PLP1-Related Disorders

Clinical Diagnosis

Imaging studies

Magnetic resonance imaging (MRI) is most helpful in establishing the diagnosis in those who have CNS involvement [Caro & Marks 1990, Ono et al 1994, Nezu et al 1998].

• PMD.  Virtually all children with the PMD phenotype have a diffuse leukoencephalopathy, best appreciated on T2-weighted or fluid-attenuated inversion recovery (FLAIR) scans. These scans show diffusely increased signal intensity in the central white matter of the cerebral hemispheres, cerebellum, and brainstem. In most children, the amount of white matter is reduced in volume and can be readily seen as thinness of the corpus callosum or general decrease in myelination [Plecko et al 2003].

  
  Because the bulk of myelination normally occurs during the first two years of life, the T2-weighted MRI images may not show definitive abnormalities until a child is at least age one or two years. However, a normal newborn should have myelination-related T1 and T2 signal changes in the pons and cerebellum, and a normal three-month-old infant should have evidence of myelination in the posterior limb of the internal capsule, in the splenium of the corpus callosum, and in optic radiations [Barkovitch 2005]. Absence of these early changes should raise the consideration for PMD or other dysmyelinating disorders.

• SPG2.  People with the SPG2 phenotype show less severe abnormalities on MRI scanning; they may have patchy abnormalities on T2-weighted scans or more diffuse leukoencephalopathy [Cambi et al 1995, Hodes et al 1999].

Magnetic resonance spectroscopy (MRS) may show reduced white matter N-acetyl aspartate (NAA) levels, especially in individuals with the PLP1 null syndrome [Bonavita et al 2001, Garbern & Hobson 2002, Plecko et al 2003]. In contrast, individuals with PLP1 duplications may have increased white matter NAA levels and may be misdiagnosed with Canavan disease [Takanashi et al 2002]. The pattern of metabolite abnormalities in individuals with PLP1 duplication appears distinctive, with increased levels of glutamine, inositol, and creatine as well as of NAA, and may assist in the discrimination of PMD from other leukodystrophies and demyelinating diseases [Hanefeld et al 2005].

Neurophysiologic studies

The features of the nystagmus and other subtle disturbances of eye movements can be quantified and characterized with sensitive ocular movement testing [Trobe et al 1991, Huygen et al 1992]. These features may be of diagnostic usefulness but have not been rigorously tested in a large number of affected individuals with verified PLP1 mutations.

Visual, auditory, and somatosensory evoked potential testing show normal to near-normal latencies of the peripheral component of the respective sensory modality, but severely prolonged or absent central latencies [Markand et al 1982, Garg et al 1983, Nezu 1995].

Except in families with PLP1 null alleles or mutations affecting the PLP1-specific region or some splice site mutations [Garbern et al 1997, Shy et al 2003, Vaurs-Barriere et al 2003], peripheral nerve conduction studies are normal [Wilkus & Farrell 1976]. When peripheral neuropathy is present, it is mild in comparison to the central nervous system disorder, and is characterized by mild slowing of conduction velocities that may be more pronounced across those regions of a limb susceptible to compression, such as the wrist and elbow.

Testing

Cytogenetic analysis.  Fewer than 1% of individuals with clinical features of PMD have interstitial duplications or more complex rearrangements of the X chromosome visible on routine cytogenetic studies [Cremers et al 1987, Carrozzo et al 1997, Hodes et al 2000].

Molecular Genetic Testing

Gene.   PLP1 is the gene most often associated with PMDand SPG2. Between 80% and 95% of males with PMD or SPG2 have an identifiable alteration in PLP1.

The classes of mutations that cause PLP1-related disorders are duplications, point mutations, and deletions. Triplication and quintuplication of the PLP1 gene also occur [Woodward, Kendall, Vetrie et al 1998; Boespflug-Tanguy et al 1999; Wolf et al 2005].

Other loci.  About 5%-20% of males with clinical findings consistent with the PLP1-related disorders do not have identifiable mutations in the PLP1 gene, suggesting that one or more additional autosomal or X-linked loci can cause these phenotypes [Osaka et al 1999] or that mutations occur in regions of the gene not routinely analyzed such as far upstream or downstream regions or introns.

• One autosomal gene, GJA12, causes a syndrome very similar to PMD [Uhlenberg et al 2004] (see Differential Diagnosis).

