OTOF mutations revealed by genetic analysis of hearing loss families including a potential temperature sensitive auditory neuropathy allele

INTRODUCTION

The majority of hearing loss in children can be accounted for by genetic causes. Non-syndromic hearing loss accounts for 80% of genetic hearing loss in children, with mutations in DFNB1/GJB2 being by far the most common cause. Among the second tier genetic causes of hearing loss in children are mutations in the DFNB9/OTOF gene.

METHODS

In total, 65 recessive non-syndromic hearing loss families were screened by genotyping for association with the DFNB9/OTOF gene. Families with genotypes consistent with linkage or uninformative for linkage to this gene region were further screened for mutations in the 48 known coding exons of otoferlin.

RESULTS

Eight OTOF pathological variants were discovered in six families. Of these, Q829X was found in two families. We also noted 23 other coding variant, believed to have no pathology. A previously published missense allele I515T was found in the heterozygous state in an individual who was observed to be temperature sensitive for the auditory neuropathy phenotype.

CONCLUSIONS

Mutations in OTOF cause both profound hearing loss and a type of hearing loss where otoacoustic emissions are spared called auditory neuropathy.

Connexin 26 gene (GJB2) mutation modulates the severity of hearing loss associated with the 1555A-->G mitochondrial mutation

We report a high prevalence of GJB2 heterozygous mutations in patients bearing the 1555A-->G mitochondrial mutation, and describe a family in which potential interaction between GJB2 and a mitochondrial gene appears to be the cause of hearing impairment. Patients who are heterozygotes for the GJB2 mutant allele show hearing loss more severe than that seen in sibs lacking a mutant GJB2 allele, suggesting that heterozygous GJB2 mutations may synergistically cause hearing loss when in the presence of a 1555A-->G mutation. The present findings indicate that GJB2 mutations may sometimes be an aggravating factor, in addition to aminoglycoside antibiotics, in the phenotypic expression of the non-syndromic hearing loss associated with the 1555A-->G mitochondrial mutation.

Reading disability and chromosome 6p21.3: evaluation of MOG as a candidate gene

Linkage analysis has localized a gene influencing specific reading disability (dyslexia) to 6p21.3. The myelin oligodendrocyte glycoprotein (MOG) gene, which maps to this region, was selected as a candidate. Myelin oligodendrocyte glycoprotein is a membrane protein, a member of the immunoglobin superfamily, that is found on the outermost lamellae of mature myelin. Although the exact function of this protein is unknown, its presence in the central nervous system and the hypothesized relationship between dyslexia and temporal processing rate as well as a suggested relationship with intelligence made this gene a candidate for dyslexia. Analysis of the coding exons and adjacent splice sites in a subset of 22 children with dyslexia from 10 sibships found a missense mutation in exon 4 in 2 of the sibships. This change from the published sequence also occurred in 86 of 96 random controls, making it considerably less frequent in this small sample of individuals with dyslexia. Subsequent typing of this single nucleotide polymorphism (SNP) in 74 nuclear families in which at least one child had reading disability showed no significant difference in frequency from the controls, however. Sib-pair linkage analysis with these families did not show significant linkage with the SNP nor with a separate polymorphic dinucleotide repeat marker in the MOG gene (MOG31/32), but association analysis identified two alleles of MOG31/32 that were associated with reading disability phenotypes with a low level of significance. Thus, although alleles in the MOG gene may be in linkage disequilibrium with a locus that contributes to reading disability, it is unlikely that the MOG gene itself is involved.

Mechanism of ascorbic acid oxidation by cytochrome b(561)

The 1 equiv reaction between ascorbic acid and cytochrome b(561) is a good model for redox reactions between metalloproteins (electron carriers) and specific organic substrates (hydrogen-atom carriers). Diethyl pyrocarbonate inhibits the reaction of cytochrome b(561) with ascorbate by modifying a histidine residue in the ascorbate-binding site. Ferri/ferrocyanide can mediate reduction of DEPC-treated cytochrome b(561) by ascorbic acid, indicating that DEPC-inhibited cytochrome b(561) cannot accept electrons from a hydrogen-atom donor like ascorbate but can still accept electrons from an electron donor like ferrocyanide. Ascorbic acid reduces cytochrome b(561) with a K(m) of 1.0 +/- 0.2 mM and a V(max) of 4.1 +/- 0.8 s(-1) at pH 7.0. V(max)/K(m) decreases at low pH but is approximately constant at pH >7. The rate constant for oxidation of cytochrome b(561) by semidehydroascorbate decreases at high pH but is approximately constant at pH <7. This suggests that the active site must be unprotonated to react with ascorbate and protonated to react with semidehydroascorbate. Molecular modeling calculations show that hydrogen bonding between the 2-hydroxyl of ascorbate and imidazole stabilizes the ascorbate radical relative to the monoanion. These results are consistent with the following mechanism for ascorbate oxidation. (1) The ascorbate monoanion binds to an unprotonated site (histidine) on cytochrome b(561). (2) This complex donates an electron to reduce the heme. (3) The semidehydroascorbate anion dissociates from the cytochrome, leaving a proton associated with the binding site. (4) The binding site is deprotonated to complete the cycle. In this mechanism, an essential role of the cytochrome is to bind the ascorbate monoanion, which does not react by outer-sphere electron transfer in solution, and complex it in such a way that the complex acts as an electron donor. Thermodynamic considerations show that no steps in this process involve large changes in free energy, so the mechanism is reversible and capable of fulfilling the cytochrome's function of equilibrating ascorbate and semidehydroascorbate.

