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ASMD Niemann-Pick Disease Type A

What is Niemann-Pick Disease Type A?

Niemann-Pick Disease Type A (NP-A) and Niemann-Pick Disease Type B (NP-B) 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.

NP-A 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 NP-B this build up mainly occurs in the liver, spleen and lungs. However, in NP-A 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 NP-A 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 NP-A life expectancy rarely exceeds three years of age.

Cause

Niemann-Pick Disease Type A (NP-A) 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 (NP-A) 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 NP-A 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 (NP-A) 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 (NP-A). However, patients will benefit from palliative treatments (individual medications that will help to treat the symptoms related to the condition).

 

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