In the current era of genetic advances, the diagnosis of genetic eye diseases is increasing and facilitated by collaborations between ophthalmologists and geneticists. Genetic testing can not only help confirm suspected diagnoses, but also provide important prognostic information and guide management for the individual affected but also help provide appropriate genetic counseling for parents. A comprehensive genetic evaluation requires a thorough clinical examination, a detailed family history, and access to advanced technology for molecular investigation.
We carried out the genetic analysis to find out molecular pattern and any significant mutations of inherited eye diseases in Nepalese children. In our study, congenital anomalies were most commonly present in terai region with 18 cases followed by 4 in mountains and 5 in hills. Five types of congenital anomalies were studied ie congenital ptosis, microphthalmos, congenital cataract, coloboma and Crouzon syndrome. Among these only crouzen syndrome was only found to be familial where mother was also affected.
Hereditary cataracts are clinically and genetically heterogeneous, often presenting as congenital or developmental cataracts that arises at birth or during the first few decades of life (7. Approximately 25% of non-syndromic cataracts are inherited (8. They can also be grouped into three major classes, based on the functions of known underlying genes, those that code for crystallins, membrane/cytoskeleton proteins, and transcription factors. At least 35 independent loci, including more than 20 known genes, have been identified for non-syndromic cataract (Cat- Map) (9,10). Mutations in crystallin genes account for the majority of missense mutations (nearly 50%) followed by mutations in the genes for cytoskeletal or membrane proteins (nearly 35%) (9). The Gap Junction Protein Alpha 8 (GJA8) and Gap Junction Protein Alpha 3 (GJA3) mutations together account for 20% of the reported total non-syndromic familial cataracts worldwide (9). We selected a known SNP c.649G > A (Val196Met) from the same gene GJA8 and tested for the children with congenital cataract. This variation was not found in our patient with congenital cataract.
We selected one alteration in the ZFHX4 gene, G12411T L4137F in the children with congenital ptosis. This alteration was also previously reported in a Japanese family (11). This change was also detected in one child with congenital ptosis.
Another group of children were with iris and chorioretinal coloboma and microphthalmia. Microphthalmia and iris and chorioretinal coloboma are related structural, congenital eye malformations which display a spectrum of severity and can occur in isolation or as part of a syndrome. Microphthalmia refers to a small eye, defined by axial length. Iris coloboma is a segmental ocular defect resembling a key hole deficiency in iris. Chorioretinal coloboma is associated with iris choloboma and visually significant if posterior pole is involved. (12). The STRA6 gene plays a key role in normal ocular development, encoding a transmembrane receptor for the retinol-binding protein (RBP) and is responsible for mediating vitamin A uptake from circulation to target organs including the eye (13). We selected two SNP in STRA6 (T > C P. Y374C and A > T P. L152M) gene which are already found in database. We could identify one of these mutations in as well.
The second gene we have selected was Crystalline Beta A 4 (CRYBA4). It is known that complex microphthalmia in association with genetic cataracts has been attributed to mutations in the CRYBA4 gene (14). We selected one SNP C > T P.R25W from CRYBA4 gene which could not be identified in our patient.
The next gene is Orthodenticle Homeobox 2 (OTX2). The OTX2 gene encodes a transcription factor critical for forebrain and eye development (15). The OTX2 protein contains a homeodomain, responsible for DNA binding, SGQFTP and SIWSPA motifs involved in protein–protein interactions, and two C-terminal tandem OTX-tail motifs responsible for transactivation (16). We have selected three SNP p. Gln104 X, p. Gln106 His, p. Thr186 Fs < frame shift from OTX2 gene which could not be identified in our patient.
We have also selected one SNP p. Ala 57 Thr G◊A from ATP Binding Cassette Sub Family B Member 6 (ABCB6) gene, which is involved in the active transport of various compounds vital for CNS development, but could not identify this mutation in our patient. Crouzen syndrome is an autosomal dominant craniosynostosis disorder that can be caused by mutation in the Fibroblast Growth Factor Receptor 2 (FGFR2) genes. (18) We also selected few SNP from FGFR2 gene, which codes for fibroblast growth factor receptor, which are mentioned in table for Crouzon syndrome but we were not able to find any mutation. (19). The limitation of our study was small sample size. Apart from the diseases included in the study, we could widen our research in other types of congenital ocular anomalies.