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The HLA 8.1 Ancestral Haplotype in schizophrenia: dual implication in neuro‐synaptic pruning and autoimmunity?
Abstract
Schizophrenia (SZ), a neurodevelopmental disorder, is one of the most severe and common psychiatric conditions, affecting globally 1% of adult population. Stemming from complex gene‐environment interactions, in a significant subset of patients, SZ is characterized by immune dysfunctions that include early inefficient anti‐infectious responses as well disease‐associated chronic low‐grade inflammation and comorbid autoimmune conditions (1). In such intersections between infection, inflammation and autoimmunity, the key genetic platform for both innate and adaptive immune processes is the major histocompatibility complex (MHC) which encompasses the prominent human leukocyte antigen (HLA) region. The HLA system is the most polymorphic region of the human genome and its allelic diversity is essential for antigen presentation to immune effector cells and downstream humoral and cellular immune responses. Hence, characterizing the HLA allelic diversity allows not only to evaluate the potential genetic relationship between the HLA system and a given disease but also to understand its impact on pathophysiological processes (2).
... C4 gene deficiencies such as C4 null alleles may be prime candidates to interrogate in future studies, as these mutations are known to lead to autoimmune susceptibilities, as are C4B partial or complete deficiencies [19,21,33,85]. It is of note that the ancestral human leukocyte antigen (HLA) 8.1 haplotype, one of the most associated haplotypes with autoimmune disorders, is thought to be protective against schizophrenia likely due to its natural lack of the C4A locus [28,86,87]. We examined CNVs of the C4 gene with the expectation that a high copy number would lead to elevated plasma C4 levels. ...
Complex brain disorders like schizophrenia may have multifactorial origins related to mis-timed heritable and environmental factors interacting during neurodevelopment. Infections, inflammation, and autoimmune diseases are over-represented in schizophrenia leading to immune system-centered hypotheses. Complement component C4 is genetically and neurobiologically associated with schizophrenia, and its dual activity peripherally and in the brain makes it an exceptional target for biomarker development. Studies to evaluate the biomarker potential of plasma or serum C4 in schizophrenia do so to understand how peripheral C4 might reflect central nervous system (CNS)-derived neuroinflammation, synapse pruning and other mechanisms. This effort, however, has produced mostly conflicting results, with peripheral C4 sometimes elevated, reduced or unchanged between comparison groups. We undertook a pilot biomarker development study to systematically identify sociodemographic, genetic, and immune-related variables (autoimmune, infection-related, gastrointestinal (GI), inflammatory), which may be associated with plasma C4 levels in schizophrenia (SCH; n=335) and/or in non-psychiatric comparison subjects (NCs; n=233). As with previously inconclusive studies, we detected no differences in plasma C4 levels between SCH and NCs. In contrast, levels of general inflammation, C-Reactive Protein (CRP), were significantly elevated in SCH compared to NCs (ANOVA, F=20.74, p<0.0001), suggestive that plasma C4 and CRP may reflect different sources or causes of inflammation. In multivariate regressions of C4 gene copy number variants (CNVs), plasma C4 levels were correlated only for C4A (not C4B, C4L, C4S) and only in NCs (R Coeff=0.39, CI=0.01-0.77, R2=0.18, p<0.01; not SCH). Other variables associated with plasma C4 levels only in NCs included sex, double-stranded DNA (dsDNA) IgG, tissue-transglutaminase (TTG) IgG, and cytomegalovirus (CMV) IgG. Toxoplasma gondii IgG was the only variable significantly correlated with plasma C4 in SCH but not in NCs. Many variables were associated with plasma C4 in both groups (body mass index (BMI), race, CRP, N-methyl-D-aspartate receptor (NMDAR) NR2 subunit IgG, TTG IgA, lipopolysaccharide (LPS)-binding protein (LBP), and soluble CD14 (sCD14). While the direction of most C4 associations was positive, autoimmune markers tended to be inverse, and associated with reduced plasma C4 levels. When NMDAR NR2 autoantibody-positive individuals were removed, plasma C4 was elevated in SCH versus NCs (ANOVA, F=5.16, p<0.02). Our study was exploratory and confirmation of the many variables associated with peripheral C4 require replication. Our preliminary results point toward autoimmune factors and exposure to the pathogen, T. gondii, as possibly significant contributors to variability of total C4 protein levels in plasma of individuals with schizophrenia.
