Pelizaeus-Merzbacher disease: an X-linked neurologic disorder of myelin metabolism with a novel mutation in the gene encoding proteolipid protein

The nosology of the inborn errors of myelin metabolism has been stymied by the lack of molecular genetic analysis. Historically, Pelizaeus-Merzbacher disease has encompassed a host of neurologic disorders that present with a deficit of myelin, the membrane elaborated by glial cells that encircles and successively enwraps axons. We describe here a Pelizaeus-Merzbacher pedigree of the classical type, with X-linked inheritance, a typical clinical progression, and a pathologic loss of myelinating cells and myelin in the central nervous system. To discriminate variants of Pelizaeus-Merzbacher disease, a set of oligonucleotide primers was constructed to polymerase-chain-reaction (PCR) amplify and sequence the gene encoding proteolipid protein (PLP), a structural protein that comprises half of the protein of the myelin sheath. The PLP gene in one of two affected males and the carrier mother of this family exhibited a single base difference in the more than 2 kb of the PLP gene sequenced, a C----T transition that would create a serine substitution for proline at the carboxy end of the protein. Our results delineate the clinical features of Pelizaeus-Merzbacher disease, define the possible molecular pathology of this dysmyelinating disorder, and address the molecular classification of inborn errors of myelin metabolism. Patients with the classical form (type I) and the more severely affected, connatal variant of Pelizaeus-Merzbacher disease (type II) would be predicted to display mutation at the PLP locus. The other variants (types III-VI), which have sometimes been categorized as Pelizaeus-Merzbacher disease, may represent mutations in genes encoding other structural myelin proteins or proteins critical to myelination.     

Molecular diagnostics for myelin proteolipid protein gene mutations in Pelizaeus-Merzbacher disease.

Pelizaeus-Merzbacher disease (PMD) is a clinically heterogeneous, slowly progressive leukodystrophy. The recent detection of mutations in the myelin proteolipid protein (PLP) gene in several PMD patients offers the opportunity both to design DNA-based tests that would be useful in diagnosing a proportion of PMD cases and, in particular, to evaluate the diagnostic utility of single-strand conformation polymorphism (SSCP) analysis for this disease. A combination of SSCP analysis and direct sequencing of PCR-amplified DNA was used to screen for PLP mutations in 24 patients affected with leukodystrophies of unknown etiology. Two heretofore undescribed mutations in the PLP gene were identified, Asp202His in exon 4 and Gly73Arg in exon 3. The ease and efficiency of SSCP analysis in detecting new mutations support the utilization of this technique in screening for PLP mutations in patients with unexplained leukodystrophies.     

Linkage of a new mutation in the proteolipid protein (PLP) gene to Pelizaeus-Merzbacher disease (PMD) in a large Finnish kindred.

The purpose of this study was to confirm linkage of the proteolipid protein gene (PLP) and Pelizaeus-Merzbacher disease (PMD). A T-->A transversion in nucleotide pair 35 of exon 4 of PLP was found in a large Finnish kindred with PMD. This mutation results in the substitution Val165-->Glu165. We used a combination of single-strand conformational polymorphism and PCR primer extension to determine the presence or absence of the point mutation in family members. A lod score of 2.6 (theta = 0) was found for linkage of the gene and the disease. We examined 101 unrelated X chromosomes and found none with the transversion. This is the second report of linkage of PMD to a missense mutation in PLP. These findings support the hypothesis that PMD in this family is a result of the missense mutation present in exon 4 of PLP.     

Familial adenomatous polyposis: identification of a new frameshift mutation of the APC gene in an Italian family.

Familial Adenomatous Polyposis (FAP) is a premalignant disease of the gastrointestinal tract inherited as an autosomal dominant trait assigned to chromosome 5q21. The 15 exons of the APC gene responsible for the defect were amplified from the DNA of one FAP patient. SSCP analysis of the amplified DNA revealed a variant conformer of exon 10. The sequencing of the cloned PCR product showed a 1 base insertion at position 1370, creating a stop codon four nucleotides downstream. SSCP analysis of 20 family members and nucleotide sequencing of exon 10 in three affected members confirmed the Mendelian inheritance of the mutant allele.     

