GENETIC ASSOCIATION OF CANNABINOID RECEPTOR 1 (CNR1) WITH
OBESITY AND OBESITY- RELATED TRAITS
Tesfaye M. Baye1, Yi Zhang1, Edward Smith1, Cecilia J. Hillard2, Jennifer Gunnell1,3, Joel Myklebust4, Roland James3,4, Ahmed H. Kissebah3,4,
Michael Olivier1,4,5, and Russell A. Wilke1,2,3,5
and Molecular Genetics Center, 2Department of Pharmacology and Toxicology, 3Department of Medicine, 4Department of Physiology
5TOPS Center for Obesity and Metabolic Research, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226
1Human
Materials and Methods
Obesity
Study population
Obesity is a major public health concern with both genetic and environmental causes
Environment:
- Obesity can be improved through diet and exercise
- However, sustained weight loss I difficult to achieve
- 2209 individuals from 507 families were recruited through the TOPS (Take Off Pounds
Sensibly, Inc.) membership in 10 Midwest states as described in poster by Yi et al.
Measured phenotypic traits
- BMI, waist and hip circumference
- Fasting glucose, insulin, and Insulin/glucose ratio
- Clinical lipid data: total cholesterol, LDL-cholesterol, HDL-cholesterol, and triglycerides
Genetics:
- The endocannabinioid (eCB) system is a target for obesity treatment
- Acquired changes in CB1 receptor function are associated with weight loss
- Genetic variability in CB1 receptor function may be associated with obesity
Endocannabinoid systems
- Endogenous signaling system
- Multiple sites of action (Figure 1) capable of altering energy homeostasis
- eCB gene polymorphisms linked to human behavior, appetite and energy
expenditure
Hypothesis
tagSNP selection procedure
- Position of the CNR1 gene was obtained from the UCSC human genome browser
- HapMap (http://www.hapmap.org) to download the CEU population
- Haploview (http://www.broad.mit.edu/mpg/haploview) to track LD and find tagSNPs
Six tagSNPs were selected from the CNR1 gene based on HapMap (Figure 2):
- two promoter SNPs,
- three exonic SNPs,
- a SNP within the 3’-UTR
Figure 2. Structure and physical location of the CNR1 gene on
Chromosome 6, along with the six tagging SNPs (outlined by
boxes) identified in European ancestry HapMap.
Genotyping
Since the isolation of Δ9 -tetrahydrocannabinol (THC) from the marijuana plant in the
1960s, the mechanisms by which cannabinoids affect the human body have been the
subject of considerable research efforts. In this report we investigate the genetic
polymporphism of endocannabinoid receptor gene (CNR1) with the hypothesis
“genetic variation in CB1 receptor gene (CNR1) is associated with anthropometric
measures of human obesity and obesity-related lipid metabolic disorders.”
- Invader Assay@, Third wave Technology, USA
- Clustering algorithm- SMP (http://nitschke.brc.mcw.edu/smp-bin/smp_interface.cgi)
Data analysis
-
FBAT- family based association
Covariance adjustment for age and gender
Hardy-Weinberg (H-W) equilibrium statistics
Additive model
Table 1. Single-locus association with obesity and obesity-related traits
trait
BMI
Waist
Hip
Chol
LDL
HDL
TG
0.025
ns
ns
0.095
0.066
ns
0.068
ns
ns
ns
0.068
ns
ns
ns
ns
ns
0.079
ns
ns
0.06
0.04
ns
ns
ns
ns
0.03
ns
0.06
ns
ns
<0.01
0.06
ns
0.04
ns
ns
0.03
<0.01
0.03
ns
ns
0.07
insulin
glucose
0.088
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
SNP
rs806366
rs806368
rs806369
rs806370
rs1049353
rs12720071
Table 2. Haplotype association with obesity and obesity-related traits
Endocannabinoid System
Role of eCB
Cannabinoid
Receptor 2
Cannabinoid Receptor 1 Blockade
CNR1 gene
Site of Action
Adipose tissue
Mechanism(s)
Immune T cells
Family-based association tests revealed significant evidence for association between
CNR1 tagSNPs and obesity related traits at both single SNP and haplotype assocaition
levels
Clinical Implications
Increase adiponectin
Decrease lipid
accumulation
Dyslipidemia
Insulin resistance
Decrease food intake
Abdominal obesity
Body weight
Liver
Decrease lipogenesis
Decrease fatty acid
synthesis
Dyslipidemia
Insulin resistance
Increase glucose uptake
Haplotype
CCAATT (H1)
CTGATC (H2)
CCGATC (H3)
TCGACT (H4)
CCGGCT (H5)
BMI
ns
ns
0.04
0.002
ns
I/G ratio
ns
ns
0.03
0.04
ns
Chol
ns
ns
ns
0.005
ns
LDL
ns
ns
ns
0.003
ns
HDL
ns
ns
ns
0.03
ns
TG
ns
ns
ns
0.004
ns
Single SNP-based association
Hypothalamus
/nucleus
Muscle
Results
Insulin resistance
Figure 1. Sites and mechanisms of action and clinical implications of
cannabinoid receptor 1 blockade
- rs806366 associated with BMI
- Most SNPs associated with lipid level
 Genetic variability in CNR1 is associated with dyslipidemia, independent of obesity
Haplotype-based association
- Five haplotypes were found with frequency >5%
- Individual with H4 has an increased risk for developing the metabolic syndrome.
- H4 was associated with insulin resistance and several clinical lipid derangements
known to accompany the metabolic syndrome (i.e., high triglycerides and low HDL).
Conclusion
.
A common
CNR1 haplotype (H4; prevalence 0.132) was associated with
multiple clinical parameters used to define the metabolic syndrome, and
single SNP association revealed several relationships between lipids and
CNR1 that are independent of body mass index
This observations provide compelling evidence in support of the strong
biological plausibility of CNR1 gene based on its gene products role in
key obesity and obesity-related metabolic pathways in humans
Acknowledgment
This work was partly funded through MCW25014, Advancing
Healthier Wisconsin (RAW) to RAW