- Email: [email protected]
Genetics of infectious disease resistance
348
Genetics of infectious disease resistance Adrian VS Hill The identification of large numbers of candidate genes and the introduction of methodologies for whole-genome screening have provided new opportunities for elucidating the molecular basis of variable susceptibility to major infectious diseases. 12 genes have been implicated in variable susceptibility to malaria and susceptibility/resistance genes for several other infectious diseases are beginning to be identified. Recent work suggests that large-scale family linkage and population association studies will be a more succesful route to human disease genes than extrapolation from mouse models of infection.
Address Wellcome Trust Centre for Human Genetics, University of Oxford, Windmill Road, Oxford OX3 7BN, UK e-mail: [email protected]
Current Opinion in Genetics & Development 1996, 6:348-353 © Current Biology Ltd ISSN 0959-437X
Abbreviations G6PD glucose-6-phosphate dehydrogenase HLA human leukocyte antigen MBP mannose-bindingprotein NRAMP1 natural resistance-associated macrophage protein 1 TNF tumour necrosis factor
Introduction
Twin studies of many infectious diseases have demonstrated the importance of host genetics in determining variable susceptibility to infectious pathogens in humans. However, with the exception of malaria, rather few susceptibility or resistance genes have been identified. Recent progress in the isolation and characterisation of candidate genes and new microsatellite-based markers for family linkage studies of the whole genome now promise to change this situation. In this short review I will discuss recent progress in the analysis of genetic susceptibility to infectious diseases in humans (the genetics of mouse models of infectious disease has been reviewed recently elsewhere [1]). Progress in particular disease categories will be summarized, followed by an outline of current genome scanning approaches to gene mapping for infectious disease and I shall finish with a brief review of some major candidate genes. Infectious disease parasitic diseases
studies
Analysis of the protective effect against malaria of deficiency of the red cell enzyme, glucose-6-phosphate dehydrogenase (G6PD), was performed by Ruwendc et al. [2°] in two African populations. Both female heterozygotes and male hemizygotes had a reduced risk, by -50%, of severe malaria. This appears to settle a long-running
debate as to whether enzyme-deficient males or only their heterozygous sisters are protected; it also allowed modelling of the rate of selection of the G6PD A- variant leading to the conclusion that some balancing selective disadvantage of the enzyme deficiency must be have been retarding the rise in frequency of G6PD-deficiency alleles in Africa. Another red cell variant, absence of the Duffy blood group antigen, was shown many years ago to be associated with resistance to Plasmodium vivax malaria. T h e underlying mutation has now been elucidated - - a substitution in the binding site for the GATA1 erythroid transcription factor at position -46 of the promoter [3]. This should facilitate further population studies of this interesting variant that has essentially reached fixation in many African populations. The puzzle is how resistance to Plasmodium vivax malaria which is, at least now, a non-fatal disease could have led to such apparently rapid selection. Further analysis of the role of the M H C in variable susceptibility to malaria has involved both association and linkage studies. In a large case-control study at Kilifi in Kenya, an overall effect of H L A - D R variation on susceptibility to severe malaria was found (SN Yates et al., unpublished data). T h e allelic associations in Kenya, however, were different to those found in a previous west African study [4]. This geographical variability in HLA associations, that is seen for several infectious diseases [5], may in this instance relate to antigenic polymorphism in the malaria parasite. McGuire eta/. [6 °°] reported that homozygosity for a position -308 promoter polymorphism of the tumour necrosis factor (TNF) gene, located in the class III region of the MHC, was associated with a seven-fold increase in risk of severe forms of cerebral malaria in T h e Gambia. Importantly, by HLA typing the same samples for HLA class I and II region variants, it was possible to show that this effect was independent of linked H L A - D R and HLA-B polymorphism. Thus, in T h e Gambia, HLA-B53, HLA-DRBI*1302 {4] and the T N F promoter variant were independently associated with altered risk of severe malaria. Cabrera eta/. [7] have studied T N F region polymorphisms in leishmaniasis patients from Brazil. Amongst 25 cases of mucocutaneous leishmaniasis, a higher frequency of the T N F -308 promoter variant was found than in 43 controls, but it is unclear whether this increase is independent of linked HLA alleles. HLA studies have also been reported recently of infections by helminthic parasites. Meyer eta/. [8] have studied a small number of onchocerciasis patients from West Africa with detailed molecular typing of the HLA class II region and suggested that HLA-DQ and H L A - D P alleles might affect susceptibility. Yazdanbaksh et al. [9] have studied patients with filariasis from Indonesia and found an increased frequency of HLA-B27 in the
Genetics of infectious disease resistance Hill 349
elephantiasis group. Although no significant association remained after correction for multiple comparisons, these two small studies encourage further larger-scale studies of the immunogenetics of chronic worm infections. Bacterial diseases Mycobacterial diseases have been of particular interest to human geneticists because of substantial variation in susceptibility to these chronic infections, but studies of mannose-binding protein (MBP) and CD32 (see below) have also encompassed acute bacterial infections. T h e role of several candidate genes for tuberculosis susceptibility has recently been assessed in Africans (C Ruwende et a/. unpublished data) who collectively may be more susceptible than Caucasians [10]. Although no effect of N R A M P 1 (natural resistance-associated macrophage protein 1; previously known as iz'h/lty/Bcg) polymorphism was found, a promoter variant of the T N F gene was associated strongly with susceptibility to tuberculosis in T h e Gambia.
T h e role of M H C variants in susceptibility to leprosy in India has been reinvestigated by Rani etal. [11] and R Roy et al. (unpublished data). T h e HLA-DR2 association with leprosy per se, found in many Asian populations [12], was dissected into its subtypes by Rani et al., who found that both common DR15 subtypes (DR2 encompasses several DR15 and DR16 alleles) were associated with both tuberculoid and lepromatous forms of leprosy in north India. Roy et al. (unpublished data), studying Bengali patients, also observed increased frequencies of both DR2 alleles in all leprosy types and went on to analyse T N F promoter variants. T h e -308 T N F promoter variant was significantly increased in frequency in lepromatous but not in tuberculoid leprosy, and this effect was independent of the HLA-DR2 association. Viral diseases 5-20% of people infected by the hepatitis B virus develop a chronic carrier state that is associated with chronic liver disease and hepatocellular carcinoma. Twin studies have identified a host genetic component affecting risk of carriage [13] and several HLA studies have compared chronic carriers with those who have cleared the infection or with uninfected controls. Almarri and Batchelor [14 °] have compared 34 chronically infected Quatari patients to 44 individuals who had cleared the virus and 100 uninfected blood donors. In a two-stage analysis, HLA-DR7 was associated with chronic carriage and HLA-DR2 was associated with viral clearance. In a large study of Gambian adults and children, Thursz et al. [15 ° ] have used more detailed molecular HLA class II typing and identified an association between H LA-DRB 1"1302 and hepatitis B virus clearance. Several factors may explain the apparent inconsistency between these studies; for example, patients with liver disease were studied in Quatar and healthy carriers in Africa, and the low H L A - D R 2 frequency in T h e Gambia makes protective association with this allele difficult to identify.
