Coexpression and regulation of the FGF-2 and FGF antisense genes in leukemic cells

FGF2 gene's antisense protein, NUDT6, plays a depressogenic role by promoting inflammation and suppressing neurogenesis without altering FGF2 signalling

Fibroblast growth factor-2 (FGF2) is involved in the regulation of affective behaviour and shows antidepressant effects through the Akt and extracellular signal regulated kinase (ERK) 1/2 pathways. Nudix hydrolase 6 (NUDT6) protein is encoded from FGF2 gene's antisense strand and its role in the regulation of affective behaviour is unknown. Here, we overexpressed NUDT6 in the hippocampus and investigated its behavioural effects and the underlying molecular mechanisms affecting the behaviour. We showed that increasing hippocampal NUDT6 results in depression-like behaviour in rats without changing FGF2 levels or activating its downstream effectors, Akt and ERK1/2. Instead, NUDT6 acted by inducing inflammatory signalling, specifically by increasing S100 calcium binding protein A9 (S100A9) levels, activating nuclear factor-kappa B-p65 (NF-κB-p65), and elevating microglia numbers along with a reduction in neurogenesis. Our results suggest that NUDT6 could play a role in major depression by inducing a proinflammatory state. This is the first report of an antisense protein acting through a different mechanism of action than regulation of its sense protein. The opposite effects of NUDT6 and FGF2 on depression-like behaviour may serve as a mechanism to fine-tune affective behaviour. Our findings open up new venues for studying the differential regulation and functional interactions of sense and antisense proteins in neural function and behaviour, as well as in neuropsychiatric disorders. KEY POINTS: Hippocampal overexpression of nudix hydrolase 6 (NUDT6), the antisense protein of fibroblast growth factor-2 (FGF2), increases depression-like behaviour in rats. Hippocampal NUDT6 overexpression triggers a neuroinflammatory cascade by increasing S100 calcium binding proteinA9 (S100A9) expression and nuclear NF-κB-p65 translocation in neurons, in addition to microglial recruitment and activation. Hippocampal NUDT6 overexpression suppresses neurogenesis. NUDT6 exerts its actions without altering the levels or downstream signalling pathways of FGF2.

Changes in the Concentrations of Proangiogenic Cytokines in Human Brain Glioma and Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) and glioma are some of the most common malignancies, with ALL most often affecting children and glioma affecting adult men. Proangiogenic cytokines and growth factors play an important role in the development of both of these tumors. Glioma is characterized by an extremely extensive network of blood vessels, which continues to expand mainly in the process of neoangiogenesis, the direct inducers of which are cytokines from the family of vascular endothelial growth factors, i.e., vascular endothelial growth factor (VEGF-A) and its receptor vascular endothelial growth factor receptor 2 (VEGF-R2), as well as a cytokine from the fibroblast growth factor family, fibroblast growth factor 2 (FGF-2 or bFGF). Growth factors are known primarily for their involvement in the progression and development of solid tumors, but there is evidence that local bone marrow angiogenesis and increased blood vessel density are also present in hematological malignancies, including leukemias. The aim of this study was to examine changes in the concentrations of VEGF-A, VEGF-R2, and FGF-2 (with a molecular weight of 17 kDa) in a group of patients divided into specific grades of malignancy (glioma) and a control group; changes of VEGF-A and FGF-2 concentrations in childhood acute lymphoblastic leukemia and a control group; and to determine correlations between the individual proteins as well as the influence of the patient's age, diet, and other conditions that may place the patient in the risk group. During the statistical analysis, significant differences in concentrations were found between the patient and control groups in samples from people with diagnosed glioma and from children with acute lymphoblastic leukemia, but in general, there are no significant differences in the concentrations of VEGF-A, VEGF-R2, and FGF-2 between different grades of glioma malignancy. Among individuals treated for glioma, there was no significant impact from the patient's gender and age, consumption of food from plastic packaging, frequency of eating vegetables and fruit, smoking of tobacco products, the intensity of physical exercise, or the general condition of the body (Karnofsky score) on the concentrations of the determined cytokines and receptor. The listed factors do not bring about an actual increase in the risk of developing brain glioma.

