An MRI and MRS study of Pelizaeus-Merzbacher disease

Leukodystrophy Imaging: Insights for Diagnostic Dilemmas

Leukodystrophies, a group of rare demyelinating disorders, mainly affect the CNS. Clinical presentation of different types of leukodystrophies can be nonspecific, and thus, imaging techniques like MRI can be used for a more definitive diagnosis. These diseases are characterized as cerebral lesions with characteristic demyelinating patterns which can be used as differentiating tools. In this review, we talk about these MRI study findings for each leukodystrophy, associated genetics, blood work that can help in differentiation, emerging diagnostics, and a follow-up imaging strategy. The leukodystrophies discussed in this paper include X-linked adrenoleukodystrophy, metachromatic leukodystrophy, Krabbe's disease, Pelizaeus-Merzbacher disease, Alexander's disease, Canavan disease, and Aicardi-Goutières Syndrome.

Identification of PMD subgroups using a myelination score for PMD

BACKGROUND

The clinical spectrum of Pelizaeus-Merzbacher disease (PMD), a common hypomyelinating leukodystrophy, ranges between severe neonatal onset and a relatively stable presentation with later onset and mainly lower limb spasticity. In view of emerging treatment options and in order to grade severity and progression, we developed a PMD myelination score.

METHODS

Myelination was scored in 15 anatomic sites (items) on conventional T2-and T1w images in controls (n = 328) and 28 PMD patients (53 MRI; n = 5 connatal, n = 3 transitional, n = 10 classic, n = 3 intermediate, n = 2 PLP0, n = 3 SPG2, n = 2 female). Items included in the score were selected based on interrater variability, practicability of scoring and importance of scoring items for discrimination between patients and controls and between patient subgroups. Bicaudate ratio, maximal sagittal pons diameter, and visual assessment of midsagittal corpus callosum were separately recorded.

RESULTS

The resulting myelination score consisting of 8 T2-and 5 T1-items differentiates patients and controls as well as patient subgroups at first MRI. There was very little myelin and early loss in severely affected connatal and transitional patients, more, though still severely deficient myelin in classic PMD, ongoing myelination during childhood in classic and intermediate PMD. Atrophy, present in 50% of patients, increased with age at imaging.

CONCLUSIONS

The proposed myelination score allows stratification of PMD patients and standardized assessment of follow-up. Loss of myelin in severely affected and PLP0 patients and progressing myelination in classic and intermediate PMD must be considered when evaluating treatment efficacy.

The neurovascular unit in leukodystrophies: towards solving the puzzle

The neurovascular unit (NVU) is a highly organized multicellular system localized in the brain, formed by neuronal, glial (astrocytes, oligodendrocytes, and microglia) and vascular (endothelial cells and pericytes) cells. The blood-brain barrier, a complex and dynamic endothelial cell barrier in the brain microvasculature that separates the blood from the brain parenchyma, is a component of the NVU. In a variety of neurological disorders, including Alzheimer's disease, multiple sclerosis, and stroke, dysfunctions of the NVU occurs. There is, however, a lack of knowledge regarding the NVU function in leukodystrophies, which are rare monogenic disorders that primarily affect the white matter. Since leukodystrophies are rare diseases, human brain tissue availability is scarce and representative animal models that significantly recapitulate the disease are difficult to develop. The introduction of human induced pluripotent stem cells (hiPSC) now makes it possible to surpass these limitations while maintaining the ability to work in a biologically relevant human context and safeguarding the genetic background of the patient. This review aims to provide further insights into the NVU functioning in leukodystrophies, with a special focus on iPSC-derived models that can be used to dissect neurovascular pathophysiology in these diseases.

