1. Introduction
Sphingolipids (SLs) comprise a very large number of amphiphilic lipid molecules, having in common a long chain aminoalcohol referred to as a sphingoid base. They are present in plants and animals, particularly in mammals. Owing to their chemical structure, most of them are constituents of cell membranes and preferentially locate in the outer leaflet of the plasma membrane. Similar to diacylglycerolipids, some of the major SLs are also components of circulating lipoproteins. Of note, sphingosine 1-phosphate (S1P) is typically found in the extracellular compartment, being relatively abundant in plasma and lymph. Membrane SLs, which are, by far, the predominant SL species, contain one molecule of fatty acid. In accordance with previous classifications of lipids, where the term “lyso” denotes a glycero(phospho)lipid lacking a radyl group
[1], lysosphingolipids (lysoSLs) are defined as SLs that lack the fatty acyl moiety.
SLs are synthesized by all mammalian cells. Their biosynthesis starts in the endoplasmic reticulum (ER) by the condensation of an acyl-CoA, mainly palmitoyl-CoA, and L-serine to form the backbone of the sphingoid base. Reduction of this initial product leads to sphinganine, which is N-acylated, i.e., linked to a fatty acyl moiety through an amide bond by a ceramide synthase. After desaturation, the resulting ceramide molecule can be further transformed into complex SLs, such as sphingomyelin or glycosphingolipids. Whereas sphingomyelin and β-glucosylceramide-based glycolipids are synthesized in the Golgi apparatus, β-galactosylceramide is formed in the ER. They all are transported to the plasma membrane. The degradation of diet-derived SLs is mediated by secreted intestinal enzymes. As to cellular SLs, their physiological catabolism mostly occurs in the lysosomes through a conserved sequence of hydrolytic steps (see Figure 1). In this pathway, the breakdown of complex SLs gradually releases the residues of the hydrophilic headgroup attached to the ceramide backbone. The last step of lysosomal degradation is catalyzed by acid ceramidase, which cleaves the amide bond and, thus, liberates sphingosine and fatty acids.
Figure 1. Sphingolipid catabolism and associated diseases. Green names correspond to the enzyme names, blue names to their activators, the red boxed texts contain the disease name, and the red and italic names indicate the corresponding lysoSL. Abbreviations: α-Gal: alpha-galactosidase; ACDase, acid ceramidase; Aryl A, arylsulfatase A; β-Gal, beta-galactosidase; Cer, ceramide; GalCase, galactosylceramidase; GalSph, galactosylsphingosine; GCase, glucosylceramidase; GlcCer, glucosylceramide; GlcSph, glucosylsphingosine; Hex, hexosaminidase; LacCer, lactosylceramide; LysoSulf, lysosulfatide; Sap, saposin; SK, sphingosine kinase; SM, sphingomyelin; SMase, sphingomyelinase; Sph, sphingosine; Sulf, sulfatide; S1P, sphingosine 1-phosphate.
Knowledge of SLs and SL metabolism has been strongly stimulated by the existence in humans of genetic conditions characterized by the accumulation of selective undegraded SL molecules. Extensive biochemical and genetic studies on these diseases, named sphingolipid storage disorders (or sphingolipidoses), led to the identification of their underlying defects more than 50 years ago. These disorders result from the disruption of the lysosomal catabolic pathway due to the deficient function of one of the hydrolases, either because this enzyme or its so-called activator protein (sphingolipid activator protein) is mutated, leading to profoundly decreased catalytic activity
[2,3][2][3]. The catabolic pathway implicating these enzymatic and activator proteins is depicted in
Figure 1, and the lipid molecules that accumulate in the corresponding sphingolipid storage disorders are listed in
Table 1. As the entry of SLs into the endolysosomal compartment is permanent, although variable, depending on cell type and external conditions, the enzymatic defect translates into substrate accumulation in the lysosomes (see
Table 1). Abnormally elevated concentrations of the undegraded SLs can be observed in organs as well as in biological fluids such as blood and urine; the presence of the accumulated SLs in the extracellular milieu may be explained by cell damage or lysosomal exocytosis.
Table 1. Primary accumulated SLs and lysoSLs in sphingolipid storage diseases. Of note, genetic conditions in which the accumulation of some SLs occurs as a secondary phenomenon are not listed here. The plasma concentrations of lysosphingolipids are indicated in brackets (control values).
The clinical presentation of patients affected with sphingolipid storage disorders is quite diverse, ranging from isolated visceral symptomatology to severe and fatal neuronopathic forms. A factor that determines organ involvement is the tissue distribution of the accumulated SLs. For instance, a neurological disease develops when the metabolism of GM1 and GM2 gangliosides or sulfatides and galactosylceramide, which are key components of neurons and myelin, respectively, are not properly degraded. On the other hand, the defective turnover of glucosylceramide (GlcCer) or sphingomyelin (SM), which are widely distributed, affects cells such as professional phagocytes, i.e., monocytes-macrophages. Of importance for the discussion below, the age of disease onset can vary widely, ranging from the antenatal period up to late adulthood. This variability is explained, at least partially, by the residual activity of the affected enzyme: the lower the activity is, the more severe the lipid storage and the symptomatology are
[4][21].
