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Diabetologia

, Volume 49, Issue 10, pp 2337–2340 | Cite as

The apolipoprotein A-V genotype and plasma apolipoprotein A-V and triglyceride levels: prospective risk of type 2 diabetes. Results from the Northwick Park Heart Study II

  • P. J. Talmud
  • J. A. Cooper
  • H. Hattori
  • I. P. Miller
  • G. J. Miller
  • S. E. Humphries
Short Communication
First Online:
Received:
Accepted:

Abstract

Aims/hypothesis

We sought to establish the relationship between plasma apolipoprotein A-V (APOA5, previously known as apoA-V) and triglyceride levels and to determine the impact of the APOA5 genotype on APOA5 levels and development of type 2 diabetes in a 15-year follow-up study of healthy UK men.

Materials and methods

APOA5 −1131T>C and S19W genotypes were determined in 2,490 men, of whom 145 subsequently developed type 2 diabetes. In a subset of 299 men, we also determined APOA5 levels.

Results

Plasma APOA5 levels positively correlated with triglycerides (r=0.18, p<0.002) and were not different in men who subsequently developed type 2 diabetes compared with healthy men (p=0.7). Carriers of either APOA5 W19 or −1131C had, as expected, higher plasma triglycerides. However, while W19 carriers had significantly higher APOA5 levels (p=0.0003), APOA5 levels were not associated with −1131T>C (p=0.63), reinforcing the idea that the reported −1131C association with triglycerides levels is due to linkage disequilibrium with variants in the APOC3 gene, and not due to the direct effect on APOA5 levels. Overall no effect of APOA5 −1131T>C or S19W was found on type 2 diabetes risk.

Conclusions/interpretation

In contrast to animal studies, in man, plasma APOA5 positively correlates with plasma triglyceride levels. In prospective analysis, with the caveat that numbers were small, APOA5 genotypes do not appear to have an impact on risk of development of type 2 diabetes.

Keywords

APOA5 ApoAV levels Gene Type 2 diabetes risk 

Abbreviations

APOA5

Apolipoprotein A-V

APOC3

Apolipoprotein C III

HR

Hazard ratio

NPHSII

Northwick Park Heart Study II

SNP

Single-nucleotide polymorphism

TG

Triglyceride

Introduction

Overexpression and knock-out mouse models of apolipoprotein A-V (APOA5, previously known as apoAV) demonstrate an inverse relationship between APOA5 and triglyceride (TG) levels [1] and show that APOA5 plays a key role in TG metabolism. This is supported by epidemiological studies reporting that variation in the APOA5 gene is consistently associated with differences in plasma TG levels in healthy individuals [2, 3] of different ethnicity [2]. Since hypertriglyceridaemia reflects the dyslipidaemia associated with type 2 diabetes, variation in APOA5 could contribute to the development of type 2 diabetes. Two haplotypes, APOA5*2 and APOA5*3, show association with raised plasma TG levels [2] and can be ‘tagged’ by the rare genotype for −1131T>C and coding single nucleotide polymorphism (SNP) 56C>G (S19W), respectively [4].

In this study we examined the correlation of plasma APOA5 with TG levels and the association with APOA5 genotypes in a subset of the men participating in the prospective Northwick Park Heart Study II (NPHSII). We also examined the association of two APOA5 SNPs with the 15-year prospective risk of developing type 2 diabetes in these middle-aged UK men, who entered the study in good health.

Subjects, materials and methods

Northwick Park Heart Study II

Between 1989 and 1994, a total of 3,012 healthy middle-aged Caucasian men were recruited from nine primary care practices in the UK for prospective surveillance. The study was approved by local ethics committees and all subjects gave written informed consent. Full details of the study are presented elsewhere [5]. In 2004, information on the development of type 2 diabetes since baseline was obtained from the medical practices by searching individual records for a diagnosis and treatment of type 2 diabetes [6]. Type 2 diabetes was diagnosed on the basis of the WHO guidelines in place at the time of diagnosis.

Determination of plasma APOA5

Plasma APOA5 levels were determined by ELISA [7] in 299 randomly selected samples.

DNA extraction and APOA5 genotyping

DNA extraction and APOA5 −1131T>C (rs662799) and S19W (rs3135506) genotyping methods have already been fully detailed [3]. Genotype was confirmed by two independent researchers blind to subject outcome, with discrepancies resolved by repeat genotyping.

Statistical analysis

Baseline characteristics for plasma TG and APOA5 levels were log transformed to a normal distribution. Results are presented as hazard ratios (HRs) obtained from Cox regression models with their corresponding 95% CIs, adjusted for age and practice (recruitment site) with further adjustment as described. Frequencies were compared by Fisher’s exact test. Statistical significance was taken as p<0.05.

Results

Subject characteristics

Of the 3,052 men recruited into the study, 76 with type 2 diabetes on entry were excluded from further analysis. Full phenotypic and APOA5 genotypic data were available on 2,490 men for −1131T>C and 2,431 men for the S19W, and in this group 145 men developed type 2 diabetes during follow-up. The baseline characteristics of these men have been reported elsewhere [6]. Baseline BMI, TG, cholesterol, HDL and blood pressure were all associated with increased risk of developing type 2 diabetes, with BMI conferring the highest risk (HR=1.86 [1.65–2.10], p<0.0001, per increase of 1 SD) [6].