Pathology of the neurovascular unit in leukodystrophies

The blood-brain barrier is a dynamic endothelial cell barrier in the brain microvasculature that separates the blood from the brain parenchyma. Specialized brain endothelial cells, astrocytes, neurons, microglia and pericytes together compose the neurovascular unit and interact to maintain blood-brain barrier function. A disturbed brain barrier function is reported in most common neurological disorders and may play a role in disease pathogenesis. However, a comprehensive overview of how the neurovascular unit is affected in a wide range of rare disorders is lacking. Our aim was to provide further insights into the neuropathology of the neurovascular unit in leukodystrophies to unravel its potential pathogenic role in these diseases. Leukodystrophies are monogenic disorders of the white matter due to defects in any of its structural components. Single leukodystrophies are exceedingly rare, and availability of human tissue is unique. Expression of selective neurovascular unit markers such as claudin-5, zona occludens 1, laminin, PDGFRβ, aquaporin-4 and α-dystroglycan was investigated in eight different leukodystrophies using immunohistochemistry. We observed tight junction rearrangements, indicative of endothelial dysfunction, in five out of eight assessed leukodystrophies of different origin and an altered aquaporin-4 distribution in all. Aquaporin-4 redistribution indicates a general astrocytic dysfunction in leukodystrophies, even in those not directly related to astrocytic pathology or without prominent reactive astrogliosis. These findings provide further evidence for dysfunction in the orchestration of the neurovascular unit in leukodystrophies and contribute to a better understanding of the underlying disease mechanism.

Glial cells in the driver seat of leukodystrophy pathogenesis

Glia cells are often viewed as support cells in the central nervous system, but recent discoveries highlight their importance in physiological functions and in neurological diseases. Central to this are leukodystrophies, a group of progressive, neurogenetic disease affecting white matter pathology. In this review, we take a closer look at multiple leukodystrophies, classified based on the primary glial cell type that is affected. While white matter diseases involve oligodendrocyte and myelin loss, we discuss how astrocytes and microglia are affected and impinge on oligodendrocyte, myelin and axonal pathology. We provide an overview of the leukodystrophies covering their hallmark features, clinical phenotypes, diverse molecular pathways, and potential therapeutics for clinical trials. Glial cells are gaining momentum as cellular therapeutic targets for treatment of demyelinating diseases such as leukodystrophies, currently with no treatment options. Here, we bring the much needed attention to role of glia in leukodystrophies, an integral step towards furthering disease comprehension, understanding mechanisms and developing future therapeutics.

Time-course of myelination and atrophy on cerebral imaging in 35 patients with PLP1-related disorders

AIM

Brain magnetic resonance imaging (MRI) motor development score (MDS) correlations were used to analyze the natural time-course of hypomyelinating PLP1-related disorders (Pelizaeus-Merzbacher disease [PMD] and spastic paraplegia type 2).

METHOD

Thirty-five male patients (ranging from 0.7-43.5y at the first MRI) with PLP1-related disorder were prospectively followed over 7 years. Patients were classified according to best motor function acquired before 5 years (MDS) into five categories (from PMD0 without motor acquisition to PMD4 with autonomous walking). We determined myelination and atrophy scores and measured corpus callosum area, volume of cerebellum, white matter and grey matter on 63 MRI.

RESULTS

Age-adjusted multivariate analysis revealed that patients with PMD0-1 had higher-severity atrophy scores and smaller corpus callosum area than did patients with PMD2 and PMD3-4. Myelination score increased until 12 years. There was evidence that the mean myelination differed in frontal white matter, arcuate fibres, and internal capsules among the groups. Most patients showed worsening atrophy (brain, cerebellum, corpus callosum), whereas grey matter and white matter proportions did not change.

INTERPRETATION

Brain atrophy and myelination of anterior cerebral regions appear to be pertinent biomarkers of motor development. The time-course of inter- and intra-individual cerebral white matter and grey matter atrophy suggests that both oligodendrocytes and neurons are involved in the physiopathology of PLP1-related disorders.

The magnetic resonance imaging spectrum of Pelizaeus-Merzbacher disease: A multicenter study of 19 patients

PURPOSE

We retrospectively evaluated the imaging spectrum of Pelizaeus-Merzbacher disease (PMD) in correlation with the clinical course and genetic abnormality.

