Genomics of Dyslipidemia
→ Trends in Cardiovascular Medicine, August 2009
George Lewis Ngo
PharmD Student
University of Georgia
College of Pharmacy
April 16, 2012
Back To The Basics



Major lipids in the body

Cholesterol

Triglycerides

Phospholipids
Transported as complex of lipid and proteins →
lipoproteins
3 major classes of lipoproteins

1) LDL (low-density lipoproteins)

2) HDL (high-density lipoproteins)

3) VLDL (very low-density lipoproteins)
Back To The Basics
Back To The Basics


Cholesterol and TGs play a
role in:

Cell membrane
formation

Hormone synthesis

Source of free fatty
acids
•
•
LDLs: ~60% cholesterol
•


HDLs: ~30% phospholipid,
~25% cholesterol, 10% TG
VLDLs: ~55-80% TGs
Definition of dyslipidemia:
•
↑ Total cholesterol
•
↑ LDL
•
↑ Triglyceride
(TGs)
Apolipoproteins: the protein
part of the lipoprotein
Chylomicrons: large TG-rich
(~95%) particles that contain
apolipoproteins B-48, B-100,
and E
Lipid Metabolism
LIVER, ADRENAL,
SMOOTH MUSCLE,
FIBROBLASTS
ACAT: Stores
LDL in cells
Classification
The Problem...

Increased risk of
cardiovascular
complications due to:
2) cerebrovascular disease

↑ total cholesterol

↑ LDL cholesterol 3) peripheral vascular


1) coronary heart disease
(CHD)
↓ HDL
cholesterol
Coronary atherosclerosis →
ischemic heart dx and
myocardial infarction (MI)
disease (PVD)
The Gameplan

National Cholesterol Education Program (NCEP)
Adult Treatment Panel III (ATP III)


Management of hypercholesterolemia
American Heart Association (AHA)

Primary and secondary prevention of CHD
Epidemiology



Total cholesterol and LDLC increase throughout life
in men and women
• < half of highest risk ppl
(sx of CHD) are rec'ving
lipid-lowering tx
~50% or nearly 105 million
American adults >20 years
have total cholesterol
≥200mg/dL
• Only 26% of pts have
optimal LDL-C (<100
mg/dL)
> half of ppl with
borderline to high risk are
unaware that they have
hypercholesterolemia
• We are not doing well in
treating dyslipidemia.
• The bottom line:
The Silver Lining



Average total cholesterol in the U.S. has fallen from
220 mg/dL in 1960-1962 to 203 mg/dL in 2002.
Compared with other countries, the U.S. has shown
the greatest decline in coronary artery dx (CAD)
(35-40%) in men and women over the last 10 years
compared to other countries
We still have a long way to go.
Framingham Risk Scoring


Risk for developing
cardiovascular (CV) dx
is related to total
cholesterol and LDL
Presence of established
CHD or prior MI ↑ risk
of MI 5 to 7 times that
seen in ppl w/o CHD
•
Additive risk factors:
•
Cigarette smoking
•
HTN
•
DM
•
↓ HDL levels
•
EKG
abnormalities
Treatment Goals
It's RX Time!
Article

Khera, Amit V, and Daniel J Rader. "Discovery And
Validation Of New Molecular Targets In Treating
Dyslipidemia: The Role Of Human Genetics."
Trends In Cardiovascular Medicine 19.6 (2009):
195-201. MEDLINE with Full Text. Web. 16
Apr. 2012.
Article Background


Careful study of pts w/ monogenic disorders has
helped us identify roles of cholesterol in
atherosclerosis and molecules for drug targets
Ex. Familial hypercholesterolemia
Some Food For Thought


JUPITER trial suggested
that measuring C-reactive
protein levels can help
identify pts w/o ↑ LDL-C
who still might benefit
from statin tx
Tx of HDL-C and TG
levels has been each
linked with clinical
outcomes in recent statin
trials
• Lipoprotein(a) levels
have been linked with
CHD and may be the
next big thing...to treat
that is
• As many as 26 drugs at
one time for lipid
disorders were in the
drug pipeline in 2009
A Little Too Enthusiastic?