Plasma APOA5 levels

Plasma APOA5 levels were measured in a subset of 299 men chosen at random. Baseline characteristics of these men were not significantly different from those in the entire cohort (data not shown). The mean plasma APOA5 levels were 113.6±50.3 ng/ml (SD) and, contrary to expectation, APOA5 was positively correlated to TG (r=0.18, p<0.002), total cholesterol (r=0.15, p=0.008), C-reactive protein (r=0.15, p=0.01) and BMI (r=0.18, p=0.002). There was no difference in APOA5 levels when stratified by future development of type 2 diabetes (non-type 2 diabetes, 112.8±50.3 ng/ml (n=273), vs type 2 diabetes, 117.8±48.1 ng/ml, p=0.71, n=16), but numbers of men with type 2 diabetes in this group are small. Carriers of W19 had significantly higher plasma APOA5 levels than S19 homozygotes (p=0.0003) independently of BMI, TG and cholesterol (p=0.0001), and in the sample as a whole had significantly higher TG levels (p=0.007) (Fig. 1). Carriers of the −1131C allele had higher TG levels than −1131TT men (p=0.003); however, there was no genotype association with plasma APOA5 levels.
Fig. 1

Mean unadjusted plasma APOA5 (±SEM) (a) and triglycerides (±SEM) (b) in men by APOA5 S19W and −1131T>C genotype. The number of each genotype class is shown at the foot of the bar

Effect of APOA5 genotype on plasma TG and future risk of diabetes

The distributions of the APOA5 S19W and −1131T>C genotypes were in Hardy–Weinberg equilibrium. There was no significant genotype or allele frequency difference between those who subsequently developed type 2 diabetes and those who did not (Table 1).
Table 1

Rare allele frequencies of APOA5 S19W and −1131T>C and hazard ratios (HR) in subjects who went on to develop type 2 diabetes and those who did not, in the Northwick Park Heart Study II

 

Non-diabetic

Diabetic

p-value for difference

HRa (95% CI)

HRb (95% CI)

S19W

SS

2,027

133

0.58

SS 1.00

1.00

SW

248

12

–

W+0.67 (0.36–1.23)

0.62 (0.33–1.15)

WW

11

0

–

p=0.20

p=0.13

Allele frequency (95% CI)

0.059 (0.05–0.07)

0.041 (0.02–0.06)

0.21

–

–

−1131T>C

TT

2,067

131

0.38

TT 1.00

1.00

TC

267

11

–

C+0.56 (0.29–1.07)

0.51 (0.27–0.98)

CC

14

0

–

p=0.08

p=0.04

Allele frequency (95% CI)

0.063 (0.06–0.07)

0.039 (0.02–0.06)

0.10

–

–

aAdjusted for age and recruitment practice

bAdjusted for age, recruitment practice and TGs

Discussion

Plasma APOA5 and TG

In contrast to the prediction from the APOA5 mouse models of an inverse relationship between APOA5 and TG levels [1], the major finding of this study is the positive correlation between plasma APOA5 and TG. Circulating APOA5 levels are approximately 300-fold lower than APOC3, but if, as suggested, APOA5 recycles between particles [8], these low levels would be sufficient for its suggested catalytic role in TG metabolism. Thus total plasma APOA5 might not be as critical as the relative distribution of APOA5 between HDL and VLDL particles. The reason for the positive correlation between APOA5 and TG in healthy men and the negative correlation between the two in mouse models is unclear, but it suggests that in this case extrapolation from animal models to man should be made with caution.

APOA5 genotypes and APOA5 levels

The functionality of S19W has been demonstrated in vitro, with the W19-signal peptide showing 50% lower secretion of a reporter protein than the S19-signal peptide [9]. It is thus surprising that, compared with S19 homozygotes, W19 carriers had significantly higher APOA5 levels. Circulating mature APOA5 no longer has a signal sequence and will be identical whether the signal peptide originally had a serine or a tryptophan at residue 19. In order to explain the discrepancy between predicted and observed plasma levels of APOA5 by genotype, these data suggest that APOA5 of W19 origin is less efficiently degraded or cleared. One possibility is that APOA5 of S19 or W19 origin could be associated with different types of lipoprotein particles of different metabolic fates. This concept is supported by the different lipoprotein profiles of people with different S19W genotypes [10].

The −1131T>C SNP was not associated with differences in APOA5 levels, although the −1131C allele was associated with higher TG levels [2, 3]. Published molecular analysis does not support the functionality of 1131T>C or other SNPs that define haplotype APOA5*2 [9]. Haplotype analysis predicts that −1131T>C acts as a genetic marker for variants within the closely linked APOC3 gene, due to strong linkage disequilibrium [3], thus the lack of association with plasma APOA5 levels is not unexpected.

