Regulation of expression of two LY-6 family genes by intron retention and transcription induced chimerism
© Calvanese et al; licensee BioMed Central Ltd. 2008
Received: 09 May 2008
Accepted: 25 September 2008
Published: 25 September 2008
Regulation of the expression of particular genes can rely on mechanisms that are different from classical transcriptional and translational control. The LY6G5B and LY6G6D genes encode LY-6 domain proteins, whose expression seems to be regulated in an original fashion, consisting of an intron retention event which generates, through an early premature stop codon, a non-coding transcript, preventing expression in most cell lines and tissues.
The MHC LY-6 non-coding transcripts have shown to be stable and very abundant in the cell, and not subject to Nonsense Mediated Decay (NMD). This retention event appears not to be solely dependent on intron features, because in the case of LY6G5B, when the intron is inserted in the artificial context of a luciferase expression plasmid, it is fully spliced but strongly stabilises the resulting luciferase transcript. In addition, by quantitative PCR we found that the retained and spliced forms are differentially expressed in tissues indicating an active regulation of the non-coding transcript. EST database analysis revealed that these genes have an alternative expression pathway with the formation of Transcription Induced Chimeras (TIC). This data was confirmed by RT-PCR, revealing the presence of different transcripts that would encode the chimeric proteins CSNKβ-LY6G5B and G6F-LY6G6D, in which the LY-6 domain would join to a kinase domain and an Ig-like domain, respectively.
In conclusion, the LY6G5B and LY6G6D intron-retained transcripts are not subjected to NMD and are more abundant than the properly spliced forms. In addition, these genes form chimeric transcripts with their neighbouring same orientation 5' genes. Of interest is the fact that the 5' genes (CSNKβ or G6F) undergo differential splicing only in the context of the chimera (CSNKβ-LY6G5B or G6F-LY6G6C) and not on their own.
In the post-genomic era biological endeavours are more and more centred on understanding the different mechanisms of regulation of gene expression. An increasing number of interacting regulatory levels are being explored and, in this amazing landscape, alternative splicing is even more interesting because, starting from a relatively limited number of genes, it is involved in increasing proteome complexity, [1–3]. In relation to this, alterations of splicing patterns or mis-splicing of genes are involved in several pathologies, [4–6] including several genetic diseases such as spinal muscular atrophy (SMA), myotonic dystrophy (MD), Alzheimer's disease (AD), and retinitis pigmentosa (for review see ). Aberrant splicing has also been linked to cancer ( and refs).
The human Major Histocompatibility Complex (MHC) is located at chromosome 6p21.3, and is ~4 Mb in length. It consists of three regions, the class I and class II regions flanking the central class III region. The class III region is ~0.9 Mb in length and contains 62–64 genes and 2–3 pseudogenes, depending on the haplotype [9, 10]. Of the predicted genes, at least 24 (41%) have a definite or potential role in the immune system. The human MHC has been linked to susceptibility to many diseases, and often these associations cannot be fully explained by variation in the class I and II genes [10, 11]. Therefore, the study of the class III region genes, especially the novel genes with a potential role in the immune system, may provide insights into the understanding of these diseases. Transcriptome studies of some MHC class III region genes indicate a high rate of different splicing events. Previously, we have defined precisely the alternative splicing patterns of a cluster of five genes of the Lymphocyte antigen-6 (LY-6) superfamily  and characterised the expression of the corresponding proteins . Strong associations have been found between Rheumatoid Arthritis and the segment of the MHC class III region which includes these LY-6 members. The characterisation of these transcripts is of great relevance for the understanding of human diseases.
LY-6 superfamily members are cysteine-rich, generally GPI-anchored, cell surface proteins which have definite or putative immune-related roles . Among these LY-6 MHC class III region genes LY6G5B and LY6G6D showed a particular behaviour in the regulation of their expression , involving an intron retention event. The intron retained is the first in the open reading frame and interrupts the protein just after the signal peptide introducing a premature stop codon. The presence of a premature block to transcription in this position should cause this intron-retaining transcript to undergo Nonsense Mediated Decay (NMD) [15–17]. However, this transcript is present and is generally more abundant than the correctly spliced partner in all cell lines and tissues analysed . Intron retention is the least characterised event of all alternative splicing types, mainly because of the exclusion of this phenomenon in many studies, due to the difficulty to differentiate it from genomic DNA or incompletely-processed transcripts. Moreover, it is not relevant to functional studies due to the introduction of premature stop codons. A number of studies indicate that up to 15% of human genes present at least one intron retention event, and that at least 22% of all informative intron-retention events are also present in the mouse transcriptome . Finally, many intron retention events occur in the 5' and 3' Untranslated Regions (UTR) , that are still incompletely characterised for most genes.
