Cells need to be able to control how much cholesterol they have (cholesterol, notably, makes up about 25% of the cell membrane). They can either synthesize it themselves, or import it from the blood.

The system for importing cholesterol can't run continuously or else it might cram the cell full of cholesterol, so there needs to be a way of turning it off (or at least slowing it down) when levels are satisfactory. PCSK9, the gene that they targeted, does just that.

So it's not a "gene for high cholesterol" per se - but if you delete it, cells will import more cholesterol, thus lowering the levels in the blood. I really don't know much about this system, but I imagine you'd have to combine this modification with a low-cholesterol diet or else it might have an adverse effect on the modified cells.