Prdm5 Regulates Collagen Gene Transcription by Association with RNA Polymerase II in Developing Bone

Abstract

PRDM family members are transcriptional regulators involved in tissue specific differentiation. PRDM5 has been reported to predominantly repress transcription, but a characterization of its molecular functions in a relevant biological context is lacking. We demonstrate here that Prdm5 is highly expressed in developing bones; and, by genome-wide mapping of Prdm5 occupancy in pre-osteoblastic cells, we uncover a novel and unique role for Prdm5 in targeting all mouse collagen genes as well as several SLRP proteoglycan genes. In particular, we show that Prdm5 controls both Collagen I transcription and fibrillogenesis by binding inside the Col1a1 gene body and maintaining RNA polymerase II occupancy. In vivo, Prdm5 loss results in delayed ossification involving a pronounced impairment in the assembly of fibrillar collagens. Collectively, our results define a novel role for Prdm5 in sustaining the transcriptional program necessary to the proper assembly of osteoblastic extracellular matrix.

Author Summary

Bone provides the essential tensile strength of the skeletal system, constitutes an important storage for minerals, and hosts the initial differentiation stages of the hematopoietic system. In addition, bones are important endocrine organs affecting organismal metabolism. Consequently, many human diseases arise from defects in bone formation or homeostasis. Hence, deciphering the molecular mechanisms underlying bone formation is essential for understanding the basis of bone and extracellular matrix-associated diseases. Here, we provide a detailed characterization of the cellular and molecular functions of the transcription factor Prdm5 during murine bone formation in vivo and find that Prdm5 is expressed in skeletal structures during development and that its loss impacts the ossification process, leading to a decrease in bone mineral density. A genome-wide mapping of Prdm5 binding sites in pre-osteoblastic cells reveals an unprecedented role for a transcription factor in targeting virtually all members of the Collagen and SLRP gene families. Interestingly, Prdm5 predominantly binds exonic regions of collagen genes and associates with RNA Polymerase II to sustain Collagen I transcription.