Mechanism and genotype-phenotype correlation of two proximal 6q deletions characterized using mBAND, FISH, array CGH, and DNA sequencing.

Proximal 6q deletions have a milder phenotype than middle and distal 6q deletions. We describe 2 patients with non-overlapping deletions of about 15 and 19 Mb, respectively, which subdivide the proximal 6q region into 2 parts. The aberrations were identified using karyotyping and analysed using mBAND and array CGH. The unaffected mother of the first patient carried a mosaic karyotype with the deletion in all metaphases analysed and a small supernumerary marker formed by the deleted material in about 77% of cells. Her chromosome 6 centromeric signal was split between the deleted chromosome and the marker, suggesting that this deletion arose through the centromere fission mechanism. In this family the location of the proximal breakpoint in the centromere prevented cloning of the deletion junction, but the junction of the more distal deletion in the second patient was cloned and sequenced. This analysis showed that the latter aberration was most likely caused by non-homologous end joining. The second patient also had a remarkably more severe phenotype which could indicate a partial overlap of his deletion with the middle 6q interval. The phenotypes of both patients could be partly correlated with the gene content of their deletions and with phenotypes of other published patients.