Research update on lysosomal disorders with special emphasis on metachromatic leukodystrophy and Krabbe disease

Lysosomal disorders as a group result from mutations in genes coding for lysosomal proteins or other proteins required for the proper processing of lysosomal enzymes and cofactors. Most patients with these disorders are diagnosed by protein-based methods using available synthetic and natural substrates. Almost all of the genes coding for lysosomal enzymes, activator proteins and protective proteins have been cloned, and mutations that result in defective protein have been identified. This may result in tests that will aid in making a prognosis in newly diagnosed cases of a given disease. In this article the problems with the accurate diagnosis of metachromatic leukodystrophy (MLD) and the difficulty in cloning the galactocerebrosidase gene, which is defective in patients with Krabbe disease, are discussed. Patients with MLD are diagnosed by the deficiency of arylsulfatase A activity in leucocytes and/or cultured skin fibroblasts. However, diagnosis is complicated by the finding of a mutation in healthy people which in a homozygous state or in the heterozygous state with a true MLD-causing mutation results in low in vitro arylsulfatase A activity. Definitive diagnosis of a new patient, and confirmation of prenatal tests can be done using a 14C-stearic acid-labeled sulfatide loading test in cultured cells. This test will also diagnose the cases of MLD caused by a defect in SAP-1. While the diagnosis of patients with Krabbe disease rests on the measurement of low galactocerebrosidase activity, there are problems with identifying carriers due to overlap with non-carriers. The gene has not yet been cloned due to the difficulty in obtaining purified enzyme and the lack of markers near the gene on chromosome 14. Galactocerebrosidase has been sufficiently purified to allow isolation of protein bands for microsequencing. This information will be used to clone the gene for future studies to improve diagnostic methods and to treat the available animal models with galactocerebrosidase deficiency.