Pelizaeus–Merzbacher disease

Pelizaeus-MerzbacherDisease

ChintalagiriShashank,

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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants

MolecularPathogenesis

Genotype -Phenotypecorrelation

References

Pelizaeus-Merzbacher DiseaseBSE638 - Structural Basis of Protein Function

Chintalagiri Shashank, [email protected]

April 29, 2010

Chintalagiri Shashank, [email protected] Pelizaeus-Merzbacher Disease



[email protected]

Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

1 IntroductionPelizaeus-Merzbacher DiseaseTypes of PMD

2 Disease Mechanism

3 PathogenesisAnimal MutantsMolecular PathogenesisGenotype - Phenotype correlation

4 References




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Pelizaeus-Merzbacher Disease[1]

Rare, progressive, degenerative central nervous systemdisorder

Coordination, motor abilities, and intellectual functiondeteriorate

One of a group of gene-linked disorders known as theleukodystrophies, affects growth of the myelin sheath.

The disease is caused by a mutation in the gene encodinga myelin protein called proteolipid protein-1 (PLP1).

PMD is inherited as an X-linked recessive trait; theaffected individuals are male and the mothers are carriersof the PLP1 mutation.

Severity and onset of the disease ranges widely, dependingon the type of PLP1 mutation.

One of a spectrum of diseases associated with PLP1.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Types of PMD[1]

There are 4 general classifications within this spectrum ofdiseases. In order of severity, they are:

Connatal PMD, which is the most severe type and involvesdelayed mental and physical development and severeneurological symptoms

Classic PMD, in which the early symptoms include muscleweakness, involuntary movements of the eyes(nystagmus), and delays in motor development within thefirst year of life

Complicated SPG2, which features motor developmentissues and brain involvement

Pure SPG2, which includes cases of PMD that do nothave neurologic complications.

Noticeable changes in the extent of myelination can bedetected by MRI analyses of the brain.Chintalagiri Shashank, [email protected] Pelizaeus-Merzbacher Disease



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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Modern morphological, biochemical, and molecular techniqueshave made this distinction obsolete, and PMD must now beconsidered a leukodystrophy with variable clinical andneuropathological phenotypes, although all cases are due tomutations of the proteolipid protein (PLP) gene.




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Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Disease Mechanism

Proposed roles for PLP1 include mediating interlamellaradhesion in compact myelin, mediating ion flux, and acting asan oligodendrocyte precursor mitogen.

PLP is a major structural component of CNS myelin, whereasDM20 which is produced earlier in CNS development may beinvolved in oligodendrocyte differentiation and survival.

Complete deficiency of PLP1 does not prevent myelination, butit does result in late-onset axonal degeneration.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Disease Mechanism

In addition to null mutations, mutations that disrupt thePLP1-specific region, a 35-amino-acid region that is spliced outduring formation of the DM20 isoform, cause both peripheralneuropathy and central axonal degeneration.

Single amino-acid changes in highly conserved regions of theDM20 protein caused the most severe forms of PMD.Substitutions of less conserved amino acids, truncations,absence of the protein and PLP-specic mutations cause themilder forms of PMD and SPG.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Disease Mechanism

Gow and Lazzarini have suggested a cellular mechanism fordisease severity in PMD. They reported that classical PMDcorrelates with misfolding and accumulation of PLP1 in theendoplasmic reticulum (ER) and transport of DM20 to the cellsurface, while connatal PMD correlates with misfolding andaccumulation of both PLP1 and DM20 in the ER.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Pathogenesis

Gencic et.al.[5] report that the underlying disturbance inmyelination of Pelizaeus-Merzbacher patients wasattributed to a failure to form myelin (dys-myelination)rather than to a breakdown of preexisting myelin(demyelination) by a pathogenic analysis by Zeman et.al.in 1964.

Zeman et.al. also predicted that the defect would involvea myelin protein or proteolipid




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Animal Mutants

PLP deficiency relates to a X-linked CNS myelin deficiency inseveral animal mutants

jimpy mouse (jp) is the oldest and best known

the myelin-deficient rat (md)

the shaking pup (sh)

the rumpshaker mouse (rsh)

the rabbit with paralytic tremor (pt)

The deficiency of PLP protein was commensurate with lowlevels of the matching messenger ribonucleic acid (mRNA).




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Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Implication of PLP

Two observations focused attention on the major structuralprotein of myelin, proteolipid protein (PLP), as a candidate formutation in Pelizaeus-Merzbacher disease.

Assignment of the PLP gene to the human X chromosomeat position Xq22, which supported the X-linkedinheritance of the disease.

Discovery that the dysmyelinating mouse mutant jimpy,which appears pathologically and genetically similar toPelizaeus-Merzbacher disease, has a mutation in the PLPgene that results in aberrantly spliced PLP transcripts(Morello et al. 1986; Nave et al. 1986, 1987; Hudson etal. 1987; Macklin et al. 1987; Ikenaka et al. 1988).




