Functional complementation of RNA interference mutants in trypanosomes
© Rusconi et al; licensee BioMed Central Ltd. 2005
Received: 10 November 2004
Accepted: 09 February 2005
Published: 09 February 2005
In many eukaryotic cells, double-stranded RNA (dsRNA) triggers RNA interference (RNAi), the specific degradation of RNA of homologous sequence. RNAi is now a major tool for reverse-genetics projects, including large-scale high-throughput screens. Recent reports have questioned the specificity of RNAi, raising problems in interpretation of RNAi-based experiments.
Using the protozoan Trypanosoma brucei as a model, we designed a functional complementation assay to ascertain that phenotypic effect(s) observed upon RNAi were due to specific silencing of the targeted gene. This was applied to a cytoskeletal gene encoding the paraflagellar rod protein 2 (TbPFR2), whose product is essential for flagellar motility. We demonstrate the complementation of TbPFR2, silenced via dsRNA targeting its UTRs, through the expression of a tagged RNAi-resistant TbPFR2 encoding a protein that could be immunolocalized in the flagellum. Next, we performed a functional complementation of TbPFR2, silenced via dsRNA targeting its coding sequence, through heterologous expression of the TbPFR2 orthologue gene from Trypanosoma cruzi: the flagellum regained its motility.
This work shows that functional complementation experiments can be readily performed in order to ascertain that phenotypic effects observed upon RNAi experiments are indeed due to the specific silencing of the targetted gene. Further, the results described here are of particular interest when reverse genetics studies cannot be easily achieved in organisms not amenable to RNAi. In addition, our strategy should constitute a firm basis to elaborate functional-dissection studies of genes from other organisms.
RNA interference (RNAi) can be triggered by introduction of long double-stranded RNA molecules (dsRNAs) in cells , and proceeds in a number of sequential steps, starting with the cleavage of long dsRNAs into shorter ≈ 21–23 nucleotide-long dsRNAs called short interfering RNAs (siRNAs; these were initially discovered in plants ). The enzyme responsible for this chopping (DICER; [3, 4]) displays RNase III activity, producing characteristic siRNAs with a phosphorylated 5' end and a two nucleotide-overhanging 3'OH end. These siRNAs enter an RNA-induced silencing complex, or RISC [5, 6]. A helicase activity unwinds the two strands of the siRNA, and RISC scans the mRNAs in the cytoplasm and cleaves the molecules that are found complementary to the RISC-contained siRNA .
RNA-silencing processes have been described in a variety of organisms: post-transcriptional gene silencing in plants [7, 8], quelling in fungi , homology-dependent gene silencing in ciliates , or RNA interference in worms , flies [11, 12], trypanosomes [13, 14] and mammals [15, 16]. It is thought that this machinery has evolved to protect cells against undesirable RNAs, like RNA viruses in plants [17, 18], or to limit the mobility of transposable elements in animals [19–21].
While RNAi and associated phenomena constitute exceptional recent basic science findings, they also provided a basis for the elaboration of powerful research tools. RNAi methodologies have been set up to perform reverse-genetics studies in a number of organisms. RNAi potency and flexibility have allowed to perform high-throughput genetic screens in several organisms [22–26]. In mammalian cells, the presence of long dsRNA (>50 base pairs) triggers the activation of sequence-unspecific interferon-related pathways [27–29]. To circumvent this difficulty, researchers resorted to the transfection of small interfering RNAs  or in vivo synthesis of small hairpin RNAs, which were demonstrated to produce gene-specific silencing [27, 30, 31]; reviewed in [32, 33].
However, an siRNA might trigger a number of potential unspecific events such as the degradation of partially complementary mRNA due to cross-hybridization, leading to unspecific RNAi, or the translational arrest due to a micro RNA-like effect where an siRNA hybridizes to a mRNA with one or few mismatches. It is thus of paramount importance to ensure that the phenotypic effects observed as a result of siRNA presence in cells are due to silencing of the target gene only. Two large-scale studies show that siRNA-induced gene silencing of transiently- or stably-expressed mRNA is highly gene-specific and does not produce secondary effects detectable by genome-wide expression profiling [34, 35]. In contrast, other works provided evidence that siRNAs can be target-unspecific, with the observation of silencing of genes that had limited sequence homology with the siRNA [36, 37]. These reports should prompt scientists to assess the specificity of RNAi-silencing in any experiment. A solution to that problem, that we devised in trypanosomes and which is described in this report, is based on the rescueing of the RNAi-mediated loss-of-function phenotype by expressing an RNAi-resistant version of the target gene.
