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The transition zone (TZ) of the cilium/flagellum serves as a diffusion barrier that controls the entry/exit of ciliary proteins. Mutations of the TZ proteins disrupt barrier function and lead to multiple human diseases. However, the systematic regulation of ciliary composition and signaling-related processes by different TZ proteins is not completely understood. Here, we reveal that loss of TCTN1 in Chlamydomonas reinhardtii disrupts the assembly of wedge-shaped structures in the TZ. Proteomic analysis of cilia from WT and three TZ mutants, tctn1, cep290, and nphp4, shows a unique role of each TZ subunit in the regulation of ciliary composition, explaining the phenotypic diversity of different TZ mutants. Interestingly, we find that defects in the TZ impair the formation and biological activity of ciliary ectosomes. Collectively, our findings provide systematic insights into the regulation of ciliary composition by TZ proteins and reveal a link between the TZ and ciliary ectosomes.

BackgroundJoubert syndrome (JS, OMIM: 213300) is a recessive developmental disorder characterized by cerebellar vermis hypoplasia and a distinctive mid‐hindbrain malformation called the “molar tooth sign” on axial magnetic resonance imaging. To date, more than 35 ciliary genes have been identified as the causative genes of JS.MethodsWhole exome sequencing was performed to detect the causative gene mutations in a Chinese patient with JS followed by Sanger sequencing. RT‐PCR and Sanger sequencing were used to confirm the abnormal transcript of centrosomal protein 104 (CEP104, OMIM: 616690).ResultsWe identified two novel heterozygous mutations of CEP104 in the proband, which were c.2364+1G>A and c.414delC (p.Asn138Lysfs*11) (GenBank: NM_014704.3) and consistent with the autosomal recessive inheritance mode.ConclusionOur study reported the fourth case of JS patients with CEP104 mutations, which expands the mutation spectrum of CEP104 and elucidates the clinical heterogeneity of JS.

IFT74 is identified as a Joubert syndrome-associated gene in a Chinese cohort. The variants disrupt intraflagellar transport and ciliary signaling.

Joubert syndrome (JBTS) and Meckel-Gruber syndrome (MKS) are rare recessive disorders caused by defects of cilia, and they share overlapping clinical features and allelic loci. Mutations of MKS1 contribute approximately 7% to all MKS cases and are found in some JBTS patients. Here, we describe a JBTS patient with two novel mutations of MKS1. Whole exome sequencing (WES) revealed c.191-1G > A and c.1058delG compound heterozygous variants. The patient presented with typical cerebellar vermis hypoplasia, hypotonia, and developmental delay, but without other renal/hepatic involvement or polydactyly. Functional studies showed that the c.1058delG mutation disrupts the B9 domain of MKS1, attenuates the interactions with B9D2, and impairs its ciliary localization at the transition zone (TZ), indicating that the B9 domain of MKS1 is essential for the integrity of the B9 protein complex and localization of MKS1 at the TZ. This work expands the mutation spectrum of MKS1 and elucidates the clinical heterogeneity of MKS1-related ciliopathies.