Can your genes make you more prone to pneumococcal disease?

Keywords::

• mannose-binding lectin
• nuclear factor-κB
• polymorphism
• PTPN22
• Streptococcus pneumoniae
• Toll-like receptors

The pneumococcus & humans: colonization versus invasive disease

Streptococcus pneumoniae (the pneumococcus) is the most common cause of community-acquired pneumonia in Europe and the USA, and more than 1 million children under 5 years of age worldwide die from pneumococcal disease each year [1,2]. Intermittent, asymptomatic colonization of the nasopharynx by S. pneumoniae is common in the general population, and a key question is why only a minority of colonized individuals develop life-threatening invasive disease (defined by the isolation of S. pneumoniae from a normally sterile site) [3]. As with other infectious diseases, the likelihood of developing clinical disease following exposure typically reflects an interaction between microbial factors (such as pneumococcal serotype and other virulence factors) and acquired and genetic host factors. Acquired host risk factors for the development of invasive pneumococcal disease (IPD) are well described and include extremes of age, cigarette smoking, malnutrition and comorbidities such as HIV infection [4–7]. However, study of the possible role of host genetic variation in susceptibility to IPD has been neglected until recently.

Susceptibility to infectious disease: the role of host genetics

Increasing evidence points to an important role for host genetics in influencing the development of infectious disease [8,9]. The classic example of genetic influence on infectious disease development is the effect of genetic red blood cell diseases on malaria susceptibility. More recently, twin studies have demonstrated higher concordance rates of TB, leprosy, poliomyelitis and hepatitis B in monozygotic (identical) than in dizygotic (non-identical) twins, suggesting a genetic component in susceptibility to these infectious diseases [8]. A landmark study of Danish adoptees reported that adopted children had a 5.8-fold increased risk of death from infectious disease if one of their biological parents had died prematurely (<50 years of age) from infection [10]. This risk was significantly higher than that demonstrated for cardiovascular disease (5.2-fold) or cancer (1.2-fold). Within the infectious disease group, most deaths were from respiratory infection. Animal studies have also supported an important role for host genetic effects, for example, revealing differences in bacterial loads and cytokine responses between different inbred mouse strains following pneumococcal infection [11,12].

Genetic architecture of infectious disease susceptibility

Genetic causes of human disease are often broadly divided into two groups: monogenic (or Mendelian) disease and complex (or multifactorial) disease. Monogenic diseases are caused by abnormal mutations in single genes; such mutations tend to be rare and exert a major effect on gene function. Complex disease results from a combination of diverse environmental factors and genetic factors, although the relative contributions of the environment and genes may vary between individuals. The genetic contribution to complex disease is often considered to comprise common variation (or polymorphism) in multiple disease-predisposing genes, each of modest individual effect. Both very rare mutations and common polymorphisms have previously been described in association with IPD [13,14]. This article will focus on recent advances, in particular the role of genetic variation within the Toll-like receptor (TLR)–nuclear factor-κB (NF-κB) inflammatory signaling pathway, the innate immune molecule mannose-binding lectin, and the autoimmune susceptibility gene PTPN22.

TLR–NF-κB signaling & susceptibility to IPD

Toll-like receptors and members of their intracellular signaling pathway play a critical role in the early recognition of invading microorganisms and the initiation of an inflammatory host response [15,16]. The TLR–NF-κB pathway is complex and remains incompletely understood despite considerable research. Viewed simply, the pathway components can be divided into four main groups: a series of activating receptors that sense different microbial products (TLRs); protein adaptors (such as MyD88 and Mal/TIRAP); kinases (e.g., the IRAKs and the IKK complex); and finally transcription factors (such as NF-κB) that control the expression of proinflammatory genes.