Further relatively small studies of HLA associations with various manifestations of HIV infection have appeared (e.g. [16]). There is substantial variation between reported series and many studies have been too small to allow useful comparisons. However, Kroner et al. [17 °] have used analysis of sibling pairs to provide more convincing evidence that the M H C influences the rate of decline of the CD4 count. Epidemiological studies are beginning to define groups that may be relatively resistant to infection [18] and genetic studies of these individuals should be of interest. Genetic linkage analysis Progress in the development of genetic and physical maps of the human genome offers new avenues for approaching the analysis of many complex multifactorial diseases. The availability of sets of highly informative markers spaced along the human genome, combined with new technologies for high-throughput fluorescence-based genotyping, now allows genetic linkage studies to scan the human genome in months rather than years. Todd's group [19 °°] used 254 microsatellite markers in a genome screen for diabetes susceptibility genes. T h e analysis usually employed is of affected sib pairs and their parents where increased sharing of markers is searched for in affected individuals. Such identity-by-descent analyses may be supplemented by identity-by-state methods when parents are unavailable. Very recently, Jepson et al. (unpublished data) have identified strong linkage between microsatellite markers in the MHC and susceptibility to clinical malaria in a study of Gambian dizygotic twins.
Except for the identification of a major disease locus, such studies require analysis of large numbers of affected sib pairs and their parents. For acute infectious diseases, such families may often be difficult to collect, particularly in places where hospital records are limited. For more chronic diseases, such as leprosy and tuberculosis, control programmes in some countries may facilitate family collection. An alternative strategy that may be particularly practical for acute infectious diseases of children is to study parent--child pairs using linkage disequilibrium mapping. This usually forms the second stage of genome-screening studies in an attempt to improve the localization of a mapped locus from a large interval o f - 1 0 M b to -1 Mb or less. Recent work on diabetes has illustrated the utility of this approach in the fine mapping of a gene on chromosome 2 [20°], employing the transmission disequilibrium test. Linkage disequilibrium mapping is dependent on contingencies of population history so that it may be necessary to perform studies in different populations to find one with sufficient disequilibrium between a functional variant and a marker allele. Also, much larger numbers of microsatellites are required to cover the genome by using linkage disequilibrium mapping than by affected sib-pair analysis; however, screening of genomic regions containing candidate genes may offer a short cut.
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An important strategic issue is whether mapping of mouse susceptibility genes using genome screening approaches will offer an effective means of identifying major human susceptibility genes. Recent evidence suggests that this approach may be difficult. In the study of diabetes, where linkage studies have been performed successfully in both mouse and human and numerous genes has been mapped, few, if any, of these non-MHC genes appear to be the same in both species [21]. Risch etal. [22] have suggested that this may turn out to be a general pattern from considerations of allele frequency variation. Moreover, two major infectious disease resistance genes have now been mapped and identified in mice, the Mx gene for influenza [23] and the Nrampl gene for certain intracellular bacteria and leishmania [24]. To date, these genes have not been found to be functionally variable in humans. T h e very early success of both affected sib-pair studies [25] and transmission disequilibrium test analysis [26] of the M H C and leprosy, however, suggest that linkage studies of non-MHC genes in human infectious diseases may be fruitful. Candidate
with at least four infectious diseases. T h e initial report by McGuire eta/. [6°°] focused on cerebral malaria and more recent work has found associations with tuberculosis, severe malarial anaemia (W McGuire et al., unpublished data), lepromatous leprosy (R Roy et al. unpublished data) and leishmaniasis [7]. It would also be of interest to assess possible associations with the rate of disease progression to AIDS in HIV infection, as T N F can lead to increased transcription rates of HIV. Table 2 Malaria resistance genes. c~-Globin ~-Globin Duff)/chemokine receptor G6PD Blood group 0 Erythrocyte band 3
HLA-B HLA -DR TNF Spectrin Glycophorin A Glycophorin B
Genes assiciated with resistance or susceptibility to malaria [6"°,27]. G6PD, glucose-6-phosphate dehydrogenase; HLA, human leukocyte antigen; TNF, tumour necrosis factor.
genes
T h e candidate gene approach (see Table 1 for a select listing) for the identification of resistance/susceptibility genes has been applied with particular success in malaria where variants of 12 genes have now been implicated in resistance to this disease (Table 2) [27]. This might relate to a relatively large number of genes that affect susceptibility to infectious disease in general. If much functional polymorphism in the human genome is a result of selection pressures by infectious diseases, many loci--particularly those involved in innate or acquired i m m u n i t y - - may affect infectious disease resistance. Some candidate genes of current interest that might affect numerous infectious diseases will be mentioned briefly.
Table 1 Candidate genes for Infectious diseases. MBP Inducible nitric oxide synthase Nramp 1 TNF FcyRII Km allotypes MxA T cell receptor variants Interferon 7 receptor Interleukin-4 Fucosyl transferase-2
/CAM- 1 HLA-DR Interleukin - 12 Interleukin-1 receptor IgE-RII Dully chemokine receptor Vitamin D receptor IgFt allotypes FcyRIII TNF-receptor 55 Interleukin- 10
A selection of some candidate genes for infectious disease resistance and susceptibility, many of which have documented functional polymorphisms. HLA, human leukocyte antigen; MBP, mannose binding protein; Nrampl, natural resistance-associated macrophage protein 1; TNF, tumour necrosis factor.
Tumour necrosis factor
Over the past year, there has been new evidence that promoter variants of the T N F gene may be associated
These associations provide evidence of pathophysiological mechanisms in these infectious diseases. In cerebral malaria, lepromatous leprosy and leishmaniasis there is evidence of increased serum levels of T N E The association with the -308 v a r i a n t - - t h a t has been associated with increased transcription rates (AG Wilson, JA Symons, T L McDowell, FS di Giovane, GW Duff, British Journal of Rheumatology 1994, 33[suppl 1]:89)--suggests that the raised T N F levels may play a part in the pathogencsis of these disorders rather than just being reactive. Functional studies of these promoter variants remain few and data arc required on the effects of appropriate specific stimuli on relevant cell types. These associations with T N F variants encourage analysis of the role of other cytokine promoter variants in infectious disease. T h e list of cytokine and cytokine receptors that might be of interest is long, but there is much current interest in the cluster of TH2-1ike cytokine genes, including interleukin-4, on chromosome 5. High total IgE levels in atopy have been mapped to this region I28"1. Mannose-binding protein
MBP is a serum lectin that plays an important role in innate immunity. MBP activates complement and acts directly as an opsonin, using the Clq receptor on macrophages. Functionally deficient variants of this collectin are common in most populations. Three mutations have been described at codons 52, 54 and 57 that alter a single amino acid and lead to low and near absent serum levels in heterozygotes and homozygotes respectively [29,30]. Recently, Madsen eta/. [31] have described further variation in the promoter region of the MBP gene that is associated with lesser degrees of variation in serum levels. MBP deficiency has been associated with repeated
Genetics of infectious disease resistance Hill 351
bacterial and fungal infections in young children [32] and recently with unusual infections in adults [33°]. Garred et aL [34 °] have found a higher frequency of homozygotes for MBP-deficiency alleles amongst patients with unexplained immunodeficiency than in controls. Studies of individual infectious diseases have begun to be reported. Small studies of serum levels of MBP in HIV infection [35] and in leprosy [36] have suggested that deficiency alleles might be associated with these diseases but no genotyping has been performed. A study of recurrent otitis media in children failed to find an association with M B P genotypes [37]. As MBP can interact with a large number of micro-organisms bearing surface carbohydrates, further studies of the influence of deficiency alleles on infectious disease risk are indicated. If only homozygotes are at increased risk, fairly large samples sizes may be required to demonstrate significant associations. It will be particularly interesting to identify why these apparently deleterious MBP deficiency alleles have reached high frequencies in so many human populations.
