Hemoglobin concentration and reproductive fitness of Tibetan women in highland Nepal

Cynthia M Beall, Ph.D.Case Western Reserve University

Geoff Childs, Ph.D.Washington University

Sienna Craig, Ph.D.Dartmouth College

April 11, 2014

The take home message

Challenge: Is a distinctive trait with heritable variation and functional significance under selection?

High altitude populations are natural experiments in evolution and adaptation.

Courtesy of Brooke Thomas

A severe, unavoidable and graded stress

Altitude, m

0 1000 2000 3000 4000 5000 6000 7000

% S

ea

Le

vel P

iO2

40

50

60

70

80

90

100

Genetic and molecular pathway for oxygen homeostasis is known.

Low Altitude High Altitude12

13

14

15

16

17

18

19

25d, 5400m56d, 4200m28d, 3800m72d, 5260m11d, 4300m21d, 4300m18d, 4300m16d, 5050m16d, 5050m2

Hem

oglo

bin

conc

entr

ation

, gm

/dL

Sensor EGLN1

Transcription Factor Subunit

EPAS1

Target Gene

EPO

Different outcomes of the natural experiment

Altitude versus Hemoglobin Concentration

Altitude, meters

0 1000 2000 3000 4000 5000

Hem

oglo

bin

co

nce

ntra

tion,

gm

/dL

14

16

18

20

22

Andean (Cosio, 1972)

Tibetan (Beall, various)

Hemoglobin concentration has functional significance.

A cost of high hemoglobin concentration is poorer pregnancy outcome.

Young adultOlder adult

05

10152025303540

TibetanAndean

Another cost is higher risk of Chronic Mountain Sickness.

% w

ith C

hron

ic M

ount

ain

Sick

ness

Hb variance is heritable among Tibetans.

1. Heritability, h2=0.64

2. Tibetan allele frequency is divergent at SNP sites in oxygen homeostasis pathway.

3. Tibetans exhibit an altitude gradient in allele frequency at those SNP sites.

4. Hb associates with SNP sites in oxygen homeostasis pathway.

http://genegeek.ca/2010/11/human-chromosomes-and-karyotype

Genome wide scan found allele - frequency differences between Tibetans and Han Chinese.

Beall et al., 2010, PNAS

(HIF2α)

“Tibetan” alleles increase with altitude

Xu S et al. Mol Biol Evol 2011;28:1003-1011

GENOTYPES

Tibetan homozygote Other homozygote

Hem

og

lob

in c

on

c., g

m/d

L

14

16

18

20

22 4200m, EPAS1

4300m, EPAS1

4300m, EPAS1 MF

4350m, EGLN1 & PPARA

3800m, EPAS1 Sh

100m, EPAS1

EPAS1 and EGLN1 associate with hemoglobin concentration

Is hemoglobin concentration under selection?

EPAS1

HYPOXIA

Hypoxia-Inducible Factor, alpha subunit

+

HIF-beta

 

 

HIF

Hypoxia-Inducible Factor 2

Hypoxia-Inducible Factor, beta subunit

HYPOXIA

O2O2

+

O2

Plausible, however there are other possibilities.

Angiogenesis (VEGF)Cell MigrationCell ProliferationCytoskeleton FormationEnergy Metabolism (GLUT1)Erythrocytosis (EPO)Growth and ApoptosisHormone ModulationImmune System (IL-6)TranscriptionTransportVasomotor Regulation

Test the hemoglobin hypothesis

% O2 saturation of hemoglobin decreased with altitude.

Altitude, m

2800 3000 3200 3400 3600 3800 4000 4200

% O

2 s

atu

rati

on

of

hem

og

lob

in

65

70

75

80

85

90

95

100

r=-0.33, p< 0.001

Hemoglobin concentration - steady across altitude.

Altitude, m

2800 3000 3200 3400 3600 3800 4000 4200

Hem

og

lob

in c

on

cen

trat

ion

, g

m/d

L

6

8

10

12

14

16

18

20

Avg 13.9 gm/dL + 1.5(SD), n=943

‘Continuously married’ sub-sample

Whole sample Continuously married subsample

N 1,011 601

Age, years 55 + 11 55 + 11

# Pregnancies 5.8 + 2.9 6.5 + 2.8

# Livebirths 5.3 + 2.8 6.2 + 2.6

# Children surviving to 15 years of age

3.7 + 2.2 (n=589) 4.1 + 2.2 (n=350)

Many factors account for variance in the number of pregnancies – hb does not.

Number of Pregnancies

Indirect, direct variables

Age Last Preg

Age First P

reg

Infant Morta

lityTwins

Husband/Natal Villa

ge

Cu

mu

lati

ve R

2

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Not Continuously MarriedContinuously Married

4.4 + 2.6, n=410

6.5 + 2.8, n=600

Hb

Conceptional and social factors account for variance in surviving children – hb does not.

3.0 + 2.0, n=239

4.1 + 2.2, n=350

Children surviving to reproductive age (15 yrs)

Indirect, direct variables

Age Last Preg

Age First P

reg

Contraceptio

n

Relative W

ealth

Infant Morta

lityTwins

Natal village

Cu

mu

lati

ve R

2

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

Not continuously married

Continuously married

Hb

The take home message

A distinctive trait with heritable variation and functional significance may not be the trait under selection OR the study design requires rethinking.

Questions?

cmb2@case.edu