms suggestive of these disorders. The use of multiplex tandem mass spectrometry urine screening (UMSMS) has meant that UFSA can be measured semi-quantitatively in a much larger cohort of patients being investigated for suspected metabolic disorders. We hypothesised that the neuraminidase of Streptococcus pneumoniae may release free sialic acid from endogenous sialylated glycoconjugates and result in increased UFSA levels. METHODS: We conducted a retrospective review of clinical records of patients who were identified as having S. pneumoniae infection and who also had UMSMS at the time of their acute infection. RESULTS: We identified three cases of increased UFSA detected by UMSMS screening that were secondary to S. pneumoniae sepsis. Additional testing ruled out genetic causes of increased UFSA in the first patient. All three patients had overwhelming sepsis with multiorgan dysfunction which was fatal. Glycosylation abnormalities consistent with the removal of sialic acid were demonstrated in serum transferrin patterns in one patient. CONCLUSIONS: We have demonstrated in a retrospective cohort that elevation of UFSA levels have been observed in cases of S. pneumoniae sepsis. This expands our knowledge of UFSA as a biomarker in human disease. This research demonstrates that infection with organisms with neuraminidase activity should be considered in patients with unexplained increases in UFSA.

Prenatal hydrops fetalis associated with infantile free sialic acid storage disease due to a novel homozygous deletion in the SLC17A5 gene.

Nonimmune hydrops fetalis, the excessive accumulation of serous fluid in the subcutaneous tissues and serous cavities of the fetus, has many possible etiologies, providing a diagnostic challenge for the physician. Lysosomal storage diseases have been reported in up to 5%-16% of nonimmune hydrops fetalis pregnancies. Infantile free sialic acid storage disease (ISSD) (OMIM #269920) is a severe form of autosomal recessive sialic acid storage disease. ISSD is caused by mutations in SLC17A5 (OMIM #604322), which encodes sialin, a lysosomal-membrane sialic acid transporter. We describe a case of fetal hydrops due to a novel homozygous deletion in the SLC17A5 gene. Prenatal single-nucleotide polymorphism (SNP) array analysis was performed on amniocytes after the discovery of fetal hydrops at 24 wk gestation revealing no copy-number variants. The SNP array, however, reported several regions of homozygosity (ROHs) including one on Chromosome 6 encompassing the SLC17A5 gene. High levels of urine sialic acid in the newborn were detected. SLC17A5 gene sequencing was initiated with no sequence variants identified; however, the assay failed to amplify exons 8 and 9, prompting an exon-level copy-number analysis that revealed a novel homozygous deletion of exons 8 and 9, inherited from heterozygous carrier parents. ISSD should be considered in the workup of patients with nonimmune hydrops fetalis, and analysis for SLC17A5 deletions should be carried out when variants are not detected by gene sequencing.

Free sialic acid storage disorder: Progress and promise.

Lysosomal free sialic acid storage disorder (FSASD) is an extremely rare, autosomal recessive, neurodegenerative, multisystemic disorder caused by defects in the lysosomal sialic acid membrane exporter SLC17A5 (sialin). SLC17A5 defects cause free sialic acid and some other acidic hexoses to accumulate in lysosomes, resulting in enlarged lysosomes in some cell types and 10-100-fold increased urinary excretion of free sialic acid. Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia. Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features. Around 200 FSASD cases are reported worldwide, with the clinical spectrum ranging from a severe infantile onset form, often lethal in early childhood, to a mild, less severe form with subjects living into adulthood, also called Salla disease. The pathobiology of FSASD remains poorly understood and FSASD is likely underdiagnosed. Known patients have experienced a diagnostic delay due to the rarity of the disorder, absence of routine urine sialic acid testing, and non-specific clinical symptoms, including developmental delay, ataxia and infantile hypomyelination. There is no approved therapy for FSASD. We initiated a multidisciplinary collaborative effort involving worldwide academic clinical and scientific FSASD experts, the National Institutes of Health (USA), and the FSASD patient advocacy group (Salla Treatment and Research [S.T.A.R.] Foundation) to overcome the scientific, clinical and financial challenges facing the development of new treatments for FSASD. We aim to collect data that incentivize industry to further develop, obtain approval for, and commercialize FSASD treatments. This review summarizes current aspects of FSASD diagnosis, prevalence, etiology, and disease models, as well as challenges on the path to therapeutic approaches for FSASD.

Inherited disorders of lysosomal membrane transporters.

