Abnormal cilia in a male-sterile mutant mouse

Biallelic Mutations in Tetratricopeptide Repeat Domain 26 (Intraflagellar Transport 56) Cause Severe Biliary Ciliopathy in Humans

BACKGROUND AND AIMS

The clinical consequences of defective primary cilium (ciliopathies) are characterized by marked phenotypic and genetic heterogeneity. Although fibrocystic liver disease is an established ciliopathy phenotype, severe neonatal cholestasis is rarely recognized as such.

APPROACH AND RESULTS

We describe seven individuals from seven families with syndromic ciliopathy clinical features, including severe neonatal cholestasis (lethal in one and necessitating liver transplant in two). Positional mapping revealed a single critical locus on chromosome 7. Whole-exome sequencing revealed three different homozygous variants in Tetratricopeptide Repeat Domain 26 (TTC26) that fully segregated with the phenotype. TTC26 (intraflagellar transport [IFT] 56/DYF13) is an atypical component of IFT-B complex, and deficiency of its highly conserved orthologs has been consistently shown to cause defective ciliary function in several model organisms. We show that cilia in TTC26-mutated patient cells display variable length and impaired function, as indicated by dysregulated sonic hedgehog signaling, abnormal staining for IFT-B components, and transcriptomic clustering with cells derived from individuals with closely related ciliopathies. We also demonstrate a strong expression of Ttc26 in the embryonic mouse liver in a pattern consistent with its proposed role in the normal development of the intrahepatic biliary system.

CONCLUSIONS

In addition to establishing a TTC26-related ciliopathy phenotype in humans, our results highlight the importance of considering ciliopathies in the differential diagnosis of severe neonatal cholestasis even in the absence of more typical features.

The Intraflagellar Transport Machinery

Eukaryotic cilia and flagella are evolutionarily conserved organelles that protrude from the cell surface. The unique location and properties of cilia allow them to function in vital processes such as motility and signaling. Ciliary assembly and maintenance rely on intraflagellar transport (IFT), the bidirectional movement of a multicomponent transport system between the ciliary base and tip. Since its initial discovery more than two decades ago, considerable effort has been invested in dissecting the molecular mechanisms of IFT in a variety of model organisms. Importantly, IFT was shown to be essential for mammalian development, and defects in this process cause a number of human pathologies known as ciliopathies. Here, we review current knowledge of IFT with a particular emphasis on the IFT machinery and specific mechanisms of ciliary cargo recognition and transport.

A mutation in the mouse ttc26 gene leads to impaired hedgehog signaling

The phenotype of the spontaneous mutant mouse hop-sterile (hop) is characterized by a hopping gait, polydactyly, hydrocephalus, and male sterility. Previous analyses of the hop mouse revealed a deficiency of inner dynein arms in motile cilia and a lack of sperm flagella, potentially accounting for the hydrocephalus and male sterility. The etiology of the other phenotypes and the location of the hop mutation remained unexplored. Here we show that the hop mutation is located in the Ttc26 gene and impairs Hedgehog (Hh) signaling. Expression analysis showed that this mutation led to dramatically reduced levels of the Ttc26 protein, and protein-protein interaction assays demonstrated that wild-type Ttc26 binds directly to the Ift46 subunit of Intraflagellar Transport (IFT) complex B. Although IFT is required for ciliogenesis, the Ttc26 defect did not result in a decrease in the number or length of primary cilia. Nevertheless, Hh signaling was reduced in the hop mouse, as revealed by impaired activation of Gli transcription factors in embryonic fibroblasts and abnormal patterning of the neural tube. Unlike the previously characterized mutations that affect IFT complex B, hop did not interfere with Hh-induced accumulation of Gli at the tip of the primary cilium, but rather with the subsequent dissociation of Gli from its negative regulator, Sufu. Our analysis of the hop mouse line provides novel insights into Hh signaling, demonstrating that Ttc26 is necessary for efficient coupling between the accumulation of Gli at the ciliary tip and its dissociation from Sufu.

