Alternative titles; symbols
HGNC Approved Gene Symbol: B9D1
Cytogenetic location: 17p11.2 Genomic coordinates (GRCh38): 17:19,334,695-19,377,913 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 17p11.2 | ?Meckel syndrome 9 | 614209 | Autosomal recessive | 3 |
| Joubert syndrome 27 | 617120 | Autosomal recessive | 3 |
B9D1 belongs to a small family of proteins that also includes B9D2 (611951) and MKS1 (609883), and all 3 B9 domain-containing proteins associate with basal bodies and primary cilia in mammalian cells (Bialas et al., 2009). These proteins localize to the transition zone complex that functions within the cilium (Dowdle et al., 2011).
By searching for genes encoding B9 domain-containing proteins, Bialas et al. (2009) identified B9D1. The deduced protein consists of little more than the approximately 115-amino acid B9 domain. Epitope- or fluorescence-tagged B9D1, B9D2, and MKS1 localized to ciliary axonemes and basal bodies of transfected ciliated mouse IMCD3 cells and to centrosomes of nonciliated IMCD3 cells. In C. elegans, the mks1, mksr1, and mksr2 genes were expressed in the transition zone at the base of sensory cilia, which corresponds to mammalian basal body only. Database analysis revealed orthologs of B9D1, B9D2, and MKS1 in the vast majority of ciliated species, but not in nonciliated organisms. The 3 B9 domain-containing proteins appeared to be evolutionarily ancient, and the duplications resulting in the 3 protein clades preceded speciation.
Bialas et al. (2009) found that disruption of Mksr1 or Mksr2 genes via RNA interference in IMCD3 cells reduced the number of ciliated cells compared with control cultures.
Williams et al. (2011) showed that the conserved proteins Mks1 (609883), Mksr1 (B9D1), Mksr2 (B9D2), Tmem67 (609884), Rpgrip1l (610937), Cc2d2a (612013), Nphp1 (607100), and Nphp4 (607215), functioned at an early stage of ciliogenesis in C. elegans. These 8 proteins localized to the ciliary transition zone and established attachments between the basal body and transition zone membrane. They also provided a docking site that restricted vesicle fusion to vesicles containing ciliary proteins.
Using tandem affinity purification and mass spectrometry to isolate proteins that purified with B9d1 in mouse IMCD3 cells and embryonic fibroblasts, Chih et al. (2012) identified several components of the B9d1-containing ciliary complex, including Tmem231 (614949), Tmem17 (614950), B9d2 (611951), Tctn1 (609863), Tctn2 (613846), Mks1, Ahi1 (608894), Cc2d2a, and Kctd10 (613421). Knockdown of B9d1, Tmem231, Tmem17, or Cc2d2a via small interfering RNA had a modest effect on cilia formation, but significantly reduced the amount of the somatostatin receptor Sstr3 (182453) that localized to cilia. Knockdown of B9d1, Tmem231, Tmem17, or Cc2d2a also interfered with sonic hedgehog signaling (see SHH, 600725) by preventing the movement of Smo (SMOH; 601500) into the ciliary membrane. Knockout of B9d1 and Tmem231 resulted in delayed ciliogenesis and cilia growth due to absence of diffusion barrier formation. Chih et al. (2012) concluded that formation of a diffusion barrier by the B9d1 complex is required for the formation and retention of cilia components.
Bialas et al. (2009) stated that the B9D1 gene maps to chromosome 17p11.2.
Meckel Syndrome 9
Hopp et al. (2011) performed exon-enriched next-generation sequencing of 31 ciliopathy genes in 12 pedigrees with Meckel syndrome in which mutations in known genes causing this disorder were not found. One fetal index case was found to carry a splice donor site mutation in the B9D1 gene (505+2T-C; 614144.0001), which was inherited from the father. Array CGH showed that the second mutation was a 1.71-Mb de novo deletion (614144.0002) at chromosome 17p11.1, eliminating the entire B9D1 locus and 18 other genes. Immunofluorescence analysis revealed a significantly lower level of ciliated patient cells compared to controls, confirming a role for B9D1 in ciliogenesis. The fetus inherited from the mother an additional, likely pathogenic novel missense change (R2210C) in a second MKS gene, CEP290 (610142), suggesting oligogenic inheritance in this case. The form of Meckel syndrome caused by mutation in the B9D1 gene is designated MKS9 (614209).
