Genetic disorders affecting white matter in the pediatric age

Etiologic Spectrum of Pediatric-Onset Leukodystrophies and Genetic Leukoencephalopathies: The Five-Year Experience of a Tertiary Care Center in Southern India

BACKGROUND

White matter (WM) disorders with a genetic etiology are classified as leukodystrophies (LDs) and genetic leukoencephalopathies (GLEs). There are very few studies pertaining to the etiologic spectrum of these disorders in the Asian Indian population.

METHODS

This study was conducted over a period of five years from January 2016 to December 2020, in the medical genetics department of a tertiary care hospital in southern India. A total of 107 patients up to age 18 years, with a diagnosis of a genetic WM disorder confirmed by molecular genetic testing and/or metabolic testing, were included in the study and categorized into LD or GLE group as per the classification suggested by the Global Leukodystrophy Initiative consortium in 2015.

RESULTS

Forty-one patients were diagnosed to have LDs, and 66 patients had GLEs. The two most common LDs were metachromatic LD (16 patients) and X-linked adrenoleukodystrophy (seven patients). In the GLE group, lysosomal storage disorders were the most common (40 patients) followed by mitochondrial disorders (nine patients), with other metabolic disorders and miscellaneous conditions making up the rest. The clinical presentations, neuroimaging findings, and mutation spectrum of the patients in our cohort are discussed.

CONCLUSIONS

This is one of the largest cohorts of genetic WM disorders reported till date from the Asian Indian population. The etiologies and clinical presentations identified in our study cohort are similar to those found in other Indian studies as well as in studies based on other populations from different parts of the world.

A hypomyelinating leukodystrophy in German Shepherd dogs

BACKGROUND

Shaking puppy syndrome is commonly attributed to abnormal myelination of the central nervous system.

HYPOTHESIS/OBJECTIVES

To report the long-term clinical course and the imaging characteristics of hypomyelinating leukodystrophy in German Shepherd dogs.

ANIMALS AND METHODS

Three related litters with 11 affected dogs.

RESULTS

The 11 affected dogs experienced coarse, side-to-side tremors of the head and trunk, which interfered with normal goal-oriented movements and disappeared at rest. Signs were noticed shortly after birth. Nine dogs were euthanized, 3 dogs underwent pathological examination, and 2 littermates were raised by their breeder. Tremors improved gradually until 6 to 7 months of age. Adult dogs walked with severe residual pelvic limb ataxia. One dog developed epilepsy with tonic-clonic seizures at 15 months of age. Conventional magnetic resonance imaging (MRI) disclosed homogenous hyperintense signal of the entire subcortical white matter in 3 affected 7-week-old dogs and a hypointense signal in a presumably unaffected littermate. Subcortical white matter appeared isointense to gray matter at 15 and 27 weeks of age on repeated MRI. Abnormal white matter signal with failure to display normal gray-white matter contrast persisted into adulthood. Cerebellar arbor vitae was not visible at any time point. Clinical signs, MRI findings, and pathological examinations were indicative of a hypomyelinating leukodystrophy. All parents of the affected litters shared a common ancestor and relatedness of the puppies suggested an autosomal recessive mode of inheritance.

CONCLUSION

We describe a novel hypomyelinating leukodystrophy in German Shepherd dogs with a suspected inherited origin.

