Comparison of juvenile and adult myasthenia gravis in a French cohort with focus on thymic histology.

Myasthenia gravis (MG) is an autoimmune disease characterized by muscle fatigability due to acetylcholine receptor (AChR) autoantibodies. To better characterize juvenile MG (JMG), we analyzed 85 pre- and 132 post-pubescent JMG (with a cutoff age of 13) compared to 721 adult MG patients under 40 years old using a French database. Clinical data, anti-AChR antibody titers, thymectomy, and thymic histology were analyzed. The proportion of females was higher in each subgroup. No significant difference in the anti-AChR titers was observed. Interestingly, the proportion of AChR+ MG patients was notably lower among adult MG patients aged between 30 and 40 years, at 69.7%, compared to over 82.4% in the other subgroups. Thymic histological data were examined in patients who underwent thymectomy during the year of MG onset. Notably, in pre-JMG, the percentage of thymectomized patients was significantly lower (32.9% compared to more than 42.5% in other subgroups), and the delay to thymectomy was twice as long. We found a positive correlation between anti-AChR antibodies and germinal center grade across patient categories. Additionally, only females, particularly post-JMG patients, exhibited the highest rates of lymphofollicular hyperplasia (95% of cases) and germinal center grade. These findings reveal distinct patterns in JMG patients, particularly regarding thymic follicular hyperplasia, which appears to be exacerbated in females after puberty.

Blocking interleukin-23 ameliorates neuromuscular and thymic defects in myasthenia gravis.

Acetylcholine receptor (AChR) myasthenia gravis (MG) is a chronic autoimmune disease characterized by muscle weakness. The AChR+ autoantibodies are produced by B-cells located in thymic ectopic germinal centers (eGC). No therapeutic approach is curative. The inflammatory IL-23/Th17 pathway is activated in the thymus as well as in the blood and the muscle, contributing to the MG pathogenic events. We aimed to study a potential new therapeutic approach that targets IL-23p19 (IL-23) in the two complementary preclinical MG models: the classical experimental MG mouse model (EAMG) based on active immunization and the humanized mouse model featuring human MG thymuses engrafted in NSG mice (NSG-MG). In both preclinical models, the anti-IL-23 treatment ameliorated MG clinical symptoms. In the EAMG, the treatment reduced IL-17 related inflammation, anti-AChR IgG2b antibody production, activated transduction pathway involved in muscle regeneration and ameliorated the signal transduction at the neuromuscular junction. In the NSG-MG model, the treatment reduced pathogenic Th17 cell population and expression of genes involved in eGC stabilization and B-cell development in human MG thymus biopsies. Altogether, these data suggest that a therapy targeting IL-23p19 may promote significant clinical ameliorations in AChR+ MG disease due to concomitant beneficial effects on the thymus and skeletal muscle defects.

Myasthenia Gravis: An Acquired Interferonopathy?

Myasthenia gravis (MG) is a rare autoimmune disease mediated by antibodies against components of the neuromuscular junction, particularly the acetylcholine receptor (AChR). The thymus plays a primary role in AChR-MG patients. In early-onset AChR-MG and thymoma-associated MG, an interferon type I (IFN-I) signature is clearly detected in the thymus. The origin of this chronic IFN-I expression in the thymus is not yet defined. IFN-I subtypes are normally produced in response to viral infection. However, genetic diseases called interferonopathies are associated with an aberrant chronic production of IFN-I defined as sterile inflammation. Some systemic autoimmune diseases also share common features with interferonopathies. This review aims to analyze the pathogenic role of IFN-I in these diseases as compared to AChR-MG in order to determine if AChR-MG could be an acquired interferonopathy.

Estrogen, estrogen-like molecules and autoimmune diseases.

In western countries, the slope of autoimmune disease (AD) incidence is increasing and affects 5-8% of the population. Mainly prevalent in women, these pathologies are due to thymic tolerance processes breakdown. The female sex hormone, estrogen, is involved in this AD female susceptibility. However, predisposition factors have to act in concert with unknown triggering environmental factors (virus, microbiota, pollution) to initiate AD. Individuals are exposed to various environmental compounds that display endocrine disruption abilities. The cellular effects of some of these molecules may be mediated through the aryl hydrocarbon receptor (AhR). Here, we review the effects of these molecules on the homeostasis of the thymic cells, the immune tolerance intrinsic factors (transcription factors, epigenetic marks) and on the immune tolerance extrinsic factors (microbiota, virus sensibility). This review highlights the contribution of estrogen and endocrine disruptors on the dysregulation of mechanisms sustaining AD development.

