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When a person goes for medical surgery in Indian hospitals, doctors could often ask two questions — is the person from the Vaishya community and is he/she allergic to Sulpha drugs.
While the second issue of allergic to drugs is under control, the known problems faced by Vaishyas, a trade and business community to anaesthesia, persist. Numbering around three millions, they are pre-dominantly in the South and are known to have traumatic response to muscle relaxants like Succinylcholine and Mivacurium, which are injected prior to surgery as anaesthesia.
Genetic variation
The reason for this, according to scientists, is that the genes responsible for producing enzymes which metabolise these anaesthesia drugs are hardly produced in people of this community. Therefore, they have to be administered a different set of drugs to facilitate surgery. The occurrence of such rare problems is due to the very little genetic variation over generations among the communities.
Terming them ‘recessive diseases’ (which is passed down families through generations), a study published on July 17 in ‘Nature Genetics’, goes on to describe how the 1.5 billion people of South Asia are particularly vulnerable to such rare genetic diseases. The study was led by co-senior authors Scientist Kumarasamy Thangaraj of CSIR-Centre for Cellular and Molecular Biology (CCMB), Hyderabad and David Reich of Harvard Medical School and the Broad Institute, Boston and Cambridge, USA, in collaboration with colleagues from many other institutes.
South Asia is inhabited by about 5,000 anthropologically well-defined populations, many of which are endogamous (marrying within a population) communities with significant barriers to gene flow due to sociological, linguistic and cultural factors that restrict inter-population marriage. Several diseases that are specific to these populations have been identified in the past. However, the genetic causes for the vast majority of population-specific disease are largely unknown.
Recessive diseases
Each person carries several mutations, which if found in two copies, would lead to serious recessive diseases. In South Asia, ‘founder events’, small numbers of ancestors carrying such mutations gave rise to large number of descendants. This in combination with endogamy causes these mutations to often be carried in two copies, leading to a higher rate of population specific diseases, explained Thangaraj.
The present study has been able to throw light on some of these trends. “We have analysed samples from more than 2,800 individuals from over 275 distinct South Asian populations who belong to various social and linguistic groups from India, Pakistan, Nepal, Sri Lanka, and Bangladesh using about 600,000 genome-wide markers”, Thangaraj.
The authors devised an algorithm to quantify the measure of the ‘founder events’ in each group based on Identity-by-Descent (IBD) segments, large stretches of DNA shared from a common founder in the last approximately one hundred generations.
The study analysed genetic data of Gujjars, Baniyas, Rajus, Reddys, Kapus to identify gene mutations and the possible susceptibility to certain genetic diseases in India.
“We found that 81 out of 263 unique South Asian groups, including 14 groups with estimated census sizes of over a million, have a strong founder event than the one that occurred in both Finns and Ashkenazi Jews in the West – these are founder groups known to have large numbers of recessive diseases”, said Reich.
He further said that this source of risk for recessive diseases is very different from that due to marriages among close relatives (consanguineous marriages), which is also a major cause of recessive disease in South Asia.
Personalised medicine
Explaining the significance of the study findings Rakesh K Mishra, Director of CCMB said it would lead to a paradigm shift, facilitating and accelerating predictive and personalised medicine.
A practical fallout of these findings is the development of the approach called Dor Yeshorim, a community genetic testing program among Orthodox Ashkenazi Jews, which screens students for common recessive disease causing mutations and enters the results into a confidential database. Matchmakers query the database whether the potential couple is “incompatible” in the sense of both being carriers for a recessive mutation at the same gene, said Reich.
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