YEAR IN REVIEW CLINICAL GENETICS IN 2014
New monogenic diseases span the immunological disease continuum Sinisa Savic & Michael F. McDermott
Three monogenic diseases, with features of both autoinflammation and autoimmunity, were described for the first time in 2014. As well as providing insights into the molecular basis of several rare immunological disorders, the discoveries have implications for their diagnosis and treatment. Savic, S. & McDermott, M. F. Nat. Rev. Rheumatol. advance online publication 23 December 2014; doi:10.1038/nrrheum.2014.215
Remarkable progress has been made in clinical genetics in the past year, particularly in finding novel disease-causing genes for a number of immune-related conditions, and also in deciphering some of the complexity of multifactorial polygenic diseases. We have been tasked with choosing three papers, under the theme of ‘clinical genetics in 2014’, that reflect progress in the field of rheumatology. From a broad range of deserving candidates, we settled on three papers that report novel genes and mechanisms for immune dysregulation in monogenic diseases. We are cognizant of the fact that these choices reflect our professional interests rather than any specific ratings system; one deciding factor was that each discovery has generated attendant diagnostic tests and potential therapies. The immunological disease continuum (IDC) categorizes the inflammatory conditions according to their predominant underlying immunopathogenic mechanisms, which range from monogenic autoinflammatory diseases at one end of the spectrum to monogenic autoimmune conditions at the other, with a multitude of conditions of presumed oligogenic or polygenic aetiology in-between.1 The IDC is based on the observation that autoinflammatory conditions are caused by aberrant innate immune responses, whereas autoimmune conditions result from aberrant adaptive immunity. However, the immunopathology of most inflammatory conditions results from a combination of both innate and adaptive pathogenic immune responses. The utility of the IDC becomes obvious when considering therapeutic options; for example, targeting the proinflammatory cytokines of the innate immune system, such as IL‑1, has
been very effective in management of many autoinflammatory diseases, whereas T‑cell and B‑cell-directed therapies, such as the anti-CD20 B‑cell-depleting agent rituximab, are effective in classical autoimmune conditions. Here, we discuss reports of three conditions that span the IDC: first, a subtype of systemic necrotizing vasculitis with a genetic cause (adenosine deaminase 2 [ADA2] deficiency);2 second, an addition to the growing list of genetically determined interferono pathies, one caused by gain-of-function mutations in the interferon-induced helicase C domain-containing protein 1 (IFIH1) gene; 3 and third, haploinsufficiency of
cytotoxic T‑lymphocyte protein 4 (CTLA‑4), an autosomal dominant immune dysregulation syndrome with multiple autoimmune clinical features (Figure 1).4 We also consider the utility of the IDC classification in light of these newly described monogenic inflammatory conditions. The discovery of biallelic mutations in CECR1, encoding ADA2, has shown that deficiency of ADA2 is a genetic cause of early-onset systemic necrotizing vasculitis in a subset of patients.2 This finding has cast light on the pathogenesis of vascular inflammation and defined a molecular pathway linking inflammation with vascular development. Monocytes and macrophages, derived from bone marrow, are the main sources of ADA2; the activity of this growth factor and enzyme was significantly reduced in serum of patients with ADA2 deficiency.2 One of the key mechanisms proposed is that monocytes from these patients readily differentiate into M1 macrophages, but few differentiate into M2 macrophages. The proinflammatory effects of M1 macrophages promote local tissue inflammation and this cell infiltration, rather than ADA2 deficiency in endothelial cells, gives rise to vasculopathy. In addition to measurement of ADA2 activity in serum as a
Features of autoinflammation Perturbations of innate immunity
Features of autoimmunity Immunological basis
Perturbations of adaptive immunity
Neutrophils Macrophages No autoantibodies
Cellular basis
B cells T cells, TREG cells Autoantibodies
Specific ‘danger’ signals (e.g. dsRNA)
Conceptual understanding
Aberrant ‘self–nonself’ discrimination, breaking of self-tolerance
Monogenic > polygenic Non-penetrance
Genetic features
Polygenic > monogenic Haploinsufficiency
B-cell immunodeficiency
Immunodeficiency
B-cell immunodeficiency T-cell immunodeficiency
Protein activity in serum Type 1 IFN signature
Diagnostic test
Protein activity in serum
Viral signature
Infectious trigger
Block cytokine cascades Replacement of protein Deficiency of ADA2
Therapy AGS7
Viral cause Block T cells, B cells or their products Replacement of protein
CTLA-4 haploinsufficiency
Figure 1 | Autoinflammatory and autoimmune features of three new monogenic Nature Reviewsdiseases. | Rheumatology Distinguishing features can be used to classify these conditions as either autoinflammatory or autoimmune. Some features overlap both categories, and AGS7 has the potential to progress from being driven by innate immunity to being an antibody-mediated disease. Abbreviations: ADA2, adenosine deaminase 2; AGS7, Aicardi–Goutières syndrome 7; CTLA-4, cytotoxic T‑lymphocyte protein 4; dsRNA, double-stranded RNA; IFN, interferon; TREG cell, T regulatory cell.