Mval polymorphism in the proteolipid protein (PLP) gene

We report a rare polymorphism in the human proteolipid protein (PLP) gene. A synonymous mutation, 168 AG, was detected in exon 2 of the PLP gene. Mutations in this gene have been reported in some cases of Pelizaeus-Merzbacher disease.     

Complete deletion of the proteolipid protein gene (PLP) in a family with X-linked Pelizaeus-Merzbacher disease.

Pelizaeus-Merzbacher disease (PMD) is an X-linked neurologic disorder characterized by dysmyelination in the central nervous system. Proteolipid protein (PLP), a major structural protein of myelin, is coded on the X chromosome. It has been postulated that a defect in the PLP gene is responsible for PMD. Different single-nucleotide substitutions have been found in conserved regions of the PLP gene of four unrelated PMD patients. Novel Southern blot patterns suggested a complex rearrangement in a fifth family. Linkage to PLP has been shown in others. We evaluated the PLP locus in a four-generation family with two living males affected with X-linked PMD. Analysis of DNA from the affected males revealed complete absence of a band, with PLP probes encompassing the promoter region, the entire coding region, and the 3' untranslated region and spanning at least 29 kb of genomic DNA. DNA from unaffected relatives gave the expected band pattern. Two obligate and one probable carrier women were hemizygous for the PLP locus by dosage analysis. Although it is unlikely, the previously described point mutations in PLP could represent polymorphisms. The finding of complete deletion of the PLP gene in our family is a stronger argument that mutations in PLP are responsible for X-linked PMD.     

Prenatal diagnosis and genetic counseling of a Chinese Alport syndrome kindred

Alport syndrome (AS) is a progressive renal disease characterized by hematuria and progressive renal failure. X-linked dominant AS (XLAS) is the major inheritance form, accounting for almost 80% of the cases, caused by mutations in COL4A5 gene. An accurate genetic diagnosis of AS is very important for genetic counseling and even prenatal diagnosis. In this study we detected mutation of COL4A5 by amplifying the entire coding sequence mRNA of peripheral blood lymphocytes using nested PCR in a Chinese XLAS family, and then performed the first prenatal diagnosis of AS in China. Mutation analysis of the fetus was performed on both cDNA-based level and DNA-based level of amniocytes. Fetus sex was determined by PCR amplification of SRY and karyotypes analysis. Maternal cell contamination was excluded by linkage analysis. There was a G-to-A substitution at position 4,271 in exon 46 of COL4A5 gene (c.G4271A) in the pregnant woman; this genetic variant has not been described previously and was a novel missense mutation. The fetus did not carry the same mutation as the mother. PCR amplification product of SRY and karyotypes analysis revealed a male fetus. Linkage analysis showed that there was no contamination of maternal cells in amniocytes.     

Genetic homogeneity of Pelizaeus-Merzbacher disease: tight linkage to the proteolipoprotein locus in 16 affected families. PMD Clinical Group.

Among the numerous leukodystrophies that have an early onset and no biochemical markers, Pelizaeus-Merzbacher disease (PMD) is one that can be identified using strict clinical criteria and demonstrating an abnormal formation of myelin that is restricted to the CNS in electrophysiological studies and brain magnetic resonance imaging (MRI). In PMD, 12 different base substitutions and one total deletion of the genomic region containing the PLP gene have been reported, but, despite extensive analysis, PLP exon mutations have been found in only 10%-25% of the families analyzed. To test the genetic homogeneity of this disease, we have carried out linkage analysis with polymorphic markers of the PLP genomic region in 16 families selected on strict diagnostic criteria of PMD. We observed a tight linkage of the PMD locus with markers of the PLP gene (cDNA PLP, exon IV polymorphism) and of the Xq22 region (DXS17, DXS94, and DXS287), whereas the markers located more proximally (DXYS1X and DXS3) or distally (DXS11) were not linked to the PMD locus. Multipoint analysis gave a maximal location score for the PMD locus (13.98) and the PLP gene (8.32) in the same interval between DXS94 and DXS287, suggesting that in all families PMD is linked to the PLP locus. Mutations of the extraexonic PLP gene sequences or of another unknown close gene could be involved in PMD. In an attempt to identify molecular defects of this genomic region that are responsible for PMD, these results meant that RFLP analysis could be used to improve genetic counseling for the numerous affected families in which a PLP exon mutation could not be demonstrated.     

