Trisomic and allelic differences influence phenotypic variability during development of Down syndrome mice.

Individuals with full or partial Trisomy 21 (Ts21) present with clinical features collectively referred to as Down syndrome (DS), although DS phenotypes vary in incidence and severity between individuals. Differing genetic and phenotypic content in individuals with DS as well as mouse models of DS facilitate the understanding of the correlation between specific genes and phenotypes associated with Ts21. The Ts1Rhr mouse model is trisomic for 33 genes (the "Down syndrome critical region" or DSCR) hypothesized to be responsible for many clinical DS features, including craniofacial dysmorphology with a small mandible. Experiments with Ts1Rhr mice showed that the DSCR was not sufficient to cause all DS phenotypes by identifying uncharacteristic craniofacial abnormalities not found in individuals with DS or other DS mouse models. We hypothesized that the origins of the larger, dysmorphic mandible observed in adult Ts1Rhr mice develop from larger embryonic craniofacial precursors. Because of phenotypic variability seen in subsequent studies with Ts1Rhr mice, we also hypothesized that genetic background differences would alter Ts1Rhr developmental phenotypes. Using Ts1Rhr offspring from two genetic backgrounds, we found differences in mandibular precursor volume as well as total embryonic volume and postnatal body size of Ts1Rhr and nontrisomic littermates. Additionally, we observed increased relative expression of Dyrk1a and differential expression of Ets2 on the basis of the genetic background in the Ts1Rhr mandibular precursor. Our results suggest that trisomic gene content and allelic differences in trisomic or nontrisomic genes influence variability in gene expression and developmental phenotypes associated with DS.