NATURE REVIEWS | RHEUMATOLOGY
ADVANCE ONLINE PUBLICATION | 1 © 2014 Macmillan Publishers Limited. All rights reserved
YEAR IN REVIEW Key advances ■■ The first description of a genetic basis for systemic necrotizing vasculitis, namely deficiency of adenosine deaminase 2 (ADA2) due to mutations in CECR1, has implications for diagnosis and therapy2 ■■ The list of genetically determined type I interferonopathies now includes an autosomal dominant syndrome, caused by mutations in IFIH1, a sensor of cytosolic double-stranded RNA3 ■■ Haploinsufficiency, combined with incomplete penetrance, could be a pathogenic mechanism by which a combination of rare monogenic lesions might drive common autoimmune diseases4
diagnostic test, a range of potential therapies are now being considered. Haematopoietic stem-cell transplantation could be an effective therapy. 5 Patients may also benefit from infusions of fresh-frozen plasma or recombinant ADA2, or from gene therapy. Mutations in a total of six genes had already been associated with aberrant type I inteferon stimulation in six different pheno types of Aicardi–Goutières syndrome (AGS); these syndromes, referred to as AGS1–AGS6, are all autosomal recessive syndromes and are associated with increased activity of type I interferons, the defining characteristic of the type I interferonopathies.6 In 2014, autosomal dominant mutations in IFIH1 (also known as MDA5) were described in patients with severe neurological impairment and immunological disease (in particular systemic lupus erythematosus [SLE]); this condition is referred to as AGS7.3 Gain-offunction mutations enable mutant IFIH1 to bind RNA with greater affinity, with increased basal and ligand-induced interferon signalling. Marked clinical variability and clinical nonpenetrance were notable features of two of the patients with AGS7 who were studied, despite complete biochemical penetrance in terms of interferon upregulation (that is, an ‘interferon signature’) in all 11 cases identified; a proposed explanation for this phenomenon is that exogenous viral RNA might be implicated in the disease process, by acting as a trigger. The type I interferon response in AGS7 raises the possibility of using anti-inflammatory therapies, including Janus kinase (JAK) inhibitors, such as tofacitinib and baricitinib, and drugs that block the interferon pathway, such as sifalimumab. If AGS progresses to antibody- mediated disease, patients might benefit from anti-B‑cell therapy such as rituximab.
