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In Silico Analysis of Missense Mutations as a First Step in Functional Studies: Examples from Two Sphingolipidoses. Int J Mol Sci 2018; 19:ijms19113409. [PMID: 30384423 PMCID: PMC6275066 DOI: 10.3390/ijms19113409] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2018] [Revised: 10/27/2018] [Accepted: 10/29/2018] [Indexed: 02/06/2023] Open
Abstract
In order to delineate a better approach to functional studies, we have selected 23 missense mutations distributed in different domains of two lysosomal enzymes, to be studied by in silico analysis. In silico analysis of mutations relies on computational modeling to predict their effects. Various computational platforms are currently available to check the probable causality of mutations encountered in patients at the protein and at the RNA levels. In this work we used four different platforms freely available online (Protein Variation Effect Analyzer- PROVEAN, PolyPhen-2, Swiss-model Expert Protein Analysis System—ExPASy, and SNAP2) to check amino acid substitutions and their effect at the protein level. The existence of functional studies, regarding the amino acid substitutions, led to the selection of the distinct protein mutants. Functional data were used to compare the results obtained with different bioinformatics tools. With the advent of next-generation sequencing, it is not feasible to carry out functional tests in all the variants detected. In silico analysis seems to be useful for the delineation of which mutants are worth studying through functional studies. Therefore, prediction of the mutation impact at the protein level, applying computational analysis, confers the means to rapidly provide a prognosis value to genotyping results, making it potentially valuable for patient care as well as research purposes. The present work points to the need to carry out functional studies in mutations that might look neutral. Moreover, it should be noted that single nucleotide polymorphisms (SNPs), occurring in coding and non-coding regions, may lead to RNA alterations and should be systematically verified. Functional studies can gain from a preliminary multi-step approach, such as the one proposed here.
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Cesani M, Lorioli L, Grossi S, Amico G, Fumagalli F, Spiga I, Filocamo M, Biffi A. Mutation Update ofARSAandPSAPGenes Causing Metachromatic Leukodystrophy. Hum Mutat 2015; 37:16-27. [DOI: 10.1002/humu.22919] [Citation(s) in RCA: 71] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2015] [Accepted: 10/08/2015] [Indexed: 12/31/2022]
Affiliation(s)
- Martina Cesani
- San Raffaele Telethon Institute for Gene Therapy; Division of Regenerative Medicine; Stem Cells and Gene Therapy; San Raffaele Scientific Institute; Milan Italy
| | - Laura Lorioli
- San Raffaele Telethon Institute for Gene Therapy; Division of Regenerative Medicine; Stem Cells and Gene Therapy; San Raffaele Scientific Institute; Milan Italy
- Vita-Salute San Raffaele University; Milan Italy
| | - Serena Grossi
- Centro di Diagnostica Genetica e Biochimica delle Malattie Metaboliche; Istituto G. Gaslini; Genova Italy
| | - Giulia Amico
- Centro di Diagnostica Genetica e Biochimica delle Malattie Metaboliche; Istituto G. Gaslini; Genova Italy
| | - Francesca Fumagalli
- San Raffaele Telethon Institute for Gene Therapy; Division of Regenerative Medicine; Stem Cells and Gene Therapy; San Raffaele Scientific Institute; Milan Italy
- Neurology Department; Division of Neuroscience; San Raffaele Scientific Institute; Milan Italy
| | - Ivana Spiga
- Clinical Molecular Biology Laboratory; San Raffaele Hospital; Milan Italy
| | - Mirella Filocamo
- Centro di Diagnostica Genetica e Biochimica delle Malattie Metaboliche; Istituto G. Gaslini; Genova Italy
| | - Alessandra Biffi
- San Raffaele Telethon Institute for Gene Therapy; Division of Regenerative Medicine; Stem Cells and Gene Therapy; San Raffaele Scientific Institute; Milan Italy
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Novoyatleva T, Tang Y, Rafalska I, Stamm S. Pre-mRNA Missplicing as a Cause of Human Disease. ALTERNATIVE SPLICING AND DISEASE 2006; 44:27-46. [PMID: 17076263 DOI: 10.1007/978-3-540-34449-0_2] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Regulated alternative splice site selection emerges as one of the most important mechanisms to control the expression of genetic information in humans. It is therefore not surprising that a growing number of diseases are either associated with or caused by changes in alternative splicing. These diseases can be caused by mutation in regulatory sequences of the pre-mRNA or by changes in the concentration of trans-acting factors. The pathological expression of mRNA isoforms can be treated by transferring nucleic acids derivatives into cells that interfere with sequence elements on the pre-mRNA, which results in the desired splice site selection. Recently, a growing number of low molecular weight drugs have been discovered that influence splice site selection in vivo. These findings prove the principle that diseases caused by missplicing events could eventually be cured.