Trypanosomes are protozoan parasites belonging to the Kinetoplastida order. These unicellular flagellated organisms diverged very early in eukaryotic evolution, and exhibit a number of original features [38–40]. Trypanosomes were amongst the first organisms where RNAi was discovered [13, 14], and a number of strategies have been devised to either transiently or permanently induce gene-specific RNAi-silencing in these cells [14, 41–43]. Examples of successful RNAi in trypanosomes used flagellar genes as targets which yielded easily monitored phenotypes . From a structural point of view, the most conserved morphological feature of eukaryotic flagella is the axoneme, which is made of nine doublets of outer microtubules plus 2 central microtubules (so-called 9+2 axonemal structure). In trypanosomes, the flagellum not only has that axone-mal structure, but it also has a lattice-like structure called the paraflagellar rod (PFR) that is positioned along the axoneme. The two main components of the PFR are TbPFR2 and TbPFR1, that share 60% primary sequence identity [45, 46]. TbPFR2 silencing leads to flagellar paralysis and trypanosomes do not swim anymore [13, 47]. During the cell cycle, the cell first replicates its mitochondrial DNA (kinetoplast) and starts to grow a new flagellum whilst maintaining the old flagellum in place. Hence, a trypanosome which has two kinetoplasts and two nuclei will be close to completion of its cell cycle and will possess an old and a new flagellum . This aspect is an interesting feature for RNAi-based studies of flagellar morphogenesis, because bi-flagellated cells have an "internal control" flagellum (the old one), while the new one has a phenotype corresponding to the RNAi-based gene knock-down. The presence of both the old and the new flagella in the same cell gives an indication of the time course of events when RNAi is induced in trypanosomes, leading to the appearance of a visible phenotype in the new flagellum while the older one is unchanged because it is not affected by gene silencing.
We previously established the degree of identity between the gene sequences capable of leading to cross-RNAi [47, 49]. However, as mentioned earlier, each time a phenotype is observed in RNAi experiments, it is necessary to ensure that it is indeed due to the specific silencing of the targeted gene(s). Inspired by the procedure with which gene knock-out is usually performed (the control experiment is done by re-introducing the knocked-out gene to ensure that the lost function gets complemented), we devised a functional complementation strategy aimed at assessing that RNAi indeed targets the intended gene. This strategy, elaborated using the TbPFR2 gene as a model system, involved the silencing of the TbPFR2 target via its UTRs and the expression of a RNAi-resistant copy of the targetted gene. The RNAi-resistant gene was either a copy of TbPFR2 with different UTRs or its Trypanosoma cruzi orthologue: TcPFR2. We found that inter-species complementation experiments were straight forward. This strategy opens a venue for functional gene dissection experiments where modified gene sequences can be tested for their ability to encode functional protein that can complement the RNAi-based loss-of-function phenotype.
Results and discussion
Multiple RNAi on trypanosomes
We wanted to establish if the co-transfection of two distinct dsRNAs, targeting two different genes, could trigger their simultaneous silencing. The genes selected were TbPFR2 and FLA1; TbPFR2 encodes one of the two major components of the paraflagellar rod and is necessary for flagellum motility ; FLA1 encodes a protein required for flagellum attachment to the cell body . These dsRNAs were transfected simultaneously in wild-type trypanosomes. As a control experiment, we used GFP dsRNA.
Silencing effciencies after transfection of various dsRNAs. WT trypanosomes were transfected with TbPFR2, FLA1 or GFP dsRNA.