Interestingly, we were also able to detect the presence of the exons of the LY6G5B and LY6G6D genes in transcripts derived from the upstream genes in the chromosome. This phenomenon, known as Transcription Induced Chimerism (TIC), or Tandem Chimerism is still largely unknown in its mechanism, but it is being promoted as a novel way to increase combinatorial complexity of the proteome [19–21]. Recent bioinformatics analyses, partially supported by experimental validation, show that this phenomenon could be quite frequent (up to 4–5% of the tandem gene pairs in the human genome) . There are also cases of TIC described in which a chimeric protein can be detected or a logical function inferred [22–24].
Here we report a precise description and quantification, of the transcripts generated by intron retention events, of the MHC LY6G5B and LY6G6D genes. As these transcripts have a premature stop codon, they should be degraded quickly by Non-sense Mediated Decay. Nevertheless, they seem to be stable and even the most abundant transcript, especially in tissue samples. This could indicate that these mis-spliced forms are real transcripts which could have potential regulatory functions. In addition, we show that the LY6G5B and LY6G6D genes can form chimeric transcripts with adjacent genes.
LY6G5B and LY6G6D transcript expression
Transcript localisation, mRNA stability and NMD escape
Treatment with Actinomycin D indicated that these mis-spliced forms were stable in the cytoplasm, though there did appear to be cell-specific differences in stability for the LY6G6D gene, which seemed to be unstable in the K562 cell line, but stable in the Raji cell line (Figures 2C and 2D). Interestingly, the mis-spliced (334 bp) form of the LY6G5B gene was also more abundant in the cytoplasmic RNA fraction of Raji cells, relative to the correctly spliced form (187 bp), compared to K562 cells (Figures 2C and 2D). In addition, there appeared to be no differences in stability between the mis-spliced and correctly spliced forms of LY6G5B as there was no obvious decay of either form after transcription was stopped. For LY6G6D we were only able to amplify the mis-splced 269 bp form, and not the 178 bp properly spliced form (Figures 2C and 2D). As control RNAs we used the housekeeping genes β-actin and GAPDH for evaluating abundance and RNA quality and the β-globin RNA as a control of a stable RNA transcript (Figure 2A) observing that the RNA was not degraded by the treatment or by the RNA isolation procedure. We also used c-Myc as a control for unstable mRNA transcripts and the intron of c-Myc as a control for genomic contamination of the cytoplasmic RNA fraction (Figure 2B). The decay of the c-Myc transcript indicated that the Actinomycin D treatment was effective and the lack of c-Myc intronic product in the cytoplasmic RNA fraction showed there was no genomic contamination in the cytoplasmic fraction. The β-globin amplification showed genomic, or splicing intermediates, in the nuclear RNA fraction (upper band of 440 bp) in addition to the correctly spliced form (lower band of 320 bp), while in the cytoplasmic RNA fraction we could only detect the correctly spliced form (320 bp), indicating no genomic contamination in the cytoplasmic fraction. In the case of β-actin and GAPDH only the correctly spliced forms of 548 bp and 612 bp, respectively, were obtained in the amplifications of the nuclear and cytoplasmic RNA fractions. No genomic products (which would be 1123 bp and 2858 bp, respectively) were observed in either RNA fraction. All the controls were also performed with Raji cell extracts and the same results were obtained (data not shown). The results from all these experiments indicate that the unspliced LY6 transcripts are real transcripts and not due to genomic contamination.
As the intron-retaining transcripts of the two genes have a PTC they should be subjected to degradation by the NMD machinery. To see whether this process acts on the non-coding transcripts we looked at the effect of translational blockage on stability, as translation of the mRNA has been shown to be required for NMD, probably for recognition of PTCs. Cycloheximide inhibits the peptidyl-transferase on the large subunit of the eukaryotic ribosome, while puromycin is a tRNA analogue that causes premature chain termination. If the NMD pathway was in some way acting on the mis-spliced transcript, we would expect to see an increase in the intron-retaining form relative to that of the correctly spliced form (observed as an increase in PCR product). K562 cells were treated with either cycloheximide or puromycin and showed no increase in stability of the mis-spliced forms of LY6G5B and LY6G6D relative to the correctly spliced forms, suggesting that these mis-spliced transcripts are not subject to NMD (Figures 2E and 2F).
EST analysis of the LY6G5B and LY6G6D genes
To better define the expression pattern of these genes in order to characterise the UTRs and to support our data on intron retention in the expressed transcripts, we performed a detailed EST analysis on the two genes. We previously  presented an EST analysis, but at that time only two ESTs were found for each of the two human genes (Acc. no AI800033 and AA535815 for LY6G6D; AI446559 and R79468 for LY6G5B), making it difficult to completely define the gene. For LY6G6D there is only one additional EST from the 2002 analysis  that aligns with the whole intron-retaining transcript; while the previously described ESTs only cover exon 3 of the gene. Thus, there is still no EST corresponding to the correctly spliced and protein expressing form, despite the fact that we could find the correctly spliced form in many human tissues . This is most likely due to the small number of ESTs, especially for low expression transcripts, and to the limitations of the EST data bank that often presents only partial sequences.
NO. Fig 5
G5B real time_spl
G5B real time_ret
To examine the expression patterns of the G6F/LY6G6D chimera, two different RT-PCR reactions were performed in a number of different human cell lines, one amplifying from exon 1 of G6F to exon 3 of LY6G6D, and the other amplifying from exon 4 of G6F to exon 3 of LY6G6D, to see whether a shorter transcript was also present (Figure 7A) as exon 4 of G6F, which normally codes for the transmembrane region of the protein, could act as a signal peptide. Southern blot analysis (Figure 7B) showed that there are different transcripts produced from the G6F exon 1 – LY6G6D exon 3 amplification (long chimera). A ~1.2 kb doublet is present mainly in the K562 cell line as well as a faint band at ~900 bp and a band at ~500 bp (Figure 7B). The 1.2 kb band is also present faintly in the HL60, Jurkat and Molt4 cell lines corresponding to the expected transcript, consisting of exons 1–4 of G6F, spliced to exons 2 and 3 of LY6G6D (1148 bp). This would translate to give a chimeric transcript containing the Ig domain and the transmembrane region of G6F fused to the LY-6 domain of LY6G6D. The upper band of the ~1.2 kb doublet consists of exons 1–5 of G6F spliced to exons 2 and 3 of LY6G6D (1248 bp) which translate to a G6F protein including the transmembrane domain and a cytoplasmic domain due to an inframe amino acid sequence encoded by exons 2 and 3 of LY6G6D. Other secondary bands have been cloned and are represented in figure 7A, but none retain the LY-6 ORF.
Splice variants of the expected ~450 bp shorter form of the chimera (exon 4 of G6F and exons 2–3 of LY6G6D) were seen in all cell lines except HeLa, with the strongest signal in the K562, Jurkat, Molt4, and 143B cell lines (Figure 7B). After sequence analysis three different splice variants were found; including the expected form of 453 bp which results in a signal peptide followed by the LY-6 domain. Other secondary bands are shown in the figure but they lose the LY-6 ORF and do not match with any known or predicted protein.
Discussion and conclusion
During the last fifteen years a major effort has been centred on describing the number and the position of all human genes and this aim has been achieved through a combination of advances in sequencing technologies and bioinformatics prediction programs. Nevertheless, there are still a considerable number of genes that remain uncharacterised, and even more where we know virtually nothing of their transcriptional control including their differential tissue expression and splicing regulation. In addition, bioinformatics tools are becoming very accurate in organising biological information and predicting the structure and function of genes, but sometimes adjustments in data processing can affect the conclusions which can be drawn. For instance, in the case presented here, the fact that non-coding transcripts sometimes are excluded from analyses and databases, to avoid the risk of considering non-fully processed intermediate transcripts, could cause a significant underestimation of the frequency of intron retention transcripts.
Alternative splicing is understood as a regulatory process, contributing to biological complexity through its ability to control the expression of proteins. An mRNA variant has been defined as being 'functional' if it is required during the life-cycle of the organism and activated in a regulated manner. In some cases, functional splice forms may not even be required in their own right, but their production is required to regulate active protein levels. Moreover, the meaning of 'required' can be generalized by defining functional splicing as that which conveys a selective advantage .
