Genetics of familial hypobetalipoproteinemia

Primary hypobetalipoproteinemias include three monogenic disorders: the relatively frequent codominant familial hypobetalipoproteinemia (FHBL), the rare recessive conditions abetalipoproteinemia (ABL) and chylomicron retention disease (CMRD). Approximately 50% of FHBL patients are carriers of mutations in the APOB gene, mostly causing the formation of truncated forms of ApoB. In some kindred, FHBL is linked to a locus on chromosome 3 (3p21), but the candidate gene is still unknown. Recently, a FHBL-like phenotype was observed in carriers of mutations of the proprotein convertase subtilisin/kexin type 9 (PCSK9) gene causing loss-of-function of the encoded protein, a proprotein convertase that regulates LDL-receptor number in the liver. Inactivation of the PCSK9 protein is associated with an increased number of LDL receptors and increased receptor-mediated hepatic uptake of plasma LDL. ABL and CMRD are due to mutations in the microsomal triglyceride transfer protein and Sar1-ADP-ribosylation GTPase 2 genes, which affect assembly and secretion of ApoB-containing lipoproteins. In this review we present the current information on the genetics and pathophysiology of these disorders affecting either the secretion or the catabolism of ApoB-containing lipoproteins.