61.2 (n = 1069)
74.2 (n = 1069)
56 (n = 586)
54 (n = 472)
TbPFR2 + GFP
56.2 (n = 301)
70.9 (n = 243)
TbPFR2 + FLA1
53.6 (n = 349)
69.7 (n = 210)
59 (n = 349)
53 (n = 210)
The transfection of TbPFR2 dsRNA yielded potent silencing, as more than 60 % of the cells showed no staining for L8C4 15 h later. Since old flagella pre-exist in cells which were affected by RNAi at the beginning of cell replication, the real percentage of silenced cells is probably higher than 60 %, which is confirmed by the fact that it built up to more than 74 % at time point 22 h. The transfection of FLA1 dsRNA produced a phenotype in which the flagellum was detached from the cell body in more than 50 % of the cells. When both dsRNAs were co-transfected, both phenotypes were indeed observed, with similar frequencies to experiments where only one dsRNA was transfected. All the transfected cell populations did show a comparable growth rate (data not shown). The trypanosomes shown in Figure 2 had been transfected with both dsRNAs and the cell on the right is starting cytokinesis. The old flagellum of that cell is detached, while the new flagellum is attached along the cell body. The new flagellum exhibits a dilation of its distal tip, probably corresponding to the accumulation of TbPFR1, that is not assembled but still transported to the distal tip of the flagellum in the absence of TbPFR2 . This observation demonstrates the usefulness of double-transfection experiments also for kinetics analysis. In our case, 22 h after dsRNAs transient transfection, the phenotype due to the FLA1 silencing is no longer visible in the new flagellum while that same flagellum still exhibits the phenotype due to the TbPFR2 silencing, clearly indicating different turn-over for TbPFR2 and FLA1 proteins.
The RNAi machinery could cope with two different dsRNA populations, without – in our conditions – any visible saturation effect. These results show the feasibility of experiments involving the use of multiple dsRNAs, thus allowing studies on complex processes in the cell physiology. However, such complex experiments can only be envisaged after ensuring that the phenotypes resulting from RNAi are specifically due to silencing of the target gene. In order to address that specific problem, we elaborated a method that involves RNAi experiments on trypanosomes that were engineered to possess an extra RNAi-resistant copy of the targeted gene, leading to functional complementation.
Gene silencing by dsRNA targeting UTRs
Silencing effciencies after transfection of various dsRNAs.
TbPFR2 silencing in the new flagellum (% bi-flagellated cells)
TbPFR2 UTRs MIX
88 % (n = 53)
62 % (n = 147)
68 % (n = 113)
2 % (n = 51)
78 % (n = 100)
72 % (n = 100)
When WT trypanosomes were transfected with TbPFR2 dsRNA complementary to only one UTR, the cells did not display any specific phenotype (data not shown). This observation is probably explained by the organization of the TbPFR2 locus: the polycistronic transcript is rapidly spliced into three different types of mRNA, each encoding one of the four copies of TbPFR2 [51–53]. Thus, even if one type of TbPFR2 RNA is destroyed, the three remaining ones would likely provide enough RNA to synthesize TbPFR2 levels compatible with normal PFR formation.
To demonstrate that the silencing observed upon transfection of WT trypanosomes with the UTRs MIX dsRNAs was due to the actual targeting of TbPFR2, we used two cell lines expressing a supplementary tagged TbPFR2 gene copy. The TbPFR2tag cell line expresses the TbPFR2-TAG protein which correctly localizes to the flagellum.
The TbPFR2tag-ΔHLA cell line expresses TbPFR2-TAG-ΔHLA, lacking the HLA tripeptide, which prevents its localization to the flagellum. To determine both the cellular localization of the tagged TbPFR2 proteins (TbPFR2-TAG and TbPFR2-TAG-ΔHLA) and the completeness of the PFR assembly, immunofluorescence experiments were carried out with the BB2 and ROD-1 antibodies; the former recognizes the Ty-1 epitope tag present on the two tagged TbPFR2 proteins , while the latter is a marker for full PFR assembly [55, 56].
We next compared TbPFR2tag trypanosomes after transfection with either CDS dsRNA or UTRs MIX dsRNAs. TbPFR2tag cells transfected with CDS dsRNA had a flagellum not (or faintly) decorated with the anti-TbPFR2 antibody, demonstrating that both the WT and the recombinant TbPFR2 gene copies were effciently silenced (Table 2). That result was confirmed with anti-TAG immunofluorescence that showed no staining of the flagellum, demonstrating that TbPFR2-TAG was absent (no red color, Figure 4B). This lack of both TbPFR2 and TbPFR2-TAG led to an incomplete assembly of the PFR, which was therefore not decorated with the ROD-1 antibody (no yellow color, Figure 4B). In contrast, cells transfected with the UTRs MIX dsRNAs exhibited a WT phenotype, with only 2 % of the cells displaying TbPFR2 silencing in the flagellum (Table 2). In this case, the tagged protein was expressed, leading to complete assembly of the PFR (yellow color, Figure 4C) because the protein is functional and localized to the flagellum (red color, Figure 4C; ). This remarkable result indicates a complementation phenomenon that is explained by the fact that the tagged TbPFR2 gene was not silenced, as it was expressed from a coding sequence flanked by UTRs from the expression vector: from the 5' UTR of the procyclin gene and from the 3'UTR of the aldolase gene (Figure 1B; ).
