THE SIGNIFICANCE OF HEREDITY IN OPHTHALMOLOGY. PRELIMINARY SURVEY OF HEREDITARY EYE DISEASES IN TASMANIA

Genetic Aspects of Keratoconus: A Literature Review Exploring Potential Genetic Contributions and Possible Genetic Relationships with Comorbidities

Introduction

Keratoconus (KC) is a complex, genetically heterogeneous, multifactorial degenerative disorder that is accompanied by corneal ectasia which usually progresses asymmetrically. With an incidence of approximately 1 per 2000 and 2 cases per 100,000 population presenting annually, KC follows an autosomal recessive or dominant pattern of inheritance and is, apparently, associated with genes that interact with environmental, genetic, and/or other factors. This is an important consideration in refractive surgery in the case of familial KC, given the association of KC with other genetic disorders and the imbalance between dizygotic twins. The present review attempts to identify the genetic loci contributing to the different KC clinical presentations and relate them to the common genetically determined comorbidities associated with KC.

Methods

The PubMed, MEDLINE, Google Scholar, and GeneCards databases were screened for KC-related articles published in English between January 2006 and November 2017. Keyword combinations of “keratoconus,” “risk factor(s),” “genetics,” “genes,” “genetic association(s),” and “cornea” were used. In total, 217 articles were retrieved and analyzed, with greater weight placed on the more recent literature. Further bibliographic research based on the 217 articles revealed another 124 relevant articles that were included in this review. Using the reviewed literature, an attempt was made to correlate genes and genetic risk factors with KC characteristics and genetically related comorbidities associated with KC based on genome-wide association studies, family-based linkage analysis, and candidate-gene approaches.

Results

An association matrix between known KC-related genes and KC symptoms and/or clinical signs together with an association matrix between identified KC genes and genetically related KC comorbidities/syndromes were constructed.

Conclusion

Twenty-four genes were identified as potential contributors to KC and 49 KC-related comorbidities/syndromes were found. More than 85% of the known KC-related genes are involved in glaucoma, Down syndrome, connective tissue disorders, endothelial dystrophy, posterior polymorphous corneal dystrophy, and cataract.

Identification of seven novel ZNF469 mutations in keratoconus patients in a Han Chinese population

PURPOSE

To test for the potential presence of novel mutations in the zinc finger protein (ZNF469) gene in patients with sporadic keratoconus (KC) from a Han Chinese population.

METHODS

Fifty-three patients with primary KC, 30 patients with high myopia (HM), and 100 unrelated population-matched healthy controls without any ocular or systemic disorders, all of Han Chinese ethnicity, were recruited. Blood samples were donated, and genomic DNA was isolated from peripheral blood leukocytes. Sequence variations in ZNF469 were initially identified in patients with KC with next-generation sequencing and subsequently confirmed using Sanger sequencing. Sequence variants identified in patients with KC were subsequently screened in 30 patients with HM and 100 healthy control subjects. Other genes that were reported to be related to KC were also screened in the patients with KC who carried the mutations in ZNF469. The Sorting Intolerant Form Tolerant (SIFT) program was used to predict the effect of amino acid substitution on the ZNF469 protein.

RESULTS

Sixteen sequence variants in the coding regions of ZNF469 were identified in this Chinese KC cohort. After five known single nucleotide polymorphisms (SNPs), one false-positive result, and three mutations that were also detected in the results of the whole-exome sequencing (WES) data performed in 220 Han Chinese individuals without ocular abnormalities were removed, seven novel mutations in ZNF469 (c.2059G>A, c.2137C>A, c.3466G>A, c.3749C>T, c.4300G>A, c.4684G>A, and c.7262G>A) that were predicted to be potentially damaging were identified. The patient with KC with the c.3466G>A mutation was also shown to carry one dedicator of cytokinesis 9 (DOCK9) mutation (c.1940C>T). None of the mutations were detected in the patients with HM or the healthy controls. All of the seven mutations in the patients with KC were heterozygote.

CONCLUSIONS

The results suggested for the first time that ZNF469 has a pathogenic role in Chinese patients with KC and have widened the mutation spectrum of KC in the Han Chinese population.

Two novel missense substitutions in the VSX1 gene: clinical and genetic analysis of families with Keratoconus from India

Background

Visual system homeobox gene (VSX1) plays a major role in the early development of craniofacial and ocular tissues including cone opsin gene in the human retina. To date, few disease-causing mutations of VSX1 have been linked to familial and sporadic keratoconus (KC) in humans. In this study, we describe the clinical features and screening for VSX1 gene in families with KC from India.

