Учебники / Genetics and Auditory Disorders Keats 2002

132 A.J. Griffith and T.B. Friedman

2.4 Mouse Models of Human Deafness

It is much easier to perform controlled studies of hereditary auditory disorders in mice, whose inner ear is sufficiently similar to that of humans to serve as a model system for such disorders. Mice are much easier to study in large numbers and at controlled time points during fetal and postnatal life, and postmortem artifacts are minimized by expeditious fixation of the tissues. The histopathologic findings associated with mouse deafness mutations were reviewed (Steel et al. 1987; Steel and Bock 1983) and a classification scheme was presented for inner ear abnormalities similar to that proposed by Ormerod for cochlear abnormalities in human temporal bones (Ormerod 1960).

Steel and Bock proposed three major patterns of inner ear pathology, including morphogenetic abnormalities, which are gross structural deformities of the inner ear. One example of this type of malformation that occurs in humans is the Mondini inner ear deformity (Schuknecht 1980). The second group comprises neuroepithelial abnormalities in which the primary defect appears to occur in the organ of Corti. In these mutants, the stria vascularis is normal, the endocochlear potential is present, Reissner’s membrane is in normal position, the auditory abnormalities are fairly symmetric between the ears, and the vestibular neurosensory end organs are variably affected (Fig. 6.3). The third pattern of abnormalities is cochleosaccular dysplasia, in which the primary defect appears to be in the stria vascularis, and the endocochlear potential is thought to be absent (Fig. 6.3). The organ of Corti may degenerate secondarily, Reissner’s membrane eventually collapses, the saccular membrane may also collapse, and the ears are often asymmetrically affected. This pattern also occurs in human temporal bones, where it is known as Scheibe dysplasia (Scheibe 1892). Although the scheme described by Steel and Bock is based upon generalized observations of mice, they provide a cognitive framework for interpreting similar studies of human hereditary hearing loss (Steel and Bock 1983).

2.5 Many Families and Multiple Alleles

In a family with one affected individual (sporadic), or in a family with just a few affected individuals, distinguishing between an acquired or a genetic etiology is difficult, and attempting to assign a mode of inheritance is problematic. In a large multigeneration family, or in an isolated population with many affected individuals, it is possible to determine the mode of inheritance by segregation analysis with greater certainty. Segregation analysis is a statistical evaluation of the mode of inheritance based on the transmission ratios of a trait in a family (Thompson 1986) (Tables 6.1, 6.2, 6.3). If segregation analysis suggests a simple Mendelian trait and the family is large, identifying a map position for the disease gene is now a routine enterprise that is becoming highly automated.

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or recessive locus, respectively. DFNA and DFNB loci have a numeric suffix, a consecutively increasing Arabic number indicating the order in which the DFNA, DFNB, or DFN locus was mapped (http://www. gene.ucl.ac.uk/nomenclature/). This nomenclature reveals nothing about the chromosomal address of the gene or the associated audiological phenotype, except in the case of X-linked DFN loci that map to the X chromosome. After publication of a new DFN locus, the gene is given a unique six-digit MIM number (see http://www3.ncbi.nlm.nih.gov/Omin/ or the printed version (McKusick 1998)).

Although recessive mutations of deafness genes usually cause congenital profound deafness (Tables 6.1 and 6.2), dominant alleles, with a few exceptions, are associated with progressive hearing loss. Mutations of dosage-sensitive genes would be expected to be dominant via a mechanism known as haploinsufficiency. Alternatively, the product of a dominant mutant allele might “poison” the function of the wild-type gene product via a dominant negative mechanism. For example, the function of a multimeric complex normally comprised of wild-type gene products may be disrupted by the incorporation of the product of a mutant allele of that gene.

Twenty-eight DFNB loci have been mapped, and six have now been identified: DFNB1, DFNB2, DFNB3, DFNB4, DFNB9 and DFNB21. Thirtyfive DFNA loci have been mapped and ten have been identified that map to nine loci: DFNA1, DFNA2, DFNA3, DFNA5, DFNA8/12, DFNA9,

DFNA11, DFNA13 and DFNA15 (Table 6.4). The first part of this review is primarily focused on those DFNA and DFNB loci that have been identified.

Surprisingly, many of the DFNA and DFNB genes are expressed in tissues other than the auditory system (Table 6.4), yet mutant alleles seem only to cause nonsyndromic hearing loss. In some families thought preliminarily to be segregating nonsyndromic hearing loss, there may well be other clinical features that were missed initially and become obvious when more is known about the tissue and developmental expression profiles of the causative gene. Alternatively, a mutant phenotype may be restricted to the auditory system because other tissues in which the gene is expressed have functional redundancy for that particular gene product. Other gene products with similar functions presumably compensate for disruption of function of the mutated gene.

The rigor of proof of identification varies for the deafness genes that have been identified to date. To demonstrate that a disease gene has been identified, a candidate gene must reside within the critical map interval of the disease gene and, if fully penetrant, the mutant allele co-segregates with the disease. Mutant alleles that alter protein sequence or expression pattern in a biologically significant manner are not usually common polymorphisms, and are absent in a random sampling of several hundred individuals. However, a high carrier rate does not necessarily mean the variant is a benign polymorphism. Some mutant alleles of disease genes have a high

carrier frequency. For example, among Caucasians, the carrier frequency is 2 to 3% for the DF508 mutant allele of CFTR, the gene for cystic fibrosis (Morral et al. 1994), and among Ashkenazi Jews there is a carrier frequency of 4% for the 167delT mutant allele of GJB2 (Morell et al. 1998).

