Confirmation and refinement of the heterozygous deletion of the small leucine-rich proteoglycans associated with posterior amorphous corneal dystrophy

PURPOSE

To present the clinical and cytogenetic features of a previously unreported family with posterior amorphous corneal dystrophy (PACD) associated with a heterozygous deletion of the small leucine-rich proteoglycan (SRLP) genes on chromosome 12.

METHODS

Clinical characterization was performed using slit lamp biomicroscopic and optical coherence tomography (OCT) imaging. Genomic DNA was collected from affected and unaffected family members, and a cytogenomic array was used to identify copy number variations (CNV) present in the PACD locus.

RESULTS

Three members of a Guatemalan family presented with clinical characteristics consistent with PACD: bilateral posterior stromal lamellar opacification, decreased corneal curvature, and iridocorneal adhesions. OCT imaging demonstrated decreased corneal thickness and hyperreflectivity of the posterior third of the corneal stroma. CNV analysis confirmed the presumed clinical diagnosis of PACD by revealing a 0.304 Mb heterozygous deletion in the PACD locus on chromosome 12 that included the four SLRP genes (KERA, LUM, DCN, and EPYC) deleted in each of the PACD families in which CNV analysis has been reported.

CONCLUSIONS

This is the first report of the OCT appearance of PACD and the second confirmation of a heterozygous deletion of chromosome 12q21.33 as the cause of PACD, highlighting the utility of array-based cytogenomics to confirm the suspected clinical diagnosis of PACD. As the smallest previously reported pathogenic deletion was 0.701 Mb, the 0.304-Mb deletion we report is the smallest identified to date and reduces the size of the PACD locus to 0.275 Mb.

Posterior amorphous corneal dystrophy caused by a de novo deletion

We present a newborn diagnosed with posterior amorphous corneal dystrophy (PACD). PACD is a rare disorder with partial or complete posterior lamellar corneal opacification. Genetic screening showed a deletion of chromosome 12q21.33-q22 containing the identified four small leucine-rich proteoglycans (SLRP's) associated with this particular dystrophy. Neither parents were carrier of the deletion. To our knowledge, this is the first report of a de novo mutation causing PACD.

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.