Monday, 28 July 2008
Indian Genome Variation Consortium: Indians are not genetically similar
July 28, 2008
By Syed Akbar
Hyderabad: Contrary to popular belief that Indian are genetically similar despite
their caste, religion and regional affiliations, a major study conducted across the nation by Indian Genome Variation Consortium has revealed that Indians are genetically different and the similarity in Indian populations is limited to certain clusters.
The IGVC carried out the largest-ever exercise in the country to arrive at the genetic affinity of various Indian people based on castes, tribes, religions, regions and customs. As many as 55 diverse endogamous Indian populations were covered under the study.
The groups included 32 large (more than one crore people) and 23 isolated populations representing a large fraction of people in the country.
"We observe high levels of genetic divergence between groups of populations that cluster largely on the basis of ethnicity and language. Indian populations not only overlap with the diversity of HapMap populations, but also contain population groups that are genetically distinct. These data and results are useful for addressing stratification and study design issues in complex traits especially for heterogeneous
populations," Dr Saman Habib, scientist in the Division of Molecular and Structural Biology, Central Drug Research Institute, Lucknow, told this correspondent.
The present study contradicts earlier reports that Indians are genetically similar despite their physical and geographical differences.
A number of research institutes across the country including Lucknow's CDRI participated in the study. Genetically isolated populations are considered to be important in dissecting complex diseases and mapping underlying genes. However, the validation of results across populations has met with limited success. Population stratification, a consequence of differences in allele frequencies across populations arising mainly due to natural selection and genetic drift, is a major problem in association studies.
It is, therefore, important to assess the nature and extent of population stratification in contemporary endogamous populations especially in the context of established or candidate disease genes, she said adding that Indians, comprising about one-sixth of the world population, with large family sizes and high levels of endogamy, provide a unique resource for dissecting complex disease aetiology and pathogenesis.
According to scientists who participated in the study, India provides a large patient pool with the majority being drug naive. Historically, the Indian population is a conglomeration of multiple culture and evolutionary histories.
"Anatomically modern man is estimated to have reached the north- western periphery of the Indian subcontinent around 70,000 years before present and moved southward into Sri Lanka in the next 20,000 years. Modern human communities may also have migrated into eastern India from Myanmar around 4500 to 11,000 ybp. The evolutionary
antiquity of Indian ethnic groups and subsequent migration from central Asia, west Asia and southern China has resulted in a rich tapestry of socio-cultural, linguistic and biological diversity," the study pointed out.
Broadly, Indians belong to Austro-Asiatic, Tibeto-Burman, Indo-European and Dravidian language families. Distinct religious communities, hierarchical castes and subcastes, and isolated tribal groups that comprise the people of India remain largely endogamous. Most of these groups have strict social rules governing mating patterns. Earlier studies using mitochondrial, Y-chromosomal and limited autosomal markers, that primarily addressed issues of origin and migrations, have demonstrated extensive genetic diversity in India In contrast, a recent study based on autosomal microsatellite markers has inferred that Indian populations show low levels of genetic differentiation. This inference was possibly due to biased recruitment
of study participants and insufficient classification based on language and ethnicity.
The representative set of genes included drug-response genes, genes involved in cancer and ageing, eye diseases, allergy and asthma, neuro-psychiatric, metabolic and cardiovascular disorders as well as genes involved in susceptibility to infections.
It is contented that the Dravidian speakers, now geographically confined to southern India, were more widespread throughout India prior to the arrival of the Indo-European speakers. They, possibly after a period of social and genetic admixture with the Indo-Europeans, retreated to southern India, a hypothesis that has been supported by mitochondrial DNA analyses.
"Our results showing genetic heterogeneity among the Dravidian speakers further supports the above hypothesis. The Indo- European speakers also exhibit a similar or higher degree of genetic heterogeneity possibly because of different extents
of admixture with the indigenous populations over different time periods after their entry into India. It is surprising that in spite of such a high levels of admixtures, the contemporary ethnic groups of India still exhibit high levels of genetic differentiation and substructuring," the study revealed.
It further said, "we note that the people of India are referred as ‘Indian’ in many population genetic studies. The implication of such usage is that the Indian population is genetically homogeneous, which, as the results of our study indicate, is evidently not true. However, we have also shown that it is possible to identify large clusters of ethnic groups that have substantial genetic homogeneity".
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