The extent to which splicing has a role in disease, as either a direct cause, a modifier or a susceptibility factor, continues to be defined. Advances in several areas will clarify the roles of splicing in disease and reveal the mechanisms involved, and will allow routine application of the knowledge gained toward diagnosis and treatment. One major advance will be to develop the ability to predict splicing outcomes associated with genetic variants and disease-causing mutations . Important insights will be gained from the full characterization of the human transcriptome, which would provide a catalogue of all the splice variants expressed from each gene and identification of the isoforms that predominate in specific cell types and tissues. This is a significant challenge that will have a huge benefit, not least being the ability to design microarrays that can be applied for the quantitative assessment of all splice variants. Finally, another key direction for the future will be the application of genome-wide microarray assays to assess splicing differences associated with normal variation and with disease. Alternative-splicing signatures are likely to provide a useful diagnostic and prognostic tool for many diseases. As for other challenges, the tools that are required to meet this challenge are developing rapidly .
Here we have presented a detailed transcriptional analysis of the LY6G5B and LY6G6D genes. The striking feature which was observed in a first analysis was the intron retention event that generated a non-coding transcript. In fact, the protein coding transcript of these genes is the one described in databases, but we found that, by the retention of the first intron, its expression is nearly completely silenced. This event seems not just to be an inherent feature of the introns because they are spliced correctly when the introns are inserted in an artificial construct, such as the luciferase expression plasmid used in this study, although they are also able to markedly stabilise the artificial transcript. In order to be retained the introns must be in the genomic environment of their particular Ly-6 genes, and for this reason might have a regulatory role in these genes. In addition, by real time RT-PCR we found that the retained and spliced forms are differentially expressed in tissues, indicating an active regulation of the non-coding transcript. We propose that when the gene is expressed the intron retention event could be regulated by a cis-acting element, acting on the processing machinery. It could be possible that this block in the expression is released only in a precise physiological response or at a particular developmental stage or in a specific pathologic process. Related to this, Yan et al. (2005)  showed a novel gene Saf, transcribed from the opposite strand of a noncoding intronic region of the Fas gene, that acts in cis and regulates alternative splicing forms of Fas. In addition, noncoding RNAs are abundantly transcribed from the introns of 74% of all RefSeq genes  and could be involved in regulation of alternative splicing in response to physiological and pathologic conditions.
Another possibility is that the chimera represents an alternative way for the expression of the protein, that borrows the promoter of the preceding gene to be expressed and then by a combination of transcript and protein processing reaches its final expressed form. In the case of the G6F-LY6G6D chimera, for example, the MEGT-1 protein would encode two extracellular Ig domains, a transmembrane segment and then the LY6G6D LY-6 domain intra-cellularly. Interestingly, the fact that exon 4 of G6F could encode a signal peptide starting with a methionine residue and that there is a long intron between exons 3 and 4 in G6F, suggests the presence of a promoter region specific for the shorter transcript. The possibility of an alternative promoter in this case is also supported by the expression results, as the short chimera seems to be expressed in nearly all analysed cell lines while the long chimera is expressed mainly in the K562 cell line, which is also the only cell line that expresses G6F.
The particular behaviour of these genes could be more general, strengthening the importance of a detailed transcriptional analysis of every gene, because their physiological and pathological roles could be based on unexpected forms of expression regulation. Susceptibility to human diseases is associated with genes in the MHC class III region. Microsatellite and SNP genotyping studies have attempted to fine map the location of these genes, finding strong associations between Rheumatoid Arthritis and a 126 Kb region in the MHC class III region, which include these LY-6 members. The characterization of the LY-6 transcripts is of great relevance for the understanding of human diseases.
Cell cultures and treatments
The cell lines Jurkat (T cell), U937 (monocyte), Raji (B cell), HL60 (monocyte), Molt4 (T cell), and K562 (undifferentiated erythroleukaemia) were grown in RPMI medium while Hek293T (embryonic kidney), HeLa (epithelial), 143B (TK-) (osteoblast) were grown in DMEM, all with 10% (v/v) foetal bovine serum (FBS), 100 IU/ml penicillin and 100 μg/ml streptomycin. Cells to be treated with Actinomycin D (5 μg/ml), Puromycin or Cycloheximide (both 300 μg/ml) were plated at 5 × 106 cells and grown for 24 hours prior to the addition of the reagent. Cells were incubated with the reagent for 0, 30 min, 60 min, 120 min and 240 min prior to harvesting.
