Localization of a gene for non-syndromic hearing loss (DFNA5) to chromosome 7p15

A gene for fluctuating, progressive autosomal dominant nonsyndromic hearing loss, DFNA16, maps to chromosome 2q23-24.3

The sixteenth gene to cause autosomal dominant nonsyndromic hearing loss (ADNSHL), DFNA16, maps to chromosome 2q23-24.3 and is tightly linked to markers in the D2S2380-D2S335 interval. DFNA16 is unique in that it results in the only form of ADNSHL in which the phenotype includes rapidly progressing and fluctuating hearing loss that appears to respond to steroid therapy. This observation suggests that it may be possible to stabilize hearing through medical intervention, once the biophysiology of deafness due to DFNA16 is clarified. Especially intriguing is the localization of several voltage-gated sodium-channel genes to the DFNA16 interval. These cationic channels are excellent positional and functional DFNA16 candidate genes.

A gene for autosomal dominant hearing impairment (DFNA14) maps to a region on chromosome 4p16.3 that does not overlap the DFNA6 locus

Non-syndromic hearing impairment is one of the most heterogeneous hereditary conditions, with more than 40 reported gene localisations. We have identified a large Dutch family with autosomal dominant non-syndromic sensorineural hearing impairment. In most patients, the onset of hearing impairment is in the first or second decade of life, with a slow decline in the following decades, which stops short of profound deafness. The hearing loss is bilateral, symmetrical, and only affects low and mid frequencies up to 2000 Hz. In view of the phenotypic similarities of this family with an American family that has been linked to chromosome 4p16.3 (DFNA6), we investigated linkage to the DFNA6 region. Lod score calculations confirmed linkage to this region with two point lod scores above 6. However, as haplotype analysis indicated that the genetic defect in this family is located in a 5.6 cM candidate region that does not overlap the DFNA6 region, the new locus has been named DFNA14.

Autosomal dominant non-syndromal low-frequency sensorineural hearing impairment linked to chromosome 4p16 (DFNA14): statistical analysis of hearing threshold in relation to age and evaluation of vestibulo-ocular functions

A Dutch kindred was studied with low-frequency sensorineural hearing impairment linked to a new locus on chromosome 4p16 (DFNA14). Of the affected individuals, 21 (aged 11-75 years) were examined and the most recent audiogram was used for cross-sectional analysis of hearing threshold in relation to age. Suitable serial audiograms were available for a longitudinal analysis in nine cases: they had been obtained from the age of six years onwards and covered a follow-up period from 14 to 36 years. The presumably congenital (offset) component of SNHI was extrapolated or estimated from average values and offset thresholds were found of about 45 dB at 0.25-1 kHz, 25 dB at 2 kHz and 10 dB at 4-8 kHz. Significant progression in hearing impairment occurred at all frequencies, but could be attributed to presbyacusis. The combination of congenital, stationary low-frequency SNHI and presbyacusis resulted in an up-sloping audiogram in the first five decades of life, which evolved into a flat-type audiogram in the sixth or seventh decade and a down-sloping audiogram at a more advanced age. With few exceptions, vestibular function was intact.

Nonsyndromic hearing impairment is associated with a mutation in DFNA5

Nonsyndromic hearing impairment is one of the most heterogeneous hereditary conditions, with more than 40 loci mapped on the human genome, however, only a limited number of genes implicated in hearing loss have been identified. We previously reported linkage to chromosome 7p15 for autosomal dominant hearing impairment segregating in an extended Dutch family (DFNA5). Here, we report a further refinement of the DFNA5 candidate region and the isolation of a gene from this region that is expressed in the cochlea. In intron 7 of this gene, we identified an insertion/deletion mutation that does not affect intron-exon boundaries, but deletes five G-triplets at the 3' end of the intron. The mutation co-segregated with deafness in the family and causes skipping of exon 8, resulting in premature termination of the open reading frame. As no physiological function could be assigned, the gene was designated DFNA5.

Nonsyndromic autosomal dominant progressive sensorineural hearing loss: audiologic analysis of a pedigree linked to DFNA2

An analysis was performed of the regression of the individual hearing threshold on age in the affected persons in a six-generation Dutch family with nonsyndromic autosomal dominant sensorineural hearing loss, which showed linkage to the DFNA2(1p34) region, similar to at least four previously reported nonrelated families. The offset threshold was significantly higher at the high frequencies (around 30 dB at 2 to 8 kHz) than at the lower ones (approximately 0 dB at 0.25 to 1 kHz). Hearing impairment at the higher frequencies may therefore have been present already at birth or in early childhood. The regression coefficient, or the 'annual threshold increase,' expressed in dB/y, was about 1 dB/y on average, but the higher frequencies (1 to 8 kHz) showed significantly more rapid progression than the lower frequencies (0.25 to 0.5 kHz).