Disorders caused by defects in lysosomal membrane transporters form a distinct subgroup of lysosomal storage disorders (LSDs). To date, defects in only 10 lysosomal membrane transporters have been associated with inherited disorders. The clinical presentations of these diseases resemble the phenotypes of other LSDs; they are heterogeneous and often present in children with neurodegenerative manifestations. However, for pathomechanistic and therapeutic studies, lysosomal membrane transport defects should be distinguished from LSDs caused by defective hydrolytic enzymes. The involved proteins differ in function, localization, and lysosomal targeting, and the diseases themselves differ in their stored material and therapeutic approaches. We provide an overview of the small group of disorders of lysosomal membrane transporters, emphasizing discovery, pathomechanism, clinical features, diagnostic methods and therapeutic aspects. We discuss common aspects of lysosomal membrane transporter defects that can provide the basis for preclinical research into these disorders.

Lysosomal storage disease spectrum in nonimmune hydrops fetalis: a retrospective case control study.

OBJECTIVES: Nonimmune hydrops fetalis (NIHF) accounts for 90% of hydrops fetalis cases. About 15% to 29% of unexplained NIHF cases are caused by lysosomal storage diseases (LSD). We review the spectrum of LSD and associated clinical findings in NIHF in a cohort of patients referred to our institution. METHODS: We present a retrospective case-control study of cases with NIHF referred for LSD biochemical testing at a single center. Cases diagnosed with LSD were matched to controls with NIHF and negative LSD testing and analyzed according to the STROBE criteria to the extent the retrospective nature of this study allowed. RESULTS: Between January 2006 and December 2018, 28 patients with NIHF were diagnosed with a LSD. Eight types of LSD were diagnosed: galactosialidosis 8/28 (28.6%), sialic acid storage disease (SASD) 5/28 (17.9%), mucopolysaccharidosis VII 5/28 (17.9%), Gaucher 4/28 (14.3%), sialidosis 2/28 (7.1%), GM1 gangliosidosis 2/28 (7.1%), Niemann-Pick disease type C 1/28 (3.6%), and mucolipidosis II/III 1/28 (3.6%). Associated clinical features were hepatomegaly 16/21 (76.2%) vs 22/65 (33.8%), P < .05, splenomegaly 12/20 (60.0%) vs 14/58 (24.1%), P < .05, and hepatosplenomegaly 10/20 (50.0%) vs 13/58 (22.4%) P < .05. CONCLUSION: The most common LSD in NIHF were galactosialidosis, SASD, mucopolysaccharidosis VII, and Gaucher disease. LSD should be considered in unexplained NIHF cases, particularly if hepatomegaly, splenomegaly, or hepatosplenomegaly is visualized on prenatal ultrasound.

Sialuria-Related Intellectual Disability in Children and Adolescent of Pakistan: Tenth Patient Described has a Novel Mutation in the GNE Gene.

BACKGROUND: Sialuria is a rare inborn error of metabolism caused by excessive synthesis of sialic acid due to the mutation in the binding site of the cytidine monophosphate-sialic acid of UDPGlcNAc 2-Epimerase/ManNAc Kinase (GNE/MNK). OBJECTIVE: This is the first study investigating the molecular basis of neuronal disorders exhibiting sialuria in Pakistani children/adolescents. METHODS: The current study genotyped GNE SNPs rs121908621, rs121908622 and rs121908623 by using PCR, RFLP, and DNA sequencing methods. Socioeconomic and clinical histories were also recorded. RESULTS: Our data suggest that clinical symptoms and financial status play a significant role in conferring sialuria related Intellectual Disability (ID). SNP: rs121908623 showed G/A substitution (R263Q) in the GNE gene. CONCLUSION: We have identified one case study in Pakistan, so this makes our research a leap forward towards the identification of the 10th case study worldwide.

Finding vacuolated lymphocytes in fetal effusions improves the prenatal diagnosis of lysosomal storage diseases.

OBJECTIVES: There are many causes of fetal effusions, including the rare lysosomal storage diseases (LSDs). Vacuolated lymphocytes (VLs) are found in the blood of infants with LSDs, and their presence in fetal effusion could increase the risk of underlying LSD. METHODS: Between 2006 and 2018, all fetal effusions samples from 43 fetal multidisciplinary centers were referred to a single laboratory. Cells were counted, and, if observed, VLs were categorized and counted. Screening for LSDs was performed by metabolite analyses on amniotic fluid supernatant. The diagnosis of an LSD was confirmed by measuring the activity of the corresponding enzyme and/or mutation analysis. RESULTS: Our laboratory received 614 ascitic fluids and 280 pleural fluids sampled between 22 and 33 weeks of gestation. The final diagnosis was LSD in 16 cases (1.8%). VLs were reported in all these 16 cases, in a mix of lymphocytes with and without vacuoles. Vacuoles in VLs varied in size and number. In most cases, VLs were easy to recognize, with numerous, large, round, well-defined vacuoles, but in three cases of LSDs, VLs were atypical. CONCLUSION: The finding of VLs in fetal effusions is an inexpensive first-line test that may help to prioritize biochemical and genetic tests for LSDs.

