47,X,idic(Y),inv dup(Y): a non-mosaic case of a phenotypically normal boy with two different Y isochromosomes and neocentromere formation.

A de novo aberrant karyotype with 47 chromosomes including 2 different-sized markers was identified during prenatal diagnosis. Fluorescence in situ hybridization (FISH) with a Y painting probe tagged both marker chromosomes which were supposed to be isochromosomes of the short and the long arm, respectively. A normal boy was born in time who shows normal physical and mental development. To characterize both Y markers in detail, we postnatally FISH-mapped a panel of Y chromosomal probes including SHOX (PAR1), TSPY, DYZ3 (Y centromere), UTY, XKRY, CDY, RBMY, DAZ, DYZ1 (Yq12 heterochromatin), SYBL1 (PAR2), and the human telomeric sequence (TTAGGG)(n). The smaller Y marker turned out to be an isochromosome containing an inverted duplication of the entire short arm, the original Y centromere, and parts of the proximal long arm, including AZFa. The bigger Y marker was an isochromosome of the rest of the Y long arm. Despite a clearly visible primary constriction within one of the DAPI- and DYZ1-positive heterochromatic regions, hybridization of DYZ3 detected no Y-specific alphoid sequences in that constriction. Because of its stable mitotic distribution, a de novo formation of a neocentromere has to be assumed.