Il-23/Th17 cell pathway: A promising target to alleviate thymic inflammation maintenance in myasthenia gravis.

IL-23/Th17 pathway has been identified to sustain inflammatory condition in several autoimmune diseases and therefore being targeted in various therapeutic and effective approaches. Patients affected with autoimmune myasthenia gravis exhibit a disease effector tissue, the thymus, that harbors ectopic germinal centers that sustain production of auto-antibodies, targeting proteins located in the neuromuscular junction, cause of the organ-specific chronic autoimmune disease. The present study aims to investigate the IL-23/Th17 cell pathway in the thymic inflammatory and pathogenic events. We found that thymuses of MG patients displayed overexpression of Interleukin-17, signature cytokine of activated Th17 cells. This activation was sustained by a higher secretion of Interleukin-23 by TEC, in addition to the increased expression of cytokines involved in Th17 cell development. The overexpression of Interleukin-23 was due to a dysregulation of interferon type I pathway. Besides, Interleukin-17 secreted, and Th17 cells were localized around thymic ectopic germinal centers. These cells expressed podoplanin, a protein involved in B-cell maturation and antibody secretion. Finally, production of Interleukin-23 was also promoted by Interleukin-17 secreted itself by Th17 cells, highlighting a chronic loop of inflammation sustained by thymic cell interaction. Activation of the IL-23/Th17 pathway in the thymus of autoimmune myasthenia gravis patients creates an unstoppable loop of inflammation that may participate in ectopic germinal center maintenance. To alleviate the physio-pathological events in myasthenia gravis patients, this pathway may be considered as a new therapeutic target.

Cultured Human Thymic-Derived Cells Display Medullary Thymic Epithelial Cell Phenotype and Functionality.

Thymic epithelial cells are one of the main components of the thymic microenvironment required for T-cell development. In this work, we describe an efficient method free of enzymatic and Facs-sorted methods to culture human medullary thymic epithelial cells without affecting the cell phenotypic, physiologic and functional features. Human medulla thymic epithelial cells (mTECs) are obtained by culturing thymic biopsies explants. After 7 days of primo-culture, mTECs keep their ability to express key molecules involved in immune tolerance processes such as autoimmune regulator, tissue-specific antigens, chemokines, and cytokines. In addition, the cells sensor their cultured environment and consequently adjust their gene expression network. Therefore, we describe and provide a human mTEC model that may be used to test the effect of various molecules on thymic epithelial cell homeostasis and physiology. This method should allow the investigations of the specificities and the knowledge of human mTECs in normal or pathological conditions and therefore discontinue the extrapolations done on the murine models.

Author Correction: Balance between Estrogens and Proinflammatory Cytokines Regulates Chemokine Production Involved in Thymic Germinal Center Formation.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

An imbalance between regulatory T cells and T helper 17 cells in acetylcholine receptor-positive myasthenia gravis patients.

A chronic autoimmune disease, myasthenia gravis (MG) is characterized in 85% of patients by antibodies directed against the acetylcholine receptor (AChR) located at the neuromuscular junction. The functional and effective balance between regulatory T cells (Treg cells) and effector T cells (Teff cells) is lost in the hyperplastic thymus of MG patients with antibodies specific for the AChR (AChR+ MG patients). The objective of this review is to describe how Treg cells and inflammatory T cells participate in this imbalance and contribute to induce a chronic inflammatory state in the MG thymus. We discuss the origins and characteristics of Treg cells and their reported dysfunctions in AChR+ MG patients. We also review the inflammatory condition observed in MG thymus, including overexpression of interleukin (IL)-1ß, IL-6, and IL-23, cytokines that promote the differentiation of T helper 17 (TH 17) cells and the expression of IL-17. We summarize the preclinical models used to determine the implication of expression of cytokines, such as IL-6, IL-12 (IL-23 subunit), IL-17, and interferon γ to the development of experimental autoimmune MG. Finally, we suggest that biological agents, such as humanized monoclonal antibodies that target the IL-23/TH 17 pathway, should be investigated in the context of MG, as they have proven efficiency in other autoimmune diseases.

AIRE: a missing link to explain female susceptibility to autoimmune diseases.

