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Arteriosclerosis, Thrombosis, and Vascular Biology. 1995;15:1704-1712

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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1995;15:1704-1712.)
© 1995 American Heart Association, Inc.

Articles

Mutations in the Gene for Lipoprotein Lipase

A Cause for Low HDL Cholesterol Levels in Individuals Heterozygous for Familial Hypercholesterolemia

Simon N. Pimstone; S. Eric Gagné; Claude Gagné; Paul J. Lupien; Daniel Gaudet; Roger R. Williams; Maritha Kotze; Paul W. A. Reymer; Joep C. Defesche; John J. P. Kastelein; Sital Moorjani; Michael R. Hayden

From the Department of Medical Genetics, University of British Columbia, Vancouver (S.N.P., S.E.G., M.R.H.), and the Lipid Research Centre, Laval University, Ste-Foy, Quebec (C.G., P.J.L., D.G., S.M.), Canada; the University of Utah, Cardiovascular Genetics, Salt Lake City (R.R.W.); the Department of Human Genetics, University of Stellenbosch, Tygerberg, South Africa (M.K.); and the Lipid Research Group, Department of Vascular Medicine, University of Amsterdam (Netherlands) (P.W.A.R., J.C.D., J.J.P.K.).

Correspondence to Dr Michael R. Hayden, Department of Medical Genetics, University of British Columbia, Rm 416-2125 East Mall, Vancouver, British Columbia V6T 1Z4, Canada.


*    Abstract
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 *Abstract
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Abstract Familial hypercholesterolemia (FH) is characterized by elevated plasma concentrations of LDL cholesterol resulting from mutations in the gene for the LDL receptor. Low HDL cholesterol levels are seen frequently in patients both heterozygous and homozygous for mutations in this gene. Suggested mechanisms for reduced HDL levels in FH patients have been altered apolipoprotein A-1 metabolism and elevated cholesteryl ester transfer protein activity, but the molecular basis for hypoalphalipoproteinemia in any of these patients has not yet been identified. We investigated four large families in which individuals were found to be double heterozygotes for both FH and lipoprotein lipase (LPL) deficiency. These double heterozygotes have significantly less HDL cholesterol than persons with FH or LPL heterozygosity alone. In the double heterozygotes, HDL particle composition is not significantly different from FH heterozygotes, suggesting a quantitative rather than qualitative defect in HDL metabolism in these persons. We propose that mutations in the LPL gene may be a cause of low HDL cholesterol levels in some individuals heterozygous for FH.


Key Words: HDL • lipoprotein lipase • familial hypercholesterolemia


*    Introduction
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 *Introduction
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 down arrowResults
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Familial hypercholesterolemia results from many different defects in the gene for the LDL receptor and is inherited as an autosomal dominant trait.1 Heterozygosity for FH occurs with a frequency of approximately 1 in 500 in the western world and is characterized by elevated levels of LDL cholesterol, tendon xanthomas, and premature coronary artery disease. Plasma levels of HDL cholesterol are significantly reduced in many FH families,2 3 4 5 although the precise frequency with which this occurs is unknown.

Currently, no clear understanding for hypoalphalipoproteinemia in patients with FH has been obtained. Kinetic studies in an FH homozygote suggest both an increased apoA-1 fractional catabolic rate and a decreased apoA-1 production rate as the mechanisms for reduced HDL cholesterol in this subject.6 It has also been suggested that enhanced activity of CETP seen in FH heterozygotes may underlie the reduced levels of HDL cholesterol in some individuals.4 Because of the large contribution (approximately 75%) made by CE to HDL cholesterol,7 any factor resulting in a decrease in the CE content of HDL could consequently reduce the plasma HDL cholesterol level. Animal models of FH (in particular, the Watanabe heritable hyperlipidemic rabbit), which also have low HDL cholesterol, have been shown to have enhanced CETP activity.8 However, the molecular basis for low HDL cholesterol levels in any patients with FH has not yet been reported.

Other genetic variants and environmental factors are known to alter the lipid phenotype and clinical outcome of FH.9 Recently, a patient homozygous for LPL deficiency and heterozygous for FH was described. A markedly reduced LDL cholesterol was noted, confirming in vivo the major role played by LPL in the conversion of TG-rich lipoproteins to LDL in human plasma.10 Here we have studied four families with individuals who were double heterozygotes for both FH and LPL deficiency. We show that double heterozygotes confirmed by molecular analysis present with alterations in their lipid profile, characterized by more marked reduction in HDL cholesterol levels, but no significant alteration in HDL particle composition when compared with family members who are heterozygotes for FH alone. Heterozygosity for partial LPL deficiency is common and may occur in approximately 5% of the general population.11 It may therefore be predicted that heterozygosity for mutations in the LPL gene might occur in approximately 5% of FH families and may represent an important cause of low HDL in some FH families.


