Brain & Development 37 (2015) 455–458 www.elsevier.com/locate/braindev
Case Report
A novel PLP1 frameshift mutation causing a milder form of Pelizaeus–Merzbacher disease Takashi Shiihara a,⇑, Mio Watanabe a, Kengo Moriyama a, Mitsugu Uematsu b, Kiyoko Sameshima c a Department of Neurology, Gunma Children’s Medical Center, Gunma 377-8577, Japan Department of Pediatrics, Tohoku University School of Medicine, Sendai 980-8574, Japan c Division of Medical Genetics, Gunma Children’s Medical Center, Gunma 377-8577, Japan
b
Received 16 April 2014; received in revised form 21 June 2014; accepted 25 June 2014
Abstract Background: Pelizaeus–Merzbacher disease (PMD), a hypomyelinating leukodystrophy, and the related but less severe allelic spastic paraplegia 2 (SPG2) are caused by mutations in the proteolipid protein 1 (PLP1) gene. Magnetic resonance imaging (MRI) is pivotal for diagnosing these disorders. The severity of PMD/SPG2 varies, and for a milder form of PMD, there have been some reports of near-normal findings in T1-weighted images but abnormal findings in T2-weighted images. Patient: We report the case of a 5-year-old boy diagnosed with a milder form of PMD caused by a novel PLP1 mutation in exon 3: c.300delC (p.I100IfsX13). He had delayed development from several months of age and was able to walk with support at 19 months in spite of the spasticity in his lower extremities. Hypomyelination was noted at 12 months by brain MRI. Motor nerve conduction studies showed decreased velocities with reduced amplitudes. Follow-up MRI at 1-year intervals from 18 months until 55 months of age showed gradual myelination progress. Discussion: The single nucleotide deletion identified in this patient can cause a frameshift and premature termination of PLP1. Via the nonsense-mediated mRNA decay mechanism of this mutation will result in loss-of-function, leading to a milder form of PMD. The present case is compatible with previously reported cases of milder form of PMD. We incidentally identified progressive myelination in this patient by T1-weighted images obtained by serial MRI. This finding adds to our understanding of the pathological stages of a milder form of PMD. Ó 2014 The Japanese Society of Child Neurology. Published by Elsevier B.V. All rights reserved.
Keywords: Pelizaeus–Merzbacher disease; Spastic paraplegia 2; Proteolipid protein; Myelination; Magnetic resonance imaging; mRNA; Nonsensemediated decay
1. Introduction Pelizaeus–Merzbacher disease (PMD; OMIM #312080), a hypomyelinating leukodystrophy, and the ⇑ Corresponding author. Address: Department of Neurology, Gunma Children’s Medical Center, 779 Shimohakoda, Hokkitsumachi, Shibukawa, Gunma 377-8577, Japan. Tel.: +81 279 52 3551; fax: +81 279 52 2045. E-mail address: [email protected] (T. Shiihara).
related but less severe allelic spastic paraplegia 2 (SPG2; OMIM #312920), are caused by mutations in the proteolipid protein 1 (PLP1) gene (OMIM *300401) [1]. PLP1, located on chromosome Xq22.2, contains 7 exons and encodes the major myelin sheath protein, proteolipid protein (PLP) and its shorterspliced isoform DM20. The severity of PMD/SPG2 varies and Cailloux et al. classified patients into five categories, according to their best motor function: form 0 with
http://dx.doi.org/10.1016/j.braindev.2014.06.011 0387-7604/Ó 2014 The Japanese Society of Child Neurology. Published by Elsevier B.V. All rights reserved.
