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Kanev, I., Zaleski, D., Van Dyke, A. Z., Hempel, T., Wiggins, M., Olney, A., Dave, B. J., & Sanger, W. G. (2004, June). A Rare Form of Down Syndrome Mosaicism and Unusual Parental Cytogenetic Characteristics. Poster presentation at the Annual Meeting of the Association of Genetic Technologists, Anaheim, CA.
A newborn female with mild phenotypic features of Down syndrome was referred to our laboratory for rapid FISH and cytogenetics studies. Interphase FISH, utilizing the LSI 13/21 DNA probe (Abbott-VYSIS) was performed on a blood smear which revealed two signals for chromosome 21 in 59% of the interphase cells and 3 signals in 41% of the cells. This was preliminarily interpreted as representing a probable mosaic form of Down syndrome. Cytogenetic studies revealed two cell lines, one with an isodicentric 21/21 and a normal 21 and a second cell line with a ring 21. Specifically, the nomenclature was interpreted as mos 46,XX,r(21)(p11.2q22.3)[32]/46,XX,+21,idic(21)(p11.2)[13]. Further metaphase FISH studies revealed the presence of 21q22, the Down syndrome critical region signal and a loss of the 21qtel in the ring 21. FISH studies also revealed the presence of two centromeres on the idic(21;21) and three copies of the 21q22 in the cell line with idic(21;21). Parental chromosome studies were normal; however, many enlarged satellites and satellite stalks were observed on the acrocentric chromosomes of both parents. Furthermore, a very high incidence of satellite association was observed in all cells examined from both parents. Cells were redropped from all cultures from both parents which revealed the same pattern. Chromosome analysis on peripheral blood from other cases performed at the same time and with the same reagents did not show any increase in satellite association. Attempts are being made to obtain a repeat blood sample from the parents to determine whether these findings are a consistent phenomenon over time. Most cytogeneticists would conclude that acrocentric polymorphisms, double satellites, and satellite associations have no clinical relevance; however, the high frequence of satellite associations and large stalks in both parents of a child with an unusual mosaic form of Down syndrome, although perhaps a coincidence, is of interest. We present this abstract to inquire whether other laboratories have observed a similar phenomenon.