*    Methods
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Study Subjects
Assessment of families for LPL mutations resulted in the serendipitous finding of four families containing individuals who were double heterozygotes for both FH and LPL deficiency (families A, B, C, and D). In all four families, mutational analysis was performed to identify the defects underlying both the LDL receptor and LPL mutations (see Table 1Down). Individuals in each family were screened for the specific LPL and LDL receptor mutation found in the proband in all families. In family C, FH was diagnosed by mutational analysis in one affected relative and in other relatives by using DNA-validated clinical criteria that were published previously.12 This family is a large FH pedigree that was reported together with a formal segregation analysis that revealed dominant transmission of elevated LDL cholesterol levels and early coronary artery disease maximum likelihood complex segregation analysis.13 In particular, all FH heterozygotes in this family had elevated total and LDL cholesterol levels for age and sex, no evidence of secondary hyperlipidemia, at least one pediatric relative with very high cholesterol, or a relative with very high cholesterol and tendon xanthomas. Dominant expression was also clearly evident in this family.


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  		Table 1. Demographic and Genetic Data for Families A, B, C, and D

The 10-kb deletion and Trp66Gly mutation in the LDL receptor in families A and B were detected by Southern blot and dot blot oligonucleotide hybridization, respectively, as described previously.14 15 In family C, a change in exon 17 of the LDL receptor gene was detected by single-strand conformational polymorphism analysis (R.R.W., unpublished data, 1994). The LDL receptor defect in family D, Val408Met, was detected by Nla III restriction endonuclease digestion after polymerase chain reaction amplification as previously described.16 The LPL mutations in family A (Asn291Ser), B (Pro207Leu), C (Glu188Gly), and D (Asn291Ser) were detected by polymerase chain reaction methods described previously11 17 18 after DNA was extracted by standard procedures.19

None of the subjects included in the study were taking any lipid-lowering drugs or any other medication known to affect lipid metabolism, including hormonal therapy. No study subject had clinical or biochemical evidence of diabetes mellitus or thyroid, hepatic, or renal disease. All individuals known to ingest more than two alcoholic drinks per day were also excluded from the study.

Laboratory Analysis
Lipid levels were measured at the same laboratory for all subjects within a family. Because no interfamilial comparison was being made, standardizing laboratories between families was unnecessary. In all four families, venous blood samples were collected after a 12-hour overnight fast according to Lipid Research Clinics Program guidelines.20 Immediately after collection, plasma and cells were separated by centrifugation in a desktop centrifuge at 3000g for 20 minutes at room temperature. Plasma total cholesterol and TG were analyzed by standard enzymatic methods in families A, B, and D.21 22 23 In families A and B, HDL cholesterol was measured after heparin manganese precipitation of VLDL and LDL.24 In family D, HDL cholesterol was determined after precipitation of VLDL cholesterol and LDL cholesterol with phosphotungstate and magnesium chloride as described previously.25 In family C, plasma lipids were determined by a microprocedure described elsewhere,26 and HDL cholesterol was measured after a MgCl2–dextran sulphate precipitation.27 The same methods of analysis were used for all family members in each family.

Detailed lipoprotein analysis was performed in family B. Lipoprotein fractions (VLDL=d<1.006 g/L; IDL=1.006<d<1.019 g/L; LDL=1.019<d<1.063 g/L; and HDL=1.063<d<1.21 g/L) were isolated by sequential ultracentrifugation using a Beckman 50.4 rotor.28 Recovery of lipids averaged 96% (range, 92% to 102%) of the values in plasma. Lipid measurements in lipoprotein fractions were performed on automated Technicon RA-1000 with enzymatic methods; reagents for cholesterol and TG were purchased from Miles Diagnostics, for UC from Boehringer Mannheim, and for PL from Wako Pure Chemicals. ApoA-I and apoB were measured in families A and B by electroimmunoassay as described previously.29 30 Heights and weights were recorded, and body mass index was calculated.