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no motor achievement; form 1 with head control; form 2 with sitting position; form 3 with supported walking; and form 4 with autonomous walking [2]. Clinical descriptions of PMD/SPG2 classifications have not been standardized and differ somewhat between studies [1–3]. According to Woodward et al., the descriptions range from the most severe to the mildest phenotype: connatal form of PMD (roughly fitting to the Cailloux et al. classification above, forms 0 and 1); classical form of PMD (forms 1 and 2); milder form of PMD (forms 2 and 3); and SPG2 (forms 3 and 4) [1–3]. Some authors further classify SPG2 into complicated and pure forms, with or without central nervous system involvement, respectively [1,3]. From a clinical viewpoint, the milder form of PMD and complicated form of SPG2 are barely distinguishable [1]. Hereafter, for the sake of clarification, when referring to SPG2, we assume a pure form of SPG2. A wide range of mutations can cause PMD/SPG2, and genotype–phenotype correlations have been elucidated [1–3,7]. Missense mutations resulting in nonconservative amino acid substitutions are mostly associated with severe connatal form of PMD. Mutations in the latter half of exon 3 (exon 3B) of PLP1 are an exception because this region is spliced-out in DM20 mRNA and does not affect its expression and function, resulting in a milder form of PMD [1–4]. Deletions or early truncating mutations are also associated with a milder form of PMD or SPG2 [1–3,5,7,9,10]. Considering that the majority of PLP1 point mutations cause more severe dysmyelinating diseases than null mutations, Inoue speculated that the profound dysmyelination resulting from PLP1 point mutations probably arises not from the absence of functional protein, but rather from the cytotoxic effect of mutant protein [3]. Most patients with PLP1 duplications, who account for more than half of
the patients with PMD/SPG2, have a classical form of PMD that results in a moderate phenotype between connatal and a milder form of PMD [1–3,7]. Magnetic resonance imaging (MRI) is pivotal in diagnosing white matter disorders, including PMD [6]. Nevertheless, no previous study has confirmed the progress of myelination of a patient with PMD in a time-dependent manner. By examining a patient with a milder form of PMD caused by a novel PLP1 mutation, we report myelination progress with serial MRI. 2. Case report The patient (a 5-year-old boy) was an only child of non-consanguineous parents. His mother, without any siblings, had glaucoma and renal stones, while his maternal great-grandmother had Parkinson’s disease. He was born at 38 weeks of gestation by cesarean section; his weight was 2620 g ( 1.0, standard deviation [SD]), his length was 48.5 cm ( 0.2, SD), and his occipito-frontal circumference (OFC) was 33.2 cm ( 0.1, SD). At 7 months, he had not attained head control and was referred to our hospital. At that time, his OFC was 44.7 cm (+0.5, SD), and his muscle tone was normal without any spasticity. No nystagmus was evident. When he was 8 months old, brain MRI showed no overt abnormalities (Fig. 1). By studying auditory brainstem responses, we found that although wave I was clearly present, subsequent waveforms were less distinct (Fig. 2). At 12 months old, follow-up brain MRI revealed a lack of progressive myelination (Fig. 1). Cytomegalovirus PCR performed using dried umbilical cord was negative. After obtaining informed parental consent, gene analysis was performed, and it revealed a novel mutation in the initial half of exon 3 (exon 3A):
Fig. 1. The upper and lower rows show magnetic resonance images of our patient (axial T1-weighted and T2-weighted images at the level of the basal ganglia) obtained at ages 8, 12, 18, 30, 42, and 55 months. These images show a hypomyelination pattern, where white matter signal abnormalities are more prominent in T2-weighted images than in T1-weighted images. As the patient aged, myelination gradually progressed, and was more evident in T1-weighted than T2-weighted images.
T. Shiihara et al. / Brain & Development 37 (2015) 455–458
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Fig. 2. Auditory brainstem responses, recorded at electrodes A1 Cz and A2 Cz, from left and right ear stimulations, with click intensities of 90– 30 dB. Wave I is clearly apparent, whereas subsequent waveforms (III–V) are less distinct with prolonged latencies.
Fig. 3. Electropherogram from direct sequencing of PLP1 exon 3, demonstrating the c.300delC mutation. DNA and amino acids sequences are also shown. This mutation results in a frameshift, introducing a stop codon 13 amino acids downstream of the mutation. The upper row shows the proband, carrying only the mutant allele. The lower row shows the proband’s mother, carrying both the wild-type and the mutant alleles.
c.300delC (p.I100IfsX13) in PLP1 (Fig. 3). This mutation resulted in a frameshift, introducing a stop codon 13 amino acids downstream of the mutation. Gene analysis of his mother confirmed her carrier state (Fig. 3). His developmental milestones were as follows, without regression; head control at 8 months, sitting unsupported at 14 months, walking with aid at 19 months, and speaking at 25 months. Since 18 months, lower extremity spasticity had become evident. At 22 months, motor nerve conduction studies of right median and ulnar nerves showed decreased conduction velocities with reduced amplitudes. Follow-up brain MRI performed at 1-year intervals from age 18 months until 55 months showed progressive myelination, more prominent in T1-weighted than in T2-weighted images (Fig. 1). 3. Discussion The present patient has a frameshift mutation that may result in a truncated PLP, less than half the size
of the wild-type protein. Complete deletion or early truncation of PLP1 causes a milder form of PMD or SPG2 [1–3,5,7]. These conditions also present with mild peripheral neuropathy, typically absent in other forms of PMD/SPG2 [3,7]. Our patient’s condition is compatible with a milder form of PMD; therefore, the expression of the truncated PLP could be negligible, probably resulting from the mRNA nonsense-mediated decay pathway [8]. Complete PLP1 deletions are extremely rare, and only one study has described detailed brain MRI findings of a patient with complete deletion [7,9]. Torisu et al. reported nearly normal white matter intensities in T1-weighted images and diffuse white matter hyperintensity in T2-weighted images for a 2-year-old boy with complete PLP1 deletion [9]. Others have described patients with a milder form of PMD caused by truncated mutations [4,10]. Matsufuji et al. reported a family with partial PLP1 deletion, where exons 2–7 were absent [10]. The proband was a 29-year-old man who could
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walk with support; however, his motor development was markedly delayed from infancy. T2-weighted imaging revealed a symmetric and extensive signal increase in his cerebral white matter, but no abnormalities were noted in T1-weighted images. His symptoms corresponded to those of a milder form of PMD. Osaka et al. reported of a 26-year-old man who developed normally until he began having difficulty walking at age 12, subsequently became unable to stand at 15, and exhibited mental regression and generalized tonic convulsions by age 20 [4]. He had a truncated mutation in the latter half of exon 3 (exon 3B) of PLP1, thus DM20 production should have been unaffected. His phenotype also corresponds to a milder form of PMD, which could be explained by the nonsense-mediated decay pathway and a PLP1-specific mutation. MRI of his brain showed nearly normal white matter intensities in T1-weighted images, but diffuse white matter hyperintensity in T2weighted images. In this study, we incidentally identified progressive myelination in the patient by T1-weighted images obtained by serial MRI. This finding may be supportive to understand the pathological stages of a milder form of PMD. The patient described here exhibits delayed myelination. The apparent delay of myelination in T2-weighted MRI does not seem specific to this disease nor this patient [6]. In the early myelinating stage (even in normal myelination), white matter structures quickly become isointense or hyperintense compared to gray matter in T1-weighted images, whereas the signal of that structure is still high in T2-weighted images. T1-weighted images are generally more informative, suggesting more myelination than T2-weighted images in hypomyelination and delayed myelination. Future T2-weighted images of the present patient may reveal a residual myelinating process.
