Arylsulfatase A Deficiency

[Metachromatic Leukodystrophy, ARSA Deficiency]

Clinical Diagnosis

Arylsulfatase A deficiency (also known as metachromatic leukodystrophy or MLD) is suspected in individuals with the following:

• Progressive neurologic dysfunction. Presenting signs may be behavioral or motor. Symptoms can occur at any age beyond one year and follow a period of normal development [Von Figura et al 2001].

• MRI evidence of a leukodystrophy

  • Diffuse symmetric abnormalities of periventricular myelin with hyperintensities on T2-weighted images. Initial posterior involvement is observed in most late- infantile cases with subcortical U-fibers and cerebellar white matter spared. As the disease progresses, MRI abnormalities become more pronounced in a rostral-to-caudal progression; cerebral atrophy develops.

  • Anterior lesions may be more common initially in individuals with later onset.

Testing

Arylsulfatase A (EC 3.1.6.8) enzyme activity

• Arylsulfatase A (ARSA) enzyme deficiency. The diagnosis of MLD is suggested by ARSA enzyme activity in leukocytes that is less than 10% of normal controls using the usual Baum type assay in which other arylsulfatases are incompletely blocked [Baum et al 1959].

  Note: (1) The use of low temperature assays can minimize interference by other arylsulfatases and lower the baseline level [Rip & Gordon 1998]. (2) Cultured skin fibroblasts have often been used to confirm deficiency of ARSA enzyme activity and to evaluate the capacity of intact cells for sulfatide breakdown. Such testing is usually not necessary for establishing the diagnosis but can be useful when the diagnosis is ambiguous (pseudodeficiency vs. late-onset MLD) or is being made presymptomatically. (3) Sulfatide loading of cultured amniocytes or CVS cells can be critical in prenatal diagnoses – see Genetic Counseling.

• ARSA enzyme pseudodeficiency. Pseudodeficiency is suggested by ARSA enzyme activity in leukocytes that is 5% to 20% of normal controls. Pseudodeficiency is difficult to distinguish from true ARSA enzyme deficiency by biochemical testing alone.

  Note: (1) As used here, the term "pseudodeficiency" only refers to very low levels of ARSA enzyme activity in an otherwise healthy individual. Pseudodeficiency was first noted in parents and relatives of individuals with MLD. (2) Although the term “pseudodeficiency” has subsequently been applied to other enzyme deficiency disorders, it does not always have the same meaning. For example, in hexosaminidase A deficiency, the term "pseudodeficiency allele" refers to mutations that are associated with reduced enzymatic activity when measured using synthetic substrate but are associated with normal enzymatic activity when measured using natural substrate.

• Because assay of ARSA enzymatic activity cannot distinguish between MLD and ARSA pseudodeficiency, the diagnosis of MLD is confirmed by one or more of the following additional tests:

  • Molecular genetic testing of the ARSA gene (see Molecular Genetic Testing)

  • Urinary excretion of specialized compounds. Sulfatides accumulate in kidney epithelial cells in MLD and eventually slough into the urine in amounts from ten- to 100-fold higher than controls as measured by thin layer chromatography, high-pressure liquid chromatography (HPLC), and/or mass spectrometric techniques. Because urine production is highly variable, urinary sulfatide excretion is referenced on the basis of urinary excretion in 24 hours or to another urinary component such as creatinine (which is a function of muscle mass).

    Note: Whitfield et al [2001] suggested that sphingomyelin, another normally excreted lipid, would be a better reference compound because individuals being evaluated for MLD often are immobile and may have reduced muscle mass. The assay of sphingomyelin is presently available in only a few laboratories.

  • Metachromatic lipid deposits in a nerve or brain biopsy. Sulfatides interact strongly with certain positively charged dyes used to stain tissues, resulting in a shift in the color of the stained tissue termed metachromasia. When frozen tissue sections are treated with acidified cresyl violet (Hirsch and Peiffer stain), sulfatide-rich storage deposits stain a golden brown. The finding of metachromatic lipid deposits in nervous system tissue is pathognomic for MLD.

    Note: (1) Fixing the tissue with alcohol before staining extracts the sulfatides such that the metachromasia is no longer observed. (2) Although still considered by some to be the diagnostic "gold standard" for MLD, this highly invasive approach is now used only in exceptional circumstances (such as confirmation of a prenatal diagnosis of MLD following pregnancy termination).

Molecular Genetic Testing

GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.

Gene. ARSA is the only gene known to be associated with arylsulfatase A deficiency (metachromatic leukodystrophy, MLD).