... Hypothetically, this could reflect the involvement of a particular immuno-genetic background in SZ etiology. We recently reported an association between late onset SZ and the Human Leukocyte Antigen (HLA)-8.1 ancestral haplotype, known to increase both autoimmunity and inflammation (Tamouza et al., 2020). Such association, if is also observed in the late onset immuno-metabolic subtype, may provide clues to understand the pathophysiological processes underpinning the associations of MetS and late onset SZ. ...
Background
Metabolic syndrome (MetS) is a highly prevalent and harmful medical disorder often comorbid with psychosis where it can contribute to cardiovascular complications. As immune dysfunction is a key shared component of both MetS and schizophrenia (SZ), this study investigated the relationship between immune alterations and MetS in patients with SZ, whilst controlling the impact of confounding clinical characteristics including psychiatric symptoms and comorbidities, history of childhood maltreatment and psychotropic treatments.
Method
A total of 310 patients meeting DSM-IV criteria for SZ or schizoaffective disorders (SZA), with or without MetS, were systematically assessed and included in the FondaMental Advanced Centers of Expertise for Schizophrenia (FACE-SZ) cohort. Detailed clinical characteristics of patients, including psychotic symptomatology, psychiatric comorbidities and history of childhood maltreatment were recorded and the serum levels of 18 cytokines were measured. A penalized regression method was performed to analyze associations between inflammation and MetS, whilst controlling for confounding factors.
Results
Of the total sample, 25% of patients had MetS. Eight cytokines were above the lower limit of detection (LLOD) in more than 90% of the samples and retained in downstream analysis. Using a conservative Variable Inclusion Probability (VIP) of 75%, we found that elevated levels of interleukin (IL)-6, IL-7, IL-12/23 p40 and IL-16 and lower levels of tumor necrosis factor (TNF)-α were associated with MetS. As for clinical variables, age, sex, body mass index (BMI), diagnosis of SZ (not SZA), age at the first episode of psychosis (FEP), alcohol abuse, current tobacco smoking, and treatment with antidepressants and anxiolytics were all associated with MetS.
Conclusion
We have identified five cytokines associated with MetS in SZ suggesting that patients with psychotic disorders and MetS are characterized by a specific “immuno-metabolic” profile. This may help to design tailored treatments for this subgroup of patients with both psychotic disorders and MetS, taking one more step towards precision medicine in psychiatry.
... The classical HLA alleles-mediated adaptive immune responses have long been associated with inflammatory responses, including the inflammatory HLA 8.1 ancestral haplotype in BD and in SZ. Although we found associations between the HLA 8.1 AH and severe forms of the BD, i.e. rapid cycling and suicidal behavior 12 , in SZ we observed that the HLA 8.1 AH confers protection or delays age of disease onset 6,13 . Given that the HLA cluster is pivotal to both immune processes and brain development, these results suggest that HLA-linked processes may be implicated in mechanisms involved in ontogenic and neurodevelopmental difference between SZ and in BD. ...
Schizophrenia (SZ) and bipolar disorders (BD) are severe mental illnesses that lack reliable biomarkers to guide diagnosis and management. As immune dysregulation is associated with these disorders, we utilized the immunoregulatory functions of the natural killer cell inhibitory HLA-E locus to investigate the relationships between HLA-E genetic and expression diversities with SZ and BD risk and severity. Four hundred and forty-four patients meeting DSM-IV criteria for SZ (N = 161) or BD (N = 283) were compared to 160 heathy controls (HC). Circulating levels of the soluble isoform of HLA-E molecules (sHLA-E) were measured and HLA-E*01:01 and HLA-E*01:03 variants genotyped in the whole sample. sHLA-E circulating levels were significantly higher in both SZ and in BD patients compared to HC (pc < 0.0001 and pc = 0.0007 for SZ and BD, respectively). High sHLA-E levels were also observed in stable SZ patients and in acute BD patients experiencing depressive episodes when comparisons were made between the acute and stable subgroups of each disorder. sHLA-E levels linearly increased along HLA-E genotypes (p = 0.0036). In conclusion, HLA-E variants and level may have utility as diagnostic biomarkers of SZ and BD. The possible roles of HLA diversity in SZ and BD etiology and pathophysiology are discussed.