Natural resistance-associated macrophage protein 1
Studies over almost 20 years have led to the identification of a gene in mice that affects resistance to several intracellular pathogens. These include Leishmania parasites, some strains of Bacille Calmette-Guerin and Salmonella. Vidal et al. [24], have isolated the gene Nrampl by positional cloning and have identified, in the recessive deficiency allele, a single amino acid change (Gly---)Asp) in a putative transmembrane domain. T h e precise function of Nrampl is unclear, but a role in macrophage activation and its equivalence with iz'h/Ity/Bcg have been confirmed using a gene-knockout mouse [43°]. Although wild-type mice with a deficiency allele have not been reported, there has been considerable speculation that variants of the human homologue of Nrampl, might play a role in variable susceptibility to some human infectious diseases. How such deleterious alleles would have been selectively maintained has not been addressed. With the mapping and identification of the human homologue, NRAMP1, both linkage and association studies have been performed. Shaw et aL [44] have studied Pakistani families with leprosy and found no linkage to the NRAMP1 region; another study of NRAMP1 and leprosy in Polynesia also failed to find linkage [45°].
Secretor
Approximately 20% of most human populations have a genetically determined inability to secrete the water soluble forms of the ABO blood group antigens into saliva and other body fluids. Such non-secretors have been the subjects of a large number of disease association studies using serological methods. Non-secretors have been reported to be at increased risk of meningococcal invasive disease [38], Candida infections [39] and recurrent urinary tract infections [40]. Interestingly, non-secretors were found to be under-represented amongst patients with a variety of respiratory viral infections, such as influenza virus and respiratory syncytial virus [41], suggesting that this apparent balanced polymorphism could be maintained by differential resistance to viral and non-viral infectious pathogens. The molecular genetic basis of the non-secretor phenotype has now been elucidated. Kelly et al. [42*] have cloned and sequenced the gene, termed Sec2, that encodes an alpha(1,2)fucosyhransferase. This gene is in close proximity to the H blood group alpha(I,2) fucosyhransferase on chromosome 19. Sequence analysis found that nonsecretors were homozygous for an enzyme-inactivating nonsense allele (Try143--~ter), and 19% of the 52 normal Americans studied were homozygous for this mutation. We have recently found this variant at high frequency in sub-Saharan Africans (S Ali et aL, unpublished data), suggesting that this may be the predominant non-secretor mutation in most populations. This definition of the molecular basis of the non-secretor phenotype should facilitate the analysis of this locus in studies of infectious disease.
Several polymorphisms in the NRAMP1 gene and its flanking sequences have now been described, facilitating disease association studies in humans [46*,47°]. Liu et al. [47"] have found no association with tuberculosis in very small studies of Hong Kong and Canadian families, and C Ruwende et al. (unpublished data) have recently found no association of NRAMP1 microsatellite alleles with tuberculosis in west Africans. Thus, as with the ilia" gene [48] which was isolated using a mouse susceptible to influenza virus, for Nrampl there is, as yet, no clear evidence of functional variation in, or infectious disease associations with, the human homologue.
FcyRII receptor Receptors for the constant region of immunoglobulins are involved in the phagocytosis of IgG-opsonized microorganisms. Three distinct receptors have been identified and the two of these that are expressed on neutrophils have diallelic functional polymorphisms: FcyRIIa (CD32) and FcyRIII (CD16). T h e single amino acid difference (His---)Arg at position 131) in the FcyRIla-R131 allele is associated with reduced phagocytosis of IgG2 opsonized bacteria. Bredius et al. [49] have phenotyped, by an indirect immunofluorescence method, cells from 25 children who had survived fulminant meningococcal septic shock and compared them to a Caucasian control group. Homozygotes for the genotype associated with reduced phagocytosis were more frequent in the cases, but heterozygotes were more frequent in the controls. No FcyRIII association was suggested. In a further Dutch study of 48 children with recurrent bacterial infections [50], homozygotes for the FcyRH allele with higher
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phagocytic activity were less frequent amongst cases than controls. T h e hypothesis suggested by these small s t u d i e s - - t h a t homozygotes for the R131 allele are at increased risk of meningococcal disease and perhaps other bacterial infections--should now be evaluated in larger series of patients.
HLA-D alleles associated with generalized disease, localized disease, and putative immunity in Onchocerca volvulus infection. Proc Natl Acad Sci USA 1gg4, 91:7515-7519.
9.
YazdanbakshM, Sartono E, Kruize YCM, Kumiawan A, Partono F, Maizels RM, Schreuder GMT, Schipper R, De Vries RRP: HLA and elephantiasis in lymphatic filariasis. Hum Immunol 1995, 44:58-61.
10.
Stead WW, Senner JW, Reddick WT, Lofgren JP: Racial differences in susceptibility to infection by Mycobacterium tuberculosis. N Eng/ J Med 1990, 322:422-427.
11.
Ran/R, Fernandez Vina MA, Zaheer SA, Beena KR, Stastny P: Study of HLA class II alleles by PeR oligotyping in leprosy patients from north India. Tissue Antigens 1993, 42:133-137.
12.
ToddJR, West Be, McDonald JC: Human leukocyte antigen and leprosy: study in northern Louisiana and review. Rev Infect Dis 1990, 12:53-74.
13.
Lin TM, Chen CJ, Wu MM, Yang CS, Chen JS, Lin CC, Kwang TY, Hsu ST, Lin SY, Hsu LC: Hepatitis B virus markers in Chinese twins. Ant~cancer Res 1989, 9:737-742.
Conclusions A variety of approaches from mouse genetics to genome scanning of human families is being used to identify a large range of new candidate genes for studies of infectious disease susceptibility in humans. Recent data suggest that genetic resistance to some, perhaps most, infections will be controlled by a large number of genes and that many resistance/susceptibility alleles will affect more than one infectious disease. T h e r e is a need for large carefully designed case-control studies in different populations to build up a picture of the relative importance of different genes. Over the next few years, this strategy should provide substantial new insights into both the pathophysiology of infectious diseases and the role of infectious agents in selecting for variation in the human genome.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: = •. 1.