To definitely demonstrate that the complementation described above is indeed due to the expression of functional TbPFR2-TAG, we transfected the same dsRNA into TbPFR2tag-ΔHLA trypanosomes expressing a modified TbPFR2 protein missing three amino acids (that is nine nucleotides out of 1800). TbPFR2tag-ΔHLA does not access the flagellar compartment and thus cannot be functional . Transfecting either CDS dsRNA or UTRs MIX dsRNAs produced cells in which the new flagellum was not (or faintly) decorated by the anti-TbPFR2 antibody (Table 2). TbPFR2-TAG-ΔHLA was not decorated by the anti-TAG antibody when cells were transfected with CDS dsRNA (red color, Figure 4E), indicating that both the WT and the tagged TbPFR2 copies were silenced, thus leading to an incomplete PFR edification (yellow color, Figure 4E). In contrast, transfection of UTRs MIX dsRNAs did not prevent the expression of the recombinant TbPFR2-TAG-ΔHLA protein, as it appeared stained by the anti-TAG antibody (red color, Figure 4F). However, that non-functional protein could not participate in the construction of the PFR, as shown by the absence of ROD-1 signal in the new flagellum, since it cannot access the flagellum (yellow color, Figure 4F).
RNA-directed RNA polymerase activity (RdRP) has been implicated as one possible step in the formation of siRNA in fungi , plants [17, 18], and worms . The fact that we could specifically silence WT TbPFR2 by targeting its UTRs, without interfering with the tagged TbPFR2 genes, suggests that spreading of silencing beyond the initial targeted sequence does not occur in trypanosomes [61–64].
Functional complementation with orthologue genes
We next asked if an RNAi-mediated loss of function could be complemented by the expression of a gene orthologue to the silenced one. The system used to answer that question involved the TbPFR2i cell line – that expresses TbPFR2 dsRNA under the control of a tetracycline-inducible promoter  – into which constitutive expression of Trypanosoma cruzi TbPFR2 orthologue (TcPFR2) was established using stable transfection procedures. TbPFR2 and TcPFR2 proteins share 90 % identity (both of them are recognized by the anti-TbPFR2 L8C4 antibody), but their gene sequences have diverged enough for us to envisage that the RNAi-silencing of TbPFR2 would not affect significantly the introduced TcPFR2 gene (83 % overall nucleotide identity). We thus created two new cell lines based on the previously described TbPFR2i cells  (see Methods). TbPFR2 expression and cell motility were analyzed.
The complementation described above shows the robustness of our strategy, because TbPFR2 and TcPFR2 are highly similar (they share 82 % identity at the nucleotide level ) and are nonetheless correctly differenciated by the RNAi machinery. However, our complementation strategy might be more diffcult to implement when the gene studied is too similar to the T. brucei counterpart. While this unfavorable case might happen with extremely evolutionarily-related organisms, studies have shown that the overall genetic sequence identity between Trypanosoma brucei and Trypanosoma cruzi, for example (the closest evolutionarily-related organisms envisaged for these studies), is roughly 80 % ( and ).  showed that this identity percentage is still compatible with an RNAi-based complementation strategy. It goes without saying that when the organisms are evolutionarily-distant, gene sequences diverge more rapidly than the protein sequences, thus laying off a field where our strategy can be implemented with good confidence that complementation will occur.
In this report, we demonstrated that RNAi-mediated silencing of a gene by targeting its UTRs is useful in studies where the loss of function resulting from this silencing must be complemented with the expression of an RNAi-resistant copy of the silenced gene, in order to demonstrate that the phenotype is indeed due to silencing of that gene, and not to inactivation of another one. The results obtained in this work are of particular interest when reverse-genetics studies cannot be easily achieved in organisms not amenable to RNAi, like Leishmania  or Trypanosoma cruzi , or where genetics experiments are hardly set up, like mammals. When genes from these organisms are to be studied, a complementation experiment can be set up as a three-step procedure whereby: 1) the ortholog gene in Trypanosoma brucei is RNAi-silenced and the loss-of-function phenotype is established; 2) T. brucei cells are engineered to ensure constitutive heterologous expression of the gene of interest, still allowing RNAi-mediated silencing of the T. brucei gene; 3) function of the investigated gene is assessed by checking if the loss-of-function phenotype observed in the first place gets complemented. Additionally, one application of the strategy described herein is genetic functional dissection, which is of interest when protein domains are to be characterized with respect to their function (e.g. the HLA tripeptide sequence in TbPFR2 that localizes the protein to the flagellum).
