Cloning of a balanced translocation breakpoint mapping in the DiGeorge syndrome critical region

DiGeorge syndrome (DGS) is a developmental defect of thymus, parathyroids and heart, which is associated with microdeletions in chromosomal region 22q11.2. A detailed physical map of the region has been established and a shortest region of overlap based on deletions and unbalanced translocations giving rise to DGS has been derived. Moreover, the breakpoint of a balanced translocation borne by a DGS patient has been localized in that critical region; thereby suggesting that the translocation breakpoint interrupts the or one of the major gene(s) implicated in DGS. We have initiated a chromosome walk by establishing cosmid contigs from probes distally flanking the breakpoint. One contig covers 150 kb of genomic DNA and a second one spans 350 kb in the region and contains the balanced translocation breakpoint. Phylogenetically conserved sequences are being searched for in the vicinity of the breakpoint to be used as probes in order to isolate cDNAs.