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Heterozygous loss-of-function mutations at the glucosecerebrosidase locus have recently been shown to be a potent risk factor for Lewy body disease. Based on this observation, we have re-evaluated the likelihood that the different PARK loci (defined using clinical criteria for disease) may be misleading attempts to find common pathways to pathogenesis. Rather, we suggest, grouping the different loci which lead to different Lewy body disease may be more revealing. Doing this, we suggest that several of the genes involved in disparate Lewy body diseases impinge on ceramide metabolism and we suggest that this may be a common theme for pathogenesis.
Parkinson’s disease (PD) is a common neurodegenerative disease which affects over 1% of people over the age of 65 years . Clinical manifestations include bradykinesia, rigidity, tremor and postural instability. From a pathological perspective, PD is characterized by dopamine neuron degeneration, which leads to depigmentation of the substantia nigra. In addition, typical PD cases have intracellular proteinaceous inclusions called Lewy bodies and Lewy neurites in the brainstem and cortical areas.
Genetic research in the past decade has changed the view of PD from an archetypical non-genetic disease to one having a clear genetic basis in a percentage of patients . Five genes have been cloned in which mutations cause parkinsonism in a mendelian fashion [3–8] (Table 1).
Table 1. Genes that cause parkinsonism in a mendelian fashion.
|Locus||Inheritance||Age at onset (years)||Chromosome||Pathology||Gene|
|PARK-1||AD||35–65||4q||Lewy body inclusions||SNCA|
|PARK-2||AR||7–60||6p||Usually no Lewy body inclusions||PRKN|
|PARK-8||AD||45–57||12q12||Usually Lewy body inclusions||LRRK2|
Classically, the approach taken to the study of genetic forms of PD has relied on a clinical definition of disease and PARK loci have been assigned on this clinical basis. It is known what clinical features are primarily associated with each locus and a great deal of attention has been focused on this association . However, if one wants to identify pathways of pathogenicity for a given disorder, arguably, one should start by analyzing the genetics of disease based on pathology. In this minireview, we start from the position that it is more likely to find a common pathway if there is a common pathology rather than common clinical characteristics. We and others have suggested that, for the early onset recessive diseases (encoded at the parkin, PINK1 and DJ-1 loci), in which Lewy bodies are either usually absent (parkin) or where no neuropathological data are available (PINK1 and DJ-1), the evidence for a mitochondrial pathway to cell death is overwhelming .
The inspiration for our attempt to re-evaluate a Lewy body pathway to cell death has come from the recent observation that mutations in glucosecerebrosidase (GBA) when homozygous, lead to Gaucher’s disease but when heterozygous, predispose to PD . GBA catalyzes the breakdown of glucosecerebroside to ceramide and glucose. Gaucher’s disease is caused by a lysosomal build up of glucosecerebroside, but this occurs only when GBA activity is almost completely lost. In the heterozygous state this is unlikely to be a problem. We therefore began to consider that ceramide metabolism, more generally, may be an initiating problem in PD.
The genes associated with Lewy bodies dealt with here are presented in Table 2. These are divided into three categories: (a) genes clearly involved in ceramide metabolism and that cause diseases in which Lewy bodies are known to be abundant; (b) genes that may be involved in ceramide metabolism and cause diseases in which Lewy bodies have been described; and (c) genes for which, although they do give rise to Lewy body disease, there is currently little or no evidence suggesting a role in ceramide metabolism.
Table 2. Genes associated with Lewy body inclusions and their role potential role in ceramide metabolism
|Ceramide metabolism and Lewy body inclusions||GBA||1q21||Lysosomal hydrolase||Gaucher’s disease/Parkinson’s disease in heterozygotes|
|PANK2||20p13-p12.3||Pantothenate kinase||Neurodegeneration with brain iron accumulation type 1 (NBIA-1)|
|PLA2G6||22q13.1||A2 phospholipase||Neurodegeneration with brain iron accumulation 2 (NBIA2)|
|Probably ceramide metabolism; possibly Lewy body inclusions||NPC1||18q11-q12||Regulation of intracellular cholesterol trafficking||Niemann–Pick disease type C1|
|SPTLC1||9q22.1-22.3||Transferase activity||Hereditary sensory neuropathy type I (HSN1)|
|Possibly ceramide metabolism; definite Lewy body inclusions||SNCA||4q21||Dopamine transmission and synaptic vesicle dynamics||Parkinson’s disease|
|Unknown ceramide; usually Lewy body inclusions||LRRK2||12q12||Protein kinase||Parkinson’s disease|
Levels of cellular ceramide are regulated by the de novo pathway and the recycling pathway. The former relates to the synthesis of ceramide through the condensation of palmitate and serine in a series of reactions that are ultimately dependent on co-enzyme A. The latter is slightly more intricate, because several outcomes are possible depending on the enzymes involved. The simplified metabolism is shown in Fig. 1.