Ezetimibe (Zetia) blocks
intestinal absorption of
LDL


Failed to show ↓
carotid medial
thickness
Fenofibrate (Tricor) is
PPARα agonist that ↓ TG
levels

Failed to show ↓
coronary events in
DMII pts
• Torcetrapib is CTEP
inhibitor that ↓ LDL levels
but ↑ HDL levels
•
Showed ↑ in all
cause mortality
• Led us to believe... maybe
there's a lot more than
just lowering LDL levels
and targeting certain
molecules in the body...
Using the Human Genome

1970s: Nobel Prizewinning work estab
familial
hypercholesterolemia


Cluster of genetic
disorders in LDL
receptor. Leads to
↑ LDL and early
onset CHD
Drugs now targeting
LDL rcptr
• Class I: LDL rcptr not made
• Class II: LDL rcptr not
transported to cell surface
• Class III: LDL rcptr not
binding LDL
• Class IV: LDL rcptr bound to
LDL not being endocytosed
• Class V: LDL rcptr not being
recycled back to cell surface
Mendelian Randomization





Tool for scientists to study genetic epidemiology
Population variation in genes can be studied to look
for effects of new drugs on specific mechanisms of
disease
Can be used to show causality
WE CAN FOCUS ON the random genetic variation
assigned at birth via processes of meiosis and
chromosomal assortment
Mimics randomization during clinical trials
More Terminology


Single nucleotide polymorphisms (SNPs)

When one nucleotide in the DNA sequence of the
genome differs btwn individuals of a population

ATCG vs ACCG
Locus → Think LOCATION




Where is the gene on the chromosome?
SNPs at >30 loci were found to be associated with plasma lipid
concentrations!!!!!
What does this mean? COMPLEXITY...
Some loci are already known to play role in lipid metabolism,
whereas others are found in regions with no prior evidence
Chromosome
Beautiful LDL Receptor Pathway
REGN727
PCSK9
Source:McPherson & Pincus: Henry's Clinical Diagnosis and Management by Laboratory Methods, 21 st
ed. Copyright 2006 W.B.Saunders Company
http://www.itd.unair.ac.id/files/ebook/Henry%27s%20Clinical%20Diagnosis%20and%20Management%20
by%20Laboratory%20Methods/1393/i4-u1.0-b1-4160-0287-1..50021-5--f3.fig.htm
I Want Examples!!!

HMG-CoA reductase (hydroxymethylglutarylcoenzyme A)


PCSK9 (protease proprotein convertase
subtilisin/kexin type 9)


Rate-limiting step of cholesterol biosynthesis
Enzyme that degrades LDL rcptrs on cell surface
SNPs @ the locus (chromosomal region) 1p13

CELSR2, PCRC1, SORT1
HMG-CoA Reductase


A common variant in the locus that codes for HMGCoA reductase has been shown to lead to small
decreases in LDL-C
Even though the SNPs lead to small effects on LDLC, pharmacologic inhibition of HMG-CoA reductase
leads to...


a much larger ↓ in LDL and ↓ in risk of coronary
events
Just because a SNP leads to ↓ enzyme activity and ↓
in LDL does not necessarily mean there will be a ↓
in mortality
PCSK9

Rare mutations in certain
pts led to ↑ in PCSK9 fxn


These individuals
had severe
hypercholesterole
mia
Mutations found in pts with
very low LDL-C showed a
↓ in PCSK9 fxn

2006: 2-3% AA pts
showed 28% ↓ in
LDL-C and 88% ↓ in
CHD risk
- 2006: Another
mutation in whites
led to a similar but
less dramatic ↓ in
LDL-C and CHD
risk
• Another study showed that
a common PCSK9 SNP
that showed a ↓ in 3mg/dL
of LDL-C and 13% ↓ CHD
risk
• Antibodies that target
PCSK9 – ex. REGN727!!!
SNPs at locus 1p13

CELSR2, PCRC1, SORT1


3 genes that may be related to LDL-C metab and
CHD
Genetic resequencing / manipulation of these genes
in model organisms may lead to changes in LDL-C
and CHD risk
Complex Interplay
Genetic polymorphisms;
Examination
of Mendelian
randomization
Drug mechanism
of action;
↓ or ↑ in certain
biomolecular
target
Clinical Efficacy
Bottom Line


What is useful clinically?

