AT-rich palindromes mediate the constitutional t(11;22) translocation

The constitutional t(11;22) translocation is the only known recurrent non-R obertsonian translocation in humans. Offspring are susceptible to der(22) s yndrome, a severe congenital anomaly disorder caused by 3:1 meiotic nondisj unction events. We previously localized the t(11;22) translocation breakpoi nt to a region on 22q11 within a low-copy repeat termed "LCR22" and within an AT-rich repeat on 11q23. The LCR22s are implicated in mediating differen t rearrangements on 22q11, leading to velocardiofacial syndrome/DiGeorge sy ndrome and cat-eye syndrome by homologous recombination mechanisms. The LCR 22s contain AT-rich repetitive sequences, suggesting that such repeats may mediate the t(11;22) translocation. To determine the molecular basis of the translocation, we cloned and sequenced the t(11;22) breakpoint in the deri vative 11 and 22 chromosomes in 13 unrelated carriers, including two de nov o cases and der(22) syndrome offspring. We found that, in all cases examine d, the reciprocal exchange occurred between similar AT-rich repeats on both chromosomes 11q23 and 22q11. To understand the mechanism, we examined the sequence of the breakpoint intervals in the derivative chromosomes and comp ared this with the deduced normal chromosomal sequence. A palindromic AT-ri ch sequence with a near-perfect hairpin could form, by intrastrand base-pai ring, on the parental chromosomes. The sequence of the breakpoint junction in both derivatives indicates that the exchange events occurred at the cent er of symmetry of the palindromes, and this resulted in small, overlapping staggered deletions in this region among the different carriers. On the bas is of previous studies performed in diverse organisms, we hypothesize that double-strand breaks may occur in the center of the palindrome, the tip of the putative hairpin, leading to illegitimate recombination events between similar AT-rich sequences on chromosomes 11 and 22, resulting in deletions and loss of the palindrome, which then could stabilize the DNA structure.