Methods

Clinical data and genomic DNA were collected from patients with clinically diagnosed KC and their family members. The study was conducted on 20 subjects of eight families from India. The coding exons of VSX1 gene were amplified using PCR and amplicons were analyzed by direct sequencing. Predictive effect of the mutations was performed using Polyphen-2, SIFT and mutation assessor algorithms. Additionally, haplotypes of VSX1 gene were constructed for affected and unaffected individuals using SNPs.

Results

In the coding region of VSX1, one novel missense heterozygous change (p.Leu268His) was identified in five KC patients from two unrelated families. Another family of three members had a novel heterozygous change (p.Ser251Thr). These variants co-segregated with the disease phenotype in all affected individuals but not in the unaffected family members and 105 normal controls. In silico analysis suggested that p.Leu268His could have a deleterious effect on the protein coded by VSX1, while p.Ser251Thr has a neutral effect on the functional properties of VSX1. Haplotype examination revealed common SNPs around the missense change (p.Leu268His) in two unrelated KC families.

Conclusions

In this study, we add p.Leu268His, a novel missense variation in the coding region of VSX1 to the existing repertoire of VSX1 coding variations observed in Indian patients with the characteristic phenotype of KC. The variant p.Ser251Thr might be a benign polymorphism, but further biophysical studies are necessary to evaluate its molecular mechanism. The shared haplotype by two families with the same variant suggests the possibility of a founder effect, which requires further elucidation. We suggest that p.Leu268His might be involved in the pathogenesis of KC, which may help in the genetic counselling of patients and their family.

Genetic and genomic perspective to understand the molecular pathogenesis of keratoconus

Keratoconus (KC; Mendelian Inheritance in Man (OMIM) 14830) is a bilateral, progressive corneal defect affecting all ethnic groups around the world. It is the leading cause of corneal transplantation. The age of onset is at puberty, and the disorder is progressive until the 3^rd–4^th decade of life when it usually arrests. It is one of the major ocular problems with significant social and economic impacts as the disease affects young generation. Although genetic and environmental factors are associated with KC, but the precise etiology is still elusive. Results from complex segregation analysis suggests that genetic abnormalities may play an essential role in the susceptibility to KC. Due to genetic heterogeneity, a recent study revealed 17 different genomic loci identified in KC families by linkage mapping in various populations. The focus of this review is to provide a concise update on the current knowledge of the genetic basis of KC and genomic approaches to understand the disease pathogenesis.

Mitochondrial complex 1 gene analysis in keratoconus

PURPOSE

Keratoconus is characterized by the thinning of corneal stroma, resulting in reduced vision. The exact etiology of keratoconus (KC) is still unknown. The involvement of oxidative stress (OS) in this disease has been reported. However, the exact mechanism of OS in keratoconus is still unknown. Thus we planned this study to screen mitochondrial complex I genes for sequence changes in keratoconus patients and controls, as mitochondrial complex I is the chief source of reactive oxygen species (ROS) production.

METHODS

A total of 20 keratoconus cases and 20 healthy controls without any ocular disorder were enrolled in this study. Mitochondrial complex I genes (ND1, 2, 3, 4, 4L, 5, and 6) were amplified in all patients and controls using 12 pairs of primers by PCR. After sequencing, DNA sequences were analyzed against the mitochondrial reference sequence NC_012920. Haplogroup frequency based Principle Component Analysis (PCA) was constructed to determine whether the gene pool of keratoconus patients is closer to major populations in India.

RESULTS

DNA sequencing revealed a total 84 nucleotide variations in patients and 29 in controls. Of 84 nucleotide changes, 18 variations were non-synonymous and two novel frame-shift mutations were detected in cases. Non-synonymous mtDNA sequence variations may account for increased ROS and decreased ATP production. This ultimately leads to OS; which is a known cause for variety of corneal abnormalities. Haplotype analysis showed that most of the patients were clustered under the haplogroups: T, C4a2a, R2'TJ, M21'Q1a, M12'G2a2a, M8'CZ and M7a2a, which are present as negligible frequency in normal Indian population, whereas only few patients were found to be a part of the other haplogroups like U7 (Indo-European), R2 and R31, whose origin is contentious.

CONCLUSIONS

Mt complex I sequence variations are the main cause of elevated ROS production which leads oxidative stress. This oxidative stress then starts a cascade of events which ultimately can lead to keratoconus. Prompt antioxidant therapy should be initiated in keratoconus patients to minimize ROS related damage.

2005 Gregg Lecture: Congenital cataract--from rubella to genetics

On the 65th anniversary of Gregg's observation "Congenital cataract following German measles in the mother", rubella has retired as the leading cause of congenital cataract, from 87% of Gregg's cohort to less than 3% over the last 25 years and almost zero now in Australia and other developed countries. However, people must keep vigilance in maintaining immunization rates and encourage immunization in developing countries. At least one-fifth of congenital cataract is familial. Understanding the genetics of familial cataract will lead to better treatment of congenital as well as age-related cataract.