Niemann-Pick Disease

Etiology

Niemann-Pick disease (NPD) is inherited in an autosomal recessive pattern, which means both copies of the gene must have mutations for the manifestation of the disease. NPD types A and B are caused by missense mutations in the sphingomyelin phosphodiesterase 1 (SMPD1) gene. Over 180 mutations in SMPD1 have been identified. NPD type C is caused by mutations in NPC1 (located on chromosome 18) and NPC2 (located on chromosome 14) genes. The mutations in these genes lead to abnormal or defective formation of proteins, which impair the movement of lipids out of the cells, leading to their accumulation within the cells.[3] 

Epidemiology

Niemann-Pick disease (NPD) types A and B affect 1 in 250,000 individuals. The prevalence is high in Ashkenazi Jewish descent, where it affects 1 in 40,000 individuals. NPD type C affects 1 in 150,000 persons. Type C is more prevalent in French-Acadian descent in Nova Scotia.[4]

Pathophysiology

Niemann-Pick disease types A and B are caused by mutations in the sphingomyelin phosphodiesterase 1 (SMPD1) gene, leading to a strongly decreased activity of acid sphingomyelinase (ASM). The enzyme ASM is mainly present in lysosomes and converts sphingomyelin (SM) to ceramide and phosphocholine. In ASMD, SM and its precursor lipids accumulate in lysosomes and cause cellular damage. There are over 180 mutations of the SMPD1 gene some with residual ASM activity up to 30%. Due to a dramatic reduction of the protein half-life, the condition may phenotypically be type A.  Allelic heterogeneity is responsible for most of the variability in severity between types A and B. The mutations can be missense, frameshift, nonsense, and frame deletions. The predominant mutation varies by region. 

Niemann-Pick disease type C (NPC) is further classified as type C1 or type C2 based on the pathogenic mutations in the NPC1 or NPC2 genes, respectively. NPC1 is the predominant subtype affecting about 95% of the patient population with over 30 different sequence alterations detected. NPC1 and NPC2 proteins are present in late endosomes and lysosomes and are involved in the transport and intracellular mobilization of cholesterol and sterols. The loss of function of NPC1 and/or NPC2 proteins blocks cholesterol egress from lysosomes, resulting in an excessive build-up of cholesterol in lysosomes. Consequently, toxic cholesterol accumulation results in cellular and organ damage.[5][6]

Histopathology

Due to the ineffective transport system, the affected cells become enlarged, sometimes as big as 90 micrometers in diameter, due to the accumulation of sphingomyelin and cholesterol. Histology shows lipid-laden macrophages in the marrow, also called foam-cells. Numerous small vacuoles of relatively uniform size are created, which give the cytoplasm a foamy appearance. Electron microscopy shows electron-opaque, concentrically laminated inclusions within the macrophage cytoplasm.

History and Physical

Nieman-Pick disease (NPD) type A presents in the first few months of life, usually before three months of age, as hepatosplenomegaly and growth retardation. By the age of one year, neurological symptoms appear as psychomotor retardation and regression of developmental milestones. All patients with NPD type A have a classical eye finding called cherry-red spot. These children usually do not survive past early childhood. NPD type B presents in mid-childhood and is not as severe as type A. These patients have hepatosplenomegaly, interstitial lung disease (ILD), causing recurrent lung infections, thrombocytopenia, and slowed bone growth. About one-third of patients with type B NPD have the cherry-red spot and neurological symptoms. NPD type C can present at any age but usually affects children. Affected individuals can have ataxia, dystonia, supranuclear gaze palsy (SNGP), dysphonia, dysphagia, and severe liver and lung disease.

The following findings can be expected while examining the body systems.

Gastrointestinal

• Hepatomegaly
• Splenomegaly

Pulmonary

• Interstitial lung disease
• Decreased diffusion capacity
• Recurrent lung infections

Integumentary

• Jaundice

Cardiovascular

• Thrombocytopenia
• Hypercholesterolemia

Rheumatologic

• Impaired growth of long bones
• Slowed mineralization of bones
• Coxa vara

Ocular

• Bright cherry-red fovea centralis surrounded by a white or pale macula
• Corneal opacification
• Brown discoloration of the anterior lens capsule

Neurologic

• Ataxia
• Dystonia
• Dysphagia
• Dysphonia
• Developmental delay and/or regression
• Mental retardation
• Peripheral neuropathy
• Gelastic catatonia
• SNGP
• Tremors[1][7][8]

Evaluation

For suspected NPD type A pr B, a blood sample is drawn, and the activity of ASM is measured in leukocytes. In the case of low enzyme activity, additional gene testing can further evaluate the disease. 

