Niemann Pick disease consists of a group of genetic disorders in which the common feature is a varying degree of sphingomyelin storage in certain tissues of the body. According to the current classification based on the enzymatic defect underlying these disorders, two main groups are distinguished. The first group, which comprises type A, which is characterized by a severe deficiency in acid sphingomyelinase activity, includes infantile neuronopathic form; and type B, an adult chronic form without neurologic symptoms. In the second heterogeneous group called type C, neuro-visceral involvement is massive and lipid metabolism is affected.

The sphingomyelin that accumulates in the lysosomes of the Niemann-Pick disease cells is thought to arise from the degradation of cells and their organelles since it is a major component of all mammalian cell membranes, the myelin sheath and the erythrocyte stroma. In Niemann-Pick type C, the main lipid accumulated in patients cells is not sphingomyelin but cholesterol, however, there is a close relationship between sphingomyelin metabolism and cholesterol metabolism.

Sphingomyelinase is an acidic lysosomal hydrolase that catalyses the cleavage of sphingomyelin to phosphoryl choline and ceramide. In patients with Pick’s disease its activity is deficient in all lysosome containing tissues. Patients with type A, the infantile form have 0.7% of the normal sphingomyelinase activity with median values in the range of 0-1% , while in patients with adult onset neuronopathic or non-neuronopathic disease the activity range is 0-19% of the normal, with median values in several tissues from 2-8% . This enzyme defect explains the massive deposition of sphingomyelin in tissues of the reticuloendothelial systems. Patients with group A variant store sphingomyelin and other lipids in the brain in elevated amounts consistent with the neuropathic features, while patients with group B form do not appear to store sphingomyelin in nervous tissue. The reason for this significant difference in neurological involvement is not clear, but it may be consistent with the level of residual sphingomyelinase activity. Fibroblasts from normal individuals and patients with Niemann- Picks disease types A and B, synthesize the sphingomyelinase polypeptide having the same molecular mass 110 Kilo Daltons (KDa) and in the same degree of abundance, during further processing the 110 KDa polypeptide is processed to a lower molecular weight 84 KDa. Deficiency of sphingomyelinase is due to intragenic defects. Experiments done so far, suggest that specific defects could be small inframe deletions or additions or point mutations. The differences in clinical course of types A and B suggest that mutations are different. Sphingomyelinase follows the same intracellular targeting and post-translational processes as the majority of the lysosomal hydrolases. However, unlike any other enzyme there are two differently sized polypeptides which are differentially distributed in tissues. In tissues like brain the smaller polypeptide (molecular weight = 80 KDa) is found, whereas kidney contains both the polypeptides (110 and 80 KDa). There is no precise explanation for the occurrence of single differently sized polypeptides in some tissues, and the presence of both forms in tissues such as kidneys.

NIEMANN - PICK’S DISEASE TYPES A AND B

Two mammalian sphingomyelinase’s (sphingomyelin choline phosphohydrolase) have been described : an acid, ubiquitous lysosomal enzyme and neutral membranous enzyme located mostly in the brain. Pick’s disease is an inherited disorder characterized by lysosomal storage of sphingomyelin as a consequence of almost complete deficiency of acid sphingomyelinase: the neutral enzyme activity is present in the tissues of patients with this desease.

Cultured human skin fibroblasts are generally used for studying the metabolism of radioactively labeled sphingomyelin in intact living cells; in such studies, the lipid is introduced into the culture medium as liposomes or other undefined dispersions, and its subsequent intracellular utilization is investigated. Fluorescent derivatives of sphingomyelin have been synthesized and used for determining sphingomyelinase activity and for diagnosing Niemann-Pick disease in vitro. In some studies, fluorescent derivatives of sphingomyelin were incorporated into the constituents of fetal calf serum and administered to cultured human skin fibroblasts from normal individuals and from a patient with Niemann pick disease type A. Differences in intracellular degradation of the fluorescent sphingomyelin between the cells with a normal metabolism, and those lacking sphingomyelinase are then analyzed. A major advantage of the use of the fluorescent sphingomyelin is that the cellular uptake of this compound can be followed by direct measurement of the fluorescence intensity of the cells.

