Julie M. Schultz

Perrault syndrome is caused by recessive mutations in CLPP, encoding a mitochondrial ATP-dependent chambered protease.

Perrault syndrome is a genetically and clinically heterogeneous autosomal-recessive condition characterized by sensorineural hearing loss and ovarian failure. By a combination of linkage analysis, homozygosity mapping, and exome sequencing in three families, we identified mutations in CLPP as the likely cause of this phenotype. In each family, affected individuals were homozygous for a different pathogenic CLPP allele: c.433A>C (p.Thr145Pro), c.440G>C (p.Cys147Ser), or an experimentally demonstrated splice-donor-site mutation, c.270+4A>G. CLPP, a component of a mitochondrial ATP-dependent proteolytic complex, is a highly conserved endopeptidase encoded by CLPP and forms an element of the evolutionarily ancient mitochondrial unfolded-protein response (UPR(mt)) stress signaling pathway. Crystal-structure modeling suggests that both substitutions would alter the structure of the CLPP barrel chamber that captures unfolded proteins and exposes them to proteolysis. Together with the previous identification of mutations in HARS2, encoding mitochondrial histidyl-tRNA synthetase, mutations in CLPP expose dysfunction of mitochondrial protein homeostasis as a cause of Perrault syndrome.

Allelic hierarchy of CDH23 mutations causing non-syndromic deafness DFNB12 or Usher syndrome USH1D in compound heterozygotes

Background Recessive mutant alleles of MYO7A, USH1C, CDH23, and PCDH15 cause non-syndromic deafness or type 1 Usher syndrome (USH1) characterised by deafness, vestibular areflexia, and vision loss due to retinitis pigmentosa. For CDH23, encoding cadherin 23, non-syndromic DFNB12 deafness is associated primarily with missense mutations hypothesised to have residual function. In contrast, homozygous nonsense, frame shift, splice site, and some missense mutations of CDH23, all of which are presumably functional null alleles, cause USH1D. The phenotype of a CDH23 compound heterozygote for a DFNB12 allele in trans configuration to an USH1D allele is not known and cannot be predicted from current understanding of cadherin 23 function in the retina and vestibular labyrinth. Methods and results To address this issue, this study sought CDH23 compound heterozygotes by sequencing this gene in USH1 probands, and families segregating USH1D or DFNB12. Five non-syndromic deaf individuals were identified with normal retinal and vestibular phenotypes that segregate compound heterozygous mutations of CDH23, where one mutation is a known or predicted USH1 allele. Conclusions One DFNB12 allele in trans configuration to an USH1D allele of CDH23 preserves vision and balance in deaf individuals, indicating that the DFNB12 allele is phenotypically dominant to an USH1D allele. This finding has implications for genetic counselling and the development of therapies for retinitis pigmentosa in Usher syndrome. Accession numbers The cDNA and protein Genbank accession numbers for CDH23 and cadherin 23 used in this paper are AY010111.2 and AAG27034.2, respectively.

Usher syndrome: hearing loss with vision loss.

Usher syndrome (USH) is a clinically heterogeneous condition characterized by sensorineural hearing loss, progressive retinal degeneration, and vestibular dysfunction. A minimum test battery is described as well as additional clinical evaluations that would provide comprehensive testing of hearing, vestibular function, and visual function in USH patients. USH is also genetically heterogeneous. At least nine genes have been identified with mutations that can cause USH. The proteins encoded by these genes are thought to interact with one another to form a network in the sensory cells of the inner ear and retina.

