Alternative titles; symbols
HGNC Approved Gene Symbol: TTC8
Cytogenetic location: 14q31.3 Genomic coordinates (GRCh38) : 14:88,824,153-88,881,079 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 14q31.3 | ?Retinitis pigmentosa 51 | 613464 | Autosomal recessive | 3 |
| Bardet-Biedl syndrome 8 | 615985 | Autosomal recessive | 3 |
TTC8 is 1 of 7 BBS proteins that form the stable core of a protein complex required for ciliogenesis (Nachury et al., 2007).
Ansley et al. (2003) fragmented the BBS4 gene (600374) into 8 overlapping segments and searched the conceptual translation of the draft human genome and the EST database. In 1 instance they observed an alignment between 3 consecutive TPRs of BBS4 and a contiguous region of the hypothetical protein TTC8. The authors identified 2 alternatively spliced isoforms. RT-PCR analysis of human tissues detected strong expression in testis, ovary, lung, brain, liver, pancreas, and fetal kidney, with lower expression in heart, kidney, placenta, thymus, and spleen. The predicted protein contains 8 C-terminal TPRs and exhibits significant similarity to a prokaryotic domain pilF, involved in twitching mobility and type IV pilus assembly.
Riazuddin et al. (2010) noted that full-length TTC8 contains 515 amino acids and includes exon 2A, whereas a shorter TTC8 isoform lacking exon 2A contains 505 amino acids. A third isoform lacking exon 2A and exon 5 contains 475 amino acids. RT-PCR analysis of mouse tissues that express Ttc8 detected the Ttc8 isoform containing exon 2A exclusively in retina, and further analysis of mouse retinal tissues obtained by laser-capture microdissection found the exon-2A isoform mostly in the photoreceptor layer.
Using RT-PCR and mouse knockout models, Murphy et al. (2015) found that full-length Ttc8, including exon 2A, was expressed almost exclusively in retinal photoreceptors. All other mouse tissues expressed Ttc8 transcripts lacking exon 2A. Inclusion of exon 2A in Ttc8 was not detected in developing mouse until postnatal day 0, after which exon 2A inclusion increased rapidly.
Ansley et al. (2003) determined that the TTC8 gene contains 14 exons. Subsequently, Riazuddin et al. (2010) identified the retina-specific exon 2A.
Murphy et al. (2015) identified intronic sequences 5-prime and 3-prime of TTC8 exon 2A that contained retina-specific splicing enhancers for inclusion of exon 2A in photoreceptor cells only.
Ansley et al. (2003) stated that the TTC8 gene maps to chromosome 14q32.11.
Ansley et al. (2003) found that TTC8 colocalizes with gamma-tubulin (see 191135), BBS4, and PCM1 (600299) in the centrosome. PCM1 is thought to be involved in centriolar replication during ciliogenesis (Kubo et al., 1999). Immunoprecipitation indicated that TTC8 binds to the C terminus of PCM1. A polyclonal antibody against TTC8 stained ciliated structures in 12-day-old mice, including maturing spermatids, the connecting cilium of the retina, and bronchial epithelial cells. In mouse embryos at 14 and 16 days, Ansley et al. (2003) detected specific localization in the telencephalon, with prominent staining at the developing ependymal cell layer and olfactory epithelium. Ansley et al. (2003) found that all C. elegans BBS homologs studied are expressed exclusively in ciliated neurons and contain regulatory elements for RFX, a transcription factor that modulates expression of genes associated with ciliogenesis and intraflagellar transport.
Nachury et al. (2007) found that BBS1 (209901), BBS2 (606151), BBS4, BBS5 (603650), BBS7 (607590), BBS8, and BBS9 (607968) copurified in stoichiometric amounts from human retinal pigment epithelium (RPE) cells and from mouse testis. PCM1 and alpha-tubulin (see 602529)/beta-tubulin (191130) copurified in substoichiometric amounts. The apparent molecular mass of the complex, which Nachury et al. (2007) called the BBSome, was 438 kD, and it had a sedimentation coefficient of 14S. The complex localized with PCM1 to nonmembranous centriolar satellites in the cytoplasm and, in the absence of PCM1, to the ciliary membrane. Cotransfection and immunoprecipitation experiments suggested that BBS9 was the complex-organizing subunit and that BBS5 mediated binding to phospholipids, predominantly phosphatidylinositol 3-phosphate. BBS1 mediated interaction with RABIN8 (RAB3IP; 608686), the guanine nucleotide exchange factor for the small G protein RAB8 (RAB8A; 165040). Nachury et al. (2007) found that RAB8 promoted ciliary membrane growth through fusion of exocytic vesicles to the base of the ciliary membrane. They concluded that BBS proteins likely function in membrane trafficking to the primary cilium.