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Affiliation(s)
- Tatyana Novoyatleva
- University of Erlangen, Institute for Biochemistry, Fahrstrasse 17, 91054 Erlangen, Germany
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Královičová J, Christensen MB, Vořechovský I. Biased exon/intron distribution of cryptic and de novo 3' splice sites. Nucleic Acids Res 2005; 33:4882-98. [PMID: 16141195 PMCID: PMC1197134 DOI: 10.1093/nar/gki811] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
We compiled sequences of previously published aberrant 3′ splice sites (3′ss) that were generated by mutations in human disease genes. Cryptic 3′ss, defined here as those resulting from a mutation of the 3′YAG consensus, were more frequent in exons than in introns. They clustered in ∼20 nt region adjacent to authentic 3′ss, suggesting that their under-representation in introns is due to a depletion of AG dinucleotides in the polypyrimidine tract (PPT). In contrast, most aberrant 3′ss that were induced by mutations outside the 3′YAG consensus (designated ‘de novo’) were in introns. The activation of intronic de novo 3′ss was largely due to AG-creating mutations in the PPT. In contrast, exonic de novo 3′ss were more often induced by mutations improving the PPT, branchpoint sequence (BPS) or distant auxiliary signals, rather than by direct AG creation. The Shapiro–Senapathy matrix scores had a good prognostic value for cryptic, but not de novo 3′ss. Finally, AG-creating mutations in the PPT that produced aberrant 3′ss upstream of the predicted BPS in vivo shared a similar ‘BPS-new AG’ distance. Reduction of this distance and/or the strength of the new AG PPT in splicing reporter pre-mRNAs improved utilization of authentic 3′ss, suggesting that AG-creating mutations that are located closer to the BPS and are preceded by weaker PPT may result in less severe splicing defects.
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Affiliation(s)
| | | | - Igor Vořechovský
- To whom correspondence should be addressed. Tel: +44 2380 796425; Fax: +44 2380 794264;
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Berná L, Gieselmann V, Poupetová H, Hrebícek M, Elleder M, Ledvinová J. Novel mutations associated with metachromatic leukodystrophy: phenotype and expression studies in nine Czech and Slovak patients. Am J Med Genet A 2005; 129A:277-81. [PMID: 15326627 DOI: 10.1002/ajmg.a.30118] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Metachromatic leukodystrophy (MLD) is an inherited demyelinating disorder caused by the deficiency of arylsulphatase A (ASA). This defect leads to an accumulation of galactosylceramide I(3)-sulphates (sulphatides) in lysosomes of different tissues. We report on mutations found in a group of nine patients from the Czech and Slovak Republics (former Czechoslovakia). Their diagnosis was confirmed by determination of the activity of arylsulphatase A in leukocytes and by abnormal urinary excretion of sulphatides. All alleles of the patients were identified and eight different mutations were found. They include four novel missense mutations in one infantile (D29N), one juvenile (C294Y), and three adult (C156R, G293S) patients. Four mutations were previously described sequence alterations (459 + 1G > A, G309S, I179S, and P426L). Polymorphisms characteristic for the ASA pseudodeficiency allele were not found in the patients. Substitutions of D29N, C294Y, and G293S in arylsulphatase A caused a severe reduction of enzyme activity in transient expression studies. In contrast, the C156R substitution reduces arylsulphatase A only to 50% of wild type ASA activity. Since no other mutations were found in this patient, the contribution of this mutation to the development of disease remains unclear.
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Affiliation(s)
- Linda Berná
- Institute of Inherited Metabolic Disorders, First Faculty of Medicine, Charles University, Prague, Czech Republic
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Stoilov P, Meshorer E, Gencheva M, Glick D, Soreq H, Stamm S. Defects in pre-mRNA processing as causes of and predisposition to diseases. DNA Cell Biol 2002; 21:803-18. [PMID: 12489991 DOI: 10.1089/104454902320908450] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
Humans possess a surprisingly low number of genes and intensively use pre-mRNA splicing to achieve the high molecular complexity needed to sustain normal body functions and facilitate responses to altered conditions. Because hundreds of thousands of proteins are generated by 25,000 to 40,000 genes, pre-mRNA processing events are highly important for the regulation of human gene expression. Both inherited and acquired defects in pre-mRNA processing are increasingly recognized as causes of human diseases, and almost all pre-mRNA processing events are controlled by a combination of protein factors. This makes defects in these processes likely candidates for causes of diseases with complicated inheritance patterns that affect seemingly unrelated functions. The elucidation of genetic mechanisms regulating pre-mRNA processing, combined with the development of drugs targeted at consensus RNA sequences and/or corresponding proteins, can lead to novel diagnostic and therapeutic approaches.
