A novel amino acid substitution (His183–>Gln) in exon 5 of the lipoprotein lipase gene results in loss of catalytic activity: phenotypic expression of the mutant gene in a heterozygous state

Pathogenic classification of LPL gene variants reported to be associated with LPL deficiency

BACKGROUND

Lipoprotein lipase (LPL) deficiency is a serious lipid disorder of severe hypertriglyceridemia (SHTG) with chylomicronemia. A large number of variants in the LPL gene have been reported but their influence on LPL activity and SHTG has not been completely analyzed. Gaining insight into the deleterious effect of the mutations is clinically essential.

METHODS

We used gene sequencing followed by in-vivo/in-vitro and in-silico tools for classification. We classified 125 rare LPL mutations in 33 subjects thought to have LPL deficiency and in 314 subjects selected for very SHTG.

RESULTS

Of the 33 patients thought to have LPL deficiency, only 13 were homozygous or compound heterozygous for deleterious mutations in the LPL gene. Among the 314 very SHTG patients, 3 were compound heterozygous for pathogenic mutants. In a third group of 51,467 subjects, from a general population, carriers of common variants, Asp9Asn and Asn291Ser, were associated with mild increase in triglyceride levels (11%-35%).

CONCLUSION

In total, 39% of patients clinically diagnosed as LPL deficient had 2 deleterious variants. Three patients selected for very SHTG had LPL deficiency. The deleterious mutations associated with LPL deficiency will assist in the diagnosis and selection of patients as candidates for the presently approved LPL gene therapy.

Lipoprotein lipase (LPL) deficiency: a new patient homozygote for the preponderant mutation Gly188Glu in the human LPL gene and review of reported mutations: 75 % are clustered in exons 5 and 6

We have investigated the lipoprotein lipase (LPL) gene of a 2-year-old patient presenting classical features of the familial LPL deficiency including undetectable LPL activity. DNA sequence analysis of exon 5 identified the patient as a homozygote for the Gly188Glu mutation, frequently involved in this disease. A review of cases of LPL deficiency with molecular study of the LPL gene showed a total number of 221 reported mutations involved in this disease. Gly188Glu was involved in 23.5 % of cases and 74.6 % of mutations were clustered in exons 5 and 6. Based on these observations, we propose a method of screening for mutations in this gene.

Familial lipoprotein lipase (LPL) deficiency: a catalogue of LPL gene mutations identified in 20 patients from the UK, Sweden, and Italy

The aim of this study was to identify mutations in the lipoprotein lipase (LPL) gene in 20 unrelated patients with familial lipoprotein deficiency (FLLD) and to investigate the genotype/phenotype relationship. The previously reported G188E mutation (Monsalve et al., J Clin Invest 86:728-734, 1990) was screened for and found to be present in seven individuals (12/40 alleles). In addition, three patients were heterozygous for the 2.0 kb insertion (Langlois et al., Proc Nalt Acad Sci US 86:948-952, 1989). Two approaches were taken for new mutation detection; single-strand conformation polymorphism and sequencing to identify micro-mutations in the proximal promoter and exons 1-9 of the LPL gene and Southern blotting to identify gross mutations. Ten different point mutations were found (W86G, A158T, H183Q, G188E, S193R, P207L, L252X, N291S, M301T, L303P). Additionally, a two nucleotide deletion in exon 6 (delta1006-1007), a six nucleotide deletion in exon 8 (delta1441-1447), and a silent substitution in the wobble position of codon E118 were identified. In vitro mutagenesis and expression in COS-B cells suggested that the A158T and S193R substitutions virtually abolished enzyme activity. In analysing the genotype/phenotype relationship, there was no strong association between age at diagnosis, severity of symptoms, lipid levels, and the nature/position of the mutation. Triglyceride levels, however, were higher in compound heterozygotes compared to true homozygotes, possibly reflecting increased instability of heterodimers. Overall, 29 of 40 (72.5%) mutant alleles were identified. Failure to identify the mutation in 11 alleles might reflect the inadequacy of the method or the possibility that mutations lie within regions of the gene not screened in the study because of lack of availability of sequence.