In a study of 96 unrelated patients with Meckel syndrome, Dowdle et al. (2011) did not identify any pathogenic mutations in the B9D1 gene.
Joubert Syndrome 27
In 2 unrelated children with Joubert syndrome-27 (JBTS27; 617120), Romani et al. (2014) identified biallelic mutations in the B9D1 gene (614144.0003-614144.0005). Functional studies of the variants and studies of patient cells were not performed. The patients were part of a group of 260 JBTS patients who were screened for mutations in ciliopathy genes.
Bialas et al. (2009) disrupted the B9 domains of C. elegans mks1, mksr1, and mksr2. In contrast to the defect found in mouse cells, C. elegans expressing single, double, or triple mks/mksr mutants showed no overt defects in ciliary structure, nor in intraflagellar transport, chemosensation, osmosensation, or lipid accumulation. However, disruption of one B9 domain-containing protein resulted in mislocalization of the others, and all possible double mks/mksr mutant combinations altered insulin signaling, leading to increased life span. The mks1/mksr1/mksr2 triple mutant did not exhibit a longevity phenotype.
Dowdle et al. (2011) found that the phenotype of B9d1-null mice resembled that of Meckel syndrome (see 249000). B9d1-null mouse mutants died at approximately embryonic day (E) 14.5. B9d1-null mouse mutants on a mixed genotypic background survived slightly longer, between E17.5 and postpartum day (P) 1. These mice showed multiple cystic kidney lesions with enlarged tubules and reduced numbers of shortened cilia compared to wildtype mice. They also showed hepatic ductal plate malformation with embryonic features, although cilia in bile ducts appeared normal. B9d1 mutants also showed preaxial polydactyly, more prominent in the hindlimbs (94%), and dextrocardia (44%), consistent with randomized heart looping. Other variable phenotypes included holoprosencephaly, microphthalmia, cleft palate, and ventricular septal defects. Embryonic nodes from mutant mice lacked almost all nodal cilia; the few that formed were short with swollen tips. Neural tubes showed proper docking of basal bodies, but failure to form axonemes. Examination of patterning and molecular signaling in the neural tube indicated a defect in hedgehog signaling. Similar to cilia in bile ducts, mouse embryonic fibroblasts showed normal ciliation, but defective hedgehog responsiveness. Mutant fibroblasts also showed a defect in ciliary localization of several proteins involved in the transition zone complex, including Smo (601500), Arl13b (608922), and ADCY3 (600291).
Chih et al. (2012) found that knockout of B9d1 or Tmem231 (614949) in mice led to lethality around embryonic day 15.5 with severe vascular defects. The phenotypes of B9d1 -/- and Tmem231 -/- mice were indistinguishable and showed signs of disrupted Shh signaling, including microphthalmia and polydactyly, and defects in patterning of the ventral spinal cord. Both B9d1 -/- and Tmem231 -/- embryos showed loss of cilia and altered Shh gene expression, including absence of Shh-positive floorplate cells.
Hopp et al. (2011) reported a single fetus affected with Meckel syndrome type 9 (MKS9; 614209) who was heterozygous for a splice site donor mutation, 505+2T-C, in B9D1, resulting in the loss of exon 4, a frameshift, and a premature stop codon (Thr82CysfsTer44) after the introduction of 44 nonconserved amino acids and a nearly complete disruption of the functional B9 domain. The mutation was inherited from the father. The other B9D1 allele was absent due to a de novo 1.71-Mb genomic deletion (614144.0002), which deleted 18 other known genes. From the mother, the fetus inherited a novel, likely pathogenic heterozygous mutation in the CEP290 gene (R2210C; 610142), which is mutant in MKS4 (611134), suggesting oligogenic inheritance in this case.
For discussion of the 1.71-Mb deletion in the B9D1 gene that was found in compound heterozygous state in a patient with Meckel syndrome type 9 (MKS9; 614209) by Hopp et al. (2011), see 614144.0001.
In a 9-year-old boy (COR363) with Joubert syndrome-27 (JBTS27; 617120), Romani et al. (2014) identified a homozygous c.467G-A transition (c.467G-A, NG_031885.1) in the B9D1 gene, resulting in an arg156-to-gln (R156Q) substitution at a highly conserved residue. Each unaffected parent was heterozygous for the mutation, which was not found in public databases. Functional studies of the variant and studies of patient cells were not performed.