Three classes of ARSA alleles resulting in low ARSA enzyme activity need to be distinguished:

1. Disease-causing ARSA-MLD alleles in the homozygous or compound heterozygous state result in ARSA enzyme activity that is insufficient to prevent sulfatide accumulation and thus cause MLD:

• Alleles that result in no functional enzyme activity are termed "I" (or "O") alleles. Presence of two "I" alleles typically results in late-infantile-onset MLD.

• Alleles that result in some residual enzyme activity are designated "A" (or "R") alleles and are associated with later-onset (i.e., juvenile or adult) MLD. Presence of two "A" alleles typically results in adult-onset MLD.

• Compound heterozygosity of one "I" allele and one "A" allele is usually associated with juvenile-onset MLD.

  Note: Exceptions to this oversimplification of the class of alleles determining age of onset of MLD occur; however, this classification provides a first-order explanation for genotype/phenotype relationships.

2. Pseudodeficiency (ARSA-PD) alleles are common polymorphisms that result in lower than average ARSA enzyme activity; however, ARSA-PD alleles still produce sufficient functional enzyme to avoid sulfatide accumulation and thus do not cause MLD in either of the following:

• The homozygous state (e.g., [ARSA-PD]+[ARSA-PD])

• The compound heterozygous state with an ARSA-MLD allele (e.g., [ARSA-PD]+[ARSA-MLD])

The most common ARSA-PD allele in the European and American populations has two sequence variants in a cis configuration (i.e., on the same chromosome), designated as c.[1049A>G; *96A>G] denoting two changes in one allele. The two changes:

• c.1049A>G (p.Asn350Ser), a glycosylation site variant that alters one of the N-glycosylation positions and results in poor targeting of the ARSA protein to the lysosome

• c.*96A>G, a polyadenylation site variant occurring in the 3' untranslated region that alters the site signaling of the polyadenylation of the mRNA and greatly reduces the amount of ARSA protein produced

The c.1049A>G (p.Asn350Ser) ARSA-PD variant occurs in isolation (without the cis c.*96A>G variant) in up to 5% of the European populations studied, in 20%-30% of the Asian populations studied, and in up to 40% of some African-derived populations. Even in the homozygous state, this mutation does not usually result in ARSA enzyme activity (the PD phenotype) sufficiently low to be within the MLD range.

The c.*96A>G variant occurs in isolation (without the cis c.1049A>G variant) only rarely [Gort et al 1999].

3. [ARSA-MLD; ARSA-PD] alleles in which a disease-causing ARSA-MLD mutation occurs in cis configuration (i.e., on the same chromosome) with an ARSA-PD allele have been reported. These are sometimes referred to ARSA-MLD-PD alleles.

Clinical testing

• Targeted mutation analysis

  • ARSA-MLD alleles. The disease-causing mutations included in targeted mutation analysis vary by laboratory. The four most common mutations occurring in the central and western European populations and their derivative North American populations (which have been most studied) include c.459+1G>A and c.1204+1G>A (the most common I-type mutations) and p.Pro426Leu and p.Ile179Ser (the most common A-type mutations).

    In general, these four alleles typically account for between 25% and 50% of the ARSA alleles in these populations (see Table 4).

    Note: The higher incidence of specific mutations in particular ethnic groups would modify the targeted mutations for such groups (e.g., Navajos or Alaskan Eskimos).

  • ARSA-PD alleles. Assays that distinguish between the isolated occurrence or cis configuration of the c.1049A>G mutation and the polyadenylation site mutation c.*96A>G are available.

    Note: The Gieselmann procedure [Gieselmann 1991, Gieselmann et al 1991] can only detect the c.1049A>G mutation and the polyadenylation site mutation c.*96A>G when they are in cis configuration.

• Sequence analysis/mutation scanning. The small size of the ARSA gene makes sequencing of the entire coding region and the majority of intronic regions feasible. Sequencing and/or mutation scanning is expected to detect 97% of ARSA mutations [Gort et al 1999]. Small deletions, insertions, and inversions within exons can be easily seen on genomic sequencing.

• Deletion/duplication analysis. There is one reported instance of a complete ARSA gene deletion in a case of MLD. No cases of full gene duplication are known. A case of dispermic chimerism has been reported where two ARSA genes were obtained from the father, one with an MLD-causing mutation and the other normal.

Table 1 summarizes molecular genetic testing for this disorder.

