Mutations in a novel isoform of TRIOBP that encodes a filamentous-actin binding protein are responsible for DFNB28 recessive nonsyndromic hearing loss

Abstract

In a large consanguineous Palestinian kindred, we previously mapped DFNB28--a locus associated with recessively inherited, prelingual, profound sensorineural hearing impairment--to chromosome 22q13.1. We report here that mutations in a novel 218-kDa isoform of TRIOBP (TRIO and filamentous actin [F-actin] binding protein) are associated with DFNB28 hearing loss in a total of nine Palestinian families. Two nonsense mutations (R347X and Q581X) truncate the protein, and a potentially deleterious missense mutation (G1019R) occurs in a conserved motif in a putative SH3-binding domain. In seven families, 27 deaf individuals are homozygous for one of the nonsense mutations; in two other families, 3 deaf individuals are compound heterozygous for the two nonsense mutations or for Q581X and G1019R. The novel long isoform of TRIOBP has a restricted expression profile, including cochlea, retina, and fetal brain, whereas the original short isoform is widely expressed. Antibodies to TRIOBP reveal expression in sensory cells of the inner ear and colocalization with F-actin along the length of the stereocilia.

Figure 1

Palestinian families with hereditary hearing loss linked to 6.3 Mb on chromosome 22q13 (DFNB28). A, Families K, AX, AY, and BD. In these families, hearing loss is linked to two haplotypes, shown in yellow and green. The order of markers was determined from the genomic sequence of chromosome 22 (UCSC Genome Browser May 2004 assembly). B, Homozygosity mapping based on 76 SNPs and microsatellite markers. Genotypes in red are shared by the yellow and green haplotypes. Black vertical lines indicate regions of 939 kb, 316 kb, and 259 kb that were identical either by descent or by state in all affected individuals. The position of TRIOBP in the 939-kb homozygous region is indicated. C, Families A17, AK, X7, and AZ. In these families, hearing loss was also linked to markers at DFNB28.

Figure 2

Determination of the genomic structure of isoforms of TRIOBP. A, Mouse and human TRIOBP isoforms amplified from cDNA derived from mouse cochlea and human fetal brain. RT-PCR primer pairs 1F-2R and 2F-1R were used to determine the mouse long isoform of TRIOBP. RT-PCR primers H1-H5 (table 1; not shown in this fig.) were used to determine the human long isoform of TRIOBP. RT-PCR primers 6F-6R and 7F-7R were used to test expression in multiple human tissues. B, Mouse cochlear and brain cDNA amplified by RT-PCR. Products were electrophoresed on agarose gels alongside a molecular-weight size marker. C, Expression of short and long transcripts of TRIOBP in human tissues. Human adult cochlear total RNA was obtained from Phylogeny and fibroblast RNA was a gift from Peter Byers (University of Washington). All other RNAs were purchased from Clontech. Total RNA (5 μg) was reverse transcribed with a primer located in the shared 3′ UTR of both isoforms (table 1). cDNA was amplified for 30 cycles with primers 6F-6R and 7F-7R, specific to each isoform (table 1), and products were electrophoresed on an agarose gel alongside a molecular-weight size marker. The original TRIOBP isoform is widely expressed in all tissues tested. The novel long isoform is expressed only in fetal brain, cochlea, and retina. Ex = exon.

Figure 3

Sequences of mutations in the long isoform of TRIOBP associated with DFNB28 hearing loss. Amino acids are indicated below the nucleotide sequences; mutant codons are indicated by red boxes. A, Sequence from a hearing individual from family K who is heterozygous for 1039C→T (R347X). B, Sequence from a deaf individual from family AK who is homozygous for 1741C→T (Q581X). C, Sequence from a deaf child who is compound heterozygous for 3055G→A (G1019R) and R347X.

Figure 4

Proline-rich domain at aa 879–1399 of the long isoform of human TRIOBP that harbors the missense mutation G1019R (G in bold and inside a box) in a 16-aa conserved repeat (underlined). This region has properties of an SH3-binding domain (Ren et al. 1993); three PXXP-containing SH3-binding domains (in boxes) are predicted by the iSPOT algorithm (Brannetti and Helmer-Citterich 2003). Ligand preferences at SH3-binding sites are determined by arginine or threonine motifs at some distance from the PXXP peptides (Kuriyan and Cowburn 1997). We speculate that the conserved motifs may be involved in specifying ligand preference and that substitution of arginine for glycine may disrupt this role. This region is truncated by both nonsense mutations.

Figure 5

Predicted functional domains of the short and long isoforms of TRIOBP

Figure 6

Expression and localization of TRIOBP in mouse cochlea. A, Western blot of two cochlea from newborn (P0) mice hybridized with monoclonal antibody TARA.27 (Seipel et al. 2001). Bands of ∼70 kDa and ∼50 kDa may represent two isoforms of original TRIOBP (UCSC Human Genome Browser May 2004 assembly). The band of >250 kDa (red arrow) may represent posttranslational modification of the predicted 219-kDa novel TRIOBP isoform. The band of ∼150 kDa may represent either a breakdown product of this isoform or an as-yet-uncharacterized alternatively spliced isoform. B–D, Top view of P0 whole-mount cochleae stained with monoclonal antibody TARA.27 against TRIOBP (B) and with rhodamine phalloidin, which stains F-actin (C). The protein is detected in the stereocilia of both the outer hair cells (OHC) and inner hair cells (IHC). Labeling indicates colocalization with actin along the length of the stereocilia and in the microvilli on the apical portion of the hair cells (D). E–G, Higher magnification showing stereocilia of outer hair cells. Scale bar is 16 μm in panels B–D and 4 μm in E–G.