METHODS

We collected the magnetic resonance imaging (MRI) findings of 19 genetically proven PMD patients (all males, aged 0-29years old) using our integrated web-based MRI data collection system from 14 hospitals. The patterns of hypomyelination were determined mainly by the signals of the cerebrum, corticospinal tract, and brainstem on T2-weighted images (T2WI). We assessed the degree of myelination age on T1-weighted images (T1WI) and T2WI independently, and we evaluated cerebellar and callosal atrophy. The clinical severity and genetic abnormalities (causal mutations of the proteolipid protein gene PLP1) were analyzed together with the imaging findings.

RESULTS

The clinical stage tended to be more severe when the whole brainstem, or corticospinal tract in the internal capsule showed abnormally high intensity on T2WI. Diffuse T2-high signal of brainstem was observed only in the patients with PLP1 point mutation. Myelination age "before birth" on T1WI is a second manifestation correlated with the clinically severe phenotypes. On the other hand, eight patients whose myelination ages were > 4months on T1WI were associated with mild clinical phenotypes. Four of them showed almost complete myelination on T1WI with a discrepancy in myelination age between T1WI and T2WI. A random and patchy pattern of myelination on T2WI was noted in one patient with PLP1 point mutation. Advanced myelination was observed in three of the seven followed-up patients. Four patients had atrophy of the cerebellum, and 17 patients had atrophy of the corpus callosum.

CONCLUSION

Our multicenter study has demonstrated a wide variety of imaging findings of PMD. Signal intensity of brainstem and corticospinal tract of internal capsule would be the points to presume clinical severity in PMD patients. The spectrum of MRI findings should be kept in mind to diagnose PMD and to differentiate from other demyelinating leukodystrophies.

Pelizaeus-Merzbacher disease: cellular pathogenesis and pharmacologic therapy

Pelizaeus-Merzbacher disease (PMD) is a rare leukodystrophy that causes severe dysmyelination in the central nervous system in infancy and early childhood. Many previous studies showed that various proteolipid protein 1 (plp1) mutations, including duplications, point mutations, and deletions, lead to oligodendrocyte dysfunction in patients with PMD. PMD onset and clinical severity range widely, depending on the type of plp1 mutation. Patients with PMD exhibit a delayed mental and physical development phenotype, but specific pharmacological therapy and clinical treatment for PMD are not yet well established. This review describes PMD pathology and establishment of new clinical treatment for PMD. These findings support the development of a new therapy for PMD and these treatments may improve the quality of life in patients with PMD.

Neuroradiologic correlates of clinical disability and progression in the X-linked leukodystrophy Pelizaeus-Merzbacher disease

OBJECTIVE

To determine whether quantitative measure of magnetic resonance imaging data from patients with the inherited leukodystrophy, Pelizaeus-Merzbacher disease (PMD) correlates with clinical severity or progression.

METHODS

In our current work we have analyzed the clinical phenotypes and MRI scans of 51 male patients with PMD and 10 female carriers for whom the PLP1 genotype had been determined. In addition, we developed a 32-point functional disability scoring (FDS) system for PMD, and validated it for inter-rater reliability. Using conventional T1- and T2-weighted MRI images of the whole brain, we measured white matter and total brain volume (WMV and TBV), inter-caudate ratio (ICR), and corpus callosum area.

RESULTS

There was a significant positive correlation of FDS with white matter fraction (WMV/TBV) and corpus callosum area. Also, when applying a median split based on FDS, patients with lower FDS showed reduced white matter fraction and corpus callosum area, and increased ICR compared to patients with relatively higher FDS, regardless of age.

CONCLUSION

Although this patient population is heterogeneous, with multiple genetic and molecular mechanisms causing PMD, these data imply that white matter atrophy is a major pathological determinant of the clinical disability in most patients. Development of reliable non-invasive quantitative biomarkers of disease activity would be useful not only for following the natural history of the disease, but also raising the potential for evaluating future therapies.

