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Lee JJ, Wang T, Wiggins K, Lu PN, Underwood C, Ochenkowska K, Samarut E, Pollard LM, Flanagan-Steet H, Steet R. Dysregulated lysosomal exocytosis drives protease-mediated cartilage pathogenesis in multiple lysosomal disorders. iScience 2024; 27:109293. [PMID: 38495824 PMCID: PMC10940929 DOI: 10.1016/j.isci.2024.109293] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2023] [Revised: 12/20/2023] [Accepted: 02/16/2024] [Indexed: 03/19/2024] Open
Abstract
The classic view of the lysosome as a static recycling center has been replaced with one of a dynamic and mobile hub of metabolic regulation. This revised view raises new questions about how dysfunction of this organelle causes pathology in inherited lysosomal disorders. Here we provide evidence for increased lysosomal exocytosis in the developing cartilage of three lysosomal disease zebrafish models with distinct etiologies. Dysregulated exocytosis was linked to altered cartilage development, increased activity of multiple cathepsin proteases, and cathepsin- and TGFβ-mediated pathogenesis in these models. Moreover, inhibition of cathepsin activity or direct blockade of exocytosis with small molecule modulators improved the cartilage phenotypes, reinforcing a connection between excessive extracellular protease activity and cartilage pathogenesis. This study highlights the pathogenic consequences in early cartilage development arising from uncontrolled release of lysosomal enzymes via exocytosis, and suggests that pharmacological enhancement of this process could be detrimental during tissue development.
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Affiliation(s)
- Jen-Jie Lee
- JC Self Research Institute, Greenwood Genetic Center, Greenwood, SC 29646, USA
| | - Tong Wang
- JC Self Research Institute, Greenwood Genetic Center, Greenwood, SC 29646, USA
| | - Kali Wiggins
- JC Self Research Institute, Greenwood Genetic Center, Greenwood, SC 29646, USA
| | - Po Nien Lu
- JC Self Research Institute, Greenwood Genetic Center, Greenwood, SC 29646, USA
| | - Christina Underwood
- Biochemical Genetics Laboratory, Greenwood Genetic Center, Greenwood, SC 29646, USA
| | - Katarzyna Ochenkowska
- Research Center, Centre hospitalier de l’Université de Montréal (CHUM), Montreal, Canada
- Department of Neuroscience, Université de Montréal, Montréal, Canada
| | - Eric Samarut
- Research Center, Centre hospitalier de l’Université de Montréal (CHUM), Montreal, Canada
- Department of Neuroscience, Université de Montréal, Montréal, Canada
| | - Laura M. Pollard
- Biochemical Genetics Laboratory, Greenwood Genetic Center, Greenwood, SC 29646, USA
| | | | - Richard Steet
- JC Self Research Institute, Greenwood Genetic Center, Greenwood, SC 29646, USA
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Richter JE, Zimmermann MT, Blackburn PR, Mohammad AN, Klee EW, Pollard LM, Macmurdo CF, Atwal PS, Caulfield TR. Protein modeling and clinical description of a novel in-frame GLB1 deletion causing GM1 gangliosidosis type II. Mol Genet Genomic Med 2018; 6:1229-1235. [PMID: 30187681 PMCID: PMC6305665 DOI: 10.1002/mgg3.454] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2017] [Revised: 03/14/2018] [Accepted: 06/12/2018] [Indexed: 12/04/2022] Open
Abstract
Background Beta‐galactosidase‐1 (GLB1) is a lysosomal hydrolase that is responsible for breaking down specific glycoconjugates, particularly GM1 (monosialotetrahexosylganglioside). Pathogenic variants in GLB1 cause two different lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB. In GM1 gangliosidosis, decreased β‐galactosidase‐1 enzymatic activity leads to the accumulation of GM1 gangliosides, predominantly within the CNS. We present a 22‐month‐old proband with GM1 gangliosidosis type II (late‐infantile form) in whom a novel homozygous in‐frame deletion (c.1468_1470delAAC, p.Asn490del) in GLB1 was detected. Methods We used an experimental protein structure of β‐galactosidase‐1 to generate a model of the p.Asn490del mutant and performed molecular dynamic simulations to determine whether this mutation leads to altered ligand positioning compared to the wild‐type protein. In addition, residual mutant enzyme activity in patient leukocytes was evaluated using a fluorometric assay. Results Molecular dynamics simulations showed the deletion to alter the catalytic site leading to misalignment of the catalytic residues and loss of collective motion within the model. We predict this misalignment will lead to impaired catalysis of β‐galactosidase‐1 substrates. Enzyme assays confirmed diminished GLB1 enzymatic activity (~3% of normal activity) in the proband. Conclusions We have described a novel, pathogenic in‐frame deletion of GLB1 in a patient with GM1 gangliosidosis type II.
