Mutation of the gap junction protein alpha 8 (GJA8) gene causes autosomal recessive cataract

Abstract

GJA8 encodes connexin-50, a gap junction protein in the eye lens. Mutations in GJA8 have been reported in families with autosomal dominant cataract. The objective of this report was to identify the disease gene in a family with congenital cataract of autosomal recessive inheritance. Eight candidate genes were screened for pathogenic alterations in affected and unaffected family members and in normal unrelated controls. A single base insertion leading to frameshift at codon 203 of connexin 50 was found to co-segregate with disease in the family. These results confirm involvement of GJA8 in autosomal recessive cataract.

BACKGROUND

Cataract is defined as any opacity of the lens resulting in partial or total loss of transparency. Hereditary cataracts are clinically and genetically heterogeneous, often presenting as congenital or developmental cataracts that arise at birth or during the first decade of life, respectively. As these opacities can cause blurring of the vision during vision development, they are clinically very important. Cataracts may account for about one-tenth of total childhood blindness in Southern India¹ and hereditary cataracts account for about one-fifth of childhood cataracts in this region.² The majority of hereditary cataracts that have been genetically characterised to date are of autosomal dominant inheritance.³ Mutations in six genes (CRYAA,⁴ LIM2,⁵ GCNT2,⁶ HSF4,⁷ CRYBB3,⁸ and BFSP1⁹) have been associated with the autosomal recessive cataracts.

CASE PRESENTATION

The proband (IV: 1 in fig 1) presented at our institution at 12 years of age. She had a history of poor vision, white opacities in both eyes, and nystagmus since birth. She had undergone cataract surgery elsewhere, and had an unaided visual acuity in both eyes of counting fingers at about 1 metre. Her brother (IV: 2 in fig 1), who was similarly affected, had a history of decreased vision since birth and on examination, had total cataracts, nystagmus and a visual acuity in both eyes of counting fingers. The pedigree obtained upon examination of all available members of the family was suggestive of autosomal recessive inheritance (fig 1).

Figure 1.

Pedigree of family with autosomal recessive cataract. The dark symbols indicate affected individuals and open symbols indicate unaffected individuals. Symbols marked with asterisks represent individuals who were analysed.

INVESTIGATIONS

We employed a candidate gene approach consisting of SSCP based screening and sequencing. We screened eight genes including six crystallin genes and two connexin genes to identify mutations. Screening of the coding regions of GJA8 revealed a single base insertion causing a frameshift at codon 203 (c.670insA; p.Thr203AsnfsX47; shown in fig 2) that was homozygous in the two affected members IV:1 and IV:2 (fig 1), and heterozygous in the parents (III:4 and III:7 in fig 1), sibling (IV:3 in fig 1) and a second degree relative of the proband (II:9 in fig 1), all of whom were unaffected. This change was not found in 75 unrelated controls. The mutation is predicted to result in a frameshift at codon 203 with a stop codon after 46 amino acids of altered reading frame, producing a truncated protein consisting of 248 amino acid residues (fig 3).

Figure 2.

Sequence of the GJA8 coding region. The sequence of GJA8 showing the insertion (arrow) of an A at position 670 of the cDNA (c.670insA) (A) homozygous in the proband and (B) heterozygous in unaffected parent. Codons are marked by brackets and amino acids indicated above. Codons and amino acids for both wild type and mutant alleles are shown in (B).

Figure 3.

Sequences of wild type and mutant GJA8 proteins. Partial protein sequences of the wild type GJA8/Cx50 (wt) and predicted sequence of the insertion mutant (mut) (c.670insA, p.Thr203AsnfsX47) are shown. The residue (position 203) at the start of the frameshift is boxed. The mutant protein terminates at 248 amino acids. Residues are numbered with respect to the wild-type Cx50 sequence.

DIFFERENTIAL DIAGNOSIS

Autosomal recessive cataract due to CRYAA, LIM2, GCNT2, HSF4, CRYBB3, or BFSP1 mutation.