Table 1. Molecular Genetic Testing Used in Arylsulfatase A Deficiency

Gene Symbol	Test Method	Mutations Detected	Mutation Detection Frequency by Test Method and Phenotype			Test Availability
			Infantile MLD	Juvenile MLD	Adult MLD
ARSA	Targeted mutation analysis	ARSA-MLD alleles 1,2	36% to 50% 3,4	40% to 50% 3,4	73% to 90% 3,4	Clinical
	Sequence analysis/ mutation scanning	ARSA-MLD sequence variants 2	90%-95% 5
	Deletion/ duplication analysis 6	ARSA partial and whole gene deletions	<1%

1. The disease-causing mutations included in targeted mutation analysis vary by laboratory.

2. This test method also detects the ARSA pseudodeficiency alleles (termed ARSA-PD), common polymorphisms that result in lower than average ARSA enzyme activity but do not cause MLD even in the homozygous state or in the compound heterozygous state with an ARSA-MLD allele.

3. Testing for the eight most common mutations (p.Arg84Gln, p.Ser96Phe, c.459+1G>A, p.Ile179Ser, p.Ala212Val, c.1204+1G>A, p.Pro426Leu, and c.1401_1411del), Berger et al [1997] determined that the mutation detection frequency in affected individuals in Austria was 36% for infantile-onset, 50% for juvenile- onset, and 90% for adult-onset.

4. Testing for the same eight mutations, Lugowska et al [2005b] determined that the mutation detection rate in affected individuals in Poland was about 50% for late-infantile onset, 45% for juvenile onset, and 73% for adult onset.

5. Using mutation scanning, Gort et al [1999] identified all of the disease-causing ARSA mutations in 18 unrelated affected persons of Spanish heritage.

6. Testing that identifies deletions/duplications not detectable by sequence analysis of genomic DNA; a variety of methods including quantitative PCR, real-time PCR, multiplex ligation dependent probe amplification (MLPA), and array CGH (see ) may be used.

Interpretation of test results

• For issues to consider in interpretation of sequence analysis results, click here.

• Because both ARSA-MLD and ARSA-PD mutations can occur in cis configuration, molecular analysis of at least one parent is useful to determine if mutant alleles are in cis configuration or in trans configuration.

Testing Strategy

Confirmation of the diagnosis in a proband. Because the most commonly used assay of ARSA enzymatic activity cannot distinguish between MLD and ARSA pseudodeficiency, the diagnosis of MLD is confirmed by one or more of the following additional tests:

• Molecular genetic testing of the ARSA gene. Molecular genetic testing is used for confirmatory diagnostic testing to determine if low ARSA enzyme activity results from either of the following:

  • A combination of known disease-causing alleles such as homozygosity for an ARSA-MLD mutation or compound heterozygosity for [ARSA-MLD]+[ARSA-MLD] mutations, which confirms the diagnosis of MLD

  • A combination of known non-disease-causing alleles such as ARSA-PD homozygosity or [ARSA-PD]+[ARSA-MLD] compound heterozygosity, which suggest the carrier state for MLD (and thus, a different explanation for the neurologic problem in a symptomatic individual)

• Urinary excretion of sulfatides

• Metachromatic lipid deposits in a nerve or brain biopsy

Carrier testing for at-risk relatives requires prior identification of the disease-causing mutations in the family.

Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutations in the family.

Genetically Related (Allelic) Disorders

Individuals with the 22q13.3 deletion syndrome often have a deletion of the ARSA gene. Presence of an ARSA-PD allele on the homologous chromosome resulting in arylsulfatase A pseudodeficiency has been reported in 22q13.3 deletion syndrome [Phelan et al 2001].

Ring 22. Coulter-Mackie et al [1995] reported an individual with a ring chromosome 22 (including deletion of the ARSA gene) who had MLD resulting from an ARSA-MLD mutation on the homologous chromosome [Koc et al 2008].

Natural History

The three clinical subtypes of arylsulfatase A deficiency (MLD) are primarily distinguished by age of onset. Late-infantile MLD comprises 50%-60% of cases, juvenile MLD approximately 20%-30%, and adult MLD approximately 15%-20%. The age of onset within a family is usually similar, but exceptions occur [Arbour et al 2000].

The presenting problems and rate of progression vary among individuals; however, all eventually have complete loss of motor and intellectual functions. The disease course may be from three to ten or more years in the late-infantile-onset form and up to 20 years or more in the juvenile- and adult-onset forms [Von Figura et al 2001]. Death most commonly results from pneumonia or other infection. Life span correlates roughly with the age of onset but can be quite variable, particularly in the later-onset forms.

