It has been known for at least a century that multiple factors play a role in the development of complex traits, and yet modern biology still holds fast to the implicit idea that complexity can be explained by reducing it to enumerable genes [Buchanan et al., 2009
]. Recent tests of the Down syndrome “critical region” hypothesis failed to support the idea that triplication of a specific gene or set of genes is sufficient to produce a number of DS phenotypes, including characteristic alterations of the cranial skeleton [Olson et al., 2004
; Olson et al., 2007
]. Since the “critical region”, which includes Ets2
, was not sufficient to cause characteristic cranial dysmorphology of DS, we used the Ts65Dn, Ets2+/−
mouse to determine whether Ets2
is necessary for those effects. Our analysis of the craniofacial phenotype and thymus weight of Ts65Dn, Ets2+/−
mice provides additional evidence against the “critical region” hypothesis and specifically, of a role for Ets2
as a single cause of these DS phenotypes. In Ts65Dn mice, returning the dosage imbalance of Ets2
to the normal two copies has little effect on the skeletal and thymus anomalies that reflect those noted in individuals with DS. Direct testing of the function of Ets2
using chromosomally engineered mice highlights the complexity of genetic interactions in the production of the final phenotypes in Down syndrome.
was necessary to produce the DS-like craniofacial and thymus anomalies in Ts65Dn mice, then Ts65Dn, Ets2+/−
mice would be more similar to euploid littermates, showing a reversal of the effects of triplicated Ets2
in Ts65Dn mice. Our comparative analyses of Ts65Dn and Ts65Dn, Ets2+/−
mice demonstrate minimal differences in the effects of trisomy on the thymus and the cranial skeleton with or without the additional copy of Ets2
. Further, the number and magnitude of significant euploid-to-Ts65Dn, Ets2+/−
contrasts are approximately equivalent to differences noted in the euploid-to-Ts65Dn contrasts. In our analysis, the exception is localized to specific dimensions of bones derived from mesoderm where trisomic-euploid shape differences of the Ts65Dn, Ets2+/−
skull were of significantly greater magnitude than those estimated for the same skull portion of the Ts65D-euploid comparison. There is a possibility that over-expression of Ets2
plays a critical role during development of mesodermal-derived components of the skull, but we should also consider that skull size and shape is influenced, though not determined, by genes and that bones of the skull respond directly to changes in soft tissue structures that surround them. Localized changes in skull morphology reflect changes in brain morphology [Richtsmeier et al., 2006
]. That part of the skull that forms from mesoderm surrounds derivatives of the mid- and hindbrain, but the locus of the measures that are statistically different between euploid and both trisomic models lie close to the cerebellum throughout murine development (), a central nervous system structure that is disproportionately affected in Down syndrome and in Ts65Dn mice [Baxter et al., 2000
]. Whether our findings reflect a differential role of Ets2 in mesoderm derived elements of the skull, or in cerebellar development cannot be determined by our analysis. The role of Ets2 should be explored in additional mesoderm-derived structures affected in DS using larger samples, as well as different aspects of the developing brain.
Figure 4 Morphological relationship of brain and skull in mice at day of birth (P0; top) and in adult mice (bottom). The top figure shows a 3D reconstruction of micro-computed tomography images of a P0 mouse skull overlying a 3D reconstruction of micro-magnetic (more ...)
The phenotypes associated with Down syndrome include structures associated with nearly every bodily system. Production of DS phenotypes is complex, requiring consideration not only of the genes at dosage imbalance, but of all genetic and nongenetic factors influencing formative cell populations and emerging phenotypic form. This study highlighted the potential importance of embryonic tissue-specific effects of Ets2 on the craniofacial skeleton and thymus, though the analyses were limited to adult mice. Thus the mechanisms by which Ets2 dosage imbalance affects specific cell populations and the structures that they form requires further investigation. To be relevant to DS, these and related investigations should include expression of the gene(s), and their networks in a well-defined temporal context.