Characterization of terminal deletions at 7q32 and 22q13.3 healed by de novo telomere addition

We have developed a strategy for the isolation of terminal deletion breakpoints from any chromosome that has been healed by de novo addition of a telomere repeat array. Breakpoints at 7q32 and 22q13.3 have been isolated and characterized in two patients (patients FB336R and AJ). Both truncated chromosomes have been healed by the addition of a novel telomere, with such an addition possibly mediated by the enzyme telomerase. The breakpoint at 7q32 in patient FB336R shows a structure similar to that of breakpoints on other chromosomes that have been healed in this way. However, the breakpoint at 22q13.3 in patient AJ has 10 nucleotides of unknown origin inserted between the sequence unique to chromosome 22q and the start of the telomere repeat array. This unusual structure is suggestive of a multistep healing event resulting in de novo telomere addition at this breakpoint, and possible mechanisms are discussed.