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Cytogenetic, FISH and molecular analysis in the girl with unbalanced t(X ; 6) and her mother presenting t(6 ; 10)

It is well known that unbalanced translocations are mostly familiar, and cytogenetic analysis of parents usually reveal the reciprocal exchange of chromosome segments identified in a given unbalanced rearrangement. Here we report on the girl with unbalanced t(X ; 6) and unusual mosaicism involving abnormalities of chromosomes 6 and 10 in the mother. Our patient is 6-year-old girl with short stature, failure to thrive, moderate mental retardation, deficiency of IgG, mild facial dysmorphism, and no malformation or abnormalities of visceral organs. Cytogenetic analysis was performed on slides obtained by peripheral blood cultures of the proband and her parents. The slides were stained by GTG and RBG banding methods. FISH was carried out using whole chromosome painting, and centromeric probes specific for chromosomes 6 and X (Vysis). In this study we used polymorphic DNA markers to investigate the parental origin of aberrant chromosome. The chromosome analysis of the proband showed a 46, X, der(X)t(X ; 6)(q22 ; p11) karyotype. The derived X was late replicating in all investigated cells with variable spreading of X chromosome inactivation onto translocated 6p. The molecular analysis revealed that the chromosome X involved in the rearrangement is of the paternal origin. The normal karyotype was observed in the father, while the mother presented 46, XX/46, XX, der(10)t(6 ; 10)(P11 ; p11). The mother is a mosaic with unbalanced t(6 ; 10) in 4% of cells. To best of our knowledge, this unusual mosaicism haw not been reported yet. We suggest that chromosome constitution in the mother is due to postzygotic recombination involving chromosome 6 and 10 at S/G2 phase of the cell cycle. Cytogenetic analysis of the proband revealed unbalanced karyotype resulting in a trisomy for the segment 6pter-p11, and loss of the segment Xqter-q22. She had only discrete facial features characteristic of partial trisomy 6p. This mild phenotypic expression is due to X chromosome inactivation spreading onto translocated 6p.

chromosome X; chromosome 6; chromosome translocations; mosaicism; molecular cytogenetics; FISH

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