Prevalence and nature of connexin 26 mutations in children with non-syndromic deafness

OBJECTIVE

To determine (1) the prevalence and nature of connexin 26 mutations in a cohort of Australian children with non-syndromic hearing loss, and (2) the carrier frequency of the common connexin 26 mutation (35delG) in the general population.

DESIGN

A cohort, case-finding study. Mutation analysis was performed on DNA extracted from white blood cells, buccal cells, or Guthrie blood spots.

SETTING

A hearing loss investigation clinic and a deafness centre in two Australian capital cities, 1 January 1998 to 31 October 2000.

PARTICIPANTS

(1) 243 children (age range, 4 weeks to 16 years; median, 4 years), attending hearing loss clinics in Sydney and Melbourne; (2) 1000 blood samples obtained from anonymous Guthrie card blood spots collected in 1984 [corrected] by the Victorian Clinical Genetics Service as part of the newborn screening program.

MAIN OUTCOME MEASURES

(1) The prevalence and types of connexin 26 mutations in a cohort of children with prelingual deafness; (2) the carrier frequency of the common connexin 26 mutation, 35delG, in the general population.

RESULTS

Connexin 26 mutations were identified and characterised in 52 (21%) of the 243 children; 14 different mutations, including four previously unreported mutations (135S, C53R, T123N and R127C), were identified. The common 35delG mutation was found in 56 of the 104 alleles (ie, 86 of the connexin 26 alleles in which a mutation was positively identified). The mutations V371 and M34T were also relatively common. The carrier frequency of connexin 26 mutations and of the common 35delG connexin 26 mutation in the Victorian population was estimated to be 1 in 54 and 1 in 100, respectively.

CONCLUSIONS

Mutations in the connexin 26 gene (especially the 35delG mutation) are a common cause of prelingual hearing loss in Australia.

Genetic analysis of the connexin-26 M34T variant: identification of genotype M34T/M34T segregating with mild-moderate non-syndromic sensorineural hearing loss

Mutations in the human gap junction beta-2 gene (GJB2) that encodes connexin-26 have been shown to cause non-syndromic sensorineural hearing loss (NSSNHL) at the DFNB1 locus on 13q11. Functional and genetic data regarding the disease causing potential of one particular GJB2 sequence variant, 101 T-->C (M34T), have proven contradictory. In this study, we found the prevalence of the M34T allele in a cohort of white sib pairs and sporadic cases with NSSNHL from the United Kingdom and Ireland to be 3.179% of chromosomes screened. Significantly, we identified the first M34T/M34T genotype cosegregating in a single family with mid to high frequency NSSNHL. Screening a control population of 630 subjects we identified 25 M34T heterozygotes; however, no M34T homozygotes were detected. Surprisingly, the majority of M34T alleles (88%) were in cis with a 10 bp deletion in the 5' non-coding sequence. This non-coding deletion was also homozygous in the homozygous M34T subjects. Microsatellite analysis of flanking loci in M34T heterozygotes and controls does not define an extensive ancestral haplotype but preliminary data suggest two common alleles in subjects with the M34T allele. In summary, we provide data that support M34T acting as a recessive GJB2 allele associated with mild-moderate prelingual hearing impairment.

High frequency hearing loss correlated with mutations in the GJB2 gene

Genetic hearing impairment affects approximately 1/2000 live births. Mutations in one gene, GJB2, coding for connexin 26 cause 10%-20% of all genetic sensorineural hearing loss. Mutation analysis in the GJB2 gene and audiology were performed on 106 families presenting with at least one child with congenital hearing loss. The families were recruited from a hospital-based multidisciplinary clinic, which functions to investigate the aetiology of sensorineural hearing loss in children and which serves an ethnically diverse population. In 74 families (80 children), the aetiology was consistent with non-syndromic recessive hearing loss. Six different connexin 26 mutations, including one novel mutation, were identified. We show that GJB2 mutations cause a range of phenotypes from mild to profound hearing impairment and that loss of hearing in the high frequency range (4000-8000 Hz) is a characteristic feature in children with molecularly diagnosed connexin 26 hearing impairment. We also demonstrate that this type of audiology and high frequency hearing loss is found in a similar-sized group of deaf children in whom a mutation could only be found in one of the connexin 26 alleles, suggesting connexin 26 involvement in the aetiology of hearing loss in these cases. In our study of the M34T mutation, only compound heterozygotes exhibited hearing loss, suggesting autosomal recessive inheritance.

