THE GENETICS

OF OBESITY

One of the most serious health problem of the XXI. century

Obesity is a condition associated

with the accumulation of excessive

body fat resulting from chronic

imbalance of energy whereby the

intake of energy exceeds

expenditure.

Worldwide, more than one billion

adults are overweight and over 300

million are obese.

ANOREXIA NERVOSA

ANOREXIGEN: DECREASING APPETITE/ FOOD INTAKE

OREXIS: appetite

OREXIGEN: INCREASING APPETITE/ FOOD INTAKE

Hypothalamus

BMI= mass (kg)/

height2 (m)

WHY IS IT BAD TO BE FAT?

WHY IS IT BAD TO BE FAT?

EARLY

DEATH

DIABETES

CARDIOVASCULAR DISEASES

CANCER

BREATHING PROBLEMS

STROKE

ARTHRITIS

PROBLEMS WITH REPRODUCTION

Manuel Uribe

Mexico 571.5 kg

WHY IS IT BAD TO BE FAT?

Prevalence:

59 million (30%) Americans are obese (BMI>= 30)

Rates are increasing faster than ever (epidemic proportions)

Energy

Intake

Genetic

Predisposition

Energy

Expenditure

Obesity

Overweight

Energy

Partitioning

Favoring Fat

Accretion

Adipogenesis

FACTORS AFFECTING OBESITY

PAL

Sedentary time

Pollutants Viruses Lactation

Prenatal factors Smoking

Energy

Intake

Energy

Expenditure

Obesity

Energy

Partitioning

Favoring Fat

Accretion

Adipogenesis

Genetic

Predisposition Overweight

Physical environment Social environment Nutrition

PHENOTYPE =

ENVIRONMENTAL EFFECTS + GENOTYPE

BM

I

“OBESOGENIC”

ENVIRONMENT

PRONE TO BE FAT

RESTRICTIVE

ENVIRONMENT

NOT PRONE

TO BE FAT

-THRIFTY GENOTYPE (J. Neel 1962)

-GLUTTONOUS GENOTYPE

-SLOTHFUL GENOTYPE

EVOLUTIONARY THEORIES OF OBESITY

Lee JH et al, IJO, 1997 BMI in Relatives

Fam

ilia

l ri

sk

20 25 30 40 45

8

6

4

2

0

FAMILIAL AGGREGATION OF

OBESITY

Proband BMI = 40+

FATTIES OFTEN MARRY FATTIES

THE KIDS OF FAT PARENTS ARE THE MOST OBESE

THE ROLE OF „FATTY FAMILIES” IN THE SPREADING OF OBESITY

GENETIC DIVERSITY OF

OBESITY

environment genes

Monogenic

high penetrance

Monogenic low penetrance

Poligenic

•Rare cases •Syndromes

•Population study •Genetic epidemiology

•Tissue investigation in clinical trials • “omic studies” Example: Adipose tissue analysis

POLIGENIC

OBESITY

SEARCHING FOR OBESITY GENES

Pre

vale

nce

Body fat

SEARCHING FOR OBESITY GENES

Quantitive Trait Locus

(QTL)

RELATION TO FUNCTIONS

Candidate

Gene

Candidate

Position

SEARCHING FOR OBESITY GENES

LO

D s

co

re

QTL

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0 cM

0

1

0.0

2

0.0

3

0.0

4

0.0

5

0.0

6

0.0

7

0.0

8

0.0

9

0.0

What is QTL?

