Cell-to-cell genetic variation in the mammalian central nervous system

The mechanisms that determine whether or not specific genes are expressed in neural cells are fundamentally important to neurobiology. This dissertation focuses on a novel mechanism controlling gene expression in the mammalian nervous system: modification of the genomes of individual neural cells by the loss and gain of whole chromosomes (i.e. aneuploidy). Aneuploidy arises through chromosome displacement and mis-segregation during the cell divisions of neural stem and progenitor cells (NPCs). Aneuploidy is observed in 33% of embryonic or postnatal murine NPCs in vivo and in 27% of fetal human NPCs in vitro, but not in cells from other tissues, as shown by spectral karyotyping (SKY), interphase fluorescence in situ hybridization (FISH), and flow-cytometric DNA content analysis. Aneuploidy is propagated from NPCs to differentiated neurons and glia in vitro and in vivo, and aneuploid cells are therefore detected at high frequency in the adult mouse and human brain. These data indicate that chromosome loss and gain among single cells are normal aspects of mammalian brain development and function.; Aneuploidy is associated with changes in gene expression and cellular function. Transgenic mice hemizygous for a ubiquitously expressed eGFP transgene present at a single locus on chromosome 15 are analyzed to identify cells that have lost a single copy of 15. Among these cells, loss of chromosome 15 eliminates transgene expression, and causes up- and down-regulation of endogenous genes on multiple chromosomes. To identify functional differences between aneuploid and euploid cells, flow sorting is used to purify euploid and aneuploid human NPCs. Among flow-sorted cells, aneuploidy is correlated with an increase in proliferation; proliferation is also increased following chemical induction of aneuploidy by transient exposure to aneugens. In sum, these data constitute direct evidence for cell-to-cell genetic variation in the mammalian nervous system, and suggest that such variation underlies differences in gene expression and cellular function among neural cells.