**5.6 Hepatic lipase (LIPC)**

Hepatic lipase (HL) is a lipolytic enzyme involved in the hydrolysis of triacylglycerols present in circulating chylomicrons providing nonesterified fatty acids and 2 monoacylglycerol for tissue utilization and phospholipids from plasma lipoproteins that participates in metabolizing intermediate-density lipoprotein and large LDL into smaller, denser LDL particles, and in converting HDL2 to HDL3 during reverse cholesterol transport (Ordovas & Corella., 2004; Fisler, Warden, 2005; Ordovas., 2006; Much et al., 2005). It may suggest a role to play as a ligand for cell-surface proteoglycans in the uptake of lipoproteins by cell-surface receptors (Fisler & Warden., 2005; Ordovas & Corella., 2004).

Given the wide spectrum of effects that HL exerts on lipoprotein metabolism, and the significance of the promoter variant (s), it is reasonable to hypothesize that genetic variation at this locus may also be involved in variability in the response to dietary therapy (Ordovas & Corella., 2004). HL deficiency is characterized by mildly elevated concentrations of triglyceride-rich LDL and HDL particles, as well as impaired metabolism of postprandial triglyceride-rich lipoproteins, which may result in premature atherosclerosis. Conversely, increased HL activity is associated with increased small, dense LDL particles and decreased HDL2 concentrations, which may also result in increased CAD risk**.** Four common single

analyzed the same data and concluded that the Apo E polymorphism did not explain a significant proportion of the variation of the response (Xu et al., 1990). In a meta-analysis of 9 studies involving 612 subjects and found that the presence of the E4 allele was associated with a significantly greater LDL response to dietary intervention (Masson et al, 2003). Four studies found significantly different HDL-cholesterol responses between genotype groups: one study found that carriers of the E4 allele had the smallest HDL-cholesterol response, whereas the other 3 studies found that carriers of the E4 allele had the largest

However, recent evidence strongly suggests that variations in a number of key genes may also be important, including common variants of the Apo E gene. The most convincing evidence to date for genotypic effects on dietary response comes from the extensively

Another gene that affects HDL levels is CETP*,* encoding for the cholesteryl ester transfer protein that exchanges cholesteryl esters and triglycerids from HDL to other lipoproteins. This protein is also called the "lipid transfer protein:' People with two copies of a common allele at position 279 of this gene tend to have low HDL levels and elevated levels of LDL and VLDL. A variation (279G>A) that decreases plasma levels of CETP is associated with increased HDL levels, decreased LDL and VLDL levels, and a lower risk of cardiovascular

A recent meta-analysis which confirms that the I405V and TaqIB variants are indeed associated with lower CETP activity and higher high-density lipoprotein-cholesterol levels

The currently available evidence suggests that several genetic variants in the CETP gene are associated with altered CETP plasma levels and activity, high-density lipoproteincholesterol plasma levels, low-density lipoprotein and high-density lipoprotein particle size,

Hepatic lipase (HL) is a lipolytic enzyme involved in the hydrolysis of triacylglycerols present in circulating chylomicrons providing nonesterified fatty acids and 2 monoacylglycerol for tissue utilization and phospholipids from plasma lipoproteins that participates in metabolizing intermediate-density lipoprotein and large LDL into smaller, denser LDL particles, and in converting HDL2 to HDL3 during reverse cholesterol transport (Ordovas & Corella., 2004; Fisler, Warden, 2005; Ordovas., 2006; Much et al., 2005). It may suggest a role to play as a ligand for cell-surface proteoglycans in the uptake of lipoproteins

Given the wide spectrum of effects that HL exerts on lipoprotein metabolism, and the significance of the promoter variant (s), it is reasonable to hypothesize that genetic variation at this locus may also be involved in variability in the response to dietary therapy (Ordovas & Corella., 2004). HL deficiency is characterized by mildly elevated concentrations of triglyceride-rich LDL and HDL particles, as well as impaired metabolism of postprandial triglyceride-rich lipoproteins, which may result in premature atherosclerosis. Conversely, increased HL activity is associated with increased small, dense LDL particles and decreased HDL2 concentrations, which may also result in increased CAD risk**.** Four common single

disease than the more common (GG) form (DeBusk, 2009; Musunuru., 2010).

by cell-surface receptors (Fisler & Warden., 2005; Ordovas & Corella., 2004).

and perhaps the risk of coronary heart disease (Boekholdt., 2004).

response (Masson et al., 2003).