A simple PCR test to detect the common 35delG mutation in the connexin 26 gene

BACKGROUND

The most common form of nonsyndromic neurosensory autosomal recessive deafness, DFNB1, is caused by mutations in the connexin 26 gene (GJB2) on chromosome 13. One mutation, in which one guanosine (G) residue is deleted from a run of 6 Gs (35delG), is found in 40% to 70% of DFNB1 cases and has an expected population frequency of one in 40 to one in 100.

METHODS AND RESULTS

Polymerase chain reaction (PCR)-based tests for the 35delG mutation were developed. They are based on mismatched PCR primers that produce novel EcoRII or DdeI restriction enzyme sites depending on the number of Gs at the 35delG locus. An EcoRII site is generated in the wild-type sequence (6 Gs), but not when the 35delG mutation is present. Alternatively, a DdeI site can be generated so that this enzyme cuts the PCR product when the 35delG mutation is present, but not the wild-type sequence.

CONCLUSIONS

These tests enable a quick and reliable screen for the common 35delG mutation.

Late diagnosis of maternal PKU in a family segregating an arylsulfatase [corrected] E mutation causing symmetrical chondrodysplasia punctata

Mutations in the arylsulfatase E gene, located on the X chromosome, have been shown to cause chondrodysplasia punctata (CDP). A substitution of arginine with serine at amino acid 12 (R12S) was identified in a patient with typical features of mild symmetrical CDP including mild mental retardation. The proband was institutionalized and was found to have seven full and half siblings all of whom were microcephalic. Six siblings are alive and all are mentally retarded. The mother is borderline retarded. The mother and three daughters are carriers of the R12S change, but do not appear to have CDP. A son and three other daughters do not carry the R12S change. Further studies revealed that the mother had phenylketonuria (PKU) and the children maternal PKU. This suggests that the R12S change is not the primary cause of short stature, microcephaly, and mental retardation in this family. The relationship between CDP and PKU, both of which can cause short statue and mental retardation, is discussed.

Connexin26 deafness in several interconnected families

Mutations in the connexin26 gene are the basis of much autosomal recessive sensorineural deafness. There is a high frequency of mutant alleles, largely accounted for by one common mutation, 35delG. We have studied a group of families, who had been brought together through marriages between Deaf persons, in which there are more than 30 Deaf people in four generations. We show that many of the several cases of deafness are the result of 35delG homozygosity or 35delG/Q57X compound heterozygosity at the connexin26 locus. A considerable range of audiographic phenotypes was observed. The combined effects of a high population frequency of mutant alleles, and of positive assortative marriage among the Deaf, led to an infrequently observed recessive pedigree pattern.

Segregation of mutations in arylsulphatase E and correlation with the clinical presentation of chondrodysplasia punctata

Sixteen males and two females with symmetrical (mild) type of chondrodysplasia punctata were tested for mutations in the X chromosome located arylsulphatase D and E genes. We identified one nonsense and two missense mutations in the arylsulphatase E gene in three males. No mutations were detected in the arylsulphatase D gene. Family studies showed segregation of the mutant genes establishing X linked inheritance for these families. Asymptomatic females and males were found in these studies. The clinical presentation varies not only between unrelated affected males, but also between affected males within the same family. We also conclude that clinical diagnosis of chondrodysplasia punctata in adults can be difficult. Finally, our results indicate that brachytelephalangy is not necessarily a feature of X linked symmetrical chondrodysplasia punctata.