Late-infantile MLD. Onset is between ages one and two years, following a period of apparently normal early development. Acquired skills such as walking and speaking deteriorate. Clumsiness, frequent falls, toe walking, and slurred speech are typical presenting signs. Symptoms may first be noted following anesthesia or an infection with fever and may even subside for several weeks before continuing on a downhill course.

In the initial stage, weakness and hypotonia are observed. Later, the child is no longer able to stand; speech becomes difficult; and mental function deteriorates. Muscle tone is increased, and pain may occur in the arms and legs. Generalized or partial seizures may occur [Wang et al 2001]. Vision and hearing are compromised with slowed sensory evoked potentials and optic atrophy. Peripheral neuropathy with slow nerve conduction velocities (NCVs) is common [Cameron et al 2004].

Eventually, the child becomes bedridden with tonic spasms and decerebrate posturing with rigidly extended extremities. Feeding usually requires the use of a gastrostomy tube. In the final stages, which may last for several years, the children are blind, have no speech or volitional movements, and appear to be generally unaware of their surroundings. Often, parents or caregivers feel that the children respond to familiar voices and touch.

The expected life span is often quoted as 3.5 years after the onset of symptoms based on earlier published cases. However, survival can be quite variable and often extends well into the second decade of life with current levels of care.

Juvenile MLD. Age of onset is between four years and sexual maturity (age 12-14 years). Although earlier descriptions of juvenile MLD included individuals with onset up to age 18 years, currently, individuals with onset between ages 14 and 18 years are considered to have adult MLD.

The initial manifestations are usually noted during the early years of schooling with a decline in school performance and the emergence of behavior problems. Early- and late-juvenile sub-variants are sometimes differentiated, neuromuscular difficulties developing first in the earlier-onset cases and behavioral issues developing first in the later-onset cases.

Clumsiness, gait problems, slurred speech, incontinence, and bizarre behaviors eventually prompt diagnostic evaluation. Seizures may occur at any stage of the disease. Balslev et al [1997] suggest that they are more commonly partial seizures.

Progression is similar to, but slower than, the late-infantile form. Survival for ten to 20 or more years after the initial diagnosis is common.

Adult MLD. Symptoms are first noted after sexual maturity (age ~14 years) but may not occur until the fourth or fifth decade. As with juvenile MLD, presenting symptoms vary.

Initial signs are often emerging problems in school or job performance associated with personality changes. Alcohol abuse, drug use, poor money management, and emotional lability often lead to psychiatric evaluation and an initial diagnosis of schizophrenia or depression. Bewilderment, inappropriate affect, and even auditory hallucinations have been reported.

In others, neurologic symptoms (weakness and loss of coordination progressing to spasticity and incontinence) predominate initially, leading to diagnoses of multiple sclerosis or other neurodegenerative diseases. Seizures have also been reported as a presenting feature.

Peripheral neuropathy is a frequent aspect of adult-onset MLD, and isolated peripheral neuropathy can be the presenting symptom [Felice et al 2000]. However, it has been completely absent in some cases [Marcao et al 2005].

The course is variable. Periods of relative stability may be interspersed with periods of decline. Inappropriate behaviors and poor decision-making become problems for the family or other caregivers. Dressing and other self-help skills deteriorate. Eventually, bowel and bladder control is lost. As the disease advances, dystonic movements, spastic quadraparesis, or decorticate posturing occur. Severe contractures and generalized seizures may occur and then resolve later. Eventually, the ability to carry on a conversation and communicate effectively is lost.

The individual usually does not lose contact with his/her surroundings until late in the disease, which may extend for two or three decades. In the end stage, the individual is blind, bedridden, and unresponsive. Pneumonia or another infection is usually the cause of death.

Other findings in MLD. In the past, findings of increased concentration of cerebrospinal fluid (CSF) protein, decreased NCVs, and abnormal auditory and visual evoked potential studies were used in diagnosis. While such tests are no longer necessary for diagnosis, they may be used in protocols for monitoring disease progression or therapeutic trials.

Involvement of the gallbladder occurs.

Pathogenesis. Arylsulfatase A deficiency is a disorder of impaired breakdown of sulfatides (cerebroside sulfate or 3-0-sulfo-galactosylceramide), sulfate-containing lipids that occur throughout the body and are found in greatest abundance in nervous tissue, kidneys, and testes. Sulfatides are critical constituents in the nervous system, where they comprise approximately 5% of the myelin lipids. Sulfatide accumulation in the nervous system eventually leads to myelin breakdown (leukodystrophy) and a progressive neurologic disorder [Von Figura et al 2001].

