Bloom syndrome (BS) is more frequent in the Ashkenazic Jewish population than in any other. There the predominant mutation, referred to as 'blm(Ash),' is a 6-bp deletion and 7-bp insertion at nucleotide position 2281 in the BLM cDNA. Using a convenient PCR assay, we have identified blm(Ash) on 58 of 60 chromosomes transmitted by Ashkenazic parents to persons with BS. In contrast, in 91 unrelated non-Ashkenazic persons with BS whom we examined, blm(Ash was identified only in 5, these coming from Spanish-speaking Christian families from the southwestern United States, Mexico, or El Salvador. These data, along with haplotype analyses, show that blm(Ash) was independently established through a founder effect in Ashkenazic Jews and in immigrants to formerly Spanish colonies. This striking observation underscores the complexity of Jewish history and demonstrates the importance of migration and genetic drift in the formation of human populations.
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