Niemann-Pick disease types A and B (NORD)

A 5-month-old girl undergoes evaluation for failure to thrive. The parents are concerned that she is no longer able to sit or roll over on her own, despite being able to a few weeks ago. She was born at term with a birth weight of 2.5-kg (5.5-lb). Her current height and weight are below the 10th percentile, while at her previous visit 2 months ago, she was in the 40th percentile for both. Vitals are within normal limits. Physical examination reveals marked hypotonia throughout, with decreased deep tendon reflexes noted bilaterally in the upper and lower extremities. Abdominal examination is notable for hepatosplenomegaly. Fundoscopic examination is shown below:

By Achim Fieß, Ömer Cal, Stephan Kehrein, Sven Halstenberg, Inez Frisch, Ulrich Helmut Steinhorst
Reproduced from Wikimedia Commons" style="max-height: 25px; max-width: 25px;">Wikimedia Commons

If further testing is performed, which of the following enzymes is most likely to be deficient in this patient?

Niemann-Pick disease type A and type B, or NPD-A and NPD-B, which are subtypes of acid sphingomyelinase or ASM deficiency, are rare, genetically inherited conditions characterized by the inability to break down a fat called sphingomyelin due to a deficiency of the enzyme, acid sphingomyelinase.

There’s also Niemann-Pick disease type C, which is known to be caused by mutations in the genes NPC1 and NPC2, and is therefore considered to be distinct from types A and B.

Sphingomyelin is a fat that's included in the membrane of many different cells.

When cells become old or damaged, they are often phagocytized, or eaten, by macrophages, which are cells of the immune system.

They contain organelles called lysosomes that are said to function as recycling centers because they break down large, potentially harmful substances to be reused by the body.

They break down sphingomyelin by using an enzyme called acid sphingomyelinase, which is a product of the sphingomyelin phosphodiesterase 1, or SMPD1 gene.

In Niemann-Pick disease types A and B, there’s a mutation in the SMPD1 gene that causes a defect in the production of sphingomyelinase, leading to an inability to break down sphingomyelin.

In NPD-A there’s almost a complete absence of sphingomyelinase activity, while NPD-B has some residual sphingomyelinase activity remaining.

While the mechanism isn’t completely understood, sphingomyelin primarily accumulates in the lysosomes of macrophages, which travel throughout the body and cause damage in multiple organs and tissues.

The macrophages develop a characteristic lipid-laden appearance under microscopes and are called “foam cells.”

Sphingomyelin can also build up in other cell types in the body, reflecting impaired intracellular recycling of membranes and damaged organelles in lysosomes due to sphingomyelinase deficiency.

Signs and symptoms of NPD-A present early in life, and are usually life-threatening.

These include enlargement of the liver and/or spleen, jaundice, feeding difficulties, and progressive loss of reflexes and muscle tone.

Infants also often develop a “cherry red spot” in the eye that affects the macula, which corresponds with a decrease in central vision.

Key Takeaways

Niemann-Pick disease (NPD) type A and type B, are rare inherited conditions characterized by the inability to break down sphingomyelin, due to a deficiency of the enzyme acid sphingomyelinase. Niemann-Pick disease type A and type B result from SMPD1 gene mutation, which normally encodes to sphingomyelinase enzyme.

NPD-A symptoms present early in life and may include hepatosplenomegaly, jaundice, feeding difficulties, and progressive loss of reflexes and muscle tone. It is also often associated with a cherry red spot � in the eye, which affects the macula and impairs central vision. Usually, NPD-A becomes fatal by the age of 3 years old.

On the other hand, NPD-B represents a less severe condition that typically does not include neurologic involvement, and can develop at any time in life. Common NPD-B symptoms include progressive splenomegaly, which causes low serum platelet and white blood cell levels, high cholesterol, and declining lung function. There is no cure for NPA and NPB, and treatment is supportive and focuses on managing symptoms.

Niemann-Pick disease types A and B (NORD)

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