A cross-sectional quantitative analysis of the natural history of free sialic acid storage disease-an ultra-orphan multisystemic lysosomal storage disorder.

PURPOSE: Quantitative definition of the natural history of free sialic acid storage disease (SASD, OMIM 604369), an orphan disorder due to the deficiency of the proton-driven carrier SLC17A5. METHODS: Analysis of published cases with SASD (N = 116) respecting STROBE criteria. MAIN OUTCOME PARAMETERS: survival and diagnostic delay. Phenotype, phenotype-biomarker associations, and geographical patient distribution were explored. RESULTS: Median age at disease onset was 0.17 years. Median age at diagnosis was 3 years with a median diagnostic delay of 2.5 years. Median survival was 11 years. The biochemical phenotype clearly predicted the disease course: patients with a urinary free sialic acid excretion below 6.37-fold or an intracellular free sialic acid storage in fibroblasts below 7.37-fold of the mean of normal survived longer than patients with biochemical values above these thresholds. Cluster analysis of disease features suggested a continuous phenotypic spectrum. Patient distribution was panethnic. CONCLUSION: Combination of neurologic symptoms, visceromegaly, and dysmorphic features and/or nonimmune hydrops fetalis should prompt specific tests for SASD, reducing diagnostic delay. The present quantitative data inform clinical studies and may stimulate and accelerate development of specific therapies. Biomarker-phenotype association is particularly important for both counseling parents and study design.

Exploration of the Sialic Acid World.

Sialic acids are cytoprotectors, mainly localized on the surface of cell membranes with multiple and outstanding cell biological functions. The history of their structural analysis, occurrence, and functions is fascinating and described in this review. Reports from different researchers on apparently similar substances from a variety of biological materials led to the identification of a 9-carbon monosaccharide, which in 1957 was designated "sialic acid." The most frequently occurring member of the sialic acid family is N-acetylneuraminic acid, followed by N-glycolylneuraminic acid and O-acetylated derivatives, and up to now over about 80 neuraminic acid derivatives have been described. They appeared first in the animal kingdom, ranging from echinoderms up to higher animals, in many microorganisms, and are also expressed in insects, but are absent in higher plants. Sialic acids are masks and ligands and play as such dual roles in biology. Their involvement in immunology and tumor biology, as well as in hereditary diseases, cannot be underestimated. N-Glycolylneuraminic acid is very special, as this sugar cannot be expressed by humans, but is a xenoantigen with pathogenetic potential. Sialidases (neuraminidases), which liberate sialic acids from cellular compounds, had been known from very early on from studies with influenza viruses. Sialyltransferases, which are responsible for the sialylation of glycans and elongation of polysialic acids, are studied because of their significance in development and, for instance, in cancer. As more information about the functions in health and disease is acquired, the use of sialic acids in the treatment of diseases is also envisaged.

Sialic acid catabolism by N-acetylneuraminate pyruvate lyase is essential for muscle function.

Sialic acids are important components of glycoproteins and glycolipids essential for cellular communication, infection, and metastasis. The importance of sialic acid biosynthesis in human physiology is well illustrated by the severe metabolic disorders in this pathway. However, the biological role of sialic acid catabolism in humans remains unclear. Here, we present evidence that sialic acid catabolism is important for heart and skeletal muscle function and development in humans and zebrafish. In two siblings, presenting with sialuria, exercise intolerance/muscle wasting, and cardiac symptoms in the brother, compound heterozygous mutations [chr1:182775324C>T (c.187C>T; p.Arg63Cys) and chr1:182772897A>G (c.133A>G; p.Asn45Asp)] were found in the N-acetylneuraminate pyruvate lyase gene (NPL). In vitro, NPL activity and sialic acid catabolism were affected, with a cell-type-specific reduction of N-acetyl mannosamine (ManNAc). A knockdown of NPL in zebrafish resulted in severe skeletal myopathy and cardiac edema, mimicking the human phenotype. The phenotype was rescued by expression of wild-type human NPL but not by the p.Arg63Cys or p.Asn45Asp mutants. Importantly, the myopathy phenotype in zebrafish embryos was rescued by treatment with the catabolic products of NPL: N-acetyl glucosamine (GlcNAc) and ManNAc; the latter also rescuing the cardiac phenotype. In conclusion, we provide the first report to our knowledge of a human defect in sialic acid catabolism, which implicates an important role of the sialic acid catabolic pathway in mammalian muscle physiology, and suggests opportunities for monosaccharide replacement therapy in human patients.

Hypogammaglobulinemia and imaging features in a patient with infantile free sialic acid storage disease (ISSD) and a novel mutation in the SLC17A5 gene.