1p Creighton FS

Dutch FS Pima FS

Quebec FS

2p San Antonio FHS

Paris-Lille FS HERITAGE FS

2q Creighton FS HERITAGE FS

3q AA FS

HERITAGE FS TOPS FS

4p Quebec FS

Utah FS

4q Creighton FS HERITAGE FS

Quebec FS

5p AA FS

TOPS FS Pima FS

Paris-Lille FS

6p AA FS

Pima FS

7q Amish FS

NHLBI FHS Quebec FS

QTL-s in the Genome associated with

obesity

8q San Antonio FHS HERITAGE FS

9q HERITAGE FS

Quebec FS

10p Amish FS Dutch FS

HERITAGE FS Paris-Lille FS

Pima FS

13q NHLBI FHS Quebec FS

14q Amish FS

HERITAGE FS TOPS FS

20q Univ Penn FS

Pima FS

18q Quebec FS Finnish FS

Pima FS

17q Quebec FS

San Antonio FHS

TOPS FS

QTL-s in the Genome associated with

obesity

GENE

FUNCTIONAL STUDIES

QTL

In vitro cell studies

Wild type and allelic versions

Sequence variants

Inventory of genes

Sequencing

Clinical studies

Population studies

Candidate gene

Transgenic animals

SEARCHING FOR OBESITY GENES

MONOGENIC

OBESITY

GENES RESULTING IN MONOGENIC

OBESITY IN THE HUMAN GENOME

Strain Inheritance Encoded Protein Defect

Obese (ob) Mouse Recessive Leptin Stop codon/promoter

defect in leptin

Diabetes (db) Mouse Recessive Leptin receptor Defect lepR

Agouti yellow (ay) Mouse

Dominant Agouti

Ectopic expression of melanocortin receptor

antagonist

Tubby (tub) Mouse

Recessive Phosphodiesterase Apoptosis in the brain?

Fat Mouse

Recessive Carboxypeptidase E

Carboxypeptidase E activity abolished

Zucker/fatty (fa) Rat Recessive Leptin receptor Defect lepR

Koletsky (kol) Rat Recessive Leptin receptor Defect lepR

Corpulent (cp) Rat Recessive Leptin receptor Defect lepR

RODENT MODELS OF MONOGENIC OBESITY

Gene Obesity Birth

weight Endocrine abnormalities Hyperphagia Inheritance Chromosome

LEP Severe Normal

Low leptin

Hypogonadism

High thyroid-stimulating hormone

High insulin

+ Recessive

7q31.1

LEPR Severe ?

High leptin

Pituitary dysfunction

Hypogonadotrophic hypogonadism

Sympathetic dysfunction

High Insulin

+ Recessive

1p31

POMC Severe

Normal

Red hair pigmentation

ACTH deficiency hypocortisolism

Low - MSH

+ Recessive

2p23.3

PC1 Severe

?

Hypogonadotrophic hypogonadism

Hypocortisolism

High proinsulin, low insulin

Postprandial hypoglycemia

High POMC

? Recessive

5q1.5-2.1

MC4-R Severe Normal Not observed + Dominant 18q22

NROB2 Mild High Mild hyperinsulinemia - Dominant 1p36.1

LEP, leptin; LEPR, leptin receptor; POMC, pro-opiomelanocortin; PC1, prohormone convertase1; MC4-R, melanocortin-4 receptor; ACTH, adrenocorticotropic hormone; - MSH, -melanocyte-stimulating hormone.

HUMAN MONOGENIC OBESITY

THE REGULATION OF

FOOD INTAKE

anorexigenic

orexigenic

THE REGULATION OF FOOD INTAKE

-MSH

HYPOTHALAMUS

anorexigenic

orexigenic

ANOREXIGENIC

SIGNALS

LEPTIN

LEPTIN RECEPTOR

-MSH

PROHORMONE

CONVERTASE 1

MELANOCORTIN 4

RECEPTOR

LEPTIN

LEPTIN

RECEPTOR

.