Figure 1. Simplified representation of ceramide metabolism. C1PP, phosphatase; CDse, ceramidase; CerS, ceramide synthase; CGT, UDP glycosyltransferase; CK, ceramide kinase; CS, ceramide synthase; DES, desaturase; GALC, galactosylceramidase; GBA, glucosylceramidase; GCS, glucosylceramide synthase; SMS, sphingomyelin synthase; SMse, sphingomyelinase; SPT, serine palmitoyl transferase. Yellow represents enzymes directly involved in ceramide metabolism, in which mutations are associated with Lewy body inclusions. Adapted from Ogretmen and Hannun .
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The gene GBA encodes a lysosomal enzyme, glucocerebrosidase, that catalyzes the breakdown of the glycolipid glucosylceramide to ceramide and glucose . Over 200 mutations have been described in GBA, most of which are known to cause Gaucher’s disease, in the homozygous or compound heterozygous condition . Gaucher patients typically present enlarged macrophages resulting from the intracellular accumulation of glucosylceramide. These patients show increased levels of the enzyme’s substrate indicates that pathogenic variants act as loss-of-function mutations. GBA mutations, in addition to causing Gaucher’s disease when homozygous, have recently been established to act as a risk factor for PD [13,14] and for Lewy body disorders .
Neurodegeneration with brain iron accumulation-1 (NBIA-1), formerly known as Hallervorden–Spatz disease is a form of neurodegeneration caused by mutations in the pantothenate kinase gene, PANK2. Clinically, the condition is characterized by progressive rigidity, first in the lower and later in the upper extremities. Both involuntary movements and rigidity may involve muscles supplied by cranial nerves, resulting in difficulties in articulation and swallowing. Mental deterioration and epilepsy occur in some. Onset is in the first or second decade and death usually occurs before the age of 30 years . Neuropathological studies have shown that patients with NBIA-1 present extensive Lewy bodies [17–19]. Pantothenate kinase is an essential regulatory enzyme in co-enzyme A biosynthesis, catalyzing the cytosolic phosphorylation of pantothenate (vitamin B5), N-pantothenoylcysteine and pantetheine . PANK2 is also involved in ceramide metabolism as the de novo pathway for ceramide formation relies on the presence of co-enzyme A . Hence, there is a direct, though not specific, connection to ceramide metabolism.
Neurodegeneration with brain iron accumulation-2 (NBIA-2) is characterized by the disruption of cellular mechanisms leading to the accumulation of iron in the basal ganglia. Mutations in the gene PLA2G6 were recently described as the cause of NBIA-2 . Phenotypically similar to NBIA-1, Lewy bodies were also described in patients with NBIA-2, particularly in the brainstem nuclei and cerebral cortex . PLA2G6 belongs to the family of A2 phospholipases, which catalyze the release of fatty acids from phospholipids and play a role in a wide range of physiologic functions . Interestingly, it has been recently demonstrated that PLA2G6 plays a role in the ceramide pathway; activation of this enzyme promotes ceramide generation via neutral sphingomyelinase-catalyzed hydrolysis of sphingomyelins . Similarly to what happens with GBA or PANK2, mutations in PLA2G6 that diminish its activity are expected to reduce the levels of ceramide formed through the breakdown of sphingomyelin.
Niemann–Pick type C (NPC) disease is an autosomal-recessive lipid storage disorder characterized by progressive neurodegeneration with a highly variable clinical phenotype. Patients with the ‘classic’ childhood-onset type C usually appear normal for 1 or 2 years with symptoms appearing between 2 and 4 years. They gradually develop neurologic abnormalities which are initially manifested by ataxia, grand mal seizures and loss of previously learned speech. Spasticity is striking and seizures are common . Approximately 95% of cases are caused by mutations in the NPC1 gene, referred to as type C1. This gene encodes a putative integral membrane protein containing motifs consistent with a role in the intracellular transport of cholesterol to post-lysosomal destinations. Cells from NPC subjects show a decrease in acid sphingomyelinase activity, leading to the accumulation of sphingomyelin . Because one of the pathways for ceramide recycling is the sphingomyelin pathway, it is conceivable that in addition to the accumulation of sphingomyelin, a decrease of ceramide may also be present. Some cases of NPC1 have been described as presenting Lewy bodies .