1) genetics plays role in different pts in terms of
how they store or metabolize LDL

2) mechanism of LDL lowering by drugs

3) magnitude of LDL lowering by drugs
It is still of utmost importance to lower the LDL
cholesterol in pts with dyslipidemia
Triglycerides

Hypertriglyceridemia


↑ chylomicrons, ↑
VLDLs, ↑ IDLs
These molecules
enter existing
lesions, are
proinflammatory,
and thus speed up
CHD
• Lipoprotein lipase
(LPL)
•
Expressed on
vascular
endothelium of
skeletal muscle &
adipose tissue
•
Catabolizes VLDL
molecules from
liver and intestine
Triglycerides


↑ LPL activity in mice is • At least 11 SNPs are
identified at different loci
assoc w/ ↓ atherosclerosis
Human genetic LPL
deficiency resulted in
HIGH chylomicrons and
TG in blood

These pts showed
early signs of
atherosclerosis
to be involved in plasma
TG levels
•
Common variants at the
LPL locus was associated
with:
•
•
•
↓ TG,
increased HDL, and
↓ risk of CHD
HDL

Increase in 1mg/dL of HDL
has been shown to decrease
cardiovascular mortality risk
by 3-4%


• CETP mutations have
led to increased HDL
•
Niacin, fibrates,
statins
These mutations have led to
decreased HDL:

ApoA-I (Milano)

ABCA1 (Tangier
dx)

LCAT
•
CETP - Transfer
protein that
exchanges TGs
from VLDL and
LDL for
cholesterol esters
from HDL
Their effect on CHD risk and
atherosclerosis still needs to be
defined more conclusively,
however...
Mutation of ApoA-I (Milano)
• Naturally occurring mutation of apolipoprotein A-1, found in
HDL, which carries cholesterol from tissues to liver
• Discovered in Milan, a small village in Northern Italy where
3.5% of the population has it
• Traced back to a single man
Giovanni Pomarelli, who lived in
this village in 18th century
• ↓ HDL and ↑ TG…
• But significantly ↓
cardiovascular disease!!!
LCAT Deficiency
• Lecithin cholesterol acyltransferase
• Converts free cholesterol to cholesteryl ester (more hydrophobic
form), which is then taken into the core of a lipoprotein molecule
• Working with ApoE and the cholesterol transporter ABCA, it
plays role in maturation of new lipoproteins and remodeling HDL
particles into more spherical forms
• S/s:
• Corneal opacities (fish eye disease)
• Anemia
• Hepatomegaly
• Splenomegaly
• Renal insufficiency
The Bottom Line


Targeting / increasing HDL levels may play less of a
role in clinical outcomes in the future
Looking at genetics to qualify HDL
**functionality** may be more important than just
measuring HDL levels
Lipoprotein(a)



Evidence from genetic
• Role of LPA still poorly
studies shows assoc. of
understood and we need
increased lipoprotein(a)
more studies
levels and CHD/MI risk • Out of all the drugs,
Two LPA gene variants
niacin shows the biggest
were shown to be assoc.
decrease / effect in LPA
w/ CHD
levels
These two SNPs
accounted for 36% of
observed variation in
LPA levels
• Statins do not
significantly decrease
LPA levels
Schematic
Conclusions


We can implement the principles of Mendelian
randomization to help direct drug development
efforts
Several genetic variables are responsible for lipid
metabolism


Segway into →
PHARMACOGENOMICS
More than meets the eye than simply lowering LDL,
increasing HDL, lowering TG...

How is the drug working?

How is the body responding to it?
Level of Evidence IIa-A
References


Khera, Amit V, and Daniel J Rader. "Discovery And
Validation Of New Molecular Targets In Treating
Dyslipidemia: The Role Of Human Genetics."
Trends In Cardiovascular Medicine 19.6 (2009):
195-201. MEDLINE with Full Text. Web. 16
Apr. 2012.
Dipiro's Pharmacotherapy