NRAS mutation causes a human autoimmune lymphoproliferative syndrome

Human genetic approaches to diseases of lymphocyte activation

Our laboratory focuses on the study of the molecular regulation of T lymphocyte homeostasis, particularly as it relates to immunological tolerance, apoptosis, and autoimmune diseases. Through intense molecular research on the regulation of lymphocyte fate, the Fas receptor and other tumor necrosis factor receptors as well as their ligands have emerged as key regulators of T lymphocyte apoptosis. We are studying genetic abnormalities of this death pathway, particularly in the context of autoimmune lymphoproliferative syndrome (ALPS) and other non-ALPS conditions affecting lymphocyte homeostasis. These studies have led to further investigations of the regulation of the NF-kappaB signaling pathway, the molecular basis for programed cell death and viral cytopathicity, mechanisms of autoimmunity, and the regulation of mature T-cell tolerance. Our investigations promise to provide insight into the molecular mechanisms behind the regulation of immune response and contribute to the development of novel diagnostic and treatment methods for autoimmune diseases.

Critical role for BIM in T cell receptor restimulation-induced death

BACKGROUND

Upon repeated or chronic antigen stimulation, activated T cells undergo a T cell receptor (TCR)-triggered propriocidal cell death important for governing the intensity of immune responses. This is thought to be chiefly mediated by an extrinsic signal through the Fas-FasL pathway. However, we observed that TCR restimulation still potently induced apoptosis when this interaction was blocked, or genetically impaired in T cells derived from autoimmune lymphoproliferative syndrome (ALPS) patients, prompting us to examine Fas-independent, intrinsic signals.

RESULTS

Upon TCR restimulation, we specifically noted a marked increase in the expression of BIM, a pro-apoptotic Bcl-2 family protein known to mediate lymphocyte apoptosis induced by cytokine withdrawal. In fact, T cells from an ALPS type IV patient in which BIM expression is suppressed were more resistant to restimulation-induced death. Strikingly, knockdown of BIM expression rescued normal T cells from TCR-induced death to as great an extent as Fas disruption.

CONCLUSION

Our data implicates BIM as a critical mediator of apoptosis induced by restimulation as well as growth cytokine withdrawal. These findings suggest an important role for BIM in eliminating activated T cells even when IL-2 is abundant, working in conjunction with Fas to eliminate chronically stimulated T cells and maintain immune homeostasis.

REVIEWERS

This article was reviewed by Dr. Wendy Davidson (nominated by Dr. David Scott), Dr. Mark Williams (nominated by Dr. Neil Greenspan), and Dr. Laurence C. Eisenlohr.

N-Ras or K-Ras inhibition increases the number and enhances the function of Foxp3 regulatory T cells

Naturally occurring regulatory T cells (Treg) driven by their transcriptional controller Foxp3 are compromised in immune-mediated disorders and confer protection when adoptively transferred. We examined the Ras-inhibitory effect on functional determinants of Treg in vivo and in vitro. Ras was inhibited in Jurkat T cells by transfection with a dominant-negative form of Ras, or by shRNA for N-Ras, K-Ras, and H-Ras, or by farnesylthiosalycylic acid, a small-molecule inhibitor. Except for H-Ras transduction with shRNA, each inhibitory mode increased expression of Foxp3 and nuclear factor of activated T cell proteins, and surface expression of CD25. Ras inhibition in PBMC and spleen-derived lymphocytes reproduced these findings. The heightened Foxp3 expression reflected both increased basal cellular protein and peripheral conversion of non-Treg to Treg. Ras inhibition enhanced Treg-induced suppression; thus, when adoptively transferred to mice, Ras-inhibited Treg reduced the incidence of diabetes. Inhibition of Foxp3 by respective siRNA reversed the enhancement. Thus, inhibition of the N- or K-Ras isoform triggers an anti-inflammatory effect by up-regulating, via Foxp3 elevation, the numbers and functional suppressive properties of Treg.

Genetic defects of apoptosis and primary immunodeficiency

Programmed cell death is important for maintaining lymphocyte homeostasis. Several human-inherited diseases with impaired apoptosis have been identified at the genetic level: autoimmune lymphoproliferative syndrome, caspase-8 deficiency state, and X-linked lymphoproliferative syndrome. These diseases feature excess lymphocyte accumulation, autoimmunity, or immunodeficiency. Elucidating their molecular pathogenesis has also provided new insights into the signaling mechanisms regulating apoptosis and lymphocyte activation.

