| SIX2 is a member of the sine oculis homeobox (SIX) family of evolutionarily conserved transcription factors, each containing a DNA binding homeodomain (HD) with a helix–turn–helix motif and an adjacent SIX domain that mediates protein–protein interactions for cofactor recruitment and transcriptional regulation. The SIX domain lies N terminal to the HD, enabling SIX2 to interact with partners such as TCF7L2 and OSR1 to modulate target gene expression without strict dependence on canonical Wnt signaling. SIX2 also plays a critical role in nephrogenesis by maintaining cap mesenchyme nephron progenitors in an undifferentiated, self renewing state downstream of HOXA2. It opposes ureteric bud inductive signals to prevent premature differentiation while cooperating with WNT9B to drive progenitor proliferation via activation of genes such as GDNF, OSR1, and SIX2 itself, and repression of differentiation markers such as WNT4. This balance ensures a sufficient nephron progenitor pool for nephrogenesis, with Six2 knockout mice exhibiting rapid depletion of nephron progenitors and kidney hypoplasia. SIX2 regulates cranial base chondrocyte differentiation for skull elongation, pyloric sphincter formation via NKX2 5/BMP/SOX9 signