Phenotypic Mutation 'taz' (pdf version)
Alleletaz
Mutation Type missense
Chromosome1
Coordinate170,986,379 bp (GRCm39)
Base Change C ⇒ T (forward strand)
Gene Mpz
Gene Name myelin protein zero
Synonym(s) Mpp, P0
Chromosomal Location 170,978,282-170,988,699 bp (+) (GRCm39)
MGI Phenotype FUNCTION: This gene is specifically expressed in Schwann cells of the peripheral nervous system and encodes a type I transmembrane glycoprotein that is a major structural protein of the peripheral myelin sheath. The encoded protein contains a large hydrophobic extracellular domain and a smaller basic intracellular domain, which are essential for the formation and stabilization of the multilamellar structure of the compact myelin. Mutations in the orthologous gene in human are associated with myelinating neuropathies. A recent study showed that two isoforms are produced from the same mRNA by use of alternative in-frame translation termination codons via a stop codon readthrough mechanism. Alternatively spliced transcript variants have also been found for this gene. [provided by RefSeq, Oct 2015]
PHENOTYPE: Mice homozygous for a spontaneous mutation exhibit premature death, infertility, neurological behavior defects, and demyelination. Mice homozygous for a knock-out allele exhibit abnormal myelination and neurological behavior defects. [provided by MGI curators]
Accession Number

NCBI RefSeq: NM_001315499 (variant 1), NM_001315500 (variant 1), NM_008623 (variant 2); MGI:103177

MappedYes 
Amino Acid Change Arginine changed to Cysteine
Institutional SourceBeutler Lab
Gene Model predicted gene model for protein(s): [ENSMUSP00000066701] [ENSMUSP00000106966]
AlphaFold no structure available at present
SMART Domains Protein: ENSMUSP00000066701
Gene: ENSMUSG00000056569
AA Change: R98C

DomainStartEndE-ValueType
signal peptide 1 29 N/A INTRINSIC
IGv 45 129 1.39e-11 SMART
transmembrane domain 155 177 N/A INTRINSIC
Pfam:Myelin-PO_C 184 248 4.3e-38 PFAM
Predicted Effect probably damaging

PolyPhen 2 Score 1.000 (Sensitivity: 0.00; Specificity: 1.00)
(Using ENSMUST00000070758)
SMART Domains Protein: ENSMUSP00000106966
Gene: ENSMUSG00000056569
AA Change: R98C

DomainStartEndE-ValueType
low complexity region 2 29 N/A INTRINSIC
IGv 45 129 1.39e-11 SMART
transmembrane domain 155 177 N/A INTRINSIC
Pfam:Myelin-PO_C 179 248 4.8e-44 PFAM
Predicted Effect probably damaging

PolyPhen 2 Score 1.000 (Sensitivity: 0.00; Specificity: 1.00)
(Using ENSMUST00000111334)
Meta Mutation Damage Score 0.9623 question?
Is this an essential gene? Probably nonessential (E-score: 0.088) question?
Phenotypic Category Autosomal Recessive
Candidate Explorer Status loading ...
Single pedigree
Linkage Analysis Data
Penetrance  
Alleles Listed at MGI

All Mutations and Alleles(23) : Chemically induced (ENU)(1) Chemically induced (other)(2) Endonuclease-mediated(2) Gene trapped(6) Spontaneous(1) Targeted(2) Transgenic(9)

Lab Alleles
AlleleSourceChrCoordTypePredicted EffectPPH Score
IGL00568:Mpz APN 1 170987571 missense possibly damaging 0.84
IGL03051:Mpz APN 1 170986380 missense probably damaging 1.00
Half-pint UTSW 1 170987204 critical splice donor site probably null
R0279:Mpz UTSW 1 170987498 splice site probably benign
R0791:Mpz UTSW 1 170986343 missense possibly damaging 0.85
R1164:Mpz UTSW 1 170986008 missense possibly damaging 0.92
R1368:Mpz UTSW 1 170987533 missense probably damaging 1.00
R4043:Mpz UTSW 1 170987340 splice site probably benign
R4857:Mpz UTSW 1 170986379 missense probably damaging 1.00
R5682:Mpz UTSW 1 170986463 missense possibly damaging 0.62
R6709:Mpz UTSW 1 170978301 unclassified probably benign
R7089:Mpz UTSW 1 170987204 critical splice donor site probably null
R7748:Mpz UTSW 1 170987509 critical splice acceptor site probably null
R7888:Mpz UTSW 1 170987204 critical splice donor site probably null
R8023:Mpz UTSW 1 170987602 missense probably damaging 1.00
Mode of Inheritance Autosomal Recessive
Local Stock Live Mice
Repository
Last Updated 2019-09-04 9:42 PM by Anne Murray
Record Created 2016-09-14 2:14 PM by Jamie Russell
Record Posted 2018-04-11
Phenotypic Description
Figure 1. Video of the taz phenotype.

