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Our results suggested a new gene involved in HHL, named BDP1, a member of the TFIIIB complex. Transcription factor TFIIIB plays key roles in transcription by RNA polymerase III. Its three components (TBP, BRF1 and BDP1) participate in crucial molecular events that include RNA polymerase recruitment, formation of the open initiation complex[20] and recycling of transcription. It has been reported that TFIIIB assembled with certain deletion mutants of its BRF1 and BDP1 subunits is competent in pol III recruitment [21] but the resulting preinitiation complex does not open the promoter.

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Nonsyndromic Hereditary Hearing Loss is a common disorder accounting for at least 60% of prelingual deafness. GJB2 gene mutations, GJB6 deletion, and the A1555G mitochondrial mutation play a major role worldwide in causing deafness, but there is a high degree of genetic heterogeneity and many genes involved in deafness have not yet been identified. Therefore, there remains a need to search for new causative mutations. In this study, a combined strategy using both linkage analysis and sequencing identified a new mutation causing hearing loss. Linkage analysis identified a region of 40 Mb on chromosome 5q13 (LOD score 3.8) for which exome sequencing data revealed a mutation (c.7873 T>G leading to p.*2625Gluext*11) in the BDP1 gene (B double prime 1, subunit of RNA polymerase III transcription initiation factor IIIB) in patients from a consanguineous Qatari family of second degree, showing bilateral, post-lingual, sensorineural moderate to severe hearing impairment. The mutation disrupts the termination codon of the transcript resulting in an elongation of 11 residues of the BDP1 protein. This elongation does not contain any known motif and is not conserved across species. Immunohistochemistry studies carried out in the mouse inner ear showed Bdp1 expression within the endothelial cells in the stria vascularis, as well as in mesenchyme-derived cells surrounding the cochlear duct. The identification of the BDP1mutation increases our knowledge of the molecular bases of Nonsyndromic Hereditary Hearing Loss and provides new opportunities for the diagnosis and treatment of this disease in the Qatari population.

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Bdp1 protein expression in the cochlea at postnatal day 5. Bdp1 shows expression in the stria vascularis (red arrow), Reissner's membrane (red arrowhead), basilar membrane (black arrowhead), and spiral ligament (black arrow). B–D, Bdp1, Kcnj10 and laminin expression in the stria vascularis. Our immunohistochemistry detected Bdp1 protein in the proximity of blood vessels (example of the same blood vessel in adjacent sections is indicated with an arrow in B, C, D). We used Kcnj10 as a marker of intermediate cells (C) and laminin as a marker of the basal lamina surrounding blood vessels (D) in the stria vascularis in the sections shown in A. The expression pattern suggests that Bdp1 is expressed in endothelial cells of the stria vascularis. Scale bars: A: 50 µm; B–D: 10 µm. sv: stria vascularis.

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determined that the TFNR gene contains 32 exons and spans about 80 kb

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DNA binding

TFIIIB-type transcription factor activity

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Triggers RNA polymerase III transcription

gene expression

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Size:

2624 amino acids

Molecular mass:

293885 Da

Quaternary structure:

Component of TFIIIB complex. The TFIIIB complex has two activities, alpha and beta. The TFIIIB-alpha and TFIIIB-beta activities are required for transcription of genes with TFIIIC-bound internal promoters and PSE transcription factor-bound external promoters, respectively. The TFIIIB-alpha activity complex is composed of TBP, BDP1, and a complex containing both BRF2 and at least four stably associated proteins; YY1 facilitates the formation of TFIIIB-alpha activity complex. The TFIIIB-beta activity complex is composed of TBP, BDP1, and BRF1. Interacts with BRF1; this interaction diminishes during mitosis resulting in the release of BDP1 from chromosomal templates. Component of TFIIIC complex. The TFIIIC complex has two activities, C1 and C2. The TFIIIC2 activity complex is only required for transcription of the classical pol III genes whereas the TFIIIC1 activity complex is required for transcription of all pol III genes. The TFIIIC1 activity complex is composed at least of BDP1. Interacts with ZBTB43.

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The product of this gene is a subunit of the TFIIIB transcription initiation complex, which recruits RNA polymerase III to target promoters in order to initiate transcription. The encoded protein localizes to concentrated aggregates in the nucleus, and is required for transcription from all three types of polymerase III promoters. It is phosphorylated by casein kinase II during mitosis, resulting in its release from chromatin and suppression of polymerase III transcription