Histone methyltransferase Setdb1 is indispensable for Meckel's cartilage development

The histone methyltransferase Setdb1 represses gene expression by catalyzing lysine 9 of histone H3 trimethylation. Given that the conventional knockout of Setdb1 is embryo-lethal at the implantation stage, its role in craniofacial development is poorly understood. Here, we investigated the role of Setdb1, using conditional knockout mice-in which Setdb1 was deleted in the Meckel's cartilage (Setdb1 CKO)-and the mouse chondrogenic cell line ATDC5-in which Setdb1 was inhibited by siRNA. Deletion of Setdb1 in Meckel's cartilage, the supportive tissue in the embryonic mandible, led to its enlargement, instead of the degeneration that normally occurs. Chondrocytes from the Meckel's cartilage of Setdb1 CKO mice showed increased size. Furthermore, at embryonic days 16.5 and 18.5, part of the perichondrium was disrupted and mineralization was observed in the Meckel's cartilage. Proliferation analysis showed that inhibition of Setdb1 caused increased proliferation in chondrocytes in the Meckel's cartilage as well as in ATDC5 cells. Quantitative RT-PCR showed decreased expression of chondrogenic genes, such as Sox9, Mmp13, Collagen II, and Aggrecan, as a result of Setdb1 inhibition in ATDC5 cells. Along with these phenomenons, SMAD-dependent BMP signaling was significantly increased by the loss of Setdb1 in both the Meckel's cartilage of Setdb1 CKO mice and ATDC5 cells. Therefore, the abnormal development of Meckel's cartilage in Setdb1 CKO mice is partly due to the enhanced SMAD-dependent BMP signaling. Overall, to our knowledge, the present study is the first to show that epigenetic regulation by Setdb1 is indispensable for the embryonic development of Meckel's cartilage.