Evidence for an essential histidine residue in the ascorbate-binding site of cytochrome b561

Cytochrome b(561) mediates equilibration of the ascorbate/semidehydroascorbate redox couple across the membranes of secretory vesicles. The cytochrome is reduced by ascorbic acid and oxidized by semidehydroascorbate on either side of the membrane. Treatment with diethyl pyrocarbonate (DEPC) inhibits reduction of the cytochrome by ascorbate, but this activity can be restored by subsequent treatment with hydroxylamine, suggesting the involvement of an essential histidine residue. Moreover, DEPC inactivates cytochrome b(561) more rapidly at alkaline pH, consistent with modification of a histidine residue. DEPC does not affect the absorption spectrum of cytochrome b(561) nor does it change the midpoint reduction potential, confirming that histidine modification does not affect the heme. Ascorbate protects the cytochrome from inactivation by DEPC, indicating that the essential histidine is in the ascorbate-binding site. Further evidence for this is that DEPC treatment inhibits oxidation of the cytochrome by semidehydroascorbate but not by ferricyanide. This supports a reaction mechanism in which ascorbate loses a hydrogen atom by donating a proton to histidine and transferring an electron to the heme.

The M34T allele variant of connexin 26

GJB2 encodes the protein Connexin 26, one of the building blocks of gap junctions. Each Connexin 26 molecule can oligomerize with five other connexins to form a connexon; two connexons, in turn, can form a gap junction. Because mutations in GJB2 are the most common cause of congenital severe-to-profound autosomal recessive nonsyndromic hearing loss, the effect of the Connexin 26 allele variants on this dynamic 'construction' process and the function of any gap junctions that do form is particularly germane. One of the more controversial allele variants, M34T, has been hypothesized to cause autosomal dominant nonsyndromic hearing loss. In this paper, we present clinical and genotypic data that refutes this hypothesis and suggests that the effect of the M34T allele variant may be dependent on the mutations segregating in the opposing allele.

Endoscopic breast subpectoral augmentation for second-degree breast ptosis

Glandular ptosis and first-degree ptosis are treated routinely with breast augmentation in select patients. Second-degree ptosis is difficult to treat with breast augmentation alone. Patients must be well informed and selected properly to obtain a satisfactory result. Historically, second-degree ptosis is treated most commonly with subglandular augmentation. The authors demonstrate that second-degree ptosis may be treated using endoscopic subpectoral augmentation. They think that the endoscopic approach gives more control and precision in the lowering of the inframammary fold and the placement of the implant. Additionally, there may be a decrease or maintenance in the distance from the clavicle to nipple because of shortening the pectoralis major as a result of dividing it from the sixth rib at the sternal attachment laterally to the serratus fascia.

An outbreak of fulminant hepatitis delta in the Waorani, an indigenous people of the Amazon basin of Ecuador

An outbreak of delta hepatitis occurred during 1998 among the Waorani of the Amazon basin of Ecuador. Among 58 people identified with jaundice, 79% lived in four of 22 Waorani communities. Serum hepatitis B surface antigen (HBsAg) was found in the sera of 54% of the jaundiced persons, and 14% of asymptomatic persons. Ninety-five percent of 105 asymptomatic Waorani had hepatitis B core (HBc) IgG antibody, versus 98% of 51 with jaundice. These data confirm that hepatitis B virus (HBV) infection is highly endemic among the Waorani. Sixteen of 23 (70%) HBsAg carriers identified at the onset of the epidemic had serologic markers for hepatitis D virus (HDV) infection. All 16 were jaundiced, where as only two of seven (29%) with negative HDV serology were jaundiced (P = .0006). The delta cases clustered in families, 69% were children and most involved superinfection of people chronically infected with HBV. The data suggest that HDV spread rapidly by a horizontal mode of transmission other than by the sexual route.

Tietz syndrome (hypopigmentation/deafness) caused by mutation of MITF

Patients with Tietz syndrome have congenital profound deafness and generalised hypopigmentation, inherited in a fully penetrant autosomal dominant fashion. The pigmentary features and complete penetrance make this syndrome distinct among syndromes with pigmentary anomalies and deafness, which characteristically have patchy depigmentation and variable penetrance. Only one family has been reported with the exact features described in the original report of this syndrome. This family was reascertained and a missense mutation was found in the basic region of the MITF gene in family members with Tietz syndrome. Mutations in other regions of this gene have been found to produce Waardenburg syndrome type 2 (WS2), which also includes pigmentary changes and hearing loss, but in contrast to Tietz syndrome, depigmentation is patchy and hearing loss is variable in WS2.

