Gaucher disease is an inborn error of metabolism that results from deficiency of β-glucocerebrosidase (D-glucosyl-N
acylsphingosine glucohydrolase, EC 18.104.22.168), encoded by the human GBA1
gene. The enzyme, a lysosomal glycoprotein, is an acid β-glucosidase. The range of natural substrates of β-glucocerebrosidase are mixtures of N
-β-D glucosides with varying acyl (fatty acid) and sphingosine moieties, including those like glucosylsphingosine that are devoid of fatty acids; the enzyme cleaves glucosylceramide (also called glucocerebroside) into glucose and ceramide. Glucosylceramide accumulates within the lysosomes of cells, particularly macrophages and related cells.4
Gaucher disease is usually regarded as the most common lysosomal storage disease, occurring in approximately one in every 75,000 births worldwide.6
Inherited as an autosomal recessive disorder, the condition is more prevalent in populations of Ashkenazi Jewish descent: according to studies of gene frequency, there are estimated to be approximately 20,000 individuals carrying two disease-related mutations (in trans) in the United States alone; two-thirds of these are of Ashkenazi origin.8
Not all of these individuals will develop symptomatic disease.
For operational purposes in clinical practice, Gaucher disease has conventionally been categorized into three clinical types. In all affected patients, the disease involves the visceral organs, bone marrow, and bone. Type 1 (Online Mendelian Inheritance in Man [OMIM] #230800), the so-called non-neuronopathic disease, is the most prevalent. It is distinguished from types 2 (OMIM #230900) and 3 (OMIM #231000) – the acute neuronopathic and subacute neuropathic variants, respectively – by the lack of characteristic involvement of the brain. However, intermediate phenotypes occur, particularly between types 2 and 3.9
In addition, there are reports of several neurological manifestations in type 1 patients that are distinct from the specific features that define types 2 and 3.10
The 7.6 kb GBA1
gene is located on chromosome 1q2 and comprises eleven exons. A 5 kb unprocessed pseudogene is located 16 kb downstream. More than 200 distinct GBA
gene mutations are listed in the Human Gene Mutation Database (http://www.hgmd.cf.ac.uk/ac/all.php
). Of these, more than 80% are single nucleotide substitutions. Complex alleles account for approximately 20% of mutations.12
Three mutant alleles, N370S (c.1226 A > G), L444P (c.1448T > C), and 84GG (c.84dupG), are the most prevalent.8
On account of its diverse phenotypic variability, prediction of disease severity based on genotype is only approximate.4
Individuals with one N370S allele are protected against neuronopathic disease, while homozygotes for L444P are likely to have neuronopathic features. Rare pathological features may accompany other mutations, such as the cardiovascular variant, which occurs in patients homozygous for the mutant human D409H allele.14
Acid glucocebrosidase activity localizes to the luminal side of the lysosomal membrane and shares properties with integral membrane proteins. The substrate, β-D-glucosylceramide (glucocerebroside), is a component of cell membranes and is widely abundant in circulating blood cells. The small sphingolipid activator protein, saposin C, which is critical for activity of glucocerebrosidase in vivo, may present glucocerebroside to the enzyme, as reviewed by Locatelli Hoops et al.16
Deficiency of saposin C, which occurs only rarely, results in a severe but distinct form of Gaucher disease.4
In patients affected by Gaucher disease, the deficiency of glucosylceramidase leads to accumulation of glucocerebroside and other glycolipids within the lysosomes of macrophages and other phagocytic cells of the monocyte-macrophage lineage. The tissue concentration of these compounds may be increased 20–100 times.17
The unacylated congener of glucosylceramide, glucosylsphingosine, is particularly elevated in patients with neuronopathic Gaucher disease, and it may have a role in the pathogenesis of Gaucher disease as well as in neurodegeneration.18
Macrophages, pathologically engorged with glycolipid material, are known as Gaucher cells and are a cardinal feature of the disease.20
In histological preparations stained by the Leishman method, Gaucher cells have a characteristic histological appearance of wrinkled tissue paper or crumpled silk. Membrane-bound inclusions filled with glucocerebroside are seen with electron microscopy. Gaucher cells have the protein expression profile of the so-called alternatively activated macrophage, a phenotype associated with chronic inflammation and fibrosis.22
As demonstrated by the effects of hematopoietic stem cell (bone marrow) transplantation, the clinical manifestations of Gaucher disease, with massive visceromegaly, are principally an indirect consequence of the accumulation of the pathological macrophages in the spleen, liver, and bone marrow. However, pathological lipid accumulation in macrophages accounts for less than 2% of the additional tissue mass in the liver and spleen; the additional increase is attributed to an inflammatory and infiltrative cellular response.23
Thrombocytopenia and anemia result from splenic sequestration, with a contribution from marrow failure, as shown by the long-term evolution of the disease after splenectomy. Growth failure, muscle wasting, and pubertal delay are features of the more general metabolic effects of the complex inflammatory response.24
Several pathological processes occur within bone: decreased mineral density, marrow infiltration, and infarction of bone.25
The mechanisms leading to decreased bone mineral density are uncertain, but they may involve failure to achieve peak bone mass,26
abnormal osteoclast regulation, or overproduction of cytokines/chemokines27
by activated macrophages.
