Development of a genotyping microarray for Usher syndrome

BACKGROUND

Usher syndrome, a combination of retinitis pigmentosa (RP) and sensorineural hearing loss with or without vestibular dysfunction, displays a high degree of clinical and genetic heterogeneity. Three clinical subtypes can be distinguished, based on the age of onset and severity of the hearing impairment, and the presence or absence of vestibular abnormalities. Thus far, eight genes have been implicated in the syndrome, together comprising 347 protein-coding exons.

METHODS

To improve DNA diagnostics for patients with Usher syndrome, we developed a genotyping microarray based on the arrayed primer extension (APEX) method. Allele-specific oligonucleotides corresponding to all 298 Usher syndrome-associated sequence variants known to date, 76 of which are novel, were arrayed.

RESULTS

Approximately half of these variants were validated using original patient DNAs, which yielded an accuracy of >98%. The efficiency of the Usher genotyping microarray was tested using DNAs from 370 unrelated European and American patients with Usher syndrome. Sequence variants were identified in 64/140 (46%) patients with Usher syndrome type I, 45/189 (24%) patients with Usher syndrome type II, 6/21 (29%) patients with Usher syndrome type III and 6/20 (30%) patients with atypical Usher syndrome. The chip also identified two novel sequence variants, c.400C>T (p.R134X) in PCDH15 and c.1606T>C (p.C536S) in USH2A.

CONCLUSION

The Usher genotyping microarray is a versatile and affordable screening tool for Usher syndrome. Its efficiency will improve with the addition of novel sequence variants with minimal extra costs, making it a very useful first-pass screening tool.

The Mass1frings mutation underlies early onset hearing impairment in BUB/BnJ mice, a model for the auditory pathology of Usher syndrome IIC

The human ortholog of the gene responsible for audiogenic seizure susceptibility in Frings and BUB/BnJ mice (mouse gene symbol Mass1) recently was shown to underlie Usher syndrome type IIC (USH2C). Here we report that the Mass1frings mutation is responsible for the early onset hearing impairment of BUB/BnJ mice. We found highly significant linkage of Mass1 with ABR threshold variation among mice from two backcrosses involving BUB/BnJ mice with mice of strains CAST/EiJ and MOLD/RkJ. We also show an additive effect of the Cdh23 locus in modulating the progression of hearing loss in backcross mice. Together, these two loci account for more than 70% of the total ABR threshold variation among the backcross mice at all ages. The modifying effect of the strain-specific Cdh23ahl variant may account for the hearing and audiogenic seizure differences observed between Frings and BUB/BnJ mice, which share the Mass1frings mutation. During postnatal cochlear development in BUB/BnJ mice, stereocilia bundles develop abnormally and remain immature and splayed into adulthood, corresponding with the early onset hearing impairment associated with Mass1frings. Progressive base-apex hair cell degeneration occurs at older ages, corresponding with the age-related hearing loss associated with Cdh23ahl. The molecular basis and pathophysiology of hearing loss suggest BUB/BnJ and Frings mice as models to study cellular and molecular mechanisms underlying USH2C auditory pathology.

Epidural analgesia in gastrointestinal surgery

BACKGROUND

The ideal perioperative analgesia should provide effective pain relief, avoid the detrimental effects of the stress response, be simple to administer without the need for intensive monitoring, and have a low risk of complications.

METHODS

This review defines the physiological effects of epidural analgesia and assesses whether the available evidence supports its preferential use in gastrointestinal surgery. All papers studied were identified from a Medline search or selected by cross-referencing.

RESULTS

Epidural analgesia is associated with a shorter duration of postoperative ileus, attenuation of the stress response, fewer pulmonary complications, and improved postoperative pain control and recovery. It does not reduce anastomotic leakage, intraoperative blood loss, transfusion requirement, risk of thromboembolism or cardiac morbidity, or hospital stay compared with that after conventional analgesia in unselected patients undergoing gastrointestinal surgery. Thoracic epidural analgesia reduces hospital costs and stay in patients at high risk of cardiac or pulmonary complications.

