WHAT IS THE CAUSE OF NPC?
If a family is affected by Niemann-Pick Type C disease apparently out of the blue, there is no way they could have prepared for the shock and no measure of prevention, which could have been planned. The tragedy is often compounded by the delayed disease onset. Children may not show symptoms for many years and diagnosis may itself take a number of years. Other children may have been born in this period and will each have a 25% risk of having the disease. There are, sadly, too many instances of families with more than one child affected.
Further complications may arise where symptoms and diagnosis of the disease in an individual are delayed to adulthood and they have families of their own. The effect is that of a time bomb. Once a diagnosis has been obtained in a family then they will be acutely aware that they are at risk of having further children affected. In addition other members of the family will be concerned and wonder if they too, could be affected.
What is it caused by?
In order to appreciate the cause of the disease, some basic understanding of the biology of the cell is needed. This is a complicated subject but it is worth investing a little time to learn about the basic components of the cell and gain a general understanding into how inheritance works.
How does one become affected?
Niemann-Pick Type C is an inherited disease.
It is inherited in an autosomal recessive manner. In simple terms, this means that both parents have to be carriers of the faulty gene. In each pregnancy of a carrier couple, there is a 25% chance that they will pass on this gene mutation to their child.
What are autosomal recessive?
All types of NPD are autosomal recessive.
Autosomes are the non-sex related chromosomes, and recessive indicates that the effects of possessing a single copy of a disease-causing gene are hidden. With a recessive condition, a person may be a carrier of a disease gene, but with no noticeable effect in their everyday lives and health.
Do both parents have to be carriers?
A positive diagnosis of say Niemann-Pick Type C disease in a child means that each of the parents is a carrier of a disease-causing mutation on this gene.
The mutations may be identical from each parent and the child will be referred to as homozygous for this mutation.
In other instances the parental mutations will be different yet disease-causing and, the child will be referred to as heterozygous for the mutations.
Niemann-Pick Disease Type C (NP-C) occurs when 2 copies of the mutated gene (one from each parent) are passed along to their child. NP-C is divided into two subtypes, NP-C1 and NP-C2, as each is caused by a different gene mutation. Approximately 95%of NP-C cases are caused by genetic mutations in the NPC1 gene, 4% caused by mutations in the NPC2 gene, and about 1% have mutations in both NPC1 and NPC2 genes
How does NPC affect patients?
NPC leads to a build up of cholesterol and other fats (glycosphingolipids) particularly in the Central Nervous System. Increasingly high toxic quantities of unesterified cholesterol, causes damage in cells and tissues.
How many people does NPC affect?
It is believed that NPC arises in 1 case per every 90,000 births.
However, it is considered highly likely that this is an underestimate due to a mixture of factors – chiefly, failure to recognise the clinical characteristics and a previous lack of definitive diagnostic tests.
NPC is present in an individual from the moment of conception but is not necessarily apparent. The symptoms are often related to the age when the disease takes hold. Yet the age of onset is an extremely variable factor, ranging from birth to old age. It is possible that babies have died prior to birth and been undiagnosed or diagnosed in a ‘blanket’ manner, eg liver disease. Similarly, people with very late onset may be incorrectly diagnosed with a disease such as Alzheimer’s. Since awareness of the disease has increased and diagnosis has been improved, the age profile of the disease is beginning to look different to that of a few years ago. What was once considered to be a childhood disease is now, equally, beginning to look like a disease of adults.