... Immune dysregulations are thought to result from complex interactions between genetic, epigenetic, and environmental factors and, as such, are major candidate targets for the improvement in diagnosis and treatment of major mood and psychotic disorders [3]. In this context, functionally relevant genetic variants of tolllike receptor loci have been associated with bipolar disorder (BP) [4], while the major histocompatibility complex (MHC) and its human leukocyte antigen (HLA) cluster have strong implications in both schizophrenia (SZ) and BP [5][6][7]. Environmental factors, including infection, heighten the risk of developing psychotic disorders. Indeed, maternal infections with influenza, Toxoplasma gondii, or herpes simplex virus (HSV) types 1 and 2 have repeatedly been found to significantly increase SZ or BP risk in offspring [8,9], while in adults, hospitalization for infection raises the risk of subsequent psychotic disorders [10]. ...
Accumulating evidence majorly implicates immune dysfunction in the etiology of psychotic disorders. In particular, altered numbers and functions of natural killer (NK) cells have been described in psychosis, but interpretation has often been confounded by a number of biases, including treatment. Eighty-one first-episode psychosis (FEP) patients who subsequently received a diagnosis of either schizophrenia (SZ; n = 30) or bipolar disorder (BP; n = 31) over a five-year follow-up period were investigated for their NK cell phenotype and compared to 61 healthy controls (HCs). We found a similar proportion of CD3⁻CD56⁺ NK cells in FEP patients and HCs. The frequency of NK cells expressing the late cell activation marker HLA-DR was significantly increased in FEP patients compared to HCs, especially in patients with BP (p < 0.0001) and, to a lesser degree, in patients with SZ (p = 0.0128). Interestingly, the expression of the activating NKG2C receptor, known to be associated with infections, was higher in patients with SZ and BP than in HCs (p < 0.0001) and correlated with HLA-DR expression, altogether defining adaptive NK cells. In terms of NK cell function, we observed a suppressed capacity of SZ-derived NK cells to mount cytotoxic responses in the presence of target cells, while NK cells from patients with BP show an inability to produce IFN-γ, a cytokine pivotal to NK function. This study strongly suggests major dysfunction of NK cells in FEP with functioning impairment correlated with psychotic, manic, and depressive symptoms in subsequently diagnosed patients with SZ and BP.
... Schizophrenia is a severe mental disorder hallmarked by altered cognition and behavior leading to social and working disability. Recent genetic studies revealed specific polymorphisms in the human leukocyte antigen (HLA) complex involved in psychiatric disorders such as schizophrenia (Tamouza et al., 2019b) and in specific forms of autism (Tamouza et al., 2019a) as developmental mental disorders accompanied by impaired social and communication skills. A recent study suggested that specific protein-encoding genes might contribute to the development of autoimmunity in psychiatric disorders (Pouget et al., 2019). ...
Background
Autoimmune-mediated encephalitis is a disease that often encompasses psychiatric symptoms as its first clinical manifestation’s predominant and isolated characteristic. Novel guidelines even distinguish autoimmune psychosis from autoimmune encephalitis. The aim of this review is thus to explore whether a wide range of psychiatric symptoms and syndromes are associated or correlate with autoantibodies.
Methods
We conducted a PubMed search to identify appropriate articles concerning serum and/or cerebrospinal fluid (CSF) autoantibodies associated with psychiatric symptoms and syndromes between 2000 and 2020. Relying on this data, we developed a diagnostic approach to optimize the detection of autoantibodies in psychiatric patients, potentially leading to the approval of an immunotherapy.
Results
We detected 10 major psychiatric symptoms and syndromes often reported to be associated with serum and/or CSF autoantibodies comprising altered consciousness, disorientation, memory impairment, obsessive-compulsive behavior, psychosis, catatonia, mood dysfunction, anxiety, behavioral abnormalities (autism, hyperkinetic), and sleeping dysfunction. The following psychiatric diagnoses were associated with serum and/or CSF autoantibodies: psychosis and schizophrenia spectrum disorders, mood disorders, minor and major neurocognitive impairment, obsessive-compulsive disorder, autism spectrum disorders (ASD), attention deficit hyperactivity disorder (ADHD), anxiety disorders, eating disorders and addiction. By relying on these symptom clusters and diagnoses in terms of onset and their duration, we classified a subacute or subchronic psychiatric syndrome in patients that should be screened for autoantibodies. We propose further diagnostics entailing CSF analysis, electroencephalography and magnetic resonance imaging of the brain. Exploiting these technologies enables standardized and accurate diagnosis of autoantibody-associated psychiatric symptoms and syndromes to deliver early immunotherapy.