The uptake of fluorescent sphingomyelin by the normal cells and Niemann-Pick cells is approximately at the same rate. However, after 24-48 hours, about 35% of sphingomyelin is degraded in normal cells, but less than 5% is degraded in the Niemann-Pick cells. After as little as 4-8 hours, the fluorescent ceramide (product of sphingomyelin degradation) can be quantified with certainty using the normal cells. The rate of degradation is essentially linear with time, paralleling the rate of internalization of fluorescent sphingomyelin. In Niemann-Pick disease cells no amount of ceramide is detected in the 4-8 hours time zone.

The conclusion that the internalized sphingomyelin is present in the cell lysosomes and degraded there, stems from the fact that cultured skin fibroblasts which contain only the acid lysosomal sphingomyelinase, lack any significant hydrolysis of sphingomyelin in Niemann-Pick disease type A and B. Furthermore, electron microscopy can be used to demonstrate the accumulation of lipids within the lysosome.

CLINICAL: Niemann- Pick disease Type A & B is a lysosomal storage disorder characterized by a deficiency of sphingomyelinase resulting in the accumulation of sphingomyelin in tissues. The storage is particularly severe in brain, bone marrow, spleen, and lungs. Type A is associated with neurological symptoms while Type B is asymptomatic.

TREATMENT: The major objectives of treating Niemann-Pick disease are to: (a) provide a lifelong source of the missing enzyme, (b) to prevent the formation of antibodies to the enzyme, © to clear the abnormal storage material and prevent further accumulation. The above three objectives can be best met by a bone marrow transplant. 1987 was the first year in which an eight year old girl was treated by bone marrow transplantation from her brother. The major source of the transplanted sphingomyelinase is the leukocytes produced by the hemopoetic bone marrow (transplanted).

NIEMANN-PICK’S DISEASE TYPE C

Niemann-pick C disease is an autosomal recessive neurovisceral lipid storage disorder. This disorder was originally classified as a sphingolipidosis, largely because of similarities in its clinical, histochemical, and biochemical presentations with Type A and B Pick diseases. However, later studies showed that the extensive enzymatic deficiencies of sphingomyelinase that characterize the primary mutation of Niemann- Pick A and B disease were not noted in type C disease. Recent studies have revealed that Niemann- Pick C disease is itself now more appropriately defined as unesterified cholesterol lipidosis rather than as a sphingomyelin lipidosis. Critical to this revised classification are the recent new findings that Type C fibroblasts accumulate excess LDL cholesterol because of a deficient induction of cellular homeostatic responses that normally control and limit intracellular levels of unesterified cholesterol. Although the primary lesion of Type C has remained elusive, these deficient homeostatic responses have been shown to be associated with a block in the intracellular translocation of exogenously derived cholesterol.

Cellular cholesterol homeostasis involves a series of integrated responses that enable cells to maintain cholesterol levels within a critical range needed for optimal growth and development under environmental conditions that include both cholesterol excess and deprivation. Receptor mediated uptake and hydrolytic lysosomal processing of LDL in cultured fibroblasts derived from Niemann Pick C patients are associated with cellular homeostatic responses that are uniformly delayed. Lipoprotein uptake by the mutant cells leads to an excessive intracellular accumulation and storage of cholesterol primarily as unesterified sterol. The internalized cholesterol is also unable to initiate timely regulatory responses such as decreased synthesis of more endogenous cholesterol.

Following 24 hours of LDL uptake, total cellular unesterified cholesterol level rise in Niemann-Pick type C cells to levels substantially higher than those found in normal cells. However, in association with this internalization of excessive LDL cholesterol by these cells, there is also less enrichment of the plasma membrane with cholesterol when compared to normal cells. This documented excessive lysosomal storage and late intracellular mobilization of cholesterol in Type C cells suggest that LDL uptake by the Type C cells results in sequestration of exogenously derived cholesterol within a metabolically silent or trapped pool.

In mutant Niemann-Pick C cells the availability of cholesterol for interaction with acyl CoA: cholesterol acyltransferase is considered potentially latent because of possible hindrances which can be envisioned to block the translocation of cholesterol to catalytic site of acyl-CoA: cholesterol acyltransferase. The latency of cholesterol esterification in Niemann-Pick C cells strongly suggests the existence of a sequestered and metabolically unavailable pool of exogenously derived cholesterol, rather than any defect in the esterifying enzyme. Ample proof of this is obtained by a partial destruction of the intracellular organelles which results in esterification of the liberated cholesterol, hence defects in the esterifying enzyme can be ruled out.