(Boekholdt., 2004).

**5.6 Hepatic lipase (LIPC)** 

studied Apo E gene variant (Lovegrove & Gitau., 2008).

**5.5 Cholesteryl ester transfer protein (CETP)** 

nucleotide polymorphisms (SNPs) on the 5\_-flanking region of the HL gene (*LIPC*) [−763 (A/G), −710 (T/C), −514 (C/T), and –250 (G/A)] are in total linkage disequilibrium and define a unique haplotype that is associated with variation in HL activity and HDLcholesterol levels (Fisler & Warden., 2005; Ordovas & Corella., 2004).

The less common A-allele of the SNP at position−250 is associated with lower HL activity and buoyant LDL particles. Normal and CAD subjects heterozygous for the A-allele have lower HL activity and significantly more buoyant LDL particles. Homozygosity for this allele (AA) is associated with an even lower HL activity. The A-allele is associated with higher HDL2-cholesterol (Ordovas & Corella., 2004).

An early intervention study with a low-saturated-fat, low-cholesterol diet found that, although significant improvements in fasting lipids occurred, there was no difference in response between genotypes at the hepatic lipase gene (LIPC) polymorphism measured. However, the study of 83 subjects may not have had adequate power to detect a modest effect of genotype (Fisler & Warden., 2005; Masson & McNeill., 2005; Ordovas., 2006).

Dietary information collected from Framingham Heart Study participants shows that subjects carrying the CC genotype react to higher contents of fat in their diets by increasing the concentrations of HDL-cholesterol, which could be interpreted as a "defense mechanism" to maintain the homeostasis of lipoprotein metabolism. Conversely, carriers of the TT genotype cannot compensate, and experience decreases on the HDL-cholesterol levels. These data could identify a segment of the population especially susceptible to dietinduced atherosclerosis. Considering the higher frequency of the T allele among certain ethnic groups (i. e., African-Americans), these data could shed some light on the impaired ability of certain ethnic groups to adapt to new nutritional environments, as clearly seen for Native Americans and Asian Indians. In this regard, they replicated the significant gene-diet interaction demonstrated in the Caucasian population of Framingham in another multiethnic cohort that consisted of Chinese, Malays, and Indians representing the population of Singapore. In addition to the significant gene-diet interactions reported in these papers, these data provides clues about the reasons why genotype-phenotype association studies fail to show consistent results. In theory, this polymorphism at the hepatic lipase gene will show dramatically different outcomes in association studies depending on the dietary environment of the population studies. The impact of these interactions will be magnified in populations with a high prevalence of the T-allele, as it is with Asians and African-Americans (Ordovas & Corella., 2004).

Three larger observational studies on the effect of a common polymorphism in the LIPC promoter gene –514C→T on the response of HDL cholesterol to dietary fat intake have been published. In examining the effects of the –514C→T LIPC polymorphism x dietary fat interaction on HDL in 2130 men and women participating in the Framingham Study, Ordovas et al found that the rarer *TT* genotype was associated with significantly higher HDL-cholesterol concentrations only in subjects consuming <30% of energy from fat. This same interaction was found for saturated and monounsaturated fats but not for polyunsaturated fat. A second association study, in an Asian population of 2170 subjects, found that Asian Indian subjects with a total fat intake of <30% of energy and with *TT* genotype at the –514C→T polymorphism had the highest HDL-cholesterol concentrations. This interaction, however, did not apply to the Chinese or Malay subjects in that study, and the significant interactions found for saturated or monounsaturated fats found by Ordovas et al were not found in the study by Tai et al. However, these 2 studies are consistent with