For NPD type C, a skin biopsy is taken and stained with a special stain, 'filipin,' to measure the enzyme activity. DNA testing can be done to look for genes that cause type C disease.[9]

Once the diagnosis is confirmed, special attention should be paid to look for disease spread and manifestations in the following systems.

• Liver
  • Liver enzymes should be measured periodically. Liver elastography or liver biopsy should be done in case of severe liver disease.
• Pulmonary
  • Spirometry should be done periodically. High-resolution computed tomography (HRCT) is done for interstitial lung disease (ILD). 
• Hematologic
  • Measure platelet counts and spleen volume.
• Cardiovascular
  • Measure HDL-cholesterol and LDL-cholesterol.
• Neurological
  • A complete neurological exam should be performed at every visit. 
• Eye
  • Fundoscopy to look for the cherry-red spot on the macula. 
• Exercise Intolerance
  • Spirometry and exercise intolerance test.
• Pain and Fatigue
  • A questionnaire should be used for grading.
• Severity of Disease
  • Measure the quantity of SM and its derivatives, macrophage markers, and oxysterols.[10][11]

Treatment / Management

For type A and B Niemann-Pick disease (NPD), there is no cure. Supportive care is the mainstay of treatment. Physicians try to keep low blood lipids levels with statins, and liver functions are monitored. If thrombocytopenia leads to bleeding episodes, transfusion of blood products may be required. For patients with ILD, oxygen is provided. Organ transplant has also been tried but with limited success. Enzyme replacement therapies and gene therapies are undergoing trials and may become the mainstay of treatment in the future.[12]

For type C disease, supportive care is also the mainstay of treatment. Physical therapy is provided for neurological symptoms. Pain is managed with analgesics. Miglustat is a glucosylceramide synthase inhibitor and helps in Niemann-Pick disease and Gaucher disease by decreasing glucocerebroside production. It is approved in Europe, Canada, and Japan but is not yet approved in the United States.[13] 

Differential Diagnosis

Other lysosomal storage diseases should be kept in the differentials, especially Gaucher disease, Tay-Sachs disease, and Metachromatic leukodystrophy. Gaucher disease also presents with hepatosplenomegaly and cytopenias, but bone pain and lesions are more prominent. The deficient enzyme in Gaucher disease is glucocerebrosidase, which leads to the accumulation of glucocerebroside inside cells instead of sphingomyelin, which is found in Niemann-Pick disease. Tay-Sachs disease, although it does not present with hepatosplenomegaly, neurodegeneration, developmental delay, and cherry-red spot on the macula, are prominent features. The deficient enzyme here is hexosaminidase A which causes a buildup of GM2 gangliosides. Metachromatic leukodystrophy causes central and peripheral demyelination and can manifest as ataxia or other neurological symptoms.[14] 

Apart from these, other diseases affecting the liver and brain should be kept in mind based on the initial presentation.

Prognosis

Type A

It is almost always fatal, and affected children are unlikely to live beyond 4 years of age.

Type B

These children have a slightly better prognosis than type A and may live till late childhood or early adulthood. But they develop many complications from the disease, so the life quality is not so good.

Type C

The prognosis depends on the time of the initial presentation. If it affects in infancy, the chances are very poor for survival beyond 5 years of age. If it affects after 5 years, then patients may live to the age of 20 years. But each patient has a slightly different outlook depending on the severity and presentation of the disease.[15]

Complications

Niemann-Pick disease is a progressive disease, and quite often, complications develop with time. 

• The initial involvement of the liver can transform into fulminant hepatic failure.
• Deterioration of the lungs can result in respiratory insufficiency.
• Progressive neurodegeneration can cause dementia, seizures, and schizophrenia-like psychosis.
• Severe thrombocytopenia can result in internal or external bleeding.
• Coronary artery and valvular heart disease
• Bones become deformed, causing enlarged bone marrow cavities, thinned cortical bone, or coxa vara.[16]