Loktev et al. (2008) found that BBIP10 (613605) copurified and cosedimented with the BBS protein complex from RPE cells. Knockdown of BBIP10 in RPE cells via small interfering RNA compromised assembly of the BBS protein complex and caused failure of ciliogenesis. Knockdown of BBS1, BBS5, or PCM1 resulted in a similar failure of ciliogenesis in RPE cells. Depletion of BBIP10 or BBS8 increased the frequency of centrosome splitting in interphase cells. BBIP10 also had roles in cytoplasmic microtubule stabilization and acetylation that appeared to be independent of its role in assembly of the BBS protein complex.
Using a protein pull-down assay with homogenized bovine retina, Jin et al. (2010) showed that ARL6 (608845) bound the BBS protein complex. Depletion of ARL6 in human RPE cells did not affect assembly of the complex, but it blocked its localization to cilia. Targeting of ARL6 and the protein complex to cilia required GTP binding by ARL6, but not ARL6 GTPase activity. When in the GTP-bound form, the N-terminal amphipathic helix of ARL6 bound brain lipid liposomes and recruited the BBS protein complex. Upon recruitment, the complex appeared to polymerize into an electron-dense planar coat, and it functioned in lateral transport of test cargo proteins to ciliary membranes.
By mass spectrometric analysis of transgenic mouse testis, Seo et al. (2011) found that Lxtfl1 (606568) copurified with human BBS4 and with the core mouse BBS complex subunits Bbs1, Bbs2, Bbs5, Bbs7, Bbs8, and Bbs9. Immunohistochemical analysis of human RPE cells showed colocalization of LXTFL1 and BBS9 in cytoplasmic punctae. Use of small interfering RNA revealed distinct functions for each BBS subunit in BBS complex assembly and trafficking. LZTFL1 depletion and overexpression studies showed a negative role for LZTFL1 in BBS complex trafficking, but no effect of LZTFL1 on BBS complex assembly. Mutation analysis revealed that the C-terminal half of Lztfl1 interacted with the C-terminal domain of Bbs9 and that the N-terminal half of Lztfl1 negatively regulated BBS complex trafficking. Depletion of several BBS subunits and LZTFL1 also altered Hedgehog (SHH; 600725) signaling, as measured by GLI1 (165220) expression and ciliary trafficking of SMO (SMOH; 601500).
Using computational analysis, Jin et al. (2010) found that the BBS protein complex shares structural features with the canonical coat complexes COPI (601924), COPII (see 610511), and clathrin AP1 (see 603531). BBS4 and BBS8 consist almost entirely of tetratricopeptide repeats (TPRs) (13 and 12.5 TPRs, respectively), which are predicted to fold into extended rod-shaped alpha solenoids. BBS1, BBS2, BBS7, and BBS9 each have an N-terminal beta-propeller fold followed by an amphipathic helical linker and a gamma-adaptin (AP1G1; 603533) ear motif. In BBS2, BBS7, and BBS9, the ear motif is followed by an alpha/beta platform domain and an alpha helix. In BBS1, a 4-helix bundle is inserted between the second and third blades of the beta propeller. BBS5 contains 2 pleckstrin (PLEK; 173570) homology domains and a 3-helix bundle, while BBIP10 consists of 2 alpha helices. Jin et al. (2010) concluded that the abundance of beta propellers, alpha solenoids, and appendage domains inside the BBS protein complex suggests that it shares an evolutionary relationship with canonical coat complexes.
Bardet-Biedl Syndrome 8
Ansley et al. (2003) screened the TTC8 gene in a cohort of 120 unrelated patients with Bardet-Biedl syndrome (see BBS8, 615985) and identified homozygous alterations in patients from 3 families. All 8 affected individuals in these 3 families had a homozygous mutant genotype and exhibited classic BBS signs. These mutations were not identified in 192 ethnically matched control chromosomes. One of the affected family members with BBS due to TTC8 mutations demonstrated situs inversus. The association between BBS and the situs inversus phenotype was not coincidental, although the presence of situs in only 1 of 3 affected family members suggested that the defect was one of randomization of left-right symmetry.
Ansley et al. (2003) demonstrated that BBS is probably caused by a defect of the basal body of ciliated cells. The TTC8 gene, mutations in which are responsible for BBS8, encodes a protein with a prokaryotic domain, pilF, involved in pilus formation and twitching mobility. In 1 family a homozygous null BBS8 mutation (608132.0002) led to BBS with randomization of left-right body axis symmetry, a defect of the nodal cilium. Ansley et al. (2003) showed that TTC8 localizes to centrosomes and basal bodies and colocalizes with gamma-tubulin (see 191135), BBS4 (600374), and PCM1 (600299). Furthermore, Ansley et al. (2003) found that all available C. elegans BBS homologs are expressed exclusively in ciliated neurons and contain regulatory elements for RFX, a transcription factor that modulates the expression of genes associated with ciliogenesis and intraflagellar transport.