Nutrigenetics and Dyslipidemia 63

al., 2011). Although TG levels were unaffected, certain LPL SNPs modulated HDL levels and may alter the susceptibility of pregnant women to developing CHD. However, further studies are required to definitively define a relationship between lipid levels, CHD, and pregnant women. Therefore, the importance of LPL in the whole-body regulation of lipid

One common LPL polymorphism, known as T495G *Hind* III, has been extensively examined and demonstrates the complexity of disease prediction associated with a single SNP. Indeed, preliminary indications suggest that this polymorphism may play a role in the onset of several important diseases, such as CHD, diabetes and obesity. This SNP has been associated with the higher plasma TG and lower HDL levels characteristic with the early onset of diabetes. Preliminary results have also suggested a positive association between the *Hind* III polymorphism and a predisposition to developing obesity. Finally, this polymorphism has been associated with variations in lipid levels and heart disease, and that these alterations were attenuated by such environmental factors as physical exercise and low calorie diets, reiterating the important interactions arising between lifestyle, nutrition, and disease. Although these associations are not conclusive, they do suggest that LPL variants play a critically important role in the regulation of whole-body lipid metabolism

A relationship was established between a low calorie diet and the circulating lipid profile in obese individuals with the *Hind* III polymorphism. Homozygotes (H2H2) were found to have significantly higher levels of plasma VLDL-TG and Apo B than heterozygotes. Caloric restriction reduced lipid levels in both H2H2 and H1 individuals to a point where no difference was observed between the groups. Although H2H2 individuals responded more strongly (larger decreases in plasma lipids) to the low calorie diet, these preliminary results identify an important relationship between LPL polymorphisms, function, and diet (Much

As such, it is difficult to draw any firm conclusions from any one gene-diet study in the absence of replication by another study that examined the same question using similar methodologies. For example, one study demonstrated that a Mediterranean-style, MUFArich diet compared to a high-carbohydrate diet increased LDL size in individuals with certain Apo E gene variants but decreased LDL size in those with other Apo E variants; this is potentially a clinically important observation, but no confirmatory study has yet emerged, calling this observation into doubt. As pointed out by others, the field would greatly benefit from increased collaboration and coordination of studies among international nutrition

Because of the heterogeneity in the type and duration of the interventions described the magnitude of the lipid response to dietary interventions varied widely: in one study the change in LDL cholesterol in the Apo B *Eco*RI *R*-*R*- genotype group was as large as 59% of the baseline concentration. In the studies that showed a significant difference in response between genotype groups, the results also varied widely: in some studies, the difference in response between 2 genotype groups was ≈20% of the baseline lipid concentration (Rantala et al., 2000; Clitfon., 1997). However, the magnitude of these differences cannot be estimated with any accuracy, largely because most studies had only a small number of subjects in the rare genotype group (Masson et al, 2003). The proportion of variance in the lipid response

metabolism has been avidly demonstrated and merits further exploration.

that may predispose an individual to the onset of several metabolic diseases.

et al., 2005).

researchers (Musunuru., 2010).

**6. Magnitude of the response** 

other studies showing that the *TT* genotype at –514C→T is associated with higher HDL concentrations, although, in 1 of those studies, this effect was attenuated by visceral obesity (Fisler & Warden., 2005).

The third association study of the interaction between dietary fat and the –514C→T polymorphism, by Zhang et al that, from a study population of 18159 men, Zhang et al selected 780 men with confirmed type 2 diabetes. After adjustment for age, smoking, alcohol intake, exercise, and BMI, higher HDL-cholesterol concentrations were found in men with the *CT* or *TT* genotype, which is consistent with previous studies. However, they found significantly higher HDL-cholesterol concentrations in men with the *CT*/*TT* genotype who consumed large amounts of dietary fat (≥32% of energy), saturated fat, and monounsaturated fat, a result that is apparently opposite to the findings of the other 2 association studies. Thus, the interaction effect of dietary fat with the –514C→T polymorphism was not replicated (Zhang et al., 2005).