Leukodystrophies

Leukodystrophies comprise a broad group of progressive, inherited disorders affecting mainly myelin. They often present after a variable period of normalcy with a variety of neurologic problems. Though the ultimate diagnosis is not found in many patients with leukodystrophies, distinctive features unique to them aid in diagnosis, treatment and prognostication. The clinical characteristics, etiologies, diagnostic testing and treatment options are reviewed in detail for some of the major leukodystrophies: X-linked adrenoleukodystrophy, Krabbe disease, metachromatic leukodystrophy, Pelizaeus-Merzbacher disease, Alexander disease, Canavan disease, megalencephalic leukoencephalopathy with subcortical cysts and vanishing white matter disease.

Neuroimaging of inherited metabolic disorders producing seizures

Both MRI with FLAIR and T2, diffusion and post-contrast T1 imaging, as well as proton MRS, allow for defining an approach to evaluating MRI and MRS in neurometabolic diseases producing seizures. Examples of this approach are given here in a limited number of cases, recognizing that a more comprehensive review is beyond the scope of this paper.

Variable expression of a novel PLP1 mutation in members of a family with Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is a rare X-linked disorder caused by mutations of the proteolipid protein 1 gene that encodes a structural component of myelin. It is characterized by progressive psychomotor delay, nystagmus, spastic quadriplegia, and cerebellar ataxia. Variable clinical expression was seen in 5 members of a family bearing a novel missense mutation in proteolipid protein 1, c.619T>C. Symptomatic patients included a 6-year-old girl, her younger brother, and their maternal uncle, a 29-year-old college graduate initially diagnosed with cerebral palsy; their brain magnetic resonance imaging studies showed diffuse dysmyelination. The mother had a history of delayed walking, achieved independently by age 3; she and the maternal grandmother were asymptomatic on presentation. Review of clinical information and family history led to consideration of Pelizaeus-Merzbacher disease. Subsequent identification of the causal mutation enabled preimplantation genetic diagnosis and the birth of an unaffected child.

A novel PLP mutation in a Japanese patient with mild Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a rare dysmyelinating disorder due to mutations in the proteolipid protein (PLP) gene. PLP gene mutations are responsible for a broad spectrum of disease, from the most severe form, connatal PMD, to a less severe form, spastic paraplegia 2 (SPG2). We describe here a very mild case of PMD in a patient who presented with nystagmus in early infancy and was unable to walk until 1 year 7 months of age. Brain magnetic resonance imaging (MRI) at 1 year 7 months of age revealed white matter abnormalities typical of PMD. Genetic testing revealed a novel mutation of the PLP gene (Gly197Arg). The patient presented with only mildly ataxic gait and slurred speech at the age of 4 years. Gly197Arg is the first novel mutation located within exon 4 of the PLP gene and associated with mild PMD/SPG2 in a Japanese patient.

Hearing profile and MRI myelination of auditory pathway in Pelizaeus-Merzbacher disease

CONCLUSIONS

This study showed that delayed auditory pathway myelination is common in Pelizaeus-Merzbacher disease (PMD), but this delay does not necessarily indicate poor hearing function.

OBJECTIVE

PMD is a rare recessively inherited X-linked leukodystrophy characterized by defective central nervous system myelination owing to a mutation in the proteolipid protein gene (PLP). The aims of this study were to evaluate the hearing function and auditory brain response (ABR) findings of patients with PMD and relate these findings to MRI-assessed myelination in the central auditory pathway.

PATIENTS AND METHODS

We retrospectively studied eight male pediatric patients with PMD. Serial auditory examinations included audiometry, behavior audiometry, distortion product otoacoustic emission (DPOAE), and ABR. MRI-assessed myelination in the auditory pathway was evaluated in the PMD patients and in 23 normal young children as a control group.

