Genetic Isolation in Casteist India Could Could Render Some People More Vulnerable to Disease

There is reduced genetic variation among the people of some subpopulations because they have been genetically isolated due to various factors – such as caste.

The occurrence of genetic diseases in certain subpopulations in India and other countries in South Asia is well known. Indian scientists now suspect that this could be due to genetic isolation caused by endogamous marriages over generations.

Endogamous marriages – which happen when people marry within a subpopulation based on caste, clan (gotra), language or culture – lead to reduced genetic variations. These are different from marriages among close relatives, known as consanguineous marriages, a practice also prevalent in parts of the country.  

In genetics, when a small number of ancestors give rise to many descendants, it known as a founder event or, alternatively, a population bottleneck. A study of anthropologically different subpopulations in South Asia has revealed that many of them are a result of strong founder events. In each of these groups, large stretches of the DNA seem to have originated from a common ancestor in the last 100 generations or so.

There is reduced genetic variation among the people of these subpopulations because they subpopulations have been genetically isolated due to various factors – including caste. Such populations are vulnerable to recessive genetic diseases, diseases in which an offspring gets disease-causing genes from both parents. This risk, researchers say, is very different from that due to marriages among close relatives.

The study, led by scientists at the Hyderabad-based Centre for Cellular and Molecular Biology (CCMB), appeared in the journalNature Genetics on July 18. Scientists analysed samples from over 2,800 individuals from over 275 distinct South Asian populations belonging to various social and linguistic groups, across India, Pakistan, Nepal, Sri Lanka and Bangladesh. They developed an algorithm to quantify the impact of founder events in each group based on stretches in DNA received from a common ‘founder’.

“We found that 81 out of 263 unique South Asian groups, including 14 groups with estimated census sizes of over a million, have had a strong founder event,” said Kumarasamy Thangaraj, who led the study along with David Reich of Harvard Medical School. These large population groups with founder events include the Gujjar (from Jammu & Kashmir), Baniya (Uttar Pradesh), Pattapu kapu (Andhra Pradesh), Vadde (Andhra Pradesh), Yadav (Puducherry), Kashtriya Aqnikula (Andhra Pradesh), Naga (Nagaland), Kumhar (Uttar Pradesh), Reddy (Telangana), Kallar (Tamil Nadu), Brahmin Manipuri (Manipur), Arunthathiyar (Tamil Nadu) and Vysya (Telangana) communities.

In their paper, the researchers highlighted the problem using the example of the Vysya, a community with more than three million members. The Vysya have about a 100-fold-higher rate of a metabolic disorder called butyrylcholinesterase (BChE) deficiency compared to other groups. Such people are highly sensitive to the anesthesia administered before undergoing surgeries.

“The next step would be to identify specific recessive diseases among various subpopulations and identify the genes responsible for them,” Thangaraj said. The research can potentially have significant public health applications. Once recessive genetic diseases specific to different groups are mapped, preventive steps like prenatal testing, premarital counselling and screening can help decrease their burden in vulnerable communities. Similar steps have been taken, for example, with the Ashkenazi Jews, the Amish and the Sardinians.

The researchers were based out of New York, Boston, Manipal, Bengaluru, Mangalore, Chennai, Noida, Hyderabad, Kolkata and Lucknow. The research was funded by grants from the Department of Science and Technology, the Department of Biotechnology and the Indian Council of Medical Research.