Figure 1. . Possible patterns of aneuploidy-dependent changes in gene expression.

The diagrams show a number of different scenarios that have been reported for how aneuploidy impacts gene expression. The examples illustrated here depict such scenarios in hypothetical diploid cells with a haploid chromosome number of 9 and carrying an extra copy of chromosome 5 (highlighted in blue). (A) Many studies have found that acquisition of an extra chromosome results in a proportional increase in the expression of genes carried on that chromosome. This scaling of gene expression may affect all the genes (as in the example shown here) or only a subset of genes on the aneuploid chromosome. (B) Other studies have reported that, in addition to an increase in the expression of genes carried by the extra chromosome, aneuploidy can also perturb the expression levels of genes on other chromosomes. Both overexpression (red) and underexpression (green) of genes on other chromosomes have been reported. (C) A number of studies have also reported some kind of dosage compensation, by which the genes on the aneuploid chromosome are not expressed at the expected levels based on gene dosage. It should be noted that dosage compensation in response to aneuploidy has been reported to occur for variable numbers of genes (from a few genes to the majority of genes on the aneuploid chromosome), depending on context. Moreover, the degree of compensation may vary. Finally, dosage compensation may occur simultaneously with effects on other chromosomes. The bottom half of the figure reports lists of organisms in which each of the different scenarios have been observed (often in combination with other effects) and corresponding key references. The references listed for ‘Down syndrome’ include studies in both human and mouse models.