Chromosomal mapping of two members of the human dynein gene family to chromosome regions 7p15 and 11q13 near the deafness loci DFNA 5 and DFNA 11

We mapped expressed tagged sequences (ESTs) corresponding to two human dynein heavy chain genes: beta heavy chain of the outer dynein arm and heavy chain isotype 1B (DYH1B), by using somatic cell hybrids and radiation hybrid panels. The EST for the beta heavy chain of the outer dynein arm mapped to chromosome region 7p15, and the EST for DYH1B mapped to 11q13.5. Two loci for nonsyndromic forms of deafness, DFNA5 and DFNA11, have previously been mapped to these two chromosomal regions. Including the gene for the axonemal light chain, hp28, we have mapped three different dynein genes near loci for different forms of nonsyndromic deafness. The hypothesis that mutations in some dynein genes are associated with nonsyndromic deafness should now be tested.

2006 expressed-sequence tags derived from human chromosome 7-enriched cDNA libraries

The establishment and mapping of gene-specific DNA sequences greatly complement the ongoing efforts to map and sequence all human chromosomes. To facilitate our studies of human chromosome 7, we have generated and analyzed 2006 expressed-sequence tags (ESTs) derived from a collection of direct selection cDNA libraries that are highly enriched for human chromosome 7 gene sequences. Similarity searches indicate that approximately two-thirds of the ESTs are not represented by sequences in the public databases, including those in dbEST. In addition, a large fraction (68%) of the ESTs do not have redundant or overlapping sequences within our collection. Human DNA-specific sequence-tagged sites (STSs) have been developed from 190 of the ESTs. Remarkably, 180 (96%) of these STSs map to chromosome 7, demonstrating the robustness of chromosome enrichment in constructing the direct selection cDNA libraries. Thus far, 140 of these EST-specific STSs have been assigned unequivocally to YAC contigs that are distributed across the chromosome. Together, these studies provide > 2000 ESTs highly enriched for chromosome 7 gene sequences, 180 new chromosome 7 STSs corresponding to ESTs, and a definitive demonstration of the ability to enrich for chromosome-specific cDNAs by direct selection. Furthermore, the libraries, sequence data, and mapping information will contribute to the construction of a chromosome 7 transcript map.

Bilateral sensorineural deafness and hydrocephalus due to foramen of Monro obstruction in sibs: a newly described autosomal recessive disorder

We identified a Canadian-Mennonite family in which a brother and sister have hydrocephalus due to obstruction at the foramen of Monro and profound bilateral sensorineural deafness. This appears to be a unique combination of anomalies and, to our knowledge, has not been reported previously. Both parents and a brother are phenotypically normal. The parents are second cousins. Thus, on the basis of consanguinity, affected sibs of both sexes, and in the absence of evidence for intrauterine infections or other adverse perinatal events, this syndrome is likely inherited in an autosomal recessive fashion.

Sensorineural hearing loss in children

Hearing loss in infants and children may be sensorineural, conductive, or mixed. Severity varies from mild to profound. Educational initiatives aimed at children, parents, and primary health care providers could help prevent needless permanent hearing impairment. Effective programs aimed at education and hearing conservation among children and adolescents are overdue. The causes of sensorineural hearing loss, the concept of multidisciplinary team evaluation, and measurement of hearing are discussed. Advances in genetics of hearing loss are reviewed.

Genes responsible for human hereditary deafness: symphony of a thousand

Hearing loss is the most frequent sensory defect in humans. Dozens of genes may be responsible for the early onset forms of isolated deafness and several hundreds of syndromes with hearing loss have been described. Both the difficulties encountered by linkage analysis in families affected by isolated deafness and the paucity of data concerning the molecular components specifically involved in the peripheral auditory process, have long hampered the identification of genes responsible for hereditary hearing loss. Rapid progress is now being made in both fields. This should allow completion of major pieces of the jigsaw for understanding the development and function of the ear.

Human sequences homologous to the gene for the cochlear protein Ocp-II do not map to currently known non-syndromic hearing loss loci

The abundant and almost exclusive expression of OCP-II protein in the mammalian cochlea has fuelled speculation that mutations in the OCP2 gene may result in inherited forms of hearing impairment. We have identified several human sequences related to OCP2 and sublocalised three of these OCP2 related loci to 4q12-p14 or 4p16.2-pter, 5q15-q21.3 and 7p22-q22 by PCR. 2 YACs with sequence consistent with the chromosome 7 locus were also used for FISH analysis and hybridised to chromosome 7q11. Our data suggest that the cytogenetic localisations of these OCP2 related sequences do not correlate with the precise chromosomal positions of deafness loci so far identified.

Genetics of deafness

The genetics of deafness is a rapidly expanding area of research. A remarkable total of twenty-two genes involved in non-syndromic deafness in humans have been localized within the past two years, compared with only one known previously. Some of the genes involved in neuroepithelial deafness, the most common type of pathology, have been identified in the past year. Two of these genes encode unconventional myosin molecules. The roles of these and other molecules identified by genetic approaches as important in hearing are being explored.