• One family with clinical features of PMD demonstrated linkage to Xq28 [Lazzarini et al 1997]. However, the observation that the duplicated PLP1 gene can be inserted elsewhere on the X chromosome might account for the failure to identify linkage to the PLP1 locus in this family and others.

Molecular genetic testing: Clinical uses

• Confirmation of diagnosis

• Carrier testing

• Prenatal diagnosis

• Preimplantation genetic diagnosis

Molecular genetic testing: Clinical methods

Duplication/deletion testing includes FISH, array genomic hybridization, Southern blot analysis, quantitative PCR, multiple ligation probe analysis (MLPA) [Wolf et al 2005, Warshawsky et al 2006], multiplex amplifiable probe hybridization (MAPH) [Combes et al 2006], and real-time PCR [Gao et al 2005, Hübner et al 2005, Regis et al 2005]:

• Duplications and other gene dosage changes of variable size are found in at least 50% of males with PLP1-related disorders [Inoue et al 1996, Woodward et al 1997, Sistermans et al 1998, Inoue et al 1999]; they are typically tandem duplications occurring in Xq22, which includes the entire PLP1 gene. In rare instances, the duplicated region can be inserted at some distance from Xq22; four insertions have been reported: at Xp22, Xq28 [Woodward, Kendall, Vetrie et al 1998; Hodes et al 2000], and 19qtel [Inoue, Osaka et al 2002] and in the Y chromosome [Woodward et al 2005, Warshawsky et al 2006].

  
  Interphase FISH analysis is a sensitive method of detecting duplications; however, it is possible that some duplications may be too small to detect by conventional FISH and are better detected by other techniques such as quantitative PCR (QPCR) or quantitative Southern blot analysis. Because the probes for FISH analysis are typically quite large (>40 kbp) and include sequences that lie outside of the PLP1 gene, which is approximately 20 kbp, it is possible that FISH testing may indicate PLP1 duplication but not actually involve the PLP1 gene [Lee et al 2006]. Therefore, more direct testing of PLP1 gene duplication should be done if the clinical syndrome is not typical of a PLP1-related disorder.

  
  Chromosomal microarray analysis (CMA) can detect PLP1 dosage changes; however, as with the PCR-based methods, CMA is not able to identify translocations or noncontiguous insertions of the supernumerary PLP1 genes [abnormalities; loci]. Because some CMA chips use large genomic fragments, false-positive or false-negative test results may occur, as with FISH analysis. One of the PCR-based methods should be used to verify the CMA results.

• Deletions of the entire PLP1 gene, which can be detected by FISH and by QPCR, occur in fewer than 2% of those with the Pelizeus-Merzbacher disease phenotype [Raskin et al 1991; Boespflug-Tanguy et al 1999; Inoue, Osaka et al 2002; Hobson, unpublished observations; Shaffer, unpublished observations]. Inoue, Osaka et al (2002) determined that the individual originally described with a PLP1 deletion has a complex rearrangement with both deletion of PLP1 and an inverted insertion of a more distal portion of the X chromosome at the deletion junction. In addition, this individual has duplication of a region 3' of PLP1 [Hobson, Ritterson et al 2002]. Partial PLP1 deletion has also been reported [Nave & Boespflug-Tanguy 1996].

• Position effect rearrangements appear to account for a small proportion of PMD and SPG2.

  • Disruption of regulatory elements of a gene neighboring PLP1 resulting in dysregulation of PLP1 expression through position effect was proposed as the explanation for a PMD-like syndrome in a child in whom conventional chromosome analysis identified an inversion of the X chromosome with a breakpoint near (70 kbp) but not including the PLP1 gene [Muncke et al 2004].

  • Position effect of a duplication identified by FISH that was near but not within the PLP1 gene has been invoked as the cause of the neurologic syndrome in a man with spastic paraplegia [Lee et al 2006].

Sequence analysis.  Point mutations, which account for approximately 15%-20% of identified PLP1 mutations, can be identified by gene sequencing [Pratt et al 1991, Pratt et al 1992, Sistermans et al 1998]. While most point mutations result in missense or frameshift changes, mutations that affect splicing also occur [Hobson et al 2000, Hobson et al 2002, Hübner et al 2005, Hobson et al 2006, Wang et al 2006].

Table 2 summarizes molecular genetic testing for this disorder.