References [1] Woodward KJ. The molecular and cellular defects underlying Pelizaeus–Merzbacher disease. Expert Rev Mol Med 2008;10:e14. [2] Cailloux F, Gauthier-Barichard F, Mimault C, Isabelle V, Courtois V, Giraud G, et al. Genotype-phenotype correlation in inherited brain myelination defects due to proteolipid protein gene mutations. Clinical European network on brain dysmyelinating disease. Eur J Hum Genet 2000;8:837–45. [3] Inoue K. PLP1-related inherited dysmyelinating disorders: Pelizaeus–Merzbacher disease and spastic paraplegia type 2. Neurogenetics 2005;6:1–16. [4] Osaka H, Koizume S, Aoyama H, Iwamoto H, Kimura S, Nagai J, et al. Mild phenotype in Pelizaeus–Merzbacher disease caused by a PLP1-specific mutation. Brain Dev 2010;32:703–7. [5] Garbern JY, Yool DA, Moore GJ, Wilds IB, Faulk MW, Klugmann M, et al. Patients lacking the major CNS myelin protein, proteolipid protein 1, develop length-dependent axonal degeneration in the absence of demyelination and inflammation. Brain 2002;125:551–61. [6] Schiffmann R, van der Knaap MS. Invited article: an MRI-based approach to the diagnosis of white matter disorders. Neurology 2009;72:750–9. [7] Grossi S, Regis S, Biancheri R, Mort M, Lualdi S, Bertini E, et al. Molecular genetic analysis of the PLP1 gene in 38 families with PLP1-related disorders: identification and functional characterization of 11 novel PLP1 mutations. Orphanet J Rare Dis 2011;6:40. [8] Inoue K, Khajavi M, Ohyama T, Hirabayashi S, Wilson J, Reggin JD, et al. Molecular mechanism for distinct neurological phenotypes conveyed by allelic truncating mutations. Nat Genet 2004;36:361–9. [9] Torisu H, Iwaki A, Takeshita K, Hiwatashi A, Sanefuji M, Fukumaki Y, et al. Clinical and genetic characterization of a 2year-old boy with complete PLP1 deletion. Brain Dev 2012;34:852–6. [10] Matsufuji M, Osaka H, Gotoh L, Shimbo H, Takashima S, Inoue K. Partial PLP1 deletion causing X-linked dominant spastic paraplegia type 2. Pediatr Neurol 2013;49:477–81.
Case Report
A novel PLP1 frameshift mutation causing a milder form of Pelizaeus–Merzbacher disease Takashi Shiihara a,⇑, Mio Watanabe a, Kengo Moriyama a, Mitsugu Uematsu b, Kiyoko Sameshima c a Department of Neurology, Gunma Children’s Medical Center, Gunma 377-8577, Japan Department of Pediatrics, Tohoku University School of Medicine, Sendai 980-8574, Japan c Division of Medical Genetics, Gunma Children’s Medical Center, Gunma 377-8577, Japan
b
Received 16 April 2014; received in revised form 21 June 2014; accepted 25 June 2014
Abstract Background: Pelizaeus–Merzbacher disease (PMD), a hypomyelinating leukodystrophy, and the related but less severe allelic spastic paraplegia 2 (SPG2) are caused by mutations in the proteolipid protein 1 (PLP1) gene. Magnetic resonance imaging (MRI) is pivotal for diagnosing these disorders. The severity of PMD/SPG2 varies, and for a milder form of PMD, there have been some reports of near-normal findings in T1-weighted images but abnormal findings in T2-weighted images. Patient: We report the case of a 5-year-old boy diagnosed with a milder form of PMD caused by a novel PLP1 mutation in exon 3: c.300delC (p.I100IfsX13). He had delayed development from several months of age and was able to walk with support at 19 months in spite of the spasticity in his lower extremities. Hypomyelination was noted at 12 months by brain MRI. Motor nerve conduction studies showed decreased velocities with reduced amplitudes. Follow-up MRI at 1-year intervals from 18 months until 55 months of age showed gradual myelination progress. Discussion: The single nucleotide deletion identified in this patient can cause a frameshift and premature termination of PLP1. Via the nonsense-mediated mRNA decay mechanism of this mutation will result in loss-of-function, leading to a milder form of PMD. The present case is compatible with previously reported cases of milder form of PMD. We incidentally identified progressive myelination in this patient by T1-weighted images obtained by serial MRI. This finding adds to our understanding of the pathological stages of a milder form of PMD. Ó 2014 The Japanese Society of Child Neurology. Published by Elsevier B.V. All rights reserved.
