Molecular insights into systemic lupus erythematosus pathogenesis.

Systemic lupus erythematosus (SLE) is a complex, heterogeneous, and chronic autoimmune disorder of an unknown origin. Its clinical symptoms range from a benign skin disorder to severe, life-threatening conditions.1 Immune effector dysfunctions are hall marks of SLE disease.2 The etiopathogenesis of the altered immune response in SLE remains unknown. SLE is characterized by the presence of auto-antibodies (AutoAbs) for a wide variety of self antigens and circulating immune complexes.1,2 The onset of lupus is variable and may affect all stages of life. The disease predominantly affiicts females in the child-bearing years about 6- to 10-fold more frequently than males. We have not had a new drug in 50 years, because of the unknown etiology of the abnormal immune response. The current lupus therapies are non-specific, symptomatic, and cause significant side effects. In this editorial, I have made an attempt to describe multistep immune alterations that pave the way for the inception and sustaining of SLE pathogenesis, and postulated molecular mechanisms involved in SLE disease onset. Such information will help in better understanding SLE etiopathogenesis and in developing effective and safer strategies to combat SLE as well as other autoimmune diseases.

A strong association has been found between elevated levels of circulating type I interferons (IFNs) and autoimmune diseases like type I diabetes and SLE.3–6 Remarkably, the therapeutic administration of type I IFNs has provoked type I diabetes, SLE, and primary Sjögren’s syndrome (pSS) in some individuals.7–9 Constitutive expression of type I IFNs was observed in SLE patients.5 The expression and up-regulation of several type I IFN-regulated genes is associated with SLE pathogenesis and disease severity.10–12 Natural IFN-α producing cells (NIPCs)/plasmacytoid dendritic cells (pDCs) play a major role in endogenous type I IFN production and NIPCs/pDCs are increased in SLE.13 Deficient expression of the type I IFN receptor reduced lupus-like disease in NZB mice.14 These studies demonstrate strong correlation between type I IFNs and SLE pathogenesis. In seeking the molecular mechanism(s) for lupus pathogenesis, I discovered editing in SLE T cell transcriptome,15 because of the up-regulation of the transcript editing gene, 150 kDa adenosine deaminase that act on RNA 1 (ADAR1).16 Studies by other investigators confirmed these findings.10,17,18 The ADARs belong to a family of mammalian RNA editing enzymes, which play an important role in several physiological and pathological processes by catalyzing hydrolytic deamination at C-6 of the adenosine (A) base in certain mRNAs, which leads to inosine (I) formation.15,16,19 Inosines are subsequently recognized as guanosine (G) by the translation machinery. Such editing will result in A to I (G) transcript mutation. The 150 kDa ADAR1 expression is regulated by type I IFNs while 110-kDA ADAR1 and ADAR2 are constitutively expressed in T cells and other cell types.

The occurrence of conserved RNA secondary structures in the human transcriptome is extensive,20 which indicates enormous amounts of potential ADAR substrates in the human transcriptome. Widespread ADARs mediated editing of exonic and intronic elements in human RNAs has been identified.21–28 Most of the editing of exonic elements is random and heterogeneous.15,16 The up-regulated 150 kDa ADAR1 randomly edit adenosines located in double stranded base-paired coding and noncoding regions and cause novel mutations in gene transcripts.16,24,25 Repeated occurrence of such exonic and intronic editing at certain base positions has been identified in SLE T cells and in normal T cells, which express up-regulated 150 kDa ADAR1, at different time points.16,24–26 In addition to ADARs induced A to I (G) editing, apolipoprotein B-editing enzyme, catalytic polypeptide-1 (APOBEC1) mediated cytidine (C) to uridine (U) editing has been well documented in human transcriptome.29 A low frequency of G to A and U to C changes were also observed only in SLE and other pathological conditions.24 The enzymatic machinery responsible for such editing and the molecular mechanism underlying such changes are unknown. Protein molecules translated from edited mRNAs have been identified in normal human B-lymphocytes.30 Recently, about hundred million A to I editing sites were identified in human transcriptome,28 which can make possible a generation of extremely diverse transcriptome. Extensive editing of human genome by activation-induced cytidine deaminase (AID) enzyme has been well documented.31,32 The AID mediated DNA editing may result in the formation of anti-DNA antibodies in addition to its role in the induction of somatic hyper mutations and class switch recombinations in immunoglobulin genes of B-lymphocytes. The occurrence of AutoAbs for mutant DNA molecules in scleroderma has been described recently.33 Peptidylarginine deiminases (PADs) edit protein molecules by deiminating arginine into citrulline and play a critical role in generating anti-citrulline antibodies in rheumatoid arthritis (RA). The association of anti-citrulline antibodies with RA pathogenesis is well established.34 The autoAbs to several proteins, RNA molecules were identified in addition to anti-DNA antibodies in SLE. Occurrence of such plethora of autoAbs in SLE by molecular mimicking (self antigens mimicking as viral and/or bacterial products) without alterations, such as editing and/or mutations in DNA, RNA, and protein molecules is impossible. Therefore, it is hypothesized that, altered and/or enhanced DNA, RNA, and protein editing will not only induce altered gene regulations and immune functions but also set the stage for production of novel auto-antigens (autoAgs). The occurrence of such process repeatedly at different time points will result in the generation of autoAbs followed by auto-immunogenicity and the onset of autoimmunity.

