Spontaneous and induced chromosomal breakage is an important cellular feature of Fanconi anemia (FA), an inherited DNA repair disorder characterized by progressive bone marrow failure, developmental abnormalities, and predisposition to leukemia. We have previously reported that in comparison to normal cells, there is a substantial increase in frequency of intragenic deletions at an endogenous locus (HPRT) in FA lymphoblasts. Taken together with the increased chromosomal instability, these observations indicated that the wild-type FA gene(s) plays an important role in the maintenance of the genomic integrity. To obtain information on the mechanism(s) underlying the genomic rearrangements in FA, the breakpoint sites of deletions in 11 FA-derived HPRT- mutants were analyzed. The results indicate that a significant proportion of deletions involving a loss of a given exon are identical and that two deletions of different size have the same 3' breakpoint. Interestingly, it appears that in most of the mutants there is a common deletion signal sequence, which suggests that the mutations in the FA gene(s) may lead to an aberrant site-specific cleavage activity that might be responsible for the deletion proneness and the chromosomal instability characteristic of the FA pathology. From the similarity or even identity of the signal sequence at some of the breakpoints with the consensus heptamer which directs cleavage and joining in the assembly of immunoglobulin and T-cell receptor genes, we speculate that steps in common with the V(D)J recombinational process may be illegitimately involved in FA cells
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