Arti Pandya

A novel deletion involving the connexin-30 gene, del(GJB6-d13s1854), found in trans with mutations in the GJB2 gene (connexin-26) in subjects with DFNB1 non-syndromic hearing impairment

Hearing impairment is a common and highly heterogeneous sensory disorder. Genetic causes are thought to be responsible for more than 60% of the cases in developed countries.1 In the majority of cases, non-syndromic hearing impairment is inherited in an autosomal recessive pattern.2 Thirty eight different loci and 20 genes for autosomal recessive non-syndromic hearing impairment (ARNSHI) have been identified to date.3 In many populations, up to 50% of all cases of ARNSHI are caused by mutations in the DFNB1 locus (MIM 220290) on 13q12.4 This locus contains the GJB2 gene (MIM 121011), encoding connexin-26 (Cx26),5 which belongs to a family of transmembrane proteins with about 20 members in humans. Hexamers of connexins (connexons) are displayed in the plasma membrane. Docking of connexons on the surfaces of two adjacent cells results in the formation of intercellular gap junction channels.6 Several different connexins, including Cx26, have been shown to participate in the complex gap junction networks of the cochlea.7,8 It has been postulated that these networks play a key role in potassium homeostasis, which is essential for the sound transduction mechanism.9 Given the high prevalence of DFNB1 deafness, molecular testing for GJB2 mutations has become the standard of care for the diagnosis of patients with non-syndromic hearing impairment of unknown cause.10 However, the finding of a large number of affected subjects with only one GJB2 mutant allele complicates the molecular diagnosis of DFNB1 deafness. In different studies, these have accounted for 10–50% of deaf subjects with GJB2 mutations.4 It was hypothesised that there could be other mutations in the DFNB1 locus but outside the GJB2 gene. This hypothesis gained support by the finding of a deletion in the DFNB1 locus outside GJB2 but truncating the neighbouring GJB6 gene (MIM 604418), which …

Origins and frequencies of SLC26A4 (PDS) mutations in east and south Asians: global implications for the epidemiology of deafness

Recessive mutations of SLC26A4 (PDS) are a common cause of Pendred syndrome and non-syndromic deafness in western populations. Although south and east Asia contain nearly one half of the global population, the origins and frequencies of SLC26A4 mutations in these regions are unknown. We PCR amplified and sequenced seven exons of SLC26A4 to detect selected mutations in 274 deaf probands from Korea, China, and Mongolia. A total of nine different mutations of SLC26A4 were detected among 15 (5.5%) of the 274 probands. Five mutations were novel and the other four had seldom, if ever, been identified outside east Asia. To identify mutations in south Asians, 212 Pakistani and 106 Indian families with three or more affected offspring of consanguineous matings were analysed for cosegregation of recessive deafness with short tandem repeat markers linked to SLC26A4. All 21 SLC26A4 exons were PCR amplified and sequenced in families segregating SLC26A4 linked deafness. Eleven mutant alleles of SLC26A4 were identified among 17 (5.4%) of the 318 families, and all 11 alleles were novel. SLC26A4 linked haplotypes on chromosomes with recurrent mutations were consistent with founder effects. Our observation of a diverse allelic series unique to each ethnic group indicates that mutational events at SLC26A4 are common and account for approximately 5% of recessive deafness in south Asians and other populations.

A Homozygous Mutation in a Novel Zinc-Finger Protein, ERIS, Is Responsible for Wolfram Syndrome 2

A single missense mutation was identified in a novel, highly conserved zinc-finger gene, ZCD2, in three consanguineous families of Jordanian descent with Wolfram syndrome (WFS). It had been shown that these families did not have mutations in the WFS1 gene (WFS1) but were mapped to the WFS2 locus at 4q22-25. A G-->C transversion at nucleotide 109 predicts an amino acid change from glutamic acid to glutamine (E37Q). Although the amino acid is conserved and the mutation is nonsynonymous, the pathogenesis for the disorder is because the mutation also causes aberrant splicing. The mutation was found to disrupt messenger RNA splicing by eliminating exon 2, and it results in the introduction of a premature stop codon. Mutations in WFS1 have also been found to cause low-frequency nonsyndromic hearing loss, progressive hearing loss, and isolated optic atrophy associated with hearing loss. Screening of 377 probands with hearing loss did not identify mutations in the WFS2 gene. The WFS1-encoded protein, Wolframin, is known to localize to the endoplasmic reticulum and plays a role in calcium homeostasis. The ZCD2-encoded protein, ERIS (endoplasmic reticulum intermembrane small protein), is also shown to localize to the endoplasmic reticulum but does not interact directly with Wolframin. Lymphoblastoid cells from affected individuals show a significantly greater rise in intracellular calcium when stimulated with thapsigargin, compared with controls, although no difference was observed in resting concentrations of intracellular calcium.

Frequency and distribution of GJB2 (connexin 26) and GJB6 (connexin 30) mutations in a large North American repository of deaf probands.