The intracellular fate of lysosomal cholesterol and the normal cellular responses to cholesterol derived from lysosomes appears to be deficient. The internalized and sequestered cholesterol of Type C cells fails to initiate the prompt homeostatic responses that serve to control and to balance intracellular cholesterol levels in normal cells. The manifestations of the diseased cells are: (a) a tardy down-regulation of the LDL receptor; a delayed suppression o HMG CoA reductase [rate limiting enzyme in the cholesterol biosynthesis]; and a defective stimulation of acyl-CoA:cholesterol acyltransferase expression.

The molecular basis for the abnormal lysosomal sequestration of LDL derived cholesterol in Niemann-Pick C disease is not known. It is speculated that the components of cholesterol transport process from lysosomes to the endoplasmic reticulum may be involved.

TREATMENT: Niemann-Pick cells accumulate excess LDL cholesterol because of a deficient induction of cellular homeostatic responses that normally control and limit intracellular levels of unesterified cholesterol.

Investigations with Dimethylsulfoxide (DMSO) have recently been reported in several studies dealing with Pick cells. Addition of 2% DMSO to the culture medium, reveals that DMSO can partially correct deficient intracellular processing of LDL derived cholesterol and substantially reverses excess lysosomal storage of cholesterol in mutant cells. Current studies have shown that the addition of 2% DMSO to the culture medium alters the intracellular processing of LDL derived cholesterol by Niemann-Pick cells. The overall influence of DMSO represents a hypocholesterolemic effect that includes an accelerated induction by LDL of cellular homeostatic responses that have been shown to limit intracellular cholesterol levels. Accompanying this reduction of excess LDL cholesterol accumulation in mutant cells treated with DMSO is a parallel activity comprising of :(a) stimulation of cholesterol ester synthesis; (b) depression of de nova cholesterol synthesis; © down regulation of LDL receptor activity.

DMSO may alter the intracellular pathways of cholesterol processing in a variety of ways. DMSO may facilitate the intracellular mobilization and translocation of exogenously derived cholesterol. The fact that DMSO enhances the ability of LDL to induce its broad spectrum of reciprocal cellular responses, suggests that the drug affects and influences a central and presumably early stage of the LDL cholesterol processing pathway,;such as facilitating cholesterol translocation across membranes. DMSO has been reported to increase the transfer of unesterified cholesterol between membranes, and abnormal cholesterol processing has itself been characterized by deficient intracellular cholesterol transport and excessive lysosomal cholesterol accumulation. Therefore, DMSO seems to be a promising avenue in the treatment of Niemann-Pick type C diseased patients.

REFERENCES:

1. Mackie, Dwyer, Vanier, Merrick. Type C Niemann-Pick Disease: Dimethyl Sulfoxide Moderates Abnormal LDL Cholesterol Processing in Mutant Fibroblasts. (1989) Biochem. et Biophysica Acta, 1006, 219--226.

2. Callahan, Sphingomyelinase and Pick Disease. (1989) Biochem, Cell Biol.:67(11-12), 801-807.

3.Vellodi, Hobbs, O’Donnell. Treatment of Niemann - Pick Disease B by Allogenic Bone Marrow Transplantation. (1987) British Medical Journal: 295(6610):1375-1376.

4. Levade, Salvayre, Maret and Blazy. Endogenous and Exogenous Sources of Sphingomyelinin Pick’s Disease A & B. (1988) Inher. Metab. Dis.: 11, 151-157.

5. Maziere, M. Lageron, Polonovski. Alterations in Cholesterol Metabolism in Cultured Fibroblast From Patients with N-P type C. (1987) Inher. Metab. Dis.: 10, 339-346.

6.Liscum and Faust. Low Density Lipoprotein Mediated Suppression of Cholesterol Synthesis: and LDL Uptake is Defective in N-P Type C Fibroblasts. J. Biol. Chem.: 262 (17002-17007).

7. Blanchette, Sokol et. al. Type C Niemann- Pick disease. (1988) J. Biol. Chem. :263, 3411-3415.