Recent Advances in the Understanding of Syndromic Forms of Hearing Loss

There are hundreds of different syndromes that include an auditory phenotype as a prominent feature and nearly as many reviews of this topic (Ahmed, Riazuddin, Riazuddin, & Wilcox, 2003; Griffith & Friedman, 2002; Gurtler & Lalwani, 2002; Lalwani, 2002; Morton, 2002; Petit, 2001; Steel, Erven, & Kiernan, 2002). Approximately 30% of individuals with hereditary hearing loss also have abnormalities of other organ systems and are considered to have a syndromic form of deafness (Gorlin, Toriello, & Cohen, 1995). The accompanying abnormalities range from subtle to obvious and may be congenital or delayed in appearance. The majority of these syndromes are inherited as monogenic disorders. Some of these genes have been mapped to a chromosomal map position and a subset of these mapped genes have been identified (cloned). In Table 1 we list many of the syndromic forms of hearing loss and some of the distinguishing clinical features. In many cases where hearing loss is listed as a part of a syndrome, the loss develops late and may simply be secondary to the general neurological decay. For virtually all inherited syndromic forms of hearing loss the Online Mendelian Inheritance in Man (www.ncbi.nlm.nih.gov/Omim/) has comprehensive descriptions of the clinical features and molecular genetics as well as an all-inclusive list of references. In this review we discuss only advances in our understanding of syndromic forms of deafness that have been made in the past few years. Several deafness syndromes are named after the clinician(s) who first or more fully described the disorder such as Jervell and Lange-Nielsen syndrome, Marshall syndrome, Stickler syndrome, Usher syndrome and Waardenburg syndrome (Table 1). Other hereditary deafness syndromes have been given labels that encompass part or all of the clinical presentation (phenotype) such as Branchio-Oto-Renal syndrome (abbreviated BOR, Table 1). As might be expected, there is a gene for each of these clinically distinct inherited syndromes that include hearing loss as one feature. However, there are exceptions to this generalization. Two phenotypically distinct syndromes may be due to different mutations of the same gene. Geneticists refer to different mutations of the same gene as allelic mutations or, more simply, as alleles. There are many examples of phenotypically distinct syndromes that are caused by allelic mutations such as Marshall syndrome (OMIM 154780) and Stickler syndrome type 2 (OMIM 604841), both of which can be caused by mutations of COL11A1 on chromosome 1p21 (Griffith, Sprunger, Sirko-Osadsa, Tiller, Meisler, & Warman, 1998) (Table 1). Another example is Waardenburg syndrome type 1 (OMIM 193500), Waardenburg syndrome type 3 (OMIM 148820) and Craniofacial-Deafness-Hand syndrome (OMIM 122880), which are clinically distinct but, in fact, are caused by allelic mutations of PAX3 on chromosome 2q35 (Asher, Sommer, Morell, & Friedman, 1996). Moreover, the converse is also true. Mutations of more than one gene may cause the identical clinical phenotype. This is referred to as genetic heterogeneity. For example, Usher syndrome type 1 is characterized by congenital, severe to profound hearing loss, retinitis pigmentosa (RP) with prepubertal onset and vestibular areflexia. Surprisingly, there are at least seven different genes that can cause clinically indistinguishable Usher syndrome type 1 (Table 1). Sometimes a patient has hearing loss that is obvious while their other associated abnormalities escape notice. There are many different reasons for incomplete diagnoses. Two examples illustrate this point. The hearing loss in Usher syndrome types 1 and 2 is congenital, while the onset of RP may be delayed and not noticed until adolescence. Patients who have Jervell and Lange-Nielsen syndrome are hearing impaired, but their heart conduction problems are easily overlooked (Table 1). Therefore, Section on Human Genetics (T.B.F., J.M.S., T.B-Y., A.L., E.R.W., S.R., Z.A., I.A.B.), Section on Gene Structure and Function (S.P.P, A.J.G.), and Hearing Section (S.P.P., A.J.G.), National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Rockville, Maryland; and Department of Pediatrics and Human Development (R.A.F.), Michigan State University, East Lansing, Michigan.