14. AlmarriA, Batchelor JR: HLA and hepatitis B infection. Lancet • 1994, 344:1194-1195. Quatari patients with chronic carriage of hepatitis B virus were found to have altered HLA-DR allele frequencies compared to controls. 15. •
ThurszMR, Kwiatkowski D, AIIsopp CE, Greenwood BM, Thomas He, Hill AV: Association between an MHC class II allele and clearance of hepatitis B virus in the Gambia. N Engl J Med 1995, 332:1065-1069. Asymptomatic carriers of hepatitis B virus in the Gambia were tess likely to have the HLA-DRBl*1302 allele, suggesting that an immune response restricted by this HLA type may facilitate viral clearance. Recent analysis by Davenport et al. [51] of the sequence of peptides bound by this molecule should facilitate molecular immunological analysis of this HLA-disease association. 16.
of special interest of outstanding interest Mcleod R, Buschman E, Arbuckle LD, Skamene E: Immunogenetics In the analysis of resistance to intracellular pathogens, Curt Opin Immuno/1995, 7:539-552.
2. •
Ruwende C, Khoo SC, Snow RW, Yates SN, Kwiatkowski D, Gupta S, Warn P, AIIsopp CE, Gilbert SC, Peschu Net al.: Natural selection of heml- and heterozygotes for G6PD deficiency in Africa by resistance to severe malaria. Nature 1g95, 376:246-249. Two large case-control studies of African children with severe malaria provide an estimate of the degree of protection afforded by this enzyme deficiency, -50o/0 reduction in risk. Both male hem/zygotes and female heterozygotes were shown to protected. 3.
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McGuire W, Hill AVS, AIIsopp CE, Greenwood BM, Kwiatkowski D: Variation In the TNF-(~ promoter region associated with susceptibility to cerebral malaria. Nature 1994, 371:508-51 O. Association of a cytokine gene variant with an infectious disease; in a casecontrol study of Gambian children, homozygotes for a promoter variant at position -308 of the TNF gene were at a substantially increased risk of cerebral malaria. This association was found to be independent of associations previously identified [3] with HLA class I and II genes. 7.
Cabrera M, Shaw M-A, Sharpies C, Williams H, Castes M, Convit C, Blackwell JM: Polymorphism in tumor necrosis factor genes associated with mucocutaneous leishmaniasis" J Exp Med 1995, 182:1259-1264.
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17. •
Kroner BL, Goedert JJ, Blattner WA, Wilson SE, Carrington MN, Mann DL: Concordance of human leukocyte antigen haplotype-sharing, CD4 decline and AIDS in hemophitic siblings. Multicenter Hemophilia Cohort and Hemophilia Growth and Development Studies. AIDS 1995, 9:275-280. An affected-sibling pair approach to assessing the influence of the major histocompatibility complex on the rate of disease progression in HIV infection. 18.
Rowland-JonesS, Sutton J, Ariyoshi K, Dong T, Gotch F, McAdam S, Whitby D, Sabally S, Gallmore A, Corrah T et al.: HIV-specific cytotoxic T-cells in HIV-exposed but uninfected Gambian women. Nat Med 1995, 1:59-64.
19. •,
Davies JL, Kawaguchi Y, Bennett ST, Copeman JB, Cordell HJ, PritchardLE, Reed PW, Gough SC, Jenkins SC, Palmer SM, Todd JA: A genome-wide search for human type 1 diabetes susceptibility genes. Nature 1g94, 371 : 130-1361 The first study to report a complete genome scan for the mapping of susceptibility genes in a multifactorial disease of humans. A similar approach to mapping and identifying genes for human infectious diseases should be feasible. 20. •
Copeman JB, Cuccia F, Hearnr CM, Comall RJ, Reed PW, RenningenKS, Undlien DE, Nistico L, Buzzetti R, Tosi R, et aL: Linkage disequilibrium mapping of a type 1 diabetes susceptibility gene (IDDM7) to chromosome 2q31-q33. Nat Genet 1995, 9:80-85. Use of the transmission disequilibrium test in parent/offspring pairs to map more finely a diabetes susceptibility gene. Such an approach may be valuable for many common acute infectious diseases of children. 21.
Todd JA: Genetic analysis of type 1 diabetes using whole genome approaches. Proc Nat/Acad Sci USA 1995, 92:8560-8565.
22.
RischN, Ghosh S, Todd JA: Statistical evaluation of multiplelocus linkage data in experimental species and its relevance to human studies: application to nonobese diabetic (NOD) mouse and human insulin-dependent diabetes mellitus (IDDM). Am J Hum Genet 1993, 53:702-714.
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Reeves RH, BF OH, Pavan WJ, Gearhart JD, Hailer O: Genetic mapping of the Mx influenza virus resistance gene within
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27
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28. •
MarshDG, Neely JD, Breazeale DR, Ghosh B, Freidhoff LR, Ehrlich Kautzky E, Schou C, Krishnaswamy G, Beaty TH: Linkage analysis of IL4 and other chromosome 5q31.1 markers and total serum immunoglobulin E concentrations. Science 1994, 264:1152-1156. Mapping of a gene for serum IgE levels to chromosome 5 in the region of a cluster of =TH2-type" cytokine genes, including IL-4 and IL-5. The polymorphic gene in this region should be a major candidate gene for several infectious disease, particularly those caused by helminth infections. 29.
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SummerfieldJA, Ryder S, Sumiya M, Thursz M, Gorchein A, Monteil MA, Turner MW: Mannose binding protein gene mutations associated with unusual and severe infections in adults. Lancet 1995, 345:886-889. Establishes mannose-binding protein as a candidate gene for infectious diseases of adults as well as children. 34. •
Garred P, Madsen HO, Hofmann B, Svejgaard A: Increased frequency of homozygosity of abnormal mannan-bindingprotein alleles in patients with suspected immunodeficiency. Lancet 1995, 346:941-943. In a large series of patients with immunodeficiency an increased frequency of individuals homozygous for mannose-binding protein deficiency alleles was found. This study suggests that homozygotes rather than heterozygotes for MBP-deficiency alleles are those at risk of infection. 35.
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37
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Blackwell CC, Jonsdottir K, Hanson M, Todd WT, Chaudhuri AK, Mathew B, Brettle RP, Weir DM: Non-secretion of ABO antigens predisposing to infection by Neisseria meningitidis and Streptococcus pneumoniae. Lancet 1986, 2:284-285.
