Progress in the study of autoinflammatory diseases appears to be in an exponential growth phase, as today’s discoveries pose tomorrow’s questions, many of which have direct clinical relevance. For the monogenic autoinflammatory diseases, three genetic questions stand out as particularly important to the clinician because of the frequency with which they arise. The first concerns the interpretation of positive genetic testing for several polymorphic variants of the periodic fever loci, including E148Q and P369S/R408Q at MEFV
, R92Q and P46L at TNFRSF1A
, and Q703K and V198M at CIAS1
. All of these variants are present at an allele frequency of greater than 1% in certain populations – for MEFV
E148Q the allele frequency may be as high as 23% among the Japanese (Sugiura et al., 2008
), and for NLRP3/CIAS1
Q703K 6.5% among Swedes (Verma et al., 2008
) – and are commonly reported among patients with undiagnosed autoinflammatory phenotypes. For each of these variants, the jury is still out, based on either careful epidemiologic data or in vitro functional studies, as to whether they actually confer a distinct phenotype, act as modifiers for other inflammatory loci, or are simply coincidental bystanders. Because of their frequency, the implications are substantial.
The second “frequently asked question” concerns FMF in particular, and is based on the fact that many of the patients with colchicine-responsive clinical FMF have only a single demonstrable MEFV
mutation, despite thorough scrutiny (Booty et al., 2009
; Marek-Yagel et al., 2009
; Ozen, 2009
). Although this widely-confirmed observation is based on a clinical definition of FMF that includes milder cases than were appreciated 20 years ago, it suggests a more complex pattern of inheritance than the simple recessive model of the textbooks. It also argues that solitary MEFV
mutations may confer a biochemical or clinical phenotype by mechanisms yet to be elucidated, perhaps in the presence of as yet unidentified modifier loci.
A third major genetic issue for the clinician is finding an explanation for the approximately 60% of patients with various autoinflammatory phenotypes who do not have mutations in any of the known genetic loci. The recent examples of DIRA (Aksentijevich et al., 2009
; Reddy et al., 2009
) and IL-10 receptor deficiencies (Glocker et al., 2009
) raise the possibility that at least some of these individuals will eventually be found to have mutations in currently unrecognized autoinflammatory genes. An important initiative currently undertaken at the NIH is to apply whole genome single nucleotide polymorphism (SNP) analyses to search for areas of homozygosity in patients from consanguineous families or isolated populations who are more likely to have recessive mutations, and to perform directed candidate gene screening or whole exome sequencing in selected other cases. Overall, in the NIH cohort alone there are over 1000 unrelated patients with genetically unexplained autoinflammatory phenotypes who may be a rich source of yet additional loci that may deepen our understanding of the human innate immunome.
Another important and largely untouched area is the identification of susceptibility loci for complex (polygenic) autoinflammatory diseases. There are a number of such disorders, including Behçet’s disease (Gül, 2005
), systemic onset juvenile idiopathic arthritis (SoJIA) (Allantaz et al., 2007
), and the syndrome of periodic fever with aphthous stomatitis, pharyngitis, and cervical adenopathy (PFAPA) (Feder and Salazar, 2009
). These illnesses are much more common than the monogenic autoinflammatory diseases, at least in certain parts of the world. Current thinking among human geneticists is that, for many of the common complex diseases, the etiology derives from permutations and combinations of common variants, each of which alone may only confer a small risk. Such variants can be detected by genome-wide association studies (GWAS). Based on the experience with GWAS conducted thus far for the classic autoimmune diseases, it is likely that the clinical implications of such studies will be more relevant to identifying pathways that may be amenable to targeting with small molecules or biologics than to establishing parameters to diagnose or predict disease (Gregersen and Olsson, 2009
). Alternatively, at least some of these genetically complex diseases may be due to high-penetrance rare mutations that only account for a few cases each (Frazer et al., 2009
), in which case screening or prediction would be possible. Of course, the most likely scenario is that many of these complex autoinflammatory diseases are due to a combination of the two models, with some cases due to low penetrance common variants and others caused by rare high-penetrance mutations.
The molecular pathophysiology of the autoinflammatory diseases is a topic of growing clinical interest. Particularly for diseases where a genetic approach has been taken, we may know the genes and mutations, and we may know the clinical phenotypes, but in many cases there is a black box between the two. Understanding this connection can have important implications for how clinicians think about human disease. There are many examples. FMF has long been a source of fascination because of the extraordinarily high carrier frequencies of multiple different mutations in Mediterranean and Middle Eastern populations, strongly suggesting a heterozygote advantage for a pathogen endemic to that part of the world (Masters et al., 2009
). Such speculation is further fueled by the fact that disease-associated mutations tend to cluster around a pocket in the C-terminal domain of pyrin, the FMF protein, that may be a binding site for such a putative pathogen (Weinert et al., 2009
). Although purely conjectural at this point, the smallpox virus appears a particularly attractive candidate binding partner, both because of the fact that this disease probably arose in Africa and was common in the ancient Mediterranean basin where there was a sufficient population density to maintain human-human spread of the disease (Hopkins, 1983
), and because poxviruses produce proteins with the canonical N-terminal pyrin domain (an interaction motif named after the FMF protein) that are thought to subvert the host innate immune response (Johnston et al., 2005
). The recent development of pyrin knockin mice by our laboratory may help to investigate the interaction between pyrin and various pathogens, although of course not the smallpox virus itself.