Purpose: Profound hearing loss occurs with a frequency of 1 in 1000 live births, half of which is genetic in etiology. The past decade has witnessed rapid advances in determining the pathogenesis of both syndromic and nonsyndromic deafness. The most significant clinical finding to date has been the discovery that mutations of GJB2 at the DFNB1 locus are the major cause of profound prelingual deafness in many countries.1 More recently, GJB2 mutations have been shown to cause deafness when present with a deletion of the GJB6 gene. We report on the prevalence of GJB2 and GJB6 mutations in a large North American Repository of DNA from deaf probands and document the profound effects of familial ethnicity and parental mating types on the frequency of these mutations in the population. Methods: Deaf probands were ascertained through the Annual Survey of Deaf and Hard of Hearing Children and Youth, conducted at the Research Institute of Gallaudet University. Educational, etiologic, and audiologic information was collected after obtaining informed consent. DNA studies were performed for the GJB2 and GJB6 loci by sequencing and PCR methods. Results: GJB2 mutations accounted for 22.2% of deafness in the overall sample but differed significantly among Asians, African-Americans and Hispanics and for probands from deaf by deaf and deaf by hearing matings, as well as probands from simplex and multiplex sibships of hearing parents. In our sample, the overall incidence of GJB2/GJB6 deafness was 2.57%. Conclusion: GJB2 mutations account for a large proportion of deafness in the US, with certain mutations having a high ethnic predilection. Heterozygotes at the GJB2 locus should be screened for the GJB6 deletion as a cause of deafness. Molecular testing for GJB2 and GJB6 should be offered to all patients with nonsyndromic hearing loss.

Genetics Evaluation Guidelines for the Etiologic Diagnosis of Congenital Hearing Loss

The advent of hearing screening in newborns in many states has led to an increase in the use of genetic testing and related genetic services in the follow-up of infants with hearing loss. A significant proportion of those with congenital hearing loss have genetic etiologies underlying their hearing loss. To ensure that those identified with congenital hearing loss receive the genetic services appropriate to their conditions, the Maternal and Child Health Bureau of the Health Resources and Services Administration funded the American College of Medical Genetics to convene an expert panel to develop guidelines for the genetic evaluation of congential hearing loss. After a brief overview of the current knowledge of hearing loss, newborn screening, and newborn hearing screening, we provide an overview of genetic services and a guideline that describes how best to ensure that patients receive appropriate genetic services. The significant contribution of genetic factors to these conditions combined with the rapid evolution of knowledge about the genetics of these conditions overlaid with the inherently multidisciplinary nature of genetic services provides an example of a condition for which a well-integrated multidisciplinary approach to care is clearly needed.The advent of hearing screening in newborns in many states has led to an increase in the use of genetic testing and related genetic services in the follow-up of infants with hearing loss. A significant proportion of those with congenital hearing loss have genetic etiologies underlying their hearing loss. To ensure that those identified with congenital hearing loss receive the genetic services appropriate to their conditions, the Maternal and Child Health Bureau of the Health Resources and Services Administration funded the American College of Medical Genetics to convene an expert panel to develop guidelines for the genetic evaluation of congential hearing loss. After a brief overview of the current knowledge of hearing loss, newborn screening, and newborn hearing screening, we provide an overview of genetic services and a guideline that describes how best to ensure that patients receive appropriate genetic services. The significant contribution of genetic factors to these conditions combined with the rapid evolution of knowledge about the genetics of these conditions overlaid with the inherently multidisciplinary nature of genetic services provides an example of a condition for which a well-integrated multidisciplinary approach to care is clearly needed.

Mutation in the mitochondrial 12S rRNA gene in two families from Mongolia with matrilineal aminoglycoside ototoxicity.

Irreversible hearing loss is a catastrophic complication of treatment with aminoglycoside antibiotics such as streptomycin, gentamycin, and kanamycin. Many kindreds showing a matrilineal pattern of inheritance of this trait have been described in China where the widespread use of aminoglycoside antibiotics accounts for approximately 25% of profound deafness in some districts. Because of the characteristic inheritance pattern, mitochondrial DNA (mtDNA) mutations were postulated to be the cause of the deafness in these pedigrees. In 1993 it was shown that an A to G substitution at base pair 1555 of the mitochondrial 12S ribosomal RNA gene was the only mutation common to all the families with aminoglycoside ototoxicity. We ascertained three Mongolian pedigrees from the School for the Deaf and Blind in Ulaanbaatar, all of which contained multiple affected subjects with streptomycin induced deafness in a pattern consistent with matrilineal transmission. Amplified mtDNA, obtained from transformed lymphoblastoid cell lines using previously described primers, showed the A to G point mutation in the 12S rRNA gene in two of the three families by restriction analysis as well as direct sequencing. No other example of this substitution was found among 400 control samples from Mongolians with normal hearing. We have thus confirmed the clinical relevance of the 1555 A to G mitochondrial mutation in the 12S rRNA gene by identifying it in affected subjects with familial aminoglycoside ototoxicity in another ethnic group. In countries where aminoglycosides are widely used, genetic counselling and screening of high risk families before the use of these drugs could have a dramatic effect on the incidence of deafness.

