The new genome era is characterised by the flood of information that is being generated in terms of genes and gene sequences, and increasingly, the definition and the approach to disease as primarily genetic. The article on Genes and Diabetic Retinopathy by Venkatesan Radha et al in this issue highlights an area which is important in more than one way. Diabetes and its complications represent public health problems of considerable magnitude in many parts of the world and probably are now assuming greater proportions in India as well. Genetic factors have been linked to diabetes susceptibility as well as to diabetic complications, in an effort to better understand the disease.
Diabetic retinopathy exemplifies a broad category of diseases that are known as complex or multifactorial diseases (examples are age-related ocular diseases such as cataract, glaucoma and macular degeneration, heart disease, cancer, Alzheimers, etc.) that have come under genetic scrutiny in recent times. As the term suggests, numerous factors both genetic and non-genetic, contribute to their aetiology. They are thus different with respect to the nature and magnitude of the underlying genetic causes when compared to the simple Mendelian diseases (such as retinitis pigmentosa, albinism, etc). Advances made from recent molecular genetics and the Human Genome Project facilitate the study of genetics of complex diseases. One of the challenges of the new era is the determination of the genetic components of complex diseases. The study of complex disorders is expected to have a greater impact on medical practice and on human health since these diseases are fairly common, unlike the rare Mendelian diseases. The contributions of genes to a complex disease may be in the form of mutations in more than one gene that determine the development of disease, or of variations (polymorphisms) in multiple genes that contribute to a person’s susceptibility to disease or determine interaction with the environment. Such subtle and complex patterns of genetic variations can be tracked with suitable ’markers’ that consist of variations in DNA sequence that differ between populations and between groups of individuals. Most notably, the discovery of Single Nucleotide Polymorphisms (SNPs), which represent variations in a single base, occurring once in every thousand bases, has opened up an enormous potential to study genetic determinants with respect to population as well as disease.
What are the possible uses of identifying genes in association with an increasing number of diseases? The objectives of the Human Genome Project explicitly state that this knowledge will be used for the common good and in particular, for the cause of human health and disease. Very broadly, such knowledge could help in early presymptomatic diagnosis, screening and detection of carriers, genetic counselling, developing correlations between genotype and phenotype that might possibly be used to predict prognosis of a disease, developing novel therapeutic agents either by means of conventional or gene therapy, and providing a better understanding of the pathology and physiology of the organ. While there are undoubted advantages to the information offered by genetic studies, there are numerous obstacles that need to be overcome before it can be envisaged as a part of routine practice. For example, presymptomatic diagnosis can be very valuable in diseases such as adult-onset glaucoma where early detection is the key. However, genetic screening is rendered cumbersome and impracticable in a number of diseases due to an extreme degree of genetic heterogeneity (this is true with most ocular disease). One has to await newer and better technologies that are also cost-effective enough to apply in routine screening.
As a result of the rapid identification of genes, there exists a gap between our knowledge of the gene sequence and its function. In order to completely understand the role of a given gene in disease, and to develop a new therapy, its function in the diseased organ has to be understood. An added goal of the HGP is to carry out large-scale studies to determine functions of genes (functional genomics). These studies will provide numerous valuable insights into our understanding of disease, and in turn open up avenues for developing therapeutic agents. Gene therapy is not on the immediate horizon for many ocular disorders and it is necessary to make improvements in the gene delivery systems before it can become practicable. A very promising recent case of success of gene therapy in a canine model of Leber congenital amaurosis has raised our hopes in this direction. We will be able to gauge the full impact of the human genome project on medicine in the near future.
Source of Support:
Conflict of Interest:
1. Venkatesan R, Mohan R, Viswanathan M. Genes and Diabetic retinopathy Indian J Ophthalmol. 2002;50:5–11
2. Website for the document on the HGP “Understanding Our Genetic Inheritance:The U.S. Human Genome Project. The First Five Years FY 1991-1995. Available at: http://www.nhgri.nih.gov/HGP/HGP_goals/5yrplan.html
3. Acland G.M, Aguirre G D, Ray J, Zhang Q, Aleman T S, Cideciyan A V, et al Gene therapy restores vision in a canine model of childhood blindness Nat Genet. 2001;28:92–95