42. •
Kelly RJ, Rouquier S, Giorgi D, Lennon GG,Lowe JB: Sequence and expression of a candidate for the human Secretor blood group alpha(1,2)fucosyltransferase gene (FUT2). Homozygosity for an enzyme-inactivating nonsense mutation commonly correlates with the non-secretor phenotype. J Bio/Chem 1995, 270:4640-4649. The molecular basis of the non-secretor phenotype defined. All alleles studied had the same stop-codon mutation. This allows the relevance of this variant to infectious disease susceptibility to be readily assessed by DNA anaysis. 43. •
Vidal S, Tremblay ML, Govoni G, Gauthier S, Sebastiani G, Malo D, Skamene E, Olivier M, Jothy S, Gros P: The Ity/Lsh/Bcg locus: natural resistance to infection with intracellular parasites is abrogated by disruption of the Nrampl gene. J Exp Med 1995, 182:655-666. Confirmation that the susceptibilty locus Lsh//ty/Bcg in mice encodes the Nrampl gene. 44.
Shaw MA, Atkinson S, Dockrell H, Hussain R, Lins Lainson Z, Shaw J, Ramos F, Silveira F, Mehdi SO,, Kaukab F, BlackweU JM: An RFLP map for 2q33-q37 from multicase mycobacterlel and leishmanlal disease families: no evidence for an Lsh/Ity/Bcg gene homologue influencing susceptibility to leprosy. Ann Hum Genet 1993, 57:251-271.
45. •
Levee G, Liu J, Gicquel B, Chanteau S, Schurr E: Genetic control of susceptibility to leprosy in French Polynesia; no evidence for linkage with markers on telomerlc human chromosome 2. /nt J Lepr Other Mycobact Dis 1994, 62:499-511. Further evidence, supporting the findings in [44], that NRAMP1 in humans may not be a susceptibility gene for leprosy. 46. •
Blackwell JM, Barton CH, White JK, Searle S, Baker A-M, Williams H, Shaw M-A: Genomic organization and sequence of the human NRAMP gene: identification and mapping of a promoter region polymorphism. Mo/Med 1995, 1:194-205. Identification of a microsatellite in the NRAMP1 promoter of value for infectious disease linkage and association studies. 4?. •
Liu J, Fujiwara TM, Buu NT, Sanchez FO, Cellier M, Paradis AJ, Frappier D, Skamene E, Gros P, Morgan K, Schurr E: Identification of polymorphisms and sequence variants in the human homologue of the mouse natural resistance-associated macrophage protein gene. Am J Hum Genet 1995, 56:845-853. Identification of several polymorphisms in the NRAMP1 gene of value for infectious disease association and linkage studies. 48.
Pavlovic J, Arzet HA, Hefti HP, Frese M, Rost D, Ernst B, Kolb E, Staeheli P, Hailer O: Enhanced virus resistance of transgenic mice expressing the human MxA protein. J Viro11995, 69:4506-4510.
49.
Bredius RG, Derkx BH, Fijen CA, De Wit TP, De Haas M, Weening RS, Van de Winkel JG, Out TA: Fc gamma receptor Ila (CD32) polymorphism in fulminant meningococcal septic shock in children. J Infect Dis 1994, 170:848-853.
50.
Sanders LA, Van de Winkel JG, Rijkers GT, Voorhorst Ogink MM, De Haas M, Capel PJ, Zegers BJ: Fc gamma receptor Ila (CD32) heterogeneity in patients with recurrent bacterial respiratory tract infections. J Infect Dis 1994, 170:854-861.
51.
Davenport MP, Quinn CL, Chicz RM, Green BN, Willis AC, Lane WS, Bell Jl, Hill AVS: Naturally processed peptides from two disease-resistance-associated HLA-DR13 alleles show related sequence motifs and the effects of the dimorphism at position 86 of the HLA-DR beta chain. Proc Nat/Acad Sci USA 1995, 92: 6567-6571.
Genetics of infectious disease resistance Adrian VS Hill The identification of large numbers of candidate genes and the introduction of methodologies for whole-genome screening have provided new opportunities for elucidating the molecular basis of variable susceptibility to major infectious diseases. 12 genes have been implicated in variable susceptibility to malaria and susceptibility/resistance genes for several other infectious diseases are beginning to be identified. Recent work suggests that large-scale family linkage and population association studies will be a more succesful route to human disease genes than extrapolation from mouse models of infection.
Address Wellcome Trust Centre for Human Genetics, University of Oxford, Windmill Road, Oxford OX3 7BN, UK e-mail: [email protected]
Current Opinion in Genetics & Development 1996, 6:348-353 © Current Biology Ltd ISSN 0959-437X
Abbreviations G6PD glucose-6-phosphate dehydrogenase HLA human leukocyte antigen MBP mannose-bindingprotein NRAMP1 natural resistance-associated macrophage protein 1 TNF tumour necrosis factor
Introduction
Twin studies of many infectious diseases have demonstrated the importance of host genetics in determining variable susceptibility to infectious pathogens in humans. However, with the exception of malaria, rather few susceptibility or resistance genes have been identified. Recent progress in the isolation and characterisation of candidate genes and new microsatellite-based markers for family linkage studies of the whole genome now promise to change this situation. In this short review I will discuss recent progress in the analysis of genetic susceptibility to infectious diseases in humans (the genetics of mouse models of infectious disease has been reviewed recently elsewhere [1]). Progress in particular disease categories will be summarized, followed by an outline of current genome scanning approaches to gene mapping for infectious disease and I shall finish with a brief review of some major candidate genes. Infectious disease parasitic diseases
studies
Analysis of the protective effect against malaria of deficiency of the red cell enzyme, glucose-6-phosphate dehydrogenase (G6PD), was performed by Ruwendc et al. [2°] in two African populations. Both female heterozygotes and male hemizygotes had a reduced risk, by -50%, of severe malaria. This appears to settle a long-running
debate as to whether enzyme-deficient males or only their heterozygous sisters are protected; it also allowed modelling of the rate of selection of the G6PD A- variant leading to the conclusion that some balancing selective disadvantage of the enzyme deficiency must be have been retarding the rise in frequency of G6PD-deficiency alleles in Africa. Another red cell variant, absence of the Duffy blood group antigen, was shown many years ago to be associated with resistance to Plasmodium vivax malaria. T h e underlying mutation has now been elucidated - - a substitution in the binding site for the GATA1 erythroid transcription factor at position -46 of the promoter [3]. This should facilitate further population studies of this interesting variant that has essentially reached fixation in many African populations. The puzzle is how resistance to Plasmodium vivax malaria which is, at least now, a non-fatal disease could have led to such apparently rapid selection. Further analysis of the role of the M H C in variable susceptibility to malaria has involved both association and linkage studies. In a large case-control study at Kilifi in Kenya, an overall effect of H L A - D R variation on susceptibility to severe malaria was found (SN Yates et al., unpublished data). T h e allelic associations in Kenya, however, were different to those found in a previous west African study [4]. This geographical variability in HLA associations, that is seen for several infectious diseases [5], may in this instance relate to antigenic polymorphism in the malaria parasite. McGuire eta/. [6 °°] reported that homozygosity for a position -308 promoter polymorphism of the tumour necrosis factor (TNF) gene, located in the class III region of the MHC, was associated with a seven-fold increase in risk of severe forms of cerebral malaria in T h e Gambia. Importantly, by HLA typing the same samples for HLA class I and II region variants, it was possible to show that this effect was independent of linked H L A - D R and HLA-B polymorphism. Thus, in T h e Gambia, HLA-B53, HLA-DRBI*1302 {4] and the T N F promoter variant were independently associated with altered risk of severe malaria. Cabrera eta/. [7] have studied T N F region polymorphisms in leishmaniasis patients from Brazil. Amongst 25 cases of mucocutaneous leishmaniasis, a higher frequency of the T N F -308 promoter variant was found than in 43 controls, but it is unclear whether this increase is independent of linked HLA alleles. HLA studies have also been reported recently of infections by helminthic parasites. Meyer eta/. [8] have studied a small number of onchocerciasis patients from West Africa with detailed molecular typing of the HLA class II region and suggested that HLA-DQ and H L A - D P alleles might affect susceptibility. Yazdanbaksh et al. [9] have studied patients with filariasis from Indonesia and found an increased frequency of HLA-B27 in the
Genetics of infectious disease resistance Hill 349
elephantiasis group. Although no significant association remained after correction for multiple comparisons, these two small studies encourage further larger-scale studies of the immunogenetics of chronic worm infections. Bacterial diseases Mycobacterial diseases have been of particular interest to human geneticists because of substantial variation in susceptibility to these chronic infections, but studies of mannose-binding protein (MBP) and CD32 (see below) have also encompassed acute bacterial infections. T h e role of several candidate genes for tuberculosis susceptibility has recently been assessed in Africans (C Ruwende et a/. unpublished data) who collectively may be more susceptible than Caucasians [10]. Although no effect of N R A M P 1 (natural resistance-associated macrophage protein 1; previously known as iz'h/lty/Bcg) polymorphism was found, a promoter variant of the T N F gene was associated strongly with susceptibility to tuberculosis in T h e Gambia.