Statistics
In this study, assumptions of normality and homoscedasticity (and possibly sample independence) were not met. Therefore, a nonparametric statistical procedure was used to determine differences in lipid, lipoprotein, and demographic data of groups within families. A Kruskal-Wallis one-way ANOVA with ranks was used. If the overall associations for a variable proved statistically significant (P<=.05), pairwise comparisons by Mann-Whitney two-sample test of association were used. Because of the known effect of sex differences in HDL values, all four families were analyzed as males and females separately. Within mutation groups, no significant sex differences were noted in the lipid values under investigation in families A, B, and D. For this reason, statistical results for these families are presented as males and females combined. Sex differences in HDL, however, were noted within FH heterozygotes in family C. The statistical analyses for this family were therefore performed separately for males and females, and the results are shown accordingly. Within all four families, mutation groups were age matched; therefore, analyses were performed without need to correct for age.


*    Results
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DNA Analysis
The molecular basis of FH and LPL deficiency in all four families is shown in Table 1Up together with the number of FH/LPL double heterozygotes, single heterozygotes for LPL deficiency and FH, and the relatives with neither mutation. Family A, of French Canadian ancestry, consisted of 25 individuals who were assessed for mutations in the genes for the LDL receptor and LPL (Fig 1Down). Mutational analysis revealed the 10-kb deletion as the underlying LDL receptor defect and the exon 6 Asn291Ser mutation as a cause for partial LPL deficiency in this family. Four individuals in the family were found to be double heterozygotes for both FH and LPL (2 males and 2 females; Table 2Down). Nine FH heterozygotes were defined (2 males and 7 females; Table 2Down), and 4 LPL heterozygotes (1 male and 3 females; Table 2Down) were studied. The control group in family A consisted of 8 related individuals (5 males and 3 females; Table 2Down).



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  		Figure 1. Pedigree of family A. This family consists of 25 individuals of French Canadian ancestry. Four FH/LPL double heterozygotes were identified in this family.


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  		Table 2. Demographic, Lipid, and Apolipoprotein Data for Family A

Family B (Fig 2Down), also of French Canadian ancestry, consisted of 145 individuals of which 8 were found to be double heterozygote for FH and LPL deficiency (3 males and 5 females; Table 3Down). Of these 8 double heterozygotes, 4 were not directly related to the family but had the same mutations, the same ancestry, and lived in the same isolated geographic area. For these reasons, they were included in this family study. Fourteen FH heterozygotes (7 males and 7 females; Table 3Down) and 21 LPL heterozygotes (12 males and 9 females; Table 3Down) were studied. A total of 102 relatives without either mutation (46 males and 56 females; Table 3Down) were assessed. The LPL heterozygotes have been analyzed further in detail to assess age and sex influences on heterozygosity for mutations in the LPL gene (manuscript in preparation). DNA analysis in this family revealed the Trp66Gly mutation as the basis for the LDL receptor defect and the Pro207Leu mutation as the molecular basis for LPL deficiency.



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  		Figure 2. Pedigree of family B. This family consists of 145 individuals of French Canadian ancestry with origins in a small, isolated town in Eastern Quebec. Only 80 individuals from this family are shown. They represent the side of this family from which the FH/LPL double heterozygotes were identified. The four unrelated FH/LPL double heterozygotes from the same geographic region are not shown.


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  		Table 3. Demographic, Lipid, and Apolipoprotein Data for Family B

Family C, of mixed European ancestry, consisted of 54 individuals (Fig 3Down) including 4 double heterozygotes (1 male and 3 females; Table 4Down). Thirteen FH heterozygotes (6 males and 7 females; Table 4Down) and 14 LPL heterozygotes (5 males and 9 females; Table 4Down) were also studied. A total of 23 related individuals (14 males and 9 females) had neither mutation (Table 4Down). The LPL mutation in this family was the Glu188Gly in exon 5 of the LPL gene; single-strand conformational polymorphism analysis revealed a new, unpublished variant in exon 17 of the LDL receptor gene, which segregated with the phenotype of FH in all 13 members of this family and was not found in individuals without the clinical phenotype in the family. The exact nature of this mutation remains unknown.



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  		Figure 3. Pedigree of family C. This family consists of 54 individuals of western European ancestry. Four FH/LPL double heterozygotes were identified in this family.


