Study Proposal: Causative Mutations in Optic Neuropathy

Optic neuropathy refers to degeneration of optic nerve. It is often described as optic atrophy that means loss of few or most of optic nerve fibers [1]. It can be symmetric or bilateral based on toxic or nutritional ( vitamin B12 or folate deficiency) insults and genetic defects [4]. In genetically inherited optic atrophies, retinal ganglion cells and optic nerve fiber layer are damaged. This damage can be focal (affects macular beam of optic nerve) or generalized [6]. The axons of retinal ganglion cells arise from retina and from optic nerve. The optic nerve enter cortex via optic disc where input signal is processed into vision. The retinal ganglion cells or nerve cells of inner retina form 1.2 million nerve fibers [1]. The key features of optic neuropathy results from death of these nerve cells or neurons.

Autosomal recessive optic atrophies (ROAs) indicate loss of nerve fibers that form optic disc, optic nerve, optic chiasm and optic tracts [net bookmark]. The only key feature of isolated ROAs is optic nerve degeneration. To date defects in genes coding for mitochondrial proteins leads to isolated ROAs [4].

The genetically inherited atrophies can be familial that follows Mendelian pattern of inheritance (X-linked recessive, autosomal recessive and autosomal dominant) or non Mendelian (mitochondrial) [2]. In autosomal recessive Mendelian pattern of inheritance, two copies of mutant allele in affected person and one copy of mutant allele in carrier are present. When two carriers mate, there is an equal chance (25%) of being affected and unaffected. There is a 50% chance of being heterozygous (unaffected carrier). But in autosomal dominant pattern of inheritance, affected individuals have one mutant copy of allele so every individual has 50% chance of being affected or being normal.

In the pure congenital autosomal recessive optic atrophy, symptoms like visual impairment appear very early and are present at birth or appear in first year of life. Affected individuals are severely impaired visually leading to visual disability or complete blindness. It is never associated with neurological disorders. It can be diagnosed very early, usually before the age of 4 years. Fundus examination reveals optic disc pallor temporally or bilaterally. A cupping may develop with age [3]. Affected individuals suffer from central scotoma, photophobia and also have red green color confusion [6]. Family history is critical for diagnosis [Hereditaryÿopticÿneuropathies: from clinical signs to diagnosis].

Clinical diagnosis involves fundoscopy, visual field testing, fluorescein angiography, optical coherence tomography scan of the retinal nerve fiber layers, color vision analysis and standard electroretinogram. Damage from optic nerve atrophy cannot be reversed [4].To clinically distinguishes it from LHON flourscene angiography was done which fails to show any peripapillary microvascular changes, beside this retinal activity is also normal confirmed by electroretinogram. Pathology of arOA is confirmed by testing visual evoked potential which was totally absent in affected individuals. Central or cecocentral faults are diagnosed through visual field testing. Lesion in the fovea or papillomacular bundle leads to cental scotoma which ultimately affects the central fixation. Thinning of retinal nerve fiber layer is diagnosed through optical coherence tomography scan [3, 4, clinical diagnosis]. Papillomacular bundle leads to the extension of cecocentral sctoma towards blind spot.

There is no effective treatment for optic atrophy because degeneration of nerve fibers is irreversible process. Although further damage can be prevented by early diagnosis and by treating underlying causes of the disorder [ net bookmark]. As environmental factor play their role in every aspect of life likewise in this disorder alcohol consumption and smoking should be strictly prohibited [Hereditaryopticneuropathies: from clinical signs to diagnosis]. Genetic counseling and proper awareness of people is very important to prevent such genetic disorders without any effective treatment.

New therapies are being formulated by designing animal models or by clinical trial on affected humans these therapies mainly focuses in preventing oxidative stress. Animal models have been designed for testing the various treatments in case of Leber hereditary optic neuropathy [Treatment of hereditaryopticneuropathies].

arOA is divided into syndromic and non-syndromic form, syndromic arOA involve multiple organs other than eye which are effected just like wolfram syndrome [6, net bookmarks]. While in non-syndromic optic nerve is affected only and it also shows familial transmission more than one members of same family can also be affected [Hereditaryopticneuropathies: from clinical signs to diagnosis].

OPA 6 and 7 are the two loci which are characterized till now, mutation in any one of two leads to arOA. Disease causing gene has been localized at chromosome 8q21–q22 (Zmax of 3.41 at h¼0 for D8S270). D8S1794 and D8S1702 markers present on OPA6 in a 12Mb interval [6]. OPA7 containing 40 known genes, only one gene TMEM126A screened as a causative agent of non-syndromic arOA [4, 7]. TMEM126A is homozygous nonsense mutation characterized as first known mutation in case of isolated arOA, this gene transcribed into mitochondrial transmembrane protein.

TMEM126A helps in early nucleation of mitochondrial complexes that is why it is also termed as mitochondria-localized mRNA (MLR) protein. It play important role in function of retinal ganglion cells by arranging protein complexes essential for the proper functioning of RGCs. It is hypothesized that TMEM126A may accelerate the rate at which protein complex assemble, which otherwise occurs slowly that ultimately affect high energy demanding RGCs. While in other tissues the effect of this mutation may be substituted by some other protein of TMEM family [4, 7].

TMEM126A transcribe single ubiquitous transcript of 770bp that contain total five exons four coding and one non-coding exon. TMEM126A present on chromosome 11 and its span is 8.5 kb. Testis, fetal retinal pigmentary epithelium (RPE), fetal retina, brain (whole), cerebellum, fetal brain and skeletal muscle are the sites where strong expression of TMEM126A has been observed. Substantial amount of specific mRNA in the ganglion cell layer, optic nerve head, the outer ellipsoide length of photoreceptor inner segments, and the outer plexiform layer is detected by the process of insitu hybridization in mouse i.e. 8 month old. In the photoreceptor outer segments and outer nuclear layer (ONL) no labeling is noticed. Mitochondria specific Alpha subunit of the ATP synthase is Immunolocalized on retinal sections of the same mouse which resulted into the same pattern of expression. So it is confirmed that TMEM126A transcribe mitochondrial localized m RNA.

Linkage analysis is the process helps in finding mutation or gene resulted into arOA. Potential functionality of genes with their chromosomal location is associated through this statistical method. During chromosomal recombination markers present closely or on the same area on chromosome will remain attached together are transmitted as such in offspring’s this idea is exploited in linkage analysis. If in an individual disease gene is transmitted along with some specific markers it means disease causing gene is present close to these markers. Those disorders that follow Mendelian inheritance pattern can easily be analyzed through this process [5].

This study will be the first step which further helps in identifying the causative gene responsible for arOA in Pakistani population and also help in designing therapeutic tools for the benefit of affected persons. By conducting these type of studies we can also aware our population about such type of rare disorders.