Table 2. Molecular Genetic Testing Used in the PLP1-Related Disorders

Test Method	Mutations Detected	Mutation Detection Rate	Test Availability
Duplication/deletion testing  1	Duplication/deletion of PLP1	50%-75%	Clinical
Sequence analysis	Point mutations of PLP1	15%-25%  2

1. Includes FISH, array genomic hybridization, Southern blot analysis, quantitative PCR, multiple ligation probe analysis (MLPA), multiplex amplifiable probe hybridization (MAPH), and real-time PCR
2. Percent of all those with a point mutation. Sequence analysis is generally only performed when a PLP1 duplication is not detected.

Interpretation of test results

• For issues to consider in interpretation of sequence analysis results, click here.

• It should be stressed that guidelines for the techniques used in duplication analysis are still under development, and equivocal or negative results from quantitative PCR or FISH should be interpreted with caution.

Testing Strategy for a Proband

• 1

  It is recommended that testing start with screening for PLP1 gene duplication, the most common type of mutation observed in males with PMD. Interphase FISH as well as quantitative PCR techniques are recommended.

  • Quantitative PCR and related techniques have the advantage of being able to detect duplications too close together to resolve by FISH, but the disadvantage of not being able to distinguish the more common tandem duplications from the less common duplications with distant insertions.

  • Metaphase FISH can identify distantly inserted duplications, whereas PCR cannot identify the location of the duplicated region.

  • Metaphase FISH can also demonstrate mosaicism for chromosomal anomalies, as in the unusual situation in which a woman was mosaic for both duplication and deletion of PLP1 and therefore had normal quantitative PCR [Woodward et al 2003].

• 2

  When appropriate, karyotype analysis should be performed to evaluate an affected male for the presence of two X chromosomes (Klinefelter syndrome) or to identify individuals who have X-chromosome rearrangements that may cause position effect-related disturbance of PLP1 expression.

• 3

  If a PLP1 duplication is not identified, sequencing of the PLP1 gene is the next step.

Genetically Related (Allelic) Disorders

No other phenotypes are associated with mutations in the PLP1 gene.

Natural History

Pelizaeus-Merzbacher disease (PMD) and X-linked spastic paraplegia 2 (SPG2) are at opposite ends of a clinical spectrum of disease caused by mutations of the gene for proteolipid protein (PLP1), which results in defective central nervous system (CNS) myelination. PMD and SPG2 have been observed in different males within the same family [Bonneau et al 1993, Hodes et al 1993, Nave & Boespflug-Tanguy 1996, Seitelberger et al 1996, Sistermans et al 1998].

Connatal PMD, the most severe form, is apparent at birth or in the first few weeks of life. Findings include pendular nystagmus, hypotonia, pharyngeal weakness, and stridor. Seizures may develop in affected infants, and motor deficits are severe.

Later, children with connatal PMD may have short stature and poor weight gain. Hypotonia later evolves into spasticity of the extremities that is usually quite severe. Children do not walk or develop effective use of the upper limbs. Verbal expression is severely limited, but comprehension may be significant. Swallowing difficulties may require consideration of feeding tube placement.

Affected children may die during infancy or childhood, usually of aspiration; with attentive care, they may live into the third decade or longer.

Classic PMD was originally described by Pelizaeus (1885) and Merzbacher (1910). Males with classic PMD usually develop nystagmus, which may not be recognized until several months of age; in rare cases, nystagmus does not develop. Affected children have hypotonia and develop titubation (tremor of the head and neck), ataxia, and spastic quadriparesis beginning in the first five years. They usually have some purposeful voluntary control of the arms. If acquired, ambulation usually requires assistive devices such as crutches or a walker and is lost as spasticity increases during later childhood or adolescence.

Cognitive abilities are impaired, but exceed those of the more severely affected children; language and speech usually develop. Extrapyramidal abnormalities, such as dystonic posturing and athetosis, may occur.

Survival into the sixth or seventh decade has been observed.

A transitional form, intermediate in onset and severity to the connatal and classic forms of PMD, has also been defined.

PLP1 null syndrome is distinguished by the absence of nystagmus and the presence of relatively mild spastic quadriparesis that mostly affects the legs, with ataxia and mild multifocal demyelinating peripheral neuropathy. Those with the PLP1 null syndrome generally ambulate better than those with classic PMD but may progress more rapidly because of degeneration of axons, inferred on the basis of magnetic resonance spectroscopy, which demonstrates reduced levels of white matter NAA [Garbern et al 2002].