Mutations in SPTLC1 are the cause of hereditary sensory neuropathy type I , a dominantly inherited sensorimotor axonal neuropathy with onset in the first or second decades of life. SPTLC1 is a key enzyme in sphingolipids biosynthesis, catalyzing the pyridoxal-5-prime-phosphate-dependent condensation of l-serine and palmitoyl-CoA to 3-oxosphinganine . Patients usually present neuropathic arthropathy, recurrent ulceration of the lower extremities, signs of radicular sensory deficiency in both the upper and the lower extremities without any motor dysfunction ; restless legs and lancinating pain are other presentations of the disorder, which often results in severe distal sensory loss and mutilating acropathy . Although mutations in SPTLC1 cause neurological disease, there is, as yet, no description of the pathology of the disorder. We would hypothesize that this disease will have Lewy body pathology.
Kufor–Rakeb syndrome is a form of autosomal recessive hereditary parkinsonism with dementia. It was recently described that loss-of-function mutations in the predominantly neuronal P-type ATPase gene ATP13A2 are the cause of Kufor–Rakeb syndrome . The clinical features of Kufor–Rakeb syndrome are similar to those of idiopathic Parkinson’s disease and pallidopyramidal syndrome, including mask-like face, rigidity and bradykinesia . Although ATP13A2 does not play an obvious role in the ceramide pathway it is a lysosomal transport protein thought to be responsible for the maintenance of the ideal pH in the lysosome. This function, albeit potentially implying a much broader effect of mutations, might also mean that ATP13A2 may be related to the recycling pathways of ceramide metabolism. Interestingly, it has been suggested that α-synuclein turnover may occur via chaperone-mediated autophagy, a specialized form of lysosomal turnover [35–39]. It has also been shown that α-synuclein turnover is slowed in mouse models of lysosomal storage disorders .
α-Synuclein (SNCA) is the major component of Lewy bodies and mutations in this gene are a rare cause of PD. Only three point mutations have been described to date, but duplication and triplication of the entire SNCA locus has also been discovered [3,41–45]. PD cases with underlying SNCA mutations have extensive Lewy bodies, because these mutations are known to increase aggregation of the protein . SNCA may also be involved, albeit in a more indirect manner, in the ceramide pathway. It has been shown that deletion of the gene decreases brain palmitate uptake  and that the presence of palmitic acid increases the de novo synthesis of ceramide significantly . However, known pathogenic mutations in SNCA are likely gain-of-function mutations, suggesting that, in these cases, the mutations drive the aggregation of α-synuclein, whereas in cases where ceramide metabolism is affected, Lewy body inclusions may be a cellular response to this altered ceramide metabolism. Also connecting the ceramide pathway to α-synuclein deposition is the recent description of an increase in α-synuclein inclusions in Caenorhabditis elegans when LASS2, a ceramide synthase, is knocked-down . This result should obviously be taken with some caution, because it was obtained in a non-mammalian organism, but nevertheless it further connects ceramide to synuclein deposition.
Mutations in the gene encoding the leucine-rich repeat kinase 2 (LRRK2) are a common cause of PD [50–52]. The function of LRRK2 is not clear, but it has been shown to possess two enzymatic domains as well as several potential protein–protein interaction motifs . The phenotype attributed to LRRK2 PD is usually not different from the idiopathic form of the disease . However, discrepant results have been presented by neuropathological studies; whereas some cases have no Lewy bodies , most have typical Lewy body disease . The mechanism of this variability is not clear. Similarly, it is not obvious that LRRK2 plays a role in the ceramide pathway as no studies of this question have been published to date.
In this minireview, we have brought together data suggesting that some of the genes involved in the genetics of Lewy body disease, have in common the fact that they impinge on ceramide metabolism. One shortfall of the present theory is the lack of neuropathological data regarding cases with PINK1 or DJ-1 mutations. However, we may see studies addressing this same issue in the near future.
A major premise of this theory is the fact that Lewy body inclusions should have a key role in our understanding of the mechanisms of the disease. We propose that pathology data will, in most cases, be more insightful than clinical data in defining the disease. This is based on what we have learned from other neurodegenerative diseases with inclusion pathology. For Alzheimer’s disease, when pathology was used as a basis to understand the disease, pathways involved became evident. This would be most unlikely to happen if, instead, clinical data was used.
These data are incomplete and there have been few relevant studies directly addressing neuronal ceramide metabolism in this context. However, the hypothesis we present has the benefit of making several predictions amongst which are: (a) mutations in other genes which alter neuronal ceramide metabolism should lead to Lewy body diseases, and plausibly ATP13A2 and hereditary sensory neuropathy type I mutation carriers should have Lewy bodies; and (b) α-synuclein and LRRK2 should have roles in ceramide metabolism.
This notion also suggests that it may be profitable to consider other genes in these pathways as risk factors for Lewy body disease, and in particular, to consider whether they influence the penetrance of the GBA mutations.