Genetic variant of TTLL11 gene and subsequent ciliary defects are associated with idiopathic scoliosis in a 5-generation UK family

Idiopathic scoliosis (IS) is a complex 3D deformation of the spine with a strong genetic component, most commonly found in adolescent girls. Adolescent idiopathic scoliosis (AIS) affects around 3% of the general population. In a 5-generation UK family, linkage analysis identified the locus 9q31.2-q34.2 as a candidate region for AIS; however, the causative gene remained unidentified. Here, using exome sequencing we identified a rare insertion c.1569_1570insTT in the tubulin tyrosine ligase like gene, member 11 (TTLL11) within that locus, as the IS causative gene in this British family. Two other TTLL11 mutations were also identified in two additional AIS cases in the same cohort. Analyses of primary cells of individuals carrying the c.1569_1570insTT (NM_194252) mutation reveal a defect at the primary cilia level, which is less present, smaller and less polyglutamylated compared to control. Further, in a zebrafish, the knock down of ttll11, and the mutated ttll11 confirmed its role in spine development and ciliary function in the fish retina. These findings provide evidence that mutations in TTLL11, a ciliary gene, contribute to the pathogenesis of IS.

www.nature.com/scientificreports/ dysfunction leads to motor or sensory ciliopathies respectively 22 . It is well known that ciliopathies are associated with a high phenotypical heterogeneity including skeletal deformity (scoliosis) 23 . Primary cilia are chemical and mechanical sensors of pericellular environment thanks to different pathways like Wnt signaling-planar cell polarity pathway, signaling at focal adhesions and at adherents junctions, hedgehog signaling, notch signaling, and the JAK-STAT signaling 24 . Primary cilia have, among others, a role in mechanotransduction of fluid flow maintaining bone homeostasis especially in growth plate 25,26 and in left-right patterning during embryogenesis 27 which is impaired in AIS patients. Moreover, the identification of POC5 [MIM: 617880], a ciliary gene, as an IS causing gene 1 , supports this hypothesis.
In this study, we identified a new candidate gene for IS. The susceptibility locus 9q31.2-q34.2 was first refined in a large UK family affected by IS presenting mainly in the adolescent period, and a mutation in the TTLL11 gene (Tubulin Tyrosine Ligase Like, member 11) [RefSeq: NM_001139442] was identified in a proband diagnosed with AIS. This gene has an important role in primary cilia integrity, and we showed that mutated TTLL11 lead to impaired ciliary glutamylation that resulted in shorter primary cilia in vitro using AIS patient's fibroblasts. Moreover, we observed spinal deformity in vivo, using CRISPR-Cas9 zebrafish model, in both larvae and adult animals.

Results
Identification of a rare variant in TTLL11 gene. Linkage analysis previously refined the region 9q31. 2-q34.2 as an AIS susceptibility locus in a 5-generation family, SC32, affected by autosomal dominant AIS2 (Fig. 1A). The curvature in this family ranged from 15° to 65°, and the Cobb angle measured for the proband (SC32.1) was 40° at the diagnosis and progressed to 56° before surgery (Fig. 1B). The linkage analysis revealed that all clinically affected individuals within family SC32 shared a common haplotype, that was absent for all unaffected individuals, confirming that disease segregated with 9q (supplemental Figure 1) 2 . To identify the causative gene, exome sequencing was performed for this specific region, 9q31.2-q34.2, on 2 affected patients, the proband SC32.1 and SC32.16 (3 meiosis distance, SC32 family). Each exome was sequenced to a mean depth of 71.39× and 62.87× with > 84.01% and > 81.18% of coding bases covered by > 20 reads respectively.
