Molecular bases of metachromatic leukodystrophy in Polish patients

Predicting clinical phenotypes of metachromatic leukodystrophy based on the arylsulfatase A activity and the ARSA genotype? - Chances and challenges

OBJECTIVES

Metachromatic leukodystrophy (MLD) is an autosomal recessive lysosomal storage disease caused by deficiency of arylsulfatase A (ARSA). Subsequent accumulation of sulfatides leads to demyelination and neurodegeneration in the central and peripheral nervous system. To date MLD is classified based on the age at onset, however, especially for late onset forms this classification provides only limited projection regarding the clinical disease course. Moreover, evolving newborn screening approaches raise the need to predict the disease onset and course in pre-symptomatic individuals. Here, we correlate the ARSA activity and the ARSA-genotype with clinical parameters in a large cohort of 96 affected individuals.

MATERIALS AND METHODS

Clinical data of 96 affected individuals with genetically and/or biochemically confirmed MLD were collected from a national database. Leukocyte samples from 69 affected individuals were re-analyzed for the ARSA activity using p-nitrocatecholsulfate as substrate with a refined ARSA assay towards the lower limit of detection. For 84 individuals genetic sequencing was conducted by Sanger or next generation sequencing (NGS).

RESULTS

The adapted ARSA assay revealed the discriminatory power to differentiate MLD subtypes as the residual enzyme activity was low in late infantile and early juvenile forms, and clearly higher in late juvenile and adult MLD (p < 0.001). A residual enzyme activity below 1% compared to controls predicted an early onset (late-infantile or early-juvenile) and rapid disease progression. A firm genotype-phenotype correlation was proven as reliable for bi-allelic protein-truncating variants in the ARSA gene resulting in minimal residual ARSA activity, an early onset of the disease and initial decline of motor functions. Although the impact of missense variants was equivocal, few variants with a recognizable clinical spectrum were identified.

DISCUSSION

ARSA activity in leukocytes as well as the ARSA genotype can predict the age of disease onset and the dynamic of disease progression for most of the early onset forms. This knowledge is relevant for patient counseling and to guide treatment decisions, especially when identifying pre-symptomatic individuals, e.g., in newborn screening. However, due to the high cumulative frequency of rare disease-causing missense variants in the ARSA gene that lead to highly variable residual enzyme activity, reiterated biochemical and genetic studies are needed to improve disease course prediction.

Multimodal imaging and genetic findings in a case of ARSG-related atypical Usher syndrome

Background: Atypical Usher syndrome has recently been associated with arylsulfatase G (ARSG) variants. In these cases, characteristic findings include progressive sensorineural hearing loss (SNHL) without vestibular involvement and ring-shaped late-onset retinitis pigmentosa (RP).Materials and Methods: One patient with atypical Usher syndrome and a novel homozygous ARSG variant was included in this study. The patient underwent a comprehensive ophthalmic examination, including multimodal imaging and genetic testing.Results: A 60-year-old male of Persian decent presented to our clinic with a history of 20 years of progressive SNHL, and 10 years of progressive peripheral vision loss and pigmentary retinopathy. Consistent with previous reports of ARSG-related atypical Usher syndrome, fundus examination revealed ring-shaped retinal hyperpigmentation and fundus autofluorescence (FAF) demonstrated a six-zone pattern of autofluorescence. Optical coherence tomography (OCT) showed extensive cystoid spaces concentrated in the ganglion cell layer. Widefield OCT angiography at the level of the choriocapillaris showed signs of atrophy that corresponded to the FAF hypofluorescent zone. The patient was homozygous for a novel ARSG variant c. 1270 C > T, p. Arg424Cys.Conclusion: We report a novel ARSG variant in a case of atypical Usher syndrome and describe multimodal imaging findings that further characterize the effect of ARSG in the pathogenesis of atypical Usher syndrome.