Until recently, only three diseases were well-characterized as monogenic auto immune disorders;1 to these three we can now add haploinsufficiency of CTLA‑4.4 Typically, CTLA‑4 is a negative regulator of T‑cell activation, functioning via several mechanisms that are not mutually exclusive. It competes with CD28 to bind B7 molecules, modulates T‑cell receptor (TCR) signalling by recruiting phosphatases, and also affects T‑cell motility.7 CTLA‑4 is constitutively expressed in regulatory T (TREG) cells, partly because it is transcriptionally regulated by FOXP3 and is critical to TREG-cell function.8 Consequently, CTLA‑4 has a central role in immune tolerance and preventing autoimmunity, by modulating TCR activation of conventional T cells and by supporting the role of TREG cells. Haploinsufficiency of CTLA‑4, caused by heterozygous germline mutations, has been described in six patients from four unrelated families.4 All affected patients had lymphocytic infiltration of non-lymphoid organs (brain, lung, gut) and autoimmune cytopenias, and two had myeloproliferation, similar to that in CTLA‑4–/– mice. All patients had reduced levels of full-length functional CTLA‑4, associated with hyperproliferative conventional T cells and reduced function of TREG cells. Therefore, replacement therapy with CTLA4–IgG fusion protein (abatacept) is a treatment option for haploinsufficiency of CTLA‑4. Interestingly, not all individuals with CTLA4 haploinsufficiency were sympto matic. It has therefore been suggested that haploinsufficiency, combined with incomplete penetrance, could be a pathogenic mechanism whereby more common auto immune diseases, such as SLE and type 1 diabetes mellitus, might result from a variety of rare monogenic lesions.9 This model challenges the traditional view of autoimmunity being a polygenic disease. With the increasing use of next-generation sequencing techniques, it is likely that similar examples of monogenic autoimmune disease will be discovered in the near future. In summary, three monogenic diseases first reported in 2014 span the IDC spectrum by having features of autoinflammatory and autoimmune disease (Figure 1). Of particular note, AGS7 is an example of a disease in which an innate immune response to a specific danger signal (double-stranded RNA) that might progress to antibody-mediated disease, whereas immunodeficiency is a feature of both ADA2 deficiency and CTLA‑4 haploinsufficiency. It is intriguing to speculate about the role of viral infections in these conditions; herpes simplex virus has
2 | ADVANCE ONLINE PUBLICATION
been associated with neonatal vasculitis but has not been reported in ADA2 deficiency, a viral signature is a feature of AGS7, and increasing evidence also implicates viral infections in autoimmune disease (for example, cytomegalovirus in Sjögren syndrome).10 Following these discoveries, a crucial next step is to determine how best to test new therapies for these conditions; this testing is likely to involve a step-change from the current practice of randomized controlled trials, which are impractical and uneconomical in rare monogenic disorders. Furthermore, the question of how auto immunity arises in a setting of immuno deficiency is raised; resolution of this paradox will require a revision of current concepts regarding the development of autoimmune aspects of various disease states. National Institute for Health Research–Leeds Musculoskeletal Biomedical Research Unit (NIHR-LMBRU), Leeds Institute of Rheumatic and Musculoskeletal Medicine (LIRMM), Wellcome Trust Brenner Building, St James University, Beckett Street, Leeds LS9 7TF, UK (S.S., M.F.M.). Correspondence to: M.F.M. [email protected] Acknowledgements The authors’ work is supported by NIHR-LMBRU. The authors apologise to colleagues whose work could not be cited because of space constraints. Competing interests The authors declare no competing interests. 1.
McGonagle, D. & McDermott, M. F. A proposed classification of the immunological diseases. PLoS Med. 3, e297 (2006). 2. Zhou, Q. et al. Early-onset stroke and vasculopathy associated with mutations in ADA2. N. Engl. J. Med. 370, 911–920 (2014). 3. Rice, G. I. et al. Gain‑of‑function mutations in IFIH1 cause a spectrum of human disease phenotypes associated with upregulated type I interferon signaling. Nat. Genet. 46, 503–509 (2014). 4. Kuehn, H. S. et al. Immune dysregulation in human subjects with heterozygous germline mutations in CTLA4. Science 345, 1623–1627 (2014). 5. Kastner, D. L., Zhou, Q. & Aksentijevich, I. Mutant ADA2 in vasculopathies. N. Engl. J. Med. 371, 480–481 (2014). 6. Crow, Y. J. Type I interferonopathies: Mendelian type I interferon up-regulation. Curr. Opin. Immunol. 32, 7–12 (2015). 7. Romo-Tena, J. Gómez-Martín, D. & AlcocerVarela, J. CTLA‑4 and autoimmunity: new insights into the dual regulator of tolerance. Autoimmun. Rev. 12, 1171–1176 (2013). 8. Walker, L. S. TREG and CTLA‑4: two intertwining pathways to immune tolerance. J. Autoimmun. 45, 49–57 (2013). 9. Rieux-Laucat, F. & Casanova, J.‑L. Immunology: autoimmunity by haploinsufficiency. Science 345, 1560–1561 (2014). 10. Schuster I. S. et al. TRAIL+ NK cells control CD4+ T cell responses during chronic viral infection to limit autoimmunity. Immunity 41, 646–656 (2014).
