Free sialic acid storage disease. A new Italian case

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.

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.

Infantile Sialic Acid Storage Disease: Two Unrelated Inuit Cases Homozygous for a Common Novel SLC17A5 Mutation

Infantile sialic acid storage disease (ISSD) is a lysosomal storage disease characterized by accumulation of covalently unlinked (free) sialic acid in multiple tissues. ISSD and Salla disease (a predominantly neurological disorder) are allelic disorders caused by recessive mutations of a lysosomal anionic monosaccharide transporter, SLC17A5. While Salla disease is common in Finland due to a founder-effect mutation (p.Arg39Cys), ISSD is comparatively rare in all populations studied.Here, we describe the clinical and molecular features of two unrelated Canadian Inuit neonates with a virtually identical presentation of ISSD. Both individuals presented antenatally with fetal hydrops, dying shortly following delivery. Urinary free sialic acid excretion was markedly increased in the one case in which urine could be obtained for testing; postmortem examination showed a picture of widespread lysosomal storage in both. Both children were homozygous for a novel splice site mutation (NM_012434:c.526-2A>G) resulting in skipping of exon 4 and an ensuing frameshift. Analysis of a further 129 pan-Arctic Inuit controls demonstrated a heterozygous carrier rate of 1/129 (~0.4 %) in our sample. Interestingly, lysosomal enzyme studies showed an unexplained ninefold increase in neuraminidase activity, with lesser elevations in the activities of several other lysosomal enzymes. Our results raise the possibility of a common founder mutation presenting as hydrops in this population. Furthermore, if confirmed in subsequent cases, the marked induction of neuraminidase activity seen here may prove useful in the clinical diagnosis of ISSD.

Sialic acid storage disease and related disorders

This paper gives an overview of the two sialic acid storage disorders, Salla disease and infantile sialic acid storage disease, and the related disorders cystinosis, sialuria, sialidosis, and galactosialidosis. Sialic acid storage disease and cystinosis are models for a deficient lysosomal transport of monosaccharides and amino acids, respectively. Several gene mutations leading to the production of the faulty membrane proteins sialin and cystinosin have been identified in recent years. Knowledge of the underlying pathophysiology is a prerequisite for future research projects, which will focus on the expression of the disease genes in living systems and the physical characterization of these proteins by X-ray crystallography and nuclear magnetic resonance spectroscopy.

Sialin expression in the CNS implicates extralysosomal function in neurons

SLC17A5 encodes a lysosomal membrane protein, sialin, which transports sialic acid from lysosomes. Mutations in sialin result in neurodegenerative sialic acid storage disorders, Salla disease (SD) and infantile sialic acid storage disease (ISSD). Here we analyzed sialin in mouse central nervous system (CNS) and primary cortical and hippocampal neurons and glia. In the CNS, sialin was predominantly expressed in neurons, especially in the proliferative zone of the prospective neocortex and the hippocampus in developing brain. In nonneuronal cells and primary glial cell cultures, mouse sialin was localized into lysosomes but interestingly, in primary neuronal cultures sialin was not targeted into lysosomes but rather revealed a punctate staining along the neuronal processes and was also seen in the plasma membrane. These data demonstrate a nonlysosomal localization of sialin in neurons and would imply a role for sialin in the secretory processes of neuronal cells.

Biochemical and molecular analyses of infantile free sialic acid storage disease in North American children

The differential diagnosis of developmental delays and growth retardation in early childhood includes the allelic lysosomal sialic acid storage disorders, Salla disease and infantile free sialic acid storage disease (ISSD). These diseases, due to defective free sialic acid transport out of lysosomes, derive from mutations in the SLC17A5 gene coding for the protein sialin. We present two patients with clinical, biochemical, and molecular data indicative of lysosomal free sialic acid storage disorders. One patient, with a severe clinical course typical of ISSD, had 86-fold elevated levels of fibroblast free sialic acid, with 62% in the lysosomal fraction. His SLC17A5 mutations include a 148-bp deletion of exon 9, due to a G >A splice site mutation in position 1 of intron 9, and a 15-bp deletion (del 801-815) in exon 6. Another patient, with "intermediate severe" Salla disease, had 9-fold elevated levels of free sialic acid in cultured fibroblasts, of which 87% resided in the lysosomal fraction. This girl is compound heterozygous for the SLC17A5 mutation commonly found in Finnish Salla disease patients (R39C) and a 15-bp deletion found in ISSD patients (del 801-815). These observations emphasize the importance of considering free sialic acid disorders in infants with developmental delays and growth retardation, regardless of whether they are of Finnish ancestry.

Infantile sialic acid storage disease and protein-losing gastroenteropathy

We report on a boy who presented at birth with gastroschisis and thereafter developed the characteristic clinical symptoms of infantile sialic acid storage disease within the first two months of life. Measurements of free sialic acid excretion (tenfold increase) in the urine and a 15-fold elevation of free sialic acid in cultured fibroblasts proved the diagnosis. The clinical course was complicated by hypertrophic cardiomyopathy, recurrent infections, hypothyroidism, and intestinal protein losses, which had never been described before in an infantile sialic acid storage disease patient. The child died at the age of 10 months. Clinical and laboratory findings are discussed and compared with other cases described in the literature.