Keywords: Pelizaeus–Merzbacher disease; Spastic paraplegia 2; Proteolipid protein; Myelination; Magnetic resonance imaging; mRNA; Nonsensemediated decay
1. Introduction Pelizaeus–Merzbacher disease (PMD; OMIM #312080), a hypomyelinating leukodystrophy, and the ⇑ Corresponding author. Address: Department of Neurology, Gunma Children’s Medical Center, 779 Shimohakoda, Hokkitsumachi, Shibukawa, Gunma 377-8577, Japan. Tel.: +81 279 52 3551; fax: +81 279 52 2045. E-mail address: [email protected] (T. Shiihara).
related but less severe allelic spastic paraplegia 2 (SPG2; OMIM #312920), are caused by mutations in the proteolipid protein 1 (PLP1) gene (OMIM *300401) [1]. PLP1, located on chromosome Xq22.2, contains 7 exons and encodes the major myelin sheath protein, proteolipid protein (PLP) and its shorterspliced isoform DM20. The severity of PMD/SPG2 varies and Cailloux et al. classified patients into five categories, according to their best motor function: form 0 with
http://dx.doi.org/10.1016/j.braindev.2014.06.011 0387-7604/Ó 2014 The Japanese Society of Child Neurology. Published by Elsevier B.V. All rights reserved.
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T. Shiihara et al. / Brain & Development 37 (2015) 455–458
no motor achievement; form 1 with head control; form 2 with sitting position; form 3 with supported walking; and form 4 with autonomous walking [2]. Clinical descriptions of PMD/SPG2 classifications have not been standardized and differ somewhat between studies [1–3]. According to Woodward et al., the descriptions range from the most severe to the mildest phenotype: connatal form of PMD (roughly fitting to the Cailloux et al. classification above, forms 0 and 1); classical form of PMD (forms 1 and 2); milder form of PMD (forms 2 and 3); and SPG2 (forms 3 and 4) [1–3]. Some authors further classify SPG2 into complicated and pure forms, with or without central nervous system involvement, respectively [1,3]. From a clinical viewpoint, the milder form of PMD and complicated form of SPG2 are barely distinguishable [1]. Hereafter, for the sake of clarification, when referring to SPG2, we assume a pure form of SPG2. A wide range of mutations can cause PMD/SPG2, and genotype–phenotype correlations have been elucidated [1–3,7]. Missense mutations resulting in nonconservative amino acid substitutions are mostly associated with severe connatal form of PMD. Mutations in the latter half of exon 3 (exon 3B) of PLP1 are an exception because this region is spliced-out in DM20 mRNA and does not affect its expression and function, resulting in a milder form of PMD [1–4]. Deletions or early truncating mutations are also associated with a milder form of PMD or SPG2 [1–3,5,7,9,10]. Considering that the majority of PLP1 point mutations cause more severe dysmyelinating diseases than null mutations, Inoue speculated that the profound dysmyelination resulting from PLP1 point mutations probably arises not from the absence of functional protein, but rather from the cytotoxic effect of mutant protein [3]. Most patients with PLP1 duplications, who account for more than half of
the patients with PMD/SPG2, have a classical form of PMD that results in a moderate phenotype between connatal and a milder form of PMD [1–3,7]. Magnetic resonance imaging (MRI) is pivotal in diagnosing white matter disorders, including PMD [6]. Nevertheless, no previous study has confirmed the progress of myelination of a patient with PMD in a time-dependent manner. By examining a patient with a milder form of PMD caused by a novel PLP1 mutation, we report myelination progress with serial MRI. 2. Case report The patient (a 5-year-old boy) was an only child of non-consanguineous parents. His mother, without any siblings, had glaucoma and renal stones, while his maternal great-grandmother had Parkinson’s disease. He was born at 38 weeks of gestation by cesarean section; his weight was 2620 g ( 1.0, standard deviation [SD]), his