T h e role of M H C variants in susceptibility to leprosy in India has been reinvestigated by Rani etal. [11] and R Roy et al. (unpublished data). T h e HLA-DR2 association with leprosy per se, found in many Asian populations [12], was dissected into its subtypes by Rani et al., who found that both common DR15 subtypes (DR2 encompasses several DR15 and DR16 alleles) were associated with both tuberculoid and lepromatous forms of leprosy in north India. Roy et al. (unpublished data), studying Bengali patients, also observed increased frequencies of both DR2 alleles in all leprosy types and went on to analyse T N F promoter variants. T h e -308 T N F promoter variant was significantly increased in frequency in lepromatous but not in tuberculoid leprosy, and this effect was independent of the HLA-DR2 association. Viral diseases 5-20% of people infected by the hepatitis B virus develop a chronic carrier state that is associated with chronic liver disease and hepatocellular carcinoma. Twin studies have identified a host genetic component affecting risk of carriage [13] and several HLA studies have compared chronic carriers with those who have cleared the infection or with uninfected controls. Almarri and Batchelor [14 °] have compared 34 chronically infected Quatari patients to 44 individuals who had cleared the virus and 100 uninfected blood donors. In a two-stage analysis, HLA-DR7 was associated with chronic carriage and HLA-DR2 was associated with viral clearance. In a large study of Gambian adults and children, Thursz et al. [15 ° ] have used more detailed molecular HLA class II typing and identified an association between H LA-DRB 1"1302 and hepatitis B virus clearance. Several factors may explain the apparent inconsistency between these studies; for example, patients with liver disease were studied in Quatar and healthy carriers in Africa, and the low H L A - D R 2 frequency in T h e Gambia makes protective association with this allele difficult to identify.
Further relatively small studies of HLA associations with various manifestations of HIV infection have appeared (e.g. [16]). There is substantial variation between reported series and many studies have been too small to allow useful comparisons. However, Kroner et al. [17 °] have used analysis of sibling pairs to provide more convincing evidence that the M H C influences the rate of decline of the CD4 count. Epidemiological studies are beginning to define groups that may be relatively resistant to infection [18] and genetic studies of these individuals should be of interest. Genetic linkage analysis Progress in the development of genetic and physical maps of the human genome offers new avenues for approaching the analysis of many complex multifactorial diseases. The availability of sets of highly informative markers spaced along the human genome, combined with new technologies for high-throughput fluorescence-based genotyping, now allows genetic linkage studies to scan the human genome in months rather than years. Todd's group [19 °°] used 254 microsatellite markers in a genome screen for diabetes susceptibility genes. T h e analysis usually employed is of affected sib pairs and their parents where increased sharing of markers is searched for in affected individuals. Such identity-by-descent analyses may be supplemented by identity-by-state methods when parents are unavailable. Very recently, Jepson et al. (unpublished data) have identified strong linkage between microsatellite markers in the MHC and susceptibility to clinical malaria in a study of Gambian dizygotic twins.
Except for the identification of a major disease locus, such studies require analysis of large numbers of affected sib pairs and their parents. For acute infectious diseases, such families may often be difficult to collect, particularly in places where hospital records are limited. For more chronic diseases, such as leprosy and tuberculosis, control programmes in some countries may facilitate family collection. An alternative strategy that may be particularly practical for acute infectious diseases of children is to study parent--child pairs using linkage disequilibrium mapping. This usually forms the second stage of genome-screening studies in an attempt to improve the localization of a mapped locus from a large interval o f - 1 0 M b to -1 Mb or less. Recent work on diabetes has illustrated the utility of this approach in the fine mapping of a gene on chromosome 2 [20°], employing the transmission disequilibrium test. Linkage disequilibrium mapping is dependent on contingencies of population history so that it may be necessary to perform studies in different populations to find one with sufficient disequilibrium between a functional variant and a marker allele. Also, much larger numbers of microsatellites are required to cover the genome by using linkage disequilibrium mapping than by affected sib-pair analysis; however, screening of genomic regions containing candidate genes may offer a short cut.
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An important strategic issue is whether mapping of mouse susceptibility genes using genome screening approaches will offer an effective means of identifying major human susceptibility genes. Recent evidence suggests that this approach may be difficult. In the study of diabetes, where linkage studies have been performed successfully in both mouse and human and numerous genes has been mapped, few, if any, of these non-MHC genes appear to be the same in both species [21]. Risch etal. [22] have suggested that this may turn out to be a general pattern from considerations of allele frequency variation. Moreover, two major infectious disease resistance genes have now been mapped and identified in mice, the Mx gene for influenza [23] and the Nrampl gene for certain intracellular bacteria and leishmania [24]. To date, these genes have not been found to be functionally variable in humans. T h e very early success of both affected sib-pair studies [25] and transmission disequilibrium test analysis [26] of the M H C and leprosy, however, suggest that linkage studies of non-MHC genes in human infectious diseases may be fruitful. Candidate
with at least four infectious diseases. T h e initial report by McGuire eta/. [6°°] focused on cerebral malaria and more recent work has found associations with tuberculosis, severe malarial anaemia (W McGuire et al., unpublished data), lepromatous leprosy (R Roy et al. unpublished data) and leishmaniasis [7]. It would also be of interest to assess possible associations with the rate of disease progression to AIDS in HIV infection, as T N F can lead to increased transcription rates of HIV. Table 2 Malaria resistance genes. c~-Globin ~-Globin Duff)/chemokine receptor G6PD Blood group 0 Erythrocyte band 3
HLA-B HLA -DR TNF Spectrin Glycophorin A Glycophorin B
Genes assiciated with resistance or susceptibility to malaria [6"°,27]. G6PD, glucose-6-phosphate dehydrogenase; HLA, human leukocyte antigen; TNF, tumour necrosis factor.