There are many other questions with clinical relevance regarding the pathophysiology of autoinflammatory disease, some of which relate to single diseases, and others that are more broadly applicable. In the case of TRAPS, it appears that the impaired TNF receptor ectodomain cleavage initially described in these pages (McDermott et al., 1999
) may have less of a proinflammatory effect than more recently described activation due to abnormal receptor trafficking (Lobito et al., 2006
), and this may have important implications for treatment. One major issue that cuts across many of these illnesses is understanding what may provoke or exacerbate the autoinflammatory phenotype and why. Although the original definition alluded to “seemingly unprovoked” inflammation, we now know that frequently there are triggers. Examples include cold exposure in FCAS, childhood immunizations in HIDS, physical trauma in TRAPS and PAPA syndrome, mechanical trauma to the skin and gastrointestinal tract in DIRA, strenuous physical exertion in FMF, and psychological stress and menstrual cycles in several of these illnesses. Longer-term gene-environment interactions are just beginning to be appreciated. One important example is the apparent relationship between country of origin and susceptibility to systemic amyloidosis in FMF, with much higher risks observed for individuals who have spent their early lives in countries with high infant mortality rates (Touitou et al., 2007
). For example, Armenians with FMF living in Armenia have a much higher risk of developing amyloidosis than Armenian-Americans. Although there are many possible explanations, one attractive possibility is that frequent, untreated exposure to bacterial infection in early childhood may predispose to amyloid deposition upon repeated inflammatory episodes later in life. Similar geographic variability in susceptibility to AA amyloidosis has been observed in a number of other conditions, including juvenile idiopathic arthritis.
Also still a mystery is why the autoinflammatory diseases exhibit a relative paucity of the usual markers of adaptive immunity. Of course, just as we are now aware that there are triggers for the “seemingly unprovoked” episodes of autoinflammation, so too is it clear that the adaptive immune system is not totally quiescent, as evidenced, for example, by the polyclonal hyperglobulinemia that is frequently seen in the hereditary recurrent fevers. Nevertheless, antinuclear antibodies and rheumatoid factors are notably absent in patients with autoinflammatory disease, despite the recent demonstration that the vaccine adjuvant alum stimulates antibody production through its activation of the inflammasome and innate immunity (Eisenbarth et al., 2008
). Perhaps the absence of some essential second signal, such as the presentation of intracellular antigens on apoptotic blebs in SLE (Suber and Rosen, 2009
), explains the relative lack of adaptive immunity in the autoinflammatory diseases. Finally, there is a brave new world of human biology that is just emerging as genetic analysis begins to uncover the loci that underlie the complex autoinflammatory diseases and the host of disorders, such as ankylosing spondylitis and psoriasis, that reside at the interface between autoinflammatory and autoimmune (McGonagle and McDermott, 2006
Although nearly any topic in the treatment of autoinflammatory disease is clinically relevant, there are two that are particularly exciting, the first concerning IL-1 and the second relating to all of the other possible therapeutic targets (See also Review by C. A. Dinarello on page XXX of this issue). As previously noted, the spectrum of disorders in which the inflammasome plays some role has grown rapidly in the last ten years, and perhaps one of the most interesting is Type 2 diabetes mellitus. By mechanisms that have not been completely elucidated, hyperglycemia may induce IL-1β production by pancreatic islet cells, leading to islet cell death, decreased insulin production, and a diabolical resonance between hyperglycemia and IL-1β production (Maedler et al., 2002
). A recent paper by Monath and colleagues suggests a possible role for anakinra, the recombinant IL-1 receptor antagonist, in maintaining glycemic control (Larsen et al., 2007
), which, if confirmed, could have a far-reaching impact on the practice of clinical medicine. On the other hand, despite the incontrovertible importance of IL-1 in both monogenic and complex autoinflammatory diseases, other pathways are already apparent (Masters et al., 2009
; Glocker et al., 2009
), and it is very likely that other yet-to-be-discovered cytokines and pathways will also be established in the next 10 years. A recent treatment of the current state of the art suggested five molecular categories of autoinflammation besides the inflammasomopathies (Masters et al., 2009
), and preliminary data from a recently completed GWAS in Behçet’s disease at the NIH suggests even further heterogeneity. Of course, the attraction of approaching these inherited diseases from the standpoint of patient cohorts with genes to be discovered is that such an approach maximizes clinical relevance while minimizing investigator bias.