No abstract

Bone abnormalities affect all individuals with Down syndrome (DS) and are linked to abnormal expression of DYRK1A, a gene found in three copies in people with DS and Ts65Dn DS model mice. Previous work in Ts65Dn male mice demonstrated that both genetic normalization of Dyrk1a and treatment with ~9 mg/kg/day Epigallocatechin-3-gallate (EGCG), the main polyphenol found in green tea and putative DYRK1A inhibitor, improved some skeletal deficits. Because EGCG treatment improved mostly trabecular skeletal deficits, we hypothesized that increasing EGCG treatment dosage and length of administration would positively affect both trabecular and cortical bone in Ts65Dn mice. Treatment of individuals with DS with green tea extract (GTE) containing EGCG also showed some weight loss in individuals with DS, and we hypothesized that weights would be affected in Ts65Dn mice after EGCG treatment. Treatment with ~20 mg/kg/day EGCG for seven weeks showed no improvements in male Ts65Dn trabecular bone and only limited improvements in cortical measures. Comparing skeletal analyses after ~20mg/kg/day EGCG treatment with previously published treatments with ~9, 50, and 200 mg/kg/day EGCG showed that increased dosage and treatment time increased cortical structural deficits leading to weaker appendicular bones in male mice. Weight was not affected by treatment in mice, except for those given a high dose of EGCG by oral gavage. These data indicate that high doses of EGCG, similar to those reported in some treatment studies of DS and other disorders, may impair long bone structure and strength. Skeletal phenotypes should be monitored when high doses of EGCG are administered therapeutically.

Trisomy 21 (Ts21) causes alterations in skeletal development resulting in decreased bone mass, shortened stature and weaker bones in individuals with Down syndrome (DS). There is a sexual dimorphism in bone mineral density (BMD) deficits associated with DS with males displaying earlier deficits than females. The relationships between causative trisomic genes, cellular mechanisms, and influence of sex in DS skeletal abnormalities remain unknown. One hypothesis is that the low bone turnover phenotype observed in DS results from attenuated osteoblast function, contributing to impaired trabecular architecture, altered cortical geometry, and decreased mineralization. DYRK1A, found in three copies in humans with DS, Ts65Dn, and Dp1Tyb DS model mice, has been implicated in the development of postnatal skeletal phenotypes associated with DS. Reduced copy number of Dyrk1a to euploid levels from conception in an otherwise trisomic Ts65Dn mice resulted in a rescue of appendicular bone deficits, suggesting DYRK1A contributes to skeletal development and homeostasis. We hypothesized that reduction of Dyrk1a copy number in trisomic osteoblasts would improve cellular function and resultant skeletal structural anomalies in trisomic mice. Female mice with a floxed Dyrk1a gene (Ts65Dn,Dyrk1afl/wt) were mated with male Osx-Cre+ (expressed in osteoblasts beginning around E13.5) mice, resulting in reduced Dyrk1a copy number in mature osteoblasts in Ts65Dn,Dyrk1a+/+/Osx-Cre P42 male and female trisomic and euploid mice, compared with littermate controls. Male and female Ts65Dn,Dyrk1a+/+/+ (3 copies of DYRK1A in osteoblasts) and Ts65Dn,Dyrk1a+/+/Osx-Cre (2 copies of Dyrk1a in osteoblasts) displayed similar defects in both trabecular architecture and cortical geometry, with no improvements with reduced Dyrk1a in osteoblasts. This suggests that trisomic DYRK1A does not affect osteoblast function in a cell-autonomous manner at or before P42. Although male Dp1Tyb and Ts65Dn mice exhibit similar skeletal deficits at P42 in both trabecular and cortical bone compartments between euploid and trisomic mice, female Ts65Dn mice exhibit significant cortical and trabecular deficits at P42, in contrast to an absence of genotype effect in female Dp1Tyb mice in trabecular bone. Taken together, these data suggest skeletal deficits in DS mouse models and are sex and age dependent, and influenced by strain effects, but are not solely caused by the overexpression of Dyrk1a in osteoblasts. Identifying molecular and cellular mechanisms, disrupted by gene dosage imbalance, that are involved in the development of skeletal phenotypes associated with DS could help to design therapies to rescue skeletal deficiencies seen in DS.

Down syndrome (DS) phenotypes result from triplicated genes, but it is generally unknown how specific three copy human chromosome 21 (Hsa21) orthologous genes or interactions between genes affect these traits. A mouse mapping panel genetically dissecting Hsa21 syntenic regions was used to investigate the contributions and interactions triplicated Hsa21 orthologous genes on mouse chromosome 16 (Mmu16). Four-month-old femurs of male and female Dp9Tyb, Dp2Tyb, Dp3Tyb, Dp4Tyb, Dp5Tyb, Dp6Tyb, Ts1Rhr, and Dp1Tyb;Dyrk1a+/+/- mice were analyzed by micro-computed tomography and 3-point bending to assess skeletal structure and mechanical properties. Male and female Dp1Tyb mice, with the entire Hsa21 homologous region of Mmu16 in three copies, display specific bone deficits similar to humans with DS and were used as a baseline comparison for the other strains in the panel. Bone phenotypes varied based on triplicated gene content, sex, and bone compartment. Three copies of Dyrk1a played a sex-specific, essential role in trabecular deficits and may interact with other genes to influence cortical deficits related to DS. Triplicated genes in Dp9Tyb and Dp2Tyb mice improved some skeletal deficits. As triplicated genes may both improve and worsen bone deficits, it is important to understand the interaction between and molecular mechanisms of skeletal alterations affected by these genes.