length was 48.5 cm ( 0.2, SD), and his occipito-frontal circumference (OFC) was 33.2 cm ( 0.1, SD). At 7 months, he had not attained head control and was referred to our hospital. At that time, his OFC was 44.7 cm (+0.5, SD), and his muscle tone was normal without any spasticity. No nystagmus was evident. When he was 8 months old, brain MRI showed no overt abnormalities (Fig. 1). By studying auditory brainstem responses, we found that although wave I was clearly present, subsequent waveforms were less distinct (Fig. 2). At 12 months old, follow-up brain MRI revealed a lack of progressive myelination (Fig. 1). Cytomegalovirus PCR performed using dried umbilical cord was negative. After obtaining informed parental consent, gene analysis was performed, and it revealed a novel mutation in the initial half of exon 3 (exon 3A):
Fig. 1. The upper and lower rows show magnetic resonance images of our patient (axial T1-weighted and T2-weighted images at the level of the basal ganglia) obtained at ages 8, 12, 18, 30, 42, and 55 months. These images show a hypomyelination pattern, where white matter signal abnormalities are more prominent in T2-weighted images than in T1-weighted images. As the patient aged, myelination gradually progressed, and was more evident in T1-weighted than T2-weighted images.
T. Shiihara et al. / Brain & Development 37 (2015) 455–458
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Fig. 2. Auditory brainstem responses, recorded at electrodes A1 Cz and A2 Cz, from left and right ear stimulations, with click intensities of 90– 30 dB. Wave I is clearly apparent, whereas subsequent waveforms (III–V) are less distinct with prolonged latencies.
Fig. 3. Electropherogram from direct sequencing of PLP1 exon 3, demonstrating the c.300delC mutation. DNA and amino acids sequences are also shown. This mutation results in a frameshift, introducing a stop codon 13 amino acids downstream of the mutation. The upper row shows the proband, carrying only the mutant allele. The lower row shows the proband’s mother, carrying both the wild-type and the mutant alleles.
c.300delC (p.I100IfsX13) in PLP1 (Fig. 3). This mutation resulted in a frameshift, introducing a stop codon 13 amino acids downstream of the mutation. Gene analysis of his mother confirmed her carrier state (Fig. 3). His developmental milestones were as follows, without regression; head control at 8 months, sitting unsupported at 14 months, walking with aid at 19 months, and speaking at 25 months. Since 18 months, lower extremity spasticity had become evident. At 22 months, motor nerve conduction studies of right median and ulnar nerves showed decreased conduction velocities with reduced amplitudes. Follow-up brain MRI performed at 1-year intervals from age 18 months until 55 months showed progressive myelination, more prominent in T1-weighted than in T2-weighted images (Fig. 1). 3. Discussion The present patient has a frameshift mutation that may result in a truncated PLP, less than half the size
of the wild-type protein. Complete deletion or early truncation of PLP1 causes a milder form of PMD or SPG2 [1–3,5,7]. These conditions also present with mild peripheral neuropathy, typically absent in other forms of PMD/SPG2 [3,7]. Our patient’s condition is compatible with a milder form of PMD; therefore, the expression of the truncated PLP could be negligible, probably resulting from the mRNA nonsense-mediated decay pathway [8]. Complete PLP1 deletions are extremely rare, and only one study has described detailed brain MRI findings of a patient with complete deletion [7,9]. Torisu et al. reported nearly normal white matter intensities in T1-weighted images and diffuse white matter hyperintensity in T2-weighted images for a 2-year-old boy with complete PLP1 deletion [9]. Others have described patients with a milder form of PMD caused by truncated mutations [4,10]. Matsufuji et al. reported a family with partial PLP1 deletion, where exons 2–7 were absent [10]. The proband was a 29-year-old man who could
