Disease relevance of GJA3

• CONCLUSIONS: The present study has identified a fifth mutation in GJA3, rendering this connexin gene one of the most common non-crystallin genes associated with autosomal dominant cataracts in humans [1].
• We have found that ROS 17/2.8 osteosarcoma cells, UMR 106-01 osteosarcoma cells, and primary rat calvarial osteoblastic cells also express another gap junction protein, Cx46 [2].

High impact information on GJA3

• While Cx43 was assembled into multimeric complexes, ROS cells contained only the monomer form of Cx46 [2].
• Both rat lens and Hela/Cx46 cells expressed 53-kD (nonphosphorylated) and 68-kD (phosphorylated) forms of Cx46; however, only the 53-kD form was produced by osteoblasts [2].
• Cx46 is a major component of plasma membrane gap junctions in lens [2].
• Loci for autosomal dominant "zonular pulverulent" cataract have been mapped to chromosomes 1q (CZP1) and 13q (CZP3) [3].
• Sequencing of a genomic clone isolated from the CZP3 candidate region identified an open reading frame coding for a protein of 435 amino acids (47,435 D) that shared approximately 88% homology with rat Cx46 [3].

Biological context of GJA3

• A novel missense mutation in the gene for gap-junction protein alpha3 (GJA3) associated with autosomal dominant "nuclear punctate" cataracts linked to chromosome 13q [1].
• RESULTS: Significant evidence of linkage was obtained at marker D13S175 (LOD score [Z]=4.11, recombination fraction [theta]=0.0) and haplotyping indicated that the disease gene lay in the about 2 Mb physical interval between D13S1316 and D13S1236, which contained the gene for gap-junction protein a3 (GJA3) or connexin46 [1].
• Haplotype analysis indicated this disease was located at 13q11, close to GJA3 [4].
• METHODS: PCR based Single Stranded Conformational Polymorphism (SSCP) analysis was used to screen sixty probands with nonsyndromic congenital cataract for mutations in the Cx46 gene (GJA3), followed by direct sequencing of samples that showed an electrophoretic shift [5].
• We studied two hCx46 mutants, N63S, a missense mutation in the first extracellular domain, and fs380, a frame-shift mutation that shifts the translational reading frame at amino acid residue 380 [6].

Anatomical context of GJA3

• This study identifies GJA3 as the sixth member of the connexin gene family to be implicated in human disease, and it highlights the physiological importance of gap-junction communication in the development of a transparent eye lens [3].
• We expressed wild-type Cx46 and the two mutants in Xenopus oocytes [6].
• Moreover, the strength of junctional coupling among cultured Schwann cells is modulated by a number of cytokines to which Schwann cells are exposed to in vivo after nerve injury, and Cx46 mRNA and protein levels correlate with the degree of coupling [7].
• In normal rat lung sections, connexin (Cx)32 was expressed by type II cells, whereas Cx43 was more ubiquitously expressed and Cx46 was expressed by occasional alveolar epithelial cells [8].
• Minimal Cx37 and Cx46 immunoreactivity was detected between occasional atrial or ventricular myocytes [9].

Associations of GJA3 with chemical compounds

• Sequencing of GJA3 detected a C->T transition in exon 2 that resulted in the gain of an Alu 1 restriction site and was predicted to cause a conservative substitution of proline to leucine at codon 59 (P59L) [1].
• Sucrose is at the exclusion limit for Cx46 channels whereas sorbitol is at the exclusion limit for Cx32E(1)43 channels [10].
• Furthermore, a point mutant of Cx46, with leucine substituted by glycine in position 35, displayed a conductance much larger than that of the wild type [11].
• It is also the first reported cataract-causing mutation in the NH2-terminal region of the Cx46 protein [12].
• Both Ca(2+) and H(+) play a role in chemical gating of gap junction channels, but, with the possible exception of Cx46 hemichannels, neither of them is likely to induce gating by a direct interaction with connexins [13].

Other interactions of GJA3

• Individual mutations of Cx46, Cx50 and Cx43 have been found in cataract or heart malformations [14].
• Substitution of the first extracellular loop (E1) domain of Cx32, an anion-preferring Cx, reduces conductance, converts Cx46 from cation to anion preferring, and changes the I-V relation form inwardly to outwardly rectifying [15].
• Wild type connexin 46 of rat (wtrCx46), and human connexin 26 (wthCx26) and derivates from rCx46 elongated at the C-terminus by 25 amino acids (rCx46Ct) as well as C-terminal truncated constructs (rCx28.1, rCx45.3) were expressed in frog oocytes of Xenopus laevis [16].
• Gap junction structures and distribution patterns of immunoreactive connexin46 (Cx46) and connexin50 (Cx50) in normal lenses and lens regrowths of rhesus monkeys were studied using electron microscopy and immunofluorescence double-labeling [17].

Analytical, diagnostic and therapeutic context of GJA3

• Assignment of connexin 26 (GJB2) and 46 (GJA3) genes to human chromosome 13q11-->q12 and mouse chromosome 14D1-E1 by in situ hybridization [18].
• In contrast, Cx46 expressed by osteoblastic cells was predominantly localized to an intracellular perinuclear compartment, which appeared to be an aspect of the TGN as determined by immunofluorescence colocalization [2].
• In this study, we characterized the effects of external divalent cations and voltage on macroscopic human connexin46 (hCx46) hemi-gap-junctional currents using the two-electrode voltage-clamp technique [19].

References