Bloom's syndrome. XIX. Cytogenetic and population evidence for genetic heterogeneity

James German, Nathan A. Ellis, Maria Proytcheva

Research output: Contribution to journalArticlepeer-review

24 Scopus citations


Cells with abnormally high rates of sister-chromatid exchange (SCE) are uniquely characteristic of Bloom's syndrome (BS). However, in one in five persons a minor population of cells with a low-SCE phenotype circulates in the blood. The origin and significance of the low-SCE cells in BS have never been understood, although they are assumed to arise by somatic mutation. In the present investigation, the enigmatic high-SCE/low-SCE mosaicism was investigated by comparing the incidence in several subpopulations of persons in the Bloom's Syndrome Registry who exhibit the two types of cells, and a striking negative correlation emerged: in persons with BS whose parents share a common ancestor, the case in approximately half of registered persons, low-SCE cells are found only rarely; conversely, the mosaicism occurs almost exclusively in persons with BS whose parents are not known to share a common ancestor. Because those who share a common ancestor are predominantly homozygous-by-descent at the mutated BS locus, the negative correlation is interpreted to mean that the emergence of low-SCE cells in BS in some way depends on the pre-existence of compound heterozygosity. A corollary to this is that BS is genetically heterogeneous.

Original languageEnglish (US)
Pages (from-to)223-231
Number of pages9
JournalClinical Genetics
Issue number5
StatePublished - May 1996


  • Bloom's syndrome
  • Compound heterozygosity
  • Consanguinity
  • Genetic heterogeneity
  • Mosaicism
  • Registries
  • Sister-chromatid exchange (SCE)

ASJC Scopus subject areas

  • Genetics
  • Genetics(clinical)


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