GM23835

GM23835 LCL from B-Lymphocyte

Description:

DEAFNESS, AUTOSOMAL RECESSIVE 1A; DFNB1A
GAP JUNCTION PROTEIN, BETA-2; GJB2 (CONNEXIN 26; CX26)

Affected:

Yes

Sex:

Male

Age:

9 YR (At Sampling)

Overview

Repository

NIGMS Human Genetic Cell Repository

Subcollection

Heritable Diseases

Biopsy Source

Peripheral vein

Cell Type

B-Lymphocyte

Tissue Type

Blood

Transformant

Epstein-Barr Virus

Sample Source

LCL from B-Lymphocyte

Family History

Relation to Proband

proband

Confirmation

Molecular characterization before cell line submission to CCR

Species

Homo sapiens

Common Name

Human

Remarks

Hearing loss; failed newborn screen, but passed the repeat; Diagnosed with Attention Deficit Disorder in HeadStart program; mulitple normal hearing tests; speech delay diagnosed in kindergarten; failed hearing test with ENT; Mild to Moderate bilateral sensorineural hearing loss; used hearing aid; donor subject is a compound heterozygote: one allele has a 1 bp deletion of one guanine in a run of six guanines extending from nucleotide 30 to 35 in exon 1 of the GJB2 gene (35delG) causing a frameshift of the coding sequence leading to premature chain termination at the 12th amino acid; the second allele has a T>C transition in exon 1 resulting in a substitution of threonine for methionine at codon 34 [Met34Thr (M34T)]; Therapy includes speech language therapy.

Characterizations

IDENTIFICATION OF SPECIES OF ORIGIN

Species of Origin Confirmed by LINE assay

Gene

GJB2

Chromosomal Location

13q11-q12

Allelic Variant 1

121011.0005; DEAFNESS, AUTOSOMAL RECESSIVE, 1; DFNB1

Identified Mutation

1-BP DEL, 35G; A mutation consisting of deletion of 1 guanine (G) in a run of 6 guanines extending from position 30 to position 35 in the GJB2 gene has been observed by several groups. Some referred to the deleted nucleotide as 30G (the first of the 6 Gs), whereas others referred to it as 35G. The second mutation found by Carrasquillo et al. [Hum. Molec. Genet. 6: 2163-2172 (1997)] to be responsible for nonsyndromic recessive deafness (220290) in a Muslim-Israeli village in the lower Galilee was a deletion of a guanine residue at cDNA position 35 (35delG), causing a frameshift of the coding sequence leading to premature chain termination at the twelfth amino acid.

