Mutations in Kinesin family member 6 reveal specific role in ependymal cell ciliogenesis and human neurological development

Cerebrospinal fluid flow is crucial for neurodevelopment and homeostasis of the ventricular system of the brain, with localized flow being established by the polarized beating of the ependymal cell (EC) cilia. Here, we report a homozygous one base-pair deletion, c.1193delT (p.Leu398Glnfs*2), in the Kinesin Family Member 6 (KIF6) gene in a child displaying neurodevelopmental defects and intellectual disability. To test the pathogenicity of this novel human KIF6 mutation we engineered an analogous C-terminal truncating mutation in mouse. These mutant mice display severe, postnatal-onset hydrocephalus. We generated a Kif6-LacZ transgenic mouse strain and report expression specifically and uniquely within the ependymal cells (ECs) of the brain, without labeling other multiciliated mouse tissues. Analysis of Kif6 mutant mice with scanning electron microscopy (SEM) and immunofluorescence (IF) revealed specific defects in the formation of EC cilia, without obvious effect of cilia of other multiciliated tissues. Dilation of the ventricular system and defects in the formation of EC cilia were also observed in adult kif6 mutant zebrafish. Finally, we report Kif6-GFP localization at the axoneme and basal bodies of multi-ciliated cells (MCCs) of the mucociliary Xenopus epidermis. Overall, this work describes the first clinically-defined KIF6 homozygous null mutation in human and defines KIF6 as a conserved mediator of neurological development with a specific role for EC ciliogenesis in vertebrates.


Introduction
The delicate balance of cerebrospinal fluid (CSF) production and flow is important for the morphogenesis and function of the brain during development and homeostasis. CSF circulation in human is largely due to gradients established by the secretion of CSF from the choroid plexuses, and its resorption at the arachnoid granulations [1]. The clinical significance of CSF stasis includes hydrocephalus and intracranial hypertension. Moreover, severely diminished CSF flow combined with increased intracranial pressure can secondarily cause ventriculomegaly, cognitive impairment, as well as degenerative and age-related dementias [2]. For these reasons, the identification of genetic risk factors involved in the pathogenesis of CSF stasis is critical for the development of genetic diagnostics and early interventions for these disorders.
One element for circulation of CSF is the multiciliated ependymal cells (ECs), which are specialized glial cells covering the ventricular walls of the brain and spinal canal [3]. In contrast, to primary cilia which are single, immotile cellular organelles extending from most cell types, ECs contain dozens of apically-arranged motile cilia, which beat in a polarized fashion to generate localized or near-wall CSF flows [4]. Defective differentiation or alterations in their stereotyped synchronous, polarized beating leads to alterations of localized CSF flow contributing to increased intracranial pressure, dilation of ventricles, and hydrocephalus in mice [5][6][7][8]. Importantly, this EC cilia-driven CSF flow is vital for regulating brain function and neurogenesis during adult development [4,9]. Impaired motility of cilia due to disruptions of the key kinesins, dyenins, or intraflagellar components, are associated with a syndromic condition known as primary ciliary dyskinesia (PCD) in humans [10,11]. While hydrocephalus can occur in some PCD patients, it is a less common manifestation of the disease in humans [11]. In contrast, genes implicated in PCD or mutations which disrupt the structure or motility of all motile cilia are strongly correlated with hydrocephalus in mouse [8]. Alternatively, some hydrocephalus in mice with dysfunctional cilia may be the result of altered function of the choroid plexus, prior to the onset of ciliadriven CSF flow [7].
KIF6 (Kinesin family member 6, OMIM: 613919) encodes a member of the kinesin-9 superfamily of microtubules motor proteins which act predominately as "plus-end" directed molecular motors that generate force and movement across microtubules [12]. Kinesins are critical for numerous cellular functions such as intracellular transport and cell division, as well as for building and maintaining the cilium in a process known as intraflagellar transport [13]. During this process, kinesins have been shown to transport cargo within the ciliary axoneme [14], establish motility and compartmentalization of the axoneme [15], or to facilitate plus-end directed microtubule disassembly and control of axonemal length [16]. As such, multiple kinesins have shown to be associated with monogenic disorders affecting a wide-spectrum of tissues, with several modes of inheritance (www.omim.org). Interestingly, KIF6 has previously been proposed as locus for susceptibility to coronary heart disease [17], while other studies did not substantiate this association [18]. We previously reported that kif6 mutant zebrafish are adult viable exhibiting larval-onset scoliosis without obvious heart defects [19]. Because of these conflicting results, and a lack of relevant mouse models, the role of KIF6 in human disease remains an open question.
Here, we present a patient with consanguineous parents, presenting with abnormal neurological morphologies and intellectual disability. Homozygosity mapping followed by wholeexome sequencing (WES) identified a novel homozygous frameshift mutation in KIF6 which is predicted to result in the truncation of the C-terminal cargo-binding domain of the kinesin motor protein. We generated an analogous frameshift mutation in mouse and found that these mutant mice displayed progressive, postnatal-onset hydrocephalus with cranial expansion, coupled with an obvious defect in EC cilia formation. In addition, we observed that kif6 mutant zebrafish also display dilation of the ventricular system, coupled with reduced EC cilia. We failed to observe cilia defects in other multiciliated tissues in Kif6 mutant mouse or zebrafish models. Together these results demonstrate that KIF6 has a specific function for the formation of EC cilia. Finally, we propose that KIF6 represents a novel locus for understanding mechanisms of neurological development and intellectual disability in humans.