Genetic heterogeneity of Usher syndrome type II: localisation to chromosome 5q

Usher syndrome is a group of autosomal recessive disorders that includes retinitis pigmentosa (RP) with hearing loss. Usher syndrome type II is defined as moderate to severe hearing loss with RP. The USH2A gene at 1q41 has been isolated and characterised. In 1993, a large Usher II family affected with a mild form of RP was found to be unlinked to 1q41 markers. Subsequent linkage studies of families in our Usher series identified several type II families unlinked to USH2A and USH3 on 3q25. After a second unlinked family with many affected members and a mild retinal phenotype was discovered, a genome search using these two large families showed another Usher II locus on 5q (two point lod = 3.1 at D5S484). To date, we have identified nine unrelated 5q linked families (maximum combined multipoint lod = 5.86) as well as three Usher II families that show no significant linkage to any known Usher loci. Haplotype analysis of 5q markers indicates that the new locus is flanked by D5S428 and D5S433. Review of ophthalmological data suggests that RP symptoms are milder in 5q linked families; the RP is often not diagnosed until patients near their third decade. Enamel hypoplasia and severe, very early onset RP were observed in two of the three unlinked families; dental anomalies have not been previously described as a feature of Usher type II.

Connexin 26: required for normal auditory function

A single base deletion mutation, 35delG, in the gene (GJB2/DFNB1)(OMIM 121011/220290) encoding the gap junction protein, connexin 26 is the most important single cause of genetic hearing loss in European and American populations. It is the cause of one of the most common human genetic disorders with a frequency similar to cystic fibrosis. Mutations in this connexin are associated with skin disorders.

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.

Prevalent connexin 26 gene (GJB2) mutations in Japanese

The gene responsible for DNFB1 and DFNA3, connexin 26 (GJB2), was recently identified and more than 20 disease causing mutations have been reported so far. This paper presents mutation analysis for GJB2 in Japanese non-syndromic hearing loss patients compatible with recessive inheritance. It was confirmed that GJB2 mutations are an important cause of hearing loss in this population, with three mutations, 235delC, Y136X, and R143W, especially frequent. Of these three mutations, 235delC was most prevalent at 73%. Surprisingly, the 35delG mutation, which is the most common GJB2 mutation in white subjects, was not found in the present study. Our data indicated that specific combinations of GJB2 mutation exist in different populations.

Human connexin 30 (GJB6), a candidate gene for nonsyndromic hearing loss: molecular cloning, tissue-specific expression, and assignment to chromosome 13q12

Mutations in connexin 26 are responsible for approximately 20% of genetic hearing loss and 10% of all childhood hearing loss. However, only about 75% of the mutations predicted to be in Cx26 are actually observed. While this may be due to mutations in noncoding regulatory regions, an alternative hypothesis is that some cases may be due to mutations in another gene immediately adjacent to Cx26. Another gap junction gene, connexin 30 (HGMW-approved symbol GJB6), is found to lie on the same PAC clone that hybridizes to chromosome 13q12. Human connexin 26 and connexin 30 are expressed in the same cells of the cochlea. Cx26 and Cx30 share 77% identity in amino acid sequence but Cx30 has an additional 37 amino acids at its C-terminus. These considerations led us to hypothesize that mutations in Cx30 might also be responsible for hearing loss. Eight-eight recessive nonsyndromic hearing loss families from both American and Japanese populations were screened for mutations. In addition, 23 dominant hearing loss families and 6 singleton families presumed to be recessive were tested. No significant mutation has been found in the dominant or recessive families.

Novel mutation in the KCNQ4 gene in a large kindred with dominant progressive hearing loss

Analysis of genotyping of a five-generation American family with nonsyndromic dominant progressive hearing loss indicated linkage to the DFNA2 locus on chromosome 1p34. This kindred consists of 170 individuals, of which 51 are affected. Pure tone audiograms, medical records, and blood samples were obtained from 36 family members. Linkage analysis with five microsatellite markers spanning the region around DFNA2 produced a lod score of 6.6 for the marker MYCL1 at straight theta = 0.0. Hearing loss in this family showed a very similar pattern as the first reported American family with the same linkage. High frequency hearing loss was detectable as early as 3 years of age, and progressed to severe to profound loss by the fourth decade. Using intronic primers, we screened the coding region of the KCNQ4 gene. Heteroduplex analysis followed by direct sequencing identified a T-->C transition at position 842, which would produce an L281S amino acid substitution. The observed mutation was shown to segregate completely with affected status in this family. The L281 residue is significantly conserved among the other members of the voltage-gated K(+) channel genes superfamily. Hydrophobicity analysis indicated that L281S substitution would lower formation of the beta structure at the P region of this ion channel. Mutation analysis of KCNQ4 was also performed on 80 unrelated probands from families with recessive or dominant nonsyndromic hearing loss. None of these cases showed a truncated mutation in KCNQ4.

