Sarra E. Jamieson

HLA and Infectious Diseases

SUMMARY Following their discovery in the early 1970s, classical human leukocyte antigen (HLA) loci have been the prototypical candidates for genetic susceptibility to infectious disease. Indeed, the original hypothesis for the extreme variability observed at HLA loci (H-2 in mice) was the major selective pressure from infectious diseases. Now that both the human genome and the molecular basis of innate and acquired immunity are understood in greater detail, do the classical HLA loci still stand out as major genes that determine susceptibility to infectious disease? This review looks afresh at the evidence supporting a role for classical HLA loci in susceptibility to infectious disease, examines the limitations of data reported to date, and discusses current advances in methodology and technology that will potentially lead to greater understanding of their role in infectious diseases in the future.

Genetic and Epigenetic Factors at COL2A1 and ABCA4 Influence Clinical Outcome in Congenital Toxoplasmosis

Background Primary Toxoplasma gondii infection during pregnancy can be transmitted to the fetus. At birth, infected infants may have intracranial calcification, hydrocephalus, and retinochoroiditis, and new ocular lesions can occur at any age after birth. Not all children who acquire infection in utero develop these clinical signs of disease. Whilst severity of disease is influenced by trimester in which infection is acquired by the mother, other factors including genetic predisposition may contribute. Methods and Findings In 457 mother-child pairs from Europe, and 149 child/parent trios from North America, we show that ocular and brain disease in congenital toxoplasmosis associate with polymorphisms in ABCA4 encoding ATP-binding cassette transporter, subfamily A, member 4. Polymorphisms at COL2A1 encoding type II collagen associate only with ocular disease. Both loci showed unusual inheritance patterns for the disease allele when comparing outcomes in heterozygous affected children with outcomes in affected children of heterozygous mothers. Modeling suggested either an effect of mothers genotype, or parent-of-origin effects. Experimental studies showed that both ABCA4 and COL2A1 show isoform-specific epigenetic modifications consistent with imprinting. Conclusions These associations between clinical outcomes of congenital toxoplasmosis and polymorphisms at ABCA4 and COL2A1 provide novel insight into the molecular pathways that can be affected by congenital infection with this parasite.

Evidence for a cluster of genes on chromosome 17q11–q21 controlling susceptibility to tuberculosis and leprosy in Brazilians

The region of conserved synteny on mouse chromosome 11/human 17q11–q21 is known to carry a susceptibility gene(s) for intramacrophage pathogens. The region is rich in candidates including NOS2A, CCL2/MCP-1, CCL3/MIP-1α, CCL4/MIP-1β, CCL5/RANTES, CCR7, STAT3 and STAT5A/5B. To examine the region in man, we studied 92 multicase tuberculosis (627 individuals) and 72 multicase leprosy (372 individuals) families from Brazil. Multipoint nonparametric analysis (ALLEGRO) using 16 microsatellites shows two peaks of linkage for leprosy at D17S250 (Zlr score 2.34; P=0.01) and D17S1795 (Zlr 2.67; P=0.004) and a single peak for tuberculosis at D17S250 (Zlr 2.04; P=0.02). Combined analysis shows significant linkage (peak Zlr 3.38) at D17S250, equivalent to an allele sharing LOD score 2.48 (P=0.0004). To determine whether one or multiple genes contribute, 49 informative single nucleotide polymorphisms were typed in candidate genes. Family-based allelic association testing that was robust to family clustering demonstrated significant associations with tuberculosis susceptibility at four loci separated by intervals (NOS2A–8.4 Mb–CCL18–32.3 kb–CCL4–6.04 Mb–STAT5B) up to several Mb. Stepwise conditional logistic regression analysis using a case/pseudo-control data set showed that the four genes contributed separate main effects, consistent with a cluster of susceptibility genes across 17q11.2.

Genetic susceptibility to infectious diseases: big is beautiful, but will bigger be even better?