The taz phenotype was identified among N-ethyl-N-nitrosourea (ENU)-mutagenized G3 mice of the pedigree R4857, some of which showed ataxia. Some mice also pulled their hind legs together close to their bodies when picked up they their tail (Figure 1).

Nature of Mutation

Figure 2. Linkage mapping of the behavioral phenotype using a recessive model of inheritance. Manhattan plot shows -log10 P values (Y-axis) plotted against the chromosome positions of 111 mutations (X-axis) identified in the G1 male of pedigree R4857. Phenotype data are shown for single locus linkage analysis without consideration of G2 dam identity.  Horizontal pink and red lines represent thresholds of P = 0.05, and the threshold for P = 0.05 after applying Bonferroni correction, respectively.

Whole exome HiSeq sequencing of the G1 grandsire identified 111 mutations. The behavioral phenotype was linked to a mutation in Mpz: a C to T transition at base pair 171,158,810 (v38) on chromosome 1, or base pair 8,098 in the GenBank genomic region NC_000067 encoding Mpz. Linkage was found with a recessive model of inheritance (P = 2.32 x 10-4), wherein four affected mice were homozygous for the variant allele, and 15 unaffected mice were either heterozygous (n = 8) or homozygous (n = 7) for the reference allele (Figure 2).

The mutation corresponds to residue 458 in the NM_008623 mRNA sequence in exon 5 of 8 total exons and residue 356 in the NM_001315499 and NM_001315500 mRNA sequences in exon 3 of 6 total exons. 

 

443 GGGACCTTCAAAGAGCGCATCCAGTGGGTAGGG

93  -G--T--F--K--E--R--I--Q--W--V--G-

The mutated nucleotide is indicated in red.  The mutation results in an arginine (R) to cysteine (C) substitution at position 98 (R98C) in both variants of the MPZ protein, and is strongly predicted by PolyPhen-2 to be damaging (score = 1.000).

Illustration of Mutations in
Gene & Protein
Protein Prediction
Figure 3. Domain organization of MPZ. The location of the taz mutation is indicated. Abbreviations: SP, signal peptide; TM, transmembrane domain.

Mpz encodes myelin protein zero (MPZ), a member of the immunoglobulin supergene family. MPZ has a 29-amino acid signal peptide that is cleaved prior the insertion of the remaining protein into the myelin sheath (1). The mature MPZ protein is a single-pass transmembrane protein (Figure 3).

The extracellular domain of MPZ is similar to an immunoglobulin variable region (Ig-like V-type) domain (2). The extracellular domain of rat MPZ forms a compact sandwich of beta-sheets held together by a disulfide bridge [PDB:1NEU; (3)]. Four MPZ extracellular domains can form cis interactions, which promote the formation of a doughnut-like structure around a large central hole (3). The MPZ homotetramers can subsequently interact in trans with the extracellular domains of MPZ tetramers from the opposing myelin wrap to form the intraperiod line of compact myelin (3). The intracellular domain of MPZ promotes MPZ-mediated homotypic adhesion. The intracellular domain also mediates compaction of the Schwann cell cytoplasm and formation of the major dense line in peripheral nervous system myelin.

MPZ undergoes several posttranslational modifications necessary for its adhesion function. Mouse MPZ is phosphorylated by PKC at Ser210 and Ser233 (UniProt). MPZ is also phosphorylated at Ser226, Ser228, Ser237, and Ser243 [UniProt; (4)]. MPZ is acylated at Cys153 (5), forms a disulfide bond between Cys50 and Cys127 (locations in mouse MPZ) (6), and is glycosylated at Asn93 (Asn122 in mouse) (7).

The taz mutation results in an arginine (R) to cysteine (C) substitution at position 98 (R98C); residue 98 is within the Ig-like V-type domain

Expression/Localization

MPZ is expressed only in myelinating Schwann cells of the peripheral nervous system. MPZ is normally targeted to the compact region of the mature myelin sheath.

Background
Figure 4. MPZ is one of several proteins within the compact region of the mature myelin sheath that function in myelin sheath formation. During myelination, a glial process wraps around the axon, forming multiple layers of myelin and elongating along the axon. The myelinating glial cells organize the axons into segments: the nodes of Ranvier, paranode, and juxtaparanode. Abbreviations: MPZ, myelin protein zero; PMP22, peripheral myelin protein-22; CX32, connexin32; MBP, myelin basic protein.

Most axons are insulated with myelin sheaths, which allows for the rapid propagation of nerve impulses.The myelin sheath is essential for the proper function of the nervous system. Myelin is a multilamellar membrane formed by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system. During myelination, a glial process wraps around the axon, forming multiple layers of myelin and elongating along the axon. The myelinating glial cells organize the axons into segments: the nodes of Ranvier, paranode, and juxtaparanode (Figure 4). MPZ, peripheral myelin protein-22 (PMP22), connexin32 (CX32), and myelin basic protein (MBP) within the compact region of the mature myelin sheath participate in forming the myelin sheath and in reducing capacitance of the internode.