Clinical features and mutation identification
We identified a Thai boy with intellectual disability and megalencephaly. His parents were first cousins. He was born at 34 weeks gestation with a head circumference of 34 cm (97 th centile). APGAR scores were 7 and 9 at 1 and 5 minutes, respectively. Neonatal hypoglycemia (blood sugar of 11 mg/dL) and neonatal jaundice were treated promptly. In the first few months of life, he was found to have delayed neurodevelopment and central hypotonia. He was able to hold his head at 5 months, rolled over at 8 months, walked and had first words at 2 years old. At the age of 9 years and 9 months, an IQ test by Wechsler Intelligence Scale for Children: 4th edition (WISC-IV) revealed that his full-scale IQ was 56, indicating intellectual disability. The patient had possible seizure activity at age 10 described as parasomnias, was found to have intermittent bifrontocentreal rhythmic theta activity, and the spells resolved after valproic acid therapy. His height and weight followed the curve of 50th centile, but his head circumference remained at 97th centile (53.5 cm and 55 cm at 6 and 9 years old, respectively). Physical examination was generally unremarkable except macrocephaly and low-set prominent anti-helical pinnae ( Fig 1A). Eye examination, hearing tests, thyroid function tests, chromosomal analysis, and nerve conduction velocity were normal. Both brain CT scans at 4 months and 8 years old and brain MRI at 7 months old showed a slight dolichocephalic cranial shape (cephalic index = 75), without overt structural brain abnormalities (Fig 1B-1D). X-ray analysis of the spine showed no obvious scoliosis at 10-years-old ( Fig 1E).
To elucidate the genetic etiology, we performed homozygosity mapping, whole genome array comparative genomic hybridization (CGH), and whole exome sequencing (WES). We identified 83 homozygous variants, which had not been reported as SNPs in dbSNP137 (S1 Table). We then selected only those located within the 63 homozygous regions found by homozygosity mapping (S2 Table). Seven candidate variants (one frameshift and six missense mutations; Table 1) were identified. Of the six missense, five were predicted to be either benign by Polyphen-2 or tolerated by SIFT prediction programs. The remaining variant, c.235G>A; p.V79M of the Carboxypeptidase E (CPE) gene, was not evolutionarily conserved among diverged species (S1 Fig). We, therefore, decided to further our study on the only candidate truncating mutation, a homozygous one base-pair deletion, c.1193delT (p.Leu398Glnfs � 2) in exon 11 of Kinesin family member 6 (KIF6) (NM_001289021.2).
KIF6 is located on human chromosome 6p21.2 and comprises 23 exons. The 2.4-kb KIF6 cDNA encodes a canonical N-terminal kinesin motor domain (amino acid positions 3-353) and three coiled-coil regions (amino acid positions 358-385, 457-493, and 633-683), predicted by SMART server [20]. Segregation of the homozygous sequence variant with the disease phenotype was confirmed by Sanger sequencing (Fig 1F) and by restriction fragment length polymorphism (RFLP) analysis of the pedigree (Fig 1G), while his parents and his unaffected brother were heterozygous for the deletion (Fig 1F and 1G). The deletion was not observed in our 1,600 in-house Thai exomes, the 1000 Genome Database, and the ExAC Database. The pedigree combined with the novelty of the mutation in KIF6 presented here, strongly suggest this C-terminal truncating mutation in KIF6 may be etiologic for neurological developmental defects.

Generation of Kif6 mutation in mouse
To test the functional consequence of the C-terminal truncating p.L398fsX2 mutation (Fig 1H), we generated an analogous frameshift mutation in exon14 of the mouse Kif6 (ENSMUST00000162854) gene, which is~150bp downstream of the frameshift mutation found in the patient (Fig 2A). After backcross of founder mice to the C57B6/J strain, we identified a nonsense allele with scarless insertion (c.1665ins) of a 3-stop donor cassette -providing integration of an ochre termination codon in all three reading frames into the endogenous Kif6 locus (S2 Fig). This endonucleasemediated insertional frameshift mutation (Kif6 em1Rgray ) is predicted to truncate the C-terminal cargo-binding domain of the kinesin motor protein (p.G555+6fs). This novel mutant allele of Kif6 (hereafter called Kif6 p.G555fs ) is predicted to encode a C-terminal truncated KIF6 protein 168 amino acids longer than is predicted for the human p.L398fsX2 variant (Fig 2A). Real time qualitative-PCR analysis of several Kif6 exon-exon boundaries found no evidence for non-sense mediated decay in Kif6 p.G555fs mutant mice ( Fig 2B). (B) MRI of the brain at 7 monthsold shows dolichocephaly with a normal brain structure. (C) and (D) CT of the brain at 8 years-old, sagittal and axial views, respectively show dolichocephalic shape of the cranium (cephalic index = 75) without demonstrable intracranial abnormality. (E) X-ray of the spine shows no scoliosis (F) Electropherograms of the patient, a control, the patient's father, mother, and unaffected brother from top to bottom. The patient is homozygous for the c.1193delT mutantion while his father, mother, and unaffected brother are all heterozygous carriers. (G) Pedigree and RFLP, using MfeI restriction enzyme: Lane M = 100 bp marker. The arrow head indicates the 500 bp band. Lanes 1-5 are controls. Lanes 6 and 7 are the proband's father and mother, respectively, showing that they are heterozygous. Lane 8 is the proband showing that he is homozygous for the c.1193delT. (H) Representative KIF6 structure. The arrow shows the position of the c.1193delT mutation.
To determine whether a more N-terminal truncated Kif6 mutation would result in a more severe hydrocephalus phenotype, we isolated a conditional-ready Kif6 allele, where exon 4 is flanked by LoxP sites (Kif6 tm1c ) (KOMP repository, see Methods and Materials). Recombination of the Kif6 tm1c allele is predicted to generate a frameshift mutation, which should generate a severely truncated, 89 amino acid, KIF6 protein (p.G83E+6fs) with a non-functional N-terminal motor domain. We generated a whole body conditional knockout by crossing the Kif6 tm1c mouse to the CMV-Cre deleter mouse [21]. We observed postnatal-onset, hydrocephalus in CMV-Cre; Kif6 tm1c/tm1c conditional mutant mice (n = 12) analogous to our observations in Kif6 p.G555fs mutant mice (S3 Fig). Interestingly, we find no evidence of non-sense mediated decay in these mutant mice despite the generation of an early premature termination codon ( Fig 2B). Because the onset and progression of hydrocephalus was equivalent comparing the whole-body conditional CMV-Cre;Kif6 tm1c/tm1c and Kif6 p.G555fs mutant mice strains we suggest that any KIF6 protein encoded by these mutant mouse strains is likely non-functional. Given its relevance to the human mutation, the majority of experiments were all done using the p. G555fs allele.
Mouse brains were analyzed histologically by hematoxylin and eosin (H&E) stained coronal sections. Our analysis of coronal sectioned brain at P14 failed to find significance when comparing the total area in section (499.2+39.9μm (Control) vs. 552.5+50.8μm (Kif6 p.G555fs ); n = 7/ genotype; p = 0.42). However, lateral and third ventricles (LV and 3V respectively) were obviously enlarged in both Kif6 p.G555fs and CMV-Cre;Kif6 tm1c/tm1c mutant mice ( Fig 2F and S3 Fig). Quantitation of LV area normalized to total brain area confirmed ventricular expansion in Kif6 p.G555fs (n = 7/genotype; p�0.05; Fig 2G) and in CMV-Cre;Kif6 tm1c/tm1c mutant animals (n = 5/genotype; p�0.05; S3 Fig). No obvious defects of the cortex or dysmorphology of other regions of the brain were apparent in these mice (Fig 2E and 2F and S3 Fig). Together these data suggest that Kif6 mutant mice display postnatal-onset, progressive hydrocephalus, without obvious overgrowth of neural cortex.