An Italian severe Salla disease variant associated with a SLC17A5 mutation earlier described in infantile sialic acid storage disease

The present study reports two Italian brothers affected by severe Salla disease (sialic acid storage disease), a slowly progressive autosomal recessive neurodegenerative disorder prevalent in the Finnish population. Mutations of the SLC17A5 gene, which encodes a protein called sialin, are the primary cause of both Salla disease and infantile sialic acid storage disease (ISSD), a clinically distinct severe disorder. All Finnish patients with Salla disease show a R39C mutation. Both patients showed moderate intellectual disability, spastic ataxic syndrome, hypomyelination and cerebellar atrophy on magnetic resonance imaging (MRI), and lysosomal storage, all typical of Salla disease. Mutation analysis of the SLC17A5 gene in the younger brother revealed no R39C mutation, but a 15-bp deletion in exon 6 on one of the alleles. This mutation is the same described in French-Canadian patients with ISSD. Salla disease must be suspected in patients with unexplained psychomotor retardation associated with ataxia and/or pyramidal symptoms, and MRI findings consistent with cerebral hypomyelination, irrespective of the patient's ethnic origin. A mutation screening based on R39C change does not exclude Salla disease outside Finland. Conversely, mutations found in ISSD can be expected, even in patients showing the Salla phenotype (e.g. symptoms at the milder end of the spectrum).

Multiple neuroendocrine disorder in Salla disease

Salla disease represents the slowly progressive adult form of the sialic acid storage diseases, a group of autosomal-recessive neurodegenerative disorders in which psychomotor development, ataxia, axial hypotonia, and spasticity in the lower limbs occur. No skeletal dysostosis or organomegaly is present, and life expectancy is normal. Short stature can also be observed. Progressive cerebral and cerebellar atrophy associated with dysmyelination and corpus callosum hypoplasia have been shown by magnetic resonance imaging studies. We report the first patient with Salla disease in whom combined growth hormone and gonadotropin deficiencies, hypothalamic pituitary in origin, have been demonstrated by neuroendocrine studies. We believe that the multiple neuroendocrine disorder may be the consequence of the abnormalities of common neuronal pathways regulating growth hormone and gonadotropin synthesis or secretion related to the brain storage of free sialic acid and/or to the neurodegenerative process occurring in Salla disease. Therefore, a complete endocrinologic evaluation of these patients is both warranted and useful.

The spectrum of SLC17A5-gene mutations resulting in free sialic acid-storage diseases indicates some genotype-phenotype correlation

Lysosomal free sialic acid-storage diseases include the allelic disorders Salla disease (SD) and infantile sialic acid-storage disease (ISSD). The defective gene, SLC17A5, coding for the lysosomal free sialic acid transporter was recently isolated by positional cloning. In the present study, we have identified a large number of mutations in SLC17A5 in patients presenting with either Salla disease or the ISSD phenotype. We also report for the first time the exon-intron boundaries of SLC17A5. All Finnish patients with SD (n=80) had a missense mutation changing a highly conserved arginine to cysteine (R39C); 91% of them were homozygotes for this old founder mutation. The compound-heterozygote patients, with the founder mutation in only one allele, presented with a more severe phenotype than did the homozygote patients. The same R39C mutation was also found both in most of the Swedish patients with SD and in a heterozygous form in five patients from central Europe who presented with an unusually severe (intermediate) SD phenotype. Ten different mutations, including deletions, insertions, and missense and nonsense mutations, were identified in patients with the most severe ISSD phenotype, most of whom were compound heterozygotes. Our results indicate some genotype-phenotype correlation in free sialic acid-storage diseases, suggesting that the phenotype associated with the homozygote R39C mutation is milder than that associated with other mutations.

Lysosomal transport disorders

In the group of lysosomal storage diseases, transport disorders occupy a special place because they represent rare examples of inborn errors of metabolism caused by a defect of an intracellular membrane transporter. In particular, two disorders are caused by a proven defect in carrier-mediated transport of metabolites: cystinosis and the group of sialic acid storage disorders (SASD). The recent identification of the gene mutations for both disorders will improve patient diagnosis and shed light on new physiological mechanisms of intracellular trafficking.

A new metabolite contributing to N-acetyl signal in 1H MRS of the brain in Salla disease

OBJECTIVE

To determine whether N-acetylaspartate (NAA) is reduced in patients with Salla disease, a neurodegenerative disorder.

BACKGROUND

1H MRS allows the brain metabolism to be studied noninvasively in vivo. N-acetyl (NA) is composed primarily of NAA, which is regarded as a neuronal marker. The NA signal in 1H MRS is reduced in several neurodegenerative disorders. Increased NA signal has thus far only been found in Canavan's disease as a result of NAA accumulation in the brain tissue. In Salla disease, an autosomal recessive free sialic acid storage disorder, N-acetylneuraminic acid (NANA), accumulates in lysosomes of brain tissue.