Additional copies of the proteolipid protein gene causing Pelizaeus-Merzbacher disease arise by separate integration into the X chromosome

The proteolipid protein gene (PLP) is normally present at chromosome Xq22. Mutations and duplications of this gene are associated with Pelizaeus-Merzbacher disease (PMD). Here we describe two new families in which males affected with PMD were found to have a copy of PLP on the short arm of the X chromosome, in addition to a normal copy on Xq22. In the first family, the extra copy was first detected by the presence of heterozygosity of the AhaII dimorphism within the PLP gene. The results of FISH analysis showed an additional copy of PLP in Xp22.1, although no chromosomal rearrangements could be detected by standard karyotype analysis. Another three affected males from the family had similar findings. In a second unrelated family with signs of PMD, cytogenetic analysis showed a pericentric inversion of the X chromosome. In the inv(X) carried by several affected family members, FISH showed PLP signals at Xp11.4 and Xq22. A third family has previously been reported, in which affected members had an extra copy of the PLP gene detected at Xq26 in a chromosome with an otherwise normal banding pattern. The identification of three separate families in which PLP is duplicated at a noncontiguous site suggests that such duplications could be a relatively common but previously undetected cause of genetic disorders.     

Detection of fetal RHD pseudogene (RHDΨ) and hybrid RHD-CE-Ds from RHD-negative pregnant women with a free DNA fetal kit

Hemolytic disease of the newborn is a clinical condition in which maternal and paternal Rh blood group antigens are incompatible and the mother is negative for the antigen whereas the father is positive. Analysis of fetal cells recovered from maternal plasma can provide a highly sensitive prenatal diagnosis. The fetal RHD gene in plasma DNA is detected by real-time PCR amplification of two different segments of the RHD gene (exons 7 and 10). Each amplicon is revealed with specific probes. We examined 40 female blood samples to verify the specificity of RHD exons (7 and 10) amplified by real-time PCR. Thirty fetuses were predicted to be RHD-positive based on analysis of plasma DNA. Seven fetuses were predicted to be RHD-negative. One fetus was negative for RHD on exon 10, and positive for RHD on exon 7 (early gestation age); two fetuses were RHD-negative on exon 7, and RHD-positive on exon 10 (RHD-CE-D(s) or RHDΨ), indicative of a maternal RHD allele. We conclude that it is necessary to analyze at least two exon regions in the RHD gene.     

Rapid identification of mutations in the glucocerebrosidase gene of Gaucher disease patients by analysis of single-strand conformation polymorphisms

To detect mutations in the glucocerebrosidase gene in Gaucher disease patients, we used the recently described technique of single-strand conformation polymorphism (SSCP) analysis in combination with selective amplification. We analyzed exon 8, 9, 10 and 11 of the glucocerebrosidase gene; these exons were sequentially amplified using the selectively amplified products as templates. We found variant SSCP patterns corresponding to the presence or absence of the 6433C mutation, which was detected by NciI digestion analysis, in exon 10. Furthermore, we detected four variant SSCP patterns in exon 8, 10 and 11. Sequencing analysis consistently revealed four single-base substitutions in the corresponding exons, three novel missense mutations (5409A, 6375G and 6682T) and one silent polymorphism (6594A). These mutations were found only in one patient; therefore, these findings have confirmed the marked genetic heterogeneity of Gaucher disease. SSCP analysis in combination with selective amplification is a rapid and sensitive procedure for the screening of the mutations in the glucocerebrosidase gene of patients with Gaucher disease.     