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Affiliation(s)
- Peter Stoilov
- University of Erlangen-Nurenberg, Institute of Biochemistry, 91054 Erlangen, Germany
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Cáceres JF, Kornblihtt AR. Alternative splicing: multiple control mechanisms and involvement in human disease. Trends Genet 2002; 18:186-93. [PMID: 11932019 DOI: 10.1016/s0168-9525(01)02626-9] [Citation(s) in RCA: 495] [Impact Index Per Article: 22.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Alternative splicing is an important mechanism for controlling gene expression. It allows large proteomic complexity from a limited number of genes. An interplay of cis-acting sequences and trans-acting factors modulates the splicing of regulated exons. Here, we discuss the roles of the SR and hnRNP families of proteins in this process. We also focus on the role of the transcriptional machinery in the regulation of alternative splicing, and on those alterations of alternative splicing that lead to human disease.
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Affiliation(s)
- Javier F Cáceres
- MRC Human Genetics Unit, Western General Hospital, Edinburgh EH4 2XU, UK.
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Fukutani Y, Noriki Y, Sasaki K, Isaki K, Kuriyama M, Kurosawa K, Ida H. Adult-type metachromatic leukodystrophy with a compound heterozygote mutation showing character change and dementia. Psychiatry Clin Neurosci 1999; 53:425-8. [PMID: 10459747 DOI: 10.1046/j.1440-1819.1999.00569.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
A 26-year-old Japanese woman slowly developed a change of character such as hypospontaneity and blunted affect, followed by obvious mental deterioration. She was diagnosed as having a disorganized type of schizophrenia at the first examination. Brain magnetic resonance imaging demonstrated diffuse high intensity in the cerebral white matter, particularly in the frontal lobes. The single photon emission computed tomography images using 123I-IMP disclosed diffuse cerebral hypofusion, especially in the frontal lobes. Electroencephalogram showed a moderate amount of 5-6Hz theta waves on the background of alpha activity. Nerve conduction velocities in the extremities were delayed. The level of leucocyte arylsulphatase was low. In the arylsulphatase A gene analysis, a compound heterozygote having the 99Gly-->Asp and 409Thr-->Ile mutations was confirmed. The patient was diagnosed as having metachromatic leukodystrophy. She gradually showed obvious dementing symptoms such as memory disturbance and disorientation. The characteristics of the psychiatric symptoms in the leukodystrophy are discussed.
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Affiliation(s)
- Y Fukutani
- Department of Neuropsychiatry, Fukui Medical University, Japan.
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Boot RG, Renkema GH, Verhoek M, Strijland A, Bliek J, de Meulemeester TM, Mannens MM, Aerts JM. The human chitotriosidase gene. Nature of inherited enzyme deficiency. J Biol Chem 1998; 273:25680-5. [PMID: 9748235 DOI: 10.1074/jbc.273.40.25680] [Citation(s) in RCA: 316] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023] Open
Abstract
The human chitinase, named chitotriosidase, is a member of family 18 of glycosylhydrolases. Following the cloning of the chitotriosidase cDNA (Boot, R. G., Renkema, G. H., Strijland, A., van Zonneveld, A. J., and Aerts, J. M. F. G. (1995) J. Biol. Chem. 270, 26252-26256), the gene and mRNA have been investigated. The chitotriosidase gene is assigned to chromosome 1q31-q32. The gene consists of 12 exons and spans about 20 kilobases. The nature of the common deficiency in chitotriosidase activity is reported. A 24-base pair duplication in exon 10 results in activation of a cryptic 3' splice site, generating a mRNA with an in-frame deletion of 87 nucleotides. All chitotriosidase-deficient individuals tested were homozygous for the duplication. The observed carrier frequency of about 35% indicates that the duplication is the predominant cause of chitotriosidase deficiency. The presence of the duplication in individuals from various ethnic groups suggests that this mutation is relatively old.