CONCLUSIONS

Epidural analgesia enhances recovery after gastrointestinal surgery. The results support the development of structured regimens of early postoperative feeding and mobilization to exploit the potential for thoracic epidural analgesia to reduce hospital stay after gastrointestinal surgery.

CDH23 mutation and phenotype heterogeneity: a profile of 107 diverse families with Usher syndrome and nonsyndromic deafness

Usher syndrome type I is characterized by congenital hearing loss, retinitis pigmentosa (RP), and variable vestibular areflexia. Usher syndrome type ID, one of seven Usher syndrome type I genetic localizations, have been mapped to a chromosomal interval that overlaps with a nonsyndromic-deafness localization, DFNB12. Mutations in CDH23, a gene that encodes a putative cell-adhesion protein with multiple cadherin-like domains, are responsible for both Usher syndrome and DFNB12 nonsyndromic deafness. Specific CDH23 mutational defects have been identified that differentiate these two phenotypes. Only missense mutations of CDH23 have been observed in families with nonsyndromic deafness, whereas nonsense, frameshift, splice-site, and missense mutations have been identified in families with Usher syndrome. In the present study, a panel of 69 probands with Usher syndrome and 38 probands with recessive nonsyndromic deafness were screened for the presence of mutations in the entire coding region of CDH23, by heteroduplex, single-strand conformation polymorphism, and direct sequence analyses. A total of 36 different CDH23 mutations were detected in 45 families; 33 of these mutations were novel, including 18 missense, 3 nonsense, 5 splicing defects, 5 microdeletions, and 2 insertions. A total of seven mutations were common to more than one family. Numerous exonic and intronic polymorphisms also were detected. Results of ophthalmologic examinations of the patients with nonsyndromic deafness have found asymptomatic RP-like manifestations, indicating that missense mutations may have a subtle effect in the retina. Furthermore, patients with mutations in CDH23 display a wide range of hearing loss and RP phenotypes, differing in severity, age at onset, type, and the presence or absence of vestibular areflexia.

Phase I study of percutaneous cryotherapy for colorectal liver metastasis

BACKGROUND

The aim was to determine the safety and feasibility of percutaneous cryotherapy for treating irresectable colorectal liver metastases.

METHODS

Liquid nitrogen cryoprobes were inserted percutaneously into metastases using the Seldinger technique under computed tomographic guidance. Single-probe treatments were performed with either 3.6- or 6.3-mm cryoprobes (ice-ball volumes 18 and 59 cm3 respectively), or dual-probe treatments with two adjacent 6.3-mm probes (ice-ball volume 205 cm3). Treatment involved a single freeze--thaw cycle.

RESULTS

Fifteen patients received 25 single-probe treatments and seven patients received 14 dual-probe treatments. The treatment-related mortality rate was zero and complications occurred after six of 39 treatments. Liver metastasis growth was significantly delayed for 2 months after dual-probe but not single-probe treatment. Metastasis cryotherapy stimulated an immediate rise, followed by a fall, in serum carcinoembryonic antigen (CEA) level, associated with immune upregulation that was significantly greater after dual-probe treatments.

CONCLUSION

Ablation zones that were approximately four times larger than those produced by previously described percutaneous techniques delayed the growth of metastases, reduced serum CEA concentration, and induced detectable inflammatory and T-lymphocyte responses. Percutaneous cryotherapy for treatment of colorectal liver metastases is feasible and may have a place in conjunction with chemotherapy.

A common ancestral origin of the frequent and widespread 2299delG USH2A mutation

Usher syndrome type IIa is an autosomal recessive disorder characterized by mild-to-severe hearing loss and progressive visual loss due to retinitis pigmentosa. The mutation that most commonly causes Usher syndrome type IIa is a 1-bp deletion, described as "2299delG," in the USH2A gene. The mutation has been identified in several patients from northern and southern Europe and from North America, and it has been found in single patients from South America, South Africa, and China. Various studies have reported a range of frequencies (.16-.44) among patients with Usher syndrome, depending on the geographic origin of the patients. The 2299delG mutation may be the one that most frequently causes retinitis pigmentosa in humans. Given the high frequencies and the wide geographic distribution of the mutation, it was of interest to determine whether the mutation resulted from an ancestral mutational event or represented a mutational hotspot in the USH2A gene. Haplotype analysis was performed on DNA samples from 116 unrelated patients with Usher syndrome type IIa; the patients were from 14 countries and represented 148 2299delG alleles. On the basis of six single-nucleotide polymorphisms within the USH2A gene, 12 core haplotypes were observed in a panel of normal chromosomes. However, in our analysis, only one core haplotype was found to be associated with the 2299delG mutation. The data indicate that the widespread geographic distribution of the 2299delG mutation is the result of an ancestral mutation that has spread throughout Europe and into the New World as a result of migration.