www.nature.com/nrrheum © 2014 Macmillan Publishers Limited. All rights reserved
New monogenic diseases span the immunological disease continuum Sinisa Savic & Michael F. McDermott
Three monogenic diseases, with features of both autoinflammation and autoimmunity, were described for the first time in 2014. As well as providing insights into the molecular basis of several rare immunological disorders, the discoveries have implications for their diagnosis and treatment. Savic, S. & McDermott, M. F. Nat. Rev. Rheumatol. advance online publication 23 December 2014; doi:10.1038/nrrheum.2014.215
Remarkable progress has been made in clinical genetics in the past year, particularly in finding novel disease-causing genes for a number of immune-related conditions, and also in deciphering some of the complexity of multifactorial polygenic diseases. We have been tasked with choosing three papers, under the theme of ‘clinical genetics in 2014’, that reflect progress in the field of rheumatology. From a broad range of deserving candidates, we settled on three papers that report novel genes and mechanisms for immune dysregulation in monogenic diseases. We are cognizant of the fact that these choices reflect our professional interests rather than any specific ratings system; one deciding factor was that each discovery has generated attendant diagnostic tests and potential therapies. The immunological disease continuum (IDC) categorizes the inflammatory conditions according to their predominant underlying immunopathogenic mechanisms, which range from monogenic autoinflammatory diseases at one end of the spectrum to monogenic autoimmune conditions at the other, with a multitude of conditions of presumed oligogenic or polygenic aetiology in-between.1 The IDC is based on the observation that autoinflammatory conditions are caused by aberrant innate immune responses, whereas autoimmune conditions result from aberrant adaptive immunity. However, the immunopathology of most inflammatory conditions results from a combination of both innate and adaptive pathogenic immune responses. The utility of the IDC becomes obvious when considering therapeutic options; for example, targeting the proinflammatory cytokines of the innate immune system, such as IL‑1, has
been very effective in management of many autoinflammatory diseases, whereas T‑cell and B‑cell-directed therapies, such as the anti-CD20 B‑cell-depleting agent rituximab, are effective in classical autoimmune conditions. Here, we discuss reports of three conditions that span the IDC: first, a subtype of systemic necrotizing vasculitis with a genetic cause (adenosine deaminase 2 [ADA2] deficiency);2 second, an addition to the growing list of genetically determined interferono pathies, one caused by gain-of-function mutations in the interferon-induced helicase C domain-containing protein 1 (IFIH1) gene; 3 and third, haploinsufficiency of
cytotoxic T‑lymphocyte protein 4 (CTLA‑4), an autosomal dominant immune dysregulation syndrome with multiple autoimmune clinical features (Figure 1).4 We also consider the utility of the IDC classification in light of these newly described monogenic inflammatory conditions. The discovery of biallelic mutations in CECR1, encoding ADA2, has shown that deficiency of ADA2 is a genetic cause of early-onset systemic necrotizing vasculitis in a subset of patients.2 This finding has cast light on the pathogenesis of vascular inflammation and defined a molecular pathway linking inflammation with vascular development. Monocytes and macrophages, derived from bone marrow, are the main sources of ADA2; the activity of this growth factor and enzyme was significantly reduced in serum of patients with ADA2 deficiency.2 One of the key mechanisms proposed is that monocytes from these patients readily differentiate into M1 macrophages, but few differentiate into M2 macrophages. The proinflammatory effects of M1 macrophages promote local tissue inflammation and this cell infiltration, rather than ADA2 deficiency in endothelial cells, gives rise to vasculopathy. In addition to measurement of ADA2 activity in serum as a
Features of autoinflammation Perturbations of innate immunity
Features of autoimmunity Immunological basis
Perturbations of adaptive immunity
Neutrophils Macrophages No autoantibodies
Cellular basis
B cells T cells, TREG cells Autoantibodies
Specific ‘danger’ signals (e.g. dsRNA)
Conceptual understanding
Aberrant ‘self–nonself’ discrimination, breaking of self-tolerance
Monogenic > polygenic Non-penetrance
Genetic features
Polygenic > monogenic Haploinsufficiency
B-cell immunodeficiency
Immunodeficiency
B-cell immunodeficiency T-cell immunodeficiency
Protein activity in serum Type 1 IFN signature
Diagnostic test
Protein activity in serum
Viral signature
Infectious trigger
Block cytokine cascades Replacement of protein Deficiency of ADA2
Therapy AGS7
Viral cause Block T cells, B cells or their products Replacement of protein
CTLA-4 haploinsufficiency
Figure 1 | Autoinflammatory and autoimmune features of three new monogenic Nature Reviewsdiseases. | Rheumatology Distinguishing features can be used to classify these conditions as either autoinflammatory or autoimmune. Some features overlap both categories, and AGS7 has the potential to progress from being driven by innate immunity to being an antibody-mediated disease. Abbreviations: ADA2, adenosine deaminase 2; AGS7, Aicardi–Goutières syndrome 7; CTLA-4, cytotoxic T‑lymphocyte protein 4; dsRNA, double-stranded RNA; IFN, interferon; TREG cell, T regulatory cell.
