Lecture 6 - Plasmodium - Resistance and susceptibility to infection

Question 1

Resistance and susceptibility to infectious diseases

Answer 1

• Infectious diseases that kill children select for genes that promote cell survival and prevent transmission of unfavourable genes for survival

P. falciparum - 20-fold higher case rate in untreated children than adults

Ability of P. falciparum to kill before reproductive age is reached has given malaria the capacity to ‘select emerging polymorphisms as rapidly as can be witnessed in evolutionary time’

‘When malaria’s effect on child mortality is considered’….‘the strongest known selective pressure in the recent history of the human genome.’

Strong selective pressure for erythrocyte polymorphisms

Question 2

Sickle cell anaemia

Answer 2

Haemoglobin - 2a and 2B chains

HbS, HbC, and HbE reached high frequency in human populations

Point mutations in B chain:
HbSb - codon 6 - Glutamic acid - valine
HbCb - codon 6 - Glutamic acid - Lysine
HbEb - codon 26 - Glutamic acid - Lysine

Under low oxygen tension - RBCs with HbS distort into sickle shapes

HbS homozygotes - sickle cell disease - high morbidity and mortality
HbS heterozygotes (HbAS) - sickle cell trait – generally asymptomatic
Normal RBCs last ~ 120 days - ‘sickled’ RBCs die after 10 to 20 days

Question 3

Heterozygotes have a protective phenotype

Answer 3

Heterozygotes 90% protected against severe malaria and 60% protected against clinical malaria

60% protection against mortality in infants

Question 4

Mechanism of resistance - sickle cell trait

Answer 4

Host microRNAs (non-coding RNAs ~22 nucleotides in length) post-transcriptionally regulate expression of parasite genes

Haemichromes (oxidized, denatured haemoglobin) accumulate in HbS-containing RBCs and result in formation of Band 3 aggregates and inhibition of knob formation

Reduced PfEMP1 expression inhibits cytoadherence and promotes splenic clearance.

Question 5

Duffy antigen recpetor for chemokines (DARC) aka ACKR1 (Atypical Chemokine Receptor 1)

Answer 5

• Acidic glycoprotein (40-45kDa)
• Extracellular N-terminal domain
• 7 transmembrane domain
• Intracellular C-terminal domain

~12,000-14,000 molecules per erythrocyte

Expression highest on young reticulocytes

Binding domain for Duffy binding protein (PvDBP - P. vivax and PkDBP - P. knowlesi) present at N-terminus of the DARC protein

Sulphation of Tyr41 (Y41) is essential for high-affinity DARC binding of PvDBP and PkDBP

Question 6

DARC function

Answer 6

Blood group antigen but also expressed on endothelial cells - binds with high affinity to 11 different chemokines

Function:
- Unable to support intracellular signalling (silent chemokine receptor) as no G-protein
- Facilitates chemokine transport across endothelium for leukocyte recruitment to sites of inflammation
- On RBC surface - scavenges excess toxic chemokines

More important role on endothelial cells, since expression on these cell types is highly conserved - whereas function on erythrocytes appears dispensable

Humans negative for Duffy blood group antigen resistant to blood-stage infection with P. vivax (Miller et al 1976 New Engl. J. Med. 295:302–304)

P. knowlesi merozoites interact with Duffy -ve human RBCs - interaction and reorientation but MJ does not develop and invasion aborted (Singh et al 2005 Molecular Microbiology 55(6):1925-1934)

Question 7

DARC gene locus

Answer 7

• Duffy locus on chromosome 1 - two major co-dominant alleles (FYB and FYA)
• FYB and FYA differ by single nucleotide at position 125 (A and G) and encoded Fyband Fyaproteins differ by a single amino acid at residue 42 (aspartic acid and glycine).

Genotypes correspond to 4 phenotypes: Fy(a+b+), Fy(a+b−), Fy(a−b+) and Fy(a−b−)

FY null phenotype:
Duffy expression in RBCs is disrupted by single base substitution in erythroid promoter region – binding site for a transcription factor (GATA-1)

Failure of GATA-1 to bind to Duffy gene promoter means that Duffy protein is absent from RBC surface - null ‘erythrocyte silent’ (ES) phenotype