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.

Clinical phenotype and mutations in connexin 26 (DFNB1/GJB2), the most common cause of childhood hearing loss

Mutations in the gene for connexin 26, GJB2, are the most common cause of hearing loss in American and European populations, with a carrier rate of about 3%-a rate similar to that for cystic fibrosis. A single mutation, 35delG, is responsible for most of this autosomal recessive hearing loss, DFNB1. A broad spectrum of mutations in GJB2 has been found to be associated with hearing loss, including another deletion mutation, 167delT, which has a carrier rate of about 4% in the Ashkenazi Jewish population. Mutations in GJB2 have also been found to be associated with dominant nonsyndromic hearing loss, DFNA3. Clinical studies have shown that the recessive hearing loss can vary from mild to profound, even within the same sibship. This type of hearing loss is nonsyndromic and is accompanied by normal vision, vestibular responses, and no malformations of the inner ear detectable by computed tomography scanning. Progressive and asymmetrical hearing loss has been noted in some cases, but it accounts for fewer than one-third of the cases of this type of hearing loss. The discovery of mutations in GJB2 that cause hearing loss has profound implications in the early diagnosis of hearing loss in general. The relative ease of diagnosis by genetic testing of Cx26 permits early identification of children with GJB2/DFNB1 hearing loss. This testing, coupled with hearing loss diagnosed by infant auditory brainstem response audiometry, will ensure that hearing-impaired children and their parents receive proper medical, audiologic, genetic, and educational counseling. Am. J. Med. Genet. (Semin. Med. Genet.) 89:130-136, 1999.

Mutations in the KCNQ4 gene are responsible for autosomal dominant deafness in four DFNA2 families

We have previously found linkage to chromosome 1p34 in five large families with autosomal dominant non-syndromic hearing impairment (DFNA2). In all five families, the connexin31 gene ( GJB3 ), located at 1p34 and responsible for non-syndromic autosomal dominant hearing loss in two small Chinese families, has been excluded as the responsible gene. Recently, a fourth member of the KCNQ branch of the K+channel family, KCNQ4, has been cloned. KCNQ4 was mapped to chromosome 1p34 and a single mutation was found in three patients from a small French family with non-syndromic autosomal dominant hearing loss. In this study, we have analysed the KCNQ4 gene for mutations in our five DFNA2 families. Missense mutations altering conserved amino acids were found in three families and an inactivating deletion was present in a fourth family. No KCNQ4 mutation could be found in a single DFNA2 family of Indonesian origin. These results indicate that at least two and possibly three genes responsible for hearing impairment are located close together on chromosome 1p34 and suggest that KCNQ4 mutations may be a relatively frequent cause of autosomal dominant hearing loss.

Clinical studies of families with hearing loss attributable to mutations in the connexin 26 gene (GJB2/DFNB1)

OBJECTIVE

This retrospective study describes the phenotype associated with the single most common cause of genetic hearing loss. The frequency of childhood deafness is estimated at 1/500. Half of this hearing loss is genetic and approximately 80% of genetic hearing loss is nonsyndromic and inherited in an autosomal recessive manner. Approximately 50% of childhood nonsyndromic recessive hearing loss is caused by mutations in the connexin 26 (Cx26) gene (GJB2/DFNB1), making it the most common form of autosomal recessive nonsyndromic hearing loss with a carrier rate estimated to be as high as 2.8%. One mutation, 35delG, accounts for approximately 75% to 80% of mutations at this gene.

METHODS

Hearing loss was examined in 46 individuals from 24 families who were either homozygous or compound heterozygous for Cx26 mutations. A subset of these individuals were examined for vestibular function, otoacoustic emissions, auditory brainstem response, temporal bone computed tomography, electrocardiography, urinalyses, dysmorphology, and thyroid function.

RESULTS

Although all persons had hearing impairment, no consistent audiologic phenotype was observed. Hearing loss varied from mild-moderate to profound, even within the group of families homozygous for the common mutation 35delG, suggesting that other factors modify the phenotypic effects of mutations in Cx26. Furthermore, the hearing loss was observed to be progressive in a number of cases. No associations with inner ear abnormality, thyroid dysfunction, heart conduction defect, urinalyses, dysmorphic features, or retinal abnormality were noted.

CONCLUSION

Newborns with confirmed hearing loss should have Cx26 testing. Cx26 testing will help define a group in which approximately 60% will have profound or severe-profound hearing loss and require aggressive language intervention (many of these patients will be candidates for cochlear implants).