Spectrum of mutations in USH2A in British patients with Usher syndrome type II

Usher syndrome (USH) is a combination of a progressive pigmentary retinopathy, indistinguishable from retinitis pigmentosa, and some degree of sensorineural hearing loss. USH can be subdivided in Usher type I (USHI), type II (USHII) and type III (USHIII), all of which are inherited as autosomal recessive traits. The three subtypes are genetically heterogeneous, with six loci so far identified for USHI, three for USHII and only one for USHIII. Mutations in a novel gene, USH2A, encoding the protein usherin, have recently been shown to be associated with USHII. The gene encodes a protein with partial sequence homology to both laminin epidermal growth factor and fibronectin motifs. We analysed 35 British and one Pakistani Usher type II families with at least one affected member, for sequence changes in the 20 translated exons of the USH2A gene, using heteroduplex analysis and sequencing. Probable disease-causing mutations in USH2A were identified in 15 of 36 (41.7%) Usher II families. The most frequently encountered mutation (11/15 families or 11/18 mutated alleles) was del2299G in exon 13, resulting in a frameshift and premature stop codon. Other mutations include insertions and point mutations, of which two are previously unreported. Five different polymorphisms were also detected. Our results indicate that mutations in this gene are responsible for disease in a large proportion of British Usher type II patients. Moreover, if screening for mutations in USH2A is considered, it is sensible to screen for the del2299G mutation first.

Identification of novel USH2A mutations: implications for the structure of USH2A protein

Usher syndrome type II is an autosomal recessive disorder, characterised by stable hearing impairment from childhood and progressive retinitis pigmentosa from the late teens. Mutations in the USH2A gene, located on 1q41, were recently shown to be responsible for Usher syndrome type IIa. We have investigated the molecular pathology of Usher type II by screening the USH2A gene for mutations in 31 unrelated patients from Denmark and Norway. Besides the frequent 2299delG mutation, which accounted for 44% of the disease alleles, a heterogeneous spectrum of mutations was identified. Sixteen new, putative disease-causing mutations were detected, of which 12 were private and four were shared by unrelated patients. The disease-causing mutations were scattered throughout the gene and included six nonsense and seven missense mutations, two deletions and one small insertion. In addition, six non-pathogenic polymorphisms were identified. All missense mutations resulted in major amino acid side-chain alterations. Four missense mutations affected the N-terminal part of USH2A, whereas three missense mutations affected the laminin-type epidermal growth factor-like (LE) domain. The structural consequences of the mutations affecting the LE domain are discussed in relation to the three-dimensional structure of a LE-module of the mouse laminin gamma1 chain.

Identification of three novel mutations in human EYA1 protein associated with branchio-oto-renal syndrome

The Branchio-oto-renal (BOR) syndrome is an autosomal dominant disorder characterized by branchial clefts, preauricular sinuses, hearing loss, and renal anomalies. Recent studies have shown that mutations in EYA1 are associated with BOR. However, the underlying molecular mechanisms by which mutations in the EYA1 gene cause BOR syndrome are unknown. We have investigated 12 unrelated Caucasian families for mutations by heteroduplex analysis and direct sequencing of products from the polymerase chain reaction. In this study, we identified two novel frameshift deletions and a single base substitution that introduces a stop codon mutation in the C-terminal region of the EYA1 gene. No obvious relationships were observed between the nature of the mutations and the variable clinical features associated with BOR syndrome.

