A RARE 6Q11- CYTOGENETIC ANALYSIS AND IN-SITU HYBRIDIZATION( HETEROMORPHISM )

A rare variant of 6q11+ heteromorphism was found in a fetus and the mo ther during amniocentesis. The G- and Q-banding and DA/DAPI stain were negative. The C-banding was positive and the C-banded segment was 3-f old longer in the variant than in its homologue. Neither of the C-band ed regions of chromosomes 6 decondensed when exposed to distamycin A o r 5-azacytidine. A DNA replication study indicated that the C-banded v ariant was late replicating. The lateral asymmetry observed in the 6q1 1 variant after one replication cycle in 5-bromodeoxyuridine may resul t from an unequal interstrand distribution of thymidine in the repetit ive DNA. Fluorescent in situ hybridization using a chromosome-6-specif ic alpha-satellite probe (D6Z1) demonstrated hybridization signals on the centromere of chromosome 6. The 6q11 variant showed a signal which was 3-fold larger than its homologue. These results indicate that the 6q11 variant is an amplification of a chromosome-6-specific alpha rep eat, and the size of the hybridization signal correlates with the size of the laterally asymmetric and C-banded region.