Regulation of fibroblast growth factor-2 expression and cell cycle progression by an endogenous antisense RNA

Basic fibroblast growth factor (FGF2) is a potent wide-spectrum mitogen whose overexpression is associated with immortalization and unregulated cell proliferation in many tumors. The FGF2 gene locus is bi-directionally transcribed to produce FGF2 mRNA from the “sense” strand and a cis-antisense RNA (NUDT6) from the NUDT6 gene on the “antisense” strand. The NUDT6 gene encodes a nudix motif protein of unknown function, while its mRNA has been implicated in the post-transcriptional regulation of FGF2 expression. FGF2 and NUDT6 are co-expressed in rat C6 glioma cells, and ectopic overexpression of NUDT6 suppresses cellular FGF2 accumulation and cell cycle progression. However, the role of the endogenous antisense RNA in regulation of FGF2 is unclear. In the present study, we employed siRNA-mediated gene knockdown to examine the role of the endogenous NUDT6 RNA in regulation of FGF2 expression and cell cycle progression. Knockdown of either FGF2 or NUDT6 mRNA was accompanied by a significant (>3 fold) increase in the complementary partner RNA. Similar reciprocal effects were observed at the protein level, indicating that these two transcripts are mutually regulatory. Remarkably, knockdown of either FGF2 or NUDT6 significantly reduced cell proliferation and inhibited S-phase re-entry following serum deprivation, implicating both FGF2 and NUDT6 in the regulation of cell transformation and cell cycle progression.

Regulation of fibroblast growth factor-2 by an endogenous antisense RNA and by argonaute-2

We have previously reported that elevated fibroblast growth factor-2 (FGF-2) expression is associated with tumor recurrence and reduced survival after surgical resection of esophageal cancer and that these risks are reduced in tumors coexpressing an endogenous antisense (FGF-AS) RNA. In the present study, we examined the role of the endogenous FGF-AS transcript in the regulation of FGF-2 expression in the human lung adenocarcinoma cell line Seg-1. FGF-2 and FGF-AS were temporally and spatially colocalized in the cytoplasm of individual cells, and knockdown of either FGF-2 or FGF-AS by target-specific siRNAs resulted in dose-dependent up-regulation of the complementary transcript and its encoded protein product. Using a luciferase reporter system, we show that these effects are mediated by interaction of the endogenous antisense RNA with the 3'-untranslated region of the FGF-2 mRNA. Deletion mapping identified a 392-nucleotide sequence in the 5823-nucleotide FGF-2 untranslated tail that is targeted by FGF-AS. Small interfering RNA-mediated knockdown of either FGF-AS or FGF-2 significantly increased the stability of the complementary partner mRNA, demonstrating that these mRNAs are mutually regulatory. Knockdown of FGF-AS also resulted in reduced expression of argonaute-2 (AGO-2) and a number of other elements of the endogenous micro-RNA/RNA interference pathways. Conversely, small interfering RNA-mediated knockdown of AGO-2 significantly increased the stability of the FGF-2 mRNA transcript and the steady-state levels of both FGF-2 mRNA and protein, suggesting a role for AGO-2 in the regulation of FGF-2 expression.

Complex sense-antisense architecture of TNFAIP1/POLDIP2 on 17q11.2 represents a novel transcriptional structural-functional gene module involved in breast cancer progression

Background

A sense-antisense gene pair (SAGP) is a gene pair where two oppositely transcribed genes share a common nucleotide sequence region. In eukaryotic genomes, SAGPs can be organized in complex sense-antisense architectures (CSAGAs) in which at least one sense gene shares loci with two or more antisense partners. As shown in several case studies, SAGPs may be involved in cancers, neurological diseases and complex syndromes. However, CSAGAs have not yet been characterized in the context of human disease or cancer.

Results

We characterize five genes (TMEM97, IFT20, TNFAIP1, POLDIP2 and TMEM199) organized in a CSAGA on 17q11.2 (we term this the TNFAIP1/POLDIP2 CSAGA) and demonstrate their strong and reproducible co-regulatory transcription pattern in breast cancer tumours. Genes of the TNFAIP1/POLDIP2 CSAGA are located inside the smallest region of recurrent amplification on 17q11.2 and their expression profile correlates with the DNA copy number of the region. Survival analysis of a group of 410 breast cancer patients revealed significant survival-associated individual genes and gene pairs in the TNFAIP1/POLDIP2 CSAGA. Moreover, several of the gene pairs associated with survival, demonstrated synergistic effects. Expression of genes-members of the TNFAIP1/POLDIP2 CSAGA also strongly correlated with expression of genes of ERBB2 core region of recurrent amplification on 17q12. We clearly demonstrate that the observed co-regulatory transcription profile of the TNFAIP1/POLDIP2 CSAGA is maintained not only by a DNA amplification mechanism, but also by chromatin remodelling and local transcription activation.