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Myelin[8]




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Introduction


Types of PMD

DiseaseMechanism

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Animal Mutants



References

Structure of PLP




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Structure of PLP

PLP is a tetraspan protein, with 4 TM α-helices spanningthe myelin membrane. Both N- and C-terminals are in thecytoplasmic side.Immunolabeling studies of PLP and several proteinstructure prediction algorithms were used to determine themost likely residues making up the α-helices.The region of PLP that is deleted in DM20 (residues 116150) is shown in yellow.Cysteines involved in the formation of the two disuldebonds in PLP (linking residues 183 227 and 200 219) areshown in blue.6 cysteine residues are thought to be acylated (shown inred), with palmitic acid sidechains attached. Alternatively,some of these cysteine residues could be involved indisulde linkages.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Structure of PLP

Structural properties of proteins specic to the myelin sheath,Amino Acids (2008)[12]

Some of the myelin proteins, belonging to the familyof tetraspanins, are amongst the most hydrophobicproteins known. One of these proteins is theproteolipid protein. This group of proteins is mostpoorly characterised structurally, and will not bediscussed in this review.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Mutation of PLP

The mutation causing PMD seems to be different fordifferent families.

PMD is always caused by a mutation of PLP.

A majority of the cases seem to be caused by duplicationsinvolving the entire gene (larger than 100 kb)

In some families, PMD is caused by the mutation of asingle amino acid.




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Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Mutations[2]

Clinical observations and studies of PLP mutations in animalsand cell cultures suggest that there at least 3 distinct geneticmechanisms that cause PMD. [2]

In addition, spastic paraplegia 2 (SPG2) is allelic to PMD andtypically caused by missense mutations in the secondextracellular domain of PLP1 or in the PLP1-specific regionthat is spliced out during formation of the DM20 isoform.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Molecular mechanisms of PMD pathogenesis[3]




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Introduction


Types of PMD

DiseaseMechanism

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Animal Mutants



References

The first of these mechanisms produces loss of PLP function,in which PLP does not accumulate in the cell. To date, 4so-called null mutations that cause PMD have been identified,including a deletion of the entire PLP gene, all of whichproduce a similar, relatively mild clinical phenotype.

Noncoding mutations affecting splicing of PLP1 and deletionshave been described in patients with PMD with the lowestfrequency.




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Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

The second of these genetic mechanisms produces again-of-toxic function. Experimental evidence supports thismechanism. Gain-of-function mutations, typically amino acidsubstitutions, prevent PLP from reaching the cell surface bydisrupting normal PLP folding. The mutant protein thenaccumulates in the endoplasmic reticulum, somehow triggeringincreased oligodendrocyte cell death by apoptosis, withresultant dysmyelination. The clinical phenotype caused by again-of-function mutation depends on the location of thealtered amino acid, as well as on the particular amino acidsubstituted.

Over 100 point mutations in the PLP1 coding region have beenidentified, and these account for approximately 15% to 20% ofPMD cases.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

The third and most common genetic mechanism is duplicationof the region of the X chromosome that contains the PLPgene. Since overexpression of PLP and/or DM20 is sufficient tocause both CNS dysmyelination and subsequent demyelinationin transgenic mice, the human duplication probably producesPMD for similar reasons. The molecular mechanismsunderlying the PLP duplication have not yet been elucidated.The breakpoints of the PLP duplication often vary betweenpatients, and inclusion of flanking genes in addition to PLPand/or disruption of a flanking gene may explain differences inphenotypic severities among patients with PLP duplication.

This mutation probably accounts for 50% to 70% of the casesof PMD.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Genotype - phenotype correlation in PLP diseaseCailloux et al, 2000

52 PMD and 28 SPG families selected for sequencing ofthe seven coding regions and the exon/intron junctions ofthe PLP gene

Identied 33 abnormalities (29 in PMD patients, 4 in SPGpatients)




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Introduction


Types of PMD

DiseaseMechanism

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Animal Mutants



References

Type and Position of Mutation

Twenty-three were missense mutations, three weredeletion/insertions with frameshifts and seven weresplice-site mutations.

Mutations were in

1 coding regions in 24 of 29 PMD patients (80%)2 exons 2 (29%), 4 (29%) and 5 (21%)3 2 of the mutations in SPG patients were in the

PLP-specific coding region, exon 3B.4 No mutations were observed in exons 1 and 7 of PLP

Of the 23 aa changes resulting from missense mutations

1 48% affected the CD loop2 each of the other locations accounted for only 4 to 13% of

the mutations.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Correlation between severity and type of mutation

Missense mutations were observed in two thirds of casesfor the severe forms and in one third of cases for themilder forms .

