Morphology, Behavior, and the Sonic Hedgehog Pathway in Mouse Models of Down Syndrome

Down Syndrome (DS) is caused by a triplication of human chromosome 21 (Hsa21). Ts65Dn, a mouse model of DS, contains a freely segregating extra chromosome consisting of the distal portion of mouse chromosome 16 (Mmu16), a region orthologous to part of Hsa21, and a non-Hsa21 orthologous region of mouse chromosome 17. All individuals with DS display some level of craniofacial dysmorphology, brain structural and functional changes, and cognitive impairment. Ts65Dn recapitulates these features of DS and aspects of each of these traits have been linked in Ts65Dn to a reduced response to Sonic Hedgehog (SHH) in trisomic cells. Dp(16)1Yey is a new mouse model of DS which has a direct duplication of the entire Hsa21 orthologous region of Mmu16. Dp(16)1Yey’s creators found similar behavioral deficits to those seen in Ts65Dn. We performed a quantitative investigation of the skull and brain of Dp(16)1Yey as compared to Ts65Dn and found that DS-like changes to brain and craniofacial morphology were similar in both models. Our results validate examination of the genetic basis for these phenotypes in Dp(16)1Yey mice and the genetic links for these phenotypes previously found in Ts65Dn , i.e., reduced response to SHH. Further, we hypothesized that if all trisomic cells show a reduced response to SHH, then up-regulation of the SHH pathway might ameliorate multiple phenotypes. We crossed Ts65Dn mice with Ptch1tm1Mps/+ mice, which up-regulate the canonical SHH pathway through the loss of function of one Ptch1 allele. The resulting four mouse genotypes were examined for craniofacial, behavioral and brain phenotypes. We found that Ptch1 heterozygotes displayed craniofacial and behavioral phenotypes that were mostly distinct from the effects of trisomy rather than complementary. However, as predicted some brain structural deficits were ameliorated in the Ts65Dn Ptch1 heterozygotes. Our studies comprise the first evaluation of Dp(16)1Yey morphology to confirm it as a DS model and our additional DS model investigations indicate a more complex pathogenicity for SHH related phenotypes in DS than constitutive uniform pathway repression

Acute upregulation of hedgehog signaling in mice causes differential effects on cranial morphology

Hedgehog (HH) signaling, and particularly signaling by sonic hedgehog (SHH), is implicated in several essential activities during morphogenesis, and its misexpression causes a number of developmental disorders in humans. In particular, a reduced mitogenic response of cerebellar granule cell precursors to SHH signaling in a mouse model for Down syndrome (DS), Ts65Dn, is substantially responsible for reduced cerebellar size. A single treatment of newborn trisomic mice with an agonist of the SHH pathway (SAG) normalizes cerebellar morphology and restores some cognitive deficits, suggesting a possible therapeutic application of SAG for treating the cognitive impairments of DS. Although the beneficial effects on the cerebellum are compelling, inappropriate activation of the HH pathway causes anomalies elsewhere in the head, particularly in the formation and patterning of the craniofacial skeleton. To determine whether an acute treatment of SAG has an effect on craniofacial morphology, we quantitatively analyzed the cranial form of adult euploid and Ts65Dn mice that were injected with either SAG or vehicle at birth. We found significant deformation of adult craniofacial shape in some animals that had received SAG at birth. The most pronounced differences between the treated and untreated mice were in the midline structures of the facial skeleton. The SAG-driven craniofacial dysmorphogenesis was dose-dependent and possibly incompletely penetrant at lower concentrations. Our findings illustrate that activation of HH signaling, even with an acute postnatal stimulation, can lead to localized dysmorphology of the skull by generating modular shape changes in the facial skeleton. These observations have important implications for translating HH-agonist-based treatments for DS