Three novel mutations and twelve polymorphisms identified in the USH2A gene in Israeli USH2 families

The Usher syndromes are autosomal recessive hereditary disorders characterized by hearing impairment and progressive visual loss due to Retinitis Pigmentosa (RP). Moderate to severe sensorineural hearing loss and progressive RP characterizes Usher syndrome type IIa (USH2A), which maps to the long arm of chromosome 1q41. Recently, three deletions carried by USH2 patients, which were found in a novel gene isolated from the critical 1q41 region, defined this gene as responsible for USH2A. The USH2A gene is predicted to encode a 1546 amino acid protein which possesses domains that are observed in basal lamina and extracellular matrix proteins and in cell adhesion molecules. Affected individuals and additional members from eleven USH2 Israeli families of diverse ethnic origin were screened for the presence of changes in all 20 coding exons of the USH2A gene. Three novel mutations (239-242insCGTA, R334W, T1515M) were identified in three families of Jewish Moroccan and Jewish Iranian origins. Twelve polymorphisms were found in the families, four of which are novel. None of the known USH2 mutations were identified in the families studied in this work. Hum Mutat 15:388, 2000.

Erratum: analysis of DNA elements that modulate myosin VIIa expression in humans

Usher syndromeIb (USH1B), an autosomal recessive disorder caused by mutations in myosin VIIa (MYO7A), is characterized by congenital profound hearing loss, vestibular abnormalities and retinitis pigmentosa. Promoter elements in the 5 kb upstream of the translation start were identified using adult retinal pigment epithelium cells (ARPE-19) as a model system. A 160 bp minimal promoter within the first intron was active in ARPE-19 cells, but not in HeLa cells that do not express MYO7A. A 100 bp sequence, 5' of the first exon, and repeated with 90% homology within the first intron, appeared to modulate expression in both cell lines. Segments containing these elements were screened by heteroduplex analysis. No heteroduplexes were detected in the minimal promoter, suggesting that this sequence is conserved. A -2568 A>T transversion in the 5' 100 bp repeat, eliminating a CCAAT element, was found only in USH1B patients. However, in all 5 families, -2568 A>T was in cis with the same missense mutation in the myosin VIIa tail (Arg1240Gln), and 4 of the 5 families were Dutch. These observations suggest either 1) linkage disequilibrium or 2)that a combination of a promoter mutation with a less active myosin VIIa protein results in USH1B.

Analysis of DNA elements that modulate myosin VIIA expression in humans

Usher syndromeIb (USH1B), an autosomal recessive disorder caused by mutations in myosin VIIa (MYO7A), is characterized by congenital profound hearing loss, vestibular abnormalities and retinitis pigmentosa. Promoter elements in the 5 kb upstream of the translation start were identified using adult retinal pigment epithelium cells (ARPE-19) as a model system. A 160 bp minimal promoter within the first intron was active in ARPE-19 cells, but not in HeLa cells that do not express MYO7A. A 100 bp sequence, 5' of the first exon, and repeated with 90% homology within the first intron, appeared to modulate expression in both cell lines. Segments containing these elements were screened by heteroduplex analysis. No heteroduplexes were detected in the minimal promoter, suggesting that this sequence is conserved. A -2568 A>T transversion in the 5' 100 bp repeat, eliminating a CCAAT element, was found only in USH1B patients. However, in all 5 families, -2568 A>T was in cis with the same missense mutation in the myosin VIIa tail (Arg1240Gln), and 4 of the 5 families were Dutch. These observations suggest either 1) linkage disequilibrium or 2)that a combination of a promoter mutation with a less active myosin VIIa protein results in USH1B.

Non-syndromic hearing loss associated with enlarged vestibular aqueduct is caused by PDS mutations

Enlarged vestibular aqueduct (EVA), known as the most common form of inner ear abnormality, has recently been of particular genetic interest because this anomaly is inherited in a recessive manner. The locus for non-syndromic sensorineural hearing loss with EVA has been mapped to the same chromosomal region, 7q31, as the Pendred syndrome locus. In the present study, seven mutations in the PDS gene (PDS), the gene responsible for Pendred syndrome, have been found in families of non-syndromic sensorineural hearing loss with EVA. One family is homozygous, three families are compound heterozygotes, and two families are heterozygous but with no other mutation detected. The present results provide evidence that mutations in PDS cause both syndromic and non-syndromic hearing loss.