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  		Table 4. Demographic, Lipid, and Apolipoprotein Data for Family C

Family D consisted of 29 individuals, all of Dutch ancestry (Fig 4Down). Four double heterozygotes were identified (2 males and 2 females; Table 5Down). Ten FH heterozygotes (7 males and 3 females; Table 5Down) and 2 LPL heterozygotes (2 males; Table 5Down) were also studied. A control group consisting of 13 related individuals (1 male and 12 females; Table 5Down) had neither mutation. DNA analysis in this family revealed the Val408Met mutation as the basis for the LDL receptor defect and the Asn291Ser mutation as the molecular basis for partial LPL deficiency.



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  		Figure 4. Pedigree of family D. This family consists of 29 individuals of Dutch ancestry. Four FH/LPL double heterozygotes were identified in this family.


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  		Table 5. Demographic and Lipid Data for Family D

Within each family, age and body mass index did not differ significantly among the groups (Tables 2 through 5UpUpUpUp). In addition, physical activity and smoking habits were recorded in families A, B, and D; no significant difference was seen among the groups in each family (data not shown).

Lipid Levels
In all four families, double heterozygotes for both FH and LPL deficiency had reduced HDL cholesterol levels compared with those individuals heterozygous for FH. Despite the small numbers of double heterozygotes, statistical significance was reached in families A, B, and D (A, P=.037; B, P=.009; D, P=.048). A similar trend was seen for family C even when separated into males and females (Table 4Up), but this did not reach significance. HDL cholesterol levels were also consistently lower in the double heterozygotes when compared with those in individuals heterozygous for LPL deficiency alone in the same family. Although these results did not reach statistical significance, a trend was noted in all four families (Tables 2 through 5UpUpUpUp).

In the four families, the ratio of total cholesterol to HDL was increased in double heterozygotes for FH and LPL deficiency when compared with individuals with FH or LPL deficiency alone, although this reached statistical significance in family B only (see Tables 2 through 5UpUpUpUp). A trend to higher TG was also noted in FH/LPL double heterozygotes in all four families when compared with individuals in the families with FH alone (A, P=.14; B, P=.11; C, P=.046; D, P=.09).

Lipoprotein Assessment
HDL Lipids
Detailed lipoprotein analysis was undertaken in family B, chosen for its extensive size. The results, illustrated in Table 6Down, indicate significant reductions in HDL lipids in FH/LPL double heterozygotes compared with FH heterozygotes alone. HDL-UC and HDL-PL were both significantly reduced in these double heterozygotes (P=.05, respectively). HDL-CE and HDL-TG were also decreased in the FH/LPL double heterozygotes compared with FH heterozygotes alone (Table 6Down), but this did not reach significance. The lipid concentrations in HDL in FH/LPL double heterozygotes were also lower than those seen in LPL heterozygotes. Both HDL-TG and HDL-PL were significantly reduced (P=.008 and P=.01, respectively). HDL-CE and HDL-UC were also lower in double heterozygotes compared with LPL heterozygotes, although significance was not reached (P=.09 and P=.10, respectively).


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  		Table 6. Lipoprotein Analysis for Family B

VLDL Lipids
Lipid concentrations in VLDL particles in FH/LPL double heterozygotes were similar to those of FH heterozygotes alone. Increased VLDL-TG was noted, but significance was not reached (P=.154; Table 6Up). VLDL lipid concentrations were not different between FH/LPL double heterozygotes and LPL heterozygotes alone.

LDL Lipids
No significant difference was seen in lipid concentration in LDL particles between FH/LPL double heterozygotes and FH heterozygotes alone. However, significantly increased levels of LDL-CE, LDL-UC, and LDL-PL were seen in FH/LPL double heterozygotes compared with LPL heterozygotes alone (P=.007 for all measurements, respectively).

Lipoprotein Particle Composition
Despite the differences noted in lipid concentrations in the various lipoprotein particles, no significant differences in lipoprotein particle composition was evident in FH/LPL double heterozygotes compared with FH heterozygotes alone (Table 7Down). Differences were seen, however, in FH/LPL double heterozygotes when compared with LPL heterozygotes alone. These differences were evident in both LDL and HDL particles. LDL-CE and LDL-UC made up a significantly greater percentage of the LDL particles in FH/LPL double heterozygotes compared with LPL heterozygotes (P=.044 and P=.025, respectively; Table 7Down). This increase in LDL cholesterol in FH/LPL double heterozygotes resulted in a significantly lower percentage of TG in LDL particles (P=.034) when compared with findings in LPL heterozygotes.