In a 7-year-old girl (COR346) with Joubert syndrome-27 (JBTS27; 617120), Romani et al. (2014) identified compound heterozygous mutations in the B9D1 gene: a c.95A-G transition (c.95A-G, NG_031885.1), resulting in a tyr32-to-cys (Y32C) substitution at a highly conserved residue, and a 3-bp in-frame deletion (c.520_522delGTG; 614144.0005), resulting in the deletion of conserved residue val174. Each unaffected parent was heterozygous for 1 of the mutations, which were not found in public databases. Functional studies of the variants and studies of patient cells were not performed.
For discussion of the 3-bp in-frame deletion (c.520_522delGTG, NG_031885.1) in the B9D1 gene, resulting in the deletion of conserved residue Val174, that was found in compound heterozygous state in a patient with Joubert syndrome-27 (JBTS27; 617120), by Romani et al. (2014), see 614144.0004.
Bialas, N. J., Inglis, P. N., Li, C., Robinson, J. F., Parker, J. D. K., Healey, M. P., Davis, E. E., Inglis, C. D., Toivonen, T., Cottell, D. C., Blacque, O. E., Quarmby, L. M., Katsanis, N., Leroux, M. R. Functional interactions between the ciliopathy-associated Meckel syndrome 1 (MKS1) protein and two novel MKS1-related (MKSR) proteins. J. Cell Sci. 122: 611-624, 2009. [PubMed: 19208769] [Full Text: https://doi.org/10.1242/jcs.028621]
Chih, B., Liu, P., Chinn, Y., Chalouni, C., Komuves, L. G., Hass, P. E., Sandoval, W., Peterson, A. S. A ciliopathy complex at the transition zone protects the cilia as a privileged membrane domain. Nature Cell Biol. 14: 61-72, 2012. [PubMed: 22179047] [Full Text: https://doi.org/10.1038/ncb2410]
Dowdle, W. E., Robinson, J. F., Kneist, A., Sirerol-Piquer, M. S., Frints, S. G. M., Corbit, K. C., Zaghloul, N. A., van Lijnschoten, G., Mulders, L., Verver, D. E., Zerres, K., Reed, R. R., Attie-Bitach, T., Johnson, C. A., Garcia-Verdugo, J. M., Katsanis, N., Bergmann, C., Reiter, J. F. Disruption of a ciliary B9 protein complex causes Meckel syndrome. Am. J. Hum. Genet. 89: 94-110, 2011. Note: Erratum: Am. J. Hum. Genet. 89: 589 only, 2011. [PubMed: 21763481] [Full Text: https://doi.org/10.1016/j.ajhg.2011.06.003]
Hopp, K., Heyer, C. M., Hommerding, C. J., Henke, S. A., Sundsbak, J. L., Patel, S., Patel, P., Consugar, M. B., Czarnecki, P. G., Gliem, T. J., Torres, V. E., Rossetti, S., Harris, P. C. B9D1 is revealed as a novel Meckel syndrome (MKS) gene by targeted exon-enriched next-generation sequencing and deletion analysis. Hum. Molec. Genet. 20: 2524-2534, 2011. [PubMed: 21493627] [Full Text: https://doi.org/10.1093/hmg/ddr151]
Romani, M., Micalizzi, A., Kraoua, I., Dotti, M. T., Cavallin, M., Sztriha, L., Ruta, R., Mancini, F., Mazza, T., Castellana, S., Hanene, B., Carlucio, M. A., Darra, F., Mate, A., Zimmermann, A., Gouider-Khouja, N., Valente, E. M. Mutations in B9D1 and MKS1 cause mild Joubert syndrome: expanding the genetic overlap with the lethal ciliopathy Meckel syndrome. Orphanet J. Rare Dis. 9: 72, 2014. Note: Electronic Article. [PubMed: 24886560] [Full Text: https://doi.org/10.1186/1750-1172-9-72]
Williams, C. L., Li, C., Kida, K., Inglis, P. N., Mohan, S., Semenec, L., Bialas, N. J., Stupay, R. M., Chen, N., Blacque, O. E., Yoder, B. K., Leroux, M. R. MKS and NPHP modules cooperate to establish basal body/transition zone membrane associations and ciliary gate function during ciliogenesis. J. Cell. Biol. 192: 1023-1041, 2011. [PubMed: 21422230] [Full Text: https://doi.org/10.1083/jcb.201012116]