RNA extraction, RT-PCR and Real Time-PCR
The SV RNA isolation kit (Promega) was used for RNA isolation from frozen pellets containing 5 × 106 cells followed by DNAse treatment. Human tissue RNAs were obtained from BioChain® (USA) http://www.biochain.com through one of their Europe distributor "ams" http://www.amsbio.com (UK). One μg of total RNA obtained from each sample was used for oligo-dT primed cDNA synthesis which was performed using the ImProm Reverse Transcription System (Promega) in a 20 μl reaction volume following the manufacturer's instructions. Initial experiments were performed to check for the presence of these gene transcripts in the cytoplasmic fraction of the cell and to optimise the fractionation procedure (data not shown). The fractionantion was performed using the Qiagen RNeasy mini kit cytoplasmic RNA extraction protocol. We improved the method for our cell types, by using 5 × 106 K562 cells and 1 × 107 Raji cells. We spun intact cells at 2000 rpm and lysed them in modified RLN buffer (10 mM Tris pH8, 100 mM NaCl, 1.5 mM MgCl2, 0.5% NP40, 1000 U/ml RNasin and 1 mM DTT). The cytoplasmic-nuclear fraction was separated by spinning at 1800 rpm for 2 minutes. The cytosolic fraction was taken and RLT buffer added and processed as described in the Qiagen protocol. To the nuclear fraction was also added FLT buffer and processed as described in the protocol. For Reverse Transcription-PCR (RT-PCR) 1 μl of cDNA was used in each PCR reaction. All RT-PCR reactions contained 2 mM MgCl2, 0.8 mM dNTPs, 0.4 μM each primer and 0.75 U Taq polymerase (Roche) in a 25 μl reaction volume. The PCR conditions were as follows: 95°C for 2 min followed by 35 cycles of 95°C for 45 s, 60°C for 30 s, 72°C for 30 s, followed by 72°C for 5 min. The primers used for b-actin were PR_13 and PR_14, for b-globin PR_15 and PR_16, for GAPDH PR_17 and PR_18, for cMyc PR_19 and PR_20, and for cMyc-inronic PR_21 and PR_22. The primers used for the amplifications of LY6G6D were PR_8 ad PR_12 and for LY6G5B were PR_1 and PR_5.
Real-time RT-PCR for LY6G5B was performed by using SYBR® green PCR master mix and the ABI PRISM® 7700 sequence detection system (Applied Biosystems). Primers for real-time RT-PCR were designed for the differential quantification of the intron retention event, with a common forward primer and two reverse primers, one spanning the exon-intron junction for the intron retaining form and the other spanning the exon-exon junction for the correctly spliced form (PR_1, 6 and 7, respectively). As the intron retaining transcript cDNA does not differ at all from the genomic DNA and the correctly spliced form differs for only 148 bases of the intron, we always performed a RT minus (RT-) reaction for each sample (a reverse transcription reaction identical to the one described in the previous section, but without adding the reverse transcriptase) to check for lack of amplification, or consistently later amplification than the corresponding RT-treated sample (more than ten cycles of difference). Quantifications were always normalised using endogenous control GAPDH (PR_23 and 24) or β-actin (PR_25 and 26). To compare levels of the two isoforms in the same sample we had to perform an absolute quantification of the two isoforms in each sample . To achieve this, we generated standard dilution curves. The two splicing isoforms of the LY6G5B gene were first amplified by nested PCR from K562 cDNA with the primers PR_1, 2, 3 and 4, then purified from a gel and cloned into the pGEM-T plasmid (Promega). The plasmid DNA was isolated, sequenced, then quantified precisely and diluted to the same copy number per millilitre. Serial dilutions of 1 in 5 volumes were then run in a real time RT-PCR assay with the primers PR_1, 6 and 7 generating the final curve to interpolate results from the cDNA samples.