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  		Table 7. Percentages of Lipoprotein Composition for Family B

HDL particle composition was also significantly different in FH/LPL double heterozygotes when compared with that in LPL heterozygotes alone. HDL-CE was significantly greater in double heterozygotes (P=.014; Table 7Up), probably because of the larger load of cholesterol in the lipoprotein fractions due to the coexisting LDL receptor defect. PL, on the other hand, made up significantly fewer of the HDL particles in FH/LPL double heterozygotes compared with LPL heterozygotes alone (P=.019).

Another mechanism by which to assess LPL functional significance is to identify ratios of lipoprotein surface to core in the lipoprotein fractions. UC and PL constitute the surface component and CE and TG the core component of the various lipoproteins. Surface-core ratios of HDL were lower in FH/LPL double heterozygotes compared with FH heterozygotes, LPL heterozygotes, and control subjects, although significance was only reached when compared with control (0.92±0.09 versus 0.96±0.07, P=.028). This significance, however, is expected in that FH heterozygotes alone had significantly lower surface-core ratios when compared with control subjects (0.96±0.07 versus 1.07±0.09, P=.006), as did LPL heterozygotes (1±0.05 versus 1.07±0.09, P=.011). In FH/LPL double heterozygotes, no significant differences in surface-core ratios were seen in both LDL and VLDL particles when compared with either FH or LPL heterozygotes (data not shown).

Apolipoprotein Assessment
In family B, apoA-1 was significantly decreased in the double heterozygotes when compared with that in FH heterozygotes alone (P=.028; Table 3Up); it was decreased, but not significantly, when compared with that in LPL heterozygotes alone (Table 3Up). In family A, nonsignificant decreases in apoA-1 were seen in FH/LPL double heterozygotes when compared with FH (P=.10) and LPL heterozygotes (P=.11) within the family (Table 2Up). ApoA-1 levels were not available for families C or D.


*    Discussion
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
 up arrowResults
 *Discussion
down arrowReferences
 
We studied four families in which individuals heterozygous for both FH and LPL deficiency present with significantly reduced HDL cholesterol levels and an elevated ratio of TC to HDL when compared with persons in the same family who are heterozygotes for either FH or LPL deficiency alone. We suggest that the various metabolic alterations seen in FH or LPL deficiency alone are additive and may account for either enhanced catabolism and/or decreased production of HDL particles, resulting in reduced HDL cholesterol levels in FH/LPL double heterozygotes.

The families in this study provided a unique opportunity to determine phenotypic expression of gene-gene interaction. In most instances, family members were living in a similar environment with similar diets; for example, all members of families B and D lived in a small town in Eastern Quebec and Holland, respectively. Within each town, family members followed similar diets. Therefore, changes in lipoprotein values within these families are unlikely to be due to environmental differences but rather reflect different genotypes for FH and LPL deficiency.

The heterozygous state for FH is clearly associated with an increased predisposition to coronary artery disease.5 31 32 Within this population, reduced levels of HDL cholesterol are seen, although the precise frequency is unknown.2 3 4 5 33 34 However, no molecular basis for this finding has been proposed. FH heterozygotes with reduced HDL levels are also prone to develop more severe premature artery disease when compared with FH heterozygotes with normal HDL cholesterol levels.34

The mechanism for hypoalphalipoproteinemia in FH alone is poorly understood. Recently Schaefer et al35 described a patient with homozygous FH in whom the fractional catabolic rate was increased and production rate of apoA-1 was decreased when compared with two normal control subjects.35 Similar results have been seen in Watanabe heritable hyperlipidemic rabbits.36 The detailed mechanism of apoA-1 catabolism in the liver and kidney is uncertain. It has been suggested that the increased HDL apoE mass seen in FH individuals37 38 may be more rapidly cleared by an LDL receptor–dependent mechanism, possibly through the remnant receptor, resulting in an enhanced fractional catabolic rate of apoA-1.35 Decreased apoA-1 would therefore be available as a cofactor for lecithin:cholesterol acyltransferase, an enzyme crucial for the generation of plasma HDL-CE formation.39

HDL metabolism is also closely linked to CE transfer. In CETP deficiency, delayed clearance of apoA-1 and elevated levels of HDL have been described.40 In contrast, enhanced CETP activity resulting in CETP-mediated transfer of CE from HDL to apoB-containing lipoproteins is reflected by reductions in plasma HDL levels.41 Different studies have reported increased CETP activity and/or hepatic CETP mRNA levels in FH individuals6 and in animal models for FH,8 possibly providing an explanation for reduced HDL levels in some patients with FH. CE transfer from HDL to apoB-containing lipoproteins is mediated not only by enhanced CETP activity but also by the size of the acceptor lipoprotein pool and by the lipid content of the respective donor and acceptor particles. In FH individuals, both enhanced CETP activity and greatly increased LDL pool size may therefore contribute to the transfer of CE from HDL particles.