The induction of autoimmunity involves two distinct phases. In the first phase autoAgs are formed by the following molecular mechanisms: (a) modulation of DNA, RNA, and proteins by editing and/or by induction of somatic mutations; (b) occurrence of same editing and/or mutation(s) at specific site(s) at different time points; (c) apoptosis of cells carrying such editing and/or mutation(s) and impaired clearance of apoptotic material by nucleases and proteases; and (d) presentation of such altered DNA, RNA, and protein molecules as non self by antigen presenting cells to T cells. Type I IFNs and/or IFN-inducible genes in the presence of autoAgs, will promote the activation and survival of naive T cells by dendritic cells (DCs), which is independent of BCL and BCLXL gene function.35 Activated T cells will induce B cell stimulation and production of autoAbs.36 This process needs constitutive and repeated occurrence of specific editing and/or mutations at the same site(s) in DNA, RNA, and/or proteins followed by impaired cellular functions and auto-immunogenicity as described earlier. Such initiated autoimmunity will be sustained by the following events, which occur as second phase; (a) the autoimmune complexes formed in the first phase act as endogenous inducers of type I IFNs, replacing exogenous type I IFNs and continuously inducing the production of type I IFNs by NIPCs; (b) continuous generation of autoAbs and autoimmune complexes is maintained this process; (c) a vicious cycle becomes established5; (d) in addition, superantigens (SAgs), products of type I IFN-regulated HERVs, target the immune system causing massive polyclonal T cell activation, cytokine release, T cell apoptosis, and/or anergy, which aid in enhancing autoimmunity.37 Therefore, such information indicates why lupus pathogenesis is so complex, variable, and hard to predict definite cause(s) for and raise the following questions. During their life time, all individuals will sustain viral infections, which are combated by endogenous and exogenous IFNs and IFN-regulated genes. However, why do only relatively few people develop autoimmunity, especially certain women during the childbearing years? Why do only a small percentage of cancer patients (20%), who are treated with IFN, express transient autoimmunity and only a fraction of them (1%) acquire SLE?7 Why do about 20% of normal subjects demonstrate the presence of antinuclear antibodies (ANAs) but fail to develop the onset of autoimmunity and why do age related increase occur in the prevalence of AutoAbs in healthy elderly subjects?38 These questions will help in hypothesizing that only some editing events and/or mutations in DNA, RNA, and protein will result in the formation of autoAgs, like how only extremely rare somatic mutations initiate cancer induction. These autoAgs will be able to produce autoAbs and induce autoimmunity only when cells containing such autoAgs undergo apoptosis followed by non clearance of apoptotic material. In addition, the induction of autoimmunity mimics the process of immunization, which needs vaccination with pathogenic material followed by repeated booster dose administration to attain good immune response for such pathogens. This may also be true in the process of attaining autoimmunity, in which repeated production of autoAbs to specific edited and/or mutated DNA, RNA, and protein molecules and their availability for developing auto-immunogenicity are important and necessary.

Based on this information, I postulate that, no present and/or future drug(s) will help in curing and/or preventing autoimmunity, specifically SLE after its onset, except for symptomatic treatment and temporary relief. Drug therapy(s) cannot modulate and/or suppress such a multistep and complex autoimmune response generated by altered plethora of self DNA, RNA, and protein molecules, before and after the onset of SLE pathogenesis. Moreover, it will be impossible to delineate the autoimmune response from normal immune response to selectively suppress it, without impairing normal immune response. Therefore, the best strategy to combat this anomaly is the multipronged approach of monitoring and regulating (a) frequent and prolonged expression of type I IFNs; (b) DNA, RNA, and protein editing; (c) apoptosis; (d) clearance of apoptotic material by nucleases and proteases during autoimmunity onset susceptible circumstances such as repeated viral and bacterial infections, radiation exposure, cancer treatment, and in women during child bearing years. Such timely and focused regimen and/or approaches could pave the way for effective and safer ways to prevent and/or control SLE as well as other autoimmune diseases.