Stoetzel et al. (2006) identified homozygous mutations in the TTC8 gene (608132.0003 and 608132.0004) in 2 of 128 BBS families. One additional family had a heterozygous mutation. Stoetzel et al. (2006) concluded that TTC8 mutations account for only about 2% of BBS families.
Retinitis Pigmentosa 51
In a large consanguineous Pakistani family segregating autosomal recessive retinitis pigmentosa (RP51; 613464) mapping to chromosome 14q, Riazuddin et al. (2010) sequenced candidate genes and identified a homozygous splice site mutation in the TTC8 gene (608132.0005) that segregated with disease and was not found in controls. None of the affected individuals had evidence of syndromic disease or any features consistent with BBS. The mutation mapped to the splice acceptor site of exon 2A of the TTC8 gene. RT-PCR analysis of mouse tissues that express Ttc8 detected Ttc8 isoforms containing exon 2A exclusively in retina, and further analysis of mouse retinal tissues obtained by laser-capture microdissection found the exon-2A isoform mostly in the photoreceptor layer.
In 3 affected individuals from a consanguineous North Indian family with nonsyndromic RP and macular degeneration, Goyal et al. (2016) identified homozygosity for a missense mutation in the TTC8 gene (Q449H; 608132.0006).
In 2 families of Saudi Arabian lineage with Bardet-Biedl syndrome (BBS8; 615985), Ansley et al. (2003) identified homozygosity for a 6-bp in-frame deletion in exon 6 of the TTC8 gene that eliminated 2 amino acids (187-188delEY). Both the E (glu) and Y (tyr) residues at the those locations are conserved in 16 TTC8 homologs.
In a consanguineous Pakistani family with Bardet-Biedl syndrome (BBS8; 615985), Ansley et al. (2003) identified a homozygous 3-bp deletion that abolished the donor sequence at the splice junction of exon 10 (IVS10+2-4delTGC) in all 3 patients, but not their unaffected sister. The authors predicted this mutation to result in a read-through culminating in an intronic stop codon. RT-PCR detected no TTC8 expression in cultured renal tubular cells and skin fibroblasts from 2 affected sibs, but detected expression in cells from their unaffected sister, who was heterozygous for the mutation, and in a control cell line. One of the affected sibs manifested situs inversus, which was not thought to be coincidental but to represent a defect of randomization of left-right symmetry.
In 3 sibs with Bardet-Biedl syndrome (BBS8; 615985), Stoetzel et al. (2006) identified a homozygous 459G-A transition affecting the last G of exon 4 of the TTC8 gene and predicted to abolish the splice site of exon 4. One affected sib also had a heterozygous mutation in the BBS7 gene (607590), but the authors concluded that it was not pathogenic, since no clinical differences were noted between the sib with the BBS7 mutation and the sibs without the BBS7 mutation. The children were born of consanguineous parents of North African descent.
In a child with Bardet-Biedl syndrome (BBS8; 615985), born of consanguineous Lebanese parents, Stoetzel et al. (2006) identified a homozygous G-to-A splice site mutation in intron 6 of the TTC8 gene.
In 4 affected members of a large consanguineous Pakistani family segregating autosomal recessive retinitis pigmentosa (RP51; 613464), Riazuddin et al. (2010) identified homozygosity for a -2A-G transition in intron 1 of the TTC8 gene, predicted to result in deletion of 10 amino acids from the protein. The mutation segregated with the disease and was not found in 384 Pakistani control chromosomes or 384 chromosomes of northern European descent. None of the affected individuals had evidence of syndromic disease or any features consistent with BBS. The mutation mapped to a highly conserved splice acceptor site of exon 2A of the TTC8 gene. RT-PCR analysis of mouse tissues that express Ttc8 detected Ttc8 isoforms containing exon 2A exclusively in retina, and further analysis of mouse retinal tissues obtained by laser-capture microdissection found the exon-2A isoform mostly in the photoreceptor layer.
Using subretinal injection and electroporation of a fluorescent splicing reporter in mice, Murphy et al. (2015) found that the -2A-G transition in intron 1 of the TTC8 gene disrupted the 5-prime splice site of exon 2A and forced the use of a cryptic splice site 7 nucleotides downstream of the mutation. Missplicing of exon 2A in retina resulted in premature termination and elimination of TTC8 protein in photoreceptors.
In 3 affected individuals from a consanguineous North Indian family with nonsyndromic RP and macular degeneration (RP51; 613464), Goyal et al. (2016) identified homozygosity for a c.1347G-C transversion (c.1347G-C, NM_144596.3) in exon 13 of the TTC8 gene, resulting in a gln449-to-his (Q449H) substitution at a highly conserved residue. The mutation segregated with disease in the family and was not found in 100 ethnically matched controls. The authors noted that the mutation involved the last nucleotide of exon 13, thereby affecting the 5-prime splice site consensus sequence; however, RNA was not available to test the effects of the mutation on splicing.
