Fish mapping of a translocation breakpoint at 6q21 (or q22) in a patient with heterotaxia

Heterotaxia is a congenital lateralization defect of visceral organs. As several single-genes that act on the formation of left-right asymmetry during embryogenesis have been identified in animals, a defect in the similar system may play a role in heterotaxia in man. We previously reported a Japanese girl with heterotaxia associated with a de novo balanced translocation (6;18)(q21 or q22;q21.3 or q22). In the present study, based on a hypothesis that one of the putative situs-determining genes is disrupted at a breakpoint of the translocation, we first isolated a yeast artificial chromosome (YAC) clone covering a breakpoint, 6q21 (or q22) of the translocation. Then, using STSs mapped on the YAC, we isolated bacterial artificial chromosome (BAG) clones spanning the breakpoint. FISH analysis using the BAC clones as probes revealed that the breakpoint is confined to a segment between two STS loci, WI-4066 and the CHLC.GATAGB06.192, within a genetic distance of 1.4 cM. The human connexin43 gene was not disrupted in our patient, although mutations of this gene have been reported in patients with complex heart disease and heterotaxia. The molecular localization of the translocation breakpoint in our patient may contribute to the positional cloning of a putative heterotaxia gene.