Phylogenetic analysis of mitochondrial DNA in Japanese pedigrees of sensorineural hearing loss associated with the A1555G mutation

Thirteen Japanese families (ten of which were from the northern part of Japan), with sensorineural hearing loss associated with the 1555 A to G (A1555G) mitochondrial mutation, a known cause of non-syndromic hearing loss, were phylogenetically analysed using data obtained by restriction fragment length polymorphism (RFLP) and D-loop sequencing of mitochondrial DNA (mtDNA). Various types of mtDNA polymorphism were detected by restriction enzymes and D-loop sequence. No common polymorphic pattern throughout the 13 families was found, though three families exhibited the same restriction patterns and the same sequence substitution in the D-loop. To find where each of the 13 families are situated in the phylogenetic tree, the 482-bp of D-loop sequence were compared with those of 62 normal Japanese subjects. Despite the three families mentioned above appearing to be clustered, the remaining 10 families were scattered along the phylogenetic tree. This indicates that there was no common ancestor for the 13 Japanese families bearing the A1555G mutation except three families, and that the A1555G mutation occurred sporadically and multiplied through evolution of the mtDNA in Japan. The present results showed that the common pathogenicity (hearing loss associated with the A1555G mutation) can occur sporadically in families which have different genetic backgrounds, even in the Japanese population.

Mutation of a gene encoding a protein with extracellular matrix motifs in Usher syndrome type IIa

Usher syndrome type IIa (OMIM 276901), an autosomal recessive disorder characterized by moderate to severe sensorineural hearing loss and progressive retinitis pigmentosa, maps to the long arm of human chromosome 1q41 between markers AFM268ZD1 and AFM144XF2. Three biologically important mutations in Usher syndrome type IIa patients were identified in a gene (USH2A) isolated from this critical region. The USH2A gene encodes a protein with a predicted size of 171.5 kilodaltons that has laminin epidermal growth factor and fibronectin type III motifs; these motifs are most commonly observed in proteins comprising components of the basal lamina and extracellular matrixes and in cell adhesion molecules.

Isolation of a novel human homologue of the gene coding for echinoderm microtubule-associated protein (EMAP) from the Usher syndrome type 1a locus at 14q32

Usher syndrome type 1 (USH1) is an autosomal recessive, genetically heterogeneous disorder causing severe congenital deafness, retinitis pigmentosa, and vestibular dysfunction. The USHla locus located on 14q32 has been linked to the genetic markers D14S250 and D14S78. Using D14S250 and D14S78, we have isolated two nonchimeric YACs, 878g10 and 844g2, and a single BAC (135i20) and PAC (194e17) clone and have arranged them into a contig spanning over the D14S250 and D14S78 markers. The analysis of the YACs, BAC, and PAC revealed that the physical distance between D14S250 and D14S78 is less than 25 kb. Iterative cDNA library screening initiated with the EST 219670 found in the vicinity of the D14S78 marker yielded a cDNA contig. The nucleotide sequence of the cDNA encodes a protein of 717 amino acids in length, showing a high level of homology to the Echinoderm 77-kDa microtubule-associated protein (EMAP). The human homologue of Echinoderm microtubule-associated protein defines a novel human gene. We propose that the human EMAP is a strong candidate for the USH1a gene based on its genomic location and the proposed function of the protein.

The genomic structure of the gene defective in Usher syndrome type Ib (MYO7A)

Usher syndrome type Ib is a recessive autosomal disorder manifested by congenital deafness, vestibular dysfunction, and progressive retinal degeneration. Mutations in the human myosin VIIa gene (MYO7A) have been reported to cause Usher type Ib. Here we report the genomic organization of MYO7A. An STS content map was determined to discover the YAC clones that would cover the critical region for Usher syndrome type Ib. Three of the YACs (802A5, 966D6, and 965F10) were subcloned into cosmids and used to assemble a preliminary cosmid contig of the critical region. Part of the gene encoding human myosin VIIa was found in the preliminary cosmid contig. A cosmid, P1, PAC, and long PCR contig that contained the entire MYO7A gene was assembled. Primers were designed from the composite cDNA sequence and used to detect intron-exon junctions by directly sequencing cosmid, P1, PAC, and genomic PCR DNA. Alternatively spliced products were transcribed from the MYO7A gene: the largest transcript (7.4 kb) contains 49 exons. The MYO7A gene is relatively large, spanning approximately 120 kb of genomic DNA on chromosome 11q13.

