abnormally low LDL

Having HDL (good cholesterol) levels of 60 mg/dL or above, is considered a "negative risk factor", since its presence allows the removal of one risk factor from the total.
Very low levels of LDL isn't normal and should be evaluated.
This result for LDL levels may indicate an inherited (genetic defect) disorders of lipoprotein metabolism like abetalipoproteinemia and familial hypobetalipoproteinemia, or it could be a secondary condition (caused by other disease) associated with liver disease, cancer, severe malnutrition.

Check this link:
http://patientrecruitment.nhlbi.nih.gov/LD L20.aspx

Best wishes!
Marija