Nitric oxide production and absorption in trachea, bronchi, bronchioles, and respiratory bronchioles of humans

Different volumes of dead-space gas were collected and analyzed for nitric oxide (NO) content, either immediately after inspiration or after a period of breath holding on clean air or NO mixtures. This allowed calculation of NO equilibrium, NO production, and NO absorption. In seven young, healthy, adult nonsmokers, the mean NO equilibrium values in parts per billion (ppb) were 56 +/- 11 (SE) in the trachea, 37 +/- 6 in the bronchi, 21 +/- 3 in the bronchioles, and 16 +/- 2 in the respiratory bronchioles. At any given NO concentration, the NO absorption rate (in nl/min) equaled the NO concentration (in ppb) times A (the absorption coefficient in l/min). A values (in l/min) were 0.11 +/- 0.01 in the trachea, 0.17 +/- 0. 04 in the bronchi, 0.66 +/- 0.09 in the bronchioles, and 1.35 +/- 0. 32 in the respiratory bronchioles. NO equilibrium concentrations and production rates in one 74-yr-old subject were three to five times as high as those found in the young subjects. Mouth equilibrium NO concentrations were 3 and 6 parts per million in two subjects who had oral production rates of 6 and 23 nl/min, respectively. In conclusion, production and absorption of NO occur throughout the first 450 ml of the airways.

Comparison between the uptake of nitrous oxide and nitric oxide in the human nose

The absorption of nitrous oxide (N2O) during unidirectional flow was compared with the rate of uptake of nitric oxide (NO). At flow rates of 10, 20, and 60 ml/min from one nostril to the other, with the soft palate closed, the N2O reached a steady-state rate of absorption in 5-15 min. The mean superficial capillary blood flow (n = 5) calculated from solubility and the steady-state rate of N2O absorption ranged from 13.3 to 15.9 ml/min. The relation between absorption of N2O in the nose and capillary blood flow fits a ventilation-perfusion model used by others to describe uptake of inert, soluble gases in the rat nose. By contrast, the rate of uptake of NO gas, which is chemically reactive, is 25-31 times as great as predicted by just its blood-to-air partition coefficient. Exogenous NO (16.9 parts/million) did not induce nasal vasodilation as measured with laser Doppler and N2O absorption methods. The difference between the measured rate of uptake of NO and the rate of uptake attributable to its partition coefficient in blood at the rate of blood flow calculated from N2O uptake is probably due to chemical reaction of NO in mucous secretions, nasal tissues, and capillary blood.

Detailed map of a region commonly amplified at 11q13-->q14 in human breast carcinoma

Amplification of loci present on band q13 of human chromosome 11 is a feature of a subset of estrogen receptor positive breast carcinomas prone to metastasis. As many as five distinct amplification units have been described on 11q13. They include particularly a genomic area encompassing the GARP gene at 11q13.5-->q14.1. We have reassessed our current knowledge of this region, located telomeric to CCND1 and EMS1, which is amplified in 7-10% of mammary tumors. The loose definition of the driving forces of these amplification events led us to map accurately the boundaries of the amplifiable region, and thus to contribute a physical and transcriptional map of a 3-Mb region of chromosome 11. Four new genes were placed on the regional map, namely CBP2, CLNS1A, UVRAG, and PAK1. We have narrowed the core of the 11q13-->q14 amplicon to a 350-kb area encompassing D11S533, mostly on its telomeric side. The map reported here represents an indispensable step toward sequencing the entire region, and thus toward uncovering gene(s) which play(s) a critical role in breast cancer progression.

Molecular approaches to the genetic analysis of specific reading disability

Specific reading disability is a complex phenotype that is under both genetic and environmental influences. There is evidence for at least one major gene, which may be detectable by parametric linkage analysis, but detection of other quantitative trait loci may require nonparametric methods. Phenotype definition may also be critical in identifying genes that affect different components of the reading process. Current research from two separate laboratories supports the localization of one gene influencing reading disability to the histocompatibility region of chromosome 6p and suggests that another gene may be located on chromosome 15. Actual identification of these and similar genes is complicated by reduced penetrance and phenocopies in families, so that delineation of the critical region using crossover analysis is problematic. This difficulty can result in the identification of many candidate genes that need to be evaluated through combinations of association analysis, functional analysis, and mutation assays.

Novel mutations in the connexin 26 gene (GJB2) that cause autosomal recessive (DFNB1) hearing loss

Mutations in the connexin 26 (Cx26) gene (GJB2) are associated with the type of autosomal recessive nonsyndromic neurosensory deafness known as "DFNB1." Studies indicate that DFNB1 (13q11-12) causes 20% of all childhood deafness and may have a carrier rate as high as 2. 8%. This study describes the analysis of 58 multiplex families each having at least two affected children diagnosed with autosomal recessive nonsyndromic deafness. Twenty of the 58 families were observed to have mutations in both alleles of Cx26. Thirty-three of 116 chromosomes contained a 30delG allele, for a frequency of .284. This mutation was observed in 2 of 192 control chromosomes, for an estimated gene frequency of .01+/-.007. The homozygous frequency of the 30delG allele is then estimated at .0001, or 1/10,000. Given that the frequency of all childhood hearing impairment is 1/1,000 and that half of that is genetic, the specific mutation 30delG is responsible for 10% of all childhood hearing loss and for 20% of all childhood hereditary hearing loss. Six novel mutations were also observed in the affected population. The deletions detected cause frameshifts that would severely disrupt the protein structure. Three novel missense mutations, Val84Met, Val95Met, and Ser113Pro, were observed. The missense mutation 101T-->C has been reported to be a dominant allele of DFNA3, a dominant nonsyndromic hearing loss. Data further supporting the finding that this mutation does not cause dominant hearing loss are presented. This allele was found in a recessive family segregating independently from the hearing-loss phenotype and in 3 of 192 control chromosomes. These results indicate that 101T-->C is not sufficient to cause hearing loss.

