Disease-causing mutations in cis with the common arylsulfatase A pseudodeficiency allele compound the difficulties in accurately identifying patients and carriers of metachromatic leukodystrophy.

Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder most often caused by mutations in the sulfatide sulfatase or arylsulfatase A (ASA) gene. This results in the storage of sulfatides in the peripheral and central nervous systems as well as in the kidneys. Patients with MLD exhibit a wide range of clinical features presenting from the late infantile period to adulthood. Testing for this disease is performed on a majority of the patient samples received for diagnostic testing in the authors' laboratory. If low ASA activity is measured, additional testing is required to confirm the diagnosis due to several factors. ASA activity is also low in individuals with multiple sulfatase deficiency and in individuals with copies of the so-called pseudodeficiency (Pd) allele. Due to the high frequency of the common Pd allele, it is possible for individuals, both with and without neurologic disease, to have low ASA activity but not have MLD. Unfortunately, the finding of the Pd mutation by molecular analysis does not rule out a diagnosis of MLD. In a recent 25 month period, this laboratory diagnosed 52 patients with MLD, and of these, 13 patients from 10 families had one or two copies of the Pd mutation. Sequencing of the ASA gene in these 10 families revealed four new mutations in cis with the Pd allele (S43R, R84Q, R311X, and E329R) and two additional new mutations (R299W, C488R). Six patients had previously reported mutations on the Pd background. Thus, a total of 14 mutations have been found to occur in cis with the Pd allele. We estimate that 1-2% of Pd alleles will have a disease-causing mutation, and this complicates the identification of patients and the assignment of risk for a couple when a copy of the Pd allele is detected.

Insights into the diagnosis and treatment of lysosomal storage diseases.

Lysosomal storage diseases (LSDs) are a group of genetic disorders that result from defective lysosomal metabolism or export of naturally occurring compounds. Signs and symptoms are variable both within and between disorders depending on the location and extent of storage. Many patients develop neurologic symptoms that become obvious from the newborn period to adulthood. Diagnosis of suspected patients can usually be made by measuring the activity of an enzyme or concentration of a metabolite in easily obtained tissue samples. Based on the considerable diagnostic experience of our laboratory, we aid the physician in selecting the appropriate tests to perform. Hematopoietic stem cell transplantation and enzyme replacement therapy are already available or in clinical trials for a number of LSDs. Early diagnosis is critical, especially since those patients who are treated before significant symptoms arise have the best chance for a positive outcome.

Lysosomal storage disorders: diagnostic dilemmas and prospects for therapy.

PURPOSE: The main purpose of this review is to address some concerns regarding the accurate and timely diagnosis of lysosomal storage disorders (LSD). METHODS: Using their experience in diagnosing LSD in more than 2500 individuals, the authors highlight several diagnostic difficulties and solutions and review the latest methods for early diagnosis and treatment. RESULTS: While "classic" patients can be accurately diagnosed using relatively simple methods in an experienced laboratory, atypical patients require more detailed studies. With a few exceptions, almost all LSD can be diagnosed in leukocytes or plasma. Methods for screening all newborns without a family history of a LSD have been proposed, but such screening may require a large amount of effort for little gain. CONCLUSIONS: With effective therapy becoming available for some LSD, early diagnosis is critically important. If the goal is to prevent serious complications related to the nervous and skeletal systems, earlier diagnosis is potentially advantageous. Accurate prognosis and assessing the need for aggressive therapy in newly diagnosed patients are problems that need further study.