Hypothalamus TRH, CRH,

GnRH

Leptin receptor

Anterior

pituitary

Fat cells

Puberty

Fertility

Food intake

Metabolic rates

Glucose metabolism

Lipid metabolism

Immune functions

Skeletal muscle

Liver

Pancreas

Angiogenesis

Other tissues

Nature, 387, pp 903-908

June 26, 1997

7q31

1 2 35’ 3’10kb 1.9kb

cEBP

AAAAcDNA= 3.5 kbGene= 18kb

167 aaLeptin

Leptin* 133aa

*

*

*

THE PROTEIN IS NOT SECRETED

LEPTIN MUTATIONS

LEPTIN: DECREASES

APPETITE &

INCREASES FAT

METABOLISM

class I cytokine receptor superfamily

The leptin receptor: (Ob-R), the diabetes (db) gene

alternate splicing: six leptin receptor (Ob-Ra-f) isoforms

Leptin signaling is blocked by suppressors of cytokine signaling-3 (SOCS-3) and by

dephosphorylation by protein tyrosine phosphatase 1B (PTP-1B).

SOCS-3 and PTP-1B deficient mice exhibit increased sensitivity to leptin and resistance to

obesity.

LEPTIN RECEPTOR SIGNALING

LEPTIN RECEPTOR MUTATIONS

LOSS OF TRANSMEMBRANE/INTRACELLULAR DOMAIN

LEPTIN: DECREASES APPETITE & INCREASES FAT

METABOLISM

In muscle Leptin

activates 5’-

AMP-

activated

protein kinase

(AMPK)

In liver,

leptin turns

down the

activity of the

gene for

stearoyl-CoA

desaturase-1

(SCD-1)

LEPTIN THERAPY: MICE & MAN

Left: Ob mouse 6 weeks post leptin therapy

Right: Ob mouse 6 weeks post saline injections A child with a mutation in the leptin

gene before and after leptin therapy

BEFORE LEPTIN THERAPY

Initial body weight, 125 kg on average

Daily leptin injection for ten months

AFTER 3 MONTHS LEPTIN

THERAPY

THEY LOST HALF OF THEIR BODY WEIGHT!

AFTER 10 MONTHS LEPTIN

THERAPY

PROOPIOMELANOCORTIN

POMC

anorexigenic

orexigenic

HYPOTHALAMUS

POMC

MSH

PC

anorexigenic

orexigenic

Modified by Coll et al., 2004

POST-TRANSLATIONAL CLEAVAGE OF POMC

PC1 cleavage site Hypothalamus products

PC2 cleavage site Anterior Pituitary products

NH2 COOH

N-teminal JP ACTH Β-LPH

γ3-MSH

γ1-MSH

α-MSH CLIP

γ-LPH

β-MSH β-end1-27

HYPOTHALAMUS

ANTERIOR PITUITARY

s s ss

MC1-R MC4-R MC2-R

Adrenal cortex Hypothalamus skin

Eumelanin pigment

synthesis

Inhibiting food

intake

Glycocorticoid

POMC

ACTH

ACTH

MSH MSH

MSH: MELANOCYTE STIMULATING HORMONE

MC4-R: MELANOCORTIN 4 RECEPTOR

human mutant POMC proteins

ARG236GLY mutation in the POMC gene leads to EARLY-ONSET OBESITY in children

prohormone convertase1

PC1

• SEVERE OBESITY

• PROHORMON

MATURATION

PROBLEMS (Proinsulin,

POMC)

PC 1 MUTATION

Fraction Number

O’Rahilly et al, NEJM, 1995 &Jackson et al, Nature Genet 1997, Jackson, Nat Genet 2003

Melanocortin

4 Receptor

MC4R

anorexigenic

orexigenic

THE REGULATION OF FOOD INTAKE

-MSH

HYPOTHALAMUS

anorexigenic

orexigenic

305

E V F V T L G

V I S L L E N I

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NH2

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Extracellular

Intracellular more than 90 mutation in the MC4R gene

2-3% of obesity cases

MC4R

MELANOCORTIN SIGNALING

MC4R homozygote ( AG 346-347)

MC4R homozygote (I166V)

MC4R heterozygote children (13)

Obese children with wild type MC4R (40)