RESULTS

Audiometry showed normal to moderate hearing impairment and the hearing threshold improved with age and became almost normal over time. DPOAEs positivity and only ABR wave I or waves I and II were found in all the patients. MRI showed delayed myelination in all the patients and the auditory pathway was myelinated up to the inferior colliculus in four cases and up to the medial geniculate body in four cases. Serial MRIs showed no progression in myelination. No clear relation was found between hearing threshold and MRI-assessed myelination in the auditory pathway.

Frameshift mutation in GJA12 leading to nystagmus, spastic ataxia and CNS dys-/demyelination

Mutations in GJA12 have been shown to cause Pelizaeus-Merzbacher-like disease (PMLD). We present two additional patients from one family carrying a homozygous frameshift mutation in GJA12. Both presented initially with nystagmus. The older girl developed ataxia first, then progressive spastic ataxia. The younger boy suffered from severe sensory neuropathy. Magnetic resonance imaging (MRI) of both children showed progressive demyelination in addition to dysmyelination, and also characteristic brainstem abnormalities. In children with nystagmus, ataxia and dysmyelination, mutation analysis of GJA12 should be considered early, especially if inheritance is autosomal recessive.

Identification of diffuse and focal brain lesions by clinical magnetic resonance spectroscopy

The purpose of this paper is to facilitate the comparison of magnetic resonance (MR) spectra acquired from unknown brain lesions with published spectra in order to help identify unknown lesions in clinical settings. The paper includes lists of references for published MR spectra of various brain diseases, including pyogenic abscesses, encephalitis (herpes simplex, Rasmussen's and subacute sclerosing panencephalitis), neurocysticercosis, tuberculoma, cysts (arachnoid, epidermoid and hydatid), acute disseminated encephalomyelitis (ADEM), adrenoleukodystrophy (ALD), Alexander disease, Canavan's disease, Krabbe disease (globoid cell leukodystrophy), Leigh's disease, megalencephalic leukoencephalopathy with cysts, metachromatic leukodystrophy (MLD), Pelizaeus-Merzbacher disease, Zellweger syndrome, HIV-associated lesions [cryptococcus, lymphoma, toxoplasmosis and progressive multifocal leukoencephalopathy (PML)], hydrocephalus and tuberous sclerosis. Each list includes information on the echo time(s) (TE) of the published spectra, whether a control spectrum is shown, whether the corresponding image and voxel position are shown and the patient ages if known. The references are listed in the approximate order of usefulness, based on spectral quality, number of spectra, range of echo times and whether the voxel positions are shown. Spectra of Zellweger syndrome, cryptococcal infection, toxoplasmosis and lymphoma are included, along with a spectrum showing propanediol (propylene glycol).

PLP1-related inherited dysmyelinating disorders: Pelizaeus-Merzbacher disease and spastic paraplegia type 2

Pelizaeus-Merzbacher disease (PMD) and its allelic disorder, spastic paraplegia type 2 (SPG2), are among the best-characterized dysmyelinating leukodystrophies of the central nervous system (CNS). Both PMD and SPG2 are caused by mutations in the proteolipid protein 1 (PLP1) gene, which encodes a major component of CNS myelin proteins. Distinct types of mutations, including point mutations and genomic duplications and deletions, have been identified as causes of PMD/SPG2 that act through different molecular mechanisms. Studies of various PLP1 mutants in humans and animal models have shed light on the genomic, molecular, and cellular pathogeneses of PMD/SPG2. Recent discoveries include complex mutational mechanisms and associated disease phenotypes, novel cellular pathways that lead to the degeneration of oligodendrocytes, and genomic architectural features that result in unique chromosomal rearrangements. Here, I review the previous and current knowledge of the molecular pathogenesis of PMD/SPG2 and delineate future directions for PMD/SPG2 studies.

