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How is Niemann-Pick type C disease inherited?

We all have 22 pairs of non-sex chromosomes (known as autosomes), which carry genetic information such as hair color, eye color and other characteristics from one generation to the next (see Figure 1). Each of us has two copies of most genes (chromosomes are made up of many genes) — one copy from our mother and one from our father. Some inherited diseases are caused by a damaged or mutated gene being passed on from only one parent, for example, in Huntington’s disease. However, in the case of Niemann-Pick type C disease (NP-C), it is the gene that carries the instructions for making the NP-C protein, which is most important. You can only develop the disease if you inherit two copies of the defective or mutated gene - one from your mother and one from your father. NP-C is therefore known as an autosomal recessive disease.1 Both parents are called carriers, but as they have one normal copy of the affected gene, they will not present with the symptoms of NP-C.

Figure 1: A pair of chromosomes with genes represented by blue and white bands (one is expanded to show the double helix of DNA ([deoxyribonucleic acid])

A pair of chromosomes with genes represented by black and white bands (one is expanded to show the double helix of DNA ([deoxyribonucleic acid])

What is the probability of having a child with NP-C?

Figure 2 shows the probability of having a child with NP-C if both mother and father carry one copy of the affected gene.1 For each pregnancy, the probability of having a child with NP-C is one in four (25%), the probability that the child will be a carrier is one in two (50%) and the probability that the child will be genetically unaffected by NP-C is one in four (25%). In all cases, the probability of developing the disease is not affected by the sex of the child.

Figure 2: Inheritance in autosomal recessive disease1

The presence of the defective gene can only be determined by genetic testing.2

 

Frequently asked questions about genetic testing

What is genetic testing?

A genetic test can help identify if there is a change or a mutation in a particular gene or chromosome. The most common reason why someone might have a genetic test is if they have a relative with an inherited condition and they want to find out if they have the condition or are a carrier of the condition. Genetic testing is also used to confirm a diagnosis of an inherited condition3 and is usually offered to individuals and families affected by NP-C, including those that are carriers or at risk of being carriers of NP-C.1,2

How long does genetic testing take?

The NP-C gene is very big and, so far, over 300 mutations have been found that cause NP-C. It may therefore take several months to identify the gene faults. However, once these have been found in the affected person it is much quicker to check family members who may also wish to know if they are carriers.4

What procedures are involved with genetic testing?

It usually involves taking a blood or tissue sample.5

I have been identified as a carrier of NP-C, what does this mean?

As a carrier of NP-C, you have a defective NP-C gene on one of your chromosomes. As you also have one normal NP-C gene, you will not present with any symptoms. If your partner does not have a NP-C defective gene, there is a 50% probability of passing this carrier-status onto your children and none of your children will be affected by the disease. However, if your partner is also a carrier of NP-C, then there is a 50% probability your child will be a carrier and a 25% probability that he or she may have NP-C. There is also a 25% probability that your children will not be affected genetically by NP-C. However, as NP-C is such a rare disease, it is unlikely that your partner would be a carrier. If you have any questions about genetic testing or the genetic inheritance of NP-C then your healthcare professional will be able to answer these for you.

What is genetic counseling?

Genetic counseling aims to provide you with all the information you need to make a decision about whether you should have a genetic test.3 It usually involves discussion with a trained professional and includes information on:3,5

  • The risk of inheriting or passing on a genetic disease
  • The implications of testing positive for the condition, including the psychological impact
  • Whether to inform relatives of your decision to be tested
  • The usual pattern of progression of the disease you are being tested for and its potential treatments.

Genetic counselors are trained to help people with the emotions they might be experiencing and are sensitive to the fact that a diagnosis can hold a different meaning to different people.

 

References

  1. Wraith JE, Imrie J. Understanding Niemann-Pick disease type C and its potential treatment. UK Blackwell Publishing, 2007.
  2. Patterson MC, Hendriksz CJ, Walterfang M, et al, on behalf of the NP-C Guidelines Working Group. Recommendations for the diagnosis and management of Niemann–Pick disease type C: An update. Mol Genet Metab 2012. 106(3):330-344.
  3. EuroGentest. What is a Genetic Test? Information for Patients and Families http://www.eurogentest.org/blocks/leaflets/pdf/english/genetic_test.pdf Last accessed September 2012.
  4. Vanier MT. Niemann-Pick disease Type C. Orphanet Journal of Rare Diseases. 2010;5:16.
  5. Patient UK. Genetic Testing, http://www.patient.co.uk/health/Genetic-Testing.htm Last accessed September 2012.