NATURE REVIEWS | RHEUMATOLOGY
ADVANCE ONLINE PUBLICATION | 1 © 2014 Macmillan Publishers Limited. All rights reserved
YEAR IN REVIEW Key advances ■■ The first description of a genetic basis for systemic necrotizing vasculitis, namely deficiency of adenosine deaminase 2 (ADA2) due to mutations in CECR1, has implications for diagnosis and therapy2 ■■ The list of genetically determined type I interferonopathies now includes an autosomal dominant syndrome, caused by mutations in IFIH1, a sensor of cytosolic double-stranded RNA3 ■■ Haploinsufficiency, combined with incomplete penetrance, could be a pathogenic mechanism by which a combination of rare monogenic lesions might drive common autoimmune diseases4
diagnostic test, a range of potential therapies are now being considered. Haematopoietic stem-cell transplantation could be an effective therapy. 5 Patients may also benefit from infusions of fresh-frozen plasma or recombinant ADA2, or from gene therapy. Mutations in a total of six genes had already been associated with aberrant type I inteferon stimulation in six different pheno types of Aicardi–Goutières syndrome (AGS); these syndromes, referred to as AGS1–AGS6, are all autosomal recessive syndromes and are associated with increased activity of type I interferons, the defining characteristic of the type I interferonopathies.6 In 2014, autosomal dominant mutations in IFIH1 (also known as MDA5) were described in patients with severe neurological impairment and immunological disease (in particular systemic lupus erythematosus [SLE]); this condition is referred to as AGS7.3 Gain-offunction mutations enable mutant IFIH1 to bind RNA with greater affinity, with increased basal and ligand-induced interferon signalling. Marked clinical variability and clinical nonpenetrance were notable features of two of the patients with AGS7 who were studied, despite complete biochemical penetrance in terms of interferon upregulation (that is, an ‘interferon signature’) in all 11 cases identified; a proposed explanation for this phenomenon is that exogenous viral RNA might be implicated in the disease process, by acting as a trigger. The type I interferon response in AGS7 raises the possibility of using anti-inflammatory therapies, including Janus kinase (JAK) inhibitors, such as tofacitinib and baricitinib, and drugs that block the interferon pathway, such as sifalimumab. If AGS progresses to antibody- mediated disease, patients might benefit from anti-B‑cell therapy such as rituximab.