Dcf1 deficiency induces hypomyelination by activating Wnt signaling

Myelination is extremely important in achieving neural function. Hypomyelination causes a variety of neurological diseases. However, little is known about how hypomyelination occurs. Here we investigated the effect of dendritic cell factor 1(Dcf1) on myelination, using in vitro and in vivo models and found that Dcf1 is essential for normal myelination, motor coordination and balance. Lack of Dcf1 downregulated myelin-associated proteins, such as myelin basic protein (MBP), myelin associated glycoprotein (MAG), and 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in the hippocampus and corpus callosum of Dcf1-null mice, as a result, the myelin sheath of these mice became thinner. Transmission electron microscopy revealed hypomyelination in Dcf1-deficient mice. Motor coordination and balance tests confirmed impaired neurological function in Dcf1-null mice. Gain-of-function analysis via In utero electroporation showed that hypomyelination could be rescued by re-expression of Dcf1 in Dcf1-null mouse brain. Dcf1-null mice exhibited a phenotype similar to that of cuprizone-induced demyelinated mice, thereby supporting the finding of hypomyelination caused by Dcf1 knockout. Mechanistically, we further revealed that insufficient Dcf1 leads to hyperactivation of the Wnt/β-catenin signaling pathway. Our work describes the role of Dcf1 in maintaining normal myelination, and this could help improve the current understanding of hypomyelination and its pathogenesis.

Canine NAPEPLD-associated models of human myelin disorders

Canine leukoencephalomyelopathy (LEMP) is a juvenile-onset neurodegenerative disorder of the CNS white matter currently described in Rottweiler and Leonberger dogs. Genome-wide association study (GWAS) allowed us to map LEMP in a Leonberger cohort to dog chromosome 18. Subsequent whole genome re-sequencing of a Leonberger case enabled the identification of a single private homozygous non-synonymous missense variant located in the highly conserved metallo-beta-lactamase domain of the N-acyl phosphatidylethanolamine phospholipase D (NAPEPLD) gene, encoding an enzyme of the endocannabinoid system. We then sequenced this gene in LEMP-affected Rottweilers and identified a different frameshift variant, which is predicted to replace the C-terminal metallo-beta-lactamase domain of the wild type protein. Haplotype analysis of SNP array genotypes revealed that the frameshift variant was present in diverse haplotypes in Rottweilers, and also in Great Danes, indicating an old origin of this second NAPEPLD variant. The identification of different NAPEPLD variants in dog breeds affected by leukoencephalopathies with heterogeneous pathological features, implicates the NAPEPLD enzyme as important in myelin homeostasis, and suggests a novel candidate gene for myelination disorders in people.

The progeroid gene BubR1 regulates axon myelination and motor function

Myelination, the process by which oligodendrocytes form the myelin sheath around axons, is key to axonal signal transduction and related motor function in the central nervous system (CNS). Aging is characterized by degenerative changes in the myelin sheath, although the molecular underpinnings of normal and aberrant myelination remain incompletely understood. Here we report that axon myelination and related motor function are dependent on BubR1, a mitotic checkpoint protein that has been linked to progeroid phenotypes when expressed at low levels and healthy lifespan when overabundant. We found that oligodendrocyte progenitor cell proliferation and oligodendrocyte density is markedly reduced in mutant mice with low amounts of BubR1 (BubR1^H/H mice), causing axonal hypomyelination in both brain and spinal cord. Expression of essential myelin-related genes such as MBP and PLP1 was significantly reduced in these tissues. Consistent with defective myelination, BubR1^H/H mice exhibited various motor deficits, including impaired motor strength, coordination, and balance, irregular gait patterns and reduced locomotor activity. Collectively, these data suggest that BubR1 is a key determinant of oligodendrocyte production and function and provide a molecular entry point to understand age-related degenerative changes in axon myelination.

A new CUL4B variant associated with a mild phenotype and an exceptional pattern of leukoencephalopathy

Cabezas type of X-linked syndromic intellectual disability (MRXSC; MIM300354) is a rare X-linked recessive intellectual disability characterized primarily by intellectual disability, short stature, hypogonadism, and gait abnormalities. It is caused by a wide spectrum of hemizygous variants in CUL4B. In a 10-year-old boy with an exceptional leukoencephalopathy pattern, we identified a new missense variant p.Leu329Gln in CUL4B using "Mendeliome" sequencing. However, his phenotype does not include the severe characteristics currently known for MRXSC. We discuss the divergent phenotype and propose a potential connection between the different CUL4B variants and corresponding phenotypes in the context of the current literature as well as 3D homology modeling.

