THE LPL GENE IN INDIVIDUALS WITH FAMILIAL COMBINED HYPERLIPIDEMIA ANDDECREASED LPL ACTIVITY

Familial combined hyperlipidemia (FCHL) is an oligogenic disorder, wit h family members having elevated apolipoprotein B-100 levels and eithe r elevated plasma cholesterol or triglyceride levels or both. Obligate heterozygous parents of children with lipoprotein lipase (LPL) defici ency express a mild FCHL phenotype. Of patients with FCHL 36% have dim inished postheparin LPL activity and mass values that are comparable w ith those of obligate heterozygotes for LPL deficiency. It is hypothes ized that heterozygosity for mutations in the LPL gene could contribut e to FCHL in this subset of patients. Single-strand conformation polym orphism (SSCP) analysis, direct DNA sequencing, and Southern blot anal ysis were used to examine exons 1 through 9 and exon-intron junctions of the LPL gene in 20 patients with FCHL and low LPL activity and mass . One subject had a substitution (GAC-->AAC) in exon 2, changing Asp(9 ) to Asn. Two subjects had a previously undescribed ''silent'' substit ution (GTG-->GTA) in exon 3 at Val(108). Three patients had a prematur e termination at codon 447 in exon 9 resulting in truncation of the ma ture protein by two amino acids. In addition to SSCP analysis, exons 4 , 5, and 6, where almost ah mutations in LPL-deficient patients have b een found, were sequenced and no additional mutations were found. Sout hern blot analysis of the LPL gene revealed one subject with heterozyg ous loss of an EcoRI site but without an abnormality in Stu I restrict ion fragments; this mutation is therefore unlikely to be functionally significant. The substitutions identified at codons 9 and 447 have pre viously been found not to affect lipolytic activity when expressed in vitro. In summary, the findings suggest that mutations in the coding s equence of the LPL gene are an infrequent cause of FCHL. Other mechani sms that regulate plasma LPL activity remain to be investigated in the pathogenesis of FCHL.