Complementation had previously been demonstrated following transformation of mammalian cells with EGFP siRNA and expression of a codon-modified, but functional, EGFP version . Our strategies are increasing flexibility for complementation studies after RNAi as unmodified genes can be used for rescue.
The procyclic T. brucei brucei strain 427 (or its derivatives) was used throughout this work. Cells were cultured at 27°C in semi-defined medium 79 (SDM 79) containing 10% foetal calf serum. PFRAi cells were described in . The TbPFR2i trypanosomes can be tetracycline-induced to express TbPFR2 dsRNA, thus eliciting an RNAi response against that gene. Note that this cell line is referred to as TbPFR2i in this article because of a change in the gene nomenclature .
RNAi assays by transient transfection
RNA was synthesized in vitro with T3 and Sp6 polymerases using PCR products as templates . The following primers (incorporating T3 or Sp6 promoters) were used:
for GFP (from the nucleotide coding sequence 476–691 of the EGFPN2 gene; Clontech), AATTAACCCTCACTAAAGGGAGAAG AACGGCATCAAGGTGAAC (T3 promoter italicized) and ATTTAGGTGACACTATAGAAG AGTGATCCCGGCGGCGGTCACG (Sp6 promoter italicized);
for FLA1, AATTAACCCTCACTAAAGGGAGA CCAAACCGTGGGCACCAAGG (T3 promoter italicized) and ATTTAGGTGAACTATAGAAGAG GTGGGATGATTAAAACGAGC (Sp6 promoter italicized);
for the TbPFR2 5' untranslated region (5' UTR; nucleotide sequence [-545→-1] upstream of TbPFR2 ATG start codon), AATTAACCCTCACTAAAGGGAGA (T3 promoter) and ATTTAGGTGACACT-ATAGAAGAG (Sp6 promoter);
for the TbPFR2 intergenic untranslated region (igUTR), AATTAACCCTCACTAAAGGGAGA CGCTGCGCTTAAATGTCTT (T3 promoter italicized) and ATTTAGGTGACACTATAGAAGA GTGATGCTTTATTGCTTTCT (Sp6 promoter italicized);
for the TbPFR2 3'untranslated region (3'UTR; nucleotide sequence [1→533] downstream of the TbPFR2 TAG stop codon), AATTAACCCTCACTAAAGGGAGA (universal T3 promoter) and ATTTAGGTGACACTATAGAAGAG (universal Sp6 promoter);
for the TbPFR2 coding sequence (CDS; nucleotide coding sequence [1084→1358]), ATTTAGGTGACACTATAGA GAGGTGAAGCGCCGTATTGAGGA (Sp6 promoter italicized) and AATTAACCCTCACTAAAGGGAGA GTTTTGTACAGGCGACGGAA (T3 promoter italicized);
Figure 1A shows the TbPFR2 locus and the position of the two dsRNA populations that were used, and their homology to either the coding sequence (labelled "CDS dsRNA") or the different 5'UTR, igUTR and 3'UTR all together (labelled "UTRs MIX dsRNAs"). A third dsRNA, homologous to the GFP gene is labelled "GFP dsRNA" throughout this work and was used as a control dsRNA. dsRNA was introduced into trypanosomes by electroporation, as described .
Plasmid pPC was generated from plasmid pSk1-GFP  as follows: pSk1-GFP was digested with Hind III and Eco RI to remove the GFP gene. Oligonucleotides AGCT GTCTAGCGATATCGGATCCG (forward) and AATT CGGATCCGATATCGCTAGCA (reverse) were annealed (protruding ends italicized) and the resulting double-strand oligonucleotide was ligated into the pSk1-GFP plasmid, resulting in the insertion of a poly-linker containing restriction sites Cla I, Hind III, Nhe I, Eco RV, Bam HI and Eco RI (Branche and Bastin; unpublished data). Plas-mid pPCTcPFR2 was generated as follows: amplification of the TcPFR2 gene was performed using Trypanosoma cruzi genomic DNA (kind gift of Cécile Gallet and Philippe Grellier, MNHN) and the two primers TcPFR2H (GAGTCTAAGCTTATGAGCTACAAGGAGGCATC) and TcPFR2ER (GCGTGGAATTCTTACTGTGTGATCTGCTGCAC). Both the amplified DNA fragment and the pPC plasmid were digested with Eco RI and Hind III. The fragment was ligated into pPC so as to yield the plasmid pPCTcPFR2 (Figure 1B).