In neuronopathic variants of Gaucher disease, profound failure of lysosomal sphingolipid breakdown leads to the accumulation of toxic macromolecules that originate from the rapid turnover of membrane gangliosides in neural cells. Occasional Gaucher cells are found in the Virchow-Robin spaces and sporadically within deep layers of the cerebral cortex and cerebellum. Neuronal death and neuronophagia by activated microglia occur in selected midbrain, brainstem, and cerebellar nuclei – changes that are accompanied by reactive astrogliosis.29
Delivery of many soluble lysosomal proteins to the lysosome is mediated by mannose 6-phosphate receptors and depends on the trafficking of the receptor to early endosomes in a manner requiring the interaction of its cytosolic domain with the GGAs (Golgi-localized gamma-ear containing, ADP-ribosylation factor binding family of multidomain coat proteins). These are highly conserved monomeric clathrin adaptor proteins that orchestrate the trafficking of the mannose 6-phosphate receptors and other cargo molecules from the trans-Golgi network to the endosome-lysosome system. When the mannose 6-phosphate pathway is defective, as in inclusion-cell (I-cell) disease, some soluble lysosomal proteins continue to traffic to the lysosomes: a protein, sortilin, is responsible for the independent targeting and has numerous cytosolic binding partners, including GGAs; the cytosolic domains of sortilin and mannose 6-phosphate receptors are functionally homologous and they share a common trafficking mechanism.
Sortilin mediates the trafficking of sphingolipid activator proteins, cathepsins D and H, and acid sphingomyelinase. Renal podocyte uptake of the therapeutic enzyme preparations for Fabry disease is also partly mediated by sortilin. 31
However, β-glucocerebrosidase and cathepsins K and L reach the lysosomes by a sortilin-independent process.
Once taken up by macrophages, the route of exogenous mannose-terminated acid β-glucocerebrosidase to the lysosome and related compartments is distinct from other lysosomal enzymes and shows marked cell/tissue selectivity. 32
In humans it appears to be used principally in the brain and kidney as well as in fibroblasts; it is less active in peripheral leukocytes. Nascent β-glucocerebrosidase binds via specific residues to a chaperone molecule on the lysosomal membrane, LIMP-2 – a mannose-6-phosphate-independent trafficking receptor for β-glucocerebrosidase. Patients with mutations in LIMP-2 develop the action myoclonus–renal failure syndrome and there appears to be no involvement of the macrophage system; their fibroblasts are deficient in acid β-glucosidase activity but the deficient trafficking is not apparent in enzymatic assays using preparations of peripheral white blood cells. Other mannose-6-phosphate-independent trafficking receptors for β-glucocerebrosidase may be active in human tissues, especially hematopoietic tissues.
Notwithstanding, the pathway for delivery of β-glucocerebrosidase clearly differs from that utilized by many other lysosomal proteins, in which the insulin-like growth factor 2/cation-independent mannose 6-phosphate receptor or sortilin pathway participates. Therapeutic delivery of exogenous mannose-terminated β-glucocerebrosidase appears to follow a unique, high-capacity pathway with specificity for macrophages, which are an important pathological focus in Gaucher disease.
The authors propose that two critical features which favor the unique salutary effects of enzyme treatment in Gaucher disease are:
- Apart from neuronopathic disease, the cells affected by storage are of monocyte-macrophage lineage; these infiltrate the liver, spleen, and bone marrow. Since they are of hematopoietic origin and are recruited to sites of disease, successful metabolic intervention has the potential to reverse the pathological infiltration and arrest ongoing tissue and organ damage.
- Macrophages display surface lectin-like receptors that mediate uptake into intracellular compartments to complement the enzyme deficiency and hence effective targeting to this cell type. Glucocerebrosidase appears to be unique in its targeting to lysosome and does not follow the well-described pathway of uptake for mannose-6-phosphate-receptor first demonstrated in acid hydrolases responsible for the degradation of glycosaminoglycans (mucopolysaccharides), and defective in mucolipidosis types 2 and 3 (I-cell disease).