Cell cycle regulation by FasL and Apo2L/TRAIL in human T-cell blasts. Implications for autoimmune lymphoproliferative syndromes

The Fas-FasL pathway plays an important role in the homeostasis of mature lymphocytes, with defects causing autoimmune lymphoproliferative syndromes (ALPS). Human T-cell blasts are not sensitive to FasL or Apo2L/TRAIL-induced apoptosis unless they get reactivated, but either of those ligands inhibits their growth in the absence of cell death induction due to a cell cycle arrest in S-G2/M. In the present work, we have studied the mechanism(s) by which FasL or Apo2L/TRAIL regulate T-cell blast cell cycle in healthy donors and in two types of ALPS patients. Our data indicate that in human CD8+ T-cell blasts, Fas ligation, and especially Apo2L/TRAIL induce the p53-dependent decrease in cyclin-B1 levels. However, the induction of the negative cell cycle regulator p21WAF1 by FasL or Apo2L/TRAIL in either CD4+ or CD8+ T-cell blasts seems to be the main regulatory mechanism. This mechanism is dependent on caspase activation and on H2O2 generation. The increase in p21 levels by FasL or Apo2L/TRAIL is concomitant with p53 increases only in CD8+ T-cell blasts, with p21 levels maintained high for longer times than p53 levels. In CD4+ T-cell blasts p21 levels are controlled through a transient and p53-independent mechanism. The present results suggest that the etiology of ALP syndromes could be related not only to defects in apoptosis induction, but also in cell cycle regulation.

Understanding systemic lupus erythematosus physiopathology in the light of primary immunodeficiencies

INTRODUCTION

Associations between systemic lupus erythematosus (SLE) and primary immunodeficiencies (PIDs) were analyzed to gain insight into the physiopathology of SLE. Some PIDs have been consistently associated with SLE or lupus-like manifestations: (a) homozygous deficiencies of the early components of the classical complement pathway in the following decreasing order: in C1q, 93% of affected patients developed SLE; in C4, 75%; in C1r/s, 57%; and in C2, up to 25%; (b) female carriers of X-linked chronic granulomatous disease allele; and (c) IgA deficiency, present in around 5% of juvenile SLE.

DISCUSSION

In the first two groups, disturbances of cellular waste-disposal have been proposed as the main mechanisms of pathogenesis. On the other hand and very interestingly, there are PIDs systematically associated with several autoimmune manifestations in which SLE has not been described, such as autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED), immunodysregulation polyendocrinopathy enteropathy X-linked (IPEX), and autoimmune lymphoproliferative syndrome (ALPS), suggesting that mechanisms considered as critical players for induction and maintenance of tolerance to autoantigens, such as (1) AIRE-mediated thymic negative selection of lymphocytes, (2) Foxp3+ regulatory T cell-mediated peripheral tolerance, and (3) deletion of auto-reactive lymphocytes by Fas-mediated apoptosis, could not be relevant in SLE physiopathology. The non-description of SLE and neither the most characteristic SLE clinical features among patients with agammaglobulinemia are also interesting observations, which reinforce the essential role of B lymphocytes and antibodies for SLE pathogenesis.

CONCLUSION

Therefore, monogenic PIDs represent unique and not fully explored human models for unraveling components of the conundrum represented by the physiopathology of SLE, a prototypical polygenic disease.

Primary immunodeficiencies unravel critical aspects of the pathophysiology of autoimmunity and of the genetics of autoimmune disease

BACKGROUND

Primary Immunodeficiencies (PIDs) represent unique opportunities to understand the operation of the human immune system. Accordingly, PIDs associated with autoimmune manifestations provide insights into the pathophysiology of autoimmunity as well as into the genetics of autoimmune diseases (AID). Epidemiological data show that there are PIDs systematically associated with AID, such as immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome (IPEX), Omenn syndrome, autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED), autoimmune lymphoproliferative syndrome (ALPS), and C1q deficiency, while strong associations are seen with a handful of other deficits.

CONCLUSION

We interpret such stringent disease associations, together with a wealth of observations in experimental systems, as indicating first of all that natural tolerance to body components is an active, dominant process involving many of the components that ensure responsiveness, rather than, as previously believed, the result of the mere purge of autoreactivities. More precisely, it seems that deficits of Treg cell development, functions, numbers, and T cell receptor repertoire are among the main factors for autoimmunity pathogenesis in many (if not all) PIDs most frequently presenting with autoimmune features. Clearly, other pathophysiological mechanisms are also involved in autoimmunity, but these seem less critical in the process of self-tolerance. Comparing the clinical picture of IPEX cases with those, much less severe, of ALPS or APECED, provides some assessment of the relative importance of each set of mechanisms.