Conclusions
We have developed a clinical diagnostic pathway for classifying subgroups of psychiatric patients whose psychiatric symptoms indicate a suspected autoimmune origin.
Human predisposition to neurological diseases such as schizophrenia, Alzheimer’s, Parkinson’s and other neuropathologies is associated with genetic and environmental factors. One of the promising directions in the area of molecular neurosciences is the study of the role of immunogenetic mechanisms in different types of pathological processes in brain. This review explores the role of complex histocompatibility genes in the pathogenesis of schizophrenia, evaluating changes in the immune repertoire of T- and B-cell receptors in neuroinflammation.
Background
The human leukocyte antigen system (HLA) is divided into two classes involved in antigen presentation: class I presenting intracellular antigens and class II presenting extracellular antigens. While susceptibility to infections is correlated with the HLA system, data on associations between HLA genotypes and Malassezia-related skin diseases (MRSD) are lacking. Thus, the objective of this study was to investigate associations between HLA alleles and MRSD.Materials and methodsParticipants in The Danish Blood Donor Study (2010–2018) provided questionnaire data on life style, anthropometric measures, and registry data on filled prescriptions. Genotyping was done using Illumina Infinium Global Screening Array, and HLA alleles were imputed using the HIBAG algorithm. Cases and controls were defined using filled prescriptions on topical ketoconazole 2% as a proxy of MRSD. Logistic regressions assessed associations between HLA alleles and MRSD adjusted for confounders and Bonferroni corrected for multiple tests.ResultsA total of 9455 participants were considered MRSD cases and 24,144 participants as controls. We identified four risk alleles B*57:01, OR 1.19 (95% CI: 1.09–1.31), C*01:02, OR 1.19 (95% CI: 1.08–1.32), C*06:02, OR 1.14 (95% CI: 1.08–1.22), and DRB1*01:01, OR 1.10 (95% CI: 1.04–1.17), and two protective alleles, DQB1*02:01, OR 0.89 (95% CI: 0.85–0.94), and DRB1*03:01, OR 0.89 (95% CI: 0.85–0.94).Conclusion
Five novel associations between HLA alleles and MRSD were identified in our cohort, and one previous association was confirmed. Future studies should assess the correlation between Malassezia antigens and antigen-binding properties of the associated HLA alleles.
The human leukocyte antigen (HLA) gene cluster, hosted by the major histocompatibility complex (MHC), encodes molecules prominently involved in the regulation of immune/inflammatory processes and the dysfunctional system could lead to a variety of (auto) immuno-inflammatory disorders ranging from infection to cancer. Also given their role in neurodevelopment and neuroplasticity, especially through regulation of microglial activation and synaptic pruning, it is predictable that this highly polymorphic gene region will have genetic influences in major psychiatric disorders. The focus of this review is to summarize the so far acquired knowledge on different aspects of the HLA genetics in major psychiatric disorders, namely, schizophrenia, bipolar disorder, and autism and to anticipate future research trends in psychiatry.
Suicide is a multifaceted phenomenon, related to an everlasting interconnection between biological, psychosocial, sociological, philosophical, and cultural aspects of human life. Immune system, which is constantly responding to changing environments and changing in response to them, seems to play a major role in this interaction. Multiple studies have reported increased risk of suicidal behaviours after or during inflammatory conditions. Individuals that engage in suicidal behaviours also have increased concentrations of inflammatory markers, such as interleukins 1β and 6, tumour necrosis factor α, and C-reactive protein, both in blood and the central nervous system (CNS). Stress, which is frequently mentioned among the major risk factors for suicidal behaviours, is also associated with a wide immune system dysregulation, entailing glucocorticoid system disruption and a low-grade inflammation. Mechanisms connecting systemic inflammation and CNS changes include damage to blood-brain barrier and interoceptive alterations, both resulting in changes in the communication between the periphery and the brain. Meanwhile, in CNS, pro-inflammatory cytokines may activate microglia causing a shift in tryptophan metabolism, preferentially generating cellular energy and toxic by-products of kynurenine pathway rather than serotonin. These changes in brain homeostasis lead to anatomical and functional brain alterations, most notably prefrontal cortex and insula, engendering maladaptive behavioural phenotypes, such as anhedonia and impulsivity that may mediate the association between inflammation and suicide. This chapter will provide a condensed overview of this complex association between inflammation and suicidal behaviours, presenting major findings in this area and explaining key pathways linking them.
