Cholesterol is an essential component of all animal cells, but too much cholesterol in the
blood is a high-risk factor for cardiovascular diseases such as heart attacks and strokes.
Cholesterol levels are measured in a blood test known as lipid panel or lipid
profile. This test typically reports: total cholesterol; low-density lipoprotein, LDL,
also known as “bad” cholesterol; high-density lipoprotein, HDL, or “good” cholesterol;
and triglycerides. A desirable profile includes less than 200mg/dL of total cholesterol,
with less than 100mg/dL of LDL and more than 40mg/dL of HDL.
The body obtains cholesterol in 2 ways: from foods of animal origin, and its own endogenous
production. Usually, cholesterol levels are kept in check by a negative feedback
control. Low levels of intracellular cholesterol induce its own production,
while high cholesterol levels inhibit it. It is noteworthy, however, that this regulation
applies to the concentration of cholesterol inside the cells, not in the blood. The high
prevalence of high blood cholesterol worldwide suggests that this control mechanism is NOT
sufficient to maintain healthy cholesterol levels when challenged by a number of factors,
including poor diet, lack of exercise, smoking, obesity, diabetes, and aging.
In addition, some people have an inherited condition called “familial hypercholesterolemia”,
FH, that causes very high levels of LDL, the “bad“cholesterol, at a young age. Left untreated,
patients are likely to have heart attacks in their 40s or 50s. LDL is basically a vehicle
that transports cholesterol from the liver to peripheral cells so it can be used in the
cell membrane. Peripheral cells take up LDL by endocytosis, using their LDL receptor, which
binds to a protein ligand on LDL surface. Most cases of FH are caused by a mutation in the LDL
receptor gene. A defective LDL receptor reduces LDL uptake, leaving more LDL in the circulation
while the cells are deficient in cholesterol. Low intracellular cholesterol levels induce
further production of endogenous cholesterol in the liver, eventually causing even higher levels
of circulating LDL. One copy of the mutated gene is enough to cause high cholesterol. The condition
is therefore inherited in an autosomal dominant manner. A parent with an altered gene has a 50%
chance of passing it to a child. If both parents have FH, each child has a 50% chance of having FH,
a 25% chance of not having FH, and a 25% chance of having two copies of the mutated gene,
called homozygous FH. Without treatment, homozygous FH patients may have heart attacks in
their 20s and may not survive past the age of 30. Treatments for high cholesterol must start with
life style changes such as healthy diets and physical exercise. On top of that, some people
may require medications to lower cholesterol. These drugs inhibit cholesterol production,
intestinal absorption, or reabsorption in the form of bile. Homozygous FH patients usually require
more drastic treatment measures which include a procedure called LDL apheresis. In this procedure,
the blood is diverted through a filtration device where LDL is removed before the remaining plasma
and blood cells are returned to the body. The procedure is repeated weekly or biweekly.