Rapid detection of alpha-1-antitrypsin deficiency by analysis of a PCR-induced TaqI restriction site

Summary A single base substitution is responsible for the PI-Z mutation in alpha-1-antitrypsin (AAT) deficiency. The Z mutation, which is in exon V of the AAT gene, was analysed directly using a primer designed with a single base substitution in the DNA sequence. During the polymerase chain reaction with this primer, a restriction enzyme site was created in the exon-V-amplified DNA sequence; this site was present in the normal allele (M form) but absent in the Z form. Here, the design of the primer and the application of the designer primer for prenatal diagnosis of chorion villus samples (CVS) for AAT deficiency is described. The method provides a simple rapid means of prenatal diagnosis of AAT deficiency within a day of the collection of the CVS. The detection of the nucleotide base change in AAT deficiency at the Z mutation site provides the opportunity for accurate prenatal diagnosis where no tissue is available from an AAT-affected individual.     

Three novel mutations causing a truncated protein within the RP2 gene in Italian families with X-linked retinitis pigmentosa

X-linked retinitis pigmentosa (XLRP) results from mutations in a number of loci, including RP2 at Xp11.3, and RP3 at Xp21.1. RP2 and RP3 genes have been identified by positional cloning. RP2 mutations are found in about 10% of XLRP patients. We performed a mutational screening of RP2 gene inpatients belonging to seven unrelated families in linkage with the RP2 locus. SSCP analysis detected three conformation variants, within exon 2 and 3. Direct sequencing of exon 2, disclosed a G-->A transition at nucleotide 449 (W150X), and a G-->T transversion in position 547 (E183X). Sequence analysis of exon 3 variant revealed an insertion (853/854insG), leading to a frameshift. In this patient, we detected an additional sequence alteration (A-->G at nucleotide 848, E283G). Each mutation was co-segregating with the disease in the affected family members available for the study. These mutations are expected to introduce a stop codon within the RP2 coding sequence probably resulting in a truncated or unstable protein.     

A point mutation at the X-chromosomal proteolipid protein locus in Pelizaeus-Merzbacher disease leads to disruption of myelinogenesis.

A group of inherited neurological disorders are the X-chromosome linked dysmyelinoses, in which myelin membranes of the CNS are missing or perturbed due to a strongly reduced number of differentiated oligodendrocytes. In animal dysmyelinoses (jimpy mouse, msd-mouse, md rat, shaking pup) mutations of the main integral myelin membrane protein, proteolipid protein, have been identified. Pelizaeus-Merzbacher disease (PMD) or sudanophilic leucodystrophy is an X-linked dysmyelinosis in humans. We report here on the molecular basis of the defect of affected males of a PMD kindred. Rearrangements of the PLP gene were excluded by Southern blot hybridisation analysis and PCR amplification of overlapping domains of the PLP gene. Sequence analysis revealed one single C----T transition in exon IV, which leads to a threonine----isoleucine substitution within a hydrophobic intramembrane domain. The impact of this amino-acid exchange on the structure of PLP in the affected cis membrane domain is discussed. A space filling model of this domain suggests a tight packing of the alpha-helices of the loop which is perturbed by the amino-acid substitution in this PMD exon IV mutant. The C----T transition in exon IV abolishes a Hph I restriction site. This mutation at the recognition site for Hph I (RFLP) and allele-specific primers have been used for mutation screening the PMD kindred.     

Combined genetic and imaging diagnosis for two large Chinese families affected with Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a rare X-linked recessive disorder characterized by nystagmus, impaired motor development, ataxia, and progressive spasticity. Genetically defective or altered levels of proteolipid protein (PLP1) or gap-junction alpha protein 12 gene have been found to be a common cause. Here we report on two large Han Chinese families affected with this disease. The probands of both families had produced sons featuring cerebral palsy that had never been correctly diagnosed. PMD was suspected after careful analysis of family history and clinical features. Three rounds of molecular testing, including RT-PCR, genetics linkage and SRY sequence analyses, in combination with fetal ultrasound and magnetic resonance imaging, confirmed the diagnosis. In Family 1, in addition to two patients, three carriers were identified, including one who was not yet married. Genetic testing indicated that a fetus did not have the disease. A healthy girl was born later. In Family 2, two patients and two carriers were identified, while a fetus was genetically normal. A healthy girl was born later. We concluded that by combining genetic testing and imaging, awareness of the symptoms of PMD and understanding of its molecular biology, there is great benefit for families that are at risk for producing offspring affected with this severe disease.     

