A COMPOUND HETEROZYGOTE FOR HEPATIC LIPASE GENE-MUTATIONS LEU334-]PHEAND THR383-]MET - CORRELATION BETWEEN HEPATIC LIPASE ACTIVITY AND PHENOTYPIC-EXPRESSION

We have characterized the molecular basis for familial hepatic lipase (HL) deficiency in a Finnish family. In the propositus, the HL deficie ncy results from compound heterozygosity for two rare HL gene mutation s, a previously unknown missense mutation designated L334F and the pre viously reported T383M mutation. These mutations were introduced into human HL cDNA by site-directed mutagenesis and the constructs expresse d in COS-1 cells. In the homogenate of COS-I cell transfected with the L334F mutant cDNA, a high amount of inactive protein accumulated. In the media of L334F transfected cells, 30% of the wild type activity an d 80% of wild type mass were detected. The lysates of COS-1 cells tran sfected with the T383M mutant cDNA contained 39% of wild type HL activ ity and 34% of wild type HL mass. In the media of COS-I cells transfec ted with the T383M cDNA construct, 50% of wild type HL mass but only 6 % of wild type activity was present. The single amino acid substitutio ns present in L334F and T383M are therefore sufficient to severely aff ect the HL enzyme. These defects explain the HL-deficient phenotype of the individual carrying the two mutations. The lipoprotein phenotype associated with compound heterozy- gosity for L334F and T383M mutation s is characterized by a slight increase in the buoyant low density lip oprotein (LDL) fraction and an increase in the light high density lipo protein (HDL) fractions, HDL(2a) and HDL(2b). These results demonstrat e that lipoprotein changes occurring in HL deficiency are difficult to identify and support the hypothesis that HL is important in HDL remod eling and metabolism in vivo.-Knudsen, P., M. Antikainen, S. Ehnholm, M. UusiOukari, H. Tenkanen, S. Lahdenpera, J. Kahri, M. Tilly-Kiesi, A . Bensadoun, M-R. Taskinen, and C. Ehnholm. A compound heterozygote fo r hepatic lipase gene mutations Leu334 --> Phe and Thr383 --> Met: cor relation between hepatic lipase activity and phenotypic expression.