Background Infantile free sialic acid storage disease (ISSD) is a severe multisystemic disorder characterized by the accumulation of free sialic acid in lysosomes. Case presentation The patient presented prenatally with fetal ascites and large scrotal hernias, without pleural or pericardial effusion. During the infantile period, he was diagnosed with permanent isolated immunoglobulin G (IgG) hypogammaglobulinemia, which thus far has rarely been associated with ISSD. The analysis of the SLC17A5 gene revealed a novel homozygous 94 bp gene deletion. We further provide a detailed description of pre- and postnatal clinical and radiographic findings. Conclusions Fetal ascites could be the first sign of several lysosomal storage diseases (LSDs), including ISSD. The analysis of LSD gene panels is an effective approach to diagnosis in the case of non-specific symptoms and when specific biochemical tests are not easily available.

Biochemical and molecular analyses of infantile sialic acid storage disease in a patient with nonimmune hydrops fetalis.

Nonimmune hydrops fetalis is the most severe clinical manifestation of lysosomal storage diseases (LSDs). Around 14 different LSDs have been accounted for as 1-15% of the cause of nonimmune hydrops fetalis. We report a Korean infant affected by an extremely rare but severe form of sialic acid storage disease. The patient presented with nonimmune hydrops fetalis, dysmorphic facial features, hepatosplenomegaly, and dysostosis multiplex and died at 39 days of age due to persistent pulmonary hypertension. LSD was suspected based on the presence of diffuse vacuolation of syncytiotrophoblast, villous stromal cells, and intermediate trophoblast in placental biopsy. Increased excretion of urinary free sialic acid was detected by liquid chromatography-tandem mass spectrometry. The patient was compound heterozygous of the c.908G>A (p.Trp303Ter) and the splicing mutation c.1259+5G>T (IVS9+5 G>T) in the SLC17A5 gene.

A New Patient With Intermediate Severe Salla Disease With Hypomyelination: A Literature Review for Salla Disease.

BACKGROUND: Likely pathogenic variants in SLC17A5 results in allelic disorders of free sialic acid metabolism including (1) infantile free sialic acid storage disease with severe global developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; (2) intermediate severe Salla disease with moderate to severe global developmental delay, hypotonia, and hypomyelination with or without coarse facial features, and (3) Salla disease with normal appearance, mild cognitive dysfunction, and spasticity. PATIENT DESCRIPTION: This five-year-old girl presented with infantile-onset severe global developmental delay, truncal hypotonia, and generalized dystonia following normal development during her first six months of life. Brain magnetic resonance imaging showed marked hypomyelination and a thin corpus callosum at age 19 months, both unchanged on follow-up at age 28 months. Urine free sialic acid was moderately elevated. Cerebrospinal fluid free sialic acid was marginally elevated. Sequencing of SLC17A5 revealed compound heterozygous likely pathogenic variants, namely, a known missense (c.291G>A) variant and a novel truncating (c.819+1G>A) variant, confirming the diagnosis of Salla disease at age 3.5 years. CONCLUSION: We report a new patient with intermediate severe Salla disease. Normal or marginally elevated urine or cerebrospinal fluid free sialic acid levels cannot exclude Salla disease. In patients with progressive global developmental delay and hypomyelination on brain magnetic resonance imaging, Salla disease should be included into the differential diagnosis.

Identification of a large intronic transposal insertion in SLC17A5 causing sialic acid storage disease.

BACKGROUND: 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. RESULTS: 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. CONCLUSIONS: 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.

Progressive leukoencephalopathy impairs neurobehavioral development in sialin-deficient mice.

Slc17a5-/- mice represent an animal model for the infantile form of sialic acid storage disease (SASD). We analyzed genetic and histological time-course expression of myelin and oligodendrocyte (OL) lineage markers in different parts of the CNS, and related this to postnatal neurobehavioral development in these mice. Sialin-deficient mice display a distinct spatiotemporal pattern of sialic acid storage, CNS hypomyelination and leukoencephalopathy. Whereas few genes are differentially expressed in the perinatal stage (p0), microarray analysis revealed increased differential gene expression in later postnatal stages (p10-p18). This included progressive upregulation of neuroinflammatory genes, as well as continuous down-regulation of genes that encode myelin constituents and typical OL lineage markers. Age-related histopathological analysis indicates that initial myelination occurs normally in hindbrain regions, but progression to more frontal areas is affected in Slc17a5-/- mice. This course of progressive leukoencephalopathy and CNS hypomyelination delays neurobehavioral development in sialin-deficient mice. Slc17a5-/- mice successfully achieve early neurobehavioral milestones, but exhibit progressive delay of later-stage sensory and motor milestones. The present findings may contribute to further understanding of the processes of CNS myelination as well as help to develop therapeutic strategies for SASD and other myelination disorders.