DIAGNOSIS AND TREATMENT OF LYSOSOMAL STORAGE DISORDERS
Experienced laboratory practitioners using relatively simple methods can accurately diagnose patients exhibiting the typical traits of LSDs. However, atypical patients usually require more detailed investigations. An early diagnosis is important to prevent serious damage to the nervous and skeletal systems. Almost all lysosomal storage diseases can be diagnosed using samples of leukocytes or plasma. Diagnosis is based upon assays that determine the reduced activities of lysosomal enzymes or an increase in substrate in skin fibroblasts, plasma or urine. For example, Gaucher’s and I-cell diseases can be confirmed by demonstrating deficiencies in β -glucocerebrosidase and N-acetylglucosylaminyl-1-phosphotransferase activities in leukocytes orfibroblasts respectively.
The prognosis for adult Gaucher’s disease patients is relatively good. The hypersplenism is usually relieved by splenectomy although this may has-ten bone deterioration. Symptomatic treatment includes analgesics for the bone pains, blood transfusions to relieve the anemia and orthopedic relief for fractures and degeneration of the hip joints. Enzyme replacement therapy, involving intravenous administration of purified A-glucocerebrosidase, is now common. Receptors on the surfaces of macrophages selectively bind man-nose residues on the enzyme and allow it to be absorbed by the cells. Within the macrophages, the enzyme is delivered to the lysosomes where it cata-lyzes the breakdown of the accumulated glucosylceramide. In the majority of cases this is an effective and safe treatment, reducing the sizes of the liver and spleen and allowing them and the bone marrow to function effectively. Again, bone marrow transplants may be useful. Gene therapy, in which a functional gene for A-glucocerebrosidase is inserted into stem cells in the bone marrow, may in the future provide a complete cure.
Treatment for I-cell disease is limited. Bone marrow transplants can potentially replace the defective hemopoietic system with stem cells from a healthy donor, providing a replacement for the defective enzymes. In a limited number of cases, bone marrow transplants have normalized lysosomal enzyme activities, but the preexisting clinical damage from the disease is usually so extensive by this stage that this option is of questionable long-term benefit. In addition, bone marrow transplants can fail and the problems of finding compatible donors are immense.
Genetic counseling and prenatal diagnosis have roles in relation to LSDs. In cases where the likelihood of a lysosomal storage disease being present in a fetus is high, prenatal diagnosis is possible and a therapeutic abortion can be offered. Methods for screening all newborns without a family history of such conditions have been proposed but such screening programs would require a large amount of effort for comparatively little gain.