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Human Genome Project and the eye

From The Hindu
July 20, 2000

HISTORY WAS made in biology when the rough draft of the human genome was released on 26th June 2000, three years ahead of schedule. In 1990, with advanced knowledge in genetics many scientists realised that a map and sequence of the human genome had to be initiated. This is equivalent to the quest and challenge for mapping the world or the human body (anatomy), during different stages of human history.

There are approximately three trillion cells in our body and each cell carries the hereditary information (the genome) in the DNA (deoxy ribonucleic acid). Computer digitizes and stores information, likewise DNA does the same in the form of four chemicals - adenine (A), thymine (T), guanine (G) and cytosine (C). English alphabets in different combinations produce various words, similarly different combinations of four DNA alphabets result in genes.

This four-alphabet language has a billion letter script in each cell and this dictates what should happen to us biologically, right from our conception till death. 'Biological syntax' error in the DNA results in many inherited genetic disorders, like retinitis pigmentosa - a night blindness disease.

This 3 billion-letter is packed in chromosomes of each cell nucleus and humans have 46 of them. Just as you pass through several miles of empty space in a highway to reach a city, you have to pass through vast `Junk' DNA regions to identify a gene. About 97 per cent of human genome is 'Junk' and the remaining 3% has 50,000 genes and presently we know more than 10,000 genetic diseases. Our (biological) life sentence is written in a four- letter DNA language that predetermines whether we should get cataract or cancer!

Mapping genes causing blindness was of vital importance to the HGP. About 1000 inherited eye diseases affect both the anterior compartment comprising of cornea, iris and lens and the posterior comprising of vitreous and retina. In landmark events, Ted Dryja at Harvard and Ed Stone at the University of Iowa in the US identified the first retinitis pigmentosa and glaucoma genes, respectively. More than 15 genes causing cataract at the time of birth (congenital) have been identified.

Presently 122 genes causing inherited retinal diseases have been mapped and 30 of them cause retinitis pigmentosa. The retinal genetic diseases could either affect the photoreceptor cones, in the central or the photoreceptor rods, in the peripheral retina. In retinitis pigmentosa (RP), patients complain of night blindness and loss of peripheral vision as rods are affected, whereas in macular degeneration they complain of loss of central vision and acuity (seeing objects clearly and reading ability) as cones are affected. Usher syndrome and Laurence-Moon-Bardet-Biedl syndrome genes have also been identified.

Sankara Nethralaya in this international collaborative HGP are responsible in mapping 7 eye-related genes: 5 RP genes, one stationary night blindness gene and an eye developmental gene. Currently we are also screening a gene for RP (RPE65 gene) to see if these patients would be benefited by vitamin A therapy. Besides we are also studying the genetics of age-related cataract, diabetic retinopathy and angle-closure glaucoma in the Indian population. Cataract caused in old age is a much larger problem than congenital cataract; there are four million people who develop cataract in India each year and equal number of surgeries are performed.

All this understanding could help us to forewarn and counsel these patients and families or plan some treatment strategy or delay the disease processes in the eye. The HGP is not devoid of controversy, Head of the HGP, Dr. Francis Collins and Head of the Celera - a private genome company, Dr. Craig Venter - two legends in this project, detest each other and are polarizing opinions regarding public accessibility of this knowledge and patenting it.

A new era has dawned and practice of medicine is never going to be the same. The way sterilization and antibiotics changed the medical scenario - carrier testing, pre-implantation, prenatal, pre-symptomatic diagnoses, i.e., predictive medicine, preventive strategies, gene therapy and molecular surgery would be the way medicine would be practiced in the near future. A new age would emerge soon, where instead of the genes deciding our fate, we would decide the fate of our genes!

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