Assessment of paediatric pain: a critical review

Pain is a complex experience, and its quantification involves many aspects including physiological, behavioural, and psychological factors. References related to the topic were selected and analysed, along with a PubMed search of the recent and earlier reports. Assessment of pain in infants and children has always been a dilemma for the clinicians. Unlike in adults, it is difficult to assess and effectively treat pain in paediatric age groups, and it often remains untreated or undertreated. Misperceptions are attributed not only to the difficulties in isolating the specific signs of pain but also in recognising and inferring the meaning of the cues available in the complex of individual differences in the reaction pattern of children to pain. In children, several parameters such as age, cognitive level, type of pain, etc. are required to be considered for the selection of appropriate pain assessment tools. Although considerable progress has been made, there is a critical need for a more accurate measurement tool for both research and clinical purposes. This review has critically analysed the various techniques available to assess pain in children with emphasis on current research and present-day status of paediatric pain assessment.

Myelin damage and repair in pathologic CNS: challenges and prospects

Injury to the central nervous system (CNS) results in oligodendrocyte cell death and progressive demyelination. Demyelinated axons undergo considerable physiological changes and molecular reorganizations that collectively result in axonal dysfunction, degeneration and loss of sensory and motor functions. Endogenous adult oligodendrocyte precursor cells and neural stem/progenitor cells contribute to the replacement of oligodendrocytes, however, the extent and quality of endogenous remyelination is suboptimal. Emerging evidence indicates that optimal remyelination is restricted by multiple factors including (i) low levels of factors that promote oligodendrogenesis; (ii) cell death among newly generated oligodendrocytes, (iii) inhibitory factors in the post-injury milieu that impede remyelination, and (iv) deficient expression of key growth factors essential for proper re-construction of a highly organized myelin sheath. Considering these challenges, over the past several years, a number of cell-based strategies have been developed to optimize remyelination therapeutically. Outcomes of these basic and preclinical discoveries are promising and signify the importance of remyelination as a mechanism for improving functions in CNS injuries. In this review, we provide an overview on: (1) the precise organization of myelinated axons and the reciprocal axo-myelin interactions that warrant properly balanced physiological activities within the CNS; (2) underlying cause of demyelination and the structural and functional consequences of demyelination in axons following injury and disease; (3) the endogenous mechanisms of oligodendrocyte replacement; (4) the modulatory role of reactive astrocytes and inflammatory cells in remyelination; and (5) the current status of cell-based therapies for promoting remyelination. Careful elucidation of the cellular and molecular mechanisms of demyelination in the pathologic CNS is a key to better understanding the impact of remyelination for CNS repair.

How big is the myelinating orchestra? Cellular diversity within the oligodendrocyte lineage: facts and hypotheses

Since monumental studies from scientists like His, Ramón y Cajal, Lorente de Nó and many others have put down roots for modern neuroscience, the scientific community has spent a considerable amount of time, and money, investigating any possible aspect of the evolution, development and function of neurons. Today, the complexity and diversity of myriads of neuronal populations, and their progenitors, is still focus of extensive studies in hundreds of laboratories around the world. However, our prevalent neuron-centric perspective has dampened the efforts in understanding glial cells, even though their active participation in the brain physiology and pathophysiology has been increasingly recognized over the years. Among all glial cells of the central nervous system (CNS), oligodendrocytes (OLs) are a particularly specialized type of cells that provide fundamental support to neuronal activity by producing the myelin sheath. Despite their functional relevance, the developmental mechanisms regulating the generation of OLs are still poorly understood. In particular, it is still not known whether these cells share the same degree of heterogeneity of their neuronal companions and whether multiple subtypes exist within the lineage. Here, we will review and discuss current knowledge about OL development and function in the brain and spinal cord. We will try to address some specific questions: do multiple OL subtypes exist in the CNS? What is the evidence for their existence and those against them? What are the functional features that define an oligodendrocyte? We will end our journey by reviewing recent advances in human pluripotent stem cell differentiation towards OLs. This exciting field is still at its earliest days, but it is quickly evolving with improved protocols to generate functional OLs from different spatial origins. As stem cells constitute now an unprecedented source of human OLs, we believe that they will become an increasingly valuable tool for deciphering the complexity of human OL identity.