Prelingual deafness: high prevalence of a 30delG mutation in the connexin 26 gene

Prelingual non-syndromic (isolated) deafness is the most frequent hereditary sensory defect. In >80% of the cases, the mode of transmission is autosomal recessive. To date, 14 loci have been identified for the recessive forms (DFNB loci). For two of them, DFNB1 and DFNB2, the genes responsible have been characterized; they encode connexin 26 and myosin VIIA, respectively. In order to evaluate the extent to which the connexin 26 gene (Cx26) contributes to prelingual deafness, we searched for mutations in this gene in 65 affected Caucasian families originating from various countries, mainly tunisia, France, New Zealand and the UK. Six of these families are consanguineous, and deafness was shown to be linked to the DFNB1 locus, 10 are small non consanguineous families in which the segregation of the trait has been found to be compatible with the involvement of DFNB1, and in the remaining 49 families no linkage analysis has been performed. A total of 62 mutant alleles in 39 families were identified. Therefore, mutations in Cx26 represent a major cause of recessively inherited prelingual deafness since according to the present results they would underlie approximately half of the cases. In addition, one specific mutation, 30delG, accounts for the majority (approximately 70%) of the Cx26 mutant alleles. It is therefore one of the most frequent disease mutations so far identified. Several lines of evidence indicate that the high prevalence of the 30delG mutation arises from a mutation hot spot rather than from a founder effect. Genetic counseling for prelingual deafness has been so far considerably impaired by the difficulty in distinguishing genetic and non genetic deafness in families presenting with a single deaf child. Based on the results presented here, the development of a simple molecular test could be designed which should be of considerable help.

The Leishmania promastigote surface antigen 2 complex is differentially expressed during the parasite life cycle

The promastigote surface antigen 2 (PSA-2) complex comprises a family of antigenically similar polypeptides of M(r) 96,000, 80,000 and 50,000, anchored to the membrane with glycosylphosphatidylinositol. Although PSA-2 was initially detected only in promastigotes, Northern blot analysis indicated that mRNA transcripts are also present in amastigotes. Unlike the situation in promastigotes, where at least four major transcripts (2.6-5.3 kb) were detected, only one major (2.6 kb) and two minor transcripts were present in amastigotes. A cDNA clone encoding a member of the PSA-2 family expressed in amastigotes was isolated using DNA probes. The predicted protein sequence of M(r) 40,000 is distinct from promastigote sequences, but shows significant similarity to previously described members of the family from L major and L amazonensis. Antibodies to the carboxyl terminal sequence conserved in all L major PSA-2 studied to date, as well as antibodies affinity purified on the amastigote cDNA-derived polypeptide recognized a major M(r) 50,000 amastigote polypeptide. Immuno-electron microscopy localized both promastigote and amastigote PSA-2 to the cell surface. The expression of PSA-2 polypeptides during the transformation of amastigotes into promastigotes was ordered in a time-dependent manner, with the promastigote M(r) 80000 polypeptide appearing first, followed by the M(r) 96000 polypeptide. In contrast to the glycosylphosphatidylinositol anchor of promastigote PSA-2, which could be hydrolysed by phosphatidylinositol-specific phospholipase C, the amastigote form was resistant to this enzyme.

