Mutations in the gene encoding cytoskeleton-associated protein 2-like (CKAP2L) have been identified as a causative factor for Filippi syndrome, a rare developmental disorder characterized by facial dysmorphism, syndactyly, and microcephaly. However, the cellular and molecular mechanisms by which CKAP2L contributes to the pathogenesis of this syndrome remain largely unknown. Here, we generate a Ckap2l knockout mouse model to investigate the in vivo and cellular roles of CKAP2L. Interestingly, Ckap2l knockout mice show no overt developmental abnormalities, with the exception of reduced male fertility, evidenced by decreased sperm count, impaired motility, and abnormally elongated flagella. At the cellular level, CKAP2L is a bona fide microtubule-associated protein that localizes to microtubule-based organelles, including the centrosome, mitotic spindle, and ciliary basal body. Depletion of CKAP2L leads to shortened mitotic spindles and cytokinesis failure, resulting in multinucleation. Furthermore, we uncover a conserved function for CKAP2L as a negative regulator of primary cilium length; its loss markedly increases ciliary length in both human and mouse cells. Collectively, these findings position CKAP2L as a multifunctional regulator of microtubule-based organelles and propose that Filippi syndrome can be classified as a 'centrosomopathy' arising from concurrent defects in cell proliferation and ciliary function.

In the developing brain, the primary cilia of radial glial cells extend from the surface of the lateral ventricle, serving as the signaling hub to integrate environmental cues critical for brain formation. Dysfunctions in ciliary proteins contribute to a wide range of brain structural abnormalities in ciliopathies, yet identifying the bona fide ciliary protein components in the brain remains a significant challenge. Here, using proximity labeling and quantitative proteomics, we systematically charted proteins localized to the cilia of radial glial cells in the dorsal and ventral regions of the embryonic brain. From this dataset, we identified cohorts of new molecules intrinsic to the cilia at distinct regions of the developing brain. We validated the cilium localization of several components of the translation machinery and studied the mechanistic roles of ciliary candidates previously linked to brain malformation, including Marcks, a key regulator of radial glial polarity, and Ckap2l, a protein associated with Filippi syndrome. These results revealed previously unrecognized ciliary mechanisms that regulate brain development. Thus, our brain proteomic dataset provides a unique resource for understanding ciliary functions in brain development and the molecular etiology of developmental disorders.

Filippi syndrome is a rare genetic disorder characterized by growth and neurodevelopmental delays, dysmorphism, and selective limb abnormalities. Although the syndrome was described approximately four decades ago, only a few families with molecularly confirmed diagnoses have been reported. In this article, we present three new patients of Filippi syndrome with unusual clinical and genetic aspects. These patients exhibited novel clinical features that have not previously been associated with Filippi syndrome, including renal hypoplasia/aplasia, renal cysts, renal cortical thinning, hypomelanotic, and hypermelanotic macules. All three patients had homozygous frameshift variants of the CKAP2L gene, specifically NM_152515.3: c.554_555del, c.981_982del, and c.1463_1467del, with the second being a novel variant. Given the limited number of reported Filippi syndrome patients to date and the ongoing discovery of new clinical aspects of the disease, exploring its potential connection with kidney and skin pigmentation abnormalities could be valuable for future research.

Pathogenic variants in CKAP2L have previously been reported in Filippi syndrome (FS), a rare autosomal recessive, craniodigital syndrome characterized by microcephaly, syndactyly, short stature, intellectual disability, and dysmorphic facial features. To date, fewer than 10 patients with pathogenic variants in CKAP2L associated with FS have been reported. All of the previously reported probands have presumed loss-of-function variants (frameshift, canonical splice site, starting methionine), and all but one have been homozygous for a pathogenic variant. Here we describe two brothers who presented with microcephaly, micrognathia, syndactyly, dysmorphic features, and intellectual disability. Whole-exome sequencing of the family identified a missense variant, c.2066G > A;p.(Arg689His), in trans with a frameshift variant, c.1169_1173del;p.(Ile390LysfsTer4), in CKAP2L To our knowledge, these are the first patients with FS to be reported with a missense variant in CKAP2L and only the second family to be reported with two variants in trans.