Given the importance of understanding the role of neuroinflammation in psychiatric illness, including its emergence and progression, it is of practical value to explore the available tools that noninvasively index neuroinflammatory markers in vivo. Various aspects of neuroinflammation can be probed by magnetic resonance imaging (MRI) measures. These aspects include water content accumulation (T2-weighted anatomical MRI, magnetization transfer imaging and diffusion MRI); blood–brain barrier dysfunction and cellular infiltration (contrast-enhanced anatomical MRI); and molecular/metabolic signatures of neuroinflammation (magnetic resonance spectroscopy). This chapter provides a nontechnical overview of these MRI measures, which have been applied to estimate putative neuroinflammatory processes in psychiatric illness.
The human leukocyte antigen (HLA) is a complex genetic system that encodes proteins which predominantly regulate immune/inflammatory processes. It can be involved in a variety of immuno-inflammatory disorders ranging from infections to autoimmunity and cancers. The HLA system is also suggested to be involved in neurodevelopment and neuroplasticity, especially through microglia regulation and synaptic pruning. Consequently, this highly polymorphic gene region has recently emerged as a major player in the etiology of several major psychiatric disorders, such as schizophrenia, autism spectrum disorder and bipolar disorder and with less evidence for major depressive disorders and attention deficit hyperactivity disorder. We thus review here the role of HLA genes in particular subgroups of psychiatric disorders and foresee their potential implication in future research. In particular, given the prominent role that the HLA system plays in the regulation of viral infection, this review is particularly timely in the context of the Covid-19 pandemic.
The complement system consists of more than 30 proteins that have long been known to participate to the immune defence against pathogens and to the removal of damaged cells. Their role, however, extends beyond immunity and clearance of altered “self” components in the periphery. In particular, complement proteins can be induced by all cell types in the brain. Recent discoveries highlight the role of complement in normal and pathological brain development. Specifically, the complement system mediates synaptic pruning, a developmental process whereby supernumerary synapses are eliminated in the immature brain. The complement system has been implicated in pathological synapse elimination in schizophrenia, West Nile virus infection, and lupus, all of which are associated with psychiatric manifestations. Complement also contributes to synapse loss in neurodegenerative conditions. This review provides a brief overview of the well-studied role of complement molecules in immunity. The contribution of complement to embryonic and adult neurogenesis, neuronal migration, and developmental synaptic elimination in the normal brain is reviewed. We discuss the role of complement in synapse loss in psychiatric and neurological diseases and evaluate the therapeutic potential of complement-targeting drugs for brain disorders.
Objective:
Bipolar Disorder (BD) is characterized by deregulated adaptive immune processes. Recent genome-wide association studies (GWAS) implicate the major histocompatibility complex (MHC) region in BD. The present study investigates the potential influence of variations in human leukocyte antigen (HLA) on BD risk and/or clinical presentations. This may have relevance to the dysregulated inflammatory processes commonly found in BD.
Method:
DNAs from 475 BD patients and 195 healthy controls (HC) were genotyped for classical HLA class I and II loci.
Results:
We found that: (i) the HLA-A*02~B*44~DRB1*07 sub-haplotype is less prevalent in BD, vs. HC (pc = 2.4 × 10-2 ); (ii) the 57.1 and the 8.1-derived ancestral haplotypes i.e. HLA-A*02~B*57~Cw*06~DRB1*07~DQB1*09 and HLA-A*02~B*08~Cw*07 are associated with rapid cycling (pc = 1.9 × 10-3 and 1.05 × 10-2 , respectively); (iii) the 8.1AH-derived HLA class II-DRB*03~HLA-DQB1*02 sub-haplotype is more frequent in BD patients with a history of suicidal behaviors (pc = 2.1 × 10-2 ); and (iv) disease onset by an hypomanic episode or by psychotic symptoms are, respectively, more frequent in BD patients bearing the 7.1 AH-derived A*03~B*07~DRB1*15 sub-haplotype (pc = 8.5 × 10-3 ) and the HLA-A*02~B*07~DRB1*15 sub-haplotype (pc = 4.0 × 10-2 ).
Conclusion:
Corroborating the established link between these HLA haplotypes/sub haplotypes and common immune disorders, our findings suggest possible HLA-mediated proinflammatory processes operating in BD.