Myosin VIIA mutation screening in 189 Usher syndrome type 1 patients

Usher syndrome type 1b (USH1B) is an autosomal recessive disorder characterized by congenital profound hearing loss, vestibular abnormalities, and retinitis pigmentosa. The disorder has recently been shown to be caused by mutations in the myosin VIIa gene (MYO7A) located on 11q14. In the current study, a panel of 189 genetically independent Usher I cases were screened for the presence of mutations in the N-terminal coding portion of the motor domain of MYO7A by heteroduplex analysis of 14 exons. Twenty-three mutations were found segregating with the disease in 20 families. Of the 23 mutations, 13 were unique, and 2 of the 13 unique mutations (Arg212His and Arg212Cys) accounted for the greatest percentage of observed mutant alleles (8/23, 31%). Six of the 13 mutations caused premature stop codons, 6 caused changes in the amino acid sequence of the myosin VIIa protein, and 1 resulted in a splicing defect. Three patients were homozygotes or compound heterozygotes for mutant alleles; these three cases were Tyr333Stop/Tyr333Stop, Arg212His-Arg302His/Arg212His-Arg302His, and IVS13nt-8c-->g/Glu450Gln. All the other USH1B mutations observed were simple heterozygotes, and it is presumed that the mutation on the other allele is present in the unscreened regions of the gene. None of the mutations reported here were observed in 96 unrelated control samples, although several polymorphisms were detected. These results add three patients to single case reported previously where mutations have been found in both alleles and raises the total number of unique mutations in MYO7A to 16.

The construction of a yeast artificial chromosome (YAC) contig in the vicinity of the Usher syndrome type IIa (USH2A) gene in 1q41

The gene for Usher syndrome type II (USH2A), an autosomal recessive syndromic deafness, has been mapped to a region of 1q41 flanked proximally by D1S217 and distally by D1S439. Using sequence-tagged sites (STSs) within the region, a total of 21 yeast artificial chromosome (YAC) clones were isolated and ordered into a single contig that spans approximately 11.0 Mb. The order of microsatellite and STS markers in this region was established as D1S505-D1S425-DXS217-D1S556-D1S237-D1S4 74-EB1-EB2-KB6-AFM144XF2-KB1-K B4-D1S229-D1S490-D1S227-TGFbeta2-D1S439. Analysis of newly positioned polymorphic markers in recombinant individuals in two Usher syndrome type IIa families has enabled us to identify DXS474 and AFM144XF2 as two flanking markers for the Usher type IIa locus. The physical distance between the two markers is 1.0 Mb. This region is covered by eight YACs from the CEPH library: 945f7, 867g9, 762a6, 919h3, 794b8, 785h4, 848b9, and 841g2. A long-range physical map of the Usher type IIa critical region, using MluI, BssHII, NotI, EagI, and SacII, has been developed.

Construction and characterization of a NotI linking library from human chromosome region 1q25-qter