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Affiliation(s)
- John E. Richter
- Department of Clinical GenomicsMayo ClinicJacksonvilleFlorida
- Center for Individualized MedicineMayo ClinicJacksonvilleFlorida
| | - Michael T. Zimmermann
- Division of Biomedical Statistics and InformaticsDepartment of Health Sciences ResearchMayo ClinicRochesterMinnesota
| | - Patrick R. Blackburn
- Division of Biomedical Statistics and InformaticsDepartment of Health Sciences ResearchMayo ClinicRochesterMinnesota
- Center for Individualized MedicineMayo ClinicRochesterMinnesota
- Department of Laboratory Medicine and PathologyMayo ClinicRochesterMinnesota
| | | | - Eric W. Klee
- Department of Clinical GenomicsMayo ClinicJacksonvilleFlorida
- Division of Biomedical Statistics and InformaticsDepartment of Health Sciences ResearchMayo ClinicRochesterMinnesota
- Center for Individualized MedicineMayo ClinicRochesterMinnesota
- Department of Laboratory Medicine and PathologyMayo ClinicRochesterMinnesota
| | - Laura M. Pollard
- Biochemical Genetics LaboratoryGreenwood Genetic CenterGreenwoodSouth Carolina
| | | | - Paldeep S. Atwal
- Department of Clinical GenomicsMayo ClinicJacksonvilleFlorida
- Center for Individualized MedicineMayo ClinicJacksonvilleFlorida
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Ferreira CR, Devaney JM, Hofherr SE, Pollard LM, Cusmano-Ozog K. Hereditary fructose intolerance mimicking a biochemical phenotype of mucolipidosis: A review of the literature of secondary causes of lysosomal enzyme activity elevation in serum. Am J Med Genet A 2017; 173:501-509. [PMID: 27797444 PMCID: PMC10506159 DOI: 10.1002/ajmg.a.38023] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/13/2016] [Accepted: 09/29/2016] [Indexed: 11/08/2022]
Abstract
We describe a patient with failure to thrive, hepatomegaly, liver dysfunction, and elevation of multiple plasma lysosomal enzyme activities mimicking mucolipidosis II or III, in whom a diagnosis of hereditary fructose intolerance (HFI) was ultimately obtained. She presented before introduction of solid foods, given her consumption of a fructose-containing infant formula. We present the most extensive panel of lysosomal enzyme activities reported to date in a patient with HFI, and propose that multiple enzyme elevations in plasma, especially when in conjunction with a normal plasma α-mannosidase activity, should elicit a differential diagnosis of HFI. We also performed a review of the literature on the different etiologies of elevated lysosomal enzyme activities in serum or plasma. © 2016 Wiley Periodicals, Inc.
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Affiliation(s)
- Carlos R. Ferreira
- National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland
- Division of Genetics and Metabolism, Children’s National Health System, Washington, District Of Columbia
| | - Joseph M. Devaney
- Division of Laboratory Medicine, Children’s National Health System, Washington, District Of Columbia
| | - Sean E. Hofherr
- Division of Laboratory Medicine, Children’s National Health System, Washington, District Of Columbia
| | - Laura M. Pollard
- Biochemical Genetics Laboratory, Greenwood Genetic Center, Greenwood, South Carolina
| | - Kristina Cusmano-Ozog
- Division of Genetics and Metabolism, Children’s National Health System, Washington, District Of Columbia
- Division of Laboratory Medicine, Children’s National Health System, Washington, District Of Columbia
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Pollard LM, Jones JR, Wood TC. Molecular characterization of 355 mucopolysaccharidosis patients reveals 104 novel mutations. J Inherit Metab Dis 2013; 36:179-87. [PMID: 22976768 DOI: 10.1007/s10545-012-9533-7] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/30/2012] [Revised: 07/25/2012] [Accepted: 07/30/2012] [Indexed: 10/27/2022]
Abstract
Mucopolysaccharidosis (MPS) disorders are heterogeneous and caused by deficient lysosomal degradation of glycosaminoglycans, resulting in distinct but sometimes overlapping phenotypes. Molecular analysis was performed for a total of 355 MPS patients with MPSI (n = 15), MPSII (n = 218), MPSIIIA (n = 86), MPSIIIB (n = 20), MPSIVA (n = 6) or MPSVI (n = 10). This analysis revealed 104 previously unreported mutations: seven in IDUA (MPSI), 61 in IDS (MPSII), 19 in SGSH (MPSIIIA), 11 in NAGLU (MPSIIIB), two in GALNS (MPSIVA) and four in ARSB (MPSVI). The intergenic comparison of the mutation data for these disorders has revealed interesting differences. Whereas IDUA, IDS, NAGLU and ARSB demonstrate similar levels of mutation heterogeneity (0.6-0.675 different mutations per total alleles), SGSH and GALNS have lower levels of mutation heterogeneity (0.282 and 0.455, respectively), due to more recurrent mutations. The type of mutation also varies significantly by gene. SGSH, GALNS and ARSB mutations are usually missense (76.5 %, 81.8 % and 85 %), while IDUA has many more nonsense mutations (56 %) than the other genes (≤20%). The mutation spectrum is most diverse for IDS, including intergenic inversions and multi-exon deletions. By testing 102 mothers of MPSII patients, we determined that 22.5 % of IDS mutations are de novo. We report the allele frequency of common mutations for each gene in our patient cohort and the exonic distribution of coding sequence alterations in the IDS, SGSH and NAGLU genes, which reveals several potential "hot-spots". This further molecular characterization of these MPS disorders is expected to assist in the diagnosis and counseling of future patients.
