May 21, 2012 at 11:58am IST

Consanguinity a factor in genetic eye disorders

KOCHI: “India is ideal for studying eye diseases caused by genetic disorders,” said Dr Mike Denton, the scientist who identified the gene used in the first successful gene therapy for genetic eye diseases. The doctor, during a visit to the city with his partner Dr Govindasamy Kumaramanickavel for the development of a genetic study centre at the Tony’s Eye Hospital in Aluva, spoke to the Express about the country’s vulnerability to genetic eye disorders due to high rates of consanguinity.
The incidence of genetic diseases is high in the case of the eyes because it a complicated organ that has a combination of a large variety of genes, Dr Mike said. “This makes it ideal for genetic studies and identifying the problems,” said the doctor, who had in the 80’s and 90’s led a research project aimed at finding the genes responsible for eye diseases by studying families suitable for genetic analysis from various parts of the Indian subcontinent.
“In genetic diseases like Retinitis Pigmentosa where patients lose complete eyesight, beginning with symptoms like night blindness, a survey has shown an incidence of about 10 times that of the global prevalence. Global RP is approximately one in every 4,000, but in some villages in Tamil Nadu, it was around one in 396,” said Dr Kumaramanickavel, who has been working on ocular genomics - primarily involved in gene mapping, mutational screening and association studiess.
There has been no comprehensive study in India to learn the prevalence of these diseases. “In India, around three to four patients come daily with diseases like RP, while in the west it is once in months. It is higher in South India than the North. In certain rural parts of Andhra Pradesh, one in two marriages are consanguineous.
In consanguinity, the property stays within the family, the danger of abuse is less between relatives and the family ties get sharpened. But then these diseases occur,” said Dr Kumaramanickavel.
The higher the degree of consanguinity, the lower the danger of disorders, said Dr Denton. First degree consanguinity is between parents and children or between siblings. The second degree is between an uncle and nieces, which is prevalent South India. Third degree consanguinity is between cousins. Marriage between community members or within a caste can be considered consanguineous, but the danger is less, said the doctor.
There are about 20,000 gene-related eye diseases. The cure for many of these are in its infancy. Identification and gene mapping will be helpful here. “India has about 20,000 ophthalmic doctors. Conducting simple screening tests for all the patients in India will be quite impossible at this rate. So if there are centres where these diseases can be identified and nurses or gene counsellors can advise the people, many patients can be warned to take steps to delay the onset of these diseases. Some tests can be done at home,”  the doctor said.