Recessive inheritance of obesity in familial non-insulin-dependent diabetes mellitus, and lack of linkage to nine candidate genes

Segregation analysis of body-mass index (BMI) supported recessive inheritance of obesity, in pedigrees ascertained through siblings with non-insulin dependent diabetes mellitus (NIDDM). BMI was estimated as 39 kg/m2 for those subjects homozygous at the inferred locus. Two-locus segregation analysis provided weak support for a second recessive locus, with BMI estimated as 32 kg/m2 for homozygotes. NIDDM prevalence was increased among those subjects presumed to be homozygous at either locus. Using both parametric and nonparametric methods, we found no evidence of linkage of obesity to any of nine candidate genes/regions, including the Prader-Willi chromosomal region (PWS), the human homologue of the mouse agouti gene (ASP), and the genes for leptin (OB), the leptin receptor (OBR/DB), the beta3-adrenergic receptor (ADRB3), lipoprotein lipase (LPL), hepatic lipase (LIPC), glycogen synthase (GYS), and tumor necrosis factor alpha (TNFA).

Common sequence variants of lipoprotein lipase: standardized studies of in vitro expression and catalytic function

We have assessed the functional activity of three common sequence variants of human lipoprotein lipase (LPL). Two of these, Asn291Ser and Asp9Asn arise from missense mutations while the third, Ser447Ter, derives from a nonsense mutation, truncating LPL by two residues. As previous in vitro studies have produced conflicting results, we have re-analyzed the catalytic function of these variants using the COS cell transfection system, under optimized and standardized experimental protocols. We found the Asn291Ser variant to manifest with a decrease in catalytic activity (57% of normal) due to a reduction in secretion and stability of the active homodimeric form. The Asp9Asn variant also showed a significant decrease in catalytic activity (85% of normal), but this was found to be due to a decreased rate of secretion only, as the homodimeric form was stable. The findings for these mutants contrasted with those of the Ser447Ter truncation variant which proved to be catalytically normal; this variant also manifested normal homodimer stability. The truncated variant did however, present with a higher total secreted mass level (131%) than control LPL. This was most likely due to enhanced secretion of the monomeric form. None of these mutations exhibited defects in binding affinity to cell surface proteoglycans. Each of these variants deviated significantly from normal as regards to their secreted activity or mass levels in the COS cell transfection system.

Development and evaluation of an ELISA method for the determination of lipoprotein lipase mass concentration--comparison with a commercial, one-step enzyme immunoassay

We developed a non-competitive, enzyme-linked, immunosorbent assay (ELISA) for the quantitation of lipoprotein lipase (LPL) in human postheparin plasma using affinity-purified antihuman milk lipoprotein lipase antibodies produced in chicken eggs and a monoclonal antibody directed against human lipoprotein lipase. We compared our ELISA method with a commercially available sandwich-enzyme immunoassay (Markit-F LPL EIA Kit, Dainippon Pharmaceutical Co, Ltd. Osaka, Japan). The reference values for lipoprotein lipase catalytic activity concentration and mass concentration in healthy Finns were determined. Lipoprotein lipase activity concentration (mean +/- SD) was 297 +/- 112 U/l in women, and mass concentration as measured by the ELISA method was 1058 +/- 367 micrograms/l. The corresponding values for men were 247 +/- 97 U/l and 815 +/- 207 micrograms/l, respectively. Across the whole concentration range of the ELISA method, the control samples' intra- and inter-assay coefficients of variation (CV) were 5.1% and 6.5%, respectively. The correlation between the ELISA and EIA methods was good, r = +0.81. The importance of the correct standardisation of immunoassays is discussed.