Genotype-Phenotype Correlations

The simple genotype-phenotype correlations proposed by Polten et al [1991] have been proven useful but are imperfect, and several discrepancies have been noted. The age of onset for a particular genotype is influenced by a variety of environmental and other genetic factors.

ARSA enzyme activity

• The genotypes ARSA-MLD/ARSA-MLD, ARSA-PD-MLD/ARSA-MLD, and ARSA-PD-MLD/ARSA-PD-MLD result in ARSA enzyme activity that is 5%-10% of control values in Baum-type assays.

• The genotype ARSA-PD/ARSA-MLD usually results in ARSA enzyme activity that is approximately 10% of control values, while the genotype ARSA-PD/ARSA-PD results in ARSA enzyme activity that is approximately 10%-20% of control values.

Age of onset of MLD is not related to the amount of apparent enzyme activity as usually measured. It does, however, correlate reasonably well with the ability of cultured fibroblasts to degrade sulfatide added to the culture medium:

• Early-onset (late-infantile) MLD. Affected individuals are usually homozygous or compound heterozygous for I-type ARSA-MLD alleles and make no detectable functional arylsulfatase A enzyme. The most common I-type alleles are c.459+1G>A, c.1204+1G>A, and p.Asp255His.

• Later-onset MLDs. Affected individuals have one or two A-type ARSA-MLD alleles that encode for an arylsulfatase A enzyme with some functional activity (≤1% when assayed with physiologic substrates). The most common A-type ARSA-MLD alleles are p.Ile179Ser and p.Pro426Leu [Fluharty et al 1991]:

  • Juvenile-onset MLD. Often, one allele provides no functional enzyme activity (I-type ARSA-MLD allele), while the other allele provides some residual enzyme activity (A-type ARSA-MLD allele).

  • Adult-onset MLD. Both alleles provide some residual enzyme activity (A-type ARSA-MLD alleles). Regis et al [2002] suggest that an A-type ARSA-MLD allele occurring in cis configuration with an ARSA-PD sequence variant may have a more severe consequence and behave as an I-type ARSA-MLD allele. The p.[Ile179Ser]+[Ile179Ser] genotype, which could be expected in late-onset cases, has not been reported to date. Information on specific mutations does correlate with initial clinical manifestations and can have prognostic implications [Baumann et al 2002, Rauschka et al 2006].

Note: In the study of Lugowska et al [2005a], these generalizations regarding genotype/phenotype correlations held up fairly well; however, in a few instances, an A-type ARSA-MLD mutation occurred in late-infantile onset MLD and an I-type ARSA-MLD mutation in adult-onset MLD.

Arylsulfatase A (ARSA) pseudodeficiency

• ARSA-MLD/ARSA-PD genotype. Associated ARSA enzyme activity is 5% to 10% of normal controls:

  • The polyadenylation site mutation, c.*96A>G, appears to contribute most strongly to the low ARSA enzyme activity characteristic of clinical pseudodeficiency [Harvey et al 1998].

  • p.Asn350Ser, the glycosylation site alteration, is associated with an increased excretion of the newly synthesized enzyme from cells and a possible decrease in the ARSA enzyme within the lysosome [Harvey et al 1998].

• ARSA-PD/ARSA-PD genotype

  • Homozygosity for the c.*96A>G mutation (almost always in conjunction with p.Asn350Ser mutation) is associated with ARSA enzyme activity that is approximately 10% of normal controls and could provide diagnostic uncertainty.

  • Homozygosity for the p.Asn350Ser mutation alone results in 50% or more of the mean control ARSA enzyme activity in leukocytes.

Nomenclature

Metachromatic leukodystrophy (metachromischen leukodystrophien) was first used by Peiffer [1959] to describe what had previously been known as “diffuse brain sclerosis.”

The term “metachromatic leukoencephalopathy” has also been used.

MLD has also been referred to as “Greenfield's disease” after the first report of the late-infantile form of MLD.

Prevalence

Arylsulfatase A deficiency. The overall prevalence of arylsulfatase A deficiency has been reported to be between 1:40,000 and 1:160,000 in different populations [Von Figura et al 2001].

The disorder seems to occur throughout the world; however, most data come from European and North American populations.

Assuming this prevalence, the overall carrier frequency is between 1:100 and 1:200.

