Brief CommunicationBiochemical and molecular findings in a patient with myoclonic epilepsy due to a mistarget of the β-glucosidase enzyme
Introduction
β-Glucosidase (βGC), a lysosomal enzyme defective in most cases of Gaucher disease (GD) [1], is target to the lysosomes through a mannose 6-phosphate receptor independent mechanism. It has been shown that the lysosomal integral membrane protein type 2 (LIMP-2), an ubiquitously expressed trans membrane protein [2] mainly found in the lysosomes and late endosomes [3], is a receptor for lysosomal mannose 6-phosphate-independent targeting of the βGC [4].
In fact, in the LIMP-2 deficient mouse the missorting of the βGC results in a reduction of intracellular βGC activity and protein levels. The phenotype is characterized by neurological and renal involvement [4].
In humans, LIMP-2 is encoded by the SCARB2 gene located on chromosome 4q13-21 [4]. Recently, an autosomal recessive disorder caused by deficiency of human LIMP-2 due to mutations in the SCARB2 gene was described in six unrelated families, three from Australia, two from Canada and one from Portugal. All of them presented with symptoms of action myoclonus-renal failure syndrome [5], [6], [7].
In this report the biochemical and molecular findings in an Italian patient affected with myoclonic epilepsy due to a mistarget of the β-glucosidase enzyme are discussed.
Section snippets
Case report
The patient is a 29-year-old female presenting with progressive myoclonic epilepsy without intellectual impairment. She manifested the first neurological symptoms at the age of 26, when she started to experience anxiety and myoclonic jerks of the lower limbs. The symptoms rapidly progressed to generalized tremors, hypotonia, ataxia and progressively worsening action myoclonus, prominent on the right upper limb. At the age of 27 she presented a single convulsive seizure and loss of
Enzyme activity assay and filipin staining
Acid β-glucosidase activity was measured using the fluorogenic substrate 4-methylumbelliferyl-β-d-glucopyranoside (Sigma, St. Louis, MO, USA) in the presence of sodium deoxytaurocholate [8].
Plasmatic chitotriosidase activity was determined using the fluorogenic substrate 4 MU-chitotrioside [9].
Filipin staining was performed using the method described by Blanchette-Mackie et al. [10].
Mutational analysis
Genomic DNA was extracted from peripheral blood leukocytes with QIAamp DNA blood Mini Kit (Qiagen GmbH, Hilden,
Results and discussion
The clinical picture of the patient prompted us to hypothesize the possible metabolic origin of the disease.
In particular, Niemann Pick type C (NPC) [12] and chronic neuronopathic GD (GD type III) [13] were considered, even if no signs of organomegaly and hematological involvement (anemia, thrombocytopenia) were evident.
Filipin staining assay clearly showed the absence of intracellular cholesterol accumulation, ruling out the diagnosis of NPC disease.
In order to determine whether the patient
Acknowledgments
The samples (patient and control subjects) were obtained from the “Cell Line and DNA Bank from Patients affected by Genetic Diseases” (G. Gaslini Institute) part of Telethon Genetic Biobank Network (Project No. GTB07001A). We gratefully acknowledge Sarah Tripepi Winteringham for her assistance with the editing of the manuscript.
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2014, Journal of the Neurological SciencesCitation Excerpt :Gene expression assays using real-time RT-PCR indicated > 80% reduction in the SCARB2 expression in the patient as compared to control (Fig. 3). After ruling out mutations in the GBA and PSAP, the striking discrepancy between lymphocyte and fibroblast GC activity in the proband indicated the possibility of a SCARB2-related disease [4,11–14]. Previously, SCARB2 mutations were identified mostly on the basis of clinical presentation, and this is the third report of a patient diagnosed as a result of initial testing for Gaucher disease [12,13,15].
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2014, Molecular Genetics and MetabolismCitation Excerpt :When GCase activity was measured, it was found that although patient fibroblasts only had 10% of control GCase activity, leukocytes and plasma showed normal activity levels, suggesting that there may be a secondary targeting mechanism by which GCase is transported to lysosomes [63]. A similar patient with PME that was heterozygous for the LIMP-2 mutation H363N had onset of symptoms at 26 years consisting of myoclonus, tremors, and ataxia [51]. In contrast to the siblings, this patient had normal GCase activity in fibroblasts [51].