Gene

GJB2

Chromosomal Location

13q11-q12

Allelic Variant 2

121011.0001; GAP JUNCTION PROTEIN, BETA-2; GJB2

Identified Mutation

MET34THR; This variant, formerly titled DEAFNESS, AUTOSOMAL RECESSIVE 1A, has been reclassified based on the findings of Shearer et al. (2014). Based on allele frequency in 8,595 controls from 12 populations (maximum minor allele frequency = 0.0200), Shearer et al. (2014) recategorized the M34T variant in the GJB2 gene as benign. In a family in which both palmoplantar keratoderma and deafness (148350) were segregating as probably independent autosomal dominant traits (Verbov, 1987), Kelsell et al. (1997) identified a heterozygous T-to-C substitution in exon 1 of the GJB2 gene, resulting in a met34-to-thr (M34T) substitution. The M34T mutation appeared to segregate with profound deafness, but not with the skin disorder, suggesting to Kelsell et al. (1997) that the mutation acted in a dominant manner. However, Kelley et al. (1998) and Scott et al. (1998) observed normal hearing in M34T heterozygotes, suggesting that the variant does not function as a dominant GJB2 allele in vivo. Moreover, Kelley et al. (1998) identified the M34T allele in 3 of 192 control chromosomes, suggesting that it may be a polymorphism. By in vitro functional studies, White et al. (1998) observed a dominant-negative effect of the M34T mutant polypeptide on the intercellular coupling activity of the wildtype GJB2 polypeptide expressed in Xenopus oocytes. Griffith et al. (2000) presented evidence that M34T is a hypomorphic allele that is insufficient in itself to cause hearing loss, but may cause hearing loss when combined with another pathogenic GJB2 allele. They reported a family with severe autosomal recessive deafness (DFNB1A; 220290) associated with a homozygous mutation in the GJB2 gene (167delT; 121011.0010). One individual who was heterozygous for M34T had normal hearing, and another who was compound heterozygous for M34T and 167delT had only mild high frequency hearing loss. Houseman et al. (2001) found the prevalence of the M34T allele in a cohort of white sib pairs and sporadic cases with nonsyndromic sensorineural hearing loss from the United Kingdom and Ireland to be 3.179% of chromosomes screened. They found the homozygous M34T/M34T genotype cosegregating with mid to high frequency deafness. In a control population of 630 individuals, they identified 25 M34T heterozygotes but no M34T homozygotes. Eighty-eight percent of the M34T alleles were in cis with a 10-bp deletion in the 5-prime noncoding sequence. This deletion was homozygous in the M34T homozygotes. Houseman et al. (2001) concluded that M34T acts as a recessive allele. Kelsell et al. (2000) investigated the possible reason for normal hearing in M34T carriers from distinct ethnic populations. They stated that no M34T homozygotes had been reported among individuals with normal hearing. They extended their analysis of a small family in which palmoplantar keratoderma and various forms of deafness were segregating. In addition to the M34T sequence variant in GJB2, 2 other sequence variants were identified: D66H, also in GJB2 (121011.0012), and R32W in GJB3 (603324). As D66H segregated with the skin disease, Kelsell et al. (2000) thought it likely to underlie the palmoplantar keratoderma. The other 2 gap junction variants identified may contribute to the type of hearing impairment and the variable severity of the skin disease in the family. D'Andrea et al. (2002) showed that CX26 proteins carrying the M34T mutation were expressed at the cell surface and showed wildtype membrane distribution following transient transfection in HeLa cells, but they did not support dye transfer. The M34T mutant also acted as a dominant inhibitor of wildtype CX26 channel activity when the 2 proteins were coexpressed to mimic the heterozygous state. In contrast, Oshima et al. (2003) found that the M34T mutation supported dye transfer in HeLa cells at levels comparable to wildtype CX26, but a CX26 protein in which the authors introduced a met34-to-ala (M34A) mutation did not. In 11 French families with nonsyndromic sensorineural hearing loss (7 familial forms and 4 sporadic cases) in which the M34T variant had been identified, Feldmann et al. (2004) found that the mutation did not segregate with deafness in 6 of the 7 families. Of the family members with normal audiograms, 8 were heterozygous for M34T and 5 were compound heterozygous for M34T and another GJB2 mutation. A screening of 116 controls demonstrated an M34T allele frequency of 1.72%, which was not significantly different from the 2.12% frequency in the deaf population cited by Feldmann et al. (2004). Feldmann et al. (2004) suggested that the M34T variant is not clinically significant in humans and is a frequent polymorphism in France. In a study of 610 hearing-impaired individuals and 294 controls, Tang et al. (2006) found no significant difference in the M34T allele frequency between cases and controls, suggesting that the M34T variant is a polymorphism. Pollak et al. (2007) studied 233 Polish patients with hearing impairment and the GJB2 35delG mutation (121011.0005) on 1 allele. Analysis of 17 patients with the M34T/35delG and 12 patients with the V37I (121011.0023)/35delG genotypes, patients with other GJB2 mutations, and controls found that the M34T and V37I were significantly overrepresented among patients with hearing impairment, consistent with both variants being pathogenic. However, both mutations showed decreased penetrance of about 10% compared to mutations of undisputed pathogenicity. Also, patients with M34T/35delG and V37I/35delG had significantly later onset of hearing impairment compared to those with other genotypes. Pollak et al. (2007) suggested that the M34T and V37I mutations cause mild hearing impairment characterized by relatively late onset and progression.