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Linkage analysis in a multigenerational family with autosomal dominant hearing loss yielded a chromosomal localisation of the underlying genetic defect in the DFNA20/26 locus at 17q25-qter. The 6-cM critical region harboured the [gamma]-1-actin (ACTG1) gene, which was considered an attractive candidate gene because actins are important structural elements of the inner ear hair cells. In this study, a Thr278Ile mutation was identified in helix 9 of the modelled protein structure. The alteration of residue Thr278 is predicted to have a small but significant effect on the [gamma] 1 actin structure owing to its close proximity to a methionine residue at position 313 in helix 11. Met313 has no space in the structure to move away. Moreover, the Thr278 residue is highly conserved throughout eukaryotic evolution. Using a known actin structure the mutation could be predicted to impair actin polymerisation. These findings strongly suggest that the Thr278Ile mutation in ACTG1 represents the first disease causing germline mutation in a cytoplasmic actin isoform.
J Med Genet 2003;40:879-884
Hereditary hearing loss is the most common sensorineural deficit and has the highest degree of genetic heterogeneity. So far, 19 genes are known to be involved in autosomal recessive forms of the disorder, 18 in autosomal dominant forms, and two in X linked types. Six of these genes can cause both autosomal dominant and recessive hearing loss. At least 36 genes remain to be identified, based on the number of known genetic loci, 17 of which are implicated in autosomal dominant forms. (1)
Autosomal recessive hearing loss is almost exclusively prelingual, severe to profound, and involves all frequencies. In contrast, autosomal dominant hearing loss shows variation in the type of hearing loss, the age of onset, and the rate of progression (for reviews, see Huygen et al (2) and Pennings et al (3) ). For individual autosomal dominant families it is often possible to suggest one or a few loci or genes that may be involved, through cross sectional or longitudinal analysis of audiograms of several family members. (4) (5)
Four families with autosomal dominant postlingual sensorineural hearing loss were found to be linked to overlapping regions of chromosome 17q25, (6-9) and two different DFNA locus numbers (DFNA20 and DFNA26) were assigned to this region. We recently described a fifth Dutch family with linkage to this interval. (10) Gently downsloping audiograms are seen in individuals at ages below 15 years. The hearing loss progresses to profound at the ages of 15-20 years and 25-40 years for the frequencies of 8 kHz and 1-4 kHz, respectively. At frequencies below 1 kHz progression is slower. The hearing loss in this family is very similar to that in the family described by DeWan et al. (9) Although hearing loss in the original DFNA20 family follows a less severe course, the progression is highest for 8 kHz, followed by 4, 2, and 1 kHz. (7)
In this paper we describe candidate gene analysis in the Dutch family, which led to the identification of a missense mutation in the ACTG1 gene which encodes [gamma]-actin 1. Using the structure of the highly homologous M[g.sup.2+]-ATP actin 1 of Dictyostelium discoideum, we provide evidence that the missense mutation has a small but significant functional effect. This represents the first description of a probable disease causing germline mutation in a cytoplasmic actin isoform.
A Dutch family with autosomal dominant hearing loss, W99-060, was ascertained (fig 1). Pure tone audiograms were obtained as described by Kemperman et al. (10) The study was approved by the local ethics committee, and written informed consent was obtained from each participating individual.
Amplification of the ACTG1 gene
DNA was isolated from blood samples using a salting out procedure. (11) Primers for amplification of exons and exon-intron boundaries of the P4HB, MAFG, CARD14, SLC26A11, and DKFZp434D1428 genes are available on request. Primers for amplification of the six exons and splice sites of the ACTG1 gene were designed using the published genomic sequence (NCBI accession No, M19283). The primers used (Invitrogen), annealing temperatures, and MgC[l.sub.2] concentrations …