The Hedgehog (Hh) signaling pathway determines pattern formation in many developing tissues, e.g., during digit formation in the limbs, by regulating proteins of the Gli family. Activation of these proteins requires their transport to the tip of the primary cilium (an antenna-like sensory structure of the cell), and subsequent dissociation from their negative regulator, Sufu. Little is known about the mechanism underlying this dissociation. To gain new insights into Hh signaling, we analyzed the mutant mouse hop-sterile (hop), whose developmental defects suggest that the primary cilia are dysfunctional. We discovered that the hop mutation lies in the Ttc26 gene, and that levels of the encoded protein are low in hop mice. Normal Ttc26 was found to bind to Intraflagellar Transport (IFT) complex B, a structure essential for building the cilium and moving proteins towards its tip. Nevertheless, unlike previously characterized mutations that affect IFT complex B, hop did not interfere with either the formation of primary cilia or the accumulation of Gli at their tips, but rather with dissociation of Gli from Sufu. Our results provide novel insights into Hh signaling, demonstrating that efficient coupling between Gli's accumulation at the ciliary tip and its dissociation from Sufu depends on Ttc26.

Primary ciliary dyskinesia associated with normal axoneme ultrastructure is caused by DNAH11 mutations

Primary ciliary dyskinesia (PCD) is an inherited disorder characterized by perturbed or absent beating of motile cilia, which is referred to as Kartagener syndrome (KS) when associated with situs inversus. We present a German family in which five individuals have PCD and one has KS. PCD was confirmed by analysis of native and cultured respiratory ciliated epithelia with high-speed video microscopy. Respiratory ciliated cells from the affected individuals showed an abnormal nonflexible beating pattern with a reduced cilium bending capacity and a hyperkinetic beat. Interestingly, the axonemal ultrastructure of these respiratory cilia was normal and outer dynein arms were intact, as shown by electron microscopy and immunohistochemistry. Microsatellite analysis indicated genetic linkage to the dynein heavy chain DNAH11 on chromosome 7p21. All affected individuals carried the compound heterozygous DNAH11 mutations c.12384C>G and c.13552_13608del. Both mutations are located in the C-terminal domain and predict a truncated DNAH11 protein (p.Y4128X, p.A4518_A4523delinsQ). The mutations described here were not present in a cohort of 96 PCD patients. In conclusion, our findings support the view that DNAH11 mutations indeed cause PCD and KS, and that the reported DNAH11 nonsense mutations are associated with a normal axonemal ultrastructure and are compatible with normal male fertility.

Absent inner dynein arms in a fetus with familial hydrocephalus-situs abnormality

We report a family in which a healthy, unrelated couple had a male fetus with bilateral ventriculomegaly, a normal liveborn girl, a hydatidiform molar pregnancy, a female fetus with ventriculomegaly and situs abnormalities, and a male fetus with hydrocephalus, a three-lobed left lung, and defective tracheal cilia with absent inner dynein arms and a single centriole. A mutation analysis of FOXJ1 and POLL in the last fetus with ciliary defect revealed no mutation within their coding regions. The presence of three affected fetuses of both sexes in a family with phenotypically normal parents suggests that the condition was inherited as an autosomal recessive trait. A thorough evaluation of the thoracic and abdominal situs is recommended before counseling a family of a child with hydrocephalus, because the recognition of situs defects may point to the diagnosis of primary ciliary defect and recurrence risk of 25% for siblings. This figure is much higher than the general risk of 1-2% for siblings of a patient with isolated hydrocephalus.

