Molecular characterisation of a mosaicism with a complex chromosome rearrangement:: evidence for coincident chromosome healing by telomere capture and neo-telomere formation

Background: Broken chromosomes must acquire new telomeric "caps'' to be structurally stable. Chromosome healing can be mediated either by telomerase through neo-telomere synthesis or by telomere capture. Aim: To unravel the mechanism(s) generating complex chromosomal mosaicisms and healing broken chromosomes. Methods: G banding, array comparative genomic hybridization (aCGH), fluorescence in-situ hybridisation (FISH) and short tandem repeat analysis (STR) was performed on a girl presenting with mental retardation, facial dysmorphism, urogenital malformations and limb anomalies carrying a complex chromosomal mosaicism. Results & discussion: The karyotype showed a de novo chromosome rearrangement with two cell lines: one cell line with a deletion 9pter and one cell line carrying an inverted duplication 9p and a non-reciprocal translocation 5pter fragment. aCGH, FISH and STR analysis enabled the deduction of the most likely sequence of events generating this complex mosaic. During embryogenesis, a double-strand break occurred on the paternal chromosome 9. Following mitotic separation of both broken sister chromatids, one acquired a telomere via neo-telomere formation, while the other generated a dicentric chromosome which underwent breakage during anaphase, giving rise to the del inv dup(9) that was subsequently healed by chromosome 5 telomere capture. Conclusion: Broken chromosomes can coincidently be rescued by both telomere capture and neo-telomere synthesis.