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.

Molecular cloning and domain structure of human myosin-VIIa, the gene product defective in Usher syndrome 1B

Myosin-VIIa is an unconventional myosin with relatively restricted expression. Cloned first from an intestinal epithelium cell line, it occurs most notably in the testis, in the receptor cells of the inner ear, and in the pigment epithelium of the retina. Defects in myosin-VIIa cause the shaker-1 phenotype in mice and Usher syndrome 1B in human, which are characterized by deafness, lack of vestibular function, and (in human) progressive retinal degeneration. Because the described cDNAs encode less than half of the protein predicted from immunoblots, we have cloned cDNAs encoding the rest of human myosin-VIIa. Two transcripts were found, one encoding the predicted 250-kDa protein and another encoding a shorter form. Both transcripts were found in highest abundance in testis, although the shorter transcript was much less abundant. Both could be detected in lymphocytes by RT-PCR. The myosin tail encoded by the long transcript includes a long repeat of approximately 460 amino acids. Each repeat contains a novel "MyTH4" domain similar to domains in three other myosins, and a domain similar to the membrane-associated portion of talin and other members of the band-4.1 family.

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.

Differential induction of mRNAs for the glycolytic and ethanolic fermentative pathways by hypoxia and anoxia in maize seedlings

Fructose-1,6-biphosphate aldolase (ALD) and enolase (ENO) from the glycolytic pathway and pyruvate decarboxylase (PDC) and alcohol dehydrogenase 2 (ADH2) from the ethanolic fermentative pathway, are enzymes previously identified as among those synthesized selectively in O2-deficient roots of maize (Zea mays L.). The present study measured levels of transcripts representing these two pathways in 5-mm root tips, root axes (the remainder of the primary seminal root), and shoots of maize seedlings to determine how closely both pathways were co-induced and how they were modulated by changes in O2 concentration. In hypoxic seedlings with the roots in solution sparged with 5% (v/v) O2 (balance N2) and the shoots in the same gaseous atmosphere, mRNAs for Pdc1 and Adh2 in root tips both increased about 15-fold during the first 12 h, followed by a decline toward initial levels by 18 to 24h. Message levels for Ald1 and Eno1 showed only small changes during hypoxia. When expression was examined under anoxia, the extent to which all four mRNAs increased in different tissues depended on whether the seedlings had been previously acclimated to hypoxia or were anoxically shocked. The results show that although all the genes examined increased expression during hypoxia and/or anoxia, they differed in the rapidity and magnitude of the response and in the time to reach maximal message levels: there was no common pattern of change of message levels for the glycolytic or for the fermantative enzymes.

Concerted proton-electron transfer between ascorbic acid and cytochrome b561

Ascorbic acid is an essential reductant in biology but its reducing power is paradoxical. At physiological pH the predominant form of ascorbate (the monoanion) is a poor electron donor because it oxidizes to the energetically unfavorable neutral free radical. The ascorbate dianion forms the relatively stable semidehydroascorbate radical anion and is a powerful electron donor but its concentration at neutral pH is insufficient to produce the reaction rates observed. For example, ascorbate rapidly reduces cytochrome b561 from adrenal medullary chromaffin vesicles. This fast reaction rate may be rationalized by a mechanism involving concerted proton-electron transfer rather than electron transfer alone. This would permit reduction of the cytochrome by the abundant ascorbate monoanion but would circumvent formation of unfavorable intermediates. This may be a general mechanism of biological ascorbic acid utilization: enzymes using ascorbic acid may react with the ascorbate monoanion via concerted proton-electron transfer.

Vitamins C and E donate single hydrogen atoms in vivo

The antioxidant vitamins, C and E, eliminate cytotoxic free radicals by redox cycling. Energetic and kinetic considerations suggest that cycling of vitamin C and vitamin E between their reduced and free radical forms occurs via the transfer of single hydrogen atoms rather than via separate electron transfer and protonation reactions. This may enable these vitamins to reduce many of the damaging free radicals commonly encountered by biological systems while minimizing the reduction of molecular oxygen to superoxide.