All other types of abnormality were observed almostexclusively in the mildest forms (80%).




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Introduction


Types of PMD

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Correlation between disease severity and theposition of exonic mutations

Of the 26 mutations in coding regions

1 15 were responsible for severe PMD forms (57%)2 9 for mild PMD forms (35%)3 2 for SPG form 4 (8%)

In terms of exons,

1 Mutations causing severe forms of PMD mappedessentially to exons 2 (40%) and 4 (33%), more rarely toexons 6 (15%), 5 (7%), and 3A (7%), and never to exons1, 3B and 7.

2 Mutations causing mild forms of PMD mappedpredominantly to exon 5 (44%), more rarely to exons 4(22%), 6 (22%) and 2 (12%).

3 SPG mutations mapped exclusively to exon 3B.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Schematic representation of PLP




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Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Severity of the disease




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Introduction


Types of PMD

DiseaseMechanism

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Animal Mutants



References

Correlation between severity the type of AAsubstitution resulting from missense mutations

Changes in the PLP-specific BC loop were responsible forthe SPG phenotype.

Ten of the 11 substitutions in the AB extracytoplasmicloop and the 4 transmembrane segments of thePLP/DM20 protein caused severe forms of PMD.

Five of the 11 substitutions in the CD loop caused severePMD and 6 mild PMD.

3 families had three different exon 4 mutations resulting inthe substitution of the same amino acid, at position 202 inthe PLP/DM20 protein (D202N, D202G,D202E) and hadthe most severe form of PMD.

2 families had two different exon 5 mutations resulting insubstitution of the same amino acid, at position 215(P215S, P215A).




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

Correlation between severity the type of AAsubstitution resulting from missense mutations

Replacement of a highly conserved amino acid, whateverthe new amino acid, caused the most severe forms ofPMD, whereas substitutions of less conserved amino acidscaused milder forms.

In 2 cases, a severe form of PMD was observed (Y174Cand A247E), despite substitution of a poorly conservedamino acid. In two cases, a severe form of PMD wasobserved (Y174C and A247E), despite substitution of apoorly conserved amino acid.




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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

List of point mutations




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Types of PMD

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Distribution of point mutations




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References

Myelin Composition[8](2009)




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References

References I

NINDS; Pelizaeus-Merzbacher Disease Information Page

http://www.ninds.nih.gov/disorders/pelizaeus_merzbacher/pelizaeus_merzbacher.htm.

James Garbern et.al.; The Molecular Pathogenesis of Pelizaeus-Merzbacher Disease, Arch. Neurol.,

VOL 56, Oct 1999

J. Y. Garbern; Pelizaeus-Merzbacher disease:Genetic and cellular pathogenesis; Cell. Mol. Life Sci. 64

(2007) 50 65

Fabrice Cailloux et.al; Genotypephenotype correlation in inherited brain myelination defects due to

proteolipid protein gene mutations; Eur. J. Hum. Genet. (2000) 8, 837845

Simonida Gencic et.al.; Pelizaeus-Merzbacher Disease: An X-linked Neurologic Disorder of Myelin

Metabolism with a Novel Mutation in the Gene Encoding Proteolipid Protein, Am. J. Hum. Genet.45:435-442, 1989

Judith M. Greer, Marjorie B. Lees; Myelin proteolipid proteinthe rst 50 years; Int. J. Biochem. & Cell

Biol. 34 211215; 2002

Eun Sil Lee et.al.; A case of complicated spastic paraplegia 2 due to a point mutation in the

proteolipid protein 1 gene; J. Neu. Sci. 224 (2004) 83 87

Olaf Jahn, Stefan Tenzer, Hauke B. Werner;Myelin Proteomics: Molecular Anatomy of an Insulating

Sheath; Mol Neurobiol (2009) 40:5572


http://www.ninds.nih.gov/disorders/pelizaeus_merzbacher/pelizaeus_merzbacher.htm



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Outline

Introduction


Types of PMD

DiseaseMechanism

Pathogenesis

Animal Mutants



References

References II

Marie-Noelle Bonnet-Dupeyron et.al.;PLP1 Splicing Abnormalities Identified in Pelizaeus-Merzbacher

Disease and SPG2 Fibroblasts Are Associated With Different Types of Mutations; Hum. Mut. 29(8),1028 - 1036, 2008

Ken Inoue;PLP1-related inherited dysmyelinating disorders:Pelizaeus-Merzbacher disease and spastic

paraplegia type 2; Neurogenetics (2005) 6: 1 16

Olaf Maier et.al.;Polarity Development in Oligodendrocytes: Sorting and Trafficking of Myelin

Components; J Mol Neurosci (2008) 35:35 53

P. Kursula; Structural properties of proteins specic to the myelin sheath; Amino Acids (2008) 34: 175

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Pelizaeus–Merzbacher disease