DISCUSSION

GJA8 encodes the gap junction protein connexin 50 (Cx50), which is one of the major lens connexins along with connexins 43 (locus GJA1) and 46 (locus GJA3). Connexins 50 and 46 are expressed in differentiating lens fibres and persist in mature fibres, and connexin 43 is expressed in lens epithelial cells.^10–14 Connexins form intercellular channels consisting of two halves or hemichannels, the connexons, each made up of six connexin monomers. Mutations in GJA3 and GJA8 are known to result in autosomal dominant cataract. Eight different mutations have been reported in the GJA8 gene (table 1), all of which are missense changes.

Table 1.

Cx50 (GJA8) mutations reported in human cataracts

Mutation	Phenotype
Arg23Thr	Progressive congenital nuclear
Val44Glu	Congenital or developmental cataract with microcornea
Glu48Lys	Zonular nuclear pulverulent
Pro88Ser	Zonular pulverulent
Pro88Gln	Lamellar pulverulent
Val79Leu	“Full moon” with Y-sutural opacity
Arg198Glu	Congenital or developmental cataract with microcornea
Ile247Met	Zonular pulverulent

The insertion described here is located in codon 203, which is predicted to be in the second extracellular domain of connexin 50; a frameshift at this position would be expected to lead to the disruption of the C-terminal half of the protein (amino acids 203–433) and thereby produce a functionally null allele. Possible consequences could be instability or non-functionality of the mutant protein, or degradation of the mRNA through the nonsense mediated decay pathway. A mechanism of disease involving loss of function at connexin loci has also been suggested in mouse models of recessive cataract. GJA3 or GJA8 homozygous knockout mice are reported to have a cataractous phenotype, whereas heterozygous knockout mice (GJA3^+/−, GJA8^+/−) have normal lenses.^15,16

Studies on various mutant connexin proteins causing dominant cataract in humans and mice have suggested varied mechanisms of action. Dominant negative effects have been proposed for the GJA8 mutant proteins Pro88Ser^17,18 and Pro88Gln¹⁹ based on studies in Xenopus oocytes. Studies on the effect of the GJA8 mutation Gly22Arg (found in Lop10 mice) in mouse lenses also revealed dominant negative effects.²⁰ In that study, the mutant proteins were found to interfere with the formation of gap junction channels. In contrast, other mutants of both GJA3 and GJA8, when tested in Xenopus oocytes, have been observed to result in loss of function without any dominant negative effects. These are the GJA8 mutant Asp47Ala (D47A) in the No2 mice²¹ and two GJA3 mutants Asn63Ser and fs380, causing cataract in humans.²² Yet another mechanism of action suggested for the GJA3 (Cx46) fs380 mutant,²³ upon expression in mammalian cells, is a gain of function, resulting in mislocalisation, caused by the frameshifted protein.²⁴ Overall, these observations do not point to any unifying mechanisms that may explain how specific connexin mutations could cause dominant versus recessive phenotypes. As has been suggested, interactions between connexin isoforms and the effects of connexins on other lens proteins may determine the phenotype.^16,25

In the present study, the homozygous GJA8 insertion mutation was associated with a severe phenotype in both affected siblings as indicated by the presence of opacities evident at birth, as well as nystagmus and amblyopia due to severe visual deprivation. One affected family member also had microcornea and microphthalmia, whereas her affected sibling was normal with respect to these parameters. Although it is not possible to conclude here as to whether the occurrence of microphthalmia is causally linked to deficiency of GJA8/Cx50, it is worth noting that microphthalmia was a feature of GJA8 knockout mice, suggesting that it is required for proper growth and development of the eye.¹⁶ This study adds to the range of phenotypes associated with GJA8 mutations and, to our knowledge, describes the first mutation in this gene to be associated with autosomal recessive inheritance of cataract.

LEARNING POINTS

• An insertion (c.670insA) in GJA8 is associated with recessive cataract.

• The majority of hereditary cataracts are inherited in a dominant fashion.

• Six genes have been identified for autosomal recessive cataract.

• Hereditary cataracts account for 1/5 of childhood cataracts in Southern India.