Two researcher discussed causes of nonreplication of genetic association studies in obesity and diabetes research that should apply to studies of dyslipidemias as well. An important cause of nonreplication is a lack of statistical power. For any polygenic model, such as models with complex phenotypes (eg, obesity, dyslipidemia, or type 2 diabetes), the effect size for any marker will be small to moderate. Thus, larger sample sizes are needed to ensure adequate power to observe an effect. In the study of Zhang et al, the problem of small sample size (despite the fact that >18000 men were screened to identify ≈800 men with type 2 diabetes) is compounded by the fact that the *TT* genotype is rare, especially in a white population. Thus, only 30 subjects in that study had homozygous *TT* genotype at –514C→T LIPC, and subjects with either the *CT* or *TT* genotype were pooled for analysis. Examination of the data of Ordovas et al and Tai et al found that the slope of predicted values for HDL cholesterol versus total fat intake as a percentage of energy is steeply negative in persons with the *TT* genotype, whereas it is positive for persons with the *CT* genotype (Ordovas, et al, 2002; Tai et al, 2003). Assuming that the data of Zhang et al followed the same pattern, combining the smaller number (*n* = 30) of persons with the *TT* genotype with the larger number (*n* = 247) of persons with the *CT* genotype would mask the effects of percentage of dietary fat and the *TT* genotype on HDL-cholesterol concentrations. An additional complexity is that BMI and obesity phenotypes may also interact with dietary fat and LIPC genotype to modulate HDL-cholesterol concentrations (Tai et al., 2003; Zhang., 2005). Thus, the nonreplication in the study by Zhang et al of the findings of Ordovas et al and Tai et al is likely due to the small number of persons with the *TT* genotype who were available in the study of Zhang et al (Fisler & Warden., 2005).

#### **5.7 Lipoprotein lipase (LPL)**

LPL, encoding lipoprotein lipase, which hydrolyzes triglycerides in chylomicrons and VLDL particles, converting the latter to LDL particles, as well facilitating cellular lipoprotein uptake (Musunuru, 2010)**.** 

Several polymorphisms have been described that disrupt normal LPL function and contribute to the premature development of CHD, primarily through the increased levels of circulating TGs (Much et al., 2005).

Indeed, several of the common LPL polymorphisms described by Merkel et al have recently been established to influence circulating lipid levels in pregnant women, whom are often characterized with high levels of circulating TGs and increased total cholesterol (Merkel et

other studies showing that the *TT* genotype at –514C→T is associated with higher HDL concentrations, although, in 1 of those studies, this effect was attenuated by visceral obesity

The third association study of the interaction between dietary fat and the –514C→T polymorphism, by Zhang et al that, from a study population of 18159 men, Zhang et al selected 780 men with confirmed type 2 diabetes. After adjustment for age, smoking, alcohol intake, exercise, and BMI, higher HDL-cholesterol concentrations were found in men with the *CT* or *TT* genotype, which is consistent with previous studies. However, they found significantly higher HDL-cholesterol concentrations in men with the *CT*/*TT* genotype who consumed large amounts of dietary fat (≥32% of energy), saturated fat, and monounsaturated fat, a result that is apparently opposite to the findings of the other 2 association studies. Thus, the interaction effect of dietary fat with the –514C→T

