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. 2012 Nov 2;91(5):958-64.
doi: 10.1016/j.ajhg.2012.10.003.

Loss-of-function Mutations in LRRC6, a Gene Essential for Proper Axonemal Assembly of Inner and Outer Dynein Arms, Cause Primary Ciliary Dyskinesia

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Free PMC article

Loss-of-function Mutations in LRRC6, a Gene Essential for Proper Axonemal Assembly of Inner and Outer Dynein Arms, Cause Primary Ciliary Dyskinesia

Esther Kott et al. Am J Hum Genet. .
Free PMC article

Abstract

Primary ciliary dyskinesia (PCD) is a group of autosomal-recessive disorders resulting from cilia and sperm-flagella defects, which lead to respiratory infections and male infertility. Most implicated genes encode structural proteins that participate in the composition of axonemal components, such as dynein arms (DAs), that are essential for ciliary and flagellar movements; they explain the pathology in fewer than half of the affected individuals. We undertook this study to further understand the pathogenesis of PCD due to the absence of both DAs. We identified, via homozygosity mapping, an early frameshift in LRRC6, a gene that encodes a leucine-rich-repeat (LRR)-containing protein. Subsequent analyses of this gene mainly expressed in testis and respiratory cells identified biallelic mutations in several independent individuals. The situs inversus observed in two of them supports a key role for LRRC6 in embryonic nodal cilia. Study of native LRRC6 in airway epithelial cells revealed that it localizes to the cytoplasm and within cilia, whereas it is absent from cells with loss-of-function mutations, in which DA protein markers are also missing. These results are consistent with the transmission-electron-microscopy data showing the absence of both DAs in cilia or flagella from individuals with LRRC6 mutations. In spite of structural and functional similarities between LRRC6 and DNAAF1, another LRR-containing protein involved in the same PCD phenotype, the two proteins are not redundant. The evolutionarily conserved LRRC6, therefore, emerges as an additional player in DA assembly, a process that is essential for proper axoneme building and that appears to be much more complex than was previously thought.

Figures

Figure 1
Figure 1
LRRC6 Mutations and Their Impact at the Protein Level in Individuals with PCD (A) Exonic organization of the human LRRC6 cDNA, in which are shown the mutations (top) and domain-organization model of the corresponding protein (middle). The mutations’ impact at the protein level is shown for the five families described in this study (bottom). The twelve exons are indicated by empty or hashed boxes, depicting translated or untranslated sequences, respectively. According to the LRRC6 domain-organization model, derived from predictions by NCBI and UniProt/Swiss-Prot, the protein contains four LRR domains (amino acids 23–42, 46–65, 68–87, and 90–112), one modified LRR domain (amino acids 115–130) and a subsequent LRRcap (amino acids 131–146), a coiled-coil domain (amino acids 178–204), a polylysine motif (amino acids 272–286), and an α-crystallin-p23-like domain (amino acids 332–381). (B) A partial protein alignment of LRRC6 shows the evolutionary conservation of the third LRR motif and the LRRcap domain, which contains the two amino acid substitutions identified in this study.
Figure 2
Figure 2
Absence of Both DAs in Respiratory Cilia and/or Spermatozoa Flagella of Individuals with LRRC6 Mutations The electron micrographs of cross-sections of cilia and/or spermatozoa flagella from a control and individuals with identified LRRC6 mutations are shown. The blue flashes and triangles show the presence of ODAs and IDAs, respectively, and red flashes and triangles show their absence in affected individuals’ axonemes. Note the partial absence of DAs in the airway cilia of DCP193 (blue flash), whereas at the sperm level, the absence of DAs seems complete. Black scale bars represent 0.1 μm.
Figure 3
Figure 3
LRRC6 Localizes to the Cytoplasm and Cilia and Is Absent in DCP18 (A–F) In control cells, LRRC6 (green) localizes to the cytoplasm and within cilia, whereas it is absent from the individual’s cells. Axoneme-specific antibodies directed against α-β-tubulin were used as a control. Nuclei were stained with DAPI (blue). Similar experiments were performed with antibodies directed against the ODA component DNAI2 (G–L) and the IDA component DNALI1 (M–R). In control cells, DNAI2 and DNALI1 localize within the cilia, whereas labeling is absent from the individual’s cells. Airway epithelial cells were examined after labeling with a mouse LRRC6 antibody (Abnova) and a secondary goat anti-mouse Alexa Fluor-488 (green) antibody (Invitrogen). For controls, we used antibodies directed against various axonemal components: α-β-tubulin (Cell Signaling Technology), DNAI2 (Abnova), and DNALI1 (Abnova) for the visualization of microtubules, ODAs, and IDAs, respectively; primary antibodies were revealed with a secondary Alexa Fluor-594 (red) or Alexa Fluor-488 (green) antibody (Invitrogen). White scale bars represent 5 μm.

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