Shaheen N. Khan

Usher Syndrome 1D and Nonsyndromic Autosomal Recessive Deafness DFNB12 Are Caused by Allelic Mutations of the Novel Cadherin-Like Gene CDH23

Genes causing nonsyndromic autosomal recessive deafness (DFNB12) and deafness associated with retinitis pigmentosa and vestibular dysfunction (USH1D) were previously mapped to overlapping regions of chromosome 10q21-q22. Seven highly consanguineous families segregating nonsyndromic autosomal recessive deafness were analyzed to refine the DFNB12 locus. In a single family, a critical region was defined between D10S1694 and D10S1737, approximately 0.55 cM apart. Eighteen candidate genes in the region were sequenced. Mutations in a novel cadherin-like gene, CDH23, were found both in families with DFNB12 and in families with USH1D. Six missense mutations were found in five families with DFNB12, and two nonsense and two frameshift mutations were found in four families with USH1D. A northern blot analysis of CDH23 showed a 9.5-kb transcript expressed primarily in the retina. CDH23 is also expressed in the cochlea, as is demonstrated by polymerase chain reaction amplification from cochlear cDNA.

Mutations of the protocadherin gene PCDH15 cause Usher syndrome type 1F.

Human chromosome 10q21-22 harbors USH1F in a region of conserved synteny to mouse chromosome 10. This region of mouse chromosome 10 contains Pcdh15, encoding a protocadherin gene that is mutated in ames waltzer and causes deafness and vestibular dysfunction. Here we report two mutations of protocadherin 15 (PCDH15) found in two families segregating Usher syndrome type 1F. A Northern blot probed with the PCDH15 cytoplasmic domain showed expression in the retina, consistent with its pathogenetic role in the retinitis pigmentosa associated with USH1F.

Dominant and recessive deafness caused by mutations of a novel gene, TMC1, required for cochlear hair-cell function.

Positional cloning of hereditary deafness genes is a direct approach to identify molecules and mechanisms underlying auditory function. Here we report a locus for dominant deafness, DFNA36, which maps to human chromosome 9q13–21 in a region overlapping the DFNB7/B11 locus for recessive deafness. We identified eight mutations in a new gene, transmembrane cochlear-expressed gene 1 (TMC1), in a DFNA36 family and eleven DFNB7/B11 families. We detected a 1.6-kb genomic deletion encompassing exon 14 of Tmc1 in the recessive deafness (dn) mouse mutant, which lacks auditory responses and has hair-cell degeneration. TMC1 and TMC2 on chromosome 20p13 are members of a gene family predicted to encode transmembrane proteins. Tmc1 mRNA is expressed in hair cells of the postnatal mouse cochlea and vestibular end organs and is required for normal function of cochlear hair cells.

Origins and frequencies of SLC26A4 (PDS) mutations in east and south Asians: global implications for the epidemiology of deafness

Recessive mutations of SLC26A4 (PDS) are a common cause of Pendred syndrome and non-syndromic deafness in western populations. Although south and east Asia contain nearly one half of the global population, the origins and frequencies of SLC26A4 mutations in these regions are unknown. We PCR amplified and sequenced seven exons of SLC26A4 to detect selected mutations in 274 deaf probands from Korea, China, and Mongolia. A total of nine different mutations of SLC26A4 were detected among 15 (5.5%) of the 274 probands. Five mutations were novel and the other four had seldom, if ever, been identified outside east Asia. To identify mutations in south Asians, 212 Pakistani and 106 Indian families with three or more affected offspring of consanguineous matings were analysed for cosegregation of recessive deafness with short tandem repeat markers linked to SLC26A4. All 21 SLC26A4 exons were PCR amplified and sequenced in families segregating SLC26A4 linked deafness. Eleven mutant alleles of SLC26A4 were identified among 17 (5.4%) of the 318 families, and all 11 alleles were novel. SLC26A4 linked haplotypes on chromosomes with recurrent mutations were consistent with founder effects. Our observation of a diverse allelic series unique to each ethnic group indicates that mutational events at SLC26A4 are common and account for approximately 5% of recessive deafness in south Asians and other populations.

Tricellulin Is a Tight-Junction Protein Necessary for Hearing

The inner ear has fluid-filled compartments of different ionic compositions, including the endolymphatic and perilymphatic spaces of the organ of Corti; the separation from one another by epithelial barriers is required for normal hearing. TRIC encodes tricellulin, a recently discovered tight-junction (TJ) protein that contributes to the structure and function of tricellular contacts of neighboring cells in many epithelial tissues. We show that, in humans, four different recessive mutations of TRIC cause nonsyndromic deafness (DFNB49), a surprisingly limited phenotype, given the widespread tissue distribution of tricellulin in epithelial cells. In the inner ear, tricellulin is concentrated at the tricellular TJs in cochlear and vestibular epithelia, including the structurally complex and extensive junctions between supporting and hair cells. We also demonstrate that there are multiple alternatively spliced isoforms of TRIC in various tissues and that mutations of TRIC associated with hearing loss remove all or most of a conserved region in the cytosolic domain that binds to the cytosolic scaffolding protein ZO-1. A wild-type isoform of tricellulin, which lacks this conserved region, is unaffected by the mutant alleles and is hypothesized to be sufficient for structural and functional integrity of epithelial barriers outside the inner ear.