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Affiliation(s)
- Laura M Pollard
- Biochemical Diagnostic Laboratory, Greenwood Genetic Center, 106 Gregor Mendel Circle, Greenwood, SC 29646, USA.
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Pollard LM, Williams NR, Espinoza L, Wood TC, Spector EB, Schroer RJ, Holden KR. Diagnosis, treatment, and long-term outcomes of late-onset (type III) multiple acyl-CoA dehydrogenase deficiency. J Child Neurol 2010; 25:954-60. [PMID: 20023066 DOI: 10.1177/0883073809351984] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
We report 4 children with late-onset (type III) multiple acyl-CoA dehydrogenase deficiency, also known as glutaric aciduria type II, which is an autosomal recessive disorder of fatty acid and amino acid metabolism. The underlying deficiency is in the electron transfer flavoprotein or electron flavoprotein dehydrogenase. Clinical presentations include fatal acute neonatal metabolic encephalopathies with/without organ system anomalies (types I and II) and late-onset acute metabolic crises, myopathy, or neurodevelopmental delays (type III). Two patients were identified in childhood following a metabolic crisis and/or neurodevelopmental delay, and 2 were identified by newborn metabolic screening. Our cases will illustrate the difficulty in making a biochemical diagnosis of late-onset (type III) multiple acyl-CoA dehydrogenase deficiency from plasma acylcarnitines and urine organic acids in both symptomatic and asymptomatic children. However, they emphasize the need for timely diagnosis to urgently implement prophylactic treatment for life-threatening metabolic crises with low protein/fat diets supplemented with riboflavin and carnitine.
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Bourn RL, Rindler PM, Pollard LM, Bidichandani SI. E. coli mismatch repair acts downstream of replication fork stalling to stabilize the expanded (GAA.TTC)(n) sequence. Mutat Res 2009; 661:71-7. [PMID: 19046977 PMCID: PMC2637364 DOI: 10.1016/j.mrfmmm.2008.11.003] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2008] [Revised: 10/31/2008] [Accepted: 11/05/2008] [Indexed: 05/27/2023]
Abstract
Expanded triplet repeat sequences are known to cause at least 16 inherited neuromuscular diseases. In addition to short-length changes, expanded triplet repeat tracts frequently undergo large changes, often amounting to hundreds of base-pairs. Such changes might occur when template or primer slipping creates insertion/deletion loops (IDLs), which are normally repaired by the mismatch repair system (MMR). However, in prokaryotes and eukaryotes, MMR promotes large changes in the length of (CTG.CAG)(n) sequences, the motif most commonly associated with human disease. We tested the effect of MMR on instability of the expanded (GAA.TTC)(n) sequence, which causes Friedreich ataxia, by comparing repeat instability in wild-type and MMR-deficient strains of Escherichia coli. As expected, the prevalence of small mutations increased in the MMR-deficient strains. However, the prevalence of large contractions increased in the MMR mutants specifically when GAA was the lagging strand template, the orientation in which replication fork stalling is known to occur. After hydroxyurea-induced stalling, both orientations of replication showed significantly more large contractions in MMR mutants than in the wild-type, suggesting that fork stalling may be responsible for the large contractions. Deficiency of MMR promoted large contractions independently of RecA status, a known determinant of (GAA.TTC)(n) instability. These data suggest that two independent mechanisms act in response to replication stalling to prevent instability of the (GAA.TTC)(n) sequence in E. coli, when GAA serves as the lagging strand template: one that is dependent on RecA-mediated restart of stalled forks, and another that is dependent on MMR-mediated repair of IDLs. While MMR destabilizes the (CTG.CAG)(n) sequence, it is involved in stabilization of the (GAA.TTC)(n) sequence. The role of MMR in triplet repeat instability therefore depends on the repeat sequence and the orientation of replication.