Lipophosphoglycan blocks attachment of Leishmania major amastigotes to macrophages

Promastigotes of the intracellular protozoan parasite Leishmania major invade mononuclear phagocytes by a direct interaction between the cell surface lipophosphoglycan found on all Leishmania species and macrophage receptors. This interaction is mediated by phosphoglycan repeats containing oligomers of beta (1-3)Gal residues specific to L. major. We show here that although amastigotes also use lipophosphoglycan to bind to both primary macrophages and a cell line, this interaction is independent of the beta (1-3)Gal residues employed by promastigotes. Binding of amastigotes to macrophages could be blocked by intact lipophosphoglycan from L. major amastigotes as well as by lipophosphoglycan from promastigotes of several other Leishmania species, suggesting involvement of a conserved domain. Binding of amastigotes to macrophages could be blocked significantly by the monoclonal antibody WIC 108.3, directed to the lipophosphoglycan backbone. The glycan core of lipophosphoglycan could also inhibit attachment of amastigotes, but to a considerably lesser extent. The glycan core structure is also present in the type 2 glycoinositolphospholipids which are expressed on the surface of amastigotes at 100-fold-higher levels than lipophosphoglycan. However, their inhibitory effect could not be increased even when they were used at a 300-fold-higher concentration than lipophosphoglycan, indicating that lipophosphoglycan is the major macrophage-binding molecule on amastigotes of L. major. In the presence of complement, the attachment of amastigotes to macrophages was not altered, suggesting that lipophosphoglycan interacts directly with macrophage receptors.

Characterization of a polymorphic family of integral membrane proteins in promastigotes of different Leishmania species

Antibodies raised against a Leishmania major recombinant promastigote surface antigen 2 (PSA-2) fragment recognized three major polypeptides of approximate M(r) 96,000, 80,000 and 50,000 in promastigotes of three Israeli isolates of L. major including the cloned line LRC-L137-V121, but detected a different array of polypeptides in other L. major isolates. The pattern was different both in number of polypeptides detected and their molecular weight. The antibodies to L. major PSA-2 also recognized polypeptides in L. tropica, L. donovani and very weakly in L. mexicana promastigotes and in Crithidia lucilliae. The number and size of the polypeptides was different in each species. In addition to the membrane-bound PSA-2 polypeptides we identified water-soluble forms of PSA-2 released in promastigote culture supernatants. Peptide maps of the various L. major PSA-2 membrane polypeptides showed they were different from each other. N-terminal amino acid sequence of the three polypeptides expressed by L. major showed they are similar but distinct, consistent with being members of a polymorphic family. Because of the extensive sequence similarity between the PSA-2 genes it has been difficult to assign protein products to individual genes. As a first step towards solving this problem, we have transfected into L. mexicana a genomic clone of a L. major PSA-2 gene and shown that it produces a M(r) 35,000 polypeptide recognized by monoclonal and polyclonal antibodies to L. major PSA-2.

Ricin-resistant mutants of Leishmania major which express modified lipophosphoglycan remain infective for mice

Glycosylation variants of the virulent Leishmania major clone V121 were generated by mutagenesis with N-methyl-N-nitroso-N-nitroguanidine and selected using the galactose-specific lectin Ricinus communis II (RCA II). Three mutants, 4B9, 1D1 and 1C12, which failed to bind RCA II, were found to have an altered expression of lipophosphoglycan (LPG), a molecule implicated in the attachment to host macrophages and survival within the phagolysosome. There were differences in the antigenicity, molecular weight and localization of LPG from mutant parasites as compared to V121. Expression of gp63, a surface molecule also implicated in attachment to macrophages, was unaltered. All 3 mutants caused disease when injected into genetically susceptible BALB/c mice but lesions developed at a much slower rate than those caused by the virulent V121 clone. This slow rate of lesion development did not correlate with promastigotes' ability to invade macrophages in vitro. Karyotype analysis showed that there was a reduction in the size of chromosome band number 2 in all 3 mutants. The differences in LPG and chromosome band 2 were retained by mutant clones following passage through mice, suggesting that these phenotypes are stable. Although the mutant parasites were infective and caused lesions, the changed structure of the LPG appeared to influence the virulence of the parasites.