10,00

12,00

14,00

16,00

18,00

20,00

22,00

24,00

26,00

28,00

30,00

32,00

34,00

36,00

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56,00

58,00

60,00

62,00

64,00

66,00

68,00

70,00

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Age (ans)

IMC

P97

P95

P90

P75

P50

P25

P5

Homo (V166I)

LepR 1

LepR 2

Non mutes

Het MC4R

97P

50P

LepR

Mutation

LepR

Mutation

Clement Nature 1998 ; Dubern, et al J Pediatr 2001 and 2007

LEPR mutation

MEMBRANE EXPRESSION

RECEPTOR ACTIVITY

functional consequences of MC4R mutations

AGRP

AC

N

C

cAMP

?

X

MSH

Food Intake

Energy homeostasis

G Prot

+ -

Weak MSH response

80% of MC4R mutations

Intracellular retention

=> 56% of MC4R mutations

Farooqi et al, NEJM, 2003

16-year-old brother with a normal genotype

9-year-old boy,

homozygous for a

mutation in MC4R

MC4R Deficiency

Severe early childhood obesity

Dominant inheritance

Marco Ferreri: La Grande Bouffe

OREXIGENIC SIGNALS

Neuropeptid Y

(NPY)

anorexigenic

orexigenic

THE REGULATION OF FOOD INTAKE

-MSH

HYPOTHALAMUS

stimulation of food intake (orexigenic)

receptor: Y1 & Y5 (GPCR)

NPY/AgRP neuronos are inhibited by leptin & insulin

NPY/AgRP neurons inhibit the POMC neurons (with GABA)

NPY/AgRP neurons activate anabolic processes & inhibit

catabolic processes

NEUROPEPTIDE Y

Ghrelin

anorexigenic

orexigenic

THE REGULATION OF FOOD INTAKE

-MSH

HYPOTHALAMUS

stimulation of food intake

(orexigenic)

stomach & hypothalamus

stimulate GH secretion

Decreases fat oxidation

Prader-Willi syndrome:

chronic hunger, high

plasma ghrelin levels

Vaccination against obesity?

Ghrelin

Ghrelin: Endogenous Ligand for

the GH Secretagogue Receptor

GHRELIN

117AA ghrelin precursor (preproghrelin)

Protease cleavage and acyl modification: 28AA mature ghrelin peptide

The acyl modification, mainly n-octanoyl modification, is essential for the activity of

ghrelin. S, signal peptide.

GHRELIN

Mature ghrelin product

Exon 1 Exon 2 Exon 3 Exon 4

G274A Arg51Gln Leu72Met

GHRELIN MUTATIONS

DGGQAEGAEDELEVRFNAPFDVGIKLSGVQ 61 90

YQQHSQALGKFLQDILWEEAKEAPADK 91 117

Mature Ghrelin Product Arg51Gln mutation

EHQRVQQRKESKKPPAKLQPRALAGWLRPE 31 60

Leu72Met polymorphism

C-terminal-processing site

* n-octanoyl modification site

MPSPGTVCSLLLLGMLWLDLAMAGSSFLSP 1 30 *

Cleavage site of a signal peptide

THE CONSEQUENCES OF

GHRELIN MUTATIONS

OREXIN, MCH, ENDOCANNABINOIDS

stimulation of food intake

(orexigenic)

Orexin-A, Orexin-B,

prepro-orexin, 130 AA

receptor: OX1R & OX2R

(GPCR)

NARCOLEPSY & OBESITY

OREXIN

stimulation of food intake

(orexigenic)

MCH, 19AA

Prohormon: Pmch

receptor: MCHR1 &

MCHR2 (GPCR)

MCH

OVEREXPRESSION: FAT

MICE

MCHR1- KO: LEAN

MICE

MELANIN-CONCENTRATING

HORMONE

anandamide & 2-arachidonyl

glycerol

receptor: CB1 cannabinoid receptor

(GPCR)

stimulation of food intake

(orexigenic)

CB1 antagonists are potential

medicines against obesity

ENDOCANNABINOIDS

Short term regulation of food ingestion buffers the swings in

nutrient availability. For example, following a meal blood glucose is high at which

time the pancreatic hormone, insulin, is secreted to lower blood glucose by

producing liver glycogen. Between meals, blood glucose falls causing secretion of

another pancreatic hormone, glucagon, which coverts glycogen back to glucose.

