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ASMD Niemann-Pick Disease Type A

What is Niemann-Pick Disease Type A?

Niemann-Pick Disease Type A (NPA) and Niemann-Pick Disease Type B (NPB) were once thought to be separate diseases, but are now understood to be opposite ends of a spectrum of the same disease. They are both caused by a deficiency of the enzyme acid sphingomyelinase (ASM) and therefore are known as Acid Sphingomyelinase Deficiency (ASMD) Niemann-Pick disease. Many variations exist within this spectrum, in terms of clinical symptoms and rate of progression.

NPA is a rare inherited lysosomal storage disorder in which harmful quantities of a fatty substance called sphingomyelin build up in the body’s cells and organs.  In NPB this build up mainly occurs in the liver, spleen and lungs. However, in NPA build up also occurs in the brain leading to aggressive neurological problems. A small number of patients may be described as having A/B variant, falling in the middle of the spectrum and exhibiting neurological problems which may become more apparent over time.

It is inherited when two copies of a faulty gene (a mutation) are passed on to a child. In every pregnancy of a couple who each carry a copy of the faulty Niemann-Pick gene, there is a 1 in 4 chance (25%) that their child will have Niemann-Pick disease. This is known as autosomal recessive inheritance.

There are three common mutations that account for NPA in the Ashkenazi Jewish population, and the estimated incidence is ~1 in 40,000. The incidence in other populations is not known, but it is considered extremely rare and estimated to be about 1 in 10 million.

Sadly, in most cases of NPA life expectancy rarely exceeds three years of age.

Cause

Niemann-Pick Disease Type A (NPA) is caused by a severe deficiency of the enzyme acid sphingomyelinase. This is required to break down a fatty substance called sphingomyelin.  Failure to break down sphingomyelin causes an accumulation within brain cells causing cell death and the enlargement of the liver and spleen.

Diagnosis

Niemann-Pick Disease Type A (NPA) is diagnosed by measuring the level of the enzyme acid sphingomyelinase, in the white blood cells. This can be done by testing a small blood sample. The diagnosis is usually confirmed by DNA sequencing to identify mutations.

The enzyme deficiency at the cause of NPA arises from mutations (or gene faults) in a gene on chromosome 11. An affected person will have inherited two faulty genes, one on each chromosome 11.

Symptoms

Symptoms of Niemann-Pick Disease Type A (NPA) develop within the first few months of life and may include a combination of:

  • Feeding difficulties
  • Prolonged jaundice
  • Swelling of the abdomen from enlargement of the liver and spleen (usually occurs around 3-6 months of age)
  • Progressive loss of early motor skills
  • Failure to thrive
  • A cherry red spot in the eye (not noticeable to parents)
  • (Generally) a very rapid decline leading to death by two to five years of age

Treatment

There are no specific treatment, or cure, for Niemann-Pick Disease Type A (NPA). However, patients will benefit from palliative treatments (individual medications that will help to treat the symptoms related to the condition).

 

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