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    Eye Disorders:

 

Aniridia ( MIM: 106210)

Aniridia is a congenital, hereditary, bilateral iris hypoplasia that may be associated with other ocular defects. It describes an extreme form of iris hypoplasia in which the iris appears absent on superficial clinical examination. This usually occurs in both eyes. Aniridia may be broadly divided into hereditary and sporadic forms. Hereditary Aniridia is usually transmitted in an autosomal dominant manner, although rarer autosomal recessive forms (such as Gillespie syndrome) have also been reported. Aniridia is caused by mutation in the paired box gene-6 (PAX6). Approximately one third of all cases of Aniridia are sporadic and these are often found to have cytogenetically detectable deletions involving 11p13 that contains the PAX6 gene.

Congenital Hereditary Endothelial Dystrophy

Congenital endothelial dystrophy (CHED) can be inherited either as an autosomal dominant disease (CHED1; MIM: 121700) or as autosomal recessive form (CHED2; MIM: 217700). CHED1, associated with mutations in CHED1 gene, is a rare disorder involving degeneration of the endothelial (inner) layer of the cornea and is characterized by a markedly opaque cornea due to stromal edema secondary to defective endothelial cells. CHED2 is a severe and rare bilateral corneal disorder that presents at birth or shortly thereafter, characterized by corneal opacification and nystagmus. The clinical diagnosis is based on the presence of a diffused corneal oedema in the absence of any other anterior segment abnormality. It is associated with mutations in SLC4A11 gene. In India there is a high frequency of CHED2, possibly related to consanguineous marriages.

           This database presently holds the relevant data for CHED2 only.

Glaucoma

Glaucoma is a heterogeneous group of optic neuropathies with a complex genetic basis. It is the second largest blinding disorder after cataract. Recent reports estimate that there would be ~60.5 million people with primary glaucoma in 2010 and 79.6 million by 2020 resulting in bilateral blindness. According to the W.H.O report, 9% of the total blind population in Indian subcontinent (India, Bangladesh, Nepal and Pakistan) is glaucomatous. According to the latest estimate approximately 12 million people will be affected with glaucoma by 2010 in India. Glaucoma can broadly be classified into three major groups: (i) primary open angle glaucoma (POAG; MIM: 137760); (ii) primary acute closed angle glaucoma (PACG; MIM: Not Available); and (iii) primary congenital glaucoma (PCG; MIM: 231300).

POAG, the commonest form of glaucoma, is described as optic nerve damage from multiple possible causes that is chronic and progresses over time, with a loss of optic nerve fibers that is characteristic of the disease. In addition POAG is characterized by open anterior chamber angles, visual field abnormalities, and intraocular pressure (IOP) that is too high for the continued health of the eye. Till date, 25 loci have been found to be linked with POAG but only three underlying genes have been identified viz. Myocilin, Optineurin and WDR36. CYP1B1 mutations are also implicated in certain POAG cases.

PCG is usually diagnosed at birth or shortly thereafter. It is characterized by improper development of the eye's drainage channel (called trabecular meshwork). Because of this, the channel that normally drains the fluid (called aqueous humor) from inside the eye does not function properly ultimately leading to high IOP. Till date 4 loci have been found to be linked with PCG. One of the underlying genes has been identified as CYP1B1, which is primarily implicated towards development of PCG.  Apart from CYP1B1, mutations in FOXC1 and LTBP2 have also been identified in a few PCG cases.

PACG is associated with a physically obstructed anterior chamber angle. It is caused by contact between the iris and trabecular meshwork, which in turn obstructs the outflow of the aqueous humor from the eye. Very little information is available regarding genetic factors related to PACG. Literature reports the association of few polymorphic variants with PACG, which suggests the possible role of underlying genetic variations in PACG.

Retinitis Pigmentosa ( MIM: 268000

Retinitis pigmentosa (RP) is a group of inherited disorders characterized by progressive peripheral vision loss and night vision difficulties (nyctalopia) that can lead to central vision loss. RP constitutes many retinal dystrophies and retinal pigment epithelium (RPE) dystrophies caused by molecular defects in more than 100 different genes. RP can be passed on by all types of inheritance: 20-25% is autosomal dominant, 15-20% is autosomal recessive, and 5-10% is X linked, while the remaining 45-50% is found in patients without any known affected relatives. The incident seems to be about one in every 4,000-5000 worldwide. In South India, Retinitis pigmentosa in the urban population has been recorded to be approximately 1 in 930 persons, while 1 in 372 of rural subjects had the disorder.

The database presently holds the relevant data for RLBP1, ABCA4, RHO, TULP1, RP1, RPE65 and RPF31 genes.

Congenital cataract ( MIM: 601286)

Here, the characteristic opacification of lenses are diagnosed at birth. In 75% of cases of congenital cataract hereditary cataract is passed from one generation to the next in autosomal dominant fashion. There are also a number of rare hereditary syndromes where the occurrence of cataract is associated with some systemic illness. Evidence for linkage was found with gamma-crystallin-1 (CRYG1) gene and also CRYGD genes. Mutations in a few other genes like LIM2 and GJ8 are also associated with congenital cataract.

The database presently holds the relevant data for LIM2, GJ8 and  CRYGS genes.