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Affiliation(s)
- R G Boot
- Department of Biochemistry, University of Amsterdam, Academic Medical Center, 1105 AZ, Amsterdam, The Netherlands
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Gieselmann V, Matzner U, Hess B, Lüllmann-Rauch R, Coenen R, Hartmann D, D'Hooge R, DeDeyn P, Nagels G. Metachromatic leukodystrophy: molecular genetics and an animal model. J Inherit Metab Dis 1998; 21:564-74. [PMID: 9728336 DOI: 10.1023/a:1005471106088] [Citation(s) in RCA: 62] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder caused by the deficiency of arylsulphatase A (ASA; EC 3.1.6.8). Deficiency of this enzyme causes intralysosomal storage of the sphingolipid cerebroside sulphate. This lipid is abundant in myelin and it may thus not be surprising that storage mainly affects oligodendrocytes. Patients suffer from a progressive demyelination causing various neurological symptoms. The disease is fatal and treatment is not available. The human ASA gene has been cloned and more than 40 mutations have been analysed that cause metachromatic leukodystrophy. Few of these alleles are frequent among patients, whereas most mutant alleles have only been found in single families. Since MLD has only been described in humans and no naturally occurring animal model has been described, ASA-deficient mice have been generated by homologous recombination. The ASA knockout mice are unable to degrade sulphatide and store the lipid intralysosomally. The pattern of lipid storage in neuronal and non-neuronal tissues resembles that described for patients. In the nervous system, lipid storage is found in oligodendrocytes, astrocytes and some neurons. Animals display an astrogliosis and a decreased average axonal diameter. Purkinje cells and Bergmann glia of the cerebellum are morphologically aberrant. Demyelination is seen in the acoustic ganglion and occurs between the ages of 6 and 12 months. The animals are deaf at this age and display various neuromotor abnormalities. However, compared to humans the mice have a surprisingly mild phenotype, since they have a normal life span and do not develop widespread demyelination. ASA-deficient mice have been transplanted with bone marrow, which was transduced with a retroviral vector expressing arylsulphatase A. The majority of transplanted animals display sustained expression of arylsulphatase A from the retroviral construct up to 5 months after transplantation. However, preliminary data suggest that this therapeutic approach does not reduce storage material.
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Affiliation(s)
- V Gieselmann
- Biochemisches Institut der Christian-Albrechts-Universität zu Kiel, Germany
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Pohlenz J, Rosenthal IM, Weiss RE, Jhiang SM, Burant C, Refetoff S. Congenital hypothyroidism due to mutations in the sodium/iodide symporter. Identification of a nonsense mutation producing a downstream cryptic 3' splice site. J Clin Invest 1998; 101:1028-35. [PMID: 9486973 PMCID: PMC508654 DOI: 10.1172/jci1504] [Citation(s) in RCA: 86] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
A 12-yr-old hypothyroid girl was diagnosed at birth as athyreotic because her thyroid gland could not be visualized by isotope scanning. Goiter development due to incomplete thyrotropin suppression, a thyroidal radioiodide uptake of < 1%, and a low saliva to plasma ratio of 2.5 suggested iodide (I-) transport defect. mRNA isolated from her thyroid gland and injected into Xenopus oocytes failed to increase I- transport. Sequencing of the entire Na+/I- symporter (NIS) cDNA revealed a C to G transversion of nucleotide (nt) 1146 in exon 6, resulting in a Gln 267 (CAG) to Glu (GAG) substitution. This missense mutation produces an NIS with undetectable I- transport activity when expressed in COS-7 cells. Although only this missense mutation was identified in thyroid and lymphocyte cDNA, genotyping revealed that the proposita and her unaffected brother and father were heterozygous for this mutation. However, amplification of cDNA with a primer specific for the wild-type nt 1146 yielded a sequence lacking 67 nt. Genomic DNA showed a C to G transversion of nt 1940, producing a stop codon as well as a new downstream cryptic 3' splice acceptor site in exon 13, responsible for the 67 nt deletion, frameshift, and premature stop predicting an NIS lacking 129 carboxy-terminal amino acids. This mutation was inherited from the mother and present in the unaffected sister. Thus, although the proposita is a compound heterozygote, because of the very low expression (< 2.5%) of one mutant allele, she is functionally hemizygous for an NIS without detectable bioactivity.
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Affiliation(s)
- J Pohlenz
- Department of Medicine, Jr., Mental Retardation Research Center, The University of Chicago, Chicago, Illinois 60637-1470, USA
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Cooper TA, Mattox W. The regulation of splice-site selection, and its role in human disease. Am J Hum Genet 1997; 61:259-66. [PMID: 9311728 PMCID: PMC1715899 DOI: 10.1086/514856] [Citation(s) in RCA: 204] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Affiliation(s)
- T A Cooper
- Department of Pathology, Baylor College of Medicine, Houston, TX 77030, USA.