As already mentioned, the deficient activity of a lysosomal hydrolase results in the accumulation of its primary substrate. However, biochemical studies performed as early as in the mid-seventies have also documented the presence and storage of deacylated SLs, e.g., galactosylsphingosine (GalSph), also called psychosine, and glucosylsphingosine (GlcSph), in organs of patients affected with Krabbe and Gaucher diseases, respectively
[5,6][8][22]. The discovery of these two lysoSLs was soon followed by the observation that other similarly N-deacylated SLs accumulate in distinct sphingolipidoses (see
Table 1). Abnormally high levels of lysoSLs are also found in patients’ biological fluids such as plasma, urine, and cerebrospinal fluid. The metabolic source of lysoSLs has long been debated. Initially, it was postulated that the synthesis of a lysoSL follows the same biosynthetic route as the conventional lipid but uses a sphingoid base rather than ceramide. Recent evidence has shown that instead, GalSph, GlcSph and lysoGb3 are generated by the action of acid ceramidase on their corresponding SL in the lysosomal compartment, i.e., where the parental lipid is stored
[7,8][23][24]. In fact, the elimination of GalSph production by ablating acid ceramidase in a mouse model of Krabbe suppressed the behavioral and histopathological features of leukodystrophy
[9][25]. Whether other lysoSLs are produced by acid ceramidase still requires further investigation. The fatty acid amide hydrolase (FAAH) enzyme has been reported to partially account for the production of lysosulfatide
[10][26].
Finally,
for the p
urpose of the preresent
paperresearch, one should recall that sphingosine 1-phosphate (S1P) is also a lysoSL. This lysophospholipid is mainly formed by the sphingosine-kinase-mediated phosphorylation of sphingosine and by the autotaxin-mediated breakdown of lysoSM
[11][27]. It is still unknown whether a ceramidase can generate S1P from ceramide 1-phosphate. So far, with the possible exception of Gaucher disease, neither accumulation of S1P nor ceramide 1-phosphate has been reported in sphingolipid storage disorders.
2. Cancer Prevalence in Sphingolipid Storage Disorders
Gaucher disease (GD) is the most frequent sphingolipid storage disease, of which the deficiency of lysosomal glucosylceramidase (GCase), encoded by the
GBA gene, results in an accumulation of GlcCer and GlcSph (
Table 1). Three disease forms are described: type 1 is the most frequent, with hepatosplenomegaly, anemia, thrombocytopenia, bone abnormalities, and lung complications without neurological involvement, the latter being only present in types 2 and 3
[28] (
Table 2). Numerous case reports, case series, and cohort studies have described the occurrence of malignancies in Gaucher patients and have suggested that the risk of developing cancer is increased in Gaucher patients with a relative risk evaluated at 1.7 (95% confidence interval (95% CI) 1.3–2.3)
[29]. In the French Gaucher Disease Registry (FGDR) (n = 657 patients),
wresearche
rs report that 27 patients presented malignancies (4.1%)
[30]. Compared to data from the French population (obtained from the International Agency for Research on Cancer (
https://gco.iarc.fr/today/home) accessed on 31 March 2021),
researchers o
ur observations suggest a marked increase in the prevalence of malignancies at an odds ratio (OR) of 1.8 (95% CI 1.03–2.81) (
Table 3). The risk of multiple myeloma seems to be particularly marked, with an OR of 23.8 (95% CI 4.9–70.2) (see
Table 2 and
Table 3), which is very consistent with the literature
[29,31,32,33][29][31][32][33]. Nevertheless, these results should be interpreted with caution because of limits relative to study design. In addition, an association with monoclonal gammopathy of undetermined significance (MGUS), a pre-malignant condition for multiple myeloma, is also suspected
[32]. In the FGDR, MGUS has been found in 59 patients (25%)
[34]. Besides the risk of hematological malignancies, the prevalence of digestive cancer and, especially, hepatocellular carcinoma in Gaucher patients also seems to be increased
[35]. However, in the FGDR, no hepatocellular carcinoma has been observed. Interestingly, in the FGDR, five Gaucher patients presented two cancers, a finding that has been previously described
[31,33,36][31][33][36] (see
Table 3).
Table 2. Overview of malignancies described in patients affected with Gaucher disease, Fabry disease, Niemann–Pick B disease, or metachromatic leukodystrophy.
Table 3.
Cancer prevalence in the French Gaucher Disease Registry and French population.