Complicated spastic paraparesis (SPG2) often includes autonomic dysfunction (such as spastic urinary bladder), ataxia, and nystagmus. A clear distinction cannot be drawn on objective criteria between complicated spastic paraplegia and relatively mild PMD (such as the PLP1 null syndrome).

Pure spastic paraparesis (SPG2) does not, by definition, include other significant CNS signs, although autonomic dysfunction, such as spastic urinary bladder, may also occur. Life span is normal.

Males with SPG2 have reproduced; males with the PMD phenotype have not.

Heterozygotes.   Women with a PLP1 gene mutation may or may not have symptoms. Several investigators have observed that in families with severely affected males, the heterozygous women are unlikely to have clinical manifestations of the PLP1-related disorders, whereas in families with mildly affected males, the heterozygous women are more likely to have symptoms [Hodes et al 1997, Sivakumar et al 1999, Hurst et al 2006]. Thus, an inverse relationship exists between the severity of manifestations in males and the likelihood of heterozygous females having neurologic signs. The risk of developing neurologic signs in heterozygous females is greatest in families in which affected males have a PLP1 null syndrome, followed by those in which affected males have an SPG2 syndrome [Hurst et al 2006]. The risk of developing neurologic signs is lowest with PLP1 duplication heterozygotes, who usually have favorably skewed X-chromosome inactivation [Woodward et al 2000].

The following explanation is offered:

• Alleles associated with a severe phenotype cause apoptosis (cell death) of oligodendrocytes (the cells that make myelin in the central nervous system) during early childhood. In heterozygous females, the oligodendrocytes that express the mutant PLP1 allele on the active X chromosome undergo apoptosis early in life but are replaced over time by oligodendrocytes that express the normal PLP1 allele on the active X chromosome. Thus, females who carry a severe PLP1 allele may develop neurologic signs because of skewed inactivation of the X chromosome with the normal PLP1 allele (as with other X-linked recessive disorders) or may have transient signs (while the oligodendrocytes expressing the mutant PLP1 are still present) from which they later recover — a phenomenon unique to PLP1-related disorders [Hodes et al 1995, Inoue et al 2001].

• Alleles associated with a mild phenotype in males do not cause apoptosis of oligodendrocytes. In heterozygous females, abnormal oligodendrocytes persist and can cause neurologic signs [Sivakumar et al 1999].

Hurst et al (2006) analyzed families with SPG2 or PMD and provided statistical support for the inverse correlation between the severity of phenotypes in affected males and their heterozygous relatives. These observations have important implications for genetic counseling and are discussed in Risks to sibs of a male proband.

Manifesting heterozygotes are usually not index cases, but rather are identified in the course of evaluating the relatives of an affected male.

Genotype-Phenotype Correlations

Some genotype-phenotype correlations exist.

Most individuals with duplications have classic PMD; however, some are classified as having connatal PMD and may have three or more copies of the PLP1 locus [Wolf et al 2005]. Variation in the extent of duplication or location(s) of the breakpoints or reinsertion sites may account for clinical variability. Other possible causes of variability include the genetic background of the individual.

The most severe clinical syndromes are typically caused by missense mutations (especially nonconservative amino acid substitutions) and other point mutations of PLP1.

The milder spastic paraplegia syndrome is most often caused by conservative amino acid substitutions in presumably less critical regions of the protein. The locations of these mutations do not provide a clear correlation between amino acid position and clinical phenotype. However, mutations in the PLP1-specific portion of the gene tend to cause less severe syndromes [Cailloux et al 2000].

Although PMD has classically been regarded as a strictly central nervous system disorder, those with null PLP1 mutations including deletion of PLP1 [Raskind et al 1991], a frameshift mutation [Garbern et al 1997], and a missense mutation affecting the initiation codon [Sistermans et al 1996] do develop a relatively mild demyelinating peripheral neuropathy, demonstrating that myelin proteolipid protein (PLP) and/or DM20 (an alternatively spliced transcript; see following) does indeed function in the peripheral as well as central nervous systems. Furthermore, the null phenotype has less severe CNS signs than those seen with the more typical forms of PMD. The null phenotype is associated with length-dependent degeneration of major central motor and sensory tracts and reduced levels of N-acetyl aspartate in cerebral white matter.