Oligodendrogenesis and myelinogenesis during postnatal development effect of glatiramer acetate

Myelinogenesis in the mammal nervous system occurs predominantly postnatally. Glatiramer acetate (GA), a drug for the treatment for multiple sclerosis (MS), has been shown to induce immunomodulation and neuroprotection in the inflamed CNS in MS and in experimental autoimmune encephalomyelitis (EAE). Here we investigated whether GA can affect myelinogenesis and oligodendrogenesis in the developing nervous system under nonpathological conditions. Towards this end we studied myelination in mice injected daily by GA, at postnatal Days 7-21. Immunohistological and ultrastructural analyses revealed significant elevation in the number of myelinated axons as well as in the thickness of the myelin encircling them and their resulting g-ratios, in spinal cords of GA-injected mice compared with their PBS-injected littermates, at postnatal Day 14. Elevation in myelinated axons was detected also in the peripheral ventral roots of the motor nerves. GA induced also an increase in axonal diameter, implying an effect on the overall development of the nervous system. A prominent elevation in the amount of progenitor oligodendrocytes and their BrdU incorporation, as well as in mature oligodendrocytes indicated that the effect of GA is linked to increased proliferation and differentiation along the oligodendroglial maturation cascade. In addition, elevation in insulin-like growth factor (IGF-1) and brain-derived neurotrophic factor (BDNF) was found in the white matter of the GA-injected mice. Furthermore, a functional advantage in rotating rod test was exhibited by GA-injected mice over their littermates at postnatal Day 21. These cumulative findings corroborate the beneficial effect of GA on oligodendrogenesis and myelination.

Genetics of dementia

25% of all people aged 55 years and older have a family history of dementia. For most, the family history is due to genetically complex disease, where many genetic variations of small effect interact to increase risk of dementia. The lifetime risk of dementia for these families is about 20%, compared with 10% in the general population. A small proportion of families have an autosomal dominant family history of early-onset dementia, which is often due to mendelian disease, caused by a mutation in one of the dementia genes. Each family member has a 50% chance of inheriting the mutation, which confers a lifetime dementia risk of over 95%. In this Review, we focus on the evidence for, and the approach to, genetic testing in Alzheimer's disease (APP, PSEN1, and PSEN2 genes), frontotemporal dementia (MAPT, GRN, C9ORF72, and other genes), and other familial dementias. We conclude by discussing the practical aspects of genetic counselling.

Magnetic resonance spectroscopy in metabolic disorders

Magnetic resonance spectroscopy (MRS) is a powerful clinical tool for investigating the metabolic characteristics of neurologic diseases. Proton ((1)H)-MRS is the most commonly used and widely available method. In this article, a brief introduction regarding technical issues of (1)H-MRS applied to the study of metabolic diseases is followed by a description of findings in some of the most common entities in this large, heterogeneous group of neurologic disorders. The aim was to provide a focused representation of the most common applications of (1)H-MRS to metabolic disorders in a routine clinical setting.

ASTROCYTES: EMERGING STARS IN LEUKODYSTROPHY PATHOGENESIS

Astrocytes are the predominant glial cell population in the central nervous system (CNS). Once considered only passive scaffolding elements, astrocytes are now recognised as cells playing essential roles in CNS development and function. They control extracellular water and ion homeostasis, provide substrates for energy metabolism, and regulate neurogenesis, myelination and synaptic transmission. Due to these multiple activities astrocytes have been implicated in almost all brain pathologies, contributing to various aspects of disease initiation, progression and resolution. Evidence is emerging that astrocyte dysfunction can be the direct cause of neurodegeneration, as shown in Alexander's disease where myelin degeneration is caused by mutations in the gene encoding the astrocyte-specific cytoskeleton protein glial fibrillary acidic protein. Recent studies point to a primary role for astrocytes in the pathogenesis of other genetic leukodystrophies such as megalencephalic leukoencephalopathy with subcortical cysts and vanishing white matter disease. The aim of this review is to summarize current knowledge of the pathophysiological role of astrocytes focusing on their contribution to the development of the above mentioned leukodystrophies and on new perspectives for the treatment of neurological disorders.