genes
T h e candidate gene approach (see Table 1 for a select listing) for the identification of resistance/susceptibility genes has been applied with particular success in malaria where variants of 12 genes have now been implicated in resistance to this disease (Table 2) [27]. This might relate to a relatively large number of genes that affect susceptibility to infectious disease in general. If much functional polymorphism in the human genome is a result of selection pressures by infectious diseases, many loci--particularly those involved in innate or acquired i m m u n i t y - - may affect infectious disease resistance. Some candidate genes of current interest that might affect numerous infectious diseases will be mentioned briefly.
Table 1 Candidate genes for Infectious diseases. MBP Inducible nitric oxide synthase Nramp 1 TNF FcyRII Km allotypes MxA T cell receptor variants Interferon 7 receptor Interleukin-4 Fucosyl transferase-2
/CAM- 1 HLA-DR Interleukin - 12 Interleukin-1 receptor IgE-RII Dully chemokine receptor Vitamin D receptor IgFt allotypes FcyRIII TNF-receptor 55 Interleukin- 10
A selection of some candidate genes for infectious disease resistance and susceptibility, many of which have documented functional polymorphisms. HLA, human leukocyte antigen; MBP, mannose binding protein; Nrampl, natural resistance-associated macrophage protein 1; TNF, tumour necrosis factor.
Tumour necrosis factor
Over the past year, there has been new evidence that promoter variants of the T N F gene may be associated
These associations provide evidence of pathophysiological mechanisms in these infectious diseases. In cerebral malaria, lepromatous leprosy and leishmaniasis there is evidence of increased serum levels of T N E The association with the -308 v a r i a n t - - t h a t has been associated with increased transcription rates (AG Wilson, JA Symons, T L McDowell, FS di Giovane, GW Duff, British Journal of Rheumatology 1994, 33[suppl 1]:89)--suggests that the raised T N F levels may play a part in the pathogencsis of these disorders rather than just being reactive. Functional studies of these promoter variants remain few and data arc required on the effects of appropriate specific stimuli on relevant cell types. These associations with T N F variants encourage analysis of the role of other cytokine promoter variants in infectious disease. T h e list of cytokine and cytokine receptors that might be of interest is long, but there is much current interest in the cluster of TH2-1ike cytokine genes, including interleukin-4, on chromosome 5. High total IgE levels in atopy have been mapped to this region I28"1. Mannose-binding protein
MBP is a serum lectin that plays an important role in innate immunity. MBP activates complement and acts directly as an opsonin, using the Clq receptor on macrophages. Functionally deficient variants of this collectin are common in most populations. Three mutations have been described at codons 52, 54 and 57 that alter a single amino acid and lead to low and near absent serum levels in heterozygotes and homozygotes respectively [29,30]. Recently, Madsen eta/. [31] have described further variation in the promoter region of the MBP gene that is associated with lesser degrees of variation in serum levels. MBP deficiency has been associated with repeated
Genetics of infectious disease resistance Hill 351
bacterial and fungal infections in young children [32] and recently with unusual infections in adults [33°]. Garred et aL [34 °] have found a higher frequency of homozygotes for MBP-deficiency alleles amongst patients with unexplained immunodeficiency than in controls. Studies of individual infectious diseases have begun to be reported. Small studies of serum levels of MBP in HIV infection [35] and in leprosy [36] have suggested that deficiency alleles might be associated with these diseases but no genotyping has been performed. A study of recurrent otitis media in children failed to find an association with M B P genotypes [37]. As MBP can interact with a large number of micro-organisms bearing surface carbohydrates, further studies of the influence of deficiency alleles on infectious disease risk are indicated. If only homozygotes are at increased risk, fairly large samples sizes may be required to demonstrate significant associations. It will be particularly interesting to identify why these apparently deleterious MBP deficiency alleles have reached high frequencies in so many human populations.
Natural resistance-associated macrophage protein 1
Studies over almost 20 years have led to the identification of a gene in mice that affects resistance to several intracellular pathogens. These include Leishmania parasites, some strains of Bacille Calmette-Guerin and Salmonella. Vidal et al. [24], have isolated the gene Nrampl by positional cloning and have identified, in the recessive deficiency allele, a single amino acid change (Gly---)Asp) in a putative transmembrane domain. T h e precise function of Nrampl is unclear, but a role in macrophage activation and its equivalence with iz'h/Ity/Bcg have been confirmed using a gene-knockout mouse [43°]. Although wild-type mice with a deficiency allele have not been reported, there has been considerable speculation that variants of the human homologue of Nrampl, might play a role in variable susceptibility to some human infectious diseases. How such deleterious alleles would have been selectively maintained has not been addressed. With the mapping and identification of the human homologue, NRAMP1, both linkage and association studies have been performed. Shaw et aL [44] have studied Pakistani families with leprosy and found no linkage to the NRAMP1 region; another study of NRAMP1 and leprosy in Polynesia also failed to find linkage [45°].
Secretor
Approximately 20% of most human populations have a genetically determined inability to secrete the water soluble forms of the ABO blood group antigens into saliva and other body fluids. Such non-secretors have been the subjects of a large number of disease association studies using serological methods. Non-secretors have been reported to be at increased risk of meningococcal invasive disease [38], Candida infections [39] and recurrent urinary tract infections [40]. Interestingly, non-secretors were found to be under-represented amongst patients with a variety of respiratory viral infections, such as influenza virus and respiratory syncytial virus [41], suggesting that this apparent balanced polymorphism could be maintained by differential resistance to viral and non-viral infectious pathogens. The molecular genetic basis of the non-secretor phenotype has now been elucidated. Kelly et al. [42*] have cloned and sequenced the gene, termed Sec2, that encodes an alpha(1,2)fucosyhransferase. This gene is in close proximity to the H blood group alpha(I,2) fucosyhransferase on chromosome 19. Sequence analysis found that nonsecretors were homozygous for an enzyme-inactivating nonsense allele (Try143--~ter), and 19% of the 52 normal Americans studied were homozygous for this mutation. We have recently found this variant at high frequency in sub-Saharan Africans (S Ali et aL, unpublished data), suggesting that this may be the predominant non-secretor mutation in most populations. This definition of the molecular basis of the non-secretor phenotype should facilitate the analysis of this locus in studies of infectious disease.
