TMEM237 is mutated in individuals with a Joubert syndrome related disorder and expands the role of the TMEM family at the ciliary transition zone

Forward genetics uncovers Transmembrane protein 107 as a novel factor required for ciliogenesis and Sonic hedgehog signaling

Cilia are dynamic organelles that are essential for a vast array of developmental patterning events, including left-right specification, skeletal formation, neural development, and organogenesis. Despite recent advances in understanding cilia form and function, many key ciliogenesis components have yet to be identified. By using a forward genetics approach, we isolated a novel mutant allele (schlei) of the mouse Transmembrane protein 107 (Tmem107) gene, which we show here is critical for cilia formation and embryonic patterning. Tmem107 is required for normal Sonic hedgehog (Shh) signaling in the neural tube and acts in combination with Gli2 and Gli3 to pattern ventral and intermediate neuronal cell types. schlei mutants also form extra digits, and we demonstrate that Tmem107 acts in the Shh pathway to determine digit number, but not identity, by regulating a subset of Shh target genes. Phenotypically, schlei mutants share several features with other cilia mutants; however, spatial restriction of mutant phenotypes and lack of left-right patterning defects in schlei animals suggest differential requirements for Tmem107 in cilia formation in distinct tissues. Also, in contrast to mutants with complete loss of cilia, schlei mutants retain some function of both Gli activator and repressor forms. Together, these studies identify a previously unknown regulator of ciliogenesis and provide insight into how ciliary factors affect Shh signaling and cilia biogenesis in distinct tissues.

The ciliopathies: a transitional model into systems biology of human genetic disease

The last decade has witnessed an explosion in the identification of genes, mutations in which appear sufficient to cause clinical phenotypes in humans. This is especially true for disorders of ciliary dysfunction in which an excess of 50 causal loci are now known; this discovery was driven partly by an improved understanding of the protein composition of the cilium and the co-occurrence of clinical phenotypes associated with ciliary dysfunction. Despite this progress, the fundamental challenge of predicting phenotype and or clinical progression based on single locus information remains unsolved. Here, we explore how the combinatorial knowledge of allele quality and quantity, an improved understanding of the biological composition of the primary cilium, and the expanded appreciation of the subcellular roles of this organelle can be synthesized to generate improved models that can explain both causality but also variable penetrance and expressivity.

Mutations in C5ORF42 cause Joubert syndrome in the French Canadian population

Joubert syndrome (JBTS) is an autosomal-recessive disorder characterized by a distinctive mid-hindbrain malformation, developmental delay with hypotonia, ocular-motor apraxia, and breathing abnormalities. Although JBTS was first described more than 40 years ago in French Canadian siblings, the causal mutations have not yet been identified in this family nor in most French Canadian individuals subsequently described. We ascertained a cluster of 16 JBTS-affected individuals from 11 families living in the Lower St. Lawrence region. SNP genotyping excluded the presence of a common homozygous mutation that would explain the clustering of these individuals. Exome sequencing performed on 15 subjects showed that nine affected individuals from seven families (including the original JBTS family) carried rare compound-heterozygous mutations in C5ORF42. Two missense variants (c.4006C>T [p.Arg1336Trp] and c.4690G>A [p.Ala1564Thr]) and a splicing mutation (c.7400+1G>A), which causes exon skipping, were found in multiple subjects that were not known to be related, whereas three other truncating mutations (c.6407del [p.Pro2136Hisfs*31], c.4804C>T [p.Arg1602*], and c.7477C>T [p.Arg2493*]) were identified in single individuals. None of the unaffected first-degree relatives were compound heterozygous for these mutations. Moreover, none of the six putative mutations were detected among 477 French Canadian controls. Our data suggest that mutations in C5ORF42 explain a large portion of French Canadian individuals with JBTS.

The ciliary transition zone: from morphology and molecules to medicine

Researchers from various disciplines, including cell and developmental biology, genetics and molecular medicine, have revealed an exceptional diversity of cellular functions that are mediated by cilia-dependent mechanisms. Recent studies have directed our attention to proteins that localize to the ciliary transition zone (TZ), a small evolutionarily conserved subcompartment that is situated between the basal body (BB) and the more distal ciliary axoneme. These reports shed light on the roles of TZ proteins in ciliogenesis, ciliary protein homeostasis and specification of ciliary signaling, and pave the way for understanding their contribution to human ciliopathies. In this review, we describe the interplay of multimeric protein complexes at the TZ, integrating morphological, genetic and proteomic data towards an account of TZ function in ciliary physiology.