Molecular characterization of inv dup del(8p): analysis of five cases

We analyzed five patients with inverted duplication deletion of 8p [inv dup del(8p)] using fluorescence in situ hybridization (FISH) and short tandem repeat polymorphism (STRP) analysis. In all patients, inv dup del(8p) consisted of a deleted distal segment, an intact in-between segment, and a duplicated proximal segment. In all of them, the proximal breakpoint of the deletion and one of the breakpoints of the duplication were identical, each located at one of the two olfactory receptor gene clusters at 8p23. FISH analysis showed all their mothers to be heterozygous carriers of an 8p23 inversion [inv(8)(p23)]. STRP analysis indicated that the deletions occurred in maternally derived chromosomes. The duplicated segments had two copies of maternal, either heterozygous or homozygous alleles. These findings support and reinforce those in 16 patients with inv dup del(8p) and their parents by Floridia et al. [1996: Am J Hum Genet 58:785-796] and subsequent additional studies of 10 of them by Giglio et al. [2001: Am J Hum Genet 68:874-883]. Based on these findings, we propose a model for the inv dup del(8p) formation. The inverted segment and its normal counterpart in inv(8)(p23) heterozygous carrier mothers form a loop at the pachytene period of meiosis I. Inv dup del(8p) with heterozygous duplication is formed through at least one meiotic recombination within the loop. Inv dup del(8p) with the homozygous duplication arises through two meiotic recombinations on the inv(8)(p23) chromosome (one within the loop and the other between the loop and centromere). Subsequent rescue by eliminating a part of the duplicated segment and a centromere enables formation of viable inv dup del(8p). The frequency of the inv(8)(p23) allele is 39% in a normal Japanese population, comparable to 26% in Europeans Giglio et al. [2001: Am J Hum Genet 68:874-883]. The proposed mechanism of formation of inv dup del(8p) requires two independent events (a recombination within the loop and subsequent rescue), which may explain its rarity.