Other types of observations strengthen the argument that a disease gene has indeed been identified, although these issues sometimes cannot be addressed easily. For example, animal models with a mutation in the orthologue of the human gene that approximates the human disease phenotype can be instrumental in understanding the pathophysiology. Moreover, correction of the mutant phenotype in an animal model by introducing the wild-type gene is a definitive demonstration that the disease gene has been correctly identified (Probst et al. 1998).

Finally, there are now cautionary tales suggesting that assignment of the “nonsyndromic” classification may be misapplied in the absence of thorough clinical data (Baldwin et al. 1995; Tranebjaerg et al. 1995) and it is certainly premature if the most common syndromes are not ruled out. The “nonsyndromic” designation for a mutant allele causing deafness should be considered tentative until the gene is identified and its spatial expression pattern is known. For nonsyndromic deafness genes that are expressed in tissues other than the auditory system, there may be associated subclinical features that were initially unnoticed.

3.2 DFNA1

3.2.1 Genetics

In an extended Costa Rican family, autosomal-dominant, progressive sensorineural hearing loss was inherited from two affected brothers who migrated to Costa Rica from Spain in the eighteenth century. Lowfrequency hearing begins to be lost in the first decade, with profound losses at all frequencies occurring by age 30 (Lalwani et al. 1998; Leon et al. 1981). The causative gene, DFNA1, was found to be linked to markers defining a ~7 cM critical interval on 5q31 (Leon et al. 1992). This was the first autosomal nonsyndromic hearing loss gene to be mapped.

3.2.2 Diaphanous is DFNA1

DFNA1 was positionally cloned and identified as the human homologue of the Drosophila diaphanous gene (Lynch et al. 1997). Human diaphanous, HDIA1, is composed of 26 exons encoding a 140 kDa protein (E. Lynch, personal communication). A G-to-T substitution in the consensus splice donor site of the penultimate exon was found in all 78 affected members of the Costa Rican family, but not in 330 normal-hearing members of this family. The G-to-T substitution results in aberrant splicing of exon 25 at a cryptic site 4 nucleotides into the intron. Splicing at this cryptic site causes a frameshift with substitution of 21 aberrant amino acids for the 32 wild-

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type HDIA1 amino acids at the carboxy terminus of the protein (Lynch et al. 1997).

The function of HDIA1 (Diaphanous) in the auditory system, as well as the pathophysiology of the HDIA1 mutation, are not known. HDIA1 is a member of the formin family of genes that are important for normal cytokinesis and establishment of cell polarity. Functional clues about HDIA1 may come from the fruit fly, Drosophila melanogaster, where a null allele of diaphanous is a pupal-lethal, a hypomorphic allele causes male sterility, and a weak allele results in polyploid cells in the central nervous system. It has been suggested that human diaphanous interacts with profilin to regulate actin filament formation, thereby affecting the intricate actin cytoskeleton of hair cells (Lynch et al. 1997). Any model for the function of human diaphanous in the inner ear should incorporate the delayed onset and progressive nature of the hearing loss associated with the HDIA1 mutation observed in the Costa Rican kindred.

3.3 DFNA2

The DFNA2 locus was mapped to 1p34 in several large families from Indonesia, the United States, Belgium and the Netherlands that were segregating dominantly inherited, progressive hearing loss (Coucke et al. 1994; Van Camp et al. 1997b). This chromosomal interval contains two good deafness candidate genes, one encoding a potassium channel (KCNQ4) and the other a gap junction family member, GJB3 (connexin 31, Cx31). Subsequently, affected individuals in two small Chinese families with hearing loss were reported to have dominant mutations in GJB3, R180X and E183K, a nonsense mutation and a missense mutation, respectively. These postulated dominant mutations do not co-segregate perfectly with the hearing loss in two small Chinese pedigrees (Xia et al. 1998). For example, an unexplained nonpenetrant individual segregating the R180X mutation indicates that the R180X is not the pathogenic mutation and/or that there are other factors modifying the hearing-loss phenotype. Therefore, the genetic mechanism of dominant GJB3 mutations such as R180X cannot be explained by a simple dominant mutation at this single locus.

Unfortunately, it is not possible to perform linkage analysis studies to corroborate these conclusions. The two Chinese pedigrees with a total of four affected individuals are too small to achieve a statistically significant Lod score. Nevertheless, recessive mutant alleles of GJB3 do appear to be associated with hearing loss. Compound heterozygous mutant alleles of

GJB3 (R180X and E183K) were reported to co-segregate with probable recessive hearing loss in two small Chinese pedigrees (Liu et al. 2000). However, the recessive deafness mutations of GJB3 in this study also lack corroborating linkage analysis.

DFNA2 is an example of locus heterogeneity in that mutant alleles of two closely linked genes, GJB3 and KCNQ4, appear to be associated with