Chromosome 1q25-qter-specific NotI linking clones have been isolated from a NotI linking library that was constructed using DNA from MCH206.1 somatic cell hybrid cells. These cells contain chromosome 1q25-qter translocated to human chromosome Xp22 as the only human genetic material in mouse background. Sixty-eight NotI linking clones have been mapped by a combination of fluorescence in situ hybridization and R-banding to cytogenetic bands on the long arm of chromosome 1. The relative order of 11 NotI clones and their relation to known chromosome 1 markers have also been determined in 1q32 and 1q41, where the genes of Van der Woude and Usher syndrome type IIa have been previously mapped: cen-chr1.14-chr1.79-chr1.56-chr1.11-chr1.9 5- chr1.58 (chr1.74)-D1S70-chr1.15-chr1.82 (chr1.143)-chr1.62-D1S81-tel. The 1q32- and 1q41-specific NotI linking clones were sequenced in the vicinity of the NotI site. They were analyzed in terms of nucleotide composition, G+C content, frequency of CpG dinucleotides, and protein coding potentials. Most of the 1q32-q41-specific NotI linking clones were derived from CpG islands. Sequences of three NotI linking clones proved to be identical with known genes. Six of the remaining eight had a high potential for coding regions and shared short homologous regions with sequences in the GenBank database. The NotI linking clones and the identified CpG islands will provide valuable resources for constructing a long-range restriction map of chromosome 1q25-q44 and for the eventual isolation of disease genes of Van der Woude syndrome (1q32-q41) and Usher syndrome type IIa (1q41).

Defective myosin VIIA gene responsible for Usher syndrome type 1B

Usher syndrome represents the association of a hearing impairment with retinitis pigmentosa and is the most frequent cause of deaf-blindness in humans. It is inherited as an autosomal recessive trait which is clinically and genetically heterogeneous. Some patients show abnormal organization of microtubules in the axoneme of their photoreceptors cells (connecting cilium), nasal ciliar cells and sperm cells, as well as widespread degeneration of the organ of Corti. Usher syndrome type 1 (USH1) is characterized by a profound congenital sensorineural hearing loss, constant vestibular dysfunction and prepubertal onset of retinitis pigmentosa. Of three different genes responsible for USH1. USH1B maps to 11q13.5 (ref. 10) and accounts for about 75% of USH1 patients. The mouse deafness shaker-1 (sh1) mutation has been localized to the homologous murine region. Taking into account the cytoskeletal abnormalities in USH patients, the identification of a gene encoding an unconventional myosin as a candidate for shaker-1 (ref. 14) led us to consider the human homologue as a good candidate for the gene that is defective in USH1B. Here we present evidence that a gene encoding myosin VIIA is responsible for USH1B. Two different premature stop codons, a six-base-pair deletion and two different missense mutations were detected in five unrelated families. In one of these families, the mutations were identified in both alleles. These mutations, which are located at the amino-terminal end of the motor domain of the protein, are likely to result in the absence of a functional protein. Thus USH1B appears as a primary cytoskeletal protein defect. These results implicate the genes encoding other unconventional myosins and their interacting proteins as candidates for other genetic forms of Usher syndrome.

Gene mapping of Usher syndrome type IIa: localization of the gene to a 2.1-cM segment on chromosome 1q41

Usher syndrome type II is associated with hearing loss and retinitis pigmentosa but not with any vestibular problems. It is known to be genetically heterogeneous, and one locus (termed USH2A) has been linked to chromosome 1q41. In an effort to refine the localization of USH2A, the genetic map of the region between and adjacent to the marker loci previously recognized as flanking USH2A (D1S70 and PPOL) is updated. Analysis of marker data on 68 Usher II families places the USH2A gene into a 2.1-cM region between the markers D1S237 and D1S229. The gene for transforming growth factor beta 2 (TGFB2) and the gene for the homeodomain box (HLX1) are both eliminated as candidates for USH2A, by virtue of their localization outside these flanking markers. The earlier finding of genetic heterogeneity was confirmed in six new families, and the proportion of unlinked Usher II families is estimated at 12.5%. The placement of the USH2A gene into this region will aid in the physical mapping and isolation of the gene itself.

Mivacurium in the myasthenic patient

We have used mivacurium in four myasthenic patients presenting for thymectomy. Supramaximal single twitch stimulation was applied to the ulnar nerve at the wrist and the force of contraction of the adductor pollicis was measured. After an initial bolus dose of 30 micrograms kg-1 (approximately one-fifth of the normal intubating dose), we observed a mean 37.5 (SEM 5.6)% reduction in evoked twitch tension. Neuromuscular block was increased with incremental doses and maintained with repeat bolus doses of 15 micrograms kg-1 at 25% recovery. The interval between maintenance bolus doses remained constant (mean 5.9 (0.7) min). Spontaneous offset was rapid with a mean recovery index (T25-T75) of 11.9 (2.1) min. Provided anticholinesterase therapy is withheld in the immediate preoperative period, mivacurium would appear to be a safe and appropriate neuromuscular blocker in this variably sensitive group of patients. The cumulative dose required to establish full neuromuscular block varied between 60 and 90 micrograms kg-1. A maintenance infusion, commencing at 3 micrograms kg-1 min-1, is recommended, guided by neuromuscular monitoring.