Women are more susceptible to autoimmune diseases than men. Autoimmunity results from tolerance breakdown toward self-components. Recently, three transcription modulators were identified in medullary thymic epithelial cells that orchestrate immune central tolerance processes: the autoimmune regulator (AIRE), FEZ family zinc finger 2 (FEZF2 or FEZ1), and PR domain zinc finger protein 1 (PRDM1). Interestingly, these three transcription modulators regulate nonredundant tissue-specific antigen subsets and thus cover broad antigen diversity. Recent data from different groups demonstrated that sex hormones (estrogen and testosterone) are involved in the regulation of thymic AIRE expression in humans and mice through direct transcriptional modulation and epigenetic changes. As a consequence, AIRE displays gender-biased thymic expression, with females showing a lower expression compared with males, a finding that could explain the female susceptibility to autoimmune diseases. So far, FEZF2 has not been related to an increased gender bias in autoimmune disease. PRDM1 expression has not been shown to display gender-differential thymic expression, but its expression level and its gene polymorphisms are associated with female-dependent autoimmune disease risk. Altogether, various studies have demonstrated that increased female susceptibility to autoimmune diseases is in part a consequence of hormone-driven reduced thymic AIRE expression.

Thymus involvement in early-onset myasthenia gravis.

It has long been established that the thymus plays a central role in autoimmune myasthenia gravis (MG) because of either thymoma or thymic hyperplasia of lymphoproliferative origin. In this review, we discuss thymic changes associated with thymic hyperplasia and their implications in the development of an autoimmune response against the acetylcholine receptor (AChR).The hyperplastic MG thymus displays all the characteristics of tertiary lymphoid organs (TLOs): neoangiogenic processes with high endothelial venule and lymphatic vessel development, chemokine overexpression favoring peripheral cell recruitment, and ectopic germinal center development. As thymic epithelial cells or myoid cells express AChR, a specific antigen presentation can easily occur within the thymus in the presence of recruited peripheral cells, such as B cells and T follicular helper cells. How the thymus turns into a TLO is not known, but local inflammation seems mandatory. Interferon (IFN)-ß is overexpressed in MG thymus and could orchestrate thymic changes associated with MG. Knowledge about how IFN-ß is induced in MG thymus and why its expression is sustained even long after disease onset would be of interest in the future to better understand the etiological and physiopathological mechanisms involved in autoimmune MG.

Balance between Estrogens and Proinflammatory Cytokines Regulates Chemokine Production Involved in Thymic Germinal Center Formation.

The early-onset form of Myasthenia Gravis (MG) is prevalent in women and associates with ectopic germinal centers (GCs) development and inflammation in the thymus. we aimed to investigate the contribution of estrogens in the molecular processes involved in thymic GCs formation. We examined expression of genes involved in anti-acetylcholine receptor (AChR) response in MG, MHC class II and α-AChR subunit as well as chemokines involved in GC development (CXCL13, CCL21and CXCL12). In resting conditions, estrogens have strong regulatory effects on thymic epithelial cells (TECs), inducing a decreased protein expression of the above molecules. In knockout mouse models for estrogen receptor or aromatase, we observed that perturbation in estrogen transduction pathway altered MHC Class II, α-AChR, and CXCL13 expression. However, in inflammatory conditions, estrogen effects were partially overwhelmed by pro-inflammatory cytokines. Interestingly, estrogens were able to control production of type I interferon and therefore play dual roles during inflammatory events. In conclusion, we showed that estrogens inhibited expression of α-AChR and HLA-DR in TECs, suggesting that estrogens may alter the tolerization process and favor environment for an autoimmune response. By contrast, under inflammatory conditions, estrogen effects depend upon strength of the partner molecules with which it is confronted to.

Preconditioned mesenchymal stem cells treat myasthenia gravis in a humanized preclinical model.

Myasthenia gravis (MG) with anti-acetylcholine receptor (AChR) Abs is an autoimmune disease characterized by severe defects in immune regulation and thymic inflammation. Because mesenchymal stem cells (MSCs) display immunomodulatory features, we investigated whether and how in vitro-preconditioned human MSCs (cMSCs) could treat MG disease. We developed a new humanized preclinical model by subcutaneously grafting thymic MG fragments into immunodeficient NSG mice (NSG-MG model). Ninety percent of the animals displayed human anti-AChR Abs in the serum, and 50% of the animals displayed MG-like symptoms that correlated with the loss of AChR at the muscle endplates. Interestingly, each mouse experiment recapitulated the MG features of each patient. We next demonstrated that cMSCs markedly improved MG, reducing the level of anti-AChR Abs in the serum and restoring AChR expression at the muscle endplate. Resting MSCs had a smaller effect. Finally, we showed that the underlying mechanisms involved (a) the inhibition of cell proliferation, (b) the inhibition of B cell-related and costimulatory molecules, and (c) the activation of the complement regulator DAF/CD55. In conclusion, this study shows that a preconditioning step promotes the therapeutic effects of MSCs via combined mechanisms, making cMSCs a promising strategy for treating MG and potentially other autoimmune diseases.