Evidence from both in vitro and animal models supports a key role for TLR signaling and NF-κB activation in the host immune response to pneumococcal infection [17–26]. Furthermore, the investigation of individuals with rare primary immunodeficiencies resulting in invasive pneumococcal infection, sometimes in association with the developmental condition anhidrotic ectodermal dysplasia, has identified causative mutations within four genes in the TLR–NF-κB pathway: NEMO (a regulatory subunit in the IKK complex), NFKBIA (which encodes an inhibitor of NF-κB), IRAK4 and MyD88[27–33]. In each case, the mutations result in impaired NF-κB activation; NEMO and NFKBIA mutations interrupt multiple innate and adaptive pathways that signal to NF-κB, including the TLR pathway, whereas mutations in IRAK4 and MyD88 appear to disrupt only TLR and IL-1 receptor signaling [27,34]. In keeping with this, the immunodeficiency resulting from hypomorphic NEMO mutations is typically severe and a wide range of pathogen susceptibilities are described, including encapsulated pyogenic bacteria, atypical mycobacteria, fungi and viruses [28–30,34]. IRAK4 and MyD88 deficiencies on the other hand appear to associate with a narrower spectrum of infectious pathogens, primarily pyogenic encapsulated bacteria, particularly S. pneumoniae[27,31,35].

These descriptions of novel primary immunodeficiencies indicate that NF-κB is critical for protective immunity against the pneumococcus in humans. The mutations appear to be extremely rare, however, with only a very small number of individuals described worldwide. Recent population-based case–control studies have identified associations between susceptibility to IPD and common polymorphisms in the genes NFKBIA, NFKBIE and NFKBIZ, each of which encodes an inhibitor of NF-κB [36,37]. Although the individual effect sizes are small, the risk variants are very common in Europeans, suggesting that these polymorphisms might make a significant contribution to the burden of IPD in this population. These findings also illustrate that both rare and common variation in the same gene (NFKBIA) may associate with severe bacterial disease. Study of the genetic control of NF-κB inhibition may be increasingly relevant in the setting of growing interest in the modulation of NF-κB activity as a treatment for infectious and inflammatory disease [38–41].

Further evidence for a role of common genetic polymorphism in IPD susceptibility comes from the study of Mal/TIRAP, an adaptor protein essential for TLR2 and TLR4 signaling [42]. A single base change has been identified that leads to an amino acid substitution of serine (S) to leucine (L) at position 180 of the Mal/TIRAP protein and affects the strength of signal after pathogen recognition. This polymorphism was found to associate not just with IPD, but also with other Gram-positive and Gram-negative bacterial infections, malaria and TB, reflecting the key role of TLR signaling in host defense [43].

Interestingly, the direction of effect was found to be one of heterozygote protection, with an overall odds ratio for infectious disease in S180L heterozygotes of approximately half. Functional studies using an in vivo model of sepsis (intravenous lipopolysaccharide challenge of human volunteers) revealed an increase in inflammatory cytokine responses in S180L heterozygotes when compared with S180 homozygotes, whereas 180L homozygotes produced the highest cytokine responses [44]. It appears that a moderately increased inflammatory response (seen in S180L heterozygotes) is beneficial, probably reflecting enhanced clearance of pathogens compared with reduced signaling in the S180 homozygote state. On the other hand, an excessively increased inflammatory response (180L homozygotes) may prove detrimental and render individuals more vulnerable to developing severe forms of malaria and bacterial disease [45–47]. It is also possible that such an excessively increased inflammatory response may lead to worse outcomes following infection – for example, the development of septic shock or adult respiratory distress syndrome – and this could prevent fixation of the leucine variant in the population [44]. Heterozygosity at S180L may therefore confer a protective phenotype characterized by intermediate levels of pathway activation and an optimal, ‘balanced’ inflammatory response.

