Identification of two separate allelic mutations in the lipoprotein lipase gene of a patient with the familial hyperchylomicronemia syndrome

The molecular defects resulting in a deficiency of lipoprotein lipase activity in a patient with the familial hyperchylomicronemia syndrome have been identified. Increased lipoprotein lipase mass but undetectable lipoprotein lipase activity in the patient's post-heparin plasma indicate the presence of an inactive enzyme. No major gene rearrangements were identified by Southern blot analysis of the patient's lipoprotein lipase gene and Northern blot hybridization revealed an lipoprotein lipase mRNA of normal size. Sequence analysis of polymerase chain reaction-amplified lipoprotein lipase cDNA identified two separate allelic mutations. A T to C transition at nucleotide 836 results in the substitution of Ile194, located near the putative interfacial recognition site of lipoprotein lipase, to a Thr. A G to A mutation at base 983 leads to the substitution of a His for Arg243 and the loss of a HhaI restriction enzyme site. Arg243 is near His241, which has been postulated to be part of the catalytic triad of lipoprotein lipase. Direct sequencing of amplified cDNA and digestion with HhaI established that the proband is a compound heterozygote for each base substitution. Transient expression of each of the mutant lipoprotein lipase cDNAs in human embryonal kidney-293 cells resulted in the synthesis of enzymically inactive proteins, establishing the functional significance of the mutations. We conclude that the Ile194 to Thr194 and Arg243 to His243 substitutions occur in lipoprotein lipase regions essential for normal enzyme activity and each mutation results in the expression of a nonfunctional enzyme leading to the hyperchylomicronemia syndrome manifested in the proband.