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Affiliation(s)
- Rebecka L. Bourn
- Department of Biochemistry & Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Paul M. Rindler
- Department of Biochemistry & Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Laura M. Pollard
- Department of Biochemistry & Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Sanjay I. Bidichandani
- Department of Biochemistry & Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
- Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
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Pollard LM, Bourn RL, Bidichandani SI. Repair of DNA double-strand breaks within the (GAA*TTC)n sequence results in frequent deletion of the triplet-repeat sequence. Nucleic Acids Res 2008; 36:489-500. [PMID: 18045804 PMCID: PMC2241870 DOI: 10.1093/nar/gkm1066] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/09/2007] [Revised: 11/07/2007] [Accepted: 11/12/2007] [Indexed: 11/13/2022] Open
Abstract
Friedreich ataxia is caused by an expanded (GAA*TTC)n sequence, which is unstable during intergenerational transmission and in most patient tissues, where it frequently undergoes large deletions. We investigated the effect of DSB repair on instability of the (GAA*TTC)n sequence. Linear plasmids were transformed into Escherichia coli so that each colony represented an individual DSB repair event. Repair of a DSB within the repeat resulted in a dramatic increase in deletions compared with circular templates, but DSB repair outside the repeat tract did not affect instability. Repair-mediated deletions were independent of the orientation and length of the repeat, the location of the break within the repeat or the RecA status of the strain. Repair at the center of the repeat resulted in deletion of approximately half of the repeat tract, and repair at an off-center location produced deletions that were equivalent in length to the shorter of the two repeats flanking the DSB. This is consistent with a single-strand annealing mechanism of DSB repair, and implicates erroneous DSB repair as a mechanism for genetic instability of the (GAA*TTC)n sequence. Our data contrast significantly with DSB repair within (CTG*CAG)n repeats, indicating that repair-mediated instability is dependent on the sequence of the triplet repeat.
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Affiliation(s)
- Laura M. Pollard
- Department of Biochemistry and Molecular Biology and Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Rebecka L. Bourn
- Department of Biochemistry and Molecular Biology and Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Sanjay I. Bidichandani
- Department of Biochemistry and Molecular Biology and Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
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Pollard LM, Chutake YK, Rindler PM, Bidichandani SI. Deficiency of RecA-dependent RecFOR and RecBCD pathways causes increased instability of the (GAA*TTC)n sequence when GAA is the lagging strand template. Nucleic Acids Res 2007; 35:6884-94. [PMID: 17932052 PMCID: PMC2175318 DOI: 10.1093/nar/gkm810] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2007] [Revised: 09/17/2007] [Accepted: 09/18/2007] [Indexed: 11/13/2022] Open
Abstract
The most common mutation in Friedreich ataxia is an expanded (GAA*TTC)n sequence, which is highly unstable in human somatic cells and in the germline. The mechanisms responsible for this genetic instability are poorly understood. We previously showed that cloned (GAA*TTC)n sequences replicated in Escherichia coli are more unstable when GAA is the lagging strand template, suggesting erroneous lagging strand synthesis as the likely mechanism for the genetic instability. Here we show that the increase in genetic instability when GAA serves as the lagging strand template is seen in RecA-deficient but not RecA-proficient strains. We also found the same orientation-dependent increase in instability in a RecA+ temperature-sensitive E. coli SSB mutant strain (ssb-1). Since stalling of replication is known to occur within the (GAA*TTC)n sequence when GAA is the lagging strand template, we hypothesized that genetic stability of the (GAA*TTC)n sequence may require efficient RecA-dependent recombinational restart of stalled replication forks. Consistent with this hypothesis, we noted significantly increased instability when GAA was the lagging strand template in strains that were deficient in components of the RecFOR and RecBCD pathways. Our data implicate defective processing of stalled replication forks as a mechanism for genetic instability of the (GAA*TTC)n sequence.