Reaction of ascorbic acid with cytochrome b561. Concerted electron and proton transfer

Rate constants for reduction of cytochrome b561 by internal ascorbate (k0A) and oxidation by external ferricyanide (k1F) were determined as a function of pH from rates of steady-state electron transfer across chromaffin-vesicle membranes. The pH dependence of electron transfer from cytochrome b561 to ferricyanide (k1F) may be attributed to the pH dependence of the membrane surface potential. The rate constant for reduction by internal ascorbate (k0A), like the previously measured rate constant for reduction by external ascorbate (k-1A), is not very pH-dependent and is not consistent with reduction of cytochrome b561 by the ascorbate dianion. The rate at which ascorbate reduces cytochrome b561 is orders of magnitude faster than the rate at which it reduces cytochrome c, despite the fact that midpoint reduction potentials favor reduction of cytochrome c. Moreover, the rate constant for oxidation of cytochrome b561 by ferricyanide (k1F) is smaller than the previously measured rate constant for oxidation by semidehydroascorbate, despite the fact that ferricyanide has a higher midpoint reduction potential. These results may be reconciled by a mechanism in which electron transfer between cytochrome b561 and ascorbate/semidehydroascorbate is accelerated by concerted transfer of a proton. This may be a general property of biologically significant electron transfer reactions of ascorbic acid.

Rate of electron transfer between cytochrome b561 and extravesicular ascorbic acid

Cytochrome b561 transfers electrons across secretory vesicle membranes in order to regenerate intravesicular ascorbic acid. To show that cytosolic ascorbic acid is kinetically competent to function as the external electron donor for this process, electron transfer rates between cytochrome b561 in adrenal medullary chromaffin vesicle membranes and external ascorbate/semidehydroascorbate were measured. The reduction of cytochrome b561 by external ascorbate may be measured by a stopped-flow method. The rate constant is 450 (+/- 190) M-1 s-1 at pH 7.0 and increases slightly with pH. The rate of oxidation of cytochrome b561 by external semidehydroascorbate may be deduced from rates of steady-state electron flow. The rate constant is 1.2 (+/- 0.5) x 10(6) M-1 s-1 at pH 7.0 and decreases strongly with pH. The ratio of the rate constants is consistent with the relative midpoint reduction potentials of cytochrome b561 and ascorbate/semidehydroascorbate. These results suggest that cytosolic ascorbate will reduce cytochrome b561 rapidly enough to keep the cytochrome in a mostly reduced state and maintain the necessary electron flux into vesicles. This supports the concept that cytochrome b561 shuttles electrons from cytosolic ascorbate to intravesicular semidehydroascorbate, thereby ensuring a constant source of reducing equivalents for intravesicular monooxygenases.

A kinetic analysis of electron transport across chromaffin vesicle membranes

Some types of secretory vesicles, such as the chromaffin vesicles of the adrenal medulla, have cytochrome b561 which is believed to mediate the transfer of electrons across the vesicle membrane. To characterize the kinetics of this process, we have examined the rate of electron transfer from ascorbate trapped within chromaffin vesicle ghosts to external ferricyanide. The rate of ferricyanide reduction saturates at high ferricyanide concentrations. The reciprocal of the rate is linearly related to the reciprocal of the ferricyanide concentration. The internal ascorbate concentration affects the y intercept of this double-reciprocal plot but not the slope. These observations and theoretical considerations indicate that the slope is associated with a rate constant k1 for the oxidation of cytochrome b561 by ferricyanide. The intercept is associated with a rate constant k0 for the reduction of cytochrome b561 by internal ascorbate. From k0 and standard reduction potentials, the rate constant k-0 for the reduction of internal semidehydroascorbate by cytochrome b561 can be calculated. Under conditions prevailing in vivo, this rate of semidehydroascorbate reduction appears to be much faster than the expected rate of semidehydroascorbate disproportionation. This supports the hypothesis that cytochrome b561 functions in vivo to reduce intravesicular semidehydroascorbate thereby maintaining intravesicular ascorbic acid.

Mechanism of ascorbic acid regeneration mediated by cytochrome b561

In summary, ascorbic acid serves as a one-electron donor for dopamine beta-hydroxylase in chromaffin vesicles and probably for peptide amidating monooxygenase in neurohypophyseal secretory vesicles. It appears that the semidehydroascorbate that is produced is reduced by cytochrome b561 to regenerate intravesicular ascorbate. Cytochrome b561, a transmembrane protein, is reduced in turn by an extravesicular electron donor, probably cytosolic ascorbic acid. It will be interesting to see whether other ascorbate-requiring enzymes in other organelles use a similar ascorbate-regenerating system to provide an intravesicular supply of reducing equivalents.

Differential induction of heat shock, SOS, and oxidation stress regulons and accumulation of nucleotides in Escherichia coli

Heat and various inhibitory chemicals were tested in Escherichia coli for the ability to cause accumulation of adenylylated nucleotides and to induce proteins of the heat shock (htpR-controlled), the oxidation stress (oxyR-controlled), and the SOS (lexA-controlled) regulons. Under the conditions used, heat and ethanol initiated solely a heat shock response, hydrogen peroxide and 6-amino-7-chloro-5,8-dioxoquinoline (ACDQ) induced primarily an oxidation stress response and secondarily an SOS response, nalidixic acid and puromycin induced primarily an SOS and secondarily a heat shock response, isoleucine restriction induced a poor heat shock response, and CdCl2 strongly induced all three stress responses. ACDQ, CdCl2, and H2O2 each stimulated the synthesis of approximately 35 proteins by factors of 5- to 50-fold, and the heat shock, oxidation stress, and SOS regulons constituted a minor fraction of the overall cellular response. The pattern of accumulation of adenylylated nucleotides during these treatments was inconsistent with a simple role for these nucleotides as alarmones sufficient for triggering the heat shock response, but was consistent with a role in the oxyR-mediated response.