Usher Syndrome

Usher syndrome is a rare disorder that is a leading cause of deaf-blindness. It is associated with a mutation in any one of 10 candidate genes. Usher syndrome is incurable at present. In this disease, deafness is associated with a defective inner ear, and the vision loss is associated with retinitis pigmentosa (RP), a degeneration of the retinal cells. Usher syndrome has three clinical subtypes, denoted as I, II and III in decreasing order of severity. People with Usher I ( MIM: 276900) are born profoundly deaf, begin to lose their vision in the first decade of life and have problems with balance in movement. Different subtypes of Usher I can be caused by mutations in any one of several different genes that function in the development and maintenance of inner ear structures and transmit sound and motion signals to the brain eg: CDH23, MYO7A, PCDH15, USH1C, and USH1G. People with Usher II are also born deaf, but do not seem to have noticeable problems with balance; they also begin to lose their vision later. Different subtypes of Usher syndrome type II (MIM: Not available) may be caused by mutations in any of three different genes: USH2A, GPR98 and DFNB31. People with Usher syndrome III are not born deaf, but experience a gradual loss of their hearing and vision; they may or may not have balance difficulties. Mutations in only one gene, the CLRN1 gene, have been linked to Usher syndrome type III ( MIM: 276902).  In India, Usher syndrome is the second most common cause of deafness.

            Considering the studies done in India to date, this database presently holds the relevant data for Usher I only caused by mutations in MYO7A.

Oculocutaneous Albinism

Oculocutaneous albinism (OCA) is a heterogeneous group of congenital disorders characterized by hypopigmentation of skin, hair, and eyes associated with other developmental ocular defects. OCA patients are often considered as legally blind and in India, it is one of the 4 major causes of childhood blindness. Long-term sun exposure greatly increases the risk of skin damage and an aggressive form of skin cancer called melanoma, in people with this condition. Mutations in genes regulating the multistep process of melanin biosynthesis cause this spectrum of disorders. The four classical types of oculocutaneous albinism are designated as OCA1 (OCA1A: MIM: 203100 and OCA1B: MIM: 606952), OCA2 ( MIM: 203200), OCA3 ( MIM: 203290) and OCA4 (MIM: 606574) caused by TYR, OCA2, TYRP1 and SLC45A2 genes, respectively. OCA1 is characterized by white hair, very pale skin, and light-colored irises. OCA2 and OCA4 have overlapping clinical features (a creamy white color and hair may be light yellow, blond, or light brown) that are generally less severe than OCA1. OCA3 affected individuals have reddish-brown skin, ginger or red hair and hazel or brown irises with milder vision abnormalities than the other forms of OCA. Overall, an estimated 1 in 20,000-30,000 people worldwide are born with oculocutaneous albinism however, the frequency differs from country to country. In India, OCA1 is the most prevalent type of OCA followed by OCA2.

Microspheropakia ( MIM: 251750)

This is a congenital, apparently autosomal recessive, usually bilateral, condition in which the crystalline lens is smaller than normal and spherical in shape. It may give rise to lenticular myopia, subluxation or glaucoma. It may occur independently or it may be associated with the Weill-Marchesani syndrome or more rarely with Marfan's syndrome, Peter's anomaly or congenital rubella. Microspherophakia can be caused by mutations in the LTBP2 gene.

Leber s congenital amaurosis ( MIM: 204100 )

Leber's congenital amaurosis is a rare inherited eye disease that appears either at birth or in the first few months of life, and affects around 1 in 80,000 of the population, typically characterized by nystagmus, sluggish or no pupillary responses, and severe vision loss or blindness. LCA is an autosomal recessive disorder thought to be caused by abnormal development of photoreceptor cells. OMIM currently recognizes 11 types of LCA, of which presently this database contains information for LCA type 2 and LCA type 5 caused by the mutations in the RPE65 and LCA5 genes.

Gelatinous drop-like corneal dystrophy ( MIM: 204870 )

 Gelatinous drop-like corneal dystrophy is an autosomal recessive disorder caused by homozygous or compound heterozygous mutation in the M1S1 gene. It is characterized by severe corneal amyloidosis. The disease clinically manifests in the first decade of life and gradually progresses leading to severely impaired visual acuity by the third decade.

  Lattice corneal dystrophy Type I  ( MIM: 122200)

Lattice corneal dystrophy, type I is an autosomal dominant eye disorder caused by mutation in the gene encoding keratoepithelin (TGFBI). It is characterized by grayish cotton threads-like lines mainly limited to a zone between the center of the cornea and the periphery, usually not extending to the limbus. The intervening cornea is relatively clear. The disease appears in adolescence and progresses to severe visual impairment by the fifth or sixth decade.

  Keratoconus  ( MIM: 605020 )

 Keratoconus is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve, thereby causing substantial distortion of vision with multiple images, streaking and sensitivity to light all often reported by the patient. It is generally diagnosed in the patient's adolescent years and attains its most severe state in the twenties and thirties. The incidence of keratoconus is 1 in 2,000 in the general population. Keratoconus is associated with mutations in the VSX1 gene and is inherited in both autosomal dominant and recssive modes.

  Granular corneal dystrophy Type I  ( MIM: 121900 )

Granular corneal dystrophy, type I is an autosomal dominant eye disorder caused by heterozygous mutation in the gene encoding keratoepithelin (TGFBI). There is corneal opacity in a disc-shaped area in the center, but the peripheral cornea is usually clear and the intervening cornea between granules is clear.

  Leber congenital amaurosis 2 ( MIM: 204100)

Leber congenital amaurosis 2 is an autosomal recessive early-onset eye disorder caused by mutations in the RPE65 gene. Patients suffer from retinal dystrophy characterized by profound vision loss, nystagmus, severe retinal dysfunction and other associated eye features.

  Macular Corneal Dystrophy ( MIM: 217800)

Macular corneal dystrophy is an autosomal recessive disorder affecting the cornea, caused by mutations in the CHST6 gene (MIM: 605294). It is a progressive disorder characterized by minute, gray, punctate corneal opacities with reduction of corneal sensitivity. Most patients suffer from painful attacks with photophobia, foreign body sensations and recurrent erosions.