MinireviewThe role of saposin C in Gaucher disease
Highlights
► We review the role of Saposin C in Gaucher disease and the result of Saposin C deficiency. ► We describe current theories regarding how Saposin C interacts with glucocerebrosidase. ► Saposin C is a potential modifier that could impact the phenotype in patients with Gaucher disease.
Introduction
The saposins are a set of four small glycoproteins, referred to as saposin (Sap) A-D, that act as enzymatic activators in multiple stages of lysosomal sphingolipid degradation, as well as lysosomal membrane digestion [1], [2], [3]. These four homologous proteins are generated in the endosome via proteolysis of a 73 kDa precursor protein called prosaposin (pSap) [4], [5]. The resulting Saps A-D each consist of approximately 80 amino acids, with six similarly-located cysteine residues that confer heat stability and a characteristic tertiary structure through the formation of three conserved disulfide bridges [3], [6].
Mature Saps A-D assist lysosomal hydrolases in the degradation of sphingolipids. Deficiencies or dysfunctions of these activators can result in lysosomal storage disorders that mimic by deficiencies of the enzyme activated by that particular Sap [7], [8], [9]. Prosaposin deficiency results in the accumulation of sphingolipid substrates that are hydrolyzed by Sap-associated enzymes including sulfatides, glucosylceramide, lactosylceramide, galactosylceramide, digalactosylceramide, ceramide, and GM2 and GM3 gangliosides [7], [10]. Patients with deficient pSap (OMIM ID: 611721), resemble type 2 (acute neuronopathic) Gaucher disease (GD; OMIM ID: 230900) [11]. Deficiencies of individual Saps also mimic specific disorders; Sap A deficiency resembles Krabbe disease [8], and inadequate Sap B resembles metachromatic leukodystrophy [12], [13], [14]. Six patients described with mutations in the Sap C domain of the PSAP gene (OMIM ID: 610539) have symptoms similar to either type 1 or type 3 (OMIM IDs: 230800, 231000) GD, despite normal glucocerebrosidase (GCase; EC 3.2.1.45) activity [9]. Sap C knockout mice also exhibit phenotypes most closely analogous to type 3 GD [15]. A specific deficiency of Sap D has not been reported in humans.
Saposin C has particular relevance for GD. It is a necessary activator for GCase, the enzyme deficient in this disorder due to mutations in the GBA gene (OMIM ID: 606463) [4], [16], [17]. GD is an autosomal recessive disorder, and the most common lysosomal storage disorder. Deficiency of GCase leads to accumulation of glucosylceramide in lysosomes, causing substrate storage in macrophages in the spleen, liver, bone marrow, and other organs. Patients with GD often exhibit hepatosplenomegaly, thrombocytopenia, bone lesions, and anemia [16]. In some cases, patients also develop neurological symptoms, including myoclonic epilepsy, ataxia, intellectual impairment, and abnormal horizontal saccadic eye movements [16], [18]. Clinically, GD is classified into three types, based on whether the patient displays neurological symptoms, and the age at which these first manifest [17]. Type 1 (non-neuronopathic) GD, the most common form, does not involve the central nervous system, but the severity ranges from significant morbidity in childhood due to complications from cytopenia, liver dysfunction, failure to thrive, or skeletal involvement, to patients that remain asymptomatic or undiagnosed for much of their life [17], [19].
Type 2 and type 3 GD, the acute and chronic neuronopathic forms, respectively, are characterized by neurological dysfunction [17]. Type 2 GD affects infants, who have a life expectancy of less than two or three years [17], [20]. These patients exhibit rapid neurological decline, severe hepatosplenomegaly, failure to thrive, and ultimately opisthotonus. A subgroup dies from hydrops fetalis or congenital ichthyosis before or shortly after birth [21], [22].
Type 3 GD results in a less severe phenotype than type 2 [17]. Neurological symptoms vary greatly, including myoclonus, seizures, ataxia, dementia, and slowed horizontal eye movements [16]. A subgroup of these patients display significant visceral involvement, including hepatosplenomegaly, and can have extensive bone disease.
