Molecular mechanisms in craniofacial anomalies of Down Syndrome (DS)

Background: The gene dosage hypothesis proposes one possible explanation as to how trisomy of Chr 21 leads to the structural changes seen in DS in humans. Ts65Dn, trisomy Chr16 mice, possess more than ¾ of the genes found on human Chr21 and exhibit specific skeletal malformations corresponding directly to the craniofacial dysmorphogenesis in DS. Objective: To test the above hypothesis, we compared developmental expression of Ets2, located on Chr16, and its gene targets involved in tooth and craniofacial development in vivo in Ts65Dn mice, and in vitro, in Ets2-transfected osteoblastic and odontoblastic cells. Methods: Ets2 and DSPP gene expression was examined in time-mated E15 and D1 mice by in-situ hybridization. Expression of Ets2, DSPP and DMP1 genes was examined by RT-PCR in Ets2-overexpression and Ets2-dominant negative-transiently transfected MC3T3-E1 osteoblastic and MO6-G3 odontoblastic cells. Results: In addition to elevated levels of Ets2 and DSPP mRNA expression in E15 and D1 DS mice, Ets2 expression was seen in some ectopic locations such as Meckel's cartilage, as compared to wildtype mice. Ets2 mRNA and protein levels were elevated in Ets2 transfected osteoblast and odontoblast cells. DSPP and DMP1 expression were elevated 2fold in transfected cells as compared to non-transfected and dominant negative transfected cells. Conclusions: These results suggest that increased “dosage” of Ets2 can affect temporospatial expression of target genes such as DSPP. These alterations could potentially explain some of the skeletal anomalies of DS. Supported by Hospital for Sick Kids Foundation grant to G.V.K. Correspondence: g.kulkarni@utoronto.ca