Summary Genetic epidemiology, including twin studies, provides robust evidence that genetic variation in human populations contributes to susceptibility to infectious disease. One of the major limitations of studies that attempt to identify the genes and mechanisms that underlie this susceptibility has been lack of power caused by small sample size. With the development of novel technologies, burgeoning information on the human genome, the HapMap project, and human genetic diversity, we are at the beginning of a new era in the study of the genetics of complex diseases. This review looks afresh at the epidemiological evidence that supports a role for genetics in susceptibility to infectious disease, examines the somewhat limited achievements to date, and discusses current advances in methodology and technology that will potentially lead to translational data in the future.

Genome-wide scans for leprosy and tuberculosis susceptibility genes in Brazilians

Genome-wide scans were conducted for tuberculosis and leprosy per se in Brazil. At stage 1, 405 markers (10 cM map) were typed in 16 (178 individuals) tuberculosis and 21 (173 individuals) leprosy families. Nonparametric multipoint analysis detected 8 and 9 chromosomal regions respectively with provisional evidence (P<0.05) for linkage. At stage 2, 58 markers from positive regions were typed in a second set of 22 (176 individuals) tuberculosis families, with 22 additional markers typed in all families; 42 positive markers in 50 (192 individuals) new leprosy families, and 30 additional markers in all families. Three regions (10q26.13, 11q12.3, 20p12.1) retained suggestive evidence (peak LOD scores 1.31, 1.85, 1.78; P=0.007, 0.0018, 0.0021) for linkage to tuberculosis, 3 regions (6p21.32, 17q22, 20p13) to leprosy (HLA-DQA, 3.23, P=5.8 × 10−5; D17S1868, 2.38, P=0.0005; D20S889, 1.51, P=0.004). The peak at D20S889 for leprosy is 3.5 Mb distal to that reported at D20S115 for leprosy in India. (151 words).

P2X7 Receptor-Mediated Killing of an Intracellular Parasite, Toxoplasma gondii, by Human and Murine Macrophages

The P2X7R is highly expressed on the macrophage cell surface, and activation of infected cells by extracellular ATP has been shown to kill intracellular bacteria and parasites. Furthermore, single nucleotide polymorphisms that decrease receptor function reduce the ability of human macrophages to kill Mycobacterium tuberculosis and are associated with extrapulmonary tuberculosis. In this study, we show that macrophages from people with the 1513C (rs3751143, NM_002562.4:c.1487A>C) loss-of-function P2X7R single nucleotide polymorphism are less effective in killing intracellular Toxoplasma gondii after exposure to ATP compared with macrophages from people with the 1513A wild-type allele. Supporting a P2X7R-specific effect on T. gondii, macrophages from P2X7R knockout mice (P2X7R−/−) are unable to kill T. gondii as effectively as macrophages from wild-type mice. We show that P2X7R-mediated T. gondii killing occurs in parallel with host cell apoptosis and is independent of NO production.

Genetics and visceral leishmaniasis: of mice and man.

Ninety per cent of the 500 000 annual new cases of visceral leishmaniasis (VL) occur in India/Bangladesh/Nepal, Sudan and Brazil. Importantly, 80–90% of human infections are sub‐clinical or asymptomatic, usually associated with strong cell‐mediated immunity. Understanding the environmental and genetic risk factors that determine why two people with the same exposure to infection differ in susceptibility could provide important leads for improved therapies. Recent research using candidate gene association analysis and genome‐wide linkage studies (GWLS) in collections of families from Sudan, Brazil and India have identified a number of genes/regions related both to environmental risk factors (e.g. iron), as well as genes that determine type 1 vs. type 2 cellular immune responses. However, until now all of the allelic association studies carried out have been underpowered to find genes of small effect sizes (odds ratios or OR < 2), and GWLS using multicase pedigrees have only been powered to find single major genes, or at best oligogenic control. The accumulation of large DNA banks from India and Brazil now makes it possible to undertake genome‐wide association studies (GWAS), which are ongoing as part of phase 2 of the Wellcome Trust Case Control Consortium. Data from this analysis should seed research into novel genes and mechanisms that influence susceptibility to VL.