Mpz-deficient (Mpz-/-) mice exhibit tremors, occasional convulsions, and defects in normal motor coordination (8). Mpz-/- mice exhibit loss of intraperiod lines and widening of the intraperiod spaces of the myelin sheath (8). In addition, Schwann cell loops were completely uncompacted and axon-Schwann cell units without myelin were observed (8;9). Mpz-/- mice exhibit severe hypomeylination with myelin decompaction (8;10;11). With age, myelin degeneration and onion bulb formation was observed. Axon degeneration was also observed in the Mpz-/- mice (8;12). Heterozygous (Mpz+/-) mice exhibited a less severe phenotype: myelin sheaths of the heterozygous mice were comparable to that in wild-type mice until four weeks of age (10). Mpz+/- mice exhibited myelin degeneration, onion bulb formation, and remyelination (10). Invading CD4+ and CD8+ lymphocytes were observed in the Mpz+/- mice (13;14). Also, an influx of macrophages were observed in close association with demyelinating or demyelinated axons (15). An ENU-induced mutation in Mpz resulted in an Asp to Gly substitution at amino acid 121. The Mpz mutation caused tremor and hypermetric gait (16). Mpz transgenic mice that overexpressed Mpz exhibited a dose-dependent, demyelinating neuropathy (17). Several of the Mpz transgenic founders exhibited ataxia, tremor, reduced weight, atrophy of the paraspinal and hindlimb musculature, and reduced lifespan (17). When bred onto the Mpz-/- genetic background, myelination was restored.

Patients with hereditary motor and sensory neuropathies often show disruption of peripheral nervous system myelination, while patients with multiple sclerosis show disrupted myelination in the central nervous system [reviewed in (18)]. Mutations in human MPZ disrupt myelination during development or disrupt axo-glial interactions in adulthood. Mutations in MPZ are linked to demyelinating neuropathies, including several variants of Charcot-Marie-Tooth disease: Charcot-Marie-Tooth disease, dominant intermediate D (OMIM: #607791), Charcot-Marie-Tooth disease, type 1B (CMT1B; OMIM: #118200), Charcot-Marie-Tooth disease, type 2I (OMIM: #607677), and Charcot-Marie-Tooth disease, type 2J (OMIM: #607736) (19-23). In general, patients with CMT can exhibit tonic pupils, deafness, and late-onset sensorimotor neuropathy, weakness and sensory loss of lower limbs, and gait disturbances. CMT1B is characterized by progressive slowing of nerve conduction and hypertrophy of Schwann cells. Mutations in MPZ are also linked to more severe polyneuropathies, including Dejerine-Sottas disease [DSS; OMIM: #145900; (24;25)], congential hypomyelinating neuropathy [CHN; OMIM: #605253; (26)], and Roussy-Levy syndrome [OMIM# 180800; (27)]. A mutation in MPZ was also linked to chronic cough (28). The coughing spasms were attributed to autonomic neuropathy.

Putative Mechanism

Mutations in MPZ are proposed to cause neuropathy by altering protein–protein interactions and inhibiting homotypic adhesion. The phenotype observed in the taz mice indicates loss of MPZtaz function.

Primers PCR Primer
taz_pcr_F: TGTCCTGGGGTTAGACAACAG
taz_pcr_R: TTTGAAATGCCTCCCAGAACC

Sequencing Primer
taz_seq_F: GCTACAGAAGGGAAAATGTTTTATGC
taz_seq_R: ATGCCTCCCAGAACCCTGTC
Genotyping

PCR program

1) 94°C 2:00
2) 94°C 0:30
3) 55°C 0:30
4) 72°C 1:00
5) repeat steps (2-4) 40x
6) 72°C 10:00
7) 4°C hold


The following sequence of 447 nucleotides is amplified (chromosome 1, + strand):


1   tgtcctgggg ttagacaaca gataggtact taaaacaacg gaaaagagaa cctgggctac
61  agaagggaaa atgttttatg ctcacaaact caactgtaga aaggtaatct gggatcctga
121 gtctggagcc aagtcttggg agcttttcac actaggattc tttcacacaa tttccatcat
181 tcctcataga tcttccacta tgccaaggga caaccttaca tcgatgaggt ggggaccttc
241 aaagagcgca tccagtgggt aggggaccct cgctggaagg atggctccat tgtcatacac
301 aacctagact acagtgacaa cggcactttc acatgtgatg tcaaaaaccc accggacata
361 gtgggcaaga cctctcaggt cacgctctat gtctttgaaa aaggtgtgag aaggaacaag
421 ggacagggtt ctgggaggca tttcaaa


Primer binding sites are underlined and the sequencing primers are highlighted; the mutated nucleotide is shown in red.

References
Science Writers Anne Murray
Illustrators Katherine Timer
AuthorsLauren Prince, Jamie Russell, and Bruce Beutler