Co-inherited mutations of Fas and caspase-10 in development of the autoimmune lymphoproliferative syndrome

BACKGROUND

Autoimmune lymphoproliferative syndrome (ALPS) is a rare inherited disorder characterized by defective function of Fas, autoimmune manifestations that predominantly involve blood cells, polyclonal accumulation of lymphocytes in the spleen and lymph nodes with lymphoadenomegaly and/or splenomegaly, and expansion of TCRalphabeta+ CD4/CD8 double-negative (DN) T cells in the peripheral blood. Most frequently, it is due to Fas gene mutations, causing ALPS type Ia (ALPS-Ia). However, other mutations, namely of the FasL gene (ALPS-Ib) and the caspase-10 gene (ALPS-II) are occasionally detected, whereas some patients do not present any known mutations (ALPS-III). Recently, mutations of the NRAS gene have been suggested to cause ALPS-IV.

RESULTS

This work reports two patients that are combined heterozygous for single nucleotide substitutions in the Fas and caspase-10 genes. The first patient carried a splice site defect suppressing allele expression in the Fas gene and the P501L substitution in caspase-10. The second had a mutation causing a premature stop codon (Q47X) in the Fas gene and the Y446C substitution in caspase-10. Fas expression was reduced and caspase-10 activity was decreased in both patients. In both patients, the mutations were inherited from distinct healthy parents.

CONCLUSION

These data strongly suggest that co-transmission of these mutation was responsible for ALPS.

Defective expression of Ras guanyl nucleotide-releasing protein 1 in a subset of patients with systemic lupus erythematosus

Dysregulation of Ras guanyl nucleotide-releasing protein 1 (RasGRP1) in mice results in a systemic lupus erythematosus (SLE)-like disorder. We therefore looked for defective isoforms and/or diminished levels of human RasGRP1 in a cohort of SLE patients. PBMCs were collected from twenty healthy individuals and thirty-two patients with SLE. mRNA was isolated and five RasGRP1 cDNAs from each subject were sequenced. T cell lysates from healthy controls and SLE patients also were evaluated for their levels of RasGRP1 protein. The accumulated data led to the identification of 13 new splice variants of the human RasGRP1 gene. Not only did our SLE patients have increased levels and types of these defective transcripts relative to normal individuals, two SLE patients were identified whose PBMCs and T cells contained very little, if any, functional RasGRP1 mRNA and protein. The presence of aberrantly spliced RasGRP1 transcripts also was correlated with lower levels of RasGRP1 protein in the patients' T cells. The lack of the normal isoform of RasGRP1 in some SLE patients and the increased prevalence of defective isoforms of RasGRP1 in others raise the possibility that dysregulation of this signaling protein contributes to the development of autoimmunity in a subset of SLE patients.

Constitutively active STAT6 predisposes toward a lymphoproliferative disorder

Signal transducer and activator of transcription 6 (STAT6) is critical for IL-4 and IL-13 responses, and necessary for the normal development of Th2 cells. We previously generated mice that express a constitutively active STAT6 (STAT6VT) under control of the CD2 locus control region, which directs expression to the T-cell compartment. We now describe that a small proportion of these mice (~5%) develop a spontaneous lymphoproliferative disease (LPD) that results in dramatic splenomegaly. The cell populations observed in the LPD spleens can be divided into 2 categories, those that are composed of mixed lineage cells and those that are predominantly T cells with a phenotype similar to that in autoimmune lymphoproliferative syndrome (ALPS) patients. These data suggest that while active STAT6 is not a transforming factor, expression in T cells predisposes toward the development of lymphoproliferative disorders.

Dominant inhibition of Fas ligand-mediated apoptosis due to a heterozygous mutation associated with autoimmune lymphoproliferative syndrome (ALPS) Type Ib

Background:

Autoimmune lymphoproliferative syndrome (ALPS) is a disorder of lymphocyte homeostasis and immunological tolerance due primarily to genetic defects in Fas (CD95/APO-1; TNFRSF6), a cell surface receptor that regulates apoptosis and its signaling apparatus.

Methods:

Fas ligand gene mutations from ALPS patients were identified through cDNA and genomic DNA sequencing. Molecular and biochemical assessment of these mutant Fas ligand proteins were carried out by expressing the mutant FasL cDNA in mammalian cells and analysis its effects on Fas-mediated programmed cell death.

Results:

We found an ALPS patient that harbored a heterozygous A530G mutation in the FasL gene that replaced Arg with Gly at position 156 in the protein's extracellular Fas-binding region. This produced a dominant-interfering FasL protein that bound to the wild-type FasL protein and prevented it from effectively inducing apoptosis.

Conclusion:

Our data explain how a naturally occurring heterozygous human FasL mutation can dominantly interfere with normal FasL apoptotic function and lead to an ALPS phenotype, designated Type Ib.