Biochemical characterization of G64W mutant of acidic beta-crystallin 4

A novel missense variant c.71G > T (p.Gly24Val) of the CRYBA4 gene contributes to autosomal-dominant congenital cataract in a Chinese family

PURPOSE

To investigate the potential genetic defects in a five-generation Chinese family with autosomal dominant congenital cataract (ADCC).

METHODS

Whole exome sequencing was performed to search the variants in the candidate genes associated with congenital cataract. Sanger sequencing was used to validate the variants and examine their co-segregation in the patients and their relatives. The potential effect of the variants was analyzed using several bioinformatic methods and further examined through Western blotting and co-immunoprecipitation.

RESULTS

A missense variant c. 71 G > T (p. Gly24Val) in the CRYBA4 gene, a known ADCC candidate gene, was identified to be heterozygously present in the patients and co-segregate with cataract in the family. The mutation was absent in all of the searched databases, including our in-house exome sequences of 10,000 Chinese. The alignments of the amino acid sequences of CRYBA4 in a variety of species revealed that the amino acid residue Gly24 was evolutionarily highly conserved, and the in silico analysis predicted that the missense mutation of Gly24Val was damaging for the protein structure and function of CRYBA4. Then, the in vitro expression analysis further revealed that the Gly24Val mutation in CRYBA4 inhibited its binding with CRYBB1. The impaired interaction of β-crystallin proteins may affect their water-solubility and contribute to the formation of precipitates in lens fiber cells.

CONCLUSION

We identified a novel missense variant in the CRYBA4 gene as a pathogenic mutation of ADCC in a Chinese family. Our finding expanded the CRYBA4 variation spectrum associated with congenital cataracts.

The use of pharmacological chaperones in rare diseases caused by reduced protein stability

Rare diseases are most often caused by inherited genetic disorders that, after translation, will result in a protein with altered function. Decreased protein stability is the most frequent mechanism associated with a congenital pathogenic missense mutation and it implies the destabilization of the folded conformation in favour of unfolded or misfolded states. In the cellular context and when experimental data is available, a mutant protein with altered thermodynamic stability often also results in impaired homeostasis, with the deleterious accumulation of protein aggregates, metabolites and/or metabolic by-products. In the last decades, a significant effort has enabled the characterization of rare diseases associated to protein stability defects and triggered the development of innovative therapeutic intervention lines, say, the use of pharmacological chaperones to correct the intracellular impaired homeostasis. Here, we review the current knowledge on rare diseases caused by reduced protein stability, paying special attention to the thermodynamic aspects of the protein destabilization, also focusing on some examples where pharmacological chaperones are being tested.

Chemical Properties Determine Solubility and Stability in βγ-Crystallins of the Eye Lens

βγ-Crystallins are the primary structural and refractive proteins found in the vertebrate eye lens. Because crystallins are not replaced after early eye development, their solubility and stability must be maintained for a lifetime, which is even more remarkable given the high protein concentration in the lens. Aggregation of crystallins caused by mutations or post-translational modifications can reduce crystallin protein stability and alter intermolecular interactions. Common post-translational modifications that can cause age-related cataracts include deamidation, oxidation, and tryptophan derivatization. Metal ion binding can also trigger reduced crystallin solubility through a variety of mechanisms. Interprotein interactions are critical to maintaining lens transparency: crystallins can undergo domain swapping, disulfide bonding, and liquid-liquid phase separation, all of which can cause opacity depending on the context. Important experimental techniques for assessing crystallin conformation in the absence of a high-resolution structure include dye-binding assays, circular dichroism, fluorescence, light scattering, and transition metal FRET.

Identification and characterization of six β-crystallin gene mutations associated with congenital cataract in Chinese families

Background

This study aims to identify the underlying genetic defects of β‐crystallin (CRYB) genes responsible for congenital cataracts in a group of Chinese families.

Methods

Detailed family history and clinical data of six Chinese families with autosomal dominant congenital cataracts were recorded. Targeted exome sequencing was applied to detect the underlying genetic defects for the families. Generated variants were confirmed by PCR and sanger sequencing. Afterward, bioinformatic analysis through several computational predictive programs was performed to assess impacts of mutations on protein structure and function.

Results

A total of 53 participants (23 affected and 30 unaffected) from six unrelated Chinese families were recruited. Cataract phenotypes covered nuclear, total, posterior polar, pulverulent, snowflake‐like, and zonular. Through targeted exome sequencing, six mutations in four β‐crystallin genes were revealed which included five missense mutations CRYBB1 p.Q70P, CRYBB2 p.E23Q, CRYBB2 p.A49V, CRYBB2 R188C, CRYBA4 p.M14K and one splice mutation CRYBB3 c.75+1 G>A. In silico results predicted pathogenic for all four missense variants except variant CRYBB2‐p.A49V yielded results as tolerant. The CRYBB3 c.75+1 G>A splice site mutation was predicted to be deleterious by leading to a broken splice site, a premature stop codon, and subsequently resulting in a short peptide of 113 amino acids, which may affect protein features.

Conclusion

The obtained results expanded mutational and phenotype spectrum of β‐crystallin genes and offer clues for pathogenesis of congenital cataracts. The data also demonstrated that targeted exome sequencing is valuable for providing molecular diagnostic information for congenital cataract patients.

In silico analysis and high-risk pathogenic phenotype predictions of non-synonymous single nucleotide polymorphisms in human Crystallin beta A4 gene associated with congenital cataract

In order to provide a cost-effective method to narrow down the number of pathogenic Crystallin beta A4 (CRYBA4) non-synonymous single nucleotide polymorphisms (nsSNPs), we collected nsSNP information of the CRYBA4 gene from SNP databases and literature, predicting the pathogenicity and possible changes of protein properties and structures using multiple bioinformatics tools. The nsSNP data of the CRYBA4 gene were collected from 4 databases and published literature. According to 12 criteria, six bioinformatics tools were chosen to predict the pathogenicity. I-Mutant 2.0, Mupro and INPS online tools were used to analyze the effects of amino acid substitution on protein stability by calculating the value of ΔΔG. ConSurf, SOPMA, GETAREA and HOPE online tools were used to predict the evolutionary conservation of amino acids, solvent accessible surface areas, and the physical and chemical properties and changes of protein structure. All 157 CRYBA4 nsSNPs were analyzed. Forty-four CRYBA4 high-risk pathogenic nsSNPs (predicted to be pathogenic by all six software tools) were detected out of the 157 CRYBA4 nsSNPs, four of which (c.283C>T, p.R95W; c.449T>A, p.V150D; c.475G>A, p.G159R; c.575G>C, p.R192P) should be focused on because of their high potential pathogenicity and possibility of changing protein properties. Thirty high-risk nsSNPs were predicted to cause a decrease of protein stability. Twenty-nine high-risk nsSNPs occurred in evolutionary conserved positions. Twenty-two high-risk nsSNPs occurred in the core of the protein. It is predicted that these high-risk pathogenic nsSNPs can cause changes in the physical and chemical properties of amino acids, resulting in structural changes of proteins and changes in the interactions between domains and other molecules, thus affecting the function of proteins. This study provides important reference value when narrowing down the number of pathogenic CRYBA4 nsSNPs and studying the pathogenesis of congenital cataracts. By using this method, we can easily find 44 high-risk pathogenic nsSNPs out of 157 CRYBA4 nsSNPs.