Reads were then aligned to the reference genome hg19 and a rare variant in TTLL11 gene was identified by variant calling (Fig. 1C). TLL11 gene codes for two transcripts, transcript 1 (NM_001139442), which is longer than transcript 2 (NM_194252). The identified mutation is an insertion of two thymine (g.124751443_124751444insTT, MAF < 1%) that results in a premature stop codon in the transcript 2 (c.1569_1570insTT), leading to a truncated protein (p.Pro524PhefsTer4) (Fig. 1D). This rare variant that we identified was not present in 3000 in house control exomes and was found on 8 alleles on 8246 according to the EVS database (Exome variant server 28 ) (supplemental Table 1). Moreover, this variant is predicted to be pathogenic for the transcript 2 by UMD-predictor 29 (UMD-score: 100) and MutationTaster 30 (score: 1; disease causing).
Subsequently, to assess for mutations in TTLL11 in a larger cohort, its 9 exons were screened by Ion AmpliSeq from Life technology in 96 AIS affected individuals from a French-Canadian and British population. The VarAFT 2.3 software was used to annotate and filter the 13 genetic variants discovered in TTLL11 gene. After filtering (minor allele frequency < 1%, single-nucleotide substitutions and small indels, pathogenicity prediction), out of 13 variants, 2 were identified as novel rare variants in TTLL11 (Supplemental Table 2). Interestingly, these two rare variants were found in 2 British AIS patients and none were identified in any of the AIS affected patients from the French-Canadian population (Supplemental Figure 2). These two mutations are predicted to be pathogenic for the transcript 1 (NM_001139442), SC203: c.1751+1G>A, (rs not known), HSF: Broken WT Donor Site and SC217: c.2214C>A; rs766983167, UMD-score: 100).
To gain more insights on the pathogenic role of the mutation in gene TTLL11 on its expression, we first assessed TTLL11 transcripts 1 and 2 expression from healthy control (DE0194) and patient-derived fibroblasts (NM_194252: c.1569_1570insTT; DE0193). In WT cells, the expression of WT TTLL11 transcript 1 was increased from 1 to 2 h following FBS privation prior to returning to regular expression ( Fig. 2A). In parallel, the mRNA expression of WT TTLL11 transcript 2 was decreased (Fig. 2B). In mutant cells, the expression of TTLL11 transcript 1 is reduced compared to WT transcript ( Fig. 2A). In parallel, mutant TTLL11 transcript 2 expression is increased compared to WT (Fig. 2B). These results suggest that the c.1569_1570insTT mutation disturbs the mRNA expression of TTLL11 transcript 1 and 2.
TTLL11 localisation and ciliogenesis. The localization of TTLL11 was assessed by immunofluorescence staining in mutant DE0193 and WT DE0194 fibroblasts after 24 h of serum starvation 31,32 . Acetylated-α-tubulin antibody binds to α-tubulin that carry acetylated K40, meaning primary cilia from proximal to distal end 33 , centrioles, mitotic spindles, midbodies and to subsets of cytoplasmic microtubules. According to these results and assuming that α-tubulin acetylation of primary cilia is independent of TTLL1 function and reliably indicates ciliary length, acetylated-α-tubulin was used as primary cilia marker. Despite the ciliary function, in fibroblasts, WT TTLL11 proteins localized in the nucleus and in the cytoplasm (Fig. 3A) but mutant TTLL11 proteins were more nuclear.