458
T. Shiihara et al. / Brain & Development 37 (2015) 455–458
walk with support; however, his motor development was markedly delayed from infancy. T2-weighted imaging revealed a symmetric and extensive signal increase in his cerebral white matter, but no abnormalities were noted in T1-weighted images. His symptoms corresponded to those of a milder form of PMD. Osaka et al. reported of a 26-year-old man who developed normally until he began having difficulty walking at age 12, subsequently became unable to stand at 15, and exhibited mental regression and generalized tonic convulsions by age 20 [4]. He had a truncated mutation in the latter half of exon 3 (exon 3B) of PLP1, thus DM20 production should have been unaffected. His phenotype also corresponds to a milder form of PMD, which could be explained by the nonsense-mediated decay pathway and a PLP1-specific mutation. MRI of his brain showed nearly normal white matter intensities in T1-weighted images, but diffuse white matter hyperintensity in T2weighted images. In this study, we incidentally identified progressive myelination in the patient by T1-weighted images obtained by serial MRI. This finding may be supportive to understand the pathological stages of a milder form of PMD. The patient described here exhibits delayed myelination. The apparent delay of myelination in T2-weighted MRI does not seem specific to this disease nor this patient [6]. In the early myelinating stage (even in normal myelination), white matter structures quickly become isointense or hyperintense compared to gray matter in T1-weighted images, whereas the signal of that structure is still high in T2-weighted images. T1-weighted images are generally more informative, suggesting more myelination than T2-weighted images in hypomyelination and delayed myelination. Future T2-weighted images of the present patient may reveal a residual myelinating process.
References [1] Woodward KJ. The molecular and cellular defects underlying Pelizaeus–Merzbacher disease. Expert Rev Mol Med 2008;10:e14. [2] Cailloux F, Gauthier-Barichard F, Mimault C, Isabelle V, Courtois V, Giraud G, et al. Genotype-phenotype correlation in inherited brain myelination defects due to proteolipid protein gene mutations. Clinical European network on brain dysmyelinating disease. Eur J Hum Genet 2000;8:837–45. [3] Inoue K. PLP1-related inherited dysmyelinating disorders: Pelizaeus–Merzbacher disease and spastic paraplegia type 2. Neurogenetics 2005;6:1–16. [4] Osaka H, Koizume S, Aoyama H, Iwamoto H, Kimura S, Nagai J, et al. Mild phenotype in Pelizaeus–Merzbacher disease caused by a PLP1-specific mutation. Brain Dev 2010;32:703–7. [5] Garbern JY, Yool DA, Moore GJ, Wilds IB, Faulk MW, Klugmann M, et al. Patients lacking the major CNS myelin protein, proteolipid protein 1, develop length-dependent axonal degeneration in the absence of demyelination and inflammation. Brain 2002;125:551–61. [6] Schiffmann R, van der Knaap MS. Invited article: an MRI-based approach to the diagnosis of white matter disorders. Neurology 2009;72:750–9. [7] Grossi S, Regis S, Biancheri R, Mort M, Lualdi S, Bertini E, et al. Molecular genetic analysis of the PLP1 gene in 38 families with PLP1-related disorders: identification and functional characterization of 11 novel PLP1 mutations. Orphanet J Rare Dis 2011;6:40. [8] Inoue K, Khajavi M, Ohyama T, Hirabayashi S, Wilson J, Reggin JD, et al. Molecular mechanism for distinct neurological phenotypes conveyed by allelic truncating mutations. Nat Genet 2004;36:361–9. [9] Torisu H, Iwaki A, Takeshita K, Hiwatashi A, Sanefuji M, Fukumaki Y, et al. Clinical and genetic characterization of a 2year-old boy with complete PLP1 deletion. Brain Dev 2012;34:852–6. [10] Matsufuji M, Osaka H, Gotoh L, Shimbo H, Takashima S, Inoue K. Partial PLP1 deletion causing X-linked dominant spastic paraplegia type 2. Pediatr Neurol 2013;49:477–81.
