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The Leukodystrophies 312 The Leukodystrophies Hannah B. Gordon, BA1 Anthea Letsou, PhD1 Joshua L. Bonkowsky, MD, PhD2,3 1 Department of Human Genetics, University of Utah School of Address for correspondence Josh Bonkowsky, MD, PhD, Department Medicine, Salt Lake City, Utah of Pediatrics, 295 Chipeta Way, Salt Lake City, UT 84108 2 Division of Pediatric Neurology, Department of Pediatrics, University (e-mail: [email protected]). of Utah School of Medicine, Salt Lake City, Utah 3 Department of Neurology, University of Utah School of Medicine, Salt Lake City, Utah Semin Neurol 2014;34:312–320. Abstract Leukodystrophies are a group of genetically determined disorders that affect develop- Keywords ment or maintenance of central nervous system myelin. Leukodystrophies have a ► leukodystrophy reported incidence of 1 in 7500 live births, but fewer than half of patients receive a ► myelin specific diagnosis. In this review, the authors discuss types of leukodystrophies: their ► adrenoleuk- prevalence, clinical presentation, symptoms, and diagnosis, as well as current and future odystrophy treatments. Diagnosis is based on a combination of history, exam, radiological, and ► metachromatic laboratory findings, including genetic testing. Leukodystrophies can present at any age leukodystrophy from infancy to adulthood, with variability in disease progression and clinical presenta- ► bone marrow tion, ranging from developmental delay to seizures to spasticity. Although there are few transplantation cures, there are significant opportunities for care and improvements in patient well- ► enzymatic being. Their high prevalence, combined with rapid advances in imaging, genetics, and replacement therapy potential treatments, makes an understanding of the leukodystrophies necessary for ► hematopoietic stem care providers in genetics and neurology. cell transplantation Leukodystrophies are a group of genetically determined One confusing issue is the definition of the term “leuko- disorders that affect development or maintenance of central dystrophy.” For the purposes of this review, a leukodystrophy – nervous system (CNS) myelin.1 4 Leukodystrophies have a will refer to a presumed genetically determined abnormality reported incidence of 1 in 7500 live births, but fewer than half in myelin development, maintenance, or turnover. In con- of patients receive a specific diagnosis.5 Diagnosis can be trast, a “leukoencephalopathy” will be defined more broadly based on a combination of history, expected prevalence, as any abnormality of the white matter, including not only physical and neurologic examination, radiological features, leukodystrophies,2 but also abnormalities of the white mat- or laboratory findings, including genetic testing.5,6 Leukodys- ter, such as caused by trauma, toxicity, or insult, for example, trophies can present at any age from infancy to adulthood, periventricular leukomalacia secondary to premature birth or with variability in disease progression and clinical presenta- white matter abnormalities caused by chemotherapy or This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. tion ranging from developmental delay to seizures to spastic- meningitis. ity. Although there are few cures, there are significant opportunities for care and improvements in patient well- Myelin Development being. Their high prevalence, combined with rapid advances in imaging, genetics, and potential treatments, makes an Myelin development begins during fetal life and continues understanding of the leukodystrophies necessary for care through adulthood. It involves a complex developmental providers in genetics, bone marrow transplantation, and orchestration of genes, proteins, and different cell types. neurology. Production of myelin requires oligodendrocytes, the glial Issue Theme Neurogenetics; Guest Copyright © 2014 by Thieme Medical DOI http://dx.doi.org/ Editor, Ali Fatemi, MD Publishers, Inc., 333 Seventh Avenue, 10.1055/s-0034-1386769. New York, NY 10001, USA. ISSN 0271-8235. Tel: +1(212) 584-4662. The Leukodystrophies Gordon et al. 313 cells that produce myelin and the myelin sheath, and inter- inflammatory demyelination?18 Research into these topics actions with other cell types, particularly neurons. Myelin is will lead to improved treatment for patients and significant necessary for normal action potential propagation and ner- insights into myelin biology. vous system function, maintenance of axonal health, and 7 nutritional support of axons. Categories and Diseases Myelin is produced by oligodendrocytes (also known as oligodendroglia). Oligodendrocytes are generated during fe- Significant confounders for discussing the different leukodys- tal and postnatal life by oligodendrocyte precursor cells trophies