Mutations in the LPL gene affecting the catalytic activity of the protein also result in expansion of the pool of apoB- and apoE-containing lipoproteins, namely, VLDL and chylomicron particles. The increased concentrations and delayed clearance of these particles in LPL-deficient subjects allow additional time for TG/CE exchange between these lipoproteins and HDL, resulting in enhanced acceptance of HDL cholesterol by these particles. Despite reported normal CETP activity in LPL heterozygotes,42 the CETP-mediated transfer of CE or exchange of CE and TG may be more efficient. Goldberg et al43 have shown that acute antibody inhibition of LPL resulted not only in marked hypertriglyceridemia but also in marked reduction in HDL-CE and a secondary increase in apoA-1 catabolism. This increased catabolism may be to be due to enhanced renal apoA-1 catabolism as a result of smaller HDL particles.

Defective HDL particle production may be another important mechanism for reduced HDL cholesterol levels in LPL deficiency. The surface coat of TG-rich lipoproteins is an important source of HDL precursors.44 45 46 Defective lipolysis of both chylomicron and VLDL particles may therefore play an important role in defective HDL production.

Increased CETP activity and apoA-1 catabolic rates and reduced apoA-1 production rates seen in FH individuals4 6 are likely to be further aggravated by the coexistence of decreased nascent HDL particle production and the accelerated catabolism of apoA-1 particles in LPL deficiency. As is evidenced by our lipoprotein particle studies in family B, no obvious significant difference is seen in lipoprotein particle composition or lipoprotein particle surface-core ratios in HDL particles, suggesting a quantitative rather than qualitative defect in HDL metabolism in FH/LPL double heterozygotes compared with FH heterozygotes in the family. Supporting this may be the finding of reduced apoA-1 levels in double heterozygotes compared with FH heterozygotes, as is seen in both families A and B in whom apoA-1 was measured. Because of the significant disturbances seen in lipoprotein metabolism in the heterozygous state for FH, the addition of partial LPL deficiency may not be enough to significantly alter lipid exchange between lipoprotein particles but may play a role in further enhancing HDL catabolism and decreasing HDL production.

The enhanced hypoalphalipoproteinemia seen in these double heterozygotes results in elevated TC-HDL ratios in these individuals. This finding has previously been associated with an increased risk for coronary artery disease.47

The trend for higher TG levels in double heterozygotes for FH and partial LPL deficiency compared with FH heterozygotes alone can be explained by the decreased lipolysis of TG-rich lipoprotein particles, an expected finding in these individuals.

LPL activity is not altered in FH, but in our laboratory all three LPL mutations described have been found to be associated with decreased LPL activity both in vivo and in vitro.11 48 49 To further highlight the functional effect of heterozygosity for LPL deficiency, it is noteworthy that HDL cholesterol was reduced in LPL heterozygotes compared with control subjects in all four families, with statistical significance being reached in families B and C (P<.001 and P=.048, respectively).

Another suggested role for LPL is the enhanced uptake of LDL particles by a receptor-dependent and/or -independent mechanism, facilitated by LPL bridging of proteoglycans present on the plasma membrane and circulating lipoproteins containing LPL.50 Studies have shown reduced uptake of lipoproteins in LDL receptor–negative fibroblasts, suggesting a role for the LDL receptor in the metabolism of LPL-lipoprotein complexes.51 In FH/LPL double heterozygotes, one might then expect to find elevated plasma levels of LDL cholesterol as a result of decreased LPL-lipoprotein complex formation and decreased complex internalization. We do not find evidence for increased LDL cholesterol levels in double heterozygotes for FH and LPL deficiency compared with FH heterozygotes alone, suggesting that, in the heterozygous state, LPL deficiency does not reduce LPL-lipoprotein complex formation sufficiently to impair its binding and that the heterozygous state for FH does not reduce the internalization of LPL-lipoprotein complexes sufficiently to result in further elevation of plasma LDL cholesterol. The uptake of these complexes is probably maintained by the functioning LDL receptors and/or by a receptor-independent mechanism.

