Lipid Metabolizing Genes

1
Lipid Metabolizing Genes
2
Cholesterol Metabolism

• CETP
• Cholesterol ester transfer protein
• Closely linked to HDL-C
• HDL-C functions in reverse cholesterol transport
• Removes excess cholesterol from periphery
• Excess cholesterol is esterified to make
  cholesterol esters
• CETP and HDL-C
• Cholesterol esters may be transferred from HDL-C
  to LDL-C
• Performed by CETP

3
CETP

• Overactive CETP transfers more cholesterol from
  HDL-C to LDL-C
• Underactive CETP results in more cholesterol
  remaining with HDL-C
• CETP Inhibitors (Stein Stein, Athersclerosis
  2005)
• Increase HDL-C
• Reduce CVD risk

4
CETP and genetic variants
16/Ex14
9/3
383/In8
VNTR-1946
-629/Prom
82/Ex15
199/In12
279/In1
1
2
3 4 5
11
12 13 14
15 16
6 7 8
9
10
Taq1B
MspI
I405V
R451Q
Thompson et al. 2003 Atherosclerosis
5
CETP 279 G?A

• Commonly referred to as Taq1B SNP
• Associated with lower CETP mass and higher HDL
• Intronic SNP
• No obvious link to functional activity
• Linked to another common SNP?
• Linked SNP tend to be inherited together
• Linkage Disequilibrium

6
Linkage Disequilibrium

• SNPs are often inherited together and travel as
  blocks from generation to generation, this is
  known as linkage disequilibrium (Duff 2006, AJCN)
• Linkage disequilibrium can be described as the
  inheritance of same chromosome SNPs together at a
  frequency greater than chance.
• Occurs when genotype frequencies at one locus are
  dependent on genotype frequencies at the second
  (locushttp//www.evotutor.org/EvoGen/EG4A.html)
• Can arise from physical linkage, genetic drift
  and selection on multilocus genotypes
• Two loci are in linkage equilibrium if genotype
  frequencies at one locus are independent of
  genotype frequencies at the second locus
  (locushttp//www.evotutor.org/EvoGen/EG4A.html)

7
Haplotypehttp//www.genome.gov/10001665

• Groups of co-inherited SNPs on the same
  chromosome (Duff 2006, AJCN)
• There could be many haplotypes in a chromosome
  region
• Recent studies are finding only a few common
  haplotypes.
• For CETP
• CETP 279G?A is closely linked to -629C?A (Klerkx
  et al 2003 Human molecular genetics)
• Four possible haplotypes for these 2 SNPs
• GC, GA, AC, AA
• Likely only two are common
• GC (normal), AA (variant)

8
Haplotype Example 1

• Region with 6 SNPs
• DNA bases that are same are not shown
• First SNP has alleles A and G
• Second SNP has alleles C and T
• 4 possible haplotypes for these 2 SNPs
• AC, AT, GC, GT
• Only AC and GT are common
• These SNPs are said to be highly associated with
  each other (ie, in linkage disequilibrium)

9
Haplotype Example 2

• If a region has only a few haplotypes
• Only a few SNPs need to be typed to determine
  which haplotype a chromosome has
• Example below
• Determine SNP 1 and 4 and will know haplotype
• Saves time and money

10
CETP and Haplotype
Knoblauch et al. 2002
11
Knoblauch et al. 2002
12
Examination of haplotype yields a more
significant effect than examination of SNPs
individually.
13
123456
1 2 3 4 5 6
A/C
G/A
Linkage Disequilibrium 279 A allele is
always inherited with the -629 C allele. A
allele is a marker only (not a functional SNP)
14
Lipid Metabolizing Genes
15
LPL (Lipoprotein Lipase)

• Hydrolyzes triglycerides of chylomicrons (CM) and
  VLDL

TG degradative products
CM
CM Remnant
Liver
Intestine
LPL
TG degradative products
Liver
LDL
Tissues
VLDL
LPL
16
LPL Activity

• Positive relationship with HDL-C
• gt LPL activity gt serum HDL-C
• Via contributing surface components to HDLC
• Negative relationship with TGs
• gtLPL activity lt serum TGs

17
LPL GeneKnoblauch et al. 2002, Human Molecular
Genetics
LPL 1595 C?G
Cytogenic Location 8p22
18
LPL C1595G

• More commonly referred to as the 447 Serine?Stop
  substitution (S447X)
• Located in exon 9
• Produces a truncated protein lacking two amino
  acids (serine-glycine) at the carboxy terminus
• Appears to be a functional polymorphism

19
X447 allele (LPL 1595 G allele)

• Associated with
• Increased enzyme activity
• Lower serum TGs
• Higher serum HDL-C
• Reduced risk of CAD (OR 0.4)
• Frequency is higher in Caucasians than African
  Americans

(Chen et al. 2001 Atherosclerosis)
20
HDL-C and TGs in carriers and non-carriers of the
LPL 1595 G allele
Plt0.01
Plt0.05
Combined data from adult Causasian and African
American men and women
(Chen et al. 2001 Atherosclerosis)
21
Lipid Metabolizing Genes
22
Apolipoprotein CIII (ApoC-III)

• Located on Chromosome 11
• 11p21 (short arm)
• Mainly synthesized in liver
• Lesser degree in intestine
• Component of TG rich lipoproteins (VLDL and CMs)
  and HDL
• Function is poorly understood

23
Proposed Function(s)

• Triglyceride Metabolism
• Invitro Studies
• Non-competitive inhibitor of LPL
• Reduced VLDL and CM catabolism
• Mice studies
• Overexpression of human apo C-III resulted in
  marked hypertriglyceridemia
• Humans
• Elevated apoC-III levels are associated with
  hypertriglyceridemia

24
ApoC-III 3175 C?G

• Exon 4 of Chromosome 11p21
• Variant is transcribed but not translated
• Does not change protein sequence
• May be linked to another variant that alters
  (increases) Apo C3 expression
• Commonly referred to as SstI
• Identified with SstI (restriction enzyme)
• May also include variants at -641, -482, -455,
  1100
• Group is often referred to as Sst1 S1/S2 site
  (Tobin et al. 2004)
• Linked to elevated triglycerides
• Associated with premature heart disease in some
  studies

25
ApoC-III 3175 C?G and TGs in controls

• Additional Comments
• Frequency of G allele
• tended (P0.06) to be
• higher in cases with CVD
• More studies are needed to
• determine how a mutation at
• this site may be linked to other
• mutations that affect protein
• function.

Ordovas et al. 1991 Atherosclerosis
26
ApoC3 and Haplotype and CVD riskC(-641)A,
C(-482)T, T(-455)C, C1100T, C3175G, T3206G
Tobin et al. 2004 European Heart Journal
27
Lipid Metabolizing Genes
28
Personalized Dietary Recommendations

• CETP 279 G?A
• Non-carriers
• More likely to have low HDL-C
• Diet has little impact on HDL-C
• Increase consumption of MUFAs (?)
• Be active (30 min /day)
• Weight loss if appropriate (i.e, BMI range
  19-26)
• LPL 1595 C?G
• Non-carriers
• More likely to have low HDL-C and high TGs
• Restrict saturated fats (i.e, lt 20 g/d)
• Restrict refined CHO intake (i.e., 40 g /d)
• Achieve or maintain appropriate body weight (i.e,
  BMI range 19-26)
• Fish oil capsules?
• APO C3 3175 C?G
• Carriers
• More likely to have high TGs
• Restrict saturated fats (i.e., 20 g/d)
• Restrict refined CHO intake