The liver monitors glucose and fatty acid titers to elicit feeding behavior via the

vagus nerve. You feel sated after a meal due to distention of the stomach and release

of hormones (CCK) into the blood. Both inhibit the satiety center in the medulla.

Long term regulation of food ingestion

involves neuropeptides which stimulate feeding (e.g., Orexin, Melanin-

concentrating hormone, and Neuropeptide Y)

or inhibit feeding (CART and alpha-MSH). These neuropeptides reside in the

Arcuate Nucleus and other hypothalamic nuclei. Leptin is a peptide hormone that

is secreted when adequate white fat stores are present. Leptin inhibits feeding

behavior to regulate body weight, and it is critical for reproductive competence.

Syndrome Name (reference)

Clinical heterogenity

Trans-mission

Loci / Genes

Prader-Willi Muscular Hypotony

Mental retardation Hyperphagia

Hypogonadism

Short stature

Autosomic dominant

imprinting

15q11

SRNPN

Micro deletion

Maternal Disomy

Bardet-Biedel

Mykytyn Nature Genet 2002

Hypogonadism

Pigmentary retinopathy

Polydactyly

Mental retardation

Autosomic recessive

BBS (1-12)

chaperonin Protein MKKS

(Chr 20)

Ciliary cells proteins

Alström

Hearn Nature Genet 2002

Collin Nature Genet 2002

Myocardiopathy Sensory deficit (retinopathy, deafness)

Dyslipidemia, diabetes

Autosomique recessive

2p14

ALMS1

Börjson-Forssman-Lehman

Lower Nature genet 2002

Morbid obesity, epilepsy

Hypogonadism, facial dysmorphy

Xq26.3 / Plant homeodomain like finger gene

Monogenic syndromes with obesity

BARDET-BIEDL SYNDROME (BBS)

Name Locus Gene Protein

BBS1 11q13 BBS1 Ciliary protein (M390R mutation 80%)

BBS2 16q21 BBS2 Ciliary Protein

BBS3 3p13 BBS3 G-ADP ribosylation

BBS4 15q22 BBS4 PCM1 recruitment (pericentriolar material protein)

BBS5 2q13 BBS5 synthesis cilia flagella

BBS6 20p12 MKKS McKusick-Kaufman

(chaperonin)

BBS7 4q27 BBS7 Ciliary protein

(close to BBS 1 & 2)

BBS8 14q32 BBS8(TTC8) Cell Motility (primary cilia)

BBS9 7p14 PTHB1 regulated by Parahormone

BBS10 12q BBS10 Chaperonin Protein (new)*

BBS11 9q33.1 BBS11 ? Unconfirmed

BBS12 4q27 BBS12 Chaperonin Protein (new)*

BBS: CILIOPATHY

Most of our cells have motile or primary cilium

Dysfunction of movement regulation (BBS7,8)

Compiled from the Obesity Research Series

1994 95 96 97 98 99 00 01 02 03

Single-gene a) -- -- -- 2 6 6 6 6 6 6

Mendelian 8 12 13 16 16 20 24 25 33 41 disorders

Animal QTLs 7 9 24 55 67 98 115 165 168 183

Human QTLs-- -- -- 3 8 14 21 33 68 139

Candidate 9 10 13 21 29 40 48 58 71 90 genes

a) Number of genes, not the number of mutations

Evolution in the Status of the Human

Obesity Gene Map