Epidemiology and Diseases
About 40 lysosomal diseases are known, with an overall prevalence of about 1:5000 to 1:8000 live births. They include, among others:
• Mucopolysaccharidosis (MPS I–VII).
• Oligosaccharidosis (fucosidosis, mannosidosis, aspartylglucosaminuria, Schindler's disease, sialidosis).
• Sphingolipidosis (Fabry's disease, Farber's disease, galactosialidosis, Gaucher's disease, GM1 gangliosidosis, GM2 gangliosidosis, Krabbe's disease, metachromatic leukodystrophy, Niemann-Pick disease types A and B).
• Lysosomal transport defects (Niemann-Pick disease type C, sialic acid storage disease).
• Glycogenosis (glycogenosis type II, Danon's disease).
• Neuronal ceroid lipofuscinosis (NCL) 1–3 (Santavuori's disease, Jansky-Bielschowsky disease, Spielmeyer-Vogt disease).
Other forms of NCL (including Kufs’ disease) are probably also lysosomal disorders.
Physiology and Etiology
Lysosomes are cell organelles of up to 2 μm in diameter; they are surrounded by a single membrane. Inside these organelles, about 70 acidic hydrolases catalyze at a low optimum pH the degradation of various macromolecules, such as glycoproteins, lipids, phospholipids, and glycosaminoglycans. Lysosomal storage disorders are caused by genetic defects in lysosomal enzymes, protein co-factors (activator proteins), or lysosomal transport proteins. They have in common the accumulation of lysosomal substrates (Wenger et al., 2003).
Clinical Features and Diagnosis
As with mitochondrial diseases, the decisive step is to think of the possibility of a lysosomal disorder in the first place. Manifestations in adults may markedly differ from the “classical” presentations in children (Rapola, 1994; Gray et al., 2000). Psychiatric and cognitive dysfunctions are not unusual first symptoms (Turpin and Baumann, 2003).
Table 17.5 gives an overview of lysosomal diseases and lists protein defects, typical symptoms, and available laboratory tests. With specific treatment options becoming more feasible for some lysosomal diseases, early diagnosis is becoming increasingly important.
Routine blood tests and CSF analysis. In various storage diseases, assessment of leukocyte differentiation under the light microscope reveals vacuolated lymphocytes. In patients with Gaucher's disease, serum acid phosphatase, angiotensin-converting enzyme (ACE), and chitotriosidase are elevated. Chitotriosidase is also elevated in Niemann-Pick disease type C. Increased CSF protein is characteristic of Krabbe's disease and metachromatic leukodystrophy.
Urinalysis. The increased urinary excretion of glycosaminoglycans in mucopolysaccharidosis can be quantitatively determined by staining with dimethylmethylene blue. Screening tests may yield normal findings in Sanfilippo's syndrome and Morquio's syndrome; if these diseases are suspected, and likewise if screening tests yield conspicuous findings, glycosaminoglycans should be analyzed in more detail by electrophoresis. Oligosaccharides and free neuraminic acid in the urine are detected by thin layer chromatography. A sample of morning urine usually suffices for these tests; it should be mixed with a preservative prior to shipment.
Enzyme defects. Lysosomal enzyme defects can be detected in fibroblasts and sometimes also in leukocytes or in serum. Serum may be shipped at room temperature. For harvesting of leukocytes, heparinized whole blood may be shipped without cooling, provided it reaches the laboratory within 24 hours. EDTA-blood is unsuitable because enzymes may become inactivated. For culturing of fibroblasts, a skin biopsy specimen should be placed in an appropriate culture medium and shipped in this medium. Enzyme activities are measured by substrate turnover.
• In metachromatic leukodystrophy caused by activator protein deficiency, arylsulfatase A activity is normal. There also exist individuals whose enzyme has limited activity for the artificial substrate used in the laboratory, but this reduced activity is enough for the natural substrate (pseudodeficiency). In both these cases, determination of sulfatides in the urine may be helpful. Gaucher's disease and GM2gangliosidosis can also be caused by activator protein deficiency. In the case of Gaucher's disease, detection of Gaucher cells in the bone marrow is helpful. The biochemical analysis of these very rare activator protein deficiencies is elaborate and expensive and is performed in only a few laboratories.
• If enzyme findings are borderline, for some diseases it may help to study the incorporation of radioactively labeled methylamine or the incorporation or degradation of radioactive sulfate in fibroblasts. Impaired methylamine incorporation may indicate Niemann-Pick disease type C. This disease often poses special diagnostic problems, particularly when the characteristic organomegaly is absent. The increased chitotriosidase activity was pointed out above. As with Niemann-Pick disease type A or B, Niemann-Pick cells and sea-blue histiocytes are present in the bone marrow (Kvasnicka and Thiele, 2002). The definitive diagnostic test is performed with fibroblasts: the lysosomal accumulation of nonesterified cholesterol is best detected by combining the determination of LDL-induced cholesteryl ester formation with a specific staining of cholesterol granules, using the fluorescent antifungal agent filipin (Vanier and Millat, 2003).
• Neuronal ceroid lipofuscinosis is another disease that frequently poses a diagnostic challenge. The deficient enzymes in infantile and late infantile NCL have recently been identified. In a patient with the typical clinical features, enzyme determination is the first step toward diagnosis. If vacuolated lymphocytes are found in the blood smear of a patient with typical presentation of classic juvenile NCL, this is virtually diagnostic of this disease. With other constellations of symptoms, particularly in the rare form of adult NCL, electron microscopic detection of specific storage material in fixed lymphocyte preparations, or in sweat glands from a deep skin biopsy, is indicative (Goebel and Wisniewski, 2004; Kohlschütter at al., 2005).
Molecular genetic analysis. For a growing number of lysosomal diseases, molecular genetic analysis is now also possible.