Ashkenazi Jewish population more diverse than people of European descent

WASHINGTON - After conducting a genomic analysis, researchers have shown that the Ashkenazi Jewish population is genetically more diverse than people of European descent, despite previous assumptions that Ashkenazi Jews have been an isolated population.

Apart from this, analyses of disease-related genes of higher prevalence in the Ashkenazi Jewish population indicate that only a minority of traits show signs of positive selection, suggesting that most have arisen through random genetic drift, revealed researchers at Emory University School of Medicine.

Investigators in the laboratory of Dr. Stephen Warren, chairman of human genetics at Emory University School of Medicine, used DNA microarray technology to read variant sites across the entire genomes of 471 Ashkenazi Jews.

The work comes from a collaboration between Warren and Ann Pulver, ScD, associate professor of psychiatry and behavioral sciences at Johns Hopkins University School of Medicine, who recruited the participants for a study of schizophrenia genetics.

Researchers looked for close to one million single nucleotide polymorphisms (SNPs): common alternative spellings in the genome, analogous to American and British spellings of words such as organize/organise.

One measure of genetic diversity in a population is heterozygosity, or how many of the SNPs inherited from the mother and father are different; a more inbred population has less heterozygosity.

“We were surprised to find evidence that Ashkenazi Jews have higher heterozygosity than Europeans, contradicting the widely-held presumption that they have been a
largely isolated group,” said first author Dr. Steven Bray, a postdoctoral fellow in Warren’s laboratory.

The researchers went on to measure linkage disequilibrium, a measure of how “chunky” a population’s genomes are.

If two individuals from separated groups have children, their descendants’ genomes are shuffled by recombination.

Successive generations continue the shuffling process, so that the linkage between traits located near each other in the genome is gradually lost over time.

High linkage disequilibrium can come either from an isolated population (for example, an island whose residents are all descendents of shipwreck survivors) or the relatively recent mixture of separate populations.

The researchers found evidence of elevated linkage disequilibrium in the Ashkenazi Jewish population, but were able to show that this matches signs of interbreeding or “admixture” between Middle Eastern and European populations.

The researchers could estimate that between 35 and 55 percent of the modern Ashkenazi genome comes from European descent.

“Our study represents the largest cohort of Ashkenazi Jews examined to date with such a high density of genetic markers, and our estimate of admixture is considerably higher than previous estimates that used the Y chromosome to calculate European admixture at between five and 23 percent,” said Bray.

The genomic analysis also provided information about selection pressures on mutations prevalent in the Ashkenazi Jewish population, such as those leading to conditions like Tay-Sachs disease or mutations in cancer susceptibility genes like BRCA1.

“Only six of the 21 disease genes that we examined showed evidence of selection. This supports the argument that most of the Ashkenazi-prevalent diseases are not generally being selected for, but instead are likely a result of a genetic bottleneck effect, followed by random drift,” said Bray.

The results of the study are published in the early edition of the Proceedings of the National Academy of Sciences. (ANI)