Moreover, after 24 h of serum starvation, around 75% of WT fibroblasts showed acetylated α-tubulin primary cilia compared to 20% for the TTLL11 mutant fibroblasts (p = 3.9 × 10 −9 ) (Fig. 3B) for which the length of acetylated α-tubulin from the primary cilium was reduced (Fig. 3C). Indeed, primary cilia of more than 5 μm are significantly reduced for mutant cells, and cilia of less than 3 μm were more prominent in mutant cells compared to WT fibroblasts (Fig. 3C). These results reveal that the identified mutation (c.1569_1570insTT) of TTLL11 leads to a mislocalisation and shorter cilia or the absence of cilia. Taken together, these results confirm the implication of TTLL11 in primary cilia integrity.  34 . TTLL11 is a polyglutamylase from the tubulin tyrosine ligase-like (TTLL) family of proteins that includes enzymes responsible for the two PTMs glutamylation and glycylation. TTLL11 is known to preferentially modify α-tubulin by extending the glutamate chain as an elongase 35 . The functionality of TTLL11 in mutant DE0193 cells and WT DE0194 cells was then assessed using both GT335 and polyE antibodies that detects the branch point of glutamate side chains or the long polyglutamate chains (≥ 3 glutamates) respectively. Interestingly, the polyglutamate chains were reduced at the cilium level in mutant cells (Fig. 4A,B), and the amount of branch point glutamate was also reduced (Stained with GT335). Additionally, the long polyglutamate chains (≥ 3 glutamates) were induced after serum starvation until 48 h prior to decrease in the WT cells while the long polyglutamate chains were strongly decreased in the mutant TTLL11 cells (Fig. 4C); leading to short cilium length. These results demonstrate that TTLL11 is required for proper long glutamate side chains elongation, and this function is impaired in TTLL11 mutant AIS cells.
Ttll11 is implicated in zebrafish spinal development. To further functionally validate the implication of TTLL11 in the pathogenesis of AIS, knockdown modelling in zebrafish was performed; a powerful model to study AIS 1,18,36,37 . Phylogenetic analysis indicated that TTLL11 was conserved during evolution and that zebrafish have a unique ortholog that shares 71% of homology with human TTLL11 gene. Interestingly, ttll11 is expressed as early as the 8-cell stage and later in development its expression is predominantly localized to the central nervous system 38 . We disrupted ttll11 expression using an antisense morpholino (MO) (Fig. 5A). Loss of function of ttll11 resulted in a significant curvature of the body axis in ttll11 morphants compared to control embryos as from 2-days postfertilization (hpf) (Fig. 5B). The body curvature in ttll11 morphants progresses through early larval stages (4-6 dpf) with a moderate or severe body curvature (Fig. 5C). Similar phenotypic variability in the appearance and severity of the curve has been observed in several zebrafish models of scoliosis 1,39,40 . We were unfortunately not able to fully characterize the spinal curvature during juvenile and adult stages using bone imaging techniques because the ttll11 morphants did not survive past 9-12 dpf, likely due to their inability to swim and reach for food. We thus created a mutant zebrafish line for ttll11 gene using CRISPR-Cas9. We identified a founder mutant zebrafish carrying an 18 bp insertion in exon 4 of ttll11 leading to a strong modification of the TTL domain of the protein (Fig. 6A). The zebrafish homozygous mutant (mut-ttll11) exhibited a high incidence of body axis curvature in the larvae compared to WT fish but at a lower penetrance than ttll11-MO (Fig. 6C). The body curvature varied with different degrees of severity (Fig. 6B). Moreover, all the mutant larvae that were showing a curly phenotype died before 2 weeks post fertilization (Fig. 6D). Fish that acquired the curvature at a later stage were fibroblasts were treated with medium without FBS for different times. Cells were then processed for mRNA isolation and the relative mRNA expression of both transcript 1 (A) and 2 (B) of TTLL11 at different time point after serum starvation were determined by qPCR. The levels of mRNAs were plotted relative to cells harvested at 0 h (n = 3). Error bars represent SD. The difference between one independent group and the control group 0 h was examined by unpaired, two-tailed Student's t-test, *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001. www.nature.com/scientificreports/ able to live until adult stage and we analysed mineralized spine of these adult fish using micro-CT that revealed a 3D deformation similar to the deformation observed in AIS patients ( Fig. 6E and supplemental videos). Indeed, the mutant spine displays curvature in two different planes with a rotation of vertebrae.