Ansley, S. J., Badano, J. L., Blacque, O. E., Hill, J., Hoskins, B. E., Leitch, C. C., Kim, J. C., Ross, A. J., Eichers, E. R., Teslovich, T. M., Mah, A. K., Johnsen, R. C., Cavender, J. C., Lewis, R. A., Leroux, M. R., Beales, P. L., Katsanis, N. Basal body dysfunction is a likely cause of pleiotropic Bardet-Biedl syndrome. Nature 425: 628-633, 2003. [PubMed: 14520415] [Full Text: https://doi.org/10.1038/nature02030]
Goyal, S., Jager, M., Robinson, P. N., Vanita, V. Confirmation of TTC8 as a disease gene for nonsyndromic autosomal recessive retinitis pigmentosa (RP51). Clin. Genet. 89: 454-460, 2016. [PubMed: 26195043] [Full Text: https://doi.org/10.1111/cge.12644]
Jin, H., White, S. R., Shida, T., Schulz, S., Aguiar, M., Gygi, S. P., Bazan, J. F., Nachury, M. V. The conserved Bardet-Biedl syndrome proteins assemble a coat that traffics membrane proteins to cilia. Cell 141: 1208-1219, 2010. [PubMed: 20603001] [Full Text: https://doi.org/10.1016/j.cell.2010.05.015]
Kubo, A., Sasaki, H., Yuba-Kubo, A., Tsukita, S., Shiina, N. Centriolar satellites: molecular characterization, ATP-dependent movement toward centrioles and possible involvement in ciliogenesis. J. Cell Biol. 147: 969-979, 1999. Note: Erratum: J. Cell Biol. 147: 1585 only, 1999. [PubMed: 10579718] [Full Text: https://doi.org/10.1083/jcb.147.5.969]
Loktev, A. V., Zhang, Q., Beck, J. S., Searby, C. C., Scheetz, T. E., Bazan, J. F., Slusarski, D. C., Sheffield, V. C., Jackson, P. K., Nachury, M. V. A BBSome subunit links ciliogenesis, microtubule stability, and acetylation. Dev. Cell 15: 854-865, 2008. [PubMed: 19081074] [Full Text: https://doi.org/10.1016/j.devcel.2008.11.001]
Murphy, D., Singh, R., Kolandaivelu, S., Ramamurthy, V., Stoilov, P. Alternative splicing shapes the phenotype of a mutation in BBS8 to cause nonsyndromic retinitis pigmentosa. Molec. Cell. Biol. 35: 1860-1870, 2015. [PubMed: 25776555] [Full Text: https://doi.org/10.1128/MCB.00040-15]
Nachury, M. V., Loktev, A. V., Zhang, Q., Westlake, C. J., Peranen, J., Merdes, A., Slusarski, D. C., Scheller, R. H., Bazan, J. F., Sheffield, V. C., Jackson, P. K. A core complex of BBS proteins cooperates with the GTPase Rab8 to promote ciliary membrane biogenesis. Cell 129: 1201-1213, 2007. [PubMed: 17574030] [Full Text: https://doi.org/10.1016/j.cell.2007.03.053]
Riazuddin, S. A., Iqbal, M., Wang, Y., Masuda, T., Chen, Y., Bowne, S., Sullivan, L. S., Waseem, N. H., Bhattacharya, S., Daiger, S. P., Zhang, K., Khan, S. N., Riazuddin, S., Hejtmancik, J. F., Sieving, P. A., Zack, D. J., Katsanis, N. A splice-site mutation in a retina-specific exon of BBS8 causes nonsyndromic retinitis pigmentosa. Am. J. Hum. Genet. 86: 805-812, 2010. [PubMed: 20451172] [Full Text: https://doi.org/10.1016/j.ajhg.2010.04.001]
Seo, S., Zhang, Q., Bugge, K., Breslow, D. K., Searby, C. C., Nachury, M. V., Sheffield, V. C. A novel protein LZTFL1 regulates ciliary trafficking of the BBSome and Smoothened. PLoS Genet. 7: e1002358, 2011. Note: Electronic Article. [PubMed: 22072986] [Full Text: https://doi.org/10.1371/journal.pgen.1002358]
Stoetzel, C., Laurier, V., Faivre, L., Megarbane, A., Perrin-Schmitt, F., Verloes, A., Bonneau, D., Mandel, J.-L., Cossee, M., Dollfus, H. BBS8 is rarely mutated in a cohort of 128 Bardet-Biedl syndrome families. J. Hum. Genet. 51: 81-84, 2006. [PubMed: 16308660] [Full Text: https://doi.org/10.1007/s10038-005-0320-2]