Conclusion

We have identified a novel TNFAIP1/POLDIP2 CSAGA and characterized its co-regulatory transcription profile in cancerous breast tissues. We suggest that the TNFAIP1/POLDIP2 CSAGA represents a clinically significant transcriptional structural-functional gene module associated with amplification of the genomic region on 17q11.2 and correlated with expression ERBB2 amplicon core genes in breast cancer. Co-expression pattern of this module correlates with histological grades and a poor prognosis in breast cancer when over-expressed. TNFAIP1/POLDIP2 CSAGA maps the risks of breast cancer relapse onto the complex genomic locus on 17q11.2.

Regulation of MYCN expression in human neuroblastoma cells

Background

Amplification of the MYCN gene in neuroblastoma (NB) is associated with a poor prognosis. However, MYCN-amplification does not automatically result in higher expression of MYCN in children with NB. We hypothesized that the discrepancy between MYCN gene expression and prognosis in these children might be explained by the expression of either MYCN-opposite strand (MYCNOS) or the shortened MYCN-isoform (ΔMYCN) that was recently identified in fetal tissues. Both MYCNOS and ΔMYCN are potential inhibitors of MYCN either at the mRNA or at the protein level.

Methods

Expression of MYCN, MYCNOS and ΔMYCN was measured in human NB tissues of different stages. Transcript levels were quantified using a real-time reverse transcriptase polymerase chain reaction assay (QPCR). In addition, relative expression of these three transcripts was compared to the number of MYCN copies, which was determined by genomic real-time PCR (gQPCR).

Results

Both ΔMYCN and MYCNOS are expressed in all NBs examined. In NBs with MYCN-amplification, these transcripts are significantly higher expressed. The ratio of MYCN:ΔMYCN expression was identical in all tested NBs. This indicates that ΔMYCN and MYCN are co-regulated, which suggests that ΔMYCN is not a regulator of MYCN in NB. However, the ratio of MYCNOS:MYCN expression is directly correlated with NB disease stage (p = 0.007). In the more advanced NB stages and NBs with MYCN-amplification, relatively more MYCNOS is present as compared to MYCN. Expression of the antisense gene MYCNOS might be relevant to the progression of NB, potentially by directly inhibiting MYCN transcription by transcriptional interference at the DNA level.

Conclusion

The MYCNOS:MYCN-ratio in NBs is significantly correlated with both MYCN-amplification and NB-stage. Our data indicate that in NB, MYCN expression levels might be influenced by MYCNOS but not by ΔMYCN.

Microgenomics profile the endogenous angiogenic phenotype in subpopulations of aggressive melanoma

Beyond the elemental role of blood vessels in tumor growth, fluid conducting networks lacking endothelium (termed vasculogenic mimicry) were identified previously in metastatic melanoma and other cancer types. The etiology remains unclear, though it appears to involve dysregulation of the tumor-specific phenotype and transdifferentiation. Instigating the molecular deciphering of this phenomenon, we established a novel technique for microdissecting the spontaneously formed vascular-like networks and the randomly arranged cells (nests) from living 3D cultures of melanoma and performed microgenomics analysis. For the first time we show that despite the shared genotype, transcription was differentially regulated among the phenotypically distinct melanoma structures in vasculogenic mimicry. Several angiogenesis-specific genes were differentially expressed in higher levels in network cells of both uveal and cutaneous melanoma with intriguing representation of the ephrin family of angiogenesis factors, which was confirmed with immunocytochemistry. Interestingly, the adjacent nest-cells over-expressed ECM-related genes. Moreover, expression of angiogenesis-specific genes in melanoma resembled that of normal microvascular cells and was enhanced in melanoma disseminating hematogenously. The findings suggest that melanoma plasticity could enable autopoiesis of vascular-mimicking elements within the tumor infrastructure with significant clinical implications, such as response to anti-angiogenic treatments. Identifying factors regulating tumor plasticity and heterogeneity at the molecular level is essential in designing effective anti-cancer therapies.