Mannose-binding lectin

Mannose-binding lectin (MBL) is a serum lectin that binds repeating sugar arrays on the surface of a wide range of microorganisms, including Gram-positive bacteria [48]. MBL plays an important role in innate immunity by promoting opsonophagocytosis, primarily through the activation of complement independently of antibody. Significant interindividual variation in baseline serum MBL levels and function occurs as a result of common genetic polymorphism, and MBL genotypic deficiency has been reported in association with a wide range of infectious phenotypes, including IPD [49–58]. However, the role of MBL deficiency in susceptibility to respiratory infection remains controversial, and recent studies have reported both a lack of association with community-acquired pneumonia and a possible association with respiratory tract infections [59–61]. Possible associations between MBL deficiency and poor clinical outcomes following pneumonia and pneumococcal infection have also been reported [59,62], although not all studies have confirmed this finding [60]. These mixed, and in some cases conflicting, results may reflect the variable disease phenotypes studied, encompassing noninvasive lower respiratory tract infection and invasive infection with a variety of bacterial species. Furthermore, the number of individuals included in analysis of clinical outcome is often small, and consequently associations with outcome should be interpreted with caution.

Inherited deficiency of the MBL-associated serine protease MASP-2 has also been described in association with recurrent pneumococcal pneumonia [63]. The contribution of this rare mutation to disease at the population level remains unclear, and a recent study demonstrated a lack of association with community-acquired pneumonia susceptibility or outcome [59].

PTPN22: autoimmunity & infection

The identification of the gene PTPN22 as a major susceptibility locus for autoimmunity represents one of the more significant recent advances in the field of human disease genetics. PTPN22 encodes the lymphoid tyrosine phosphatase Lyp, which acts to negatively regulate T-cell signaling by dephosphorylating and inactivating T-cell-receptor-associated kinases [64,65]. A common, functional PTPN22 polymorphism has been described that encodes the amino acid substitution of arginine to tryptophan at position 620 of Lyp [64], and multiple reports have established that the Trp620 variant associates with an increased risk of autoimmune disease [64–76]. The Trp620 variant was subsequently shown to be a gain-of-function mutation, which increases phosphatase activity and is a more potent inhibitor of T-cell-receptor and B-cell signaling [76–78]. In keeping with such a downregulatory effect on T- and B-cell responses, the Trp620 variant was also found to associate with susceptibility to IPD [79]. This variant is notably rare in African individuals, perhaps reflecting a selective pressure from the huge burden of bacterial disease in these populations [79]. Although it is often assumed that susceptibility alleles for common autoimmune diseases have been selected by providing resistance to major infectious causes of mortality [80], the observation that the same PTPN22 variant associates with both autoimmune disease and IPD is not in keeping with this hypothesis.

Future research directions

Recent research has described both rare genetic mutations (NEMO, IRAK4, MyD88 and MASP-2) and common polymorphisms (NFKBIA, NFKBIE, NFKBIZ, TIRAP, MBL and PTPN22) that associate with susceptibility to IPD. Many more genes are likely to be involved, however. Future approaches will utilize ‘genome-wide’ scanning technology, which enables genotyping of hundreds of thousands of polymorphisms spanning the genome [81]. Unlike previous ‘candidate’ gene approaches, which are limited to the study of a relatively small number of loci with pre-existing evidence for a role in disease development, such a genome-wide approach makes no assumptions about the location of causal variants. Indeed, the ability of well-conducted genome-wide association studies to identify previously unsuspected genetic associations with common disease has now been clearly demonstrated, although very large sample sizes are required [74,81]. Another area of major interest concerns the relative contribution of multiple rare mutations and common genetic polymorphism to disease susceptibility at the population level [82,83]. Advances in sequencing technology will soon permit the large-scale and systematic study of uncommon and rare variants in susceptibility to infectious disease.

Finally, a significant but essential challenge will be to develop strategies for translating our knowledge of novel susceptibility genes into improved patient outcomes from pneumococcal disease. Given the considerable interindividual variation observed in both susceptibility to IPD and outcomes from this disease, it is hoped that subtle manipulation of the host immune response may translate into clinical benefit. Genomics offers a potentially powerful approach to dissect the relevant pathways, and may offer novel therapeutic targets for immunomodulatory drugs. Definition of the molecules and pathways that are important in individual patients may eventually lead to a personalized approach to care, with prophylaxis and therapy tailored on the basis of an individual’s genetic background.

Financial & competing interests disclosure

This work was supported by The Wellcome Trust, UK, and NIHR Oxford Biomedical Research Centre. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.