Tradeoffs centrally mediate the expression of human adaptations. We propose that tradeoffs also influence the prevalence and
forms of human maladaptation manifest in disease. By this logic, increased risk for one set of diseases commonly engenders
decreased risk for another, diametric, set of diseases. We describe evidence for such diametric sets of diseases from epidemiological,
genetic, and molecular studies in four clinical domains: (1) psychiatry (autism versus psychotic-affective conditions), (2)
rheumatology (osteoarthritis versus osteoporosis), (3) oncology and neurology (cancer versus neurodegenerative disorders),
and (4) immunology (autoimmunity versus infectious disease). Diametric disorders are important to recognize because genotypes
or environmental factors that increase risk for one set of disorders protect from opposite disorders, thereby providing novel
and direct insights into disease causes, prevention and therapy. Ascertaining the mechanisms that underlie disease-related
tradeoffs should also indicate means of circumventing or alleviating them, and thus reducing the incidence and impacts of
human disease in a more general way.
Schizophrenia is a highly heritable disorder. Genetic risk is conferred by a large number of alleles, including common alleles of small effect that might be detected by genome-wide association studies. Here we report a multi-stage schizophrenia genome-wide association study of up to 36,989 cases and 113,075 controls. We identify 128 independent associations spanning 108 conservatively defined loci that meet genome-wide significance, 83 of which have not been previously reported. Associations were enriched among genes expressed in brain, providing biological plausibility for the findings. Many findings have the potential to provide entirely new insights into aetiology, but associations at DRD2 and several genes involved in glutamatergic neurotransmission highlight molecules of known and potential therapeutic relevance to schizophrenia, and are consistent with leading pathophysiological hypotheses. Independent of genes expressed in brain, associations were enriched among genes expressed in tissues that have important roles in immunity, providing support for the speculated link between the immune system and schizophrenia.
Schizophrenia is a heritable brain illness with unknown pathogenic mechanisms. Schizophrenia's strongest genetic association at a population level involves variation in the major histocompatibility complex (MHC) locus, but the genes and molecular mechanisms accounting for this have been challenging to identify. Here we show that this association arises in part from many structurally diverse alleles of the complement component 4 (C4) genes. We found that these alleles generated widely varying levels of C4A and C4B expression in the brain, with each common C4 allele associating with schizophrenia in proportion to its tendency to generate greater expression of C4A. Human C4 protein localized to neuronal synapses, dendrites, axons, and cell bodies. In mice, C4 mediated synapse elimination during postnatal development. These results implicate excessive complement activity in the development of schizophrenia and may help explain the reduced numbers of synapses in the brains of individuals with schizophrenia.
Complex interactions between the immune system and the brain might have important aetiological and therapeutic implications for neuropsychiatric brain disorders. A possible association between schizophrenia and the immune system was postulated over a century ago, and is supported by epidemiological and genetic studies pointing to links with infection and inflammation. Contrary to the traditional view that the brain is an immunologically privileged site shielded behind the blood-brain barrier, studies in the past 20 years have noted complex interactions between the immune system, systemic inflammation, and the brain, which can lead to changes in mood, cognition, and behaviour. In this Review, we describe some of the important areas of research regarding innate and adaptive immune response in schizophrenia and related psychotic disorders that, we think, will be of interest to psychiatric clinicians and researchers. We discuss potential mechanisms and therapeutic implications of these findings, including studies of anti-inflammatory drugs in schizophrenia, describe areas for development, and offer testable hypotheses for future investigations.
