Type 3 hyperlipidaemia
Type 3 hyperlipidaemia, or Type 3 for short, is a rare inherited condition. It is similar to the more common condition familial combined hyperlipidaemia (FCH) in that both cause higher levels of cholesterol and triglycerides – types of blood fats.
Type 3 is also known as dysbetalipoproteinaemia and remnant hyperlipidaemia. About 1 in 5,000 to 1 in 10,000 people are thought to have it. In genetic terms, it is known as an autosomal recessive condition.
How is Type 3 diagnosed?
Your cholesterol and triglycerides can be tested with a blood test.
In people with Type 3:
- total cholesterol levels are usually between 7mmol/L and 10mmol/L (healthy levels are below 5 mmol/L)
- triglycerides are usually between 4mmol/L and 8mmol/L (healthy levels are below 2.3 mmol/L).
In more severe cases, Type 3 can lead to the build up of fatty deposits in the creases of the hands (called palmer creases) and on the elbows and knees (known as tuberoeruptive xanthomas). Your doctor can look for these signs to help make a diagnosis.
What causes Type 3?
After a meal, triglycerides are carried from the gut around the body to be used for energy or stored for later. They can't travel loosely in the blood, so they are carried in parcels made of fat and protein known as chylomicrons.
Once the fat has been removed, the chylomicrons shrink to what are known as 'chylomicron remnants'. These are very 'sticky', meaning they easily build up in the lining of the blood vessels, but they're normally removed from the blood quickly by the liver.
What is ApoE?
Apolipoprotein E (ApoE) is the protein which allows the liver cells to recognise the chylomicron remnants so they can be removed from the blood – the liver cells have receptors on them which bind to ApoE. Type 3 happens when there is a problem with ApoE.
There are three common forms of ApoE
These are known as ApoE2, ApoE3 and ApoE4, and the type you have is determined by your genes.
Problems can occur with ApoE2 and ApoE4.
- ApoE2 causes most cases of Type 3. It has a "shallow handle" which makes it harder for the receptors in the liver to bind to them, and it binds less readily than ApoE3 and ApoE4. If you inherit the gene for Apo2 from both parents, you may have more remnant chylomicrons in the blood. ApoE2 does not normally cause Type 3 without another underlying cause such as type 2 diabetes or insulin resistance and being very overweight.
- ApoE3 is the "normal" form. It is easily recognised by the liver which means the chylomicron remnants can be removed from the blood quickly.
- ApoE4 “holds on” to the receptors in the liver, which means the liver can’t bind to other chylomicron remnants so they will build up in the blood.
What are the long term effects of Type 3?
If Type 3 is left untreated, or is not brought under control, fats can build up in the blood vessel walls. This causes a very high risk of diseases of the heart and blood vessels such as coronary heart disease and peripheral vascular disease, where the arteries supplying legs, arms and major organs become narrowed, reducing the blood flow.
How is Type 3 treated?
If your doctor thinks you might have Type 3, they should refer you to a lipid clinic for an assessment by a specialist.
Your specialist should give you blood tests to check your cholesterol, triglycerides and ApoB levels. Genetic testing is also sometimes available to find out which type of ApoE you have.
Similarly to FCH, treatment for Type 3 should be with a statin together with a fibrate. This will help improve how ApoE2 works which will lead to healthy cholesterol and triglyceride levels for most people. With treatment, chylomicron remnants in the blood will fall and the fatty deposits in the skin should disappear.
Type 3 needs to be treated with medicines, but a healthy diet and lifestyle will help keep your heart and blood vessels healthy.