The control plasmid pGL3 (Promega) was modified to contain an insertion site (Pst I and Eco RV) in the 3'UTR of the firefly luciferase open reading frame (ORF). This was used as the starting plasmid for all the constructs in Figure 4A. Constructs containing introns were made by PCR amplification of the intron from genomic DNA using gene specific primers containing different restriction sites: Hin dIII for the LY6G5B intron (PR_27, 28), Hin dIII and Nco I for the LY6G6D intron (PR_30, 31) for the 5'UTR cloning, and Pst I and Eco RV for the 3'UTR cloning (PR_32, 33, 34, 35). Either pGL4.11 or pHRL (Promega) containing the renilla luciferase ORF were used as control for transfection efficiency. In order to be able to determine the role of the 5' flanking region in the intron of LY6G5B, and to be able to look at the splicing of the intron by RT-PCR, we created another construct by inserting the synthetic oligonucleotide (PR_36, 37), representing the last 25 bases of the LY6G5B exon 1, in the Hin dIII restriction site of the pGL3 control plasmid described above. The same insert of the LY6G5B intron was inserted in this new control plasmid in both orientations as well as a mutated version of the intron where the GT...AG boundaries were mutated to CA...GA with the primers PR_38 and PR_39 and Hin dIII digestion. This plasmid was used as control for intron retention. All constructs were sequenced with primers PR_40 and PR_41 to confirm the presence of the correct sequence. For the transfection Hek293T cells (1 × 106) were seeded in 60 mm dishes and 24 hours later were transfected with 2 μg DNA (Luciferase plasmid and Renilla plasmid in a 20:1 quantitative proportion), using PolyFect (Qiagen) following the manufacturer's instructions. Two days after the transfection, cells were harvested in PBS, then a tenth of the total cells for each plate were transferred to a 96-well plate in duplicate and luciferase and renilla activity were sequentially measured using the Dual-Glo™ Luciferase Assay System (Promega) kit. The remaining cells were pelleted and the RNA extracted to be quantified by Real Time-PCR as described above.
EST Data-base analysis
The EST analysis was performed for the two genes by submitting the sequence of each exon and of the first intron to a BLAST analysis in the human EST database at the National Center for Biotechnology Information (NCBI) http://www.ncbi.nlm.nih.gov/BLAST, and the matching EST clones identified and aligned.
Nested RT-PCR and Southern blot analysis for chimeric transcripts
For the CSNK2B-LY6G5B chimera the first round of PCR was performed with external primers in exons 1, 5 or 6 (PR_42, 44, 46) of CSNK2B and in the 3'UTR of LY6G5B (PR_4), and the second round using primers just downstream of the first round ones (PR_43, 45, 47 and 2). PCR reagents and conditions were the same as described above. For the second round 1 μl of a 1:10 dilution of the first round product was used as template. The PCR products were gel purified, cloned and sequenced (at least three clones for each band) then aligned to the genomic sequence. For the G6F-LY6G6D chimera the first round of PCR was performed with external primers in exons 1 or 4 of G6F (PR_50, 52) and in the 3'UTR of LY6G6D (PR_11), and the second round using primers just downstream of the first round ones (PR_51, 53 and 9). To verify that the products obtained were specific Southern blot analysis was performed. Specific products were detected using a probe consisting of exon 3 and the 3' UTR of LY6G6D, created by digesting IMAGE clone 2321242 (Accession number AI800033). The probe was labeled with fluorescein-11-dUTP as detailed in the manufacturer's protocol for ECL random prime labelling and detection systems, version II (Amersham Life Science). The signal was detected by Enhanced Chemiluminescence (ECL) and visualised by autoradiography. The PCR products of the K562 cell line for the PR_51 and 9 amplification (long chimera) and of the HL60 cell line for the PR_53 and 9 amplification (short chimera) were also cleaned (Qiagen PCR columns) and subcloned into the pGEM-T vector for sequencing.
The results are expressed as mean ± s.e.m. of the number of experiments indicated in the figure legends. The data were analysed by ANOVA and a probability level of P < 0.05 was considered to be statistically significant.
We would like to thank Chris Sanderson and Chris Hunter for their incredible help developing the luciferase assays and Francisco Iborra for all his help on the NMD work. Fernando Carrasco and Aitor Sanchez, from the genomics service at the CBMSO performed the real time assays. We thank the Fundación Ramón Areces for Departmental support.
Medical Research Council, Ministerio de Educación y Ciencia (Plan Nacional BFU2005-03683), Comunidad de Madrid (200620M078, and GR/SAL/0670/2004). VC was supported by a FPU (Ministerio de Educación y Ciencia, Spain) studentship. MM was supported by a MRC Studentship. BA holds a Ramón y Cajal Programme Fellowship.
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