Kif6 is expressed specifically in the ECs of the mouse brain
To determine the endogenous expression patterns of Kif6 in the mouse, we also isolated a Kif6-LacZ reporter mouse (Kif6-LacZ tm1b ) (KOMP repository, see Methods and Materials). Hemizygous Kif6-LacZ tm1b/+ mice appeared unremarkable and exhibited no evidence of hydrocephalus. Intercrosses of Kif6-LacZ tm1b/+ hemizygous mice failed to generate litters with Kif6-LacZ tm1b/tm1b homozygous mice, suggesting that the homozygosity of the lacZ expressing allele is embryonic lethal (0/13 Kif6-LacZ tm1b/tm1b homozygous mutant mice from 3 independent litters). At P10 and P21, Kif6-LacZ tm1b/+ transgenic mice showed lacZ expression in the ECs of the ventricular system (red arrows; Fig  Interestingly at P10, other multi-ciliated tissues such as the oviduct or trachea were not labeled in these transgenic mice, despite the clear presence of cilia observed by oblique lighting (red arrows Fig 3B' and 3C') as well as by IF using acetylated-tubulin to label axonemes in adjacent sections of these tissues ( Fig 3B" and 3C"). No obvious changes to oviduct or trachea cilia were observed in Kif6 p.G555fs mutant mice at P21 (S5 Fig), suggesting that Kif6 expression and function are tightly restricted to the multiciliated ECs in mouse. Taken together these data suggested a cellular mechanism centered on defective ECs underlying the development of hydrocephalus in Kif6 mutant mice.