Genetic contributions to the midsagittal area of the corpus callosum

The degree to which genes and environment determine variations in brain structure and function is fundamentally important to understanding normal and disease-related patterns of neural organization and activity. We studied genetic contributions to the midsagittal area of the corpus callosum (CC) in pedigreed baboons (68 males, 112 females) to replicate findings of high genetic contribution to that area of the CC reported in humans, and to determine if the heritability of the CC midsagittal area in adults was modulated by fetal development rate. Measurements of callosal area were obtained from high-resolution MRI scans. Heritability was estimated from pedigree-based maximum likelihood estimation of genetic and non-genetic variance components as implemented in Sequential Oligogenic Linkage Analysis Routines (SOLAR). Our analyses revealed significant heritability for the total area of the CC and all of its subdivisions, with h2 = .46 for the total CC, and h2 = .54, .37, .62, .56, and .29 for genu, anterior midbody, medial midbody, posterior midbody and splenium, respectively. Genetic correlation analysis demonstrated that the individual subdivisions shared between 41% and 98% of genetic variability. Combined with previous research reporting high heritability of other brain structures in baboons, these results reveal a consistent pattern of high heritability for brain morphometric measures in baboons.

Systematic approaches to central nervous system myelin

Rapid signal propagation along vertebrate axons is facilitated by their insulation with myelin, a plasma membrane specialization of glial cells. The recent application of 'omics' approaches to the myelinating cells of the central nervous system, oligodendrocytes, revealed their mRNA signatures, enhanced our understanding of how myelination is regulated, and established that the protein composition of myelin is much more complex than previously thought. This review provides a meta-analysis of the > 1,200 proteins thus far identified by mass spectrometry in biochemically purified central nervous system myelin. Contaminating proteins are surprisingly infrequent according to bioinformatic prediction of subcellular localization and comparison with the transcriptional profile of oligodendrocytes. The integration of datasets also allowed the subcategorization of the myelin proteome into functional groups comprising genes that are coregulated during oligodendroglial differentiation. An unexpectedly large number of myelin-related genes cause-when mutated in humans-hereditary diseases affecting the physiology of the white matter. Systematic approaches to oligodendrocytes and myelin thus provide valuable resources for the molecular dissection of developmental myelination, glia-axonal interactions, leukodystrophies, and demyelinating diseases.

Cognition in nephropathic cystinosis: pattern of expression in heterozygous carriers

Individuals with cystinosis exhibit specific cognitive deficits in visual spatial function. The purpose of the current study was to examine if obligate heterozygotes of the CTNS mutation have the same pattern of cognitive functioning seen in homozygotes, namely aberrant visual-spatial functioning against a background of relatively intact visual-perceptual functioning and overall cognitive ability. Study participants were 254 adults (100 heterozygotes and 154 controls), ages 17 years 10 months through 74 years 9 months. Tests of intelligence, visual perceptual, and visual spatial functioning were administered. Our results showed that cystinosis heterozygotes demonstrated intelligence within the normal range, and performed similarly to controls on tests of visual-perceptual ability. In contrast, the heterozygotes performed significantly more poorly on each of the visual-spatial tests when compared to controls. Obligate heterozygotes for the CTNS mutation display a similar pattern of visual processing decrements as do individuals with cystinosis. Namely, carriers demonstrate relative weaknesses in visual-spatial processing, while maintaining normal visual perceptual ability and intelligence in the normal range. The visual spatial decrements in heterozygotes were not as marked as those found in individuals with cystinosis, suggesting a gene dosing effect. This study provides an impetus for other studies of gene-behavior relationships in recessive disorders, and may stimulate further interest in the role of aberrant genes on "individual differences" in behavior.