The different constructs used to transform trypanosomes are shown on Figure 1B. The cell lines were established as follows.
WT-derived trypanosomes constitutively expressing TbPFR2-TAG proteins
The TbPFR2tag cell line was derived from the WT cell line into which the pTbPFR2TAG430 plasmid  was transfected. The recombinant cells constitutively expressed the TbPFR2-TAG protein, that is localized in the flagellum (Fig 4D). Tagged TbPFR2 is known to be functional [56, 72]. In contrast, transformation of WT cells with the pTbPFR2TAGΔHLA430 plasmid lead to the expression of slightly modified TbPFR2 protein, missing only three amino acids, that failed to enter the flagellum compartment and hence was found in the cell body cytoplasm  (Fig 4G). This cell line was called TbPFR2tag-ΔHLA. After electroporation , cells were grown overnight and then distributed in 24-well plates in the presence of phleomycin (2 μg/mL) for selection.
TbPFR2i-derived trypanosomes constitutively expressing GFP and TcPFR2
TbPFR2i cells  constituted the genetic background into which we established the PCGFP and PCTcPFR2 new cell lines. The PCGFP cell line was established by transfecting TbPFR2i cells with plasmid pPCGFP after linearization with BstX I. For establishing the PCTcPFR2 cell line, the pPCTcPFR2 plasmid was linearized with BstX I and transfected into TbPFR2i cells. Recombinant cells were selected by addition of puromycin (1 μg/mL), phleomycin (2 μg/mL), G418 (15 μg/mL) and hygromycin (20 μg/mL) to the culture medium.
Immunofluorescence and microscopy
Three different monoclonal antibodies were used as hybridoma supernatants: L8C4, IgG recognizing T. brucei TbPFR2 and cross-reacting with T. cruzi orthologue TcPFR2 ; BB2, IgG recognizing the Ty-1 tag of the TbPFR2-TAG and TbPFR2-TAG-ΔHLA recombinant proteins ; and ROD-1, IgM recognizing a doublet of minor PFR proteins . For immunofluorescence, trypanosomes were spread onto poly-L-lysine-coated slides, fixed in cold methanol and processed as described . Experiments involving the use of L8C4 only were performed with an FITC-conjugated anti-mouse IgG secondary antibody. Double-staining experiments using BB2 and ROD-1 were performed with a TRITC-conjugated specific anti-mouse IgG secondary antibody and an FITC-conjugated specific anti-mouse IgM secondary antibody. DNA was systematically stained with 4',6-diamidino-2-phenylindole (DAPI). Slides were examined with a Leica DMR microscope, images were captured using a cooled CCD camera (Cool Snap HQ, Roper Scientific) and processed with the GNU image manipulation program version 2 .
Cell sedimentation assay
The trypanosome sedimentation assay was performed as described in . Briefly: trypanosomes were grown at ≈ 5.106 cells/mL in normal culture medium, with or without 48 hour tetracycline induction. 1 mL of these cultures was dispensed to 5 plastic spectrophotometry cuvettes, for time points 0, 2, 4, 6, 8 hours, and left still. At each time point, the optical density at 600 nm was measured twice: first without mixing (O.D.no mix) and second after mixing the cuvette (O.D.mix). Data were plotted as a function of time.
We wish to thank Linda Kohl for live image acquisition, Carole Branche for providing pPC, Cécile Gal-let and Philippe Grellier for providing T. cruzi genomic DNA and Sabrina Benghanem for helpful discussions. M.D.-D. is supported by a "Bourse de formation recherche du Gouvernement luxembourgeois". This work was financed with the following grants: "ACI dynamique et réactivité des assemblages biologiques", (CNRS and Ministère de la recherche), "ACI biologie du développement et physiologie intégrative" (Ministère de la recherche), "GIS recherche sur les maladies rares" (INSERM and Institut des Maladies rares).
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