Loss of Cilia in Kif6 mutant mice
Defects of EC cilia are known to cause hydrocephalus in mouse [8]. To assay this we first utilized scanning electron microscopy (SEM) to directly visualize the LV en face. Heterozygous Kif6 p.G555fs/+ mice displayed a high-density of regularly spaced EC multiciliated tufts along the LV surface ( Fig 4A and 4A'), typical at P21 in mouse development [22]. In contrast, homozygous Kif6 p.G555fs mutant mice displayed a marked reduction of multiciliated tufts across the LV wall (p = 0.029; Avg. # ECs /section het = 225; Avg. # ECs /section mutant = 129.3), coupled with a reduction in the density of ciliary axonemes within these ciliary tufts (Fig 3B and 3B'). This phenotype was even more obvious at P28 in Kif6 mutant mice (p = 0.001; Avg. # ECs / section het = 146; Avg. # ECs /section mutant = 0) (S6 Fig). Together, these data suggested that hydrocephalus may result from either a reduction in EC differentiation and/or defects in EC cilia during postnatal development.
To address the differentiation status of the ECs, we utilized IF in coronal sectioned brain tissues to image known proteins components of the EC and their cilia. At P21, we observed the expression of the ependymal cell-marker S100B [22] throughout the epithelium lining luminal surface of the ventricles, as well as, the presence of apically localized γ-tubulin-positive basal bodies within these ECs in both WT ( Fig 4C and 4C'') and Kif6 p.G555fs mutant mice ( Fig 4D  and 4D''). Conversely, we observed a obvious reduction in the density of CD133-positive EC axonemes [23] extending into the ventricular lumen in Kif6 p.G555fs mutant mice ( Fig 4C and  4C‴), compared with WT ( Fig 4D and 4D‴). Quantitation of binned mean fluorescence intensity from of CD133-positive axonemes confirmed a severe reduction of EC axonemes in Kif6 mutant mice (n = 9 mice/genotype, p�0.001) (Fig 4E).
In order to address whether Kif6 is required for EC cilia formation or ciliogenesis, we performed IF at P14, the time point at which the ECs are fully ciliated across the ventricular system in mouse [22]. We observe significant alterations in the extension of EC ciliary axonemes (CD133-positive) (n = 5 mice/genotype, p�0.01), without an alterations in the specification of ECs or their apical polarity (S7 Fig). The reduction of EC cilia was also observed at both P14 and P21 in Kif6 p.G555fs mutant mice using an acetylated tubulin antibody, which labels both primary and motile cilia as well as neuronal cell types in the cortex (S8 Fig). Taken together, these results suggest that the onset of hydrocephalus in Kif6 mutant mice is primarily due to a general defect of cilia formation (ciliogenesis) and not the result of defects in differentiation of the ECs or a loss of EC cilia as the result of the onset of hydrocephalus.
Alterations of the motility or synchronicity of EC cilia beating are known to cause hydrocephaly in mouse [8]. In order to address if Kif6 is required for normal EC cilia beating we performed live ex vivo imaging of lateral wall explants taken from both Kif6 mutant and littermate control mice. We consistently observed multiple tufts of EC cilia in WT animals that beating in a synchronous fashion at P21, with obvious flow generation, demonstrated by the movement of fortuitous particles within the media (S1 Movie). As expected, we observed an obvious reduction in formation of EC cilia coupled with reduced particle flow in explants from Kif6 mutant mice (S2 Movie). Interestingly, on the rare occasions where we did observed EC cilia in these mutant mice, the motility of these EC cilia appeared typical. These results in conjunction with our IF and SEM analysis (Fig 4, S6-S8 Figs) suggest that the onset of progressive hydrocephaly in Kif6 mutant mice is the result of defective ciliogenesis of ECs leading to a reduction in near-wall CSF flow.

