Based on studies in mice, RUNX2 binds to the basic helix-loop-helix (bHLH) protein HAND2. The interaction involves the N-terminal part of HAND2 and the Runt DNA binding domain of RUNX2. Interaction with HAND2 prevents binding of RUNX2 to the DNA, thus inhibiting transcriptional activation of RUNX2 target genes. HAND1 is also able to bind to RUNX2 and inhibit binding of RUNX2 to the DNA, but this interaction is not considered to be physiologically relevant as RUNX2 and HAND1 are not coexpressed in vivo, and targeted deletion of Hand1 in mice does not lead to skeletal defects. Mice hypomorphic for a mutant Hand2 allele, however, show abnormal membranous bone phenotype, accompanied with elevated Runx2 expression and precocious and accelerated osteoblast differentiation (Funato et al. 2009).
In mouse tissues, Hand2 expression is also associated with Runx2 downregulation (Funato et al. 2009, Barron et al. 2011). This could be due to the suggested Hand2-mediated transcriptional repression of Dlx5 and Dlx6, two transcription factors involved in stimulation of Runx2 expression (Barron et al. 2011).