Scientific Reports
Ttll11 is implicated in retinal integrity in zebrafish. Ciliary defects cause a group of diseases called ciliopathies that are especially characterized by retinal defects and scoliosis. Cilia, as a complex structure, comprises more than 900 genes that are involved in ciliary structure and function 41 , and those genes are called ciliary genes. Because TTLL11 is a ciliary gene, and ciliary genes, such as POC5, were already related to retinal function [42][43][44][45] , the ciliary retinal tissue was then investigated. Zebrafish ttll11 has been reported to be expressed within the CNS and neural tube 38 . Moreover, function and structure of photoreceptor cilia are well understood 46 and zebrafish retina is easy to access. Retinal sections of WT and mut-ttll11 zebrafish with and without scoliosis were stained by hematoxylin eosin (Fig. 7) and revealed a disorganization of the cone cell layer of mut-ttll11 adult zebrafish compared to control. To validate this finding, retinal sections were then labeled with different retinal layer specific antibodies: 3A10 antibody, specific to neurofilaments (rods and cones), zpr1 antibody, specific to cones and zpr3 that is specific to rods (Fig. 8). We observed a disorganisation of the cone cell layer (Fig. 8A,B) and especially of the double cones in mutant fish compared to controls. Indeed, lack of staining is observed in cones cell body layer (Fig. 8B, white arrows) and the layer of double cones cilium seems to be impaired (Fig. 8B, black arrows). On the other hand, no anomalies were observed in the rod layer between mutant ttll11 zebrafish and controls (Fig. 8C). These results support the implication of the ciliary protein ttll11 in retinal layer integrity in a zebrafish model.

Discussion
Idiopathic Scoliosis is a complex disease with a multifactorial aetiology 47 including genetic, epigenetic 48 , environmental, biomechanical 49 and hormonal 50 factors. Many studies identified candidate genes that are probably involved in pathogenesis of IS and AIS but almost all of them are susceptibility genes, not causative genes 51,52 . In this study, we reported a new causative gene in a five-generation UK family, TTLL11, a ciliary gene. TTLL11 is a ciliary protein and a member of Tubulin Tyrosine Ligase Like (TTLL) family, which play a role in α -and β-tubulin mono-and polyglutamylation, a post-translational modification (PTM). This PTM is crucial for ciliary integrity, cell cycle and cell differentiation 53,54 . Indeed, polyglutamylation has been shown to maintain a proper alignment of outer doublet in ciliary axoneme 55,56 . Values shown as mean ± SD, the difference between two independent groups was examined by unpaired, twotailed Student's t-test, ***P ≤ 0.001.  1 reported a causative gene, POC5, a ciliary-related gene, which explains 10% of AIS family cases. TTLL11 and POC5 seem to be involved in a similar biological process, which could be involved in the physiopathogenesis of AIS. Indeed, these genes have an important role in the integrity of cilia especially primary cilia. It is accepted that ciliopathies could include skeletal deformity and scoliosis.
Polyglutamylation is a reversible modification characterized by the addition of glutamate residue, carefully regulated by the TTLL proteins family and tubulin deglutamylase of the Cytoplasmic Carboxyl Peptidase (CCP) proteins family 57 . This post-translational modification allows the interaction with the microtubule-associated proteins (MAPs) 54 and play a role on intraflagellar transport (IFT) 58 .
In human fibroblasts, we showed that TTLL11 is localized in the nucleus. Similar findings were observed when TTLL11 was expressed in HeLa cells 59 . In contrast, ttll11 was shown to localize in the cilium in C. elegans 60 and in the basal body in MDCK cells 35 . However, these are limited studies and further work is required to better characterize its cellular localization in mammalian cells, especially in human cells.
According to the literature, mouse Ttll11 shows preference for polyglutamylation elongation of α-tubulin when overexpressed in HeLa cells 35 , but in the nematode C. elegans, a recent study highlighted the crucial role of ttll11 to initiate the glutamate side chain 61 . This recent analysis is consistent with our data in human AIS cells, indeed we showed that TTLL11 was required to add the initiating glutamate to the side chain and proper long glutamate side chain. We noticed that the GT335 signal was not completely lost suggesting that some residual branch point glutamates were still added, suggesting a residual activity of mutant TTLL11, or a residual activity of one of the other initiase from the TTLLs family. For instance, TTLL5 is responsible of the proper glutamylation of RPGR, an X-linked retinitis pigmentosa GTPase regulator 62 . To date, no non-tubulin substrate for TTLL11 has been described, but it can't be excluded.
