The lipid storage diseases are a group of diseases in which a strategic metabolic enzyme is missing or inactive, usually as a result of a single gene deletion (see Table 10.5). Because of this missing enzyme, undigested metabolic products accumulate in cells and cell integrity is affected. Cells of the reticuloendothelial system (RES) are most often affected. The RES is a network of cells seen throughout the circulation and tissues that provide the phagocytic defense system. Histiocytes, monocytes, macrophages, and the cells of the bone marrow, liver, spleen, and lymph nodes comprise this network. Consequently, large, easily identifiable cells specific to each disease are located in the bone marrow and are part of the diagnostic picture of many of these disorders. For this reason, these disorders are not frequently observed in the clinical laboratory.
Common Features of a Few of the Lipid Storage Diseases
Gaucher's, Tay-Sachs, and Niemann-Pick are the three most common lipid storage diseases, and they have many common features. All are autosomal recessive disorders as a result of a single-gene mutation. Abnormal facial features and liver enlargement are seen in all but Tay-Sachs. Although there is a wide range of clinical presentation, from infant onset to adult onset, those individuals with infant onset have a more severe clinical presentation and a shorter life span. Many of these diseases have a high incidence in the northeast European Jewish population (the Ashkenazi), and for this reason prenatal counseling and genetic screening are highly recommended in affected or extended families. Central nervous system involvement is often a feature of infantile forms of the disease, especially Tay-Sachs, and short life spans usually prevail.12 There is no cure for the lipid storage diseases, and for the most severe manifestations,
supportive therapy is all that can be offered. Enzyme replacement therapy using biosynthetic enzyme material is available in limited qualities.13 Bone marrow transplantation is also available, yet the risks and benefits of this procedure in young children must be carefully considered.
Gaucher's and Niemann-Pick diseases each have specific bone marrow cells that are representative of the particular disorder. For Gaucher's disease, the cell is large, 20 to 100 pm, with rod-shaped inclusions that appear like crinkled tissue paper in the bone marrow (Fig. 10.13). For Niemann-Pick, the cell is equally large but appears round with evenly sized lipid accumulations (Fig. 10.14). In their own right, each of these cells is
Figure 10.17 Precipitated stain, not bacteria.
striking on bone marrow examination because they are infrequently observed. In Tay-Sachs disease, there is no large identifiable bone marrow cell, yet most of these individuals have a deficiency of hexosamindadase A, which can be tested for prenatally.14 The lymphocytes in each of these disorders may show vacuolization, and although it is a common finding, it is not specific for the lipid storage disorders.
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