PLP1 encodes two major alternatively spliced transcripts: myelin proteolipid protein (PLP), encoded by the full-length PLP1 gene; and DM20, which lacks the PLP1-specific domain that encodes residues 117-151. Peripheral neuropathy as well as a relatively mild CNS syndrome result from mutations that affect only the PLP1-specific region [Shy et al 2003]. The CNS syndrome can be milder than that observed in individuals with the null phenotype.

Penetrance

PLP1 mutations are believed to be completely penetrant in males. The complete conservation of amino acid sequence between rodent and human PLP suggests that variation from the normal sequence is detrimental.

Anticipation

Anticipation has not been reported for PLP1 mutations.

Nomenclature

Older terms for myelin proteolipid protein include: lipophilin, Folch-Lees protein, major myelin proteolipid and proteolipid protein 1.

Proteolipid protein 1 was previously called proteolipid protein. After discovery of a similar gene that is predominantly expressed in gut, numerical designation was added.

Prevalence

In the US, the prevalence of PMD in the population is estimated to be about 1/200,000 to 1/500,000.

In a survey of leukodystrophies in Germany, the incidence of PMD was about 0.13 per 100,000 live births [Heim et al 1997].

Seeman et al (2003) reported that in the Czech Republic PLP1 mutations were detected in one per 90,000 births. While this may reflect a situation particular to the Czech Republic, it suggests that the prevalence of PMD may be higher than is generally recognized.

Evaluations at Initial Diagnosis to Establish the Extent of Disease

• Physical examination to determine extent and severity of respiratory and feeding difficulties, weakness, hypotonia, spasticity, scoliosis, ataxia, visual impairment, cognitive impairment, contractures, joint dislocations, and ambulation

• In infants and children, developmental assessment to determine capabilities and needs for cognitive, physical and other symptomatic therapies

• Brain MRI (more helpful ≥age 9 months) to determine severity of myelination abnormalities; in older children and adults, magnetic resonance spectroscopy to ascertain axonal dysfunction

• Assessment of peripheral nerve function using NCV to identify individuals with the PLP1 null syndrome

• Family history to identify other affected or at-risk individuals

Treatment of Manifestations

A multidisciplinary team comprising specialists in neurology, physical medicine, orthopedics, pulmonary medicine, and gastroenterology is optimal for care.

Early attention to swallowing difficulties and airway protection, especially in the most severely affected individuals, is important. Those with severe dysphagia may require feeding by gastrostomy.

Seizures are usually restricted to individuals with the most severe (connatal) syndrome and may not always be associated with electroencephalographic evidence for epileptiform waveforms, although they generally respond to antiepileptic agents such as carbamazepine.

Spasticity management may include physical therapy and exercises, including regular stretching exercises. Antispasticity medications such as baclofen (including intrathecal administration), diazepam, and tizanidine may be helpful, especially in combination with physical therapy, exercise, orthotics, and other assistive devices. In advanced cases, surgery to release joint contractures may be required.

Proper wheelchair seating and physical therapy may help prevent scoliosis, which when severe may result in pulmonary compromise as well as discomfort, especially with position changes. With severe scoliosis, corrective surgery may be necessary to preserve pulmonary function.

Specialized schooling arrangements are typically necessary for children with PLP1-related disorders. Developmental assessments can accurately assess a child's capabilities, which may be greater than is apparent because of severe motor deficits. Electronic or other communication devices may facilitate communication especially in children with visual and auditory impairment.

Surveillance

Semiannual to annual neurologic and physical medicine evaluation is indicated to monitor developmental progress during childhood and to monitor and treat spasticity and orthopedic complications as needed.

Agents/Circumstances to Avoid

No specific agents or exposures are known to worsen or accelerate the disease.

High temperature, as with fever, may aggravate neurologic signs and symptoms, as occurs in individuals with multiple sclerosis (Uthoff phenomenon).

Therapies Under Investigation

Partial myelination of the CNS by donor oligodendrocytes has been demonstrated in research involving animal models of PMD [Duncan et al 1997, Learish et al 1999]. This approach is not available for humans and is of uncertain benefit.

Pharmacologic agents that lower expression of PLP1 should be of theoretical benefit in individuals with extra copies of PLP1, and those with severe point mutations. Preclinical studies are underway to test drugs of this type.

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Other

No controlled studies, either successful or unsuccessful, have been reported for therapy of PMD or SPG2.

Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

See Consumer Resources for disease-specific and/or umbrella support organizations for this disorder. These organizations have been established for individuals and families to provide information, support, and contact with other affected individuals.

