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Mutations in GJB2, the gene encoding connexin-26 at the DFNB1 locus on 13q12, are found in as many as 50% of subjects with autosomal recessive, nonsyndromic prelingual hearing impairment. However, genetic diagnosis is complicated by the fact that 10%-50% of affected subjects with GJB2 mutations carry only one mutant allele. Recently, a deletion truncating the GJB6 gene (encoding connexin-30), near GJB2 on 13q12, was shown to be the accompanying mutation in approximately 50% of these deaf GJB2 heterozygotes in a cohort of Spanish patients, thus becoming second only to 35delG at GJB2 as the most frequent mutation causing prelingual hearing impairment in Spain. Here, we present data from a multicenter study in nine countries that shows that the deletion is present in most of the screened populations, with higher frequencies in France, Spain, and Israel, where the percentages of unexplained GJB2 heterozygotes fell to 16.0%-20.9% after screening for the del(GJB6-D13S1830) mutation. Our results also suggest that additional mutations remain to be identified, either in DFNB1 or in other unlinked genes involved in epistatic interactions with GJB2. Analysis of haplotypes associated with the deletion revealed a founder effect in Ashkenazi Jews and also suggested a common founder for countries in Western Europe. These results have important implications for the diagnosis and counseling of families with DFNB1 deafness.

Eleven affected members of a large German-American family segregating recessively inherited, congenital, non-syndromic sensorineural hearing loss (SNHL) were found to be homozygous for the common 35delG mutation of GJB2, the gene encoding the gap junction protein Connexin 26. Surprisingly, four additional family members with bilateral profound SNHL carried only a single 35delG mutation. Previously, we demonstrated reduced expression of both GJB2 and GJB6 mRNA from the allele carried in trans with that bearing the 35delG mutation in these four persons. Using array comparative genome hybridization (arrayCGH), we have now identified on this allele a deletion of 131.4 kb whose proximal breakpoint lies more than 100 kb upstream of the transcriptional start sites of GJB2 and GJB6. This deletion, del(chr13:19,837,968,698), segregates as a completely penetrant DFNB1 allele in this family. It is not present in 528 persons with SNHL and monoallelic mutation of GJB2 or GJB6, nor have we identified any other candidate pathogenic copy number variation by arrayCGH in a subset of 10 such persons. Characterization of distant GJB2/GJB6 cis-regulatory regions evidenced by this allele may be required to find the 'missing' DFNB1 mutations that are believed to exist.

Globoid cell leukodystrophy is an autosomal recessive inherited disease caused by deficiency of the lysosomal enzyme galactocerebrosidase (GALC). Although the severe, rapidly progressing infantile form is the most common, late‐onset forms have been described. We investigated the molecular basis of GALC deficiency in a patient with a late‐life mild form of globoid cell leukodystrophy who survived into the eighth decade. Since material suitable for mutation analysis was no longer available from the proband, her GALC genotype was reconstructed by analyzing this gene in her six obligate carrier offspring. One allele contained the mutation 809G>A (G270D) in the 1637C background, while the other allele contained three sequence variants: 1609G>A (G537R), 1873G>A (A625T), and 1650T>A (V550V) in the 1637T background. These mutations were confirmed in the proband's genomic DNA isolated from a sural nerve biopsy. Expression studies indicated that the G537R is a disease‐causing mutation, as it resulted in no GALC activity, either alone or together with the A625T. This A625T sequence variant did not affect the enzyme activity, at least when expressed in the 1637T background. The mild clinical phenotype was likely to be associated with the 809G>A, since residual GALC activity, about 17% of the control activity, was detected in the expression studies of this mutation. This mutation has been found in several other patients with late‐onset GLD. Hum Mutat 14:256–262, 1999. © 1999 Wiley‐Liss, Inc.

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