Molecular Genetic Study of Primary Congenital Glaucoma

Glaucoma is a group of eye disorders that result from the high pressure of circulating fluid (aqueous humor) in the globe, usually higher than the eye can tolerate over a long period of time. This pressure (intraocular pressure) occurs because excessive fluid is formed or because the channel (canal of Schlemm) through which the fluid normally drains is fully or partially blocked. This elevated pressure within the eye can constrict the blood vessels that nourish the sensitive visual structures in the back of the eye leading to the reduction of blood supply and the eventual death of visual cells, resulting in some loss of vision. As this condition progress, more nerve cells are damaged, and the range of vision becomes narrower. This process eventually leads to total blindness. Glaucoma is generally divided into two broad groups, primary and secondary. By definition, primary glaucoma is unrelated to any other ocular disease. Secondary glaucoma occurs as a consequence of another recognizable ocular abnormality or disease, such as intraocular tumor, inflammation, vascular disorder, or injury. Primary glaucoma is further subdivided into several categories such as Adult Glaucoma, Congenital Glaucoma, and Absolute Glaucoma. The Congenital Glaucoma, sometimes defined as glaucoma occurring in individuals under 30 years of age is very rare. Although it is divided into infantile and juvenile types, the age limit and the terminology indicate either congenital origin, or predisposition in this age group. This proposal describes a genetic linkage study of autosomal recessive form of congenital glaucoma known as Primary Congenital Glaucoma (PCG). The purpose of this investigation is to establish the chromosomal location of the gene responsible for this type of pediatric glaucoma. The methods by which this will be achieved is known as positional mapping. For this, one usually proceeds as follow: 1. Families in which the PCG is inherited in their pedigrees are first identified and confirmation of diagnosis for each affected individual is requested from the patient’s Ophthalmologist. 2. Consent for participation of appropriate family members are first requested and their blood samples are then taken. 3. The obtained blood samples are taken to the laboratory and the DNA (DeoxyriboNucleic Acid, the ultimate molecule of life or genetic message) are then extracted by standard techniques. 4. Unlike the classical markers (blood groups, enzymes and proteins) that are known to have a biological functions, most of the DNA markers used in positional mapping are the random pieces of the DNA that has been isolated by a series of molecular biology techniques. Many different types of DNA markers are now identified. These are generally representing regions of the DNA where a particular short sequence of DNA is “repeated” many times. The number of times that each sequence is “repeated” among a population of individuals varies greatly and as a result, these regions produce different types of polymorphisms. In the last 5 years, an enormous amount of DNA polymorphisms have been identified and localized to a specific region of a chromosome. A comprehensive map of these DNA markers for all human chromosomes have recently been published in the SCIENCE and NATURE. DNA’s from all the family members are usually genotyped at these polymorphic sites by a technique known as Polymerase Chain Reaction (PCR). With this technique a direct amplification of the region containing t he sequence “repeats” is obtained. 5. The resultant genotypes are transferred to each pedigree and the likelihood (odds) for co-inheritance (or Linkage) of the PCG gene with a particular type of the DNA marker is then estimated by the LOO score method. This term refers to the natural log of likelihood of linkage versus no linkage. If the value of the LOD score is > 3 (likelihood of 1000:1 for linkage of PCG to the DNA marker), then the two loci are accepted to be genetically linked. In this case, the gene for PCG will be on the same region of that particular chromosome where the DNA marker was originally selected from. 6. When the chromosomal location of the disease gene is identified, it would be possible to saturate the map of that region by using additional DNA markers. Once two such markers flanking the PCG locus are recognized, they can be used to determine whether ALL the families are linked to the same genetic markers. If not, the PCG may have resulted from different mutations in single (allelic heterogeneity) or multiple (genetic heterogeneity) genes. 7. The obtained information on the map location of the PCG locus can be used to develop new molecular genetic methods for prenatal, presymptomatic and early diagnosis of affected individuals in the families. A more comprehensive risk prediction for families can then be provided to the genetic counselling professionals. The ultimate goal of this proposal is the isolation of the PCG gene and characterization of the genetic defects causing this condition. This should help us to identify the resultant abnormal protein product which in turn provides valuable new insights into the pathogenesis of the PCG. The information of this kind can lead to the development of new drug therapy which can provide an efficient treatment for the affected individuals. The ultimate beneficiary of this kind of research are our patients population who will have a better chance of retaining their vision and have a normal life not only as a child but also as an adult.