Mode of Inheritance

The PLP1-related disorders are inherited in an X-linked manner.

Risk to Family Members

Parents of a male proband

• The father of a proband will not have the disease nor will he be a carrier of the mutation.

• Women who have an affected child and one other affected relative are obligate heterozygotes (carriers).

• If pedigree analysis reveals that the proband is the only affected family member, it is possible that his mother is a carrier or that he has a de novoPLP1 point mutation.

  • Most mothers of a proband are carriers of a PLP1 gene mutation regardless of family history.

  • De novo mutations have been reported for several PLP1 point mutations [Dlouhy et al 1993, Otterbach et al 1993, Pratt et al 1995, Hodes et al 1998] but not for PLP1 duplications that appear to arise in the male germline [Woodward, Kendall, Vetrie, Malcolm 1998; Mimault et al 1999].

Sibs of a male proband

• The risk to the sibs depends upon the carrier status of the mother.

• Women with a PLP1 mutation have a 50% chance of transmitting the mutation to each child. Male sibs who inherit the mutation will be affected; female sibs who inherit the mutation will be carriers and may manifest mild to moderate signs of the disease. Of note, PLP1 alleles that cause relatively mild neurologic signs in affected males are more likely to be associated with neurologic manifestations in heterozygotes.

• Germline mosaicism could occur in this condition. Thus, even if the disease-causing mutation has not been identified in the mother's DNA, sibs of the proband are still at increased risk of inheriting the disease-causing mutation [Woodward et al 2003].

• Female sibs are more likely to develop neurologic signs if the phenotype in affected males is relatively mild (complicated or pure spastic paraparesis) [Hurst et al 2006]. The risk of a heterozygous female being clinically affected is highest when the brother has the PLP1 null syndrome, and lowest when he has a PLP1 duplication. Heterozygous females with PLP1 duplication have been reported to have favorably skewed X inactivation [Woodward et al 2000].

Offspring of a male proband.  Males with typical PMD do not reproduce, but those with spastic paraplegia may be able to father children. Affected males transmit the PLP1 mutation to all of their daughters and to none of their sons.

Other family members of a male proband.  The proband's maternal aunts and their offspring may be at risk of being carriers or of being affected (depending upon their gender, family relationship, and the carrier status of the proband's mother). Females who are heterozygous are more likely to have neurologic signs (usually adult-onset spastic paraparesis) if affected males have a less severe syndrome.

Carrier Detection

Heterozygotes are usually neurologically normal but may manifest mild-to-moderate signs of the disease. Carriers can be identified through molecular genetic testing if the disease-causing PLP1 mutation has been identified in an affected family member or if linkage analysis in the family is informative.

Related Genetic Counseling Issues

Phenotypic variability.  It is important for couples at risk to be aware that varying phenotypes can coexist in the same kindred or sibship; thus, families in which males have had a milder phenotype are at risk of having affected offspring who may display a more severe phenotype.

Distantly inserted duplications.  While distantly inserted duplications are a rare cause of PLP1-related disorders, they may raise potentially difficult genetic counseling issues because the inheritance pattern may not be X linked [Hodes et al 2000; Inoue, Osaka et al 2002].

Family planning.  The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.

DNA banking.  DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. DNA banking is particularly relevant in situations in which the sensitivity of currently available testing is less than 100%. See DNA Banking for a list of laboratories offering this service.

Prenatal Testing

Prenatal testing is possible for pregnancies of women who are carriers if the PLP1 mutation has been identified in a family member [Maenpaa et al 1990, Bridge et al 1991, Strautnieks et al 1992, Woodward et al 1999, Regis et al 2001]. The usual procedure is to determine fetal sex by DNA analysis or cytogenetic analysis of cells obtained from chorionic villus sampling (CVS) at about ten to 12 weeks' gestation or amniocentesis usually performed at about 15-18 weeks' gestation. If the fetus is male, DNA from fetal cells can be analyzed for the known disease-causing mutation. The phenotype of an affected fetus cannot be accurately predicted as widely varying phenotypes often coexist in the same kindred or sibship; thus, families in which males have had a milder phenotype are at risk of having affected offspring with a more severe phenotype. In addition, females who inherit the mutation will be carriers and may manifest mild to moderate signs of the disease.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutation has been identified in an affected family member.