[Autoimmune disease predisposition: Aire « protects ¼ men]. / Prédisposition aux pathologies auto-immmunes - Les hommes ne manquent pas « d'Aire ¼.

Autoimmune diseases are a group of about 80 different diseases affecting 5-8% of the population. They are due to a deregulation of the immune system that attacks specific molecules and/or cells in the body. The thymus is the school of T cells that must be able to react to foreign molecules penetrating into the body. This education process is mediated by interactions between T cells and thymic epithelial cells (TEC) that express specific proteins of the peripheral tissues (TSA, "tissue-specific antigen"). This complex mechanism is called central tolerance. Most of the autoimmune diseases display a common feature : women are more susceptible to these diseases than men. Since the thymus is the main organ of central tolerance, we conducted a comparative study of thymic transcriptome of women and men. Our data revealed sex-associated differences in the expression of TSAs that are controlled by the autoimmune regulator (AIRE), a key factor in central tolerance. By studying human and murine cell models, we analyzed the relationship between gender, hormones and AIRE. Our work shows that AIRE is less expressed in women than in men after puberty. Furthermore, we show that estrogen induces decreased thymic AIRE expression by epigenetic modifications through increased number of methylation sites within the AIRE promoter. Consequently, these data suggest that from puberty, women have a reduced effectiveness of central tolerance process, leading to increased number of autoreactive lymphocytes, and as a result, increased susceptibility to autoimmune diseases. Together, these data may question the impact of exposure to "estrogen-like" molecules on the growing incidence of autoimmune diseases.

Estrogen-mediated downregulation of AIRE influences sexual dimorphism in autoimmune diseases.

Autoimmune diseases affect 5% to 8% of the population, and females are more susceptible to these diseases than males. Here, we analyzed human thymic transcriptome and revealed sex-associated differences in the expression of tissue-specific antigens that are controlled by the autoimmune regulator (AIRE), a key factor in central tolerance. We hypothesized that the level of AIRE is linked to sexual dimorphism susceptibility to autoimmune diseases. In human and mouse thymus, females expressed less AIRE (mRNA and protein) than males after puberty. These results were confirmed in purified murine thymic epithelial cells (TECs). We also demonstrated that AIRE expression is related to sexual hormones, as male castration decreased AIRE thymic expression and estrogen receptor α-deficient mice did not show a sex disparity for AIRE expression. Moreover, estrogen treatment resulted in downregulation of AIRE expression in cultured human TECs, human thymic tissue grafted to immunodeficient mice, and murine fetal thymus organ cultures. AIRE levels in human thymus grafted in immunodeficient mice depended upon the sex of the recipient. Estrogen also upregulated the number of methylated CpG sites in the AIRE promoter. Together, our results indicate that in females, estrogen induces epigenetic changes in the AIRE gene, leading to reduced AIRE expression under a threshold that increases female susceptibility to autoimmune diseases.

Central role of interferon-beta in thymic events leading to myasthenia gravis.