APOA5 genotype and type 2 diabetes risk

This study had 80% power at the 5% level to detect a relative risk of 1.77 for diabetes for 19W+vs 19SS (or relative risk of 0.37 if association is negative), and a relative risk of 1.74 (or 0.38) for −1131C+vs −1131TT. This limited power to detect an effect of APOA5 genotypes on future risk of type 2 diabetes suggests that any such risk would be less than 1.77. Since numbers of affected men who carried the rare alleles of either APOA5 SNP in this study were low, these results need to be confirmed in a larger study, and in addition in women. One of the limitations of this study is the absence of any fasting glucose or insulin measures at the baseline assessment or subsequent follow-up visits to confirm diagnosis. Identification of the men with type 2 diabetes by the medical record search is unlikely to include any false-positive diagnosis but in the absence of a full recall for fasting glucose testing some subjects may have been missed.

Most of what we know about APOA5 is inferred from animal and in vitro studies, which suggested that the potential actions of APOA5 are intracellular inhibition of hepatic VLDL secretion and the promotion of catabolism of TG-rich plasma lipoproteins [11]. These present results highlight the need for additional in vivo and in vitro studies to clarify the role of APOA5 in human TG metabolism.

Notes

Acknowledgements

We thank the subjects and general practices involved in the study. P. J. Talmud, J. A. Cooper and S. E. Humphries are supported by the British Heart Foundation (FS/2002/083 and RG2005/15). The Northwick Park Heart Study II was supported by the British Medical Research Council, the US National Institute of Health (grant NHLBI 33014) and Du Pont Pharma, Wilmington, DE, USA.

Duality of interest

The authors have stated that there was no duality of interest associated with this study.

References

  1. 1.
    Pennacchio LA, Olivier M, Hubacek JA et al (2001) An apolipoprotein influencing triglycerides in humans and mice revealed by comparative sequencing. Science 294:169–173PubMedCrossRefGoogle Scholar
  2. 2.
    Pennacchio LA, Olivier M, Hubacek J, Krauss RM, Rubin EM, Cohen JC (2002) Two independent apolipoprotein A5 haplotypes influence human plasma triglyceride levels. Hum Mol Gen 11:3031–3038PubMedCrossRefGoogle Scholar
  3. 3.
    Talmud PJ, Hawe E, Martin S et al (2002) Relative contribution of variation within the APOC3-A4-A5 gene cluster in determining plasma triglycerides. Hum Mol Gen 11:3039–3046PubMedCrossRefGoogle Scholar
  4. 4.
    Olivier M, Wang X, Cole R et al (2004) Haplotype analysis of the apolipoprotein gene cluster on human chromosome 11. Genomics 83:912–923PubMedCrossRefGoogle Scholar
  5. 5.
    Miller GJ, Bauer KA, Barzegar S, Cooper JA, Rosenberg RD (1996) Increased activation of the haemostatic system in men at high risk of fatal coronary heart disease. Thromb Haemost 75:767–771PubMedGoogle Scholar
  6. 6.
    Gable DR, Stephens JW, Cooper JA, Miller GJ, Humphries SE (2005) Variation in the UCP2-UCP3 gene cluster predicts the development of type 2 diabetes in the Northwick Park Heart study II. Diabetes 55:1504–1511CrossRefGoogle Scholar
  7. 7.
    Ishihara M, Kujiraoka T, Iwasaki T et al (2005) A sandwich enzyme-linked immunosorbent assay for human plasma apolipoprotein A-V concentration. J Lipid Res 46:2015–2022PubMedCrossRefGoogle Scholar
  8. 8.
    Merkel M, Heeren J (2005) Give me A5 for lipoprotein hydrolysis! J Clin Invest 115:2694–2696CrossRefGoogle Scholar
  9. 9.
    Talmud PJ, Palmen J, Putt W, Lins L, Humphries SE (2005) Determination of the functionality of common APOA5 polymorphisms. J Biol Chem 280:28215–28220PubMedCrossRefGoogle Scholar
  10. 10.
    Talmud PJ, Martin S, Taskinen MR et al (2004) APOA5 gene variants, lipoprotein particle distribution and progression of coronary heart disease: results from the LOCAT study. J Lipid Res 45:750–756PubMedCrossRefGoogle Scholar
  11. 11.
    van Dijk KW, Rensen PC, Voshol PJ, Havekes LM (2004) The role and mode of action of apolipoproteins CIII and AV: synergistic actors in triglyceride metabolism? Curr Opin Lipidol 15:239–246PubMedCrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • P. J. Talmud
    • 1
  • J. A. Cooper
    • 1
  • H. Hattori
    • 2
  • I. P. Miller
    • 3
  • G. J. Miller
    • 4
  • S. E. Humphries
    • 1
  1. 1.Division of Cardiovascular Genetics, Department of MedicineRoyal Free and University College Medical SchoolLondonUK
  2. 2.Department of Advanced Medical Technology and DevelopmentBiomedical Laboratories IncSaitamaJapan
  3. 3.Cardiovascular Genetics Division, Department of Internal MedicineUniversity of Utah School of MedicineSalt Lake CityUSA
  4. 4.Medical Research Council Cardiovascular Group, Centre for Environmental and Preventive MedicineWolfson Institute of Preventive MedicineLondonUK

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