Anti-galactosyl antibodies that react with unmodified agarose: a potential source of artifacts in immunoaffinity chromatography

Rabbits were immunized with glycolipid and membrane glycoprotein extracts of the promastigotes of Leishmania major partially purified by two-phase extraction with Triton X-114. The resulting antibodies were affinity purified on agarose immunoadsorbents to which a recombinant DNA-produced L. major polypeptide had been coupled. In addition to the expected reaction with the polypeptide, it was found that the affinity-purified antibodies reacted strongly with lipophosphoglycan of L. major amastigotes and to a lesser extent with the lipophosphoglycan of promastigotes. Antibodies with this reactivity could be affinity purified on unmodified agarose and were probably directed against beta-1,3-galactosyl determinants shared between the agarose matrix and Leishmania. In this and other special situations the properties of the support matrix can be exploited to obtain antibodies of defined specificity which may be useful probes for the identification and characterization of carbohydrate structures. This work also points to a potential source of artifacts in affinity chromatography, in which the bead matrix is usually considered as inert. Carbohydrate antigens are ubiquitous, particularly in micro-organisms, and the presence of such antibodies may be more common than previously recognized.

Identification, characterisation and genomic cloning of a O-linked N-acetylglucosamine-containing cytoplasmic Leishmania glycoprotein

Antibodies against Leishmania major wheat germ agglutinin-binding glycoproteins were used to select from a genomic lambda gt11 expression library a clone coding for a L. major glycoprotein. The partial DNA sequence indicated the presence of a mosaic of repetitive sequences. Southern blot hybridisation on genomic DNA using the cloned gene as a probe at high stringency suggested a single gene, which was localised to chromosome band 18. Northern blot analysis of L. major mRNA detected a major transcript of 7.5 kb and a minor 4.0-kb transcript. Antibodies affinity-purified on the fusion protein identified a complex of two water-soluble cytoplasmic polypeptides of approximately 96 kDa and 92 kDa in L. major promastigotes and amastigotes. They also recognised polypeptides in other Leishmania species, in Crithidia lucilliae and very weakly in Leptomonas. The apparent molecular weight of these polypeptides, while conserved within each species, varied between species. A peptide map of the two polypeptides from L. major generated an identical pattern suggesting a close relatedness at the protein level. This protein complex was not hydrolysed by N-glycanase and was not affected by tunicamycin, but treatment with anhydrous hydrogen fluoride suggested that it is O-glycosylated. The glycan moiety appears to be N-acetylglucosamine, and N-acetylglucosamine beta-1,4-galactosyltransferase was capable of adding [3H]galactose to it. This was susceptible to beta elimination and beta-galactosidase treatment. Taken together, the data indicates that gp96/92 belongs to the newly described class of cytoplasmic and nuclear glycoproteins containing O-linked N-acetylglucosamine.

Cloning and characterization of a Golgi-associated GTP-binding protein homologue from Leishmania major

This paper describes the cloning of a Golgi-associated GTP-binding protein homologue from Leishmania major. The gene was isolated using degenerate oligonucleotides to conserved sequences amongst the small GTP-binding proteins in a polymerase chain reaction on genomic DNA of the L. major cloned line V121. The reading frame of one clone showed high similarity to the rab/YPT subfamily of small GTP-binding proteins. A full length copy of the gene was isolated from a lambda gt10 V121 genomic library and sequenced. At the amino acid level the gene showed highest similarity to the human/rat rab1 A gene and the mouse/yeast YPT gene and was named LmYPT. The LmYPT gene was present as a single copy gene in both the L. major and L. donovani genomes. Karyotype analysis localized the LmYPT gene to chromosome band 18 in V121, but it was located on a larger chromosome in the different L. major isolate L119. The LmYPT gene was transcribed as a 3.9-kb transcript in both the promastigote and amastigote forms of the parasite. Western blot analysis, using a polyclonal rabbit antiserum raised against an Escherichia coli expressed portion of the molecule, identified a doublet at 20 and 23 kDa in total promastigote protein. Immunoelectron microscopy in combination with peroxidase staining localized the LmYPT molecule to the Leishmania Golgi apparatus.