METHODS

The authors studied eight patients with Salla disease (age range, 6 to 44 years) and eight age-matched healthy volunteers using quantitative 1H MRS. The spectra were obtained from two selected 8-cm3 volumes of interest localized in the basal ganglia and in the parietal white matter using conventional 1.5-T MRI equipment. The spectral resonance lines of NA groups, creatine and phosphocreatine (Cr), and choline-containing compounds (Cho) were analyzed quantitatively. All MR images were evaluated to verify the state of myelination.

RESULTS

1H MRS from parietal white matter revealed 34% higher NA and 47% higher Cr concentrations, and a 35% lower Cho concentration in the patients with Salla disease compared with the age-matched control subjects. The patients had a 22% higher water content in their parietal white matter, whereas in the basal ganglia the water concentrations did not differ significantly. In the patients' basal ganglia the Cr concentration was 53% higher.

CONCLUSIONS

NAA is considered to be a neuronal marker that, except for Canavan's disease, has been found or assumed to be either stable or reduced. However, in Salla disease the high NA signal may have a contribution from accumulated lysosomal NANA, which offsets the possible loss of NAA. The high Cr is in line with the increased glucose uptake found in our earlier 2-fluoro-2-deoxy-D-glucose-PET study, reflecting increased energy demand. It is worth noting that in a conventional 1H MRS ratio-based analysis these underlying abnormalities would have remained undetected. Our study thus emphasizes the importance of a quantitative assessment of metabolite concentrations in 1H MRS for detecting altered brain metabolism.

Salla disease--rare or underdiagnosed?

Salla disease is described in two English children. Eighty-seven of the 89 cases so far reported come from Finland. It may be genuinely rare outside Finland or possibly underdiagnosed. Although a lysosomal disorder, it lacks many of their more characteristic features. Deterioration, for example, in the paediatric age range is rare. The clinical features are, however, consistent and specific. Definitive diagnosis is achieved by demonstrating increased amounts of free sialic acid in cultured skin fibroblasts. If the colorimetric method in widespread use is employed for this, a false negative result may be obtained. High-pressure liquid chromatography is sufficiently sensitive. It is possible therefore that Salla disease is under-reported, both from lack of clinical awareness and from lack of appropriate laboratory confirmation.

Salla disease variant in a Dutch patient. Potential value of polymorphonuclear leucocytes for heterozygote detection

A Dutch child with psychomotor retardation, impaired speech, ataxia, sialic acid storage and vacuolized skin fibroblasts and lymphocytes was diagnosed as having free sialic acid storage disease. Slight corneal opacities, pale optic disks at the fundus oculi and vertebral abnormalities, not earlier reported in Salla disease, were peculiar to this case. Free sialic acid was about tenfold increased in urine and cultured fibroblasts, without changes in the glycoconjugate-bound sialic acid pool. A subsequent pregnancy of the patient's mother was monitored by assay of sialic acid in chorionic villi and amniotic fluid. An unaffected foetus was predicted. Sialic acid was also assayed in peripheral blood total leucocytes, and in mononuclear and polymorphonuclear (PMN) leucocyte subpopulations. Each of these leucocyte fractions from the patient showed 10- to 30-fold increase in sialic acid content. The PMN subpopulation provided the most restricted range of control values and showed slightly increased values for the patient's parents. These results suggest that the assay of sialic acid in PMN might be useful for the identification of heterozygotes in sialic acid storage disease. Studies on a larger number of obligate heterozygotes are needed to confirm this observation.

Sialic acid storage diseases. A multiple lysosomal transport defect for acidic monosaccharides

A defective efflux of free sialic acid from the lysosomal compartment has been found in the clinically heterogeneous group of sialic acid storage disorders. Using radiolabeled sialic acid (NeuAc) as a substrate, we have recently detected and characterized a proton-driven carrier for sialic acid in the lysosomal membrane from rat liver. This carrier also recognizes and transports other acidic monosaccharides, among which are uronic acids. If no alternative routes of glucuronic acid transport exist, the disposal of uronic acids can be affected in the sialic acid storage disorders. In this study we excluded the existence of more than one acidic monosaccharide carrier by measuring uptake kinetics of labeled glucuronic acid [( 3H]GlcAc) in rat lysosomal membrane vesicles. [3H]GlcAc uptake was carrier-mediated with an affinity constant of transport (Kt) of 0.3 mM and the transport could be cis-inhibited or trans-stimulated to the same extent by sialic acid or glucuronic acid. Human lysosomal membrane vesicles isolated from cultured fibroblasts showed the existence of a similar proton-driven transporter with the same properties as the rat liver system (Kt of [3H]GlcAc uptake 0.28 mM). Uptake studies with [3H]NeuAc and [3H]GlcAc in resealed lysosome membrane vesicles from cultured fibroblasts of patients with different clinical presentation of sialic acid storage showed defective carrier-mediated transport for both sugars. Further evidence that the defective transport of acidic sugars represents the primary genetic defect in sialic acid storage diseases was provided by the observation of reduced, half-normal transport rates in lymphoblast-derived lysosomal membrane vesicles from five unrelated obligate heterozygotes. This study reports the first observation of a human lysosomal transport defect for multiple physiological compounds.