Alternative splicing and differential subcellular localization of the rat FGF antisense gene product

Background

GFG/NUDT is a nudix hydrolase originally identified as the product of the fibroblast growth factor-2 antisense (FGF-AS) gene. While the FGF-AS RNA has been implicated as an antisense regulator of FGF-2 expression, the expression and function of the encoded GFG protein is largely unknown. Alternative splicing of the primary FGF-AS mRNA transcript predicts multiple GFG isoforms in many species including rat. In the present study we focused on elucidating the expression and subcellular distribution of alternatively spliced rat GFG isoforms.

Results

RT-PCR and immunohistochemistry revealed tissue-specific GFG mRNA isoform expression and subcellular distribution of GFG immunoreactivity in cytoplasm and nuclei of a wide range of normal rat tissues. FGF-2 and GFG immunoreactivity were co-localized in some, but not all, tissues examined. Computational analysis identified a mitochondrial targeting sequence (MTS) in the N-terminus of three previously described rGFG isoforms. Confocal laser scanning microscopy and subcellular fractionation analysis revealed that all rGFG isoforms bearing the MTS were specifically targeted to mitochondria whereas isoforms and deletion mutants lacking the MTS were localized in the cytoplasm and nucleus. Mutation and deletion analysis confirmed that the predicted MTS was necessary and sufficient for mitochondrial compartmentalization.

Conclusion

Previous findings strongly support a role for the FGF antisense RNA as a regulator of FGF2 expression. The present study demonstrates that the antisense RNA itself is translated, and that protein isoforms resulting form alternative RNA splicing are sorted to different subcellular compartments. FGF-2 and its antisense protein are co-expressed in many tissues and in some cases in the same cells. The strong conservation of sequence and genomic organization across animal species suggests important functional significance to the physical association of these transcript pairs.

Selective cyclooxygenase-2 inhibition suppresses basic fibroblast growth factor expression in human esophageal adenocarcinoma

Inhibition of cyclooxygenase (COX)-2 is reported to suppress growth and induce apoptosis in human esophageal adenocarcinoma (EADC) cells, although the precise biologic mechanism is unclear. In this study we tested the hypothesis that the antitumor activity of COX-2 inhibitors may involve modulation of basic fibroblast growth factor (FGF-2), which is overexpressed in EADC. We evaluated the effects of NS-398, a selective COX-2 inhibitor, on FGF-2 expression and proliferation of EADC cell lines that express COX-2 and those that do not. We also correlated COX-2 and FGF-2 expression with clinico-pathologic findings and outcome in a well-characterized series of surgically resected EADC tissues. Seg-1 cells robustly expressed COX-2 and FGF-2, whereas Bic-1 cells expressed neither transcript. FGF-2 was reduced to undetectable levels in Seg-1 cells following NS-398 treatment, but increased within 4 h of drug removal. NS-398 significantly inhibited the growth of Seg-1 cells, and this effect was ameliorated by addition of exogenous FGF-2. In contrast, NS-398 had no effect on Bic-1 cell proliferation and FGF-2 alone had no effect on proliferation of either cell line. NS-398, or a neutralizing anti-FGF-2 antibody, induced apoptosis in Seg-1 cells, and these effects were inhibited by addition of exogenous FGF-2. COX-2 protein was strongly expressed in 46% (10/22) of EADCs, and was associated with a trend towards reduced disease-free survival. These findings indicate that the antitumor effects of COX-2 inhibition in EADC cells may be mediated via suppression of FGF-2, and that COX-2 may be a clinically relevant molecular marker in the management of human EADC.

Alternative splicing of the FGF antisense gene: differential subcellular localization in human tissues and esophageal adenocarcinoma