Several polymorphisms in the NRAMP1 gene and its flanking sequences have now been described, facilitating disease association studies in humans [46*,47°]. Liu et al. [47"] have found no association with tuberculosis in very small studies of Hong Kong and Canadian families, and C Ruwende et al. (unpublished data) have recently found no association of NRAMP1 microsatellite alleles with tuberculosis in west Africans. Thus, as with the ilia" gene [48] which was isolated using a mouse susceptible to influenza virus, for Nrampl there is, as yet, no clear evidence of functional variation in, or infectious disease associations with, the human homologue.
FcyRII receptor Receptors for the constant region of immunoglobulins are involved in the phagocytosis of IgG-opsonized microorganisms. Three distinct receptors have been identified and the two of these that are expressed on neutrophils have diallelic functional polymorphisms: FcyRIIa (CD32) and FcyRIII (CD16). T h e single amino acid difference (His---)Arg at position 131) in the FcyRIla-R131 allele is associated with reduced phagocytosis of IgG2 opsonized bacteria. Bredius et al. [49] have phenotyped, by an indirect immunofluorescence method, cells from 25 children who had survived fulminant meningococcal septic shock and compared them to a Caucasian control group. Homozygotes for the genotype associated with reduced phagocytosis were more frequent in the cases, but heterozygotes were more frequent in the controls. No FcyRIII association was suggested. In a further Dutch study of 48 children with recurrent bacterial infections [50], homozygotes for the FcyRH allele with higher
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phagocytic activity were less frequent amongst cases than controls. T h e hypothesis suggested by these small s t u d i e s - - t h a t homozygotes for the R131 allele are at increased risk of meningococcal disease and perhaps other bacterial infections--should now be evaluated in larger series of patients.
HLA-D alleles associated with generalized disease, localized disease, and putative immunity in Onchocerca volvulus infection. Proc Natl Acad Sci USA 1gg4, 91:7515-7519.
9.
YazdanbakshM, Sartono E, Kruize YCM, Kumiawan A, Partono F, Maizels RM, Schreuder GMT, Schipper R, De Vries RRP: HLA and elephantiasis in lymphatic filariasis. Hum Immunol 1995, 44:58-61.
10.
Stead WW, Senner JW, Reddick WT, Lofgren JP: Racial differences in susceptibility to infection by Mycobacterium tuberculosis. N Eng/ J Med 1990, 322:422-427.
11.
Ran/R, Fernandez Vina MA, Zaheer SA, Beena KR, Stastny P: Study of HLA class II alleles by PeR oligotyping in leprosy patients from north India. Tissue Antigens 1993, 42:133-137.
12.
ToddJR, West Be, McDonald JC: Human leukocyte antigen and leprosy: study in northern Louisiana and review. Rev Infect Dis 1990, 12:53-74.
13.
Lin TM, Chen CJ, Wu MM, Yang CS, Chen JS, Lin CC, Kwang TY, Hsu ST, Lin SY, Hsu LC: Hepatitis B virus markers in Chinese twins. Ant~cancer Res 1989, 9:737-742.
Conclusions A variety of approaches from mouse genetics to genome scanning of human families is being used to identify a large range of new candidate genes for studies of infectious disease susceptibility in humans. Recent data suggest that genetic resistance to some, perhaps most, infections will be controlled by a large number of genes and that many resistance/susceptibility alleles will affect more than one infectious disease. T h e r e is a need for large carefully designed case-control studies in different populations to build up a picture of the relative importance of different genes. Over the next few years, this strategy should provide substantial new insights into both the pathophysiology of infectious diseases and the role of infectious agents in selecting for variation in the human genome.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: = •. 1.
14. AlmarriA, Batchelor JR: HLA and hepatitis B infection. Lancet • 1994, 344:1194-1195. Quatari patients with chronic carriage of hepatitis B virus were found to have altered HLA-DR allele frequencies compared to controls. 15. •
ThurszMR, Kwiatkowski D, AIIsopp CE, Greenwood BM, Thomas He, Hill AV: Association between an MHC class II allele and clearance of hepatitis B virus in the Gambia. N Engl J Med 1995, 332:1065-1069. Asymptomatic carriers of hepatitis B virus in the Gambia were tess likely to have the HLA-DRBl*1302 allele, suggesting that an immune response restricted by this HLA type may facilitate viral clearance. Recent analysis by Davenport et al. [51] of the sequence of peptides bound by this molecule should facilitate molecular immunological analysis of this HLA-disease association. 16.
of special interest of outstanding interest Mcleod R, Buschman E, Arbuckle LD, Skamene E: Immunogenetics In the analysis of resistance to intracellular pathogens, Curt Opin Immuno/1995, 7:539-552.
2. •
Ruwende C, Khoo SC, Snow RW, Yates SN, Kwiatkowski D, Gupta S, Warn P, AIIsopp CE, Gilbert SC, Peschu Net al.: Natural selection of heml- and heterozygotes for G6PD deficiency in Africa by resistance to severe malaria. Nature 1g95, 376:246-249. Two large case-control studies of African children with severe malaria provide an estimate of the degree of protection afforded by this enzyme deficiency, -50o/0 reduction in risk. Both male hem/zygotes and female heterozygotes were shown to protected. 3.
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Kroner BL, Goedert JJ, Blattner WA, Wilson SE, Carrington MN, Mann DL: Concordance of human leukocyte antigen haplotype-sharing, CD4 decline and AIDS in hemophitic siblings. Multicenter Hemophilia Cohort and Hemophilia Growth and Development Studies. AIDS 1995, 9:275-280. An affected-sibling pair approach to assessing the influence of the major histocompatibility complex on the rate of disease progression in HIV infection. 18.
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Davies JL, Kawaguchi Y, Bennett ST, Copeman JB, Cordell HJ, PritchardLE, Reed PW, Gough SC, Jenkins SC, Palmer SM, Todd JA: A genome-wide search for human type 1 diabetes susceptibility genes. Nature 1g94, 371 : 130-1361 The first study to report a complete genome scan for the mapping of susceptibility genes in a multifactorial disease of humans. A similar approach to mapping and identifying genes for human infectious diseases should be feasible. 20. •
Copeman JB, Cuccia F, Hearnr CM, Comall RJ, Reed PW, RenningenKS, Undlien DE, Nistico L, Buzzetti R, Tosi R, et aL: Linkage disequilibrium mapping of a type 1 diabetes susceptibility gene (IDDM7) to chromosome 2q31-q33. Nat Genet 1995, 9:80-85. Use of the transmission disequilibrium test in parent/offspring pairs to map more finely a diabetes susceptibility gene. Such an approach may be valuable for many common acute infectious diseases of children. 21.
Todd JA: Genetic analysis of type 1 diabetes using whole genome approaches. Proc Nat/Acad Sci USA 1995, 92:8560-8565.
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RischN, Ghosh S, Todd JA: Statistical evaluation of multiplelocus linkage data in experimental species and its relevance to human studies: application to nonobese diabetic (NOD) mouse and human insulin-dependent diabetes mellitus (IDDM). Am J Hum Genet 1993, 53:702-714.
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