Two researcher discussed causes of nonreplication of genetic association studies in obesity and diabetes research that should apply to studies of dyslipidemias as well. An important cause of nonreplication is a lack of statistical power. For any polygenic model, such as models with complex phenotypes (eg, obesity, dyslipidemia, or type 2 diabetes), the effect size for any marker will be small to moderate. Thus, larger sample sizes are needed to ensure adequate power to observe an effect. In the study of Zhang et al, the problem of small sample size (despite the fact that >18000 men were screened to identify ≈800 men with type 2 diabetes) is compounded by the fact that the *TT* genotype is rare, especially in a white population. Thus, only 30 subjects in that study had homozygous *TT* genotype at –514C→T LIPC, and subjects with either the *CT* or *TT* genotype were pooled for analysis. Examination of the data of Ordovas et al and Tai et al found that the slope of predicted values for HDL cholesterol versus total fat intake as a percentage of energy is steeply negative in persons with the *TT* genotype, whereas it is positive for persons with the *CT* genotype (Ordovas, et al, 2002; Tai et al, 2003). Assuming that the data of Zhang et al followed the same pattern, combining the smaller number (*n* = 30) of persons with the *TT* genotype with the larger number (*n* = 247) of persons with the *CT* genotype would mask the effects of percentage of dietary fat and the *TT* genotype on HDL-cholesterol concentrations. An additional complexity is that BMI and obesity phenotypes may also interact with dietary fat and LIPC genotype to modulate HDL-cholesterol concentrations (Tai et al., 2003; Zhang., 2005). Thus, the nonreplication in the study by Zhang et al of the findings of Ordovas et al and Tai et al is likely due to the small number of persons with the *TT* genotype who were available in the

LPL, encoding lipoprotein lipase, which hydrolyzes triglycerides in chylomicrons and VLDL particles, converting the latter to LDL particles, as well facilitating cellular lipoprotein

Several polymorphisms have been described that disrupt normal LPL function and contribute to the premature development of CHD, primarily through the increased levels of

Indeed, several of the common LPL polymorphisms described by Merkel et al have recently been established to influence circulating lipid levels in pregnant women, whom are often characterized with high levels of circulating TGs and increased total cholesterol (Merkel et

(Fisler & Warden., 2005).

polymorphism was not replicated (Zhang et al., 2005).

study of Zhang et al (Fisler & Warden., 2005).

**5.7 Lipoprotein lipase (LPL)** 

circulating TGs (Much et al., 2005).

uptake (Musunuru, 2010)**.** 

al., 2011). Although TG levels were unaffected, certain LPL SNPs modulated HDL levels and may alter the susceptibility of pregnant women to developing CHD. However, further studies are required to definitively define a relationship between lipid levels, CHD, and pregnant women. Therefore, the importance of LPL in the whole-body regulation of lipid metabolism has been avidly demonstrated and merits further exploration.

One common LPL polymorphism, known as T495G *Hind* III, has been extensively examined and demonstrates the complexity of disease prediction associated with a single SNP. Indeed, preliminary indications suggest that this polymorphism may play a role in the onset of several important diseases, such as CHD, diabetes and obesity. This SNP has been associated with the higher plasma TG and lower HDL levels characteristic with the early onset of diabetes. Preliminary results have also suggested a positive association between the *Hind* III polymorphism and a predisposition to developing obesity. Finally, this polymorphism has been associated with variations in lipid levels and heart disease, and that these alterations were attenuated by such environmental factors as physical exercise and low calorie diets, reiterating the important interactions arising between lifestyle, nutrition, and disease. Although these associations are not conclusive, they do suggest that LPL variants play a critically important role in the regulation of whole-body lipid metabolism that may predispose an individual to the onset of several metabolic diseases.

A relationship was established between a low calorie diet and the circulating lipid profile in obese individuals with the *Hind* III polymorphism. Homozygotes (H2H2) were found to have significantly higher levels of plasma VLDL-TG and Apo B than heterozygotes. Caloric restriction reduced lipid levels in both H2H2 and H1 individuals to a point where no difference was observed between the groups. Although H2H2 individuals responded more strongly (larger decreases in plasma lipids) to the low calorie diet, these preliminary results identify an important relationship between LPL polymorphisms, function, and diet (Much et al., 2005).

As such, it is difficult to draw any firm conclusions from any one gene-diet study in the absence of replication by another study that examined the same question using similar methodologies. For example, one study demonstrated that a Mediterranean-style, MUFArich diet compared to a high-carbohydrate diet increased LDL size in individuals with certain Apo E gene variants but decreased LDL size in those with other Apo E variants; this is potentially a clinically important observation, but no confirmatory study has yet emerged, calling this observation into doubt. As pointed out by others, the field would greatly benefit from increased collaboration and coordination of studies among international nutrition researchers (Musunuru., 2010).