Ventricular dilation and Reduced EC cilia in kif6 mutant zebrafish
Defects in EC cilia in the central canal leads to impaired CSF flow and hydrocephalus in zebrafish embryos [24]. Our previous studies in kif6 sko mutant zebrafish observed late-onset scoliosis in larval zebrafish, without defects of CSF flow, EC cilia in the central canal, or hydrocephalus during early embryonic development [19]. However, in contrast to the defined period of EC development in the mouse, zebrafish demonstrate continuous differentiation of ECs within the ventricular system throughout adult development [25]. Interestingly, recent studies in a variety mutant zebrafish demonstrate that reduced CSF flow, ventricular dilation, and loss of EC cilia may underlie the onset late-onset scoliosis in larval zebrafish [26]. In order to determine if kif6 mutant zebrafish might also display changes in the ventricular system in adults, we used iodine contrast-enhanced, micro computed tomography (μCT) [27] to generate high-resolution (5 μm voxel size) images of aged matched, 3-month-old WT and kif6 sko homozygous mutant zebrafish (Fig 5A and 5B'). After reconstruction and alignment of 3D tomographic datasets in the coronal plane, we utilized visualization software (Avizo Lite v.9.5) for 3D-reconstruction and segmentation of a virtual endocast to represent the ventricular volume in wildtype (WT) and kif6 sko mutant zebrafish (Fig 5A-5D'). Analysis of the ventricles in the endocast of the adult zebrafish brain highlighted several dysmorphophic regions in kif6 sko mutants including dilation of the diencephalic ventricle (DiV) (Fig 5D) and dilation of regions of the central canal (red arrows; Fig 5D and 5D'). At the same time, we observe that while some ventricles patent in wild-type zebrafish, were completely obstructed or less open in kif6 sko mutants (asterisks; Fig 5C' and 5D').
To further describe and quantify alterations of the ventricular system we utilized individual transverse optical slices from these contrast-enhanced μCT datasets, using stereotyped landmarks of the zebrafish brain and spinal cord to compare equivalent axial sections. At distinct axial levels of the brain (Fig 5E), we observed consistent dilation of the ventricular system and central canal in kif6 sko homozygous mutant zebrafish (yellow arrows; Fig 5G, 5I and 5K), compared to the stereotyped anatomy described for the adult zebrafish brain (Fig 5F, 5H and 5J) [28]. Multiple regions of kif6 sko mutant zebrafish brain were found to be structurally abnormal in kif6 sko mutants compared to WT zebrafish (S3, S4 Movies). We next quantified the areas of two anatomically distinctive ventricles in our tomographic datasets: (i) the medial tectal ventricle (TecV) at the medial division of valvula cerebelli (Vam) (Fig 5F and 5G) and (ii) a region of the rhombencephalic ventricle (RV) just posterior to the lobus facialis (Fig 5H and 5I) [28]. We observed a significant increase in the area (dashed yellow line, Fig 5F-5I) of the medial TecV and the posterior RV in kif6 sko mutant zebrafish comparing several optical sections from independent aged-matched zebrafish (n = 3 fish/genotype; p<0.0001). The central canal was also clearly dilated in kif6 sko mutant zebrafish (yellow arrow, Fig 5K). However, we were unable to reliably quantify this area in WT samples at the current imaging resolution. Our previous observations in kif6 sko mutant zebrafish embryos failed to find phenotypes that are characteristic of cilia defects, such as hydrocephalus, situs inversus, or kidney cysts [19]. Moreover, we observed normal development and function of motile EC cilia within the central canal in embryonic mutant zebrafish [19]. These data, coupled with our new observations of ventricular dilation in adult kif6 mutants (Fig 5), suggest that Kif6 is required for the postembryonic, robust development of the EC cilia within the ventricles of the brain as was reported in other zebrafish mutants displaying similar late-onset scoliosis, as observed in kif6 mutant zebrafish [19,26].
In order to assay whether EC cilia were affected during adult development in zebrafish, we isolated a stable transgenic allele, Tg(Foxj1a:GFP) dp1 previously reported to label multiciliated Foxj1a-positive multi-ciliated cells (MCCs), including ECs, with cytoplasmic EGFP in zebrafish [26]. Using this transgenic approach, we observed no differences in the specification of Foxj1a:GFP-positive ECs comparing adult (90dpf) heterozygous kif6 sko/+ phenotypically wildtype and homozygous kif6 sko mutant fish (Fig 6A and 6B). Cytoplasmic GFP can freely diffuse through the transition zone of cilia and label the axoneme [29]. As such, we were also able to observe GFP-filled EC axonemes projecting into the ventricular lumen in Tg(Foxj1a:GFP) dp1 ; kif6 sko/+ heterozygous fish (red arrowheads; Fig 6A). In contrast, these GFP-filled EC axonemes were reduced or absent in Tg[Foxj1a:GFP] dp1 ; kif6 sko mutant fish (Fig 6B). To further support our model that EC cilia were affected in kif6 mutant zebrafish, we generated a noveltransgenic fish line Tg(Foxj1a:Arl13b-GFP) dp22 to allow for the direct fluorescent-labeling of ciliary axonemes with mouse Arl13b-GFP specifically in fox1a-expressing lineages. Analysis of kif6 sko ; Tg(Foxj1a:Arl13b-GFP) dp22 transgenic adult mutant zebrafish (90dpf) demonstrated an obvious reduction in Arl13b-GFP labeled axonemes (Fig 6D), in comparison to robust labeling of EC cilia in heterozygous kif6 sko/+ ;Tg(Foxj1a:Arl13b-GFP) dp22 sibling fish (Fig 6C). Fluorescently-tagged Kif6 (Kif6-GFP) localizes to the basal body of Kuper's vesicle motile cilia in zebrafish [30]. Indeed our own attempts to visualize Kif6-EGFP by microinjection demonstrate that Kif6-EGFP localizes to microtubule-rich spindle poles and mitotic spindle during cell divisions in the rapidly dividing blastomeres of the early embryo (S10 Fig). Our attempts to drive Kif6-EGFP within the ECs using the foxj1a promoter in transgenic zebrafish have failed for this purpose, whereas clear signal was obtained for similar foxj1a-driven GFP and Arl13bGFP transgenic constructs (Fig 6). Unfortunately, IF and Westerns using several commercially-available KIF6 antibodies were unsuccessful to report on endogenous KIF6 in both mouse and zebrafish tissues (S11 Fig). For these reasons, we turned to the muco-ciliated Xenopus laevis epidermis in order to address Kif6 localization in an analogous MCCs lineage. The Xenopus mucociliary epithelium is analogous to the airway epithelium of the mammalian trachea [31]. Importantly, this system provides a robust model system for both genetic analysis [32] and robust visualization of fluorescently-tagged proteins in MCCs in vivo [33,34]. By microinjection of synthetic Xenopus Kif6-GFP RNA, we observed Kif6-GFP localization at the basal bodies of MCCs (Fig 7D and  7F) and co-localized with a basal body marker; Centrin-BFP [35] (Fig 7E and 7F). Similarly, we observed Kif6-GFP puncta within the axoneme (co-labeled with pan-membrane bound RFP) (Fig 7A-7C). Interestingly, similar punctate localization at the basal bodies and within the axoneme has been previously been shown in Xenopus MCCs using various fluorescentlytagged IFT proteins [36]. Together these data suggest a model in which Kif6 may act as a component of IFT trafficking uniquely and specifically in EC cilia, which is dispensable for the function of other MCCs in mouse and zebrafish.