Despite clinical categorization of GD into these three types, a wide spectrum of phenotypic heterogeneity is observed in this disorder. As an essential activator of GCase, Sap C is a potential disease modifier, and subtle changes in its expression may contribute to the array of phenotypes observed in GD.
Sap C was discovered by Ho and O'Brien in 1971 [23]. It was extracted from spleen homogenate from a 12-year-old female with type 3 GD, following splenectomy. Further experiments showed that it was heat-stable and capable of restoring mutant GCase activity in vitro [23]. The common genetic origin of Sap C and Sap B, which was discovered in 1964, was confirmed in the late 1980s, when the cDNAs encoding each protein were cloned independently, and it was found that both derive from proteolytic processing of a 73 kDa precursor protein, later confirmed to be pSap [24], [25], [26]. In total, four homologous domains were found in the pSap protein. All were approximately 80 amino acid residues in length and had similarly located cysteine and proline residues, suggesting common secondary and tertiary structures. In addition, each domain had at least one glycosylation site. These results indicated the existence of two other mature Sap proteins, which correspond to Saps A and D [27], [28], [29].
The nomenclature for the Sap proteins has evolved over the years, and thus the literature is often confusing. The current term “saposin,” derived from “sphingolipid activator protein,” was coined by O'Brien and Kishimoto [28], [29]. Sap C has taken many names, originally called factor P by its discoverers [23]. It was also referred to as heat stable factor, A1 activator, and co-β-glucosidase in the early literature. In 1984, the term sphingolipid activator protein (SAP) was applied to the two Saps known at the time. Sap B and Sap C were called SAP-1 and SAP-2, respectively [30]. The currently accepted saposin A-D nomenclature system arose from an improved understanding of the genetic basis for pSap and the mature Sap proteins. The Saps were named sequentially based on their position in the pSap amino acid sequence, starting from the N-terminus. This nomenclature identifies the individual Saps based on their amino acid sequences, rather than their activities, highlights their common origin from PSAP, and distinguishes them from other sphingolipid activator proteins [31].
Section snippets
The saposin precursor
Prosaposin is encoded by the ~ 17 kb PSAP gene, located on chromosome 10q21 [32]. The gene contains 15 exons and 14 introns. The four homologous gene domains, each encoding one Sap protein, suggest that the PSAP gene arose from multiple duplications of an ancient gene that encoded a single Sap. Three isoforms of pSap result from alternative splicing at exon 8, which consists of nine base pairs encoding three amino acids in Sap B. Human cDNA libraries contain cDNAs with all nine, the last six, or
The role of saposin C in the pathogenesis of Gaucher disease
Reduced GCase activity in GD is primarily caused by protein misfolding and improper trafficking of the enzyme to the lysosome, or the total absence of the protein resulting from null alleles [69]. However, some mutations in GCase result in diminished residual enzyme activity in vitro by causing structural or biochemical changes that interfere with the mechanism of activation by Sap C [70]. The N370S mutation causes an amino acid substitution that limits the mutant enzyme's capacity to interact
Clinical consequences of saposin C deficiency
Six patients with Sap C deficiency due to mutations in the PSAP gene have been reported in the literature, all of whom had symptoms resembling GD [9], [77], [78], [79], [80], [81], [82], [83]. Two of these patients exhibited a type 1 GD phenotype, while two had neurological involvement resembling type 3 GD [9], [77], [80]. The two remaining patients, an adult brother and sister, were originally described as having type 1 GD, but a recent update reports that they have begun to display mild
Conclusions
It is becoming increasingly apparent that GD encompasses a spectrum of phenotypes, and that there is great phenotypic heterogeneity associated with this single-gene disorder [84]. Over 250 mutations associated with GD have been identified in GBA, yet direct correlations between genotype and phenotype are limited [68]. Two notable exceptions are N370S, which is found exclusively in type 1 GD, and L444P, which when homozygous, is typically associated with neurological involvement [85]. However,
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgments
This work was supported by the Intramural Research Program of the National Human Genome Research Institute and National Institutes of Health. The authors thank Darryl Leja for his assistance with Fig. 3.
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