IL6 −174 G/C Promoter Polymorphism Influences Susceptibility to Mucosal but Not Localized Cutaneous Leishmaniasis in Brazil

BACKGROUND Mucosal leishmaniasis (ML) is associated with exaggerated tumor necrosis factor- alpha and interferon- gamma responses and tissue destruction. ML follows localized cutaneous leishmaniasis (CL) caused by Leishmania braziliensis infection. Interleukin (IL)-6 down-regulates T helper (Th) cell type 1 differentiation and drives Th2 cell differentiation. The IL6 -174 G/C polymorphism is associated with proinflammatory diseases and IL-6 regulation. METHODS The -174 G/C polymorphism was genotyped in population samples and families with CL and ML from Brazil. Genotype frequencies were compared among patients with ML, patients with CL, and 2 control groups by logistic regression and family-based association test (FBAT) analysis. IL-6 levels were measured in macrophages. RESULTS The C allele was more common in patients with ML than in patients with CL (odds ratio [OR], 2.55 [95% confidence interval {CI}, 1.32-4.91]; P=.005), than in patients who were leishmanin skin-test positive (OR, 2.23 [95% CI, 1.23-4.05]; P=.009), and than in neighborhood control subjects (OR, 2.47 [95% CI, 1.24-4.90]; P=.01). FBAT analysis confirmed an association between allele C and ML under both additive (z=4.295; P=.000017) and dominant (z=4.325; P=.000015) models. Significantly lower levels of IL-6 were measured in unstimulated macrophages from CC individuals than from GG individuals (P=.003) as well as after stimulation with soluble leishmania antigen (P=.009). CONCLUSIONS IL-6 may regulate type 1 proinflammatory responses, putting individuals with low macrophage IL-6 levels at increased risk for ML.

Evidence for associations between the purinergic receptor P2X 7 (P2RX7) and toxoplasmosis

Congenital Toxoplasma gondii infection can result in intracranial calcification, hydrocephalus and retinochoroiditis. Acquired infection is commonly associated with ocular disease. Pathology is characterized by strong proinflammatory responses. Ligation of ATP by purinergic receptor P2X7, encoded by P2RX7, stimulates proinflammatory cytokines and can lead directly to killing of intracellular pathogens. To determine whether P2X7 has a role in susceptibility to congenital toxoplasmosis, we examined polymorphisms at P2RX7 in 149 child/parent trios from North America. We found association (FBAT Z-scores ±2.429; P=0.015) between the derived C(+)G(−) allele (f=0.68; OR=2.06; 95% CI: 1.14–3.75) at single-nucleotide polymorphism (SNP) rs1718119 (1068T>C; Thr-348-Ala), and a second synonymous variant rs1621388 in linkage disequilibrium with it, and clinical signs of disease per se. Analysis of clinical subgroups showed no association with hydrocephalus, with effect sizes for associations with retinal disease and brain calcifications enhanced (OR=3.0–4.25; 0.004<P<0.009) when hydrocephalus was removed from the analysis. Association with toxoplasmic retinochoroiditis was replicated (FBAT Z-scores ±3.089; P=0.002) in a small family-based study (60 families; 68 affected offspring) of acquired infection in Brazil, where the ancestral T(+) allele (f=0.296) at SNP rs1718119 was strongly protective (OR=0.27; 95% CI: 0.09–0.80).

Association between SLC11A1 (formerly NRAMP1) and the risk of sarcoidosis in Poland

Sarcoidosis (SA) is a systemic granulomatous disorder of unknown etiology characterized by T helper 1-type inflammatory responses at sites of disease with signs of B cell hyperactivity. Like rheumatoid arthritis and diabetes, an infectious etiology has frequently been postulated but no single infectious trigger definitively identified. Polymorphic alleles at SLC11A1 have previously been associated with susceptibility to both the putative infectious agents and to these autoimmune disorders. We therefore investigated its candidacy as a genetic determinant of SA in Poland in an association-based study comparing 86 SA patients with 85 tuberculosis (TB) patients and 93 control subjects. The functional promoter (GT)n polymorphism and four of 10 other single nucleotide or insertion/deletion polymorphisms genotyped across SLC11A1 were informative in our sample. Consistent with previous autoimmune disease studies, allele 3 at the functional (GT)n promoter region repeat polymorphism was significantly associated with SA when compared with healthy controls (odds ratio 1.68; 95% CI: 1.01–2.81; P=0.04) or with TB patients (odds ratio 1.69; 95% CI: 1.042–0.78; P=0.03).