Null mutations in LTBP2 cause primary congenital glaucoma

Primary congenital glaucoma (PCG) is an autosomal-recessive condition characterized by high intraocular pressure (IOP), usually within the first year of life, which potentially could lead to optic nerve damage, globe enlargement, and permanent loss of vision. To date, PCG has been linked to three loci: 2p21 (GLC3A), for which the responsible gene is CYP1B1, and 1p36 (GLC3B) and 14q24 (GLC3C), for which the genes remain to be identified. Here we report that null mutations in LTBP2 cause PCG in four consanguineous families from Pakistan and in patients of Gypsy ethnicity. LTBP2 maps to chromosome 14q24.3 but is around 1.3 Mb proximal to the documented GLC3C locus. Therefore, it remains to be determined whether LTBP2 is the GLC3C gene or whether a second adjacent gene is also implicated in PCG. LTBP2 is the largest member of the latent transforming growth factor (TGF)-beta binding protein family, which are extracellular matrix proteins with multidomain structure. It has homology to fibrillins and may have roles in cell adhesion and as a structural component of microfibrils. We confirmed localization of LTBP2 in the anterior segment of the eye, at the ciliary body, and particularly the ciliary process. These findings reveal that LTBP2 is essential for normal development of the anterior chamber of the eye, where it may have a structural role in maintaining ciliary muscle tone.

Targeted capture and next-generation sequencing identifies C9orf75, encoding taperin, as the mutated gene in nonsyndromic deafness DFNB79.

Targeted genome capture combined with next-generation sequencing was used to analyze 2.9 Mb of the DFNB79 interval on chromosome 9q34.3, which includes 108 candidate genes. Genomic DNA from an affected member of a consanguineous family segregating recessive, nonsyndromic hearing loss was used to make a library of fragments covering the DFNB79 linkage interval defined by genetic analyses of four pedigrees. Homozygosity for eight previously unreported variants in transcribed sequences was detected by evaluating a library of 402,554 sequencing reads and was later confirmed by Sanger sequencing. Of these variants, six were determined to be polymorphisms in the Pakistani population, and one was in a noncoding gene that was subsequently excluded genetically from the DFNB79 linkage interval. The remaining variant was a nonsense mutation in a predicted gene, C9orf75, renamed TPRN. Evaluation of the other three DFNB79-linked families identified three additional frameshift mutations, for a total of four truncating alleles of this gene. Although TPRN is expressed in many tissues, immunolocalization of the protein product in the mouse cochlea shows prominent expression in the taper region of hair cell stereocilia. Consequently, we named the protein taperin.

Mutations of MYO6 Are Associated with Recessive Deafness, DFNB37

Cosegregation of profound, congenital deafness with markers on chromosome 6q13 in three Pakistani families defines a new recessive deafness locus, DFNB37. Haplotype analyses reveal a 6-cM linkage region, flanked by markers D6S1282 and D6S1031, that includes the gene encoding unconventional myosin VI. In families with recessively inherited deafness, DFNB37, our sequence analyses of MYO6 reveal a frameshift mutation (36-37insT), a nonsense mutation (R1166X), and a missense mutation (E216V). These mutations, along with a previously published missense allele linked to autosomal dominant progressive hearing loss (DFNA22), provide an allelic spectrum that probes the relationship between myosin VI dysfunction and the resulting phenotype.

Alterations of the CIB2 calcium- and integrin-binding protein cause Usher syndrome type 1J and nonsyndromic deafness DFNB48

Sensorineural hearing loss is genetically heterogeneous. Here, we report that mutations in CIB2, which encodes a calcium- and integrin-binding protein, are associated with nonsyndromic deafness (DFNB48) and Usher syndrome type 1J (USH1J). One mutation in CIB2 is a prevalent cause of deafness DFNB48 in Pakistan; other CIB2 mutations contribute to deafness elsewhere in the world. In mice, CIB2 is localized to the mechanosensory stereocilia of inner ear hair cells and to retinal photoreceptor and pigmented epithelium cells. Consistent with molecular modeling predictions of calcium binding, CIB2 significantly decreased the ATP-induced calcium responses in heterologous cells, whereas mutations in deafness DFNB48 altered CIB2 effects on calcium responses. Furthermore, in zebrafish and Drosophila melanogaster, CIB2 is essential for the function and proper development of hair cells and retinal photoreceptor cells. We also show that CIB2 is a new member of the vertebrate Usher interactome.

Growth Factor Preconditioning Increases the Function of Diabetes-Impaired Mesenchymal Stem Cells

Bone marrow-derived mesenchymal stem cells (MSCs) possess multilineage differentiation potential and can be used for the treatment of diabetic heart failure. However, hyperglycemia can affect the function of MSCs adversely and merits the requirement for a strategy to correct this anomaly. MSCs were isolated from the tibias and femurs of C57BL/6 wild-type mice at 60 days after induction of diabetes by streptozotocin. MSCs were characterized by flow cytometry for CD44 (97.7%), CD90 (95.4%), and CD105 (92.3%) markers and were preconditioned with insulin-like growth factor-1 (IGF-1) (50 ng/mL) and fibroblast growth factor-2 (FGF-2) (50 ng/mL) in combination for 1 h in serum-free Iscoves modified Dulbeccos medium. This was followed by hypoxic and high glucose insults to mimic diabetic heart microenvironment and to study the effect of preconditioning. Diabetic MSCs after treatment showed upregulation of IGF-1, FGF-2, Akt, GATA-4, and Nkx 2.5 and downregulation of p16(INK4a), p66(shc), p53, Bax, and Bak. Under hypoxic stress, preconditioned diabetic MSCs showed high superoxide dismutase activity (52.3%) compared with untreated cells (36.9%). This was concomitant with low numbers of annexin-V-positive cells, high in vitro tube-forming ability, and high chemotactic mobility to stromal cell-derived factor-1α after preconditioning in diabetic MSCs. Upregulation of Ang-I and VEGF and downregulation of p16(INK4a) were also observed in preconditioned cells under conditions of high glucose insult. Therefore, preconditioning with IGF-1 and FGF-2 in combination represents a novel strategy to augment MSC function affected by diabetes and holds significance for future strategies to treat diabetic heart failure.