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Ullal AJ, Pham H, Singh R, Ross P, Graham CA, Norton SM, Nuffer MH, Burns DS, Eckhardt AE, Escolar M, Bali D, Pamula VK. Fluorimetric assay with a novel substrate for quantification of galactocerebrosidase activity in dried blood spot specimens. Pract Lab Med 2019; 18:e00141. [PMID: 31720353 PMCID: PMC6838535 DOI: 10.1016/j.plabm.2019.e00141] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2019] [Revised: 09/19/2019] [Accepted: 10/08/2019] [Indexed: 12/02/2022] Open
Abstract
Background Decreased galactocerebrosidase (GALC) enzyme activity is causative for Krabbe disease, a lysosomal storage disorder with devastating neurodegenerative consequences. Quantitative fluorimetric assays for GALC activity in isolated blood and skin cells have been described; however, no such assay has been described using dried blood spot (DBS) specimens. Methods GALC enzyme activity was measured quantitatively using fluorescence from a novel glycosidic substrate: carboxy derived from 6-hexadecanoylamino-4-methylumbelliferone. GALC activity was demonstrated on newborn DBS specimens, known Krabbe disease patient specimens, proficiency testing and quality control samples. Results We present data on characterization of the novel substrate and assay, including pH optimization and enzyme kinetics using a fluorimetric profile. Single and multi-day precision analyses revealed tight analytical measurements with %CV ranging from 5.2% to 14.1%. GALC enzyme activity was linear over the range of 0.31 - 12.04 μmol/l/h with a limit of detection of 0.066 μmol/l/h. Our results with this assay show a clear discrimination between GALC activities in samples from Krabbe disease patients versus presumed normal newborn samples. Conclusions A fluorimetric assay for GALC enzyme activity measurement on dried blood spot specimens is feasible. Improvements to the assay including novel substrate design, increased substrate concentration and removal of sodium chloride maximize the specificity of the assay and minimize interference from β-galactosidase. First fluorescent assay to measure GALC enzyme activity in dried blood spots. Linear range covers the activity normally seen in newborn dried blood spots. Successfully discriminates deficient from normal enzyme activity in DBS specimens.
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Key Words
- CHMU, 3-Carboxy 6-hexadecanoylamino-4-methylumbelliferone
- CHMUG, 3-Carboxy 6-hexadecanoylamino-4-methylumbelliferone β-D-galactopyranoside substrate
- CLSI, Clinical and Laboratory Standards Institute
- DBS, Dried blood spot
- Dried blood spot
- Fluorescence enzyme assay
- GALC, galactocerebrosidase
- Galactocerebrosidase
- HMU, 6-hexadecanoylamino-4-methylumbelliferone
- HMUG, 6-hexadecanoylamino-4-methylumbelliferone β-D-galactopyranoside substrate
- Krabbe disease
- LoB, Limit of the blank
- LoD, Limit of detection
- LoQ, Limit of quantitation
- MUG, 4-methylumbelliferyl-β-D-galactopyranoside substrate
- NBS, Newborn screening
- NEH, Non-enzymatic hydrolysis
- Newborn screening
- PT, Proficiency testing
- QCBP, Quality control base pool
- QCH, Quality control high sample
- QCL, Quality control low sample
- QCM, Quality control medium sample
- RFU, Relative fluorescence units
- rhGALC, Recombinant human galactocerebrosidase
- rhβ-gal, Recombinant human β-galactosidase
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Affiliation(s)
| | - Hong Pham
- Baebies, Inc., PO Box 14403, Durham, NC, 27709, USA
| | | | - Peter Ross
- Baebies, Inc., PO Box 14403, Durham, NC, 27709, USA
| | | | | | | | | | - Allen E Eckhardt
- Advanced Liquid Logic Inc., PO Box 14025, Durham, NC, 27709, USA
| | - Maria Escolar
- University of Pittsburgh, One Children's Hospital Way, 4401 Penn Ave, Pittsburgh, PA, 15224, USA
| | - Deeksha Bali
- Duke University, Biochemical Genetics Laboratory, 801 Capitola Dr., Suite 6, Durham, NC, 27713, USA
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Tuncer FN, Iseri SAU, Yapici Z, Demir M, Karaca M, Calik M. A novel homozygous GALC variant has been associated with Krabbe disease in a consanguineous family. Neurol Sci 2018; 39:2123-2128. [PMID: 30209698 DOI: 10.1007/s10072-018-3556-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2018] [Accepted: 09/04/2018] [Indexed: 11/26/2022]
Abstract
Krabbe disease (KD) or globoid cell leukodystrophy is an autosomal recessive lysosomal storage disorder involving the white matter of the peripheral and the central nervous systems. It is caused by a deficiency of galactocerebrosidase enzyme activity. The most common manifestation is the classical early onset KD that leads to patient's loss before the age of 2. Herein, we report the evaluation of a consanguineous family with three affected children manifesting severe neurological findings that ended with death before the age of 2, in an attempt to provide genetic diagnosis to the family. One of the children underwent detailed physical and neurological examinations, including brain magnetic resonance imaging (MRI) and scalp electroencephalography (EEG) evaluations. GALC genetic testing on this child enabled identification of a novel homozygous variant (NM_000153.3: c.1394C>T; p.(Thr465Ile)), which confirmed diagnosis as KD. Familial segregation of this variant was performed by PCR amplification and Sanger sequencing that revealed the parents as heterozygous carriers. We believe this novel GALC variant will not only help in genetic counseling to this family but will also aid in identification of future KD cases.
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Affiliation(s)
- Feyza Nur Tuncer
- Department of Genetics, Aziz Sancar Institute of Experimental Medicine, Istanbul University, Vakif Gureba Cad., 34093, Fatih/Istanbul, Turkey.
| | - Sibel Aylin Ugur Iseri
- Department of Genetics, Aziz Sancar Institute of Experimental Medicine, Istanbul University, Vakif Gureba Cad., 34093, Fatih/Istanbul, Turkey
| | - Zuhal Yapici
- Division of Child Neurology, Department of Neurology, Istanbul Medical Faculty, Istanbul University, Istanbul, Turkey
| | - Mahmut Demir
- Department of Pediatrics, Harran University Faculty of Medicine, Sanliurfa, Turkey
| | - Meryem Karaca
- Pediatric Metabolism Disorder Department, Harran University Faculty of Medicine, Sanliurfa, Turkey
| | - Mustafa Calik
- Department of Pediatric Neurology, Harran University Faculty of Medicine, Sanliurfa, Turkey
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Civallero G, de Kremer R, Giugliani R. High-Risk Screening and Diagnosis of Inborn Errors of Metabolism. JOURNAL OF INBORN ERRORS OF METABOLISM AND SCREENING 2018. [DOI: 10.1177/2326409818792065] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Affiliation(s)
- Gabriel Civallero
- Medical Genetics Service, HCPA, Porto Alegre, Brazil
- Department of Genetics, UFRGS, Porto Alegre, Brazil
| | - Raquel de Kremer
- Centro de Estudios de las Metabolopatías Congénitas, CEMECO, Facultad de Ciencias Médicas, Universidad Nacional de Córdoba, Argentina
| | - Roberto Giugliani
- Medical Genetics Service, HCPA, Porto Alegre, Brazil
- Department of Genetics, UFRGS, Porto Alegre, Brazil
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