Sixteen novel mutations in the arylsulfatase A gene causing metachromatic leukodystrophy

Predicting disease severity in metachromatic leukodystrophy using protein activity and a patient phenotype matrix

BACKGROUND

Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder caused by mutations in the arylsulfatase A gene (ARSA) and categorized into three subtypes according to age of onset. The functional effect of most ARSA mutants remains unknown; better understanding of the genotype-phenotype relationship is required to support newborn screening (NBS) and guide treatment.

RESULTS

We collected a patient data set from the literature that relates disease severity to ARSA genotype in 489 individuals with MLD. Patient-based data were used to develop a phenotype matrix that predicts MLD phenotype given ARSA alleles in a patient's genotype with 76% accuracy. We then employed a high-throughput enzyme activity assay using mass spectrometry to explore the function of ARSA variants from the curated patient data set and the Genome Aggregation Database (gnomAD). We observed evidence that 36% of variants of unknown significance (VUS) in ARSA may be pathogenic. By classifying functional effects for 251 VUS from gnomAD, we reduced the incidence of genotypes of unknown significance (GUS) by over 98.5% in the overall population.

CONCLUSIONS

These results provide an additional tool for clinicians to anticipate the disease course in MLD patients, identifying individuals at high risk of severe disease to support treatment access. Our results suggest that more than 1 in 3 VUS in ARSA may be pathogenic. We show that combining genetic and biochemical information increases diagnostic yield. Our strategy may apply to other recessive diseases, providing a tool to address the challenge of interpreting VUS within genotype-phenotype relationships and NBS.

Infantile Metachromatic Leukodystrophy (MLD): A Rare Case

Metachromatic leukodystrophy (MLD) is the typical white matter disease belonging to the lysosomal sphingolipid storage group and is a genetic autosomal recessive disorder. Early presentation is in the form of learning disability and behavioral issues; the subsequent involvement is gait and balance. Prenatal blood testing for genetic screening is available for arylsulfatase A (ARSA) deficiency is indicated if the family history is positive for MLD. Diagnostic tools for MLD are- absence or low-level arylsulfatase activity in genetic screening, sulphatides in urine, and magnetic resonance image (MRI) showing frontal horns and atrial periventricular leukodystrophy. The typical finding is known as the trigonid pattern. A two and half-year-old boy was born out of marriage in near blood relation. No prenatal screening was done. As narrated by the mother, the patient was alright six months back when he gradually developed lower limb weakness. Due to this, he stopped walking, which he could initially do without support. The parent also complained that he used to speak fifteen to twenty words, and now he is not saying a single word. With the above complaint, the patient was taken to the local hospital, where an MRI showed periventricular leukodystrophy, suggesting metachromatic leukodystrophy of periventricular white matter. The practice of prenatal and newborn genetic screening could enhance the efficacy of management, as early interventions are more effective.

Systematic Collaborative Reanalysis of Genomic Data Improves Diagnostic Yield in Neurologic Rare Diseases

Many patients experiencing a rare disease remain undiagnosed even after genomic testing. Reanalysis of existing genomic data has shown to increase diagnostic yield, although there are few systematic and comprehensive reanalysis efforts that enable collaborative interpretation and future reinterpretation. The Undiagnosed Rare Disease Program of Catalonia project collated previously inconclusive good quality genomic data (panels, exomes, and genomes) and standardized phenotypic profiles from 323 families (543 individuals) with a neurologic rare disease. The data were reanalyzed systematically to identify relatedness, runs of homozygosity, consanguinity, single-nucleotide variants, insertions and deletions, and copy number variants. Data were shared and collaboratively interpreted within the consortium through a customized Genome-Phenome Analysis Platform, which also enables future data reinterpretation. Reanalysis of existing genomic data provided a diagnosis for 20.7% of the patients, including 1.8% diagnosed after the generation of additional genomic data to identify a second pathogenic heterozygous variant. Diagnostic rate was significantly higher for family-based exome/genome reanalysis compared with singleton panels. Most new diagnoses were attributable to recent gene-disease associations (50.8%), additional or improved bioinformatic analysis (19.7%), and standardized phenotyping data integrated within the Undiagnosed Rare Disease Program of Catalonia Genome-Phenome Analysis Platform functionalities (18%).