Discussion
This study demonstrates the importance of KIF6 for EC cilia formation and homeostasis of the ventricular system in vertebrates, and potentially implicates a novel locus for understanding neurological defects in humans. This is supported by several lines of evidence including the discovery of a novel nonsense-mutation of KIF6 in a child with intellectual disability and megalencephaly and underscored by functional analysis in both mouse and zebrafish Kif6 mutant models (Table 2).
We identified a homozygous KIF6 c.1193delT mutation in a child with macrocephaly and cognitive impairment that segregated with this phenotype in his family, and leads to a loss of the C-terminal second and third coiled-coil regions which are important for dimerization and cargo selectivity of kinesin motors [13]. We engineered an analogous, C-terminal truncating mutation of KIF6 in mouse, which displays severe hydrocephalus and defects of EC cilia providing strong evidence for pathogenicity of the mutation in the child. Other than the case described here, no prior mutation directly attributed to human disease has been described for KIF6. Taken together, the clinical data reported here suggest that biallelic mutations in KIF6 may underlie some unexplained intellectual disability and neurological developmental defects. Future analyses of KIF6 mutations in these patient groups are warranted.
In addition, our analyses of several independent loss-of-function Kif6 mutant animal models found no evidence of obvious heart abnormalities to explain the prior association of the common variant KIF6 p.W719R in some [17], but not all [18], studies of coronary heart disease in humans. Because expressed sequence tag clones of KIF6 have not been reported from cDNAs libraries derived from human heart or vascular tissues (UniGene 1956991-Hs.588202), any possible functional effects of KIF6 on heart function remains unexplained. However, detailed analysis of coronary function was not explored in our models, therefore it is possible that subtle defects may be present.
Previous reports of an ENU-derived Kif6 splice acceptor site mutant mouse strain, predicted to delete the 3rd exon of KIF6 (Kif6Δ 3/ Δ 3 ), also did not show cardiac or lipid abnormalities [37]. Of note this mutant mouse was also not reported to have hydrocephalus. Our analysis shows that the loss of exon 3 in Kif6Δ 3/ Δ 3 mutant mouse generates an inframe deletion of only 25 amino acids in the N-terminal motor domain of the KIF6 protein, otherwise generating a mostly full-length KIF6 protein (Table 2). In contrast, here we report two novel Kif6 mutant mice: (i) a C-terminal Kif6 p.G555fs/p.G555fs deletion mutant, predicted to truncate 248 amino acids of the C-terminal domain, which are important for cargo binding in Kinesin motor proteins [13]; and (ii) a conditional CMV-Cre;Kif6 tm1c/tm1c mutant which recombines exon 4 leading to an early frame shift mutation predicted to generate a N-terminal truncated 122 amino acid KIF6 protein ( Table 2), both of which display indistinguishable progressive, hydrocephalus with EC ciliogenesis defects. The most parsimonious explanation for the difference in phenotypes in these mutant mice is that the Kif6Δ 3 allele encodes a functional KIF6 protein. Analysis of these mutations in trans or quantitative analysis of these kinesin motor proteins in vitro is warranted to more fully address these conflicting observations.
There are noticeable differences in the phenotypes among the human, mouse, and zebrafish associated with mutations in KIF6. For example, kif6 mutant zebrafish display post-natal onset scoliosis, mirroring adolescent idiopathic scoliosis (IS) in humans [38]. The formation of IS-like defects in zebrafish has been shown to be the result of a loss of CSF flow, associated with a loss of EC cilia and ventricular dilation during a defined window of larval zebrafish development [26]. Interestingly, we did not observe scoliosis in the Kif6 mutant mice (S12 Fig), despite being of an appropriate age when IS-like scoliosis can manifest in mouse [39]. Moreover, we do not observe scoliosis in the patient at the age of 10 years, though it is possible that he may yet develop scoliosis during adolescence. The mechanisms behind these differences may reflect distinctions in the functional input of the ventricular system for spine stability amongst teleosts and amniotes.
Furthermore, while we observe a clear role for KIF6 in maintaining the ventricular system in mouse and zebrafish, the patient does not have obvious hydrocephalus. However, his relative macrocephaly and slightly enlarged ventricles by MRI (Fig 1B-1D) may suggest an element of what is commonly referred to as arrested hydrocephalus [40]. Moreover, the contribution of EC cilia beating to bulk CSF flow might be species dependent. For instance, the majority of CSF flow in humans is thought to occur via the generation of a source-sink gradients; partly from the secretion of the choroid plexus and exchanges of the interstitial fluids, coupled with absorption at the arachnoid villi and lymphatics [41]. In contrast, localized or near-wall CSF flow [4], generated by polarized beating of EC cilia, are clearly important for the formation of hydrocephalus in rodents [8]; however, there have been limited evidence of EC cilia defects causing hydrocephalus in humans. Regardless there is growing evidence suggesting that EC cilia dependent CSF flow is crucial for the regulation of brain function and neurogenesis [4], and for adult neural stem cell proliferation [9]. It is possible that a specific loss of EC cilia in humans may only have minor effects on CSF bulk flow and ventricular homeostasis, while causing severe defects of neurogenesis leading to intellectual disability and other neurological disease. It will be important to determine (i) if the loss of KIF6 function during adult development in mouse will lead to a reduction in EC cilia; and (ii) whether the loss of EC cilia function contributes to ventricular dilation and decline of neurological function. Finally, KIF6 now joins five other kinesin genes, KIF1C, KIF2A, KIF4A, KIF5C and KIF7 that were previously reported to be associated with neurological abnormalities in humans [42][43][44][45]. Here we suggest that KIF6 has a uniquely specific function in the EC cilia in vertebrates, resulting in both cognitive impairment and macrocephaly in a child with a homozygous onebase pair deletion. Using a cell biological and transgenic approaches in both mouse and zebrafish, we identified specific loss of EC cilia these Kif6 mutant models suggesting a strong conservation of KIF6 function in ventricular system in vertebrates. Furthermore, we utilized imaging of fluorescently-tagged Kif6 in MCCs of the Xenopus epidermis, which are anatomically and functionally analogous to ECs lining the ventricles of the brain. Using this heterologous system we demonstrated that Kif6-GFP localizes to the basal bodies and as puncta within the ciliary axonemes in these MCCs, which is reminiscent of observations of canonical IFT proteins [36,46]. This imaging data coupled with our findings of defective ciliogenesis in ECs in both Kif6 mutant mice and zebrafish suggest a model where KIF6 is acting in concert with one or more anterograde Kinesin-II motors to promote robust ciliogenesis specifically in ECs. Indeed, tissue specific accessory IFT motors have been described in amphid sensory neurons in C. elegans [47] and zebrafish photoreceptors [48,49]. Interestingly, a homologue of KIF6, KIF9B, has been shown to be critical for flagellar motility and for the stabilization of the paraflagellar rod structure which tightly abutted the flagella in the protist, Trypanosoma brucei [15]. It remains to be determined whether KIF6 has an analogous, structural-functional role in ECs which may provide a unique functional role in these cilia, which is not critical for other analogous MCC lineages such as the trachea or oviduct. The confinement of LacZ expression specifically with EC in the Kif6-LacZ tm1b gene trap mouse suggest a large part of this specificity of function is simply due to strict regulation of Kif6 expression to the EC lineages, which would implicate the action of a tightly regulated, EC specific-promoter driving expression of the Kif6 locus. Both models will be important to test in future studies to better understand the critical components and pathways important for EC cilia development.