Hyccin, the molecule mutated in the leukodystrophy hypomyelination and congenital cataract (HCC), is a neuronal protein

“Hypomyelination and Congenital Cataract”, HCC (MIM #610532), is an autosomal recessive disorder characterized by congenital cataract and diffuse cerebral and peripheral hypomyelination. HCC is caused by deficiency of Hyccin, a protein whose biological role has not been clarified yet. Since the identification of the cell types expressing a protein of unknown function can contribute to define the physiological context in which the molecule is explicating its function, we analyzed the pattern of Hyccin expression in the central and peripheral nervous system (CNS and PNS). Using heterozygous mice expressing the b-galactosidase (LacZ) gene under control of the Hyccin gene regulatory elements, we show that the gene is primarily expressed in neuronal cells. Indeed, Hyccin-LacZ signal was identified in CA1 hippocampal pyramidal neurons, olfactory bulb, and cortical pyramidal neurons, while it did not colocalize with oligodendroglial or astrocytic markers. In the PNS, Hyccin was detectable only in axons isolated from newborn mice. In the brain, Hyccin transcript levels were higher in early postnatal development (postnatal days 2 and 10) and then declined in adult mice. In a model of active myelinogenesis, organotypic cultures of rat Schwann cells (SC)/Dorsal Root Ganglion (DRG) sensory neurons, Hyccin was detected along the neurites, while it was absent from SC. Intriguingly, the abundance of the molecule was upregulated at postnatal days 10 and 15, in the initial steps of myelinogenesis and then declined at 30 days when the process is complete. As Hyccin is primarily expressed in neurons and its mutation leads to hypomyelination in human patients, we suggest that the protein is involved in neuron-to-glia signalling to initiate or maintain myelination.

FA2H-related disorders: a novel c.270+3A>T splice-site mutation leads to a complex neurodegenerative phenotype

Homozygous mutations in the gene for fatty acid 2-hydroxylase (FA2H) have been associated in humans with three neurodegenerative disorders: complicated spastic paraplegia (SPG35), leukodystrophy with spastic paraparesis and dystonia, and neurodegeneration with brain iron accumulation. Here, we describe a novel homozygous c.270+3A>T mutation in an Italian consanguineous family. In two affected brothers (age at molecular diagnosis 22y and 15y; age at last follow-up 24y and 17y), altered FA2H function led to a severe phenotype, with clinical features overlapping those of the three FA2H-associated disorders. Both patients showed childhood onset progressive spastic paraparesis, mild pyramidal and cerebellar upper limb signs, severe cognitive impairment, white-matter disease, and cerebellar, brainstem, and spinal cord atrophy. However, absence of dystonia, drowsiness episodes, and a subtle globus pallidus involvement suggested that FA2H mutations result in a clinical spectrum, rather than causing distinct disorders. Although clinical heterogeneity is apparent, larger numbers of patients are needed to establish more accurate genotype-phenotype correlations.

Identification of a new mtDNA mutation (14724G>A) associated with mitochondrial leukoencephalopathy

We report a novel 14724G>A mutation in the mitochondrial tRNA glutamic acid gene in a 4-year-old boy with myopathy and leukoencephalopathy. A muscle biopsy showed cytochrome c oxidase-negative ragged-red fibers and biochemical analysis of the respiratory chain enzymes in muscle homogenate revealed partial complex I and complex IV deficiencies. The mutation, which affects the dihydrouridine arm at a conserved site, was nearly homoplasmic in muscle and heteroplasmic in blood DNA of the proband, but it was absent in peripheral leukocytes from the asymptomatic mother, sister, and two maternal aunts, suggesting that it arose de novo. This report proposes to look for variants in the mitochondrial genome when dealing with otherwise undetermined leukodystrophies of childhood.