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Affiliation(s)
- Laura M. Pollard
- Department of Biochemistry and Molecular Biology and Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Yogesh K. Chutake
- Department of Biochemistry and Molecular Biology and Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Paul M. Rindler
- Department of Biochemistry and Molecular Biology and Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
| | - Sanjay I. Bidichandani
- Department of Biochemistry and Molecular Biology and Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
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M. Rindler P, Clark RM, Pollard LM, De Biase I, Bidichandani SI. Replication in mammalian cells recapitulates the locus-specific differences in somatic instability of genomic GAA triplet-repeats. Nucleic Acids Res 2006; 34:6352-61. [PMID: 17142224 PMCID: PMC1669776 DOI: 10.1093/nar/gkl846] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2006] [Revised: 10/09/2006] [Accepted: 10/09/2006] [Indexed: 11/21/2022] Open
Abstract
Friedreich ataxia is caused by an expanded (GAA.TTC)n sequence in intron 1 of the FXN gene. Small pool PCR analysis showed that pure (GAA.TTC)44+ sequences at the FXN locus are unstable in somatic cells in vivo, displaying both expansions and contractions. On searching the entire human and mouse genomes we identified three other genomic loci with pure (GAA.TTC)44+ sequences. Alleles at these loci showed mutation loads of <1% compared with 6.3-30% for FXN alleles of similar length, indicating that somatic instability in vivo is regulated by locus-specific factors. Since distance between the origin of replication and the (CTG.CAG)n sequence modulates repeat instability in mammalian cells, we tested if this could also recapitulate the locus-specific differences for genomic (GAA.TTC)n sequences. Repeat instability was evaluated following replication of a (GAA.TTC)115 sequence in transfected COS1 cells under the control of the SV40 origin of replication located at one of five different distances from the repeat. Indeed, depending on the location of the SV40 origin relative to the (GAA.TTC)n sequence, we noted either no instability, predominant expansion or both expansion and contraction. These data suggest that mammalian DNA replication is a possible mechanism underlying locus-specific differences in instability of GAA triplet-repeat sequences.
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Affiliation(s)
- Paul M. Rindler
- Department of Biochemistry and Molecular BiologyOklahoma City, OK 73104, USA
| | - Rhonda M. Clark
- Department of Biochemistry and Molecular BiologyOklahoma City, OK 73104, USA
| | - Laura M. Pollard
- Department of Biochemistry and Molecular BiologyOklahoma City, OK 73104, USA
| | - Irene De Biase
- Department of Biochemistry and Molecular BiologyOklahoma City, OK 73104, USA
| | - Sanjay I. Bidichandani
- Department of Biochemistry and Molecular BiologyOklahoma City, OK 73104, USA
- Department of Pediatrics, University of Oklahoma Health Sciences CenterOklahoma City, OK 73104, USA
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Pollard LM, Sharma R, Gómez M, Shah S, Delatycki MB, Pianese L, Monticelli A, Keats BJB, Bidichandani SI. Replication-mediated instability of the GAA triplet repeat mutation in Friedreich ataxia. Nucleic Acids Res 2004; 32:5962-71. [PMID: 15534367 PMCID: PMC528813 DOI: 10.1093/nar/gkh933] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2004] [Revised: 10/22/2004] [Accepted: 10/22/2004] [Indexed: 11/14/2022] Open
Abstract
Friedreich ataxia is caused by the expansion of a polymorphic and unstable GAA triplet repeat in the FRDA gene, but the mechanisms for its instability are poorly understood. Replication of (GAA*TTC)n sequences (9-105 triplets) in plasmids propagated in Escherichia coli displayed length- and orientation-dependent instability. There were small length variations upon replication in both orientations, but large contractions were frequently observed when GAA was the lagging strand template. DNA replication was also significantly slower in this orientation. To evaluate the physiological relevance of our findings, we analyzed peripheral leukocytes from human subjects carrying repeats of similar length (8-107 triplets). Analysis of 9400 somatic FRDA molecules using small-pool PCR revealed a similar mutational spectrum, including large contractions. The threshold length for the initiation of somatic instability in vivo was between 40 and 44 triplets, corresponding to the length of a eukaryotic Okazaki fragment. Consistent with the stabilization of premutation alleles during germline transmission, we also found that instability of somatic cells in vivo and repeats propagated in E.coli were abrogated by (GAGGAA)n hexanucleotide interruptions. Our data demonstrate that the GAA triplet repeat mutation in Friedreich ataxia is destabilized, frequently undergoing large contractions, during DNA replication.
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Affiliation(s)
- Laura M Pollard
- Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73104, USA
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