The complete amino Acid sequence for the anaerobically induced aldolase from maize derived from cDNA clones

A cDNA library was synthesized from maize anaerobic root mRNA and screened with cDNA specific to the anaerobically induced Zea mays cytoplasmic aldolase. At least 1% of the cDNA of the library corresponded to maize cytoplasmic aldolase. The sequence of four overlapping cDNA clones encoded a protein of molecular weight 38,611 homologous to aldolase. These cDNAs were polymorphic at three bases and one of these cDNAs had a different, shorter 3'-untranslated region. No known eukaryotic poly(A) addition site was detected. The derived amino acid sequences of maize was compared to the sequence of aldolase of trypanosome, Drosophila, and two mammalian isozymes, A and B. Of these, maize cytoplasmic aldolase was found to have the highest homology (55%) with rabbit aldolase A.

Cytochrome b561 spectral changes associated with electron transfer in chromaffin-vesicle ghosts

The involvement of cytochrome b561, an integral membrane protein, in electron transfer across chromaffin-vesicle membranes is confirmed by changes in its redox state observed as changes in the absorption spectrum occurring during electron transfer. In ascorbate-loaded chromaffin-vesicle ghosts, cytochrome b561 is nearly completely reduced and exhibits an absorption maximum at 561 nm. When ferricyanide is added to a suspension of these ghosts, the cytochrome becomes oxidized as indicated by the disappearance of the 561 nm absorption. If a small amount of ferricyanide is added, it becomes completely reduced by electron transfer from intravesicular ascorbate. When this happens, cytochrome b561 returns to its reduced state. If an excess of ferricyanide is added, the intravesicular ascorbate becomes exhausted and the cytochrome b561 remains oxidized. The spectrum of these absorbance changes correlates with the difference spectrum (reduced-oxidized) of cytochrome b561. Cytochrome b561 becomes transiently oxidized when ascorbate oxidase is added to a suspension of ascorbate-loaded ghosts. Since dehydroascorbate does not oxidize cytochrome b561, it is likely that oxidation is caused by semidehydroascorbate generated by ascorbate oxidase acting on free ascorbate. This suggests that cytochrome b561 can reduce semidehydroascorbate and supports the hypothesis that the function of cytochrome b561 in vivo is to transfer electrons into chromaffin vesicles to reduce internal semidehydroascorbate to ascorbate.

Coordinate induction of alcohol dehydrogenase 1, aldolase, and other anaerobic RNAs in maize

Anaerobiosis results in the selective synthesis of a particular set of polypeptides in the maize root including the two alcohol dehydrogenases (Sachs, M. M., Freeling, M., and Okimoto, R. (1980) Cell 20, 761-768), pyruvate decarboxylase (Wignarajah, K., and Greenway, H. (1976) New Phytol. 77, 575-584; Laszlo, A., and St. Lawrence, P. (1983) Mol. Gen. Genet. 192, 110-117), glucose phosphate isomerase (Kelley, P. M., and Freeling, M. (1984) J. Biol. Chem. 259, 673-677) and aldolase (Kelley, P. M., and Freeling, M. (1984) J. Biol. Chem. 259, 14180-14183). This report describes the identification and characterization of cDNA clones to five different mRNA species induced upon anaerobic shock. Immunoprecipitation of hybrid-selected translation polypeptides has determined the identity of the cDNA clone for fructose-1,6-diphosphate aldolase mRNA. Quantitative hybridization analysis of anaerobic mRNAs using the cDNA clones has shown that there is not a simultaneous accumulation of anaerobic mRNAs. Upon reintroduction of air, the anaerobic mRNAs disappear rapidly and at approximately the same rate. A translocation line that generates progeny that contain 1, 2, and 3 doses of the long arm of chromosome one (1L) allowed us to test for clustering of the anaerobic genes; two of the anaerobic genes tested do not reside with Adh 1 and Phi 1 on the long arm of chromosome 1.

Anaerobic expression of maize fructose-1,6-diphosphate aldolase

The anaerobic proteins of maize are a set of 10 major and 10 minor polypeptides selectively synthesized in anaerobic seedling roots. 1) Anaerobiosis resulted in the selected labeling of a protein which bound to Blue Sepharose and was eluted by fructose 1,6-diphosphate. 2) This protein elicited antiserum which recognized a single protein with molecular weight of approximately 40,000. 3) By Western blot analysis, this antiserum recognized a maize fructose-1,6-diphosphate aldolase purified to homogeneity. We show that two major anaerobic proteins of maize, ANP35.5 and ANP33A, correspond to a cytoplasmic fructose-1,6-diphosphate aldolase.