Mutation analysis of 12 genes in Chinese families with congenital cataracts

• Introduction
• Methods
• Results
• Discussion

Abstract

Purpose: To identify mutations in 12 genes in Chinese families with congenital cataracts.

Methods: Twenty five families with congenital cataracts involved in this study. The coding exons and adjacent intronic regions of 12 genes were analyzed by cycle sequencing, including the alpha A crystallin (CRYAA), alpha B crystallin (CRYAB), beta A1 crystallin (CRYBA1), beta A4 crystallin (CRYBA4), beta B1 crystallin (CRYBB1), beta B2 crystallin (CRYBB2), beta B3 crystallin (CRYBB3), gamma C crystallin (CRYGC), gamma D crystallin (CRYGD), gamma S crystallin (CRYGS), alpha 3 gap junction protein (GJA3), and alpha 8 gap junction protein (GJA8) genes. Novel variants were further evaluated in 96 normal controls.

Results: Nine mutations were identified in 10 of the 25 families (40%), including 5 novel (c.350_352delGCT in CRYAA, c.205C>T in CRYAB, c.106G>C in CRYGD, c.77A>G in CRYGS, c.1143_1165del23 in GJA3) and 4 known (c.292G>A in CRYAA; c.215+1G>A and c.272_274delGAG in CRYBA1, and c.176C>T in GJA3). All novel mutations were predicted to be pathogenic and were not present in 96 controls.

Conclusions: Mutations in the 12 genes encoding crystallins and connexins were responsible for 40% Chinese families with congenital cataracts. Our results enriched our knowledge on the molecular basis of congenital cataracts in Chinese population.

Congenital cataract is a leading cause of childhood blindness, accounting for 10~38% of blindness in children. The prevalence of congenital cataract is estimated to be 0.6~6 per 10,000 live births [1,2]. Various etiological factors have been reported, including infection, neonatal asphyxia, malnutrition, and genetic defects. It was reported that 8.3%~25% of congenital cataracts were inherited [3]. Several genes have been identified to be associated with congenital cataracts [3,4], such as genes encoding crystallins, connexins and other membrane proteins, beaded filament proteins, growth and transcription factors, and others.

For cataract families with identified mutations, it has been suggested that about half had mutations in crystallins and a quarter in connexins (gap junction proteins) [3]. So far, mutations in 10 crystallin genes and 2 connexin genes have been identified to be responsible for congenital cataracts, including alpha A crystallin (CRYAA, OMIM 123580) [5-12], alpha B crystallin (CRYAB, OMIM 123590) [10,13-15], beta A1 crystallin (CRYBA1, OMIM 123610) [16-23], beta A4 crystallin (CRYBA4, OMIM 123631) [24,25], beta B1 crystallin (CRYBB1, OMIM 600929) [26-29], beta B2 crystallin (CRYBB2, OMIM 123620) [30-34], beta B3 crystallin (CRYBB3, OMIM 123630) [35,36], gamma C crystallin (CRYGC, OMIM 123680) [12,37-40], gamma D crystallin (CRYGD, OMIM 123690) [37,41-43], gamma S crystallin (CRYGS, OMIM 123730) [10,44,45], alpha 3 gap junction protein (GJA3, OMIM 121015) [34,46-50], and alpha 8 gap junction protein (GJA8, OMIM 600897) [40,51-56]. Several mutations in these genes have been identified but most reports are either based on one to a few gene(s) or one to a few family(ies). The exact mutation frequency of these genes in congenital cataract is unclear as comprehensive analysis of all the 12 genes in the same set of families is absent although a few studies reported analysis of some of them [10,29,36].

In this study, molecular genetic analysis was performed on all these 12 genes in 25 Chinese families with congenital cataract. Nine mutations, including 5 novel and 4 known, were identified in 10 of the 25 families (40%).

Based on complete analysis of the coding exons and their adjacent intronic regions in the 12 genes in 25 families, nine heterozygous mutations were detected in 6 genes in 10 families (Table 2, Figure 1). Of the nine mutations, five were novel (c.350_352delGCT in CRYAA, c.205C>T in CRYAB, c.106G>C in CRYGD, c.77A>G in CRYGS, c.1143_1165del23 in GJA3) and four were known (c.292G>A in CRYAA; c.215+1G>A and c.272_274delGAG in CRYBA1, and c.176C>T in GJA3). The c.272_274delGAG mutation in CRYBA1 was present in two unrelated families. The pedigrees and cosegregation analyses of the 10 families with identified mutations were shown in Figure 2. All 5 novel mutations were not present in 96 normal controls.

Three of the five novel variants were missense mutations. Of the three, the c.205C>T (p.Arg69Cys) in CRYAB and the c.77A>G (p.Asp26Gly) in CRYGS involved highly conserved residues while the other one (p.Ala36Pro in CRYGD) replaced a nonconserved residue (Figure 3). The novel c.350_352delGCT mutation in CRYAA resulted in substitution of arginine at position 117 and deletion of tyrosine at position 118, where the two residues are highly conserved (Figure 3). The c.1143_1165del23 mutation resulted in frameshift with additional 48 new residues from residue 381.

The clinical data of the available patients with mutations were listed in Table 3 and cataract phenotypes for some patients were shown in Figure 4.

In this study, nine heterozygous mutations in the 12 genes encoding crystallins and connexins were identified in 10 out of 25 Chinese families (40%) with congenital cataract. Eight of the 25 families (32%) had mutations in crystallin genes and two of them (8%) had mutations in connexin genes.

For the congenital cataract families with identified mutations reported before, about three-fourths of the families had mutations in the 10 crystallin and 2 connexin genes, which was mostly based on studies of individual gene [3]. However, this mutation proportion may not reflect the actual frequency of these genes in congenital cataract, since the genes encoding crystallins and connexins might be more frequently analyzed. In fact, varied frequencies of mutation detection, mostly lower, were reported in several studies involving analysis of multiple genes. Devi et al. found crystallin mutations in 16.6% (10 of 60) Indian families by analyzing the 10 crystallin genes [10]. Burdon et al. [63] detected crystallin mutations in 5.3% (2/38) Australian families by analyzing 7 crystallin genes. Hansen et al. [36] detected crystallin and connexin mutations in 35.7% (10/28) and 21.4% (6/28) Danish families, respectively. Kumar et al. [40] detected mutations in 20% (6/30) Indian families by analyzing 4 of the 12 genes. Wang et al. [29] identified mutations in 15% (3/20) Chinese families by analyzing 10 of the 12 genes. None of these studies performed analysis of all the 12 genes in the same set of families. In this study, we detected mutations in 40% families by analyzing all of the 12 genes. Overall, the frequencies of mutations in the 12 genes varied significantly in different ethnic populations but were more likely to be less than three fourth of families with congenital cataracts. The over-representation of mutations in these 12 genes is more likely due to more frequent studies on these genes.

Although several mutations in the 12 genes have been identified so far, we still identified five novel mutations in the Chinese families with congenital cataracts. Our results expand the mutation spectrum of these genes in Chinese population. The families without identified mutations will be good candidates for future study in screening for additional causative genes.

The authors thank the family members for their participation. This study was supported by the National Science Fund for Distinguished Young Scholars (30725044 to Q.Z.), National 973 Plan of China (2010CB529904 to Q.Z.), and the Fundamental Research Funds of State Key Lab of Ophthalmology, Sun Yat-sen University.