Ethics statement
The collection and use of human DNA samples in this study was approved by the Institutional Review Board Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand (IRB 381/ 61). All subjects provided written informed consent prior to inclusion in the study.
All animal research was conducted according to federal, state, and institutional guidelines and in accordance with protocols approved by Institutional Animal Care and Use Committees at University of Texas at Austin (AUP-2015-00185; AUP-2015-00187; and AUP-2018-00225).

Whole exome sequencing (WES)
The patient's genomic DNA of patient was extracted from peripheral blood leukocyte using AchivePure DNA Blood Kit (5 Prime Inc., Gaithersburg, MD). The sample was sent to Macrogen, Inc. (Seoul, Korea) for whole exome sequencing. The 4 ug of DNA sample was enriched by TruSeq Exome Enrichment Kit and was sequenced onto Hiseq 2000. The raw data per exome was mapped to the human reference genome hg19 using CASAVA v1.7. Variants calling were detected with SAMtools.

Homozygosity mapping
The sample was sent to Macrogen, Inc. (Seoul, Korea) for genotyping. The DNA sample was genotyped by HumanOmni 2.5-4v1 DNA BeadChip (Illumina) which contain 2,443,177 SNPs. The experiment was performed by the array protocol. PLINK was used to analyze for the homozygous regions.

Mutation analysis
We performed resequencing of KIF6 pathogenic region in patient and patient's family. Primers for the amplification of the candidate variant were designed using Primer 3 software

PCR-RFLP
One hundred chromosomes and patient's trio were genotyped by PCR-RFLP. Primer KIF6 MfeI F 5'-TGGCTTCACTATAAATTTCACTTTGTCAATG-3' and mutagenic primer KIF6 mutagenic MfeI R 5'-TCCTGGTCTTCCAAAAAGGATGCAAT-3'were used to amplify KIF6 T-deletion. The 20 ul of PCR reaction contained 50-100 ng of genomic DNA, 200 uM of each dNTP, 150 nM of each primer, 1.5 mM MgCl 2 and 0.5 unit of Taq DNA polymerase (Fermentas Inc., Glen Burnie, MD). The PCR condition was started with 95 C for 5 min for pre-denaturation following with the 35 cycles of 94 C for 30 sec, 60 C for 30 sec and 72 C for 30 sec. The product size of these primers is around 223 bp. The PCR product was incubated with 10U of Mfe-HF (New England Biolabs, Ipswich, MA) at 37 C overnight. Three percent of agarose gel electrophoresis was used to detect the different cut sizes of PCR product. A 196 bp and 26 bp bands were present in one base deletion sample.

Mice
All mouse studies and procedures were approved by the Animal Studies Committee at the University of Texas at Austin (AUP-2015-00185). The Kif6 p.G555fs mutant mouse were developed using CRISPR-Cas9-mediated genome editing. Using the CHOP-CHOP online tool [50], we identified a suitable 20-nucleotide site (GGAGATGTCACTGGGACGCC) targeting exon 14 of mouse Kif6 (ENSMUST00000162854.1) in order to generate a C-terminal truncation allele. The gene specific and universal tracrRNA oligonucleotides (S3 Table) were annealed, filled in with CloneAmp HiFi PCR premix, column purified, and directly used for in vitro transcription of single-guide RNAs (sgRNAs) with a T7 Polymerase mix (M0255A NEB). All sgRNA reactions were treated with RNAse free-DNAse. We utilized a ssDNA oligo (S3 Table) to insert a frameshift mutation in all three reading frames, along with 8-cutter restriction sites for genotyping (3- Table). PCR products in isolated alleles were cloned to pCRII TOPO (ThermoFisher) to identify scarless integration of the 3-stop donor at the Kif6 locus using gene specific flanking primers (S3 Table).
Kif6-LacZ tm1b mice were generated by injection of embryonic stem cell clones obtained from the Knockout Mouse Project (KOMP) Repository. Three Kif6 tm1a(KOMP)Mbp embryonic stem (ES) cell clones (KOMP: EPD0736_3_G01; EPD0736_3_H02; and EPD0736_3_A03) all targeting exon 4 of the Kif6 gene with a promoter-driven targeting cassette for the generation of a 'Knockout-first allele' [52]. Pronuclear injections of all clones were done using standard procedures established by the UT MGEF. After screening for germline transmission, we isolated and confirmed a single heterozygous founder male (Kif6 tm1a(KOMP)Mbp ) carrier derived from the G01 clone. We confirmed the locus by long-range PCR, several confirmation PCR strategies targeting specific transgene sequences, and Sanger sequencing of the predicted breakpoints (S3 Table). After several backcrosses to the WT C57BL/6J substrain (JAX), we crossed a hemizygous Kif6 tm1a/+ mutant male to a homozyogus CMV-Cre female (B6.C-Tg (CMV-cre)1Cgn/J) (JAX, 006054) to convert the Kif6 tm1a allele to a stable LacZ expressing Kif6 tm1b allele (Kif6-LacZ tm1b ). Mutant F1 offspring from this cross were backcrossed to WT C57BL6/J mice and the F2 progeny were genotyped to confirm the Kif6-LacZ tm1b allele and the presence/absence of the CMV-Cre transgene. A single founder Kif6-LacZ tm1b with the desired genotype (Kif6-LacZ tm1b hemizygous, Cre transgene absent) was used to expand a colony for spatial expression analysis.
Kif6 tm1c conditional ready mice were generated by outcross of the Kif6 tm1a(KOMP)Mbp allele described above to a ubiquitously expressed Flippase strain (129S4/SvJaeSor-Gt(ROSA) 26Sor tm1(FLP1)Dym /J) (JAX, 003946). F1 offspring were genotyped and sequenced at several breakpoints to ensure proper flip recombination and a single F1 founder was used to backcross to C57B6/J for propagation of the Kif6 tm1c strain. Analysis of recombination of the floxed Kif6 tm1c was performed by crossing homozygous Kif6 tm1c/tm1c to a compound heterozygous CMV-Cre; Kif6 tm1c/+ mouse. Recombination of the exon 4 of Kif6 was confirmed by PCR-gel electrophoresis analysis (S3 Table).