Overexpression of FGF-2 is associated with tumor recurrence and reduced survival after surgical resection of esophageal cancer, and these risks are reduced in tumors co-expressing the FGF antisense (FGF-AS) RNA. The aim of this study was to characterize the expression of alternatively spliced FGF-AS transcripts and encoded nudix-motif proteins in normal human tissues and in esophageal adenocarcinoma, and to correlate their expression with clinicopathologic findings and outcome. Three alternatively spliced FGF-AS transcripts encoding GFG/NUDT6 isoforms with distinct N termini were detected in various human tissues including esophageal adenocarcinoma. Expression of each isoform as a fusion protein with enhanced green fluorescent protein revealed differential subcellular trafficking: hGFGa is localized to mitochondria by an N-terminal targeting sequence (MTS), whereas hGFGb and hGFGc were localized in the cytoplasm and nucleus. Mutation/deletion analysis confirmed that the predicted MTS was necessary and sufficient for mitochondrial compartmentalization. The predominant FGF-AS mRNA expressed in esophageal tumors was splice variant b. GFG immunoreactivity was detected in the cytoplasm of all esophageal adenocarcinomas and in 88% of tumor cell nuclei. Although we found a trend towards reduced disease-free survival in patients with FGF-2 overexpressing esophageal adenocarcinomas, significantly worse disease-free survival was noted among patients whose tumors did not also overexpress the FGF-AS b isoform (p = 0.03). Tetracycline-inducible FGF-AS b expression in stably transfected human Seg-1 esophageal adenocarcinoma cells resulted in a significant suppression of steady state FGF-2 mRNA content and cell proliferation. Our data implicate the FGF-AS b isoform in modulation of FGF-2 expression and clinical outcome in esophageal adenocarcinoma.

Stress-induced glucocorticoids suppress the antisense molecular regulation of FGF-2 expression

Psychological stress can upregulate basic fibroblast growth factor (FGF-2) expression. Because glucocorticoids can also upregulate FGF-2 expression, the present studies investigated whether stress-induced glucocorticoids mediate the effects of stress on FGF-2. FGF-2 is regulated by an FGF-2 antisense (AS) molecular mechanism and so the present experiments also, for the first time, assessed the effects of stress on FGF-2-AS mRNA, as well as the mediating role of glucocorticoids. The effects of either escapable shock (ES) or yoked-inescapable tail shock (IS) on FGF-2 and FGF-2-AS were determined. To test whether glucocorticoids mediate the effect of stress on FGF-2 and FGF-2-AS, animals were pretreated with temporary corticosterone (CORT) synthesis inhibitors and exposed to IS. To test whether glucocorticoids are sufficient to modulate FGF-2 and FGF-2-AS mRNA, animals were injected with CORT and mRNA measured. ES and IS similarly downregulated FGF-2-AS mRNA at 0 h post-stress and upregulated FGF-2 mRNA 2 h post-stress. Inhibition of CORT synthesis abrogated the effect of IS on both FGF-2-AS and FGF-2 mRNA. Exogenous CORT mimicked the effects of ES and IS on FGF-2, but not FGF-2-AS mRNA. The present study demonstrates that glucocorticoids mediate the effects of stress on FGF-2 and FGF-2-AS.

The fibroblast growth factor-2 antisense gene inhibits nuclear accumulation of FGF-2 and delays cell cycle progression in C6 glioma cells

Fibroblast growth factor-2 (FGF-2) is a potent heparin-binding protein with growth-promoting and anti-apoptotic activity. Transcription of the GFG/NUDT6 gene on the opposite DNA strand generates an overlapping antisense RNA (FGF-AS) implicated in the post-transcriptional regulation of FGF-2. C6 glioma cells coordinately express FGF-2 and FGF-AS mRNA in a cell cycle-dependent manner. Cellular FGF-2 immunoreactivity was also cell cycle-dependent, with marked nuclear accumulation during S-phase. Stable transfection and overexpression of the FGF-AS RNA resulted in suppression of total cellular FGF-2, and a reduction in nuclear accumulation of FGF-2 isoforms. Serum stimulation of growth-arrested wild-type cells evoked a rapid nuclear translocation of FGF-2, and cell cycle re-entry. FGF-AS transfectants, in contrast, showed a significant delay in recovery of both nuclear FGF-2 staining and S-phase re-entry. Similar results were observed when cells were released from aphidicolin-induced G1 arrest or subjected to heat shock. These findings indicate that FGF-AS RNA inhibits expression and cell cycle-dependent nuclear accumulation of FGF-2, and this is associated with a marked delay in S-phase progression. The results suggest that the endogenous FGF antisense RNA may play a significant functional role in the regulation of FGF-2 dependent cell proliferation in FGF-2 expressing cells.