In the following consanguineous populations, the disease prevalence can be much higher (figures are approximate):

• 1:75 in Habbanite Jews in Israel

• 1:8000 in Israeli Arabs

• 1:10,000 in Christian Israeli Arabs

• 1:2500 for the western portion of the Navajo Nation in the US

ARSA-PD alleles. The most common ARSA-PD allele in the European and American populations has two sequence variants in a cis configuration (i.e., on the same chromosome), designated as c.[1049A>G; *96A>G] (see Molecular Genetic Testing).

The homozygous ARSA-PD genotype occurs in as many as 0.5%-2% of the European/ Euro-American population and may be even more common in Asian and African populations. Thus, an ARSA-PD homozygous genotype is more than 400-fold more common than the ARSA-MLD homozygous genotype, and an [ARSA-PD]+[ARSA-MLD] compound heterozygous genotype is 30- to 50-fold more common than the ARSA-MLD homozygous genotype.

An ARSA-MLD mutation is as likely to be found on an ARSA-PD allele as on a wild type allele, implying that 0.5%-1% of ARSA-PD alleles are associated with a cis ARSA-MLD mutation (so-called [ARSA-PD; ARSA-MLD] alleles).

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with arylsulfatase A deficiency, the following evaluations are recommended:

• If the diagnosis is made presymptomatically, baseline measures of ARSA enzyme activity, urinary sulfatide excretion, and myelin integrity by MRI to monitor disease progression and evaluate the need for possible intervention

• Baseline assessment of development/cognitive abilities and behavior to monitor disease progression or changes with attempted therapy

• Examination of the peripheral nervous system

Treatment of Manifestations

Treatment of specific symptoms and the provision of supportive therapies to maximize the retention of physical and intellectual functions are the usual approach to the management of MLD:

• Seizures, if they occur, should be treated with antiepileptic drugs.

• Contractures, if they occur, should be treated with muscle relaxants.

• Every effort should be made to maximize intellect, neuromuscular function, and mobility through aggressive physical therapy and an enriched environment. The parents and/or caregivers should be aware of the likely progression of the disorder in order to anticipate decisions on walking aids, wheelchairs, feeding tubes, and other supportive measures.

• Because MLD affects the whole family, management should include a team of professionals to provide genetic counseling and family support through what will be a long disease process. Even children with late-infantile MLD may survive for five to ten years, with a progressive loss of function and continually changing care needs. It is important for most families to develop a network of support services and other families who have faced similar situations.

Prevention of Primary Manifestations

Bone marrow transplantation (BMT) or hematopoetic stem cell transplantation (HSCT). BMT is the only presently available therapy that attempts to treat the primary central nervous system manifestations of MLD [Krivit et al 1999, Peters & Steward 2003, Krivit 2004]. Not all individuals with MLD are suitable candidates for these procedures, and not all families are willing to undertake the risks involved. Although identification of adequately matched donors and treatments for BMT complications are constantly improving, BMT remains controversial because of its substantial risk and somewhat uncertain long-term effects. In the absence of alternative approaches, BMT needs to be considered, particularly for the more slowly progressing late-onset forms of MLD.

The best outcomes are observed in individuals undergoing transplantation before symptoms occur. At best, it appears to slow disease progression and does not seem to alleviate peripheral nervous system manifestations [Koc et al 2002].

BMT in a presymptomatic neonate has been reported, but complications were encountered and disease progression was not halted [Bredius et al 2007].

Görg et al [2007] reported 13-year follow-up of an individual with juvenile MLD treated with BMT.

Meuleman et al [2008] reported minimal complications in an adult who underwent reduced intensity conditioning accompanied by mesenchymal stromal cell infusion.

Although the availability of hematopoietic stem cells from cord blood enhances the chances of obtaining a suitable source of donor cells, the results reported to date indicate that considerable problems remain [Martin et al 2006].

Prevention of Secondary Complications

Prevention of joint contractures by maintaining joint mobility facilitates nursing care in the later stages of the disorder.

Affected individuals remain susceptible to the full range of childhood and adult diseases. The pediatrician or general care physician should be involved in developing comprehensive care plans.