Severe hypomyelination associated with increased levels of N-acetylaspartylglutamate in CSF

BACKGROUND

Two unrelated girls had early onset of nystagmus and epilepsy, absent psychomotor development, and almost complete absence of myelin on cerebral MRI. The clinical features and MR images of both patients resembled the connatal form of Pelizaeus-Merzbacher disease (PMD), which is an X-linked recessive disorder caused by duplications or mutations of the proteolipid protein gene (PLP).

OBJECTIVE

To define a unique neurometabolic disorder with failure of myelination.

METHOD

S AND RESULTS: 1H-NMR of CSF in both girls was performed repeatedly, and both showed highly elevated concentrations of N-acetylaspartylglutamate (NAAG). The coding sequence of the gene coding for glutamate carboxypeptidase II, which converts NAAG to N-acetylaspartate (NAA) and glutamate, was entirely sequenced but revealed no mutations. Even though both patients are girls, the authors sequenced the PLP gene and found no abnormality.

CONCLUSIONS

NAAG is an abundant peptide neurotransmitter whose exact role is unclear. NAAG is implicated in two cases of unresolved severe CNS disorder. Its elevated concentration in CSF may be the biochemical hallmark for a novel neurometabolic disorder. The cause of its accumulation is still unclear.

A case of complicated spastic paraplegia 2 due to a point mutation in the proteolipid protein 1 gene

Pelizaeus-Merzbacher disease (PMD) is a rare X-linked dysmyelinating disorder resulting from mutation of the proteolipid protein gene (PLP1). Clinical features of PMD include progressive psychomotor developmental delay, nystagmus, spastic quadriplegia, dystonia, and cerebellar ataxia. PMD is clinically classified into three subtypes according to the severity of the disease: connatal, transitional, and classic forms. Patients with PMD have been identified with duplication, point mutations, and deletion of PLP1. In addition, spastic paraplegia 2 (SPG2) is allelic to PMD and typically caused by missense mutations in the second extracellular domain of PLP1 or in the PLP1-specific region that is spliced out during formation of the DM20 isoform. The authors describe a Korean boy diagnosed with SPG2 caused by a mutation that results in a Pro215Leu substitution in the second extracellular domain. Analysis of phenotypes resulting from mutations affecting PLP1 has been valuable in identifying functional domains of this still incompletely understood major myelin protein. Null mutations and mutations affecting the PLP1-specific domain cause peripheral neuropathy. The PLP1-specific domain also is important in the long-term maintenance of axonal integrity. This patient's phenotype was relatively mild, in contrast with other mutations at position 215 of PLP1 that cause severe PMD. One of these severe mutations is also a missense mutation substituting an aliphatic residue, alanine, for proline. The distinct severity difference between the Pro215Leu and Pro215Ala substitutions suggests that this region of the protein is very sensitive to subtle structural changes and likely plays a critical role in PLP1 function.

Pelizaeus-Merzbacher disease: diffusion MR imaging and proton MR spectroscopy findings

A 5-Year-old boy is reported with genetically confirmed Pelizaeus-Merzbacher disease. On chemical-shift spectroscopic imaging choline (Cho) peaks were prominently decreased in the white matter resulting in markedly high NAA/Cho ratios, and low Cho/Cr ratios, compared to five control cases. Low Cho levels could be indicative of the dysmyelinating disorder in the disease. On b=1000 s/mm(2) images of diffusion MRI, a tigroid pattern was evident, and there was no apparent signal abnormality. However, on ADC maps high signal and high ADC values were evident in the white matter ranging from 1.16 to 1.52 X 10(-3) mm(2)/s, compared to the ADC values of nine control cases, consistent with some disintegration of the white matter secondary to lack of myelination. On the other hand, the ADC values of the cortex were normal (0.79-0.95 X 10(- 3) mm(2)/s). These findings on spectroscopy and diffusion MRI likely represented deficient myelination in the disease.

