Functional disomies of the X chromosome influence the cell selection and hence the X inactivation pattern in females with balanced X-autosome translocations: a review of 122 cases

Am J Med Genet. 1992 Jan 15;42(2):161-9. doi: 10.1002/ajmg.1320420205.


We reviewed 122 cases of balanced X-autosome translocations in females, with respect to the X inactivation pattern, the position of the X break point and the resulting phenotype. In 77% of the patients the translocated X chromosome was early replicating in all cells analysed. The break points in these cases were distributed all along the X chromosome. Most of these patients were either phenotypically normal or had gonadal dysgenesis, some had single gene disorders, and less than 9% had multiple congenital anomalies and/or mental retardation. In the remaining 23% of the cases the translocated X chromosome was late replicating in a proportion of cells. In these cells only one of the translocation products was reported to replicate late, while the remaining portion of the X chromosome showed the same replication pattern as the homologous part of the active, structurally normal X chromosome. The analysis of DNA methylation in one of these cases confirmed noninactivation of the translocated segment. Consequently, these cells were functionally disomic for a part of the X chromosome. The presence of disomic cells was highly prevalent in translocations with break points at Xp22 and Xq28, even though spreading of X inactivation onto the adjacent autosomal segment was noted in most of these cases. This suggests that selection against cells with a late replicating translocated X is driven predominantly by a functional disomy X, and that the efficiency of this process depends primarily on the position of the X break point, and hence the size of the noninactivated region.(ABSTRACT TRUNCATED AT 250 WORDS)

Publication types

  • Review

MeSH terms

  • Animals
  • Chromosome Aberrations / genetics
  • Chromosome Disorders
  • Chromosome Mapping
  • Diploidy
  • Dosage Compensation, Genetic*
  • Female
  • Humans
  • Phenotype
  • Translocation, Genetic / genetics*
  • X Chromosome*