The high frequency of LPL mutations in the general population (such as the change Asn291Ser) would suggest that these LPL mutations and other less common LPL defects (such as the Pro207Leu and the Glu188Gly described here) may be found in at least one in every 20 to 30 FH families and possibly with higher frequency in areas where LPL carrier rates are high. These LPL defects decrease HDL cholesterol levels and apoA-1 levels in these FH individuals and therefore may alter the prognosis in such persons with both FH and partial LPL deficiency, although this remains to be proven. Underlying LPL mutations may also account for the variability of HDL and TG values within families with FH, since the LPL mutations segregate independently of the mutation in the gene for the LDL receptor.

In summary, we present four families in which individuals were found to be double heterozygotes for both LDL receptor defects and mutations in the LPL gene. Such individuals present with an altered lipid phenotype characterized by significant hypoalphalipoproteinemia and an increased TC-HDL ratio. A nonsignificant trend for higher TG levels was also noted in these individuals. Further studies are needed to see whether these changes in lipoproteins predict an increased susceptibility to atherosclerosis.


*    Selected Abbreviations and Acronyms
 
apoA-1 = apolipoprotein A-1
CE = cholesteryl ester
CETP = cholesteryl ester transfer protein
FH = familial hypercholesterolemia
LPL = lipoprotein lipase
PL = phospholipid
TG = triglyceride
UC = unesterified cholesterol


*    Acknowledgments
 
This work was supported by the MRC Canada and the Heart and Stroke Foundation of British Columbia and the Yukon. Most (3 of the 4) of the families were identified by the International MED-PED collaboration, and we thank all the investigators participating in this project. Thanks also go to Drs Jiri Frohlich and Haydn Pritchard of the Atherosclerosis Specialty Unit, St Pauls' Hospital, Vancouver, BC, for their valuable comments and criticisms. Dr Hayden is an established investigator of the British Columbia Children's Hospital.

Received May 18, 1995; accepted August 2, 1995.


*    References
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
 up arrowResults
 up arrowDiscussion
 *References
 

  1. Goldstein J, Hobbs HH, Brown MS. Familial hypercholesterolemia. In: The Metabolic Basis of Inherited Disease. 7th ed. Scriver CR, Beaudet AC, Sly WS, Valle D, eds. New York, NY: McGraw Hill Book Co; 1995:1981-2030.
  2. Seftel HC, Baker SG, Sandler MP, Forman MB, Joffe HI, Mendelsohn D, Jenkins T, Mieny CJ. A host of hypercholesterolaemic homozygotes in South Africa. BMJ. 1980;281:633-636.
  3. Kwiterovitch PO. Familial hypercholesterolemia (one form of familial type II hyperlipoproteinaemia): a study of its biochemical, genetic and clinical presentation in childhood. J Clin Invest. 1973;53:1237-1249.
  4. Inazu A, Koizumi J, Mabuchi H, Kajinami K, Takeda R. Enhanced cholesteryl ester transfer protein activities and abnormalities of high density lipoproteins in familial hypercholesterolemia. Horm Metab Res. 1992;24:284-288. [Medline] [Order article via Infotrieve]
  5. Gagné C, Moorjani S, Brun D, Toussaint M, Lupien P-J. Heterozygous familial hypercholesterolemia: relationship between plasma lipids, lipoproteins, clinical manifestations and ischemic heart disease in men and women. Atherosclerosis. 1979;34:13-24. [Medline] [Order article via Infotrieve]
  6. Schaefer JR, Rader DJ, Ikewaki K, Fairwell T, Zech LA, Kindt MR, Davignon J, Gregg RE, Brewer HB Jr. In vivo metabolism of apolipoprotein A-1 in a patient with homozygous familial hypercholesterolemia. Arterioscler Thromb. 1992;12:843-848. [Abstract/Free Full Text]
  7. Kunz F, Pechlaner C, Erhart R, Fend F, Muhlberger V. HDL and plasma phospholipids in coronary artery disease. Arterioscler Thromb. 1994;14:1146-1150. [Abstract/Free Full Text]
  8. Son Y-SC, Zilversmit DB. Increased lipid transfer activities in hyperlipidemic rabbit plasma. Arteriosclerosis. 1986;6:345-351. [Abstract/Free Full Text]
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