are several. First, there is disagreement over the (OPCs). Oligodendrocyte precursor cells can persist into adult exact definition of a leukodystrophy.5,19 Second, only about life,8 and there is evidence that new oligodendrocytes formed half of all causes of leukodystrophies are known.5,20 Finally, during adult life generate new myelin.9 Each oligodendrocyte limited understanding of the pathogenesis of the leukodys- extends many myelin sheet-forming processes that envelope trophies makes classification difficult. axonal projections. Different processes from a single oligo- Currently, leukodystrophies appear to fall into three broad dendrocyte typically envelope different axons.10 Thus, on any categories (►Table 1): (1) hypomyelination,inwhichthereis axon, different oligodendrocytes can be contributing to absent or diminished myelin production; (2) dysmyelination, neighboring stretches of myelin. Myelin is composed of in which there is abnormal myelin development; and (3) proteins (30%) and lipids (70%), and its production is highly demyelination, in which there is loss and/or destruction of energy-dependent. The most abundant protein components previously established myelin. Although widely accepted (or are myelin basic protein (MBP) and proteolipid protein (PLP). perhaps tacitly assumed) that demyelinating inflammatory The lipid components of myelin are cholesterol, phospholi- diseases such as multiple sclerosis or neuromyelitis optica pids, and glycolipids; glycosphingolipids, particularly galac- constitute a different diagnostic category than demyelinating tocerebrosides, are the predominant glycolipids.11,12 leukodystrophies,21 it will be interesting in the next decade to In addition to the oligodendrocyte-specific determinants see if shared common elements of pathogenesis are discov- of myelin formation, neuron-oligodendrocyte interactions ered.22 This being said, trials of immune modulatory therapy are also necessary for normal myelination.13 Oligodendro- for demyelinating leukodystrophies have not shown cytes must migrate to the correct locations in the brain to efficacy.23 effect axonal ensheathment by myelin,14 and the extent of However, certain leukodystrophies do not fit clearly into myelination is affected by neuronal activity.15 the above categories. For example, Rett syndrome and Batten During brain development, myelination proceeds in an disease can have white matter changes and clinical features inside–outside and caudal-to-rostral fashion, with myelina- that would initially suggest a leukodystrophy before subse- tion of deeper structures occurring first. Myelination begins quent testing leads to their ultimate diagnosis. Or some by the fourth month of gestation. Essentially all areas of the patients have syndromic dysmorphic physical features, and brain are myelinated by the second year of life, although the are found to have chromosome microdeletions or micro- total myelin content of the brain does not peak until the third duplications, such as in the 18q- syndrome24; yet the primary decade of life.16 CNS pathology can consist of leukodystrophy. It is apparent, then, that a large number of insults or An alternative method of classification is based on the processes, during development or in adulthood, could affect primary site of action of the gene product implicated in the myelin development or the maintenance of myelin, and hence leukodystrophy. In this regard, leukodystrophies such as lead to a leukodystrophy. The biological complexity of myelin metachromatic leukodystrophy or Krabbe disease can be development requires both neurons and glia, and requires linked to defects in the lysosome, while X-linked adreno- their interactions. Therefore, problems with any of these leukodystrophy can be linked to defects in the peroxisome.4 three could produce a leukodystrophy. In addition, leukodys- A drawback to the site-of-action classification approach is trophies can arise from defects in intrinsic/biochemical path- the imperfect correlation between gene function and ways that are common to all cells, but that are magnified in pathology of the leukodystrophy. For example, X-linked This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. glia due to their high energy requirements and extensive adrenoleukodystrophy is caused by mutation of the membrane turnover. For example, mutations in the intrinsic ABCD1 gene producing a peroxisomal transporter protein, myelin protein PLP1 lead to Pelizaeus-Merzbacher disease, yet levels and presence of very-long chain fatty acids and vanishing white matter disease (VWMD) is caused by correlate with neither the development of disease nor the mutations in the ubiquitously expressed translation initiation timing of disease onset. factor
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