Little is known about the role of TTLL11 and about its potential role in IS but recent studies demonstrate that cilia hypoglutamylation promotes cilia disassembly 63 and affect Hh signalling through anterograde IFTdependant mechanism 58,63 . TTLL11 gene lead to two different transcripts, transcript 1 composed by 9 exons and transcript 2 that has 4 exons, but their roles are not well documented yet.
Our experiments support the hypothesis of the implication of cilia pathway in AIS physiopathogenesis. Moreover, the TTLL protein family was found to be involved in Joubert syndrome [MIM; 612291], a ciliopathy characterized by mid-hindbrain malformation, hypotonia, developmental delay and skeletal defects such as scoliosis 64 . Indeed, TTLL6, which is also an α-tubulin polyglutamylase 53 , was found to be mislocalized in Joubert Syndrome 65 . The characterization of TTLL11 mRNA expression and its localization in AIS patient fibroblasts highlight an impairment due to the c.1569_1570insTT mutation and this seems to drastically affect ciliogenesis. An increase of transcript 2 was observed, and although in-silico prediction, we also observed a modification of TTLL11 transcript 1 mRNA expression, probably to balance the over-expression of transcript 2. Altogether, our results suggest that the altered TTLL11 protein leads to a hypoglutamylated tubulin resulting in shorter primary cilia. Supporting this hypothesis several studies have shown the involvement of glutamylation in ciliogenesis, IFT   66 , which is involved in both endochondral and intramembranous ossification 67 , that are implicated in spine development 68 . In addition, Ihh pathway is associated with height 69,70 , an important parameter for the evolution of the spine curvature occurring in AIS 71 . Primary cilia host also the polycystin pathway which is involved in skeletogenesis and bone development 72,73 . Moreover, cilia sensory functions are exhibited by the TRP family receptors and Ca 2+ influxes and interestingly, cilia respond to the sex hormones 74,75 that are significantly increased during puberty.
To support the implication of this gene in IS physiopathogenesis we did functional studies in an animal model, zebrafish (Danio rerio). The phenotype that we observed is similar to the phenotype observed in fish when ttll3 and ttll6, two other members of the TTLL protein family, are knocked down 38 , which supports the implication of this family in skeletal development. Because this approach is not highly specific and because these larvae did not live for more than 2 weeks, a mutant zebrafish line was designed using the genome-editing technology CRISPR-Cas9. With the selected guide, only one mutation was obtained, an 18pb insertion in exon 4 of ttll11 leading to an insertion of 6 amino acids on the TTL domain of the protein. Almost no larvae with spine curvature were able to stay alive suggesting that this deformation is not viable for more than 2 weeks. This effect could be explained by the fact that the curvature affects their swimming capacity and prevents them from feeding. However, some fish developed this deformity at a later stage and were able to grow until adult stage. (C) Incidence of scoliosis phenotype in 8 days old mut-ttll11 zebrafish. 6% of the mut-ttll11 zebrafish show a 3D curvature of the body axis. n = 250. (D) Lethality of scoliosis phenotype in 8 days old mut-ttll11 zebrafish. All the mut-ttll11 zebrafish expressing 3D curvature died after 2 weeks, 20% the first week and 80% the second week. All the WT and mut-ttll11 no curved were alive after 2 weeks. n = 15. (E) lateral view (left panels) of WT, mut-ttll11 and mut-ttll11 with scoliosis adult zebrafish. MicroCT lateral (middle panels) and dorsal (right panels) images reveal a 3D curvature for the mut-ttll11 with scoliosis fish compared to WT and mut-ttll11 without scoliosis phenotype. www.nature.com/scientificreports/ The curvature observed by micro-CT analysis in this ttll11 mutant zebrafish line is a 3D spine curvature with vertebral rotation (supplemental videos). Because TTLL11 is a ciliary protein and because the TTLL5 (also from the TTLLs family) and the ciliary protein POC5 was found to be implicated in scoliosis and retinal function 1,42,62 , histology of mut-ttll11 retinal tissue was assessed and revealed a disorganisation of the double cone cell layer. Interestingly, no retinal defect was initially observed in Ttll5 −/− mice 76 , but further additional assays at older stage revealed a progressive photoreceptor degeneration 62 . These studies describe that retinal defect was seen in older animals but AIS is a disease that develop during the puberty, at younger stage of life.