LacZ staining protocol
Mice were perfused with LacZ fixative and post fixed for 2 hours at RT. Whole brains were then stained in X-gal solution overnight at 37˚C followed by post-fixation in 4% PFA overnight at 4˚C. The samples were then prepped for cryosectioning in 30% sucrose/OCT and sectioned. Sections were counter stained in Nuclear Fast Red stain (Sigma).

X-ray analyses of mice
Radiographs of the mouse skeleton were generated using a Kubtec DIGIMUS X-ray system (Kubtec, T0081B) with auto exposure under 25 kV.

Zebrafish manipulations and transgenesis
All zebrafish studies and procedures were approved by the Animal Studies Committee at the University of Texas at Austin (AUP-2015-00187). Adult zebrafish of the AB were maintained and bred as previously described [53]. Individual fish were used for analysis and compared to siblings and experimental control fish of similar size and age. Independent experiments were repeated using separate clutches of animals. Strains generated for this study: Tg(Foxj1a: GFP) dp1 and Tg(foxj1a::Arl13b-GFP) dp15 . Transgenic lines were generated using a Gatewaycompatible middle entry cloning containing mouse Arl13b open reading frame [54]was modified to include a C-terminal GFP by megaprimer PCR to generate pME-Arl13b GFP. This clone was recombined with p5E-foxj1aP [26], p3E-polyA [55] and pDEST pDestTol2pA-CryGFP to generate a final transgenesis vector. Embryos were injected at the one-cell stage with 25 pg of assembled transgene and 25 pg Tol2 mRNA. Embryos were sorted at 48 hpf for reporter expression (GFP+ eyes) and were subsequently grown to adulthood. Individuals were bred to TU wild-type zebrafish to generate a stable F1 line, and subsequently bred into a kif6 sko mutant background. pDestTol2pACryGFP was a gift from Joachim Berger & Peter Currie (Addgene plasmid # 64022). Previously published strains: kif6 sko [19].

Xenopus manipulation and analysis
Xenopus embryo manipulations were carried out using standard protocols [56]. Full length of Xenopus Kif6 cDNA sequence was provided from Xenbase (www.xenbase.org) and amplified from Xenopus cDNA library by PCR and inserted in-frame into pCS10R-eGFP. 5'-capped Kif6-GFP RNA was synthesized using mMESSAGE mMACHINE SP6 transcription kit (Invitrogen Ambion). Synthetic 5'-capped RNAs: Kif6-GFP, membrane RFP and Centrin4-BFP [35] were injected into two ventral blastomeres at the 4-cell stage with~40 pg/RNA/injection. Live images were captured with a Zeiss LSM700 laser scanning confocal microscope using a plan-APOCHROMAT 63X 1.4 NA oil objective lens (Zeiss).

Mouse and zebrafish perfusions and embedding of brain tissues
Mice were humanely euthanized by extended CO 2 exposure and transferred to chemical hood where the mouse was perfused with buffered saline followed by 4% PFA. Whole brains were placed in 4% PFA 4 hours at RT, then at 4˚C overnight. Zebrafish were euthanized by exposure to lethal, extended dose of Tricane (8%) followed by decapitation. Zebrafish brains were extracted and fixed in 4% PFA at 4˚C overnight. For paraffin embedding, the fixed brains were embedded and cut using standard paraffin embedding and sectioning protocols. Paraffin sections were stained with standard hematoxylin-eosin solution.
For frozen sections both mouse or zebrafish brains were fixed as above and then equilibrated to 30% or 35% sucrose, respectively at 4˚C overnight. Whole brains were then placed in O.C.T. Compound (Tissue-Tek) and flash in cold ethanol bath. All blocks were stored at -80˚Celsius until sectioning on a cryostat (Leica). All sections were dried at RT for~2hrs. and stored at -80˚C until use.

Ex-Vivo ventricle imaging
Brains were isolated from freshly euthanized mice and were dissected in cold DMEM/F12 media. The brain was cut into thin coronal slices, promptly placed onto cover slip with PBS, and imaged using oblique lighting with a Keyence BZ-X800 microscope.

Statistical analysis and image measures
GraphPad Prism version 7.0c for Mac (GraphPad Software) was used to analyze and plot data. Images for measurement were opened in FIJI (Image J) [57], and measures were taken using the freehand tool to draw outlines on ventricular area or whole brain area. Statistically significant differences between any two groups were examined using a two-tailed Student's t-test, given equal variance. P values were considered significant at or below 0.05.
Supporting information S1 Fig. Clustal Alignment of CPE (p.V79M)     advice and assistance with the SEM imaging in this study, and Jessie Maisano and Matthew W. Colbert for advice for iodine-contrast μCT and 3D segmentation approaches used in this study.