Pelizaeus-Merzbacher disease and spastic paraplegia type 2: two faces of myelin loss from mutations in the same gene

Pelizaeus-Merzbacher disease and X-linked spastic paraplegia type 2 are two sides of the same coin. Both arise from mutations in the gene encoding myelin proteolipid protein. The disease spectrum for Pelizaeus-Merzbacher disease and spastic paraplegia type 2 is extraordinarily broad, ranging from a spastic gait in the pure form of spastic paraplegia type 2 to a severely disabling form of Pelizaeus-Merzbacher disease featuring hypotonia, respiratory distress, stridor, nystagmus, and profound myelin loss. The diverse disease spectrum is mirrored by the underlying pathogenesis, in which a blockade at any stage of myelin proteolipid protein synthesis and assembly into myelin spawns a unique phenotype. The continuing definition of pathogenetic mechanisms operative in Pelizaeus-Merzbacher disease and spastic paraplegia type 2, together with advances in neural cell transplant therapy, augurs well for future treatment of the severe forms of Pelizaeus-Merzbacher disease.

High-resolution 3D proton spectroscopic imaging of the human brain at 3 T: SNR issues and application for anatomy-matched voxel sizes

In a systematic study on the interdependence of linewidth, signal-to-noise ratio (SNR), and spatial resolution in 3D proton spectroscopic imaging ((1)H-SI) at 3 T, we demonstrate reduced linewidths with increased spatial resolution due to reduced magnetic inhomogeneity within the brain. High-precision quantitative data (0.75-0.094 cm(3)) were obtained for all resolutions, enabling the creation of metabolic maps that display details such as the ventricles, sulci, and gyri. High-resolution (1)H-SI allows differences in metabolic ratios to be estimated for anatomically defined regions in gray (GM) and white matter (WM). Seven distinct regions in a healthy brain were anatomically segmented and their metabolic ratios were compared quantitatively. Data from a tumor patient are also presented to demonstrate potential clinical applications. Because of the high resolution, the metabolite ratios could be determined for distinct pathologic regions within the tumor and its surroundings. The method was additionally applied to a patient with patchy Pelizaeus Merzbacher disease (PMD), and compared to single-voxel spectroscopy performed in the same session. High-resolution SI data were demonstrated in our study to allow the direct matching of anatomic and metabolic images. This may enhance the clinical value of (1)H-SI.

Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) can now be defined as an X-linked recessive leukodystrophy that is caused by a mutation in the proteolipid protein (PLP) gene on chromosome Xq22. The most common mutation is gene duplication followed in frequency by missense mutations, insertions, and deletions. The clinical spectrum ranges from severe neonatal cases to relatively benign adult forms and X-linked recessive spastic paraplegia type 2. The lack of PLP is accompanied by deficits in the other myelin proteins of the central nervous system, including myelin basic protein, myelin-associated glycoprotein, and cyclic nucleotide phosphodiesterase. Surprisingly, the total absence of PLP due to gene deletion or a null allele causes a relatively benign form of PMD. Abnormal PLP is thought to impair protein trafficking and to induce apoptosis in oligodendroglia. Immunocytochemistry with specific antibodies reveals the PLP deficiency and insufficient generation of myelin sheaths with the remaining proteins. Both excessive biosynthesis of PLP, as in gene duplications, or conformational change of the protein, as in missense mutations, are detrimental to myelination. Several naturally occurring and transgenic animal models with PLP gene mutations or deletions have contributed to our understanding of dysmyelination in PMD and the general knowledge of myelination and myelin repair.

Abnormal myelination in a patient with deletion 14q11.2q13.1

A male carrying an interstitial deletion of chromosome 14, presumably del(14)(q11.2q13), and presenting with abnormal myelination on magnetic resonance imaging is described. The abnormal myelination was evidenced as a high-signal intensity on T(2)-weighted magnetic resonance imaging. The patient had severe neurologic signs, various dysmorphic features, and a marked microcephaly. To our knowledge, this case is the first patient reported with abnormal myelination and a deletion of chromosome 14.