Clinical, Biochemical, and Molecular Characterization of Metachromatic Leukodystrophy Among Egyptian Pediatric Patients: Expansion of the ARSA Mutational Spectrum

Metachromatic leukodystrophy (MLD) is a neurodegenerative disorder characterized by progressive demyelination due to deficiency of the enzyme arylsulfatase A (ARSA) in leukocytes, and consequently leads to impaired degradation and accumulation of cerebroside-3-sulfate (sulfatide). This study aimed to sequence the ARSA gene in a total of 43 patients with metachromatic leukodystrophy descendant from 40 Egyptian families. In addition, four carrier parents from two families with children who had died from MLD came to the clinic for genetic analysis. Prenatal diagnosis was performed for four families with molecularly diagnosed MLD sibs. Different mutations were characterized in our cohort, including missense, nonsense, splice, and deletion. Overall, 21 different mutations in the ARSA gene were detected, with 12 novel mutations, i.e. p.Arg60Pro, p.Tyr65*, p.Val112Asp, p.Arg116*, p.Gly124Asp, p.Pro193Ser, p.Gln238*, p.Gln456*, p.Thr276Lys, and p.Gly311Arg, in addition to two new acceptor splice-site mutations 685-1G > A and c.954_956 delCTT. The amniotic fluid samples revealed two carrier fetuses with heterozygous monoallelic mutations, and two affected fetuses had the homozygous biallelic mutations. In conclusion, the current study sheds light on the underlying ARSA gene defect, with an expansion of the mutation spectrum. To our knowledge, this is the first molecular study of MLD among the Egyptian population.

Identification of Novel ARSA Mutations in Chinese Patients with Metachromatic Leukodystrophy

Objective

Metachromatic leukodystrophy (MLD) is an inherited disease caused by a deficiency of the enzyme arylsulfatase A (ARSA) that leads to severe physiologic and developmental problems. Our study is aimed at elucidating the clinical and genetic characteristics of Chinese MLD patients.

Methods

Clinical data of 21 MLD patients was collected. All coding exons of ARSA and their flanking intronic sequences were amplified by polymerase chain reaction and subjected to direct sequencing.

Results

All 21 patients were diagnosed with MLD clinically and genetically, out of which 17 patients were late infantile and 4 were juvenile types. A total of 34 ARSA mutations, including 28 novel mutations (22 missense, 1 splicing, 1 nonsense, 3 small insertions, and 1 small deletion mutation) and 6 known mutations (5 missense and 1 small insertion mutation), were identified. Prenatal diagnosis was performed for four pedigrees. One fetus was a patient, two fetuses were carriers, and two were wild type.

Conclusions

The present study discovered 28 novel ARSA mutations and widely expanded the mutation spectrum of ARSA. Four successful prenatal diagnoses provided critical information for MLD families.

Enzyme activities of α-glucosidase in Japanese neonates with pseudodeficiency alleles

Lysosomal storage disorders (LSDs) are caused by defective enzyme activities in lysosomes, characterized by the accumulation of sphingolipids, glycolipids, oligosaccharides, mucopolysaccharides, the oxidation products of cholesterol, and other biological substances. A growing number of clinical studies have suggested the enhanced efficacy of existing therapies, including enzyme replacement therapy, which is effective when it is initiated during the presymptomatic period. Thus, the identification of disease-affected individuals by newborn screening has been considered an effective platform. Previous studies have suggested that the discrimination of infantile-onset Pompe disease (IOPD) requires multi-step examination of GAA enzyme activity using the fluorometric technique. In sharp contrast, the MS/MS-based technique can identify the population of IOPD and the pseudodeficiency alleles of the GAA enzyme [Liao HC et al. Clin Chem (2017) in press; doi: http://dx.doi.org/10.1373/clinchem.2016.269027]. To determine whether MS/MS-based assay can identify these two populations in Japanese neonates, we first performed a validation study of this assay using flow-injection analysis (FIA)-MS/MS and liquid chromatography (LC)-MS/MS followed by examination of GAA enzyme activity in our population. By minimizing the effect of substrate-derived in-source decomposition products, the activities of 6 LSD enzymes were quantified in FIA-MS/MS and LC-MS/MS. The mean value of GAA activity with IOPD, pseudodeficiency alleles, and healthy controls by FIA-MS/MS were 1.0 ± 0.3 μmol/h/L (max, 1.3; min, 0.7; median, 1.2; n = 3), 2.7 ± 0.7 μmol/h/L (max, 4.5; min, 1.5; median, 2.5; n = 19), and 12.9 ± 5.4 μmol/h/L (max, 29.6; min, 2.5; median, 11.0; n = 83), respectively. These results suggest that the population of GAA with pseudodeficiency alleles has approximately 20% of GAA enzyme activity compared to controls, providing the preliminary evidence to estimate the cut-off values in the Japanese population using this technique.