Apolipoprotein E genotype and LRP1 polymorphisms in patients with different clinical types of metachromatic leukodystrophy

Metachromatic leukodystrophy (MLD) is a severe, neurodegenerative, metabolic disease which is caused by deficient activity of arylsulfatase A (ARSA). Sulfatides and other substrates of ARSA are stored in central and peripheral nervous systems, and in some other organs. Accumulated sulfatides are especially toxic to oligodendrocytes and Schwann cells leading to progressive demyelination. The kind of apolipoprotein E (apoE) isoform is of essential significance for the modulation of sulfatide quantity in the brain as apoE4 contains more sulfatides than apoE3. Taking into consideration the fact that apoE4 leads to the loss of sulfatides in CSF of Alzheimer's disease patients, we examined if apoE isoforms display any impact on clinical outcome in patients with different forms of MLD in whom sulfatides accumulate. The significant association of age at the onset of MLD symptoms with APOE ε2/ε3/ε4 and LRP1 c.766C>T polymorphisms was shown in multivariate stepwise regression analysis, in which other factors known to affect age at onset of the disease, i.e. clinical type of MLD, family connection of the patient and sex were also analyzed. As expected, the clinical type of MLD explained about 80% of the variance of the dependent variable. The impact of both polymorphisms on age of onset of the disease was considerably lower: 2.0% in the case of APOE polymorphism and 1.0% in the case of LRP1 polymorphism. Thus, the clinical outcome in MLD patients is related principally to the genotype of the ARSA gene, while the polymorphisms in the APOE and LRP1 genes are only slightly modifying factors.

Effects of glycosylation and pH conditions in the dynamics of human arylsulfatase A

Arylsulfatase A (ARSA) is a lysosomal sulfatase that catalyzes the hydrolysis of cerebroside sulfate. Its deficiency results in Metachromatic Leukodystrophy, whereas a minor condition called ARSA pseudodeficiency occurs in healthy individuals, which has been associated with the substitution of the glycosylated Asn350 by a Ser and with the loss of the polyadenylation signal. In this work, we have investigated ARSA dynamics employing molecular dynamics simulations in response to (1) different pH's, as, beyond its natural lysossomal environment, it has been recently identified in cytoplasmatic medium and (2) glycan occupancies, including its normal glycosylation state, presenting three high mannose-type oligosaccharides. Accordingly, four systems were studied considering ARSA under different conditions: (1) nonglycosylated at pH ∼ 7 (ARSApH7); (2) non-glycosylated at pH ∼ 5 (ARSApH5); (3) triple glycosylated at pH ∼ 5 (ARSAglyc,pH5); and (4) ARSA-N350S mutant at pH ∼ 5 (ARSAN350S,pH5). Lowering pH and increasing glycosylation was found to reduce the flexibility of the enzyme. In addition, at acidic pH, the glycosylated enzyme presented a higher secondary conformational stability when compared to its nonglycosylated counterpart, supporting experimental findings on triple glycosylation as the essential state of ARSA. The N350S mutant exhibited a consistent degree of unfolding, which may be related to its in vitro reduced stability. Finally, the obtained data are discussed in the search for structural evidences able to contribute to the understanding of biological activity of ARSA and molecular etiology of ARSA pseudodeficiency, as determined by ARSA-N350S in the absence of polyadenylation defect.

Population carrier rates of pathogenic ARSA gene mutations: is metachromatic leukodystrophy underdiagnosed?

Background

Metachromatic leukodystrophy (MLD) is a severe neurometabolic disease caused mainly by deficiency of arylsulfatase A encoded by the ARSA gene. Based on epidemiological surveys the incidence of MLD per 100 000 live births varied from 0.6 to 2.5. Our purpose was to estimate the birth prevalence of MLD in Poland by determining population frequency of the common pathogenic ARSA gene mutations and to compare this estimate with epidemiological data.

Methodology

We studied two independently ascertained cohorts from the Polish background population (N∼3000 each) and determined carrier rates of common ARSA gene mutations: c.459+1G>A, p.P426L, p.I179S (cohort 1) and c.459+1G>A, p.I179S (cohort 2).

Principal Findings

Taking into account ARSA gene mutation distribution among 60 Polish patients, the expected MLD birth prevalence in the general population (assuming no selection against homozygous fetuses) was estimated as 4.0/100 000 and 4.1/100 000, respectively for the 1^st and the 2^nd cohort with a pooled estimate of 4.1/100 000 (CI: 1.8–9.4) which was higher than the estimate of 0.38 per 100 000 live births based on diagnosed cases. The p.I179S mutation was relatively more prevalent among controls than patients (OR = 3.6, P = 0.0082, for a comparison of p.I179S frequency relative to c.459+1G>A between controls vs. patients).

Conclusions/Significance

The observed discrepancy between the measured incidence of metachromatic leukodystrophy and the predicted carriage rates suggests that MLD is substantially underdiagnosed in the Polish population. The underdiagnosis rate may be particularly high among patients with p.I179S mutation whose disease is characterized mainly by psychotic symptoms.