These results suggested a link between vision and the proper development of the spine as a link between vision and AIS was already showed in human 77 , this field warrants further investigations. Noteworthy, no visual disturbance was observed in the 5-generation UK family based on clinical record.
In parallel, several studies showed that cilia could be involved in AIS by their implication in CSF (cerebrospinal fluid). Indeed, motile cilia of the surrounding epithelium of CSF could play an important role in distribution of this flow 78 , and a defect of the CSF flow in zebrafish influences body axis formation and spine morphogenesis 18,79,80 possibly through CSF-contacting neurons that line the central canal of the spinal cord and the brain ventricles 80 .
Finally, identifying TTLL11, as a novel gene for IS with role in cilia biology is a crucial step to clarify pathways potentially involved in this condition. Based on this study, in addition to the previously identified ciliary genes in Human 1,81-85 and animal model 19,37,39 , this pathology could be considered as a cilia-associated disorder, or a subtype of ciliopathy.
Further studies of larger cohort are needed to establish the prevalence of mutated TTLL11 in AIS and to clarify the involvement of ciliary pathways. Understanding these mechanisms can contribute, in the long term, to the early diagnosis of scoliosis and may help to establish strategies to prevent the progression of deformity. Meanwhile, in affected families were causative or predisposing mutations are identified, offspring can be screened in the new-born period. Those identified as carrying the family mutation can be followed at regular intervals and early spinal curvature treated optimally to prevent severe deformity.

Patients.
A five-generation British family (SC32), with eighteen known affected members (clinical data published in 2 , supplemental Table 3), was recruited by the orthopaedic surgeon at St George's Hospital, London, UK. The proband SC32.1 was diagnosed as having adolescent idiopathic scoliosis (AIS). Among the family members, 7 males and 11 females (Fig. 1A) were affected. In total, there are nine affected members living. All are affected by a lateral spine curvature of at least 10° with vertebral rotation (Fig. 1B)  Whole exome sequencing. Whole exome sequencing was performed by the Division of Genetics and Molecular Medicine (King's College London, UK) using DNA extracted from whole blood in two affected individuals from family SC32.1 and SC32. 16. This strategy, where we assumed that rare or low-frequency variants shared between the two exomes sequenced affected relatives in a family, were highly likely to be identical, optimised the information gained and reduce the cost compared to WES of all the affected individuals. A total of 3 μg of genomic DNA was sheared to a mean fragment size of 150 bp [Covaris], and the fragments used for Illumina paired-end DNA library preparation and enrichment for target sequences [SureSelect Human All Exon 50Mb . Sequencing reads were aligned to the reference human genome sequence hg19 using the Novoalign software 86 . Duplicate and multiply mapping reads were excluded, and the depth and breadth of sequence coverage were calculated using custom scripts and the BedTools package 87 . Single-nucleotide substitutions and small indels were identified with SAMtools 88 , annotated with the ANNOVAR software 89 . The identified SNPs were then compared to a 3000 controls cohort and EVS database 90 .
Sanger sequencing. DNA was sequenced using ABI BigDye terminator cycle sequencing chemistry 3.   -GGC TGA TTT GTT ATC TCA TCT AGG T-3′, while that of the control-MO, was 5′-CCT CTT ACC TCA  GTT ACA ATT TAT A-3′. MOs were diluted to 1 ng/nl, and approximately 3nl was injected into fertilised zebrafish eggs at the one-to two-cell stage using a Picospritzer III microinjector. Embryos were stage matched, anaesthetised using 0.02% MS222 in E3 embryo medium and observed under a stereomicroscope (SZX16 Olympus).