Genetic heterogeneity of Usher syndrome type II

Usher syndrome is an autosomal recessive disorder characterised by retinitis pigmentosa and congenital sensorineural hearing loss. A gene for Usher syndrome type II (USH2) has been localised to chromosome 1q32-q41. DNA from a family with four of seven sibs affected with clinical characteristics of Usher syndrome type II was genotyped using markers spanning the 1q32-1q41 region. These included D1S70 and D1S81, which are believed to flank USH2. Genotypic results and subsequent linkage analysis indicated non-linkage of this family to these markers. The A test analysis for heterogeneity with this family and 32 other Usher type II families was statistically significant at p < 0.05. Further clinical evaluation of this family was done in light of the linkage results to determine if any phenotypic characteristics would allow for clinical identification of the unlinked type. No clear phenotypic differences were observed; however, this unlinked family may represent a previously unreported subtype of Usher type II characterised by a milder form of retinitis pigmentosa and mild vestibular abnormalities. Heterogeneity of Usher syndrome type II complicates efforts to isolate and clone Usher syndrome genes using linkage analysis and limits the use of DNA markers in early detection of Usher type II.

Linkage of Usher syndrome type I gene (USH1B) to the long arm of chromosome 11

Usher syndrome is the most commonly recognized cause of combined visual and hearing loss in technologically developed countries. There are several different types and all are inherited in an autosomal recessive manner. There may be as many as five different genes responsible for at least two closely related phenotypes. The nature of the gene defects is unknown, and positional cloning strategies are being employed to identify the genes. This is a report of the localization of one gene for Usher syndrome type I to chromosome 11q, probably distal to marker D11S527. Another USH1 gene had been previously localized to chromosome 14q, and this second localization establishes the existence of a new and independent locus for Usher syndrome.

Localization of Usher syndrome type II to chromosome 1q

Usher syndrome is characterized by congenital hearing loss, progressive visual impairment due to retinitis pigmentosa, and variable vestibular problems. The two subtypes of Usher syndrome, types I and II, can be distinguished by the degree of hearing loss and by the presence or absence of vestibular dysfunction. Type I is characterized by a profound hearing loss and totally absent vestibular responses, while type II has a milder hearing loss and normal vestibular function. Fifty-five members of eight type II Usher syndrome families were typed for three DNA markers in the distal region of chromosome 1q: D1S65 (pEKH7.4), REN (pHRnES1.9), and D1S81 (pTHH33). Statistically significant linkage was observed for Usher syndrome type II with a maximum multipoint lod score of 6.37 at the position of the marker THH33, thus localizing the Usher type II (USH2) gene to 1q. Nine families with type I Usher syndrome failed to show linkage to the same three markers. The statistical test for heterogeneity of linkage between Usher syndrome types I and II was highly significant, thus demonstrating that they are due to mutations at different genetic loci.

Usher syndrome: clinical findings and gene localization studies

The issue of genetic heterogeneity is a critical problem in the localization of the gene(s) for Usher syndrome. Based on the data obtained on families studied to date, the differences between type I and type II Usher syndrome appear quite distinct with regard to auditory and vestibular function. Although the majority of families can be confidently diagnosed as typical type I or type II, clinical investigations revealed four families with findings that did not fit into either of the two more common subtypes. These findings emphasize the critical importance of an in-depth clinical analysis concomitant with the linkage investigation to assure accurate subtyping of Usher syndrome. Based on an analysis of only those families with definite type I or type II Usher syndrome, approximately 17% of the genome can be excluded as a potential site of the gene for type I, and 14% can be excluded as the site for the type II gene. This study will continue until the Usher gene(s) is successfully localized.