Evaluating plague and smallpox as historical selective pressures ...
Transcript of Evaluating plague and smallpox as historical selective pressures ...
Evalua&ng plague and smallpox as historical selec&ve pressures for the CCR5-‐Δ32 HIV-‐resistance
allele
Alison P. Galvani & Montgomery Slatkin University of California, Berkeley
Proceedings of the Na1onal Academy of Sciences, 100(25), 15276-‐15279. 2008
CCR5
Ø CCR5 is a chemokine receptor in leucocytes, e.g. macrophages
Ø involved in inflammaNon processes
Ø located at the surface of the membrane
CCR5-‐Δ32
Ø MutaNon of CCR5
-‐> Receptor Protein stays in the cytoplasma Ø recent origin: esNmated 700a
CCR5-‐Δ32 – Distribu&on in Europa
CCR5-‐Δ32 – Distribu&on
Ø high frequency in Europa: Homozygote: 1% Heterozygote: 10%
Ø low frequency in Africa, Asia, middle east and in naNve American populaNons
HIV can not be responsible for frequencies
-‐> first scienNfic descripNon in 1983
CCR5-‐Δ32 and resistance
Ø CCR5 is the gateway for HIV to enter immune system cells in the primary phase
Ø CCR5-‐ Δ32 -‐> Protein is not expressed at the cell surface
Homozygote: almost complete HIV-‐Resistance Heterozygote: parNal resistance and slower disease progression à Strong selecNon for CCR5-‐Δ32 where HIV is frequent
Bubonic plague
1346-‐1352: «Black death» pandemic 25-‐40% of Europes populaNon died
Series of smaller epidemic outbreaks
total mortality < 5% 1665-‐1666: «great plague» pandemic
15-‐20 % of Europe's populaNon died Decline to eradicaNon: 1667 England, 1722 France, 1750 enNre Europe
Bubonic plague – spread during «Black death»
Yersinia pesNs
Bubonic plague
Ø used as the explanaNon for high CCR5-‐Δ32 in Europe and it’s distribuNon
Ø widely accepted, despite the lack of evidence à Represents the classic example for «historical selecNon on a clinically important locus»
smallpox
smallpox
Ø Variola major / Variola minor
Ø smallpox occurred frequently (recently eradicNonned) Ø childhood disease: most likely before the age of 10
weaker immune system Ø Hight case fatality/mortality: 30%
Aim of the study
Ø HIV cannot be responsible for high frequencies Ø highly accepted hypothesis:
selecNve pressure from bubonic plague is responsible
but….
Aim of the study
But…
… there is no evidence for plague hypothesis -‐> we test it
… plague is transmijed via flea reservoir
… plague induces strong, but episodic selecNon
Aim of the study
Small pocks might be the reason for the high frequency: Ø human to human transmission Ø conNnuous, weak selecNon Ø geographic distribuNon would fit bejer
-‐> Small pox are more consistent
Aim of the study
Which disease is more likely responsible for the high CCR5-‐ Δ32 frequency?
Methods
PopulaNon geneNc model framework
-‐> including temporal pajerns -‐> including age dependence of the diseases
Methods
Ø 55 age classes, each 1 year: x Ø survival probability for all age classes: µ(x) Ø Number of offsprings from females for each age class m(x) Survival and fecundity based on empirical data and esNmaNons from the 19th century in europa
Methods
Example of a age-‐structured modell (without geneNcs)
Methods
Ø historic progression/temporal pajern of the diseases is also included EsNmaNons based on historic data
Ø addiNonal mortality for infected individuals for each age class
EsNmaNons base on historic data
Ø disNncNon between homozygote and heterozygote based on Hardy-‐Weinberg because of the sufficient populaNons size
Methods
Ø Model run for 28 generaNons (~25 years per generaNon)
à The two diseases are comparable, because p0 (frequency of CCR5-‐Δ32) is the same for both
Important: average selecNon coefficient per generaNon
Results -‐ Dominance/Homozygot
splague = 0.18 ssmallpox = 0.24 à > 10x higher CCR5-‐Δ32 frequency over 700 years
à result fits perfectly for smallpox à Burbonic plague not capable of sufficient evoluNonary pressure
CCR5-‐Δ32 frequency maximum at 1% during presence of plague
Results – incomplete Dominance/Heterozygot
AssumpNon: 50% resistance -‐> case fatality halved
splague = 0.09 ssmallpox = 0.17
à bubonic plague sNll not capable of reaching p(0) à smallpocks need 1135 years to reach p(0)
-‐> fits with other predicNons about the age of CCR5-‐Δ32 [700, 1400]a
à Longer Nmespan reduces splague even further to 0.06 because of it’s restricted temporal range
Results
Interpreta&on
Ø bubonic plague kills more people per Nme than smallpox Ø smallpox killed much more people over Nme
Ø smallpocks affect young people and therefore remove non CCR5-‐Δ32 individuals faster from reproducNon
Ø plague cannot be responsible in any case
Interpreta&on
Ø Micro satelite analysis esNmate CCR5-‐Δ32 to be 1400 years old Ø smallpox do exist over 2000 years
higher Nmespans strengthen the support for small pox Ø mutaNon occured first in north europa with disperal to the south
consistant with smallpox inconsistant with plague
Interpreta&on
Ø disappearance of smallpox meet occurence of HIV -‐> no decline in CCR5-‐Δ32 frequency
Ø smallpox and HIV use the same mechanism to enter leucocyte Ø plague uses very different pathways
Conclusion
CCR5-‐Δ32 may also have negaNv fitness effects Result could also mean, that heterozygots are 100% resistant Responsible for the current CCR5-‐Δ32 frequency has to be a disease with high case fatality and con=nuity.
Is this knowledge applicable? Hüjer, G., Nowak, D., Mossner, M., Ganepola, S., Müßig, A., Allers, K., ... & Thiel, E. (2009). Long-‐term control of HIV by CCR5 Delta32/Delta32 stem-‐cell transplantaNon. New England Journal of Medicine, 360(7), 692-‐698. Stem-‐cell transplantaNon from a homozygote CCR5-‐Δ32 donor to a HIV paNent
-‐> Stem-‐cells transplanted once -‐> no severe effects from the intrusion
Is this knowledge applicable?
Stem-‐cell transplantaNon from a homozygote CCR5-‐Δ32 donor to a HIV paNent Results: PCR-‐assay
Is this knowledge applicable?
Stem-‐cell transplantaNon from a homozygote CCR5-‐Δ32 donor to a HIV paNent Results: Number of T-‐Cells
Is this knowledge applicable?
Results:
SCT: Stem cell transplantaNon HAART: highly acNve anNretroviral therapy
Is this knowledge applicable?
Stem-‐cell transplantaNon from a homozygote CCR5-‐Δ32 donor to a HIV paNent Results: Successful transplantaNon of stemcells No acNve, replicaNng HIV detectable azer 20 months PaNent disconNnued anNretroviral therapy PaNent remained vital