Abstract 1198: Non-truncating APOB Gene Mutations that Cause Familial Hypobetalipoproteinemia Decrease Apolipoprotein B-containing Lipoprotein Secretion in Stably Transfected McA-RH7777 Cells
Familial hypobetalipoproteinemia (FHBL), an autosomal co-dominant disorder, is characterized by decreased plasma concentrations (<5th percentile for age and sex) of apolipoprotein B-containing lipoproteins. In most cases, FHBL is associated with mutations in the APOB gene, resulting in translation of carboxyl-terminally truncated apoB proteins. Three years ago, we discovered the first missense mutation, namely R463W, in an extended FHBL kindred. Expression of a recombinant human apoB100 carrying the RW mutation (designated apoB100RW) in stably transfected rat hepatoma McA-RH7777 cells resulted in markedly decreased secretion efficiency as compared with the wild type apoB100. Expression of apoB100RW in these cells had no effect on the secretion of endogenous rat apoB100. Recently, we identified a second APOB gene mutation, namely L343V, in another extended FHBL kindred. Heterozygotes for L343V (n=10) had a mean plasma apoB concentration of 0.31 vs. 0.80 g/L for unaffected family members (n=22). The effect of the L343V mutation on apoB secretion was tested using recombinant human apoB48 that was stably expressed in McA-RH7777 cells. The mutant B48LV was secreted poorly compared to its wild type counterpart; pulse-chase analysis showed that secretion efficiency of 35S-B48LV was 20% lower than wild-type B48, compared with a 40% reduction found with B48RW. Moreover, similarly reduced secretion efficiency was also observed with the mutant B17LV, an apoB truncate that has limited ability to assemble lipids. Sequence analysis reveals that Leu343, like Arg463, is conserved among mammals and both residues are located within the predicted alpha-helical domain at the amino-terminus of apoB. These results suggest that proper folding of the alpha-helical domain of apoB, independent of lipid association, may play a key role in governing efficient secretion of apoB.