Until recently, only three diseases were well-characterized as monogenic auto immune disorders;1 to these three we can now add haploinsufficiency of CTLA‑4.4 Typically, CTLA‑4 is a negative regulator of T‑cell activation, functioning via several mechanisms that are not mutually exclusive. It competes with CD28 to bind B7 molecules, modulates T‑cell receptor (TCR) signalling by recruiting phosphatases, and also affects T‑cell motility.7 CTLA‑4 is constitutively expressed in regulatory T (TREG) cells, partly because it is transcriptionally regulated by FOXP3 and is critical to TREG-cell function.8 Consequently, CTLA‑4 has a central role in immune tolerance and preventing autoimmunity, by modulating TCR activation of conventional T cells and by supporting the role of TREG cells. Haploinsufficiency of CTLA‑4, caused by heterozygous germline mutations, has been described in six patients from four unrelated families.4 All affected patients had lymphocytic infiltration of non-lymphoid organs (brain, lung, gut) and autoimmune cytopenias, and two had myeloproliferation, similar to that in CTLA‑4–/– mice. All patients had reduced levels of full-length functional CTLA‑4, associated with hyperproliferative conventional T cells and reduced function of TREG cells. Therefore, replacement therapy with CTLA4–IgG fusion protein (abatacept) is a treatment option for haploinsufficiency of CTLA‑4. Interestingly, not all individuals with CTLA4 haploinsufficiency were sympto matic. It has therefore been suggested that haploinsufficiency, combined with incomplete penetrance, could be a pathogenic mechanism whereby more common auto immune diseases, such as SLE and type 1 diabetes mellitus, might result from a variety of rare monogenic lesions.9 This model challenges the traditional view of autoimmunity being a polygenic disease. With the increasing use of next-generation sequencing techniques, it is likely that similar examples of monogenic autoimmune disease will be discovered in the near future. In summary, three monogenic diseases first reported in 2014 span the IDC spectrum by having features of autoinflammatory and autoimmune disease (Figure 1). Of particular note, AGS7 is an example of a disease in which an innate immune response to a specific danger signal (double-stranded RNA) that might progress to antibody-mediated disease, whereas immunodeficiency is a feature of both ADA2 deficiency and CTLA‑4 haploinsufficiency. It is intriguing to speculate about the role of viral infections in these conditions; herpes simplex virus has
2 | ADVANCE ONLINE PUBLICATION
been associated with neonatal vasculitis but has not been reported in ADA2 deficiency, a viral signature is a feature of AGS7, and increasing evidence also implicates viral infections in autoimmune disease (for example, cytomegalovirus in Sjögren syndrome).10 Following these discoveries, a crucial next step is to determine how best to test new therapies for these conditions; this testing is likely to involve a step-change from the current practice of randomized controlled trials, which are impractical and uneconomical in rare monogenic disorders. Furthermore, the question of how auto immunity arises in a setting of immuno deficiency is raised; resolution of this paradox will require a revision of current concepts regarding the development of autoimmune aspects of various disease states. National Institute for Health Research–Leeds Musculoskeletal Biomedical Research Unit (NIHR-LMBRU), Leeds Institute of Rheumatic and Musculoskeletal Medicine (LIRMM), Wellcome Trust Brenner Building, St James University, Beckett Street, Leeds LS9 7TF, UK (S.S., M.F.M.). Correspondence to: M.F.M. [email protected] Acknowledgements The authors’ work is supported by NIHR-LMBRU. The authors apologise to colleagues whose work could not be cited because of space constraints. Competing interests The authors declare no competing interests. 1.
McGonagle, D. & McDermott, M. F. A proposed classification of the immunological diseases. PLoS Med. 3, e297 (2006). 2. Zhou, Q. et al. Early-onset stroke and vasculopathy associated with mutations in ADA2. N. Engl. J. Med. 370, 911–920 (2014). 3. Rice, G. I. et al. Gain‑of‑function mutations in IFIH1 cause a spectrum of human disease phenotypes associated with upregulated type I interferon signaling. Nat. Genet. 46, 503–509 (2014). 4. Kuehn, H. S. et al. Immune dysregulation in human subjects with heterozygous germline mutations in CTLA4. Science 345, 1623–1627 (2014). 5. Kastner, D. L., Zhou, Q. & Aksentijevich, I. Mutant ADA2 in vasculopathies. N. Engl. J. Med. 371, 480–481 (2014). 6. Crow, Y. J. Type I interferonopathies: Mendelian type I interferon up-regulation. Curr. Opin. Immunol. 32, 7–12 (2015). 7. Romo-Tena, J. Gómez-Martín, D. & AlcocerVarela, J. CTLA‑4 and autoimmunity: new insights into the dual regulator of tolerance. Autoimmun. Rev. 12, 1171–1176 (2013). 8. Walker, L. S. TREG and CTLA‑4: two intertwining pathways to immune tolerance. J. Autoimmun. 45, 49–57 (2013). 9. Rieux-Laucat, F. & Casanova, J.‑L. Immunology: autoimmunity by haploinsufficiency. Science 345, 1560–1561 (2014). 10. Schuster I. S. et al. TRAIL+ NK cells control CD4+ T cell responses during chronic viral infection to limit autoimmunity. Immunity 41, 646–656 (2014).
www.nature.com/nrrheum © 2014 Macmillan Publishers Limited. All rights reserved