Over several decades, various forms of genomic analysis of the human major histocompatibility complex (MHC) have been extremely successful in picking up many disease associations. This is to be expected, as the MHC region is one of the most gene-dense and polymorphic stretches of human DNA. It also encodes proteins critical to immunity, including several controlling antigen processing and presentation. Single-nucleotide polymorphism genotyping and human leukocyte antigen (HLA) imputation now permit the screening of large sample sets, a technique further facilitated by high-throughput sequencing. These methods promise to yield more precise contributions of MHC variants to disease. However, interpretation of MHC-disease associations in terms of the functions of variants has been problematic. Most studies confirm the paramount importance of class I and class II molecules, which are key to resistance to infection. Infection is likely driving the extreme variation of these genes across the human population, but this has been difficult to demonstrate. In contrast, many associations with autoimmune conditions have been shown to be specific to certain class I and class II alleles. Interestingly, conditions other than infections and autoimmunity are also associated with the MHC, including some cancers and neuropathies. These associations could be indirect, owing, for example, to the infectious history of a particular individual and selective pressures operating at the population level. Expected final online publication date for the Annual Review of Genomics and Human Genetics Volume 14 is August 31, 2013. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
An individual's major histocompatibility complex (MHC) ancestral haplotype (AH) is the dearest single determinant of susceptibility to MHC associated immunopathological disease, as it defines the alleles carried at all loci in the MHC. However, the direct effects of any of the 150–200 genes that constitute the MHC are difficult to determine since recombination only occurs at defined hotspots. This review concerns the 8.1 AH (HLA-A1, C7, B8, C4AQ0, C4B1, DR3, DQ2), which is carried by most Caucasians with HLA-B8. It is associated with accelerated human immunodeficiency virus (HIV) disease, and susceptibility to insulin-dependent diabetes mellitns (IDDM), systemic lupus erythematosus, dermatitis herpetiformis, common variable immunodeficiency and IgA deficiency, myasthenia gravis and several other conditions. We have mapped susceptibility genes for HIV, IDDM and myasthenia gravis co the central MHC between HLA-B and the tumour necrosis factor or complement genes. Here we consider which of the remaining 8.1-associated diseases are more closely associated with HLA-DR3 and/or DQ2. Several candidate genes in the central MHC have the potential to modulate immune or inflammatory responses in an antigen-independent manner, as is seen in studies of cultured cells from healthy carriers of the 8.1 AH. Hence these genes may act as a common co-factor in the diverse immunopathological conditions associated with the 8.1 AH.
An individual's major histocompatibility complex (MHC) ancestral haplotype (AH) is the clearest single determinant of susceptibility to MHC associated immunopathological disease, as it defines the alleles carried at all loci in the MHC. However, the direct effects of any of the 150-200 genes that constitute the MHC are difficult to determine since recombination only occurs at defined hotspots. This review concerns the 8.1 AH (HLA-A1, C7, B8, C4AQ0, C4B1, DR3, DQ2), which is carried by most Caucasians with HLA-B8. It is associated with accelerated human immunodeficiency virus (HIV) disease, and susceptibility to insulin-dependent diabetes mellitus (IDDM), systemic lupus erythematosus, dermatitis herpetiformis, common variable immunodeficiency and IgA deficiency, myasthenia gravis and several other conditions. We have mapped susceptibility genes for HIV, IDDM and myasthenia gravis to the central MHC between HLA-B and the tumour necrosis factor or complement genes. Here we consider which of the remaining 8.1-associated diseases are more closely associated with HLA-DR3 and/or DQ2. Several candidate genes in the central MHC have the potential to modulate immune or inflammatory responses in an antigen-independent manner, as is seen in studies of cultured cells from healthy carriers of the 8.1 AH. Hence these genes may act as a common co-factor in the diverse immunopathological conditions associated with the 8.1 AH.
Subjects with certain HLA alleles have a higher risk of specific autoimmune diseases than those without these alleles. The 8.1 ancestral haplotype (AH) is a common Caucasoid haplotype carried by most people who type for HLA-B8,DR3. It is unique in its association with a wide range of immunopathological diseases. To gain insight into the identification of the mechanism(s) of disease susceptibility of 8.1 AH carriers, we have investigated the prevalence of circulating immune complexes and non-organ-specific autoantibodies in healthy carriers of the haplotype. The results show that carriers of 8.1 AH display both a significant increased prevalence of immune complexes and higher titers of anti-nuclear autoantibodies. This AH carries a single segment characterized by no C4A gene. This null allele does not code for a functional C4A protein that likely plays an anti-inflammatory role being specialized in the opsonization and immunoclearance processes. So, this genetic defect has been claimed to allow that an increased production of autoantibodies directed vs. cells that have undergone apoptosis and are not efficiently disposed because a reduced antigenic clearance. The results obtained in the present study fit very well with this hypothesis. In the AH carriers the simultaneous high setting of tumor necrosis factor (TNF)-alpha may supply the autoantigens (providing an excess of apoptotic cells) that drive the autoimmune response. In conclusion, the C4 defect associated to the increased spontaneous release of TNF-alpha, modifying a certain number of immunological parameter may be the most characterizing feature of the 8.1 AH. In the majority of individuals, an autoimmune response clinically relevant will develop only in the presence of other immunological abnormalities.
Diametrical diseases reflect evolutionary‐genetic tradeoffs
- BJ Crespi
- MC Go