A patient was diagnosed with an isolated increase in LDL. His father and brother had the same disease with increased cholesterol. The likely diagnosis is
LDL receptor mutation 

Isolated increase in LDL with positive family history suggests the diagnosis of familial hypercholesterolemia.
Familial hypercholesterolemia (type IIa) is due to deficiency of functional LDL receptors as a result of different types of mutations.
Familial type III hyperlipoproteinemia
Abetalipoproteinemia
Familial lipoprotein lipase deficiency (type 1)
LDL receptor mutation