A novel mutation in the ABCD1 gene of a Korean boy diagnosed with X-linked adrenoleukodystrophy

X-linked adrenoleukodystrophy (ALD; MIM #300100) is a neurodegenerative disorder caused by mutations in the ABCD1 adrenoleukodystrophy protein gene. The ABCD1 gene mutations have been reported by laboratories in China and Japan, but not in Korea. This case report describes a Korean boy diagnosed with X-ALD. Direct sequencing for the ABCD1 gene in this boy and his mother detected Tyr620His missense mutation, caused by cDNA nucleotide change 1858 T>C in exon 8 (c.1858T>C). This missense variant was novel and predicted to be possibly damaging by the PolyPhen and SIFT prediction software. Moreover, this is the first report in Korean.     

Novel and recurrent LDLR gene mutations in Pakistani hypercholesterolemia patients

The majority of patients with the autosomal dominant disorder familial hypercholesterolemia (FH) carry novel mutations in the low density lipoprotein receptor (LDLR) that is involved in cholesterol regulation. In different populations the spectrum of mutations identified is quite different and to date there have been only a few reports of the spectrum of mutations in FH patients from Pakistan. In order to identify the causative LDLR variants the gene was sequenced in a Pakistani FH family, while high resolution melting analysis followed by sequencing was performed in a panel of 27 unrelated sporadic hypercholesterolemia patients. In the family a novel missense variant (c.1916T > G, p.(V639G)) in exon 13 of LDLR was identified in the proband. The segregation of the identified nucleotide change in the family and carrier status screening in a group of 100 healthy subjects was done using restriction fragment length polymorphism analysis. All affected members of the FH family carried the variant and none of the non-affected members nor any of the healthy subjects. In one of the sporadic cases, two sequence changes were detected in exon 9, one of these was a recurrent missense variant (c.1211C > T; p.T404I), while the other was a novel substitution mutation (c.1214 A > C; N405T). In order to define the allelic status of this double heterozygous individual, PCR amplified fragments were cloned and sequenced, which identified that both changes occurred on the same allele. In silico tools (PolyPhen and SIFT) were used to predict the effect of the variants on the protein structure, which predicted both of these variants to have deleterious effect. These findings support the view that there will be a novel spectrum of mutations causing FH in patients with hypercholesterolaemia from Pakistan.     

A Single Nucleotide Difference in the Gene for Myelin Proteolipid Protein Defines the Jimpy Mutation in Mouse

We have previously shown that, in the myelin-deficient jimpy mutant mouse, 74 nucleotides are absent from the mRNA for proteolipid protein (PLP) as a result of aberrant RNA processing. To define the exact site of the jimpy mutation, we have analyzed the PLP gene obtained from a jimpy mouse genomic library. We find that the nucleotide sequence that is absent from jimpy PLP mRNA is fully preserved in the jimpy PLP gene. The missing segment corresponds to a separate exon, equivalent to exon 5 of the human PLP gene. The nucleotide sequence at the 3' end of intron 4 in the jimpy PLP gene contains a single point mutation. A base change A----G in the 3' acceptor splice site has altered a position that is 100% conserved in all published splice acceptor sequences. We conclude that the primary genetic defect of the jimpy mouse is a single base change in the PLP gene disabling an invariant recognition sequence of RNA splicing.     

A (G-to-A) mutation in the initiation codon of the proteolipid protein gene causing a relatively mild form of Pelizaeus-Merzbacher disease in a Dutch family

Pelizaeus-Merzbacher disease (PMD) is an X-linked recessive disorder that is characterized by dysmyelination of the central nervous system resulting from mutations in the proteolipid protein (PLP) gene. Mutations causing either overexpression or expression of a truncated form of PLP result in oligodendrocyte cell death because of accumulation of PLP in the endoplasmic reticulum. It has therefore been hypothesized that absence of the protein should result in a less severe phenotype. However, until now, only one patient has been described with a complete deletion of the PLP gene. We report a Dutch family with a relatively mild form of PMD, in which the disease cosegregates with a (G-to-A) mutation in the initiation codon of the PLP gene. This mutation should cause the total absence of PLP and is therefore in agreement with the hypothesis that absence of PLP leads to a mild form of PMD.