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Pomponio RJ, Reynolds TR, Mandel H, Admoni O, Melone PD, Buck GA, Wolf B. Profound biotinidase deficiency caused by a point mutation that creates a downstream cryptic 3' splice acceptor site within an exon of the human biotinidase gene. Hum Mol Genet 1997; 6:739-45. [PMID: 9158148 DOI: 10.1093/hmg/6.5.739] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Biotinidase recycles the vitamin biotin from biocytin upon the degradation of the biotin-dependent carboxylases. We have identified a novel point mutation within the biotinidase gene that encodes the signal peptide in two unrelated individuals with profound biotinidase deficiency. Sequence analysis of genomic DNA from these individuals revealed a G to A transition (G100-->A) located 57 bases downstream of the authentic splice acceptor site in exon B. Although this mutation predicts a G34S substitution, it also generates a 3' splice acceptor site. Sequence of the PCR-amplified cDNA from the homozygous child revealed that all the product was shorter than that of normal individuals and was the result of aberrant splicing. The aberrantly spliced transcript lacked 57 bases, including a second in-frame ATG, that encode most of the putative signal peptide and results in an in-frame deletion of 19 amino acids. The mutation results in failure to secrete the aberrant protein into the blood. This is the first reported example in which a point mutation creates a cryptic 3' splice acceptor site motif that is used preferentially over the upstream authentic splice site. The preferential usage of the downstream splice site is not consistent with the 5'-3' scanning model, but is consistent with the exon definition model of RNA splicing.
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Affiliation(s)
- R J Pomponio
- Department of Human Genetics, Medical College of Virginia, Virginia Commonwealth University, Richmond 23298, USA
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Regis S, Carrozzo R, Filocamo M, Serra G, Mastropaolo C, Gatti R. An AT-deletion causing a frameshift in the arylsulfatase A gene of a late infantile metachromatic leukodystrophy patient. Hum Genet 1995; 96:233-5. [PMID: 7635478 DOI: 10.1007/bf00207387] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
A metachromatic leukodystrophy (MLD) patient affected with the late infantile form was found to be homozygous for an AT-deletion (2324delAT) in the arylsulfatase A gene. The mutation causes a frameshift at the beginning of exon 8 leading to an early termination codon. The parents and unaffected brother of the patient were heterozygous for the microdeletion. The mutation was not detected in another 31 MLD Italian patients. No aberrant transcript caused by the mutation was revealed by the reverse transcription-polymerase chain reaction method.
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Affiliation(s)
- S Regis
- Laboratorio di Diagnosi Pre e Post-Natale di Malattie Metaboliche, Istituto G. Gaslini, Genova, Italy
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Affiliation(s)
- V Gieselmann
- Department of Biochemistry II, Georg August Universität, Göttingen, Germany
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Gieselmann V, Zlotogora J, Harris A, Wenger DA, Morris CP. Molecular genetics of metachromatic leukodystrophy. Hum Mutat 1994; 4:233-42. [PMID: 7866401 DOI: 10.1002/humu.1380040402] [Citation(s) in RCA: 86] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Metachromatic leukodystrophy is an autosomal recessive inherited lysosomal storage disease. It can be caused by mutations in two different genes, the arylsulfatase A and the prosaposin gene. These genes encode two proteins that are needed for the proper degradation of cerebroside sulfate, a glycolipid mainly found in the myelin membranes. Deficiency of arylsulfatase A or of a proteolytic product of prosaposin leads to the accumulation of cerebroside sulfate, which causes a lethal progressive demyelination. Mutations in the arylsulfatase A gene are far more frequent than those of the prosaposin gene. So far 31 amino acid substitutions, one nonsense mutation, three small deletions, three splice donor site mutations, and one combined missense/splice donor site mutation have been identified in the arylsulfatase A gene. Two of these mutant alleles are frequent, accounting for about one-half of all mutant alleles, whereas the remainder are heterogeneous. Amino acid substitutions cluster in exons 2 and 3, a region that shows a high degree of conservation among sulfatases of different function and origin. Different mutations are associated with phenotypes of different severity, but there is a remarkable variability of severity when patients with identical genotypes are compared. Demonstration of an arylsulfatase A deficiency is not a proof of metachromatic leukodystrophy, since a substantial deficiency without any clinical consequences is frequent in the general population. This deficiency is caused by an arylsulfatase A allele, which due to certain mutations encodes greatly reduced amounts of functional enzyme. However, these amounts are sufficient to sustain a normal phenotype. In the diagnosis and genetic counseling, these deficiencies must be differentiated from those causing metachromatic leukodystrophy. So far only six patients with mutations in the prosaposin gene have been described, in which three defective alleles two with amino acid substitutions and one with a 33-bp insertion have been identified.
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Affiliation(s)
- V Gieselmann
- Institut für Biochemie II, Georg-August-Universität Göttingen, Germany
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