Evidence of a mechanism for isodicentric chromosome Y formation in a 45,X/46,X,idic(Y)(p11.31)/46,X,del(Y)(p11.31) mosaic karyotype.

Abnormalities involving sex chromosomes account for approximately 0.5% of live births. The phenotypes of individuals with mosaic cell lines having structural aberrations of the X and Y chromosomes are variable and hard to accurately predict. Phenotypes associated with sex chromosome mosaicism range from Turner syndrome to males with infertility, and often present with ambiguous genitalia. Previous studies of individuals with an 45,X/46,X,idic(Y)(p11) karyotype suggest that the presence of both cell lines should result from an intermediate, 46,XY cell line. Here we report a 2.5 year old female with phenotypic features of Turner syndrome with an isodicentric Y chromosome and a cell line with a deleted Y with a final karyotype of 45,X/46,X,idic(Y)(p11.31)/46,X,del(Y)(p11.31). Fluorescence in situ hybridization (FISH) mapping of the Y chromosome breakpoint revealed very low percentages of the deleted Y cells, but suggested a potential mechanism for the formation of the isodicentric Y chromosome. To our knowledge, the 46,X,del(Y) intermediate cell line in our patient has not been previously reported in individuals with mosaic sex chromosome structural abnormalities.