An unusual presentation of macular corneal dystrophy associated with uniparental isodisomy and a novel Leu173Pro mutation

CHST6 mutations identified in Iranian MCD patients and CHST6 mutations reported worldwide identify targets for gene editing approaches including the CRISPR/Cas system

PURPOSE

To identify CHST6 mutations in Iranians macular corneal dystrophy (MCD) patients and also to assess distribution of amino acids in the encoded protein that are affected by CHST6 mutations reported hitherto in various populations in order to predict gene regions that may be appropriate targets for gene editing approaches including the CRISPR/Cas system. The analysis will also reveal biologically and functionally important regions of the protein.

METHODS

Mutation screening of CHST6 by sequencing was performed on 21 Iranian MCD-affected probands. Previously reported MCD causing CHST6 mutations were identified by searches in NCBI.

RESULTS

Nineteen CHST6 mutations were found among the 21 Iranian patients, most of which were missense mutations and six of which were novel. Totally, 189 mutations among 375 MCD patients have been found worldwide, and 134 of these are missense mutations. The distribution of 88 amino acids affected by missense mutations along the length of the encoded protein was not random, and four regions of possible mutation clustering were noted. 25% of patients harbored mutations in a DNA region consisting of only 36 nucleotides.

CONCLUSION

Similar to most populations, CHST6 mutations among Iranians are very heterogeneous as indicated by finding 19 different mutations among 21 MCD patients. Nevertheless, identification of four potential mutation clusters identifies regions that are most suitable for gene therapy targeting by the CRISPR/Cas approach. Additionally, the mutation clusters identify regions with potential structural and/or functional importance. Consistent with this, the amino acids in these regions are well conserved among various membrane-bound sulfotransferases.

A comprehensive evaluation of 181 reported CHST6 variants in patients with macular corneal dystrophy

Macular corneal dystrophy (MCD) is an autosomal recessive disease featured by bilateral progressive stromal clouding and loss of vision, consequently necessitating corneal transplantation. Variants in CHST6 gene have been recognized as the most critical genetic components in MCD. Although many CHST6 variants have been described until now, the detailed mechanisms underlying MCD are still far from understood. In this study, we integrated all the reported CHST6 variants described in 408 MCD cases, and performed a comprehensive evaluation to better illustrate the causality of these variants. The results showed that majority of these variants (165 out of 181) could be classified as pathogenic or likely pathogenic. Interestingly, we also identified several disease causal variants with ethnic specificity. In addition, the results underscored the strong correlation between mutant frequency and residue conservation in the general population (Spearman's correlation coefficient = -0.311, P = 1.20E-05), thus providing potential candidate targets for further genetic manipulation. The current study highlighted the demand of further functional investigations to evaluate the causality of CHST6 variants, so as to promote earlier accurate diagnosis of MCD and future development of potential targets for genetic therapy.

Exclusion of positional candidate gene coding region mutations in the common posterior polymorphous corneal dystrophy 1 candidate gene interval

PURPOSE

Posterior polymorphous corneal dystrophy (PPCD) is an autosomal-dominant disorder of the corneal endothelium associated with visually significant corneal edema and glaucoma. Statistical genetic analysis of 4 families with PPCD has demonstrated linkage to a 2.4 cM common support interval on chromosome 20 bordered by the markers D20S182 and D20S139. We sought to identify the genetic basis of PPCD linked to chromosome 20 (PPCD1) by screening the 26 positional candidate genes between these markers in a family previously mapped to the PPCD1 region.

METHODS

The coding regions of the 26 positional candidate genes mapped to the common PPCD1 support interval were amplified and sequenced in affected and unaffected individuals from a family previously linked to the PPCD1 locus. Nine other genes positioned just outside of the common PPCD1 support interval but within the autosomal-dominant congenital hereditary endothelial dystrophy interval were also screened.

RESULTS

Four DNA sequence variants in 3 of the positional candidate genes demonstrated complete segregation with the affected phenotype: p.Thr109Thr (rs6111803) in OVOL2, p.Arg56Gln (novel variant-RPSnovel) in RPS19P1, and p.Thr85Thr (rs1053834) and p.Pro99Ser (rs1053839) in C20orf79. Each of these 4 sequence variants demonstrated significant linkage with the affected phenotype in this family (P = 2.5 x 10 for RPSnovel, rs1053834 and rs1053839; P = 8.6 x 10 for rs6111803). However, we also identified each of these 4 sequence variants in > or = affected control individuals. The haplotype on which the disease-causing mutation is segregating was found to have a population frequency of 4.2% in the CEPH HapMap trios. Although a number of other previously described and novel single nucleotide polymorphisms were identified in the 35 positional candidate genes located within the PPCD1 and congenital hereditary endothelial dystrophy intervals, none segregated with the affected phenotype.

CONCLUSIONS

We report the absence of a presumed pathogenic coding region mutation in the common PPCD1 support interval. Although minor alleles of 4 single nucleotide polymorphisms were identified that segregated with the affected phenotype, the relatively high frequency of each minor allele in the general population indicates that none is a candidate for the causal variant for PPCD. Instead, the causal variant is most likely a coding region deletion or a variant in a noncoding region of the PPCD1 common support interval.

The IC3D classification of the corneal dystrophies

BACKGROUND

The recent availability of genetic analyses has demonstrated the shortcomings of the current phenotypic method of corneal dystrophy classification. Abnormalities in different genes can cause a single phenotype, whereas different defects in a single gene can cause different phenotypes. Some disorders termed corneal dystrophies do not appear to have a genetic basis.

PURPOSE

The purpose of this study was to develop a new classification system for corneal dystrophies, integrating up-to-date information on phenotypic description, pathologic examination, and genetic analysis.

METHODS

The International Committee for Classification of Corneal Dystrophies (IC3D) was created to devise a current and accurate nomenclature.

RESULTS

This anatomic classification continues to organize dystrophies according to the level chiefly affected. Each dystrophy has a template summarizing genetic, clinical, and pathologic information. A category number from 1 through 4 is assigned, reflecting the level of evidence supporting the existence of a given dystrophy. The most defined dystrophies belong to category 1 (a well-defined corneal dystrophy in which a gene has been mapped and identified and specific mutations are known) and the least defined belong to category 4 (a suspected dystrophy where the clinical and genetic evidence is not yet convincing). The nomenclature may be updated over time as new information regarding the dystrophies becomes available.

CONCLUSIONS

The IC3D Classification of Corneal Dystrophies is a new classification system that incorporates many aspects of the traditional definitions of corneal dystrophies with new genetic, clinical, and pathologic information. Standardized templates provide key information that includes a level of evidence for there being a corneal dystrophy. The system is user-friendly and upgradeable and can be retrieved on the website www.corneasociety.org/ic3d.