Lipoproteinemia

Abnormalities of lipoprotein metabolism occur at the sites of production or utilization of lipoproteins causing hyper or hypolipoproteinemia. These disorders may be due to a defect in apolipoproteins, enzyme or cellular receptors.

The classification of disorders of lipid transport is based on concentrations of the major classes of lipoproteins in plasma.

Hyperlipoproteinemia:

       Hyperlipoproteinemia are detected by measuring plasma cholesterol and triacylglycerols after 12- 14 hours fasting. They can be primary or secondary.

A) Primary hyperlipoproteinemia: are due to a single gene defect or a combination of genetic defects. Hyperlipoproteinemia are 5 types which can be characterized by:

1.  Hupercholesterolaemia (Type II a ).
2. Hypertriglyceridemia (type I, IV, V ).
3. Combined form (type IIb. III).

Familial Hypercholesterolemia (type II a ):

It is an inborn error of metabolism due to a defect in LDL receptor. Defect in receptor function can be due to:

1. The receptor may not be synthesized at all.
2. The receptor may fail to bind LDL.
3. Receptor may fail to cluster in coated pits.
4. The receptor may fail to release LDL in the endosome.

     In individuals who are homozygous for the gene defects, the receptors are few or absent. They rarely syrvive the second decade of life due to ischaemic heart disease. Heterozygous persons have half the normal number of LDL receptors.

In all cases, blood cholesterol levels are very high and affected individuals develop atherosclerosis in early childhood. Endogenous cholesterol synthesis continues despite high serum cholesterol because extracellular cholesterol cannot enter cells to regulate intracellular cholesterol synthesis.

B) Secondary hyperlipoproteinemias:

 are due to diabetes mellitus, hypothyrodism, nephritic syndrome, alcohol abuse and intake of oral contraceptives.

Hypolipoproteinemias:

• Abetahypolipoproteinemia: A genetic defect prevents synthesis of apo B.

     Lipoprotein containing apo B are affected (chylomicrons, LDL and VLDL).

There is malabsorption of fat. Serum levels of cholesterol, triglycerides and phospholipids are extremely low. Blindness may occur as a result of degenerative changes in the retina.

• Alpha –Lipoprotein Deficiency (Tangier disease): It is a relatively benign disease. It is due to accelerated catabolism of apo A1 and HDL. Cholesterol esters are accumulated in tissues, which is manifested are large orange yellow tonsils, muscles atrophy, hepato- and spleenomegaly. Atherosclerosis is common.