Whole-exome sequencing identification of a recurrent CRYBB2 variant in a four-generation Chinese family with congenital nuclear cataracts

Congenital cataracts is the most common cause of visual impairment and blindness in children. Although there have been extensive studies into the pathogenesis of congenital cataracts, the pathogenic mechanism underlying the recurrent variant CRYBB2:c.62T>A(p.I21N) has not been previously reported. Thus, the present study aimed to use whole-exome sequencing (WES) to identify potential genetic variants and investigate how they may have induced the occurrence of cataracts in a four-generation Chinese family with congenital nuclear cataracts. The medical history of this family was recorded and WES was conducted for one proband. Sanger sequencing was used to verify the presence of the putative variant in all participants. PolyPhen-2, SIFT and ProtScale were used to analyze the effect of the identified variants on protein function and hydrophobicity, and Pymol was used to show the structure of the wild-type (Wt) and mutant β-crystallin B2 (CRYBB2) protein. Full-length Wt-CRYBB2 or mutant-CRYBB2 (I21N-CRYBB2) were fused to green fluorescent protein (GFP), and the recombinant plasmids were transfected into HeLa cells. Reverse transcription-quantitative PCR and western blotting were used to detect the expression levels of CRYBB2 mRNA and protein. Immunofluorescence and flow cytometry analyses were used to detect protein localization and apoptosis, respectively. A recurrent variant CRYBB2:c.62T>A(p.I21N) was identified in a four-generation Chinese family with congenital nuclear cataracts. Multiple-sequence alignment of CRYBB2 demonstrated that codon 21 was highly conserved. Pymol revealed that the structure of the I21N-CRYBB2 protein was distinct from that of Wt-CRYBB2. PolyPhen-2 predicted that it had a variant provean score 1.0, suggesting it was ‘probably damaging’, and SIFT predicted it had a variant provean score of -5.113, indicating it was ‘deleterious’. ProtScale indicated that the hydrophobicity of the mutation site was significantly reduced. The protein expression levels of the I21N-CRYBB2 were decreased compared with the Wt-CRYBB2. Immunofluorescence analysis revealed that the variant I21N-CRYBB2 protein tended to accumulate around the nucleus, and flow cytometry analysis indicated that it increased cell apoptosis. Furthermore, I21N-CRYBB2 induced the activation of the unfolded protein response (UPR). In conclusion, a pathogenic variant of CRYBB2:c.62T>A(p.I21N) was identified via WES in a four-generation Chinese family with congenital nuclear cataracts. Through biological analysis, it was found that the variant induced abnormal protein aggregation, activated the UPR and triggered excessive cell apoptosis, which may lead to the occurrence of congenital nuclear cataracts in this family.