PGD has been used successfully not only for gender selection [Grifo et al 1994, de Die-Smulders et al 1998] but also to select against pathologic PLP1 point mutations [Verlinsky et al 2006]. PGD has not yet been attempted in families that have a PLP1 duplication, but it should be possible to use linked markers and/or unique junction fragments to distinguish the abnormal X chromosome from the normal X chromosome. (Duplications may be difficult to distinguish from newly replicated DNA using a PCR-based assay.) For laboratories offering PGD, see .

Molecular Genetic Pathogenesis

For a review of molecular pathophysiology of Pelizaeus-Merzbacher disease (PMD), see Hudson, (2003), Hudson et al (2004), and Garbern et al (in press).

Yool et al (2000) reviewed features of PLP1 mutations in both animals and humans. The clinical syndromes caused by small mutations are quite broad, ranging from mild spastic paraparesis and ataxia [Hodes et al 1997] to severe spastic quadriplegia, seizures, stridor, and childhood fatality [Trofatter et al 1989]. A compelling hypothesis to explain the variation in clinical severities of different PLP1 mutations has been put forward by Gow and colleagues [Gow & Lazzarini 1996, Southwood & Gow 2001]. Missense mutations and other small mutations presumably cause misfolding of myelin proteolipid protein (PLP) or DM20. These misfolded proteins fail to progress through the intracellular processing pathway and are retained in the endoplasmic reticulum (ER), failing to be incorporated into the cell membrane and to activate the unfolded protein response (UPR) [Southwood et al 2002]. As oligodendrocytes attempt to myelinate their axons, UPR-activated apoptosis ensues if both PLP and DM20 are retained in the ER. However, if only PLP but not DM20 is ER-retained, oligodendrocytes survive and myelinate, resulting in a less severe syndrome such as spastic paraplegia [Schneider et al 1992].

Overexpression of PLP is presumed to be the mechanism triggered by increased PLP1 gene dosage. Experimental studies suggest that increased PLP expression results in mislocalization of PLP along with cholesterol and lipids to the late endosomal/lysosomal compartment [Simons et al 2000].

Deletion of the PLP1 gene, and probably some splice-site mutations that preclude expression of PLP, result in mild myelin defects but more severe axonal degeneration [Garbern et al 2002].

Some intrafamilial phenotypic variability can occur, presumably the result of genetic background or environmental effects that are not understood. An interesting animal model highlights the effect of genetic background. The I187T missense ("rumpshaker") mutation causes a mild syndrome with normal life span (~2 years) in the C3H mouse strain but causes a fatal syndrome in the C57BL/6 strain background [Al-Saktawi et al 2003].

Normal allelic variants: PLP1 comprises seven exons spanning about 15 kbp. Several non-disease-causing polymorphisms have been described. Thus far, four polymorphic missense mutations, most of which are conservative substitutions, have been reported in the single nucleotide polymorphism database, indicating that some amino acid sequence changes are tolerated:

• c.43C>A (p.15P>T)

• c.44C>G (p.15P>R)

• c.266G>T (p.89E>D)

• c.480G>A (p.161V>I)

Additional changes that do not affect the coding sense of the gene have been reported:

• -445C>G (5' of transcription initiation site) in the promoter [Kawanishi et al 1997]

• -220-210del (5' of transcription initiation site) [Hübner et al 2005]

• c.-102C>T [Hübner et al 2005]

• Dinucleotide (CA) repeat polymorphism in intron 1 [Mimault et al 1995]

• c.5-111T>C, Msp1 polymorphism in intron 1, [Hobson et al 2001]

• c.168A>G, an Mval polymorphism in exon 2 [Osaka et al 1995]

• c.243T>C in exon 3 [Hobson et al, unpublished observation]

• c.606T>C, an AhaII/BsaHI polymorphism in exon 4 [Trofatter et al 1991]

• c.622+28C>G in intron 4 [Hobson and Sistermans, unpublished observations; Hübner et al 2005]

• c.*101C>T, reported as insT932 by Podulso et al (1993) [Hobson et al, unpublished observations]

Pathologic allelic variants:

Note: Historically, numbering of PLP1 residues began with the glycine, since the initiating methionine is post-translationally cleaved [Lees et al 1983]; until recently, publications have used this numbering system. This entry uses the more conventional numbering from the initiation methionine and methionine codon as described by den Dunnen & Antonarakis 2001 and accepted by the Human Genome Variation Society.