Cytotoxicity of a Novel Fibroblast Growth Factor Receptor Targeted Immunotoxin on a Human Ovarian Teratocarcinoma Cell Line

Basic fibroblast growth factor (bFGF) is an important tumor-associated growth factor that contributes to proliferation and angiogenesis of tumor. The high-affinity receptor for bFGF, fibroblast growth factor receptor (FGFR), is found to be overexpressed in a number of tumor cells. For the purpose of exploring the significance of bFGF/FGFR in tumor-targeted therapy, a recombinant immunotoxin contained the N-terminal 389 residues of diphtheria toxin (DT), and the full length of human bFGF was designed, expressed, and purified. The bioactivity of the product was evaluated by testing the cytotoxicity on PA-1 cells (a human ovarian teratocarcinoma cell line with high-level expression of FGFR) in vitro. The immunotoxin showed a significant cytotoxicity on PA-1 cells (IC(50) 8 - 9 ng/mL), and this effect could be antagonized by equine diphtheria antitoxin (DAT), bFGF, anti-bFGF monoclonal antibody (MAb), and anti- FGFR polyclonal antibody (PAb), respectively. Additionally, Chinese hamster ovary (CHO) cells and Hep-2 cells (a human epidermoid laryngocarcinoma cell line) with low expression of FGFR were tested to be resistant to the immunotoxin. The results indicated that FGFR might be an effective target for tumor therapy, and bFGF-mediated immunotoxin could be a potential candidate in the treatment of cancer.

Basic fibroblast growth factor (FGF-2) overexpression is a risk factor for esophageal cancer recurrence and reduced survival, which is ameliorated by coexpression of the FGF-2 antisense gene

PURPOSE

The basic fibroblast growth factor (FGF-2) gene is bidirectionally transcribed to generate overlapping sense and antisense (FGF-AS) mRNAs. FGF-AS has been implicated in the post-transcriptional regulation of FGF-2 expression. The aim of this study was to characterize FGF-2 and FGF-AS in esophageal cancer and to correlate their expression with clinicopathologic findings and outcome.

EXPERIMENTAL DESIGN

Reverse transcription-PCR was used to study FGF-2 and FGF-AS mRNA expression (normalized to glyceraldehyde-3-phosphate dehydrogenase) in 48 esophageal cancers relative to matched histologically normal esophageal epithelia (internal control). We used Cox proportional hazards analysis to calculate hazard ratios for recurrence and survival of patients with underexpression relative to the overexpression of FGF-2 and/or FGF-AS.

RESULTS

Overexpression of FGF-2 mRNA, by comparison with tumors underexpressing FGF-2, was associated with significantly increased risk for tumor recurrence (hazard ratio, 3.80; 95% confidence interval, 1.64-8.76) and reduced overall survival (hazard ratio, 2.11; 95% confidence interval, 1.0-4.58). When the effects of FGF-2 and FGF-AS were considered simultaneously, the association of FGF-2 mRNA overexpression with recurrence and mortality was even more pronounced, whereas FGF-AS mRNA overexpression was associated with reduced risk for recurrence and improved survival.

CONCLUSIONS

Overexpression of FGF-2 mRNA is associated with tumor recurrence and reduced survival after surgical resection of esophageal cancer and that these risks are reduced in tumors coexpressing the FGF-AS mRNA. These data support the hypothesis that FGF-AS is a novel tumor suppressor that modulates the effect of FGF-2 expression and may have potential clinical application to the development of novel therapeutic strategies.

Natural antisense transcripts: sound or silence?

Antisense RNA was a rather uncommon term in a physiology environment until short interfering RNAs emerged as the tool of choice to knock down the expression of specific genes. As a consequence, the concept of RNA having regulatory potential became widely accepted. Yet, there is more to come. Computational studies suggest that between 15 and 25% of mammalian genes overlap, giving rise to pairs of sense and antisense RNAs. The resulting transcripts potentially interfere with each other’s processing, thus representing examples of RNA-mediated gene regulation by endogenous, naturally occurring antisense transcripts. Concerns that the large-scale antisense transcription may represent transcriptional noise rather than a gene regulatory mechanism are strongly opposed by recent reports. A relatively small, well-defined group of antisense or noncoding transcripts is linked to monoallelic gene expression as observed in genomic imprinting, X chromosome inactivation, and clonal expression of B and T leukocytes. For the remaining, much larger group of bidirectionally transcribed genes, however, the physiological consequences of antisense transcription as well as the cellular mechanism(s) involved remain largely speculative.