THE 2ND CASE OF A T(17-22) IN A FAMILY WITH NEUROFIBROMATOSIS TYPE-1 - SEQUENCE-ANALYSIS OF THE BREAKPOINT REGIONS

A reciprocal t(17;22)(q11.2;q11.2) was found in a female patient with neurofibromatosis type 1 (NF1) and in her affected daughter. Sequence analysis of cloned junction fragments traversing the breakpoints allow ed the identification of the structures involved in the rearrangement. Aberrant bands in Southern hybridizations of restriction enzyme-diges ted DNA of the patient pointed to the disruption of the NF1 gene in in tron 31. Semispecific polymerase chain reaction analysis of the genomi c DNA of the patient with the specific primer anchored at NF1 exon 31 was used to obtain the breakpoint-spanning fragment of the derivative chromosome 17. The intron 31 sequence turned out to be interrupted wit hin a large irregular (AT) repeat. The chromosome 22-derived sequence of the der(17) junction fragment allowed us to identify cosmids of the corresponding region from a chromosome 22-specific cosmid library. Wi th the support of the break-point-spanning cosmids, the chromosome 22 region upstream of the fragment carried by the der(17) was characteriz ed. Primers deduced from the sequence of this up-stream region were us ed in combination with a primer in NF1 intron 31 distal to the breakpo int on chromosome 17 to amplify the der(22) junction fragment. The str ucture of the junction sequences suggested that the translocation had arisen by unequal homologous recombination between (AT)-rich repeats o n chromosome 22 and on chromosome 17 in intron 31 of the NF1 gene. How ever, our data support the assumption of additional rearrangements pri or to, or in the course of, the recombination event, leading to a loss of the sequences between the involved (AT) repeats on chromosome 22. In the direct vicinity of these (AT) repeats, two members of a previou sly undescribed low-copy repetitive sequence have been found, copies o f which are also present on human chromosome 13.