Surveillance

The following are appropriate:

• A program of periodic MRI monitoring developed by the neurologist and primary care physician

• Monitoring of changes in locomotion, communication, and behavior which could indicate a need to alter care and support systems, e.g., introduction of walking aids and/or a wheel chair

• Monitoring for onset of seizures and/or contractures which could indicate a need to change medical management and physical therapy

• Monitoring for behavioral changes, inappropriate emotions or actions, problems in following directions, memory loss and/or incontinence, which indicate a need for increasing physical restriction and loss of independence

• Monitoring for difficulties in swallowing or weight loss, which trigger consideration of gastrostomy

• Special attention following general anesthesia or an infection with a high fever as these may trigger exacerbation of disease progression

Agents/Circumstances to Avoid

While environmental factors are thought to influence the onset and severity of MLD symptoms, no specific exacerbating agents are known. Initial symptoms are often noted following a febrile illness or other stress, but it is unclear if a high fever actually accelerates progression.

Excessive alcohol and drug use are often associated with later-onset MLD, but it is unclear if this is caused by the disease or is simply an attempt at self-medication in the face of increasing cognitive difficulties [Alvarez-Leal et al 2001].

Exacerbation of symptoms has been noted following anesthesia because affected individuals may have altered responses to sedatives and anesthetics.

Testing of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Attempts at improving the effectiveness of bone marrow transplantation include combined therapy with either genetically engineered ARSA enzyme [Martino et al 2005] or mesenchymal stem cells [Koc et al 2002, Meuleman et al 2008].

Enzyme replacement therapy (ERT) is, at present, considered impractical because of the difficulty of bypassing the blood-brain barrier; but human ARSA enzyme is now available, and animal studies have suggested that it may be a useful supplement to other type of therapies [Martino et al 2005, Matzner et al 2005]. Clinical testing of recombinant human enzyme has been initiated.

Nevo et al [1996] reported that immunomodulation (immunosuppression or treatment with immunoglobulin) improved strength and slowed functional regression, but the effect on the long-term outcome was unclear.

The large number of papers published recently on experimental gene therapies for arylsulfatase A deficiency are reviewed by Biffi & Naldini [2007], Sevin et al [2007], and Gieselmann [2008]. Viral vectors for introducing the ARSA gene into the enzyme-deficient mouse model have been investigated [Matzner & Gieselmann 2005]. Kurai et al [2007] observed that coexpression of the gene encoding the formylglycine-generating enzyme (deficient in multiple sulfatase deficiency) is necessary for efficient gene replacement and correction in the mouse model; Matzner et al [2008] evaluated parameters affecting enzyme replacement that could be an adduct to therapy; and Capotondo et al [2007] evaluated over-expression of the ARSA gene. Human gene replacement trials can be anticipated in the near future, but the prospects for clinical effectiveness are uncertain and substantial regulatory concerns remain.

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Other

Lead has been reported to enhance secretion of ARSA enzyme by cells in culture and to lower cellular enzyme levels [Poretz et al 2000]. Minimizing lead exposure is already an important public health goal, and it is uncertain if additional steps would be useful in individuals with MLD.

Genetics clinics are a source of information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

Support groups have been established for individuals and families to provide information, support, and contact with other affected individuals. The Resources section may include disease-specific and/or umbrella support organizations.

Mode of Inheritance

Arylsulfatase A deficiency is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

• The parents of an affected child are usually obligate heterozygotes (i.e., carriers of one mutant allele).

• Heterozygotes (carriers) are asymptomatic.

Sibs of a proband

• At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.

• Once an at-risk sib is known to be unaffected, the risk of his/her being a carrier is 2/3.

• Heterozygotes (carriers) are asymptomatic.

• In instances in which one copy of the ARSA gene has been deleted, a single ARSA-MLD mutation on the remaining allele results in MLD [Eng et al 2004]. Therefore, in instances of apparent homozygosity for an ARSA-MLD mutation in a proband, it is appropriate to establish the presence of the disease-causing ARSA mutation in both parents when possible to assure accurate use of molecular genetic testing in clarifying the genetic status of at-risk relatives.

Offspring of a proband. The offspring of an individual with arylsulfatase A deficiency are obligate heterozygotes (carriers) for a disease-causing mutation in the ARSA gene.

Other family members of a proband. Each sib of the proband's parents is at a 50% risk of being a carrier.

Carrier Detection

Biochemical testing. Analysis of arylsulfatase A enzyme activity in leukocytes or cultured fibroblasts for carrier detection is fairly reliable if the range of enzyme activity within a family is known; however, it is much less reliable for testing individuals with no family history of MLD because of the substantial variation in "normal" enzyme activity resulting from the high frequency of pseudodeficiency alleles.

Molecular genetic testing. Molecular genetic methods can be used for carrier testing for at-risk family members if the ARSA mutations in the family have been identified.

Related Genetic Counseling Issues

Family planning

• The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.

• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, are carriers, or are at risk of being carriers.

DNA banking. DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. DNA banking is particularly relevant when the sensitivity of currently available testing is less than 100%. See for a list of laboratories offering DNA banking.

Prenatal Testing

Biochemical testing. Analysis of arylsulfatase A enzyme activity in cultured amniotic fluid cells or chorionic villus cells has been used for the prenatal diagnosis of arylsulfatase A deficiency. Cells for culturing are obtained by amniocentesis usually performed at approximately 15-18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation. The chief limitation to this approach is the need to grow sufficient cells for testing, which may take two to three weeks after amniocentesis or CVS sampling.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

In instances in which one parent has a very low level of arylsulfatase A enzyme activity caused by a pseudodeficiency allele, prenatal diagnosis using the assay of enzyme activity is unreliable. Either sulfatide loading of the cultured cells or molecular genetic testing can be used in these instances to clarify diagnostic issues.

Molecular genetic testing. Prenatal diagnosis for pregnancies at increased risk is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis or chorionic villus sampling. Both disease-causing alleles of an affected family member must have been identified before prenatal testing can be performed by this method.

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutations have been identified. For laboratories offering PGD, see .

Molecular Genetic Pathogenesis

The molecular pathologic processes involved in MLD are poorly understood. While the accumulation of sulfatides in oligodendrocytes and Schwann cells is thought to somehow be responsible for the loss of these cells and the resultant demyelination, these lipids have not proven to be toxic in cell cultures. Psychosine sulfate (lyso-sulfatide) is elevated in tissues from individuals with MLD, and a cytotoxic role parallel to that of psychosine in Krabbe disease has been suggested.

Normal allelic variants. ARSA contains eight exons in a relatively short coding region of 3.2 kilobases (kb) and is translated to a 2.1-kb mRNA. The 5' untranslated region is typical of a housekeeping gene but lacks a TATA or CAAT box typical of lysosomal enzymes. The gene extends for nearly 3 kb beyond the stop codon. Additional mRNA products of 3.7 and 4.8 kb are detected in cells, but their significance remains uninvestigated.

Several normal allelic variants of ARSA have been identified. The most common is p.Thr391Ser, which was found in approximately half of the Euro/American population initially studied. The c.1049A>G(p.Asn350Ser) site (ARSA-PD glycosylation site alteration) is a common normal variant occurring in 15%-40% of individuals, depending on the population studied. A number of other relatively rare polymorphisms and neutral base changes have also been reported.

Pathologic allelic variants. More than 150 mutations of ARSA associated with arylsulfatase A deficiency have been reported. Disease-causing ARSA-MLD mutations are as likely to be found in cis configuration with a ARSA-PD sequence variant in wild-type alleles. Complete deletion of one ARSA gene has been reported in one individual with MLD [Eng et al 2004].

Normal gene product. Arylsulfatase A has a precursor polypeptide of approximately 62 kd that is then processed by N-linked glycosylation, phosphorylation, sulfation, and proteolytic cleavage to a complex mixture of isoforms that differs from tissue to tissue. A magnesium or calcium ion also becomes tightly bound near the active site. During post-synthetic processing, the Cys69 must be converted to formylglycine before the sulfatase becomes active [Lukatela et al 1998, Dierks et al 2005].

As isolated at neutral pH, the arylsulfatase A enzyme is dimeric (~100 to 120 kd) with two subunits, which may not be identical. At acid pH such as that occurring in the lysosome, the enzyme aggregates further to an octamer, the form present in the crystalline enzyme [Vagedes et al 2002].

Abnormal gene product. In general, the splice-site mutations and insertions or deletions do not lead to any active enzyme (I-type ARSA-MLD mutations). Approximately half of the mutations involving an amino acid substitution also fall into this class but are more likely to express an immuno cross-reactive material.

Between 20% and 25% of the single amino acid changes are associated with a low level (≤1%) of ARSA enzyme activity (A-type ARSA-MLD mutations). In those cases in which the properties of the mutant ARSA enzyme have been explored, processing and stability have been affected, leading to altered enzyme or altered ability of the protein to self-associate and an enhanced turnover of the mutant protein [von Bulow et al 2002, Poeppel et al 2005].

Literature Cited

Published Statements and Policies Regarding Genetic Testing

No specific guidelines regarding genetic testing for this disorder have been developed.

Revision History

• 23 September 2008 (me) Comprehensive update posted live

• 30 May 2006 (me) Review posted to live Web site

• 15 November 2004 (mf) Original submission