A mutation of MET, encoding hepatocyte growth factor receptor, is associated with human DFNB97 hearing loss

Background Hearing loss is a heterogeneous neurosensory disorder. Mutations of 56 genes are reported to cause recessively inherited non-syndromic deafness. Objective We sought to identify the genetic lesion causing hearing loss segregating in a large consanguineous Pakistani family. Methods and results Mutations of GJB2 and all other genes reported to underlie recessive deafness were ruled out as the cause of the phenotype in the affected members of the participating family. Homozygosity mapping with a dense array of one million SNP markers allowed us to map the gene for recessively inherited severe hearing loss to chromosome 7q31.2, defining a new deafness locus designated DFNB97 (maximum logarithm of the odds score of 4.8). Whole-exome sequencing revealed a novel missense mutation c.2521T>G (p.F841V) in MET (mesenchymal epithelial transition factor), which encodes the receptor for hepatocyte growth factor. The mutation cosegregated with the hearing loss phenotype in the family and was absent from 800 chromosomes of ethnically matched control individuals as well as from 136 602 chromosomes in public databases of nucleotide variants. Analyses by multiple prediction programmes indicated that p.F841V likely damages MET function. Conclusions We identified a missense mutation of MET, encoding the hepatocyte growth factor receptor, as a likely cause of hearing loss in humans.

Genetic Analysis through OtoSeq of Pakistani Families Segregating Prelingual Hearing Loss

Objective To identify the genetic cause of prelingual sensorineural hearing loss in Pakistani families using a next-generation sequencing (NGS)–based mutation screening test named OtoSeq. Study Design Prospective study. Setting Research laboratory. Subjects and Methods We used 3 fluorescently labeled short tandem repeat (STR) markers for each of the known autosomal recessive nonsyndromic (DFNB) and Usher syndrome (USH) locus to perform a linkage analysis of 243 multigenerational Pakistani families segregating prelingual hearing loss. After genotyping, we focused on 34 families with potential linkage to MYO7A, CDH23, and SLC26A4. We screened affected individuals from a subset of these families using the OtoSeq platform to identify underlying genetic variants. Sanger sequencing was performed to confirm and study the segregation of mutations in other family members. For novel mutations, normal hearing individuals from ethnically matched backgrounds were also tested. Results Hearing loss was found to co-segregate with locus-specific STR markers for MYO7A in 32 families, CDH23 in 1 family, and SLC26A4 in 1 family. Using the OtoSeq platform, a microdroplet PCR-based enrichment followed by NGS, we identified mutations in 28 of the 34 families including 11 novel mutations. Sanger sequencing of these mutations showed 100% concordance with NGS data and co-segregation of the mutant alleles with the hearing loss phenotype in the respective families. Conclusion Using NGS-based platforms like OtoSeq in families segregating hearing loss will contribute to the identification of common and population-specific mutations, early diagnosis, genetic counseling, and molecular epidemiology.

Cone Responses in Usher Syndrome Types 1 and 2 by Microvolt Electroretinography

PURPOSE Progressive decline of psychophysical cone-mediated measures has been reported in type 1 (USH1) and type 2 (USH2) Usher syndrome. Conventional cone electroretinogram (ERG) responses in USH demonstrate poor signal-to-noise ratio. We evaluated cone signals in USH1 and USH2 by recording microvolt level cycle-by-cycle (CxC) ERG. METHODS Responses of molecularly genotyped USH1 (n = 18) and USH2 (n = 24) subjects (age range, 15-69 years) were compared with those of controls (n = 12). A subset of USH1 (n = 9) and USH2 (n = 9) subjects was examined two to four times over 2 to 8 years. Photopic CxC ERG and conventional 30-Hz flicker ERG were recorded on the same visits. RESULTS Usher syndrome subjects showed considerable cone flicker ERG amplitude losses and timing phase delays (P < 0.01) compared with controls. USH1 and USH2 had similar rates of progressive logarithmic ERG amplitude decline with disease duration (-0.012 log μV/y). Of interest, ERG phase delays did not progress over time. Two USH1C subjects retained normal response timing despite reduced amplitudes. The CxC ERG method provided reliable responses in all subjects, whereas conventional ERG was undetectable in 7 of 42 subjects. CONCLUSIONS Cycle-by-cycle ERG showed progressive loss of amplitude in both USH1 and USH2 subjects, comparable to that reported with psychophysical measures. Usher subjects showed abnormal ERG response latency, but this changed less than amplitude with time. In USH syndrome, CxC ERG is more sensitive than conventional ERG and warrants consideration as an outcome measure in USH treatment trials.