In Vivo NMR Studies of the Brain with Hereditary or Acquired Metabolic Disorders

Metabolic disorders, whether hereditary or acquired, affect the brain, and abnormalities of the brain are related to cellular integrity; particularly in regard to neurons and astrocytes as well as interactions between them. Metabolic disturbances lead to alterations in cellular function as well as microscopic and macroscopic structural changes in the brain with diabetes, the most typical example of metabolic disorders, and a number of hereditary metabolic disorders. Alternatively, cellular dysfunction and degeneration of the brain lead to metabolic disturbances in hereditary neurological disorders with neurodegeneration. Nuclear magnetic resonance (NMR) techniques allow us to assess a range of pathophysiological changes of the brain in vivo. For example, magnetic resonance spectroscopy detects alterations in brain metabolism and energetics. Physiological magnetic resonance imaging (MRI) detects accompanying changes in cerebral blood flow related to neurovascular coupling. Diffusion and T1/T2-weighted MRI detect microscopic and macroscopic changes of the brain structure. This review summarizes current NMR findings of functional, physiological and biochemical alterations within a number of hereditary and acquired metabolic disorders in both animal models and humans. The global view of the impact of these metabolic disorders on the brain may be useful in identifying the unique and/or general patterns of abnormalities in the living brain related to the pathophysiology of the diseases, and identifying future fields of inquiry.

Late infantile metachromatic leukodystrophy: Clinical manifestations of five Taiwanese patients and Genetic features in Asia

BACKGROUND

This study was conducted to describe the clinical and genetic features of patients with late infantile metachromatic leukodystrophy.

METHODS

Clinical and genetic manifestations of five Taiwanese patients with late infantile metachromatic leukodystrophy from January 2003 to April 2014 were reviewed. The genetic features of such patients reported in Asian countries during a period of 20 years were also analyzed.

RESULTS

The median age at disease onset was 1 year and 3 months with the first clinical symptom being gait disturbance. All five patients became bed-ridden at a median age of 2 years and 5 months. Nerve conduction velocity revealed demyelinating polyneuropathy and brain MRI disclosed tigroid and leopard skin pattern of dysmyelination in all 5 patients. All patients had decreased ARSA activities in leukocytes accounting for 15.88% to 30.75% of controls. Five novel mutations, p.A316D, p.G303R, p.Q176X, p.R293X, and c.749 insGCGGGCCA, were identified in our case series. Eighteen patients, including our 5 patients, were reported in Asian countries. A total of 22 different disease-causing alleles were found, in which p.W320X was identified in Taiwan and China, and p.G101V was found in Taiwan and Korea.

CONCLUSIONS

Patients with late infantile metachromatic leukodystrophy exhibited a rapid and devastating clinical course. The pattern of dysmyelination on brain MRI together with peripheral demyelination polyneuropathy indicates that evaluation of ARSA activity in leukocytes is warranted. A wide diversity of ARSA gene mutations was noted in Asia.

Metachromatic leukodystrophy: Disease spectrum and approaches for treatment

Metachromatic leukodystrophy is an inherited lysosomal disorder caused by recessive mutations in ARSA encoding arylsulfatase A. Low activity of arylsulfatase A results in the accumulation of sulfatides in the central and peripheral nervous system leading to demyelination. The disease is classified in a late-infantile, juvenile and adult onset type based on the age of onset, all characterized by a variety of neurological symptoms, which eventually lead to death if untreated. There is no curative treatment for all types and stages. This review discusses diagnostic process and efficacy of current and possible future therapies such as hematopoietic stem cell transplantation, enzyme replacement therapy and gene therapy. A systematic evaluation regarding the efficacy of hematopoietic stem cell transplantation and a longer follow up period for gene therapy are needed to come to a general conclusion and improve treatment options for metachromatic leukodystrophy.

Mesenchymal stem cells as cellular vectors for pediatric neurological disorders

Lysosomal storage diseases are a heterogeneous group of hereditary disorders characterized by a deficiency in lysosomal function. Although these disorders differ in their etiology and phenotype those that affect the nervous system generally manifest as a profound deterioration in neurologic function with age. Over the past several decades implementation of various treatment regimens including bone marrow and cord blood cell transplantation, enzyme replacement, and substrate reduction therapy have proved effective for managing some clinical manifestations of these diseases but their ability to ameliorate neurologic complications remains unclear. Consequently, there exists a need to develop alternative therapies that more effectively target the central nervous system. Recently, direct intracranial transplantation of tissue-specific stem and progenitor cells has been explored as a means to reconstitute metabolic deficiencies in the CNS. In this chapter we discuss the merits of bone marrow-derived mesenchymal stem cells (MSCs) for this purpose. Originally identified as progenitors of connective tissue cell lineages, recent findings have revealed several novel aspects of MSC biology that make them attractive as therapeutic agents in the CNS. We relate these advances in MSC biology to their utility as cellular vectors for treating neurologic sequelae associated with pediatric neurologic disorders.