Attitudes of deaf and hard of hearing subjects towards genetic testing and prenatal diagnosis of hearing loss

Hearing loss is an economically and socially important cause of human morbidity. It is estimated that at least 20% of the population develop clinically significant hearing loss at some time during their lives.1 Hereditary hearing loss occurs in approximately 1/2000 newborns.2 During the past five years, dramatic advances have been made in the mapping of more than 60 loci for non-syndromic deafness. Interestingly, quite contrary to the assumption that genetic deafness was caused by a large number of equally rare genes, it has been shown that in many populations mutations at the connexin 26 ( Cx26 ) locus can account for as much as 50-80% of recessive genetic deafness.3 The ability to diagnose specific forms of recessive deafness by molecular testing even in simplex families with only one affected child is rapidly becoming the standard of care for the management of such cases. However, the imminent availability of widespread testing in the deaf community has raised a number of ethical issues, some of which are unique to the genetics of deafness. For instance, some would consider prenatal diagnosis for hearing status unacceptable owing to the unique perspective on hearing loss among different groups. Differences in the way people view the field of genetics and genetic testing for hearing loss are quite likely related to cultural differences in attitudes about hearing loss or deafness. The medical community tends to view deafness as a disability and a condition to be cured. Hearing subjects tend to share this “pathological” perspective on deafness. While most subjects with hearing loss become well integrated into the hearing world, many deaf people regard deafness and manual communication as distinctive features that define the separate, closely knit culture of the Deaf community (denoted with a capital D).4 Sociologists, linguists, and anthropologists now recognise Deaf people as …

Connective Tissue and Related Disorders and Preterm Birth: Clues to Genes Contributing to Prematurity

To identify candidate genes contributing to preterm birth, we examined the existing literature on the association between known disorders of connective tissue synthesis and metabolism and related diseases and prematurity. Our hypothesis was that abnormal matrix metabolism contributes to prematurity by increasing risk of preterm premature rupture of membranes (PPROM) and cervical incompetence. Based on this review, we identified gene mutations inherited by the fetus that could predispose to preterm birth as a result of PPROM. The responsible genes include COL5A1, COL5A2, COL3A1, COL1A1, COL1A2, TNXB, PLOD1, ADAMTS2, CRTAP, LEPRE1 and ZMPSTE24. Marfan syndrome, caused by FBN1 mutations, and polymorphisms in the COL1A1 and TGFB1 genes have been associated with cervical incompetence. We speculate that an analysis of sequence variation at the loci noted above will reveal polymorphisms that may contribute to susceptibility to PPROM and cervical incompetence in the general population.

Relation between choice of partner and high frequency of connexin-26 deafness

Recessive mutations at the connexin-26 gene locus are now recognised as the cause of nearly half of all cases of genetic deafness in many populations. We suggest that this high frequency is only seen in populations with a long tradition of intermarriage among deaf people. Available data are consistent with the hypothesis that such marriages might well have contributed to the high frequency of connexin-26 deafness in the USA, and could represent a novel mechanism for maintaining specific genotypes at unexpectedly high frequencies.

Does universal newborn hearing screening identify all children with GJB2 (Connexin 26) deafness? Penetrance of GJB2 deafness.

Objective: Deafness is the most common neurosensory defect at birth, and GJB2 (connexin 26) mutations are the most frequent genetic cause of hearing loss in many populations. The hearing loss caused by GJB2 mutations is usually congenital in onset and moderate to profound in degree. Considerable phenotypic variation has been noted however, including two anecdotal cases of apparent non penetrance at birth. The objective of this study is to document nine additional children with two pathogenic GJB2 mutations who had non penetrance of hearing loss at birth. Design: Subjects were identified through a national repository which includes deaf probands ascertained primarily from the United States through the Annual Survey of Deaf and Hard of Hearing Children and Youth conducted at the Research Institute at Gallaudet University. The hearing of each of these children had been screened at birth using standard audiologic techniques. Parents were interviewed and available medical records were reviewed. Testing for GJB2 mutations was performed by PCR and sequencing of the entire coding exon in all nine individuals. Results: Using parent interviews and medical records, we documented that all nine children passed newborn audiologic hearing screening. The age at which the hearing loss was subsequently identified in these nine children ranged from 12–60 mo. Of these nine children, 3 were compound heterozygotes and six were homozygous for the 35delG mutation in the GJB2 gene. Conclusion: These nine cases demonstrate that current newborn hearing screening does not identify all infants with two GJB2 mutations. These cases suggest that the frequency of non penetrance at birth is approximately 3.8% or higher. It is important to consider connexin deafness in any child with recessive nonsyndromic hearing loss as well as simplex cases with no history of other affected family members even when the newborn hearing screening results were within the normal range.