By definition, Familial Hypercholesterolemia is a genetic defect of the receptor responsible for clearing cholesterol-containing droplets from the blood.

I understand that, but my point was that it's in principle possible that the exact effects of a particular genetic disease are hard to discover. Serum cholesterol is easy to measure, so it could be just the easiest to notice sign of a more complex metabolic modification induced by the specific genes. I doubt we are at a point where we can look at a gene and say exactly what it affects in the entire body.