Genetic analysis of inherited hydrocephalus in a rat model

Congenital hydrocephalus is a serious neurological disorder with a diverse etiology. Although there is strong evidence for genetic causes, few genes have been identified in humans. The rodent model, the H-Tx rat, has hydrocephalus with an onset in late gestation and a complex mode of inheritance. Ventricular dilatation is associated with abnormalities in the cerebral aqueduct and subcommissural organ. Quantitative trait locus (QTL) mapping was performed on DNA from the progeny of a backcross with the non-hydrocephalic Fischer F344 strain, using DNA microsatellite markers. The hydrocephalus trait was quantified by measuring the severity of the ventricular dilatation. Four chromosomes, each with a locus for hydrocephalus (Chrs 9, 10, 11, and 17), were mapped using additional markers and DNA from four subsets of backcross progeny with allelic recombination at or near each locus. The genetic positions for the markers and the loci were located using the Ensemble Rat Genome Browser. For each chromosome studied, the interval containing the locus was examined for known rat genes and for human genes identified from human-rat homology. Genes expressed in brain and with a function associated with known causes of hydrocephalus were identified as possible candidate genes. Future studies to characterize the causative genes in this animal model will improve the understanding of genetic causes in humans.

Unusual inheritance of primary ciliary dyskinesia (Kartagener's syndrome)

Primary ciliary dyskinesia syndrome is characterised by chronic sinusitis, bronchiectasis, and, in 50% of cases, dextrocardia. It is generally believed to be inherited as an autosomal recessive disorder. In this report, we describe a family consisting of a mother and her five male children, the offspring of three different fathers, all of whom have this syndrome. This argues for either an X linked or autosomal dominant pattern of inheritance. Cytogenetic and FISH (fluorescent in situ hybridisation) analyses were done on the mother and one son and were found to be normal.

Pathophysiology of the ciliated epithelium of the respiratory mucosa in humans. Disorders of ciliogenesis

If we want to interpret morphological alterations of the ciliated epithelium of the respiratory tract, we have to consider the physiological differentiation of ciliated cells and cilia. For this, we suggest a system in which disorders are distinguished at three distinct arrest levels of the ciliary development: (1) Disorders in the production of procentrioles (secondary centrioles) lead to partial or total aplasia of the ciliary apparatus. (2) Disorders in the development of kinetosomes from secondary centrioles result in a reduced number of cilia and malformations of ciliary components. (3) Disorders in the outgrowth of cilia result in morphological abnormalities of the peripheral ciliary shaft (i.e., immotile cilia syndrome). We suppose that at all three arrest levels hereditary factors may be important, whereas at levels 2 and 3 exogenous influences may cause these alterations too. Correlation between morphology and clinical status is made difficult by conditions of auxiliary mechanisms, like cough, which compensate the mucociliary dysfunctions. For the purpose of morphometric evaluation of the biopsies we established a method of light microscopical semiquantitative analysis of brush biopsies. The regenerative power of the ciliary apparatus with respect to these frequently observed abnormalities is discussed.

The immotile cilia syndrome. Mice versus man

When homozygous the recessive, pleiotropic, mutation hpy (hydrocephalic-polydactyl) produces post-natal hydrocephalus, complete sterility in males, and reduced reproductive performance in females. Because the fertility problems and the development of hydrocephalus could arise as consequences of defective flagella and ciliary axonemes, this mutant type might serve as a useful animal model for the immotile cilia syndrome. Ultrastructural defects seen in axonemes of flagella, and of cilia from the trachea, oviduct, and ependyma included: a deficiency of inner dynein arms (the most frequent defect); an absence of one or both central-pair tubules; extra central tubules; a displacement of one outer doublet and/or the central-pair tubules. Some axonemes showed more than one of these defects. The frequency of dynein-deficient axonemes in all three tissues was similar (about 35%) and fell within the range reported for human patients with the immotile cilia syndrome. On this basis, this mutant type might be considered as a useful animal model for such studies. There were no indications of situs inversus, nor was there a marked increase in respiratory problems. So hpy/hpy mice do not exhibit all of the clinical symptoms characteristic of the human condition.