A Missense Mutation in the Human Connexin50 Gene (GJA8) Underlies Autosomal Dominant “Zonular Pulverulent” Cataract, on Chromosome 1q

Abstract

CZP1, a locus for autosomal dominant "zonular pulverulent" cataract, previously had been linked with the Duffy blood-group-antigen locus on chromosome 1q. Here we report genetic refinement of the CZP1 locus and show that the underlying mutation is present in GJA8, the gene for connexin50. To map the CZP1 locus we performed linkage analysis using microsatellite markers on two distantly related branches of the original Ev. pedigree, which now spans eight generations. Significantly positive two-point LOD score (Z) values were obtained for markers D1S2669 (maximum Z [Zmax] = 4.52; maximum recombination frequency [thetamax] = 0) and D1S514 (Zmax = 4.48; thetamax = 0). Multipoint analysis gave Zmax = 5.22 (thetamax = 0) at marker D1S2669. Haplotyping indicated that CZP1 probably lies in the genetic interval D1S2746-(20.6 cM)-D1S2771. Sequence analysis of the entire protein-coding region of the GJA8 gene from the pedigree detected a C-->T transition in codon 88, which introduced a novel MnlI restriction-enzyme site that also cosegregated with the cataract. This missense mutation is predicted to result in the nonconservative substitution of serine for a phylogenetically conserved proline (P88S). These studies provide the first direct evidence that GJA8 plays a vital role in the maintenance of human lens transparency and identify the genetic defect believed to underlie the first inherited disease to be linked to a human autosome.

Functional Role of the Carboxyl Terminal Domain of Human Connexin 50 in Gap Junctional Channels

“…Changes in pH to a value of 6.8 would not have a major impact upon gap junctional intercellular communication between fiber cells, since wild-type human Cx50 channels remain open at that pH. Thus, our data suggest that changes in intracellular pH within the lens can have enormous effects for the survival of fiber cells and lens homeostasis and transparency, since alterations in gap junctional intercellular communication can lead to the formation of cataracts (White et al, 1998;Gong et al, 1997;Mathias, Rae & Baldo, 1997;Shiels et al, 1998;Pal et al, 1999). Moreover, because lens fiber cells also contain Cx46, consequences for lens survival would be influenced by the pH dependence of Cx46 channels and heteromeric Cx50-Cx46 channels.…”

“…Moreover, because lens fiber cells also contain Cx46, consequences for lens survival would be influenced by the pH dependence of Cx46 channels and heteromeric Cx50-Cx46 channels. (A) Schematic drawing of the membrane topology of wild-type human Cx50 and two truncation mutants (top) and the aligned sequences of the regions of ovine and human Cx50 flanking the truncation sites (bottom) (based on Yang & Louis, 1996;Lin et al, 1997;Shiels et al, 1998 Transitions between different states of human Cx50 wild-type (A) and Cx50A294stop (B) gap junctional channels. A single gap junctional channel recorded from a cell pair expressing wildtype Cx50 at V j of +100 mV is shown in A and the circled region is shown below on an expanded time scale.…”

“…Wild-type human Cx50 (Shiels et al, 1998;Pal et al, 1999) was subcloned into the EcoRI site of the eukaryotic expression vector pSFFV-neo (Fuhlbrigge et al, 1988). Carboxyl terminal deletion mutants of Cx50 were obtained by PCR using a sense primer, 5′-ggaattcaccatgggcgactgg-3′, and two antisense primers, 5′-ccggaattctcactcggtcaagggg-3′ and 5′-ccggaattctcaaggcagtgggctgg-3′ to generate Cx50 polypeptides that ended at valine-284 (Cx50V284stop) and alanine-294 (Cx50A294stop), respectively (Fig.…”

“…Previous studies have shown that disruption of either Cx46 or Cx50 leads to cataract formation (Gong et al, 1997;White et al, 1998). Furthermore, certain types of congenital cataracts in humans and mice are associated with mutations in the genes for these connexins Berry et al, 1999;Shiels et al, 1998;Steele et al, 1998). However, the specific roles of these two connexins in the lens are still not completely understood.…”

Regulation of Intermuscular Electrical Coupling by theCaenorhabditis elegansInnexininx-6

“…Several human diseases have been linked to connexin polymorphisms (Bergoffen et al, 1993;Kelsell et al, 1997;Shiels et al, 1998) and directed mutations in mice have revealed diverse roles for connexins in transplacental nutrient transfer (Gabriel et al, 1998), ovulation (Simon et al, 1997), and cardiac development and function (Cx40, Cx43, and Cx45) (Kanter et al, 1994;Reaume et al, 1995;Ewart et al, 1997;Simon et al, 1998).…”