Thus far, over 100 PLP1 mutations have been reported; see Table 3 (pdf) for a summary of reported mutations. Most result in missense substitutions, but there are small deletions and insertions as well as mutations at or near splice sites or possible splice-regulatory regions [e.g., Hobson, Huang et al 2002]. One mutation in the 5' UTR that is associated with a PMD phenotype has been reported [Kawanishi et al 1996].

Duplication of a genomic region that includes PLP1 is the most frequent mutation associated with PMD. Most duplications are tandem direct repeats with endpoints that vary among families, leading to duplications of different sizes. Duplications appear to arise from an intrachromosomal recombination in the male germline [Woodward, Kendall, Vetrie, Malcolm 1998; Mimault et al 1999; Inoue et al 1999]. Sequence analysis across abnormal duplication junctions suggests that duplications arise by a coupled homologous and nonhomologous recombination mechanism [Woodward et al 2005]. Models involving repair of a double-stranded break by sister chromatid invasion, DNA synthesi,s and nonhomologous end-joining have been proposed [Woodward et al 2005, Lee et al 2006]. The PMD duplication mechanism differs from that observedin other genomic diseases such as Charcot-Marie-Tooth disease 1A (CMT1A), where it has been proposed that homologous low-copy repeat (LCR) regions flanking the duplicated regions mediate nonallelic homologous recombination [Lupski 1998]. The PLP1 gene does not have flanking homologous LCRs, but it has been proposed that a large distal LCR and multiple small proximal repeats [Woodward et al 2005] or the distal LCR and a small proximal LCR [Lee et al 2006] are involved in stimulating duplication events. In at least three cases, the duplicated region is inserted at a different locus on the X chromosome [Woodward, Kendall, Vetrie, Malcolm 1998; Hodes et al 2000]. Triplication and even quintuplication of the PLP1 gene have been observed [Wolf et al 2005]. The molecular mechanism of these anomalies is not known.

Deletion of the PLP1 locus also appears to utilize nonhomologous end-joining [Inoue, Osaka et al 2002].

Normal gene product: Myelin proteolipid protein (PLP) is the predominant protein constituent of central nervous system myelin, constituting about 50% of the myelin protein mass. Among mammals, PLP is highly conserved, with the mouse, rat, and human PLP sequences being completely identical over the 276-amino acid sequence. Other mammalian PLPs differ at only a handful of residues. In addition to myelin proteolipid protein, at least one additional gene product, DM20, is encoded by the PLP1 gene. PLP and DM20 are both predicted to be transmembrane proteins that span the lipid bilayer four times. In addition, protein is anchored further to cell membranes through covalent acyl linkages to fatty acids [Bizzozero & Good 1990]. Although it is thought that PLP probably cements adjacent leaflets of myelin, additional or alternative functions are also possible [Griffiths et al 1998].

A product of alternative splicing, DM20 is also found in the nervous system and other tissues. The splicing variation occurs at exon 3, where the 3' half of the exon is spliced out in DM20 mRNA, resulting in the in-frame deletion of amino acid residues 117-151.

Post-translational proteolytic cleavage of PLP/DM20 appears to occur in vivo [Bizzozero et al 2002]. There is evidence that PLP-derived peptides promote oligodendrocyte mitogenesis [Yamada et al 1997].

Abnormal gene product: Duplication of PLP1 presumably results in overexpression of myelin proteolipid protein (PLP), leading to dysfunction and death of oligodendrocytes, the myelin-forming cells in the CNS. Transgenic mice that overexpress normal PLP1 develop both dysmelination and demyelination of the CNS [Griffiths et al 1995, Griffiths et al 1998, Yool et al 2000].

Literature Cited

Published Statements and Policies Regarding Genetic Testing

No specific guidelines regarding genetic testing for this disorder have been developed.

Suggested Readings

Author Notes

Dr. Garbern’s Web site: cmmg.biosci.wayne.edu/jgarbern/jgarbern-home.html

Acknowledgments

The authors are grateful to the PMD foundation, the Children's Research center of Michigan, the Nemours Foundation, the NIH and the PMD families.

Revision History

• 15 September 2006 (me) Comprehensive update posted to live Web site

• 7 October 2004 (jg) Revision: Table 3

• 11 June 2004 (me) Comprehensive update posted to live Web site

• 20 March 2002 (me) Comprehensive update posted to live Web site

• 15 June 1999 (me) Review posted to live Web site

• 28 January 1999 (jg) Original submission