Molecular pathobiology of the human lipoprotein lipase gene

Lipoprotein lipase (LPL; E.C. 3.1.1.34) is a key enzyme in the metabolism of lipids. Many diseases, including obesity, coronary heart disease, chylomicronemia (pancreatitis), and atherosclerosis, appear to be directly or indirectly related to abnormalities in LPL function. Human LPL is a member of a superfamily of lipases that includes hepatic lipase and pancreatic lipase. These lipases are characterized by extensive homology, both at the level of the gene and the mature protein, suggesting that they have a common evolutionary origin. A large number of natural mutations have been discovered in the human LPL gene, which are located at different sites in the gene and affect different functions of the mature protein. There is a high prevalence of two of these mutations (207 and 188) in the Province of Québec, and one of them (207) is almost exclusive to the French-Canadian population. A study of these and other naturally occurring mutant LPL molecules, as well as those created in vitro by site-directed mutagenesis, indicate that the sequence of LPL is organized into multiple structural and functional units that act in concert in the normal enzyme. In this review, we discuss the interrelationships of LPL structure and its function, the molecular etiology of abnormal LPL in humans, and the clinical and therapeutic aspects of LPL deficiency.

A novel nonsense mutation in exon 1 and a transition in intron 3 of the lipoprotein lipase gene

We examined the lipoprotein lipase (LPL) gene by single strand conformation polymorphism (SSCP) and by restriction fragment length polymorphism (RFLP) analysis in 106 patients with hypertriglyceridemia to screen for novel mutations and to study the contribution of LPL genetic defects in hypertriglyceridemia. We found a single incidence of a homozygous novel nonsense mutation (216G-->A; -14Tryptophan-->stop codon) in exon 1 and 6 cases heterozygous for a single transition (C-->T) at six bp upstream from splicing acceptor site of intron 3. These mutations were not found in 105 normolipidemic controls. The proband homozygous for the nonsense mutation in exon 1, a 74 year old woman, had mild hyperchylomicronemia and her post-heparin plasma showed no LPL protein. However, four heterozygous among family members did not demonstrate hypertriglyceridemia. The frequency of heterozygosity for the C-->T transition in intron 3 was significantly different from that in normolipidemic controls. Therefore, it was suggested that the mutation is involved in hypertriglyceridemia. All of the heterozygotes were men with 4 patients having impaired glucose tolerance or diabetes mellitus. These observations suggest that this polymorphism in intron 3 combined with other as yet undefined factors may be related to hypertriglyceridemia.

A common variant in the gene for lipoprotein lipase (Asp9-->Asn). Functional implications and prevalence in normal and hyperlipidemic subjects

Subjects with combined hyperlipidemia (CHL) were screened for mutations in the lipoprotein lipase (LPL) gene by single-strand conformational polymorphism, and a previously reported G-->A DNA sequence change in exon 2, causing substitution of Asp by Asn at position 9, was identified in 2 individuals. Because this substitution destroys a recognition site for Taq I, pooling of DNA samples, amplification, and digest with Taq I allowed the rapid screening of 1563 healthy individuals and patients of Dutch, Swedish, English, and Scottish origin. In the general populations of all four countries, healthy carriers of the mutation were detected at a frequency of 1.6% to 4.4% (mean, 3.0%; 95% confidence interval, 2.0% to 4.0%). The frequency of carriers was roughly twice as high (range, 4.0% to 9.8%) in selected patients with CHL or type IV hyperlipoproteinemia or in subjects with angiographically assessed atherosclerosis; the frequency was consistently higher in each patient group compared with its matched control group. In 773 healthy men from two general practices in the United Kingdom, 25 carriers and 2 homozygotes for the mutation were identified. In these 27, plasma triglyceride but not plasma cholesterol levels were significantly higher than in noncarriers (2.25 versus 1.82 mmol/L, P < .02), and this difference was maintained in three subsequent annual measurements. Postheparin LPL activity data were available for some carriers and for 7 of 9 individuals from the patient groups, and 6 of 6 individuals from the control groups had LPL activity that was lower than the respective group mean. In vitro mutagenesis and transient expression in COS cells showed that compared with the LPL-Asp9 construct, LPL-Asn9 activity and mass were reduced by 20% to 30% in the culture media. Overall however, LPL-Asn9 had only slightly reduced specific activity (by 18%). Thus, although the precise mechanism of the effect is unclear, the data strongly suggest that the LPL-Asn9 variant is associated with and may play a direct role in predisposing carriers to develop hypertriglyceridemia.