Ttll11 mutant zebrafish. We used the CRISPRscan software 93 to design the sgRNA to target the sequence GAG GTA GAT GCC ATC CCCTT GGG (with PAM, Protospacer Adjacent Motif). The designed sgRNA, taatac-gactcactataGGG GTA GAT GCC ATC CCC TTgttttagagctagaa has no identified off-targets using CRISPR-scan. All injections were performed in 1-cell stage embryos from WT strain. Each embryo was injected with around 1.7 nl solution containing 30 ng/µ l of sgRNA and 100 ng/µ l of Cas9 mRNA. Mutants were selected by using HRM (High-resolution melting) as described previously 94 . To date, the transgenic lines have been outcrossed for more than 4 generations to dilute out potential off target mutations.
Three-dimensional imaging and reconstruction. Three months old WT and ttll11 mutant zebrafish were collected and fixed in 4% paraformaldehyde (PFA) overnight at 4 °C and three-dimensional Imaging and Reconstruction of Zebrafish spine underwent a micro-CT scan (SkyScan 1072 High Resolution Desktop Micro-CT System, Microtomograph, SkyScan) for three-dimensional (3D) visualization of the skeleton after 3D imaging and subsequent reconstruction. Acquisition parameters for the scan were as follows: 35 kV, 215 µ A, step rotation of 0.9°, pixel size 4-7 microns; images were reconstructed using NRecon (Version: 1.6.1.3). The general body shape of 8 days-old larvae was also analysed under a stereomicroscope (Leica M205 FA).
Retinal histology and immunostaining. Adult zebrafish were decalcified after micro-CT examination and embedded in paraffin wax prior to transverse sectioning. Eye tissue sections (6 μm) were then mounted on microscope slides and stained with hematoxylin eosin (H&E) following standard protocol. For immunostaining, eye tissue sections were deparaffinized in xylene, rehydrated in a graded series of ethanol, washed 3 times in PBS, and permeabilized for 30 min in 4% Triton X-100 containing 10% goat serum and 2% bovine serum albumin (BSA). Then retinal sections were incubated with the zpr1, zpr3 antibodies obtained from the Zebrafish International Resource Center 95 or 3A10 antibody obtained from Hybridomas Bank (cat# AB_531874) (1/500) and acetylated-α-tubulin (Sigma Aldrich cat #T7451) (1/2000) simultaneously during 24 h at 4 °C. Tissue sections were washed several times in PBS and then incubated with the secondary antibody conjugated with Alexa Fluor 488 (life technologies cat# A11008 1/500) and Alexa Flour 555 (Life technologies cat # A21422 1/500) for 1 h at RT. Images were acquired using confocal microscope.

Statistical analysis.
All data values are given as means ± SD. Statistical analyses were performed, and data were plotted in SigmaPlot 11.0 (Systat Software Inc., San Jose, CA, USA). One-way ANOVAs and Fisher LSD tests were used to determine significance of normally distributed and equal variance data. Kruskal-Wallis ANOVA and Dunn's method of comparison were used for non-normal distributions.
Study approval. All animal procedures applied in this study were carried out in accordance with the guidelines set out by the Canadian Council for Animal Care (CCAC), the CHU Sainte-Justine Research Center, and the Comité de Déontologie de l'Expérimentation sur les Animaux (CDEA), which is the local animal care committee at the University of Montreal, Canada. The study was carried out in compliance with the ARRIVE guidelines (Animal Research: Reporting of In Vivo Experiments). This study was approved by the ethics committee for CHU Sainte-Justine Research Center, University of Montreal (ZF-09-60/Category B). All procedures involving humans were carried out in accordance with the guidelines set out by the ethics committee of CHU Sainte-Justine Research Center. Written informed consent was received from participants and from legally authorised representative of minor participants prior to enrollment.