The thymus plays a primary role in early-onset Myasthenia Gravis (MG) mediated by anti-acetylcholine receptor (AChR) antibodies. As we recently showed an inflammatory and anti-viral signature in MG thymuses, we investigated in detail the contribution of interferon (IFN)-I and IFN-III subtypes in thymic changes associated with MG. We showed that IFN-I and IFN-III subtypes, but especially IFN-ß, induced specifically α-AChR expression in thymic epithelial cells (TECs). We also demonstrated that IFN-ß increased TEC death and the uptake of TEC proteins by dendritic cells. In parallel, we showed that IFN-ß increased the expression of the chemokines CXCL13 and CCL21 by TECs and lymphatic endothelial cells, respectively. These two chemokines are involved in germinal center (GC) development and overexpressed in MG thymus with follicular hyperplasia. We also demonstrated that the B-cell activating factor (BAFF), which favors autoreactive B-cells, was overexpressed by TECs in MG thymus and was also induced by IFN-ß in TEC cultures. Some of IFN-ß effects were down-regulated when cell cultures were treated with glucocorticoids, a treatment widely used in MG patients that decreases the number of thymic GCs. Similar changes were observed in vivo. The injections of Poly(I:C) to C57BL/6 mice triggered a thymic overexpression of IFN-ß and IFN-α2 associated with increased expressions of CXCL13, CCL21, BAFF, and favored the recruitment of B cells. These changes were not observed in the thymus of IFN-I receptor KO mice injected with Poly(I:C), even if IFN-ß and IFN-α2 were overexpressed. Altogether, these results demonstrate that IFN-ß could play a central role in thymic events leading to MG by triggering the overexpression of α-AChR probably leading to thymic DC autosensitization, the abnormal recruitment of peripheral cells and GC formation.

Oral benzo[a]pyrene: understanding pharmacokinetics, detoxication, and consequences--Cyp1 knockout mouse lines as a paradigm.

Benzo[a]pyrene (BaP) is a prototypical polycyclic aromatic hydrocarbon (PAH); this ubiquitous environmental carcinogenic agent is found in tobacco smoke, charcoal-grilled foods, and PAH-contaminated surfaces of roofs, playgrounds, and highways. Cytochrome P450 1 wild-type, Cyp1a2(-/-), Cyp1b1(-/-), or Cyp1a2/1b1(-/-) knockouts, and mice with Cyp1a1 expression deleted in hepatocytes can ingest large oral BaP doses (125 mg/kg/d) without apparent toxicity. Cyp1a1(-/-) and Cyp1a1/1a2(-/-) knockouts and mice with Cyp1a1 expression deleted in gastrointestinal (GI) tract epithelial cells develop immunotoxicity and die within 32 days, indicating that GI tract inducible CYP1A1 is absolutely required for detoxication of oral BaP. Cyp1a1/1b1(-/-) and Cyp1a1/1a2/1b1(-/-) mice are rescued from immunosuppression and early death due to absent metabolic activation of BaP by CYP1B1 in immune cells. Ten-fold lower oral BaP doses result in adenocarcinoma of the proximal small intestine (PSI) in Cyp1a1(-/-) mice; Cyp1a1/1b1(-/-) double-knockout mice show no PSI cancer but develop squamous cell carcinoma of the preputial gland duct (PGD). BaP-metabolizing CYP1B1 in the PSI and CYP3A59 in the PGD are the most likely candidates to participate in tumor initiation in the epithelial cells of these two tissues; oncogenes and tumor-suppressor genes upregulated and downregulated during tumorigenesis are completely different between these tissues. This "oral BaP Cyp1" mouse paradigm represents a powerful teaching tool, showing that gene-environment interactions depend on route-of-administration: the same oral, but not intraperitoneal, BaP exposure leads to dramatic differences in target-organ toxicity and tumor type as a function of dose and Cyp1 genotype.

Implication of double-stranded RNA signaling in the etiology of autoimmune myasthenia gravis.

OBJECTIVE: Myasthenia gravis (MG) is an autoimmune disease mediated mainly by anti-acetylcholine receptor (AChR) antibodies. The thymus plays a primary role in MG pathogenesis. As we recently showed an inflammatory and antiviral signature in MG thymuses, we investigated whether pathogen-sensing molecules could contribute to an anti-AChR response. METHODS: We studied the effects of toll-like receptor agonists on the expression of α-AChR and various tissue-specific antigens (TSAs) in human thymic epithelial cell (TEC) cultures. As polyinosinic-polycytidylic acid (poly[I:C]), which mimics double-stranded RNA (dsRNA), stimulated specifically α-AChR expression, the signaling pathways involved were investigated. In parallel, we analyzed the expression of dsRNA-signaling components in the thymus of MG patients, and the relevance of our data was investigated in vivo in poly(I:C)-injected mice. RESULTS: We demonstrate that dsRNA signaling induced by poly(I:C) specifically triggers the overexpression of α-AChR in TECs and not of other TSAs. A poly(I:C) effect was also observed on MG TECs. This induction is mediated through toll-like receptor 3 (TLR3) and protein kinase R (PKR), and by the release of interferon (IFN)-ß. In parallel, human MG thymuses also display an overexpression of TLR3, PKR, and IFN-ß. In addition, poly(I:C) injections specifically increase thymic expression of α-AChR in wild-type mice, but not in IFN-I receptor knockout mice. These injections also lead to an anti-AChR autoimmune response characterized by a significant production of serum anti-AChR antibodies and a specific proliferation of B cells. INTERPRETATION: Because anti-AChR antibodies are highly specific for MG and are pathogenic, dsRNA-signaling activation could contribute to the etiology of MG.

Organ-specific roles of CYP1A1 during detoxication of dietary benzo[a]pyrene.

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed environmental toxicants derived from sources that include cigarette smoke, petroleum distillation, gas- and diesel-engine exhaust, and charcoal-grilled food. The gastrointestinal tract is the principal route of PAH exposures, even when inhaled. The most thoroughly studied prototype of PAHs is benzo[a]pyrene (BaP), well known to be toxic, mutagenic, and carcinogenic in various tissues and cell types. This lab has previously shown that Cyp1a1(-/-) global knockout mice treated by oral administration of BaP die at 28 to 32 days with immunosuppression, whereas wild-type mice remain healthy for 1 year on high BaP doses (125 mg/kg/day). Thus, for oral BaP, CYP1A1 is more important in detoxication than in metabolic activation. After several days of oral BaP, we found surprisingly low CYP1A1 levels in liver, compared with that in small intestine; we postulated that this finding might reflect efficient detoxication of oral BaP in proximal small intestine such that significant amounts of the inducer BaP no longer reach the liver. In the present study, many parameters were therefore compared in wild-type, Cyp1a1(-/-) global knockout, intestinal epithelial cell-specific Cyp1a1 knockout, and hepatocyte-specific Cyp1a1 knockout mice as a function of long-term oral exposure to BaP. The peak of CYP1A1 (mRNA, protein) expression in liver occurred at 12 h, whereas highly induced CYP1A1 in small intestine persisted throughout the 30-day experiment. Hepatocyte-specific Cyp1a1 knockout mice remained as healthy as wild-type mice; intestinal epithelial cell-specific Cyp1a1 knockout mice behaved like Cyp1a1(-/-) mice, dying with immunosuppression approximately 30 days on oral BaP. We conclude that small intestine CYP1A1, and not liver CYP1A1, is critically important in oral BaP detoxication.

Oral benzo[a]pyrene-induced cancer: two distinct types in different target organs depend on the mouse Cyp1 genotype.

Benzo[a]pyrene (BaP) is a prototypical polycyclic aromatic hydrocarbon (PAH) found in combustion processes. Cytochrome P450 1A1 and 1B1 enzymes (CYP1A1 and CYP1B1) can both detoxify PAHs and activate them to cancer-causing reactive intermediates. Following high dosage of oral BaP (125 mg/kg/day), ablation of the mouse Cyp1a1 gene causes immunosuppression and death within ∼28 days, whereas Cyp1(+/+) wild-type mice remain healthy for >12 months on this regimen. In this study, male Cyp1(+/+) wild-type, Cyp1a1(-/-) and Cyp1b1(-/-) single-knockout and Cyp1a1/1b1(-/-) double-knockout mice received a lower dose (12.5 mg/kg/day) of oral BaP. Tissues from 16 different organs-including proximal small intestine (PSI), liver and preputial gland duct (PGD)-were evaluated; microarray cDNA expression and >30 mRNA levels were measured. Cyp1a1(-/-) mice revealed markedly increased CYP1B1 mRNA levels in the PSI, and between 8 and 12 weeks developed unique PSI adenomas and adenocarcinomas. Cyp1a1/1b1(-/-) mice showed no PSI tumors but instead developed squamous cell carcinoma of the PGD. Cyp1(+/+) and Cyp1b1(-/-) mice remained healthy with no remarkable abnormalities in any tissue examined. PSI adenocarcinomas exhibited striking upregulation of the Xist gene, suggesting epigenetic silencing of specific genes on the Y-chromosome; the Rab30 oncogene was upregulated; the Nr0b2 tumor suppressor gene was downregulated; paradoxical overexpression of numerous immunoglobulin kappa- and heavy-chain variable genes was found-although the adenocarcinoma showed no immunohistochemical evidence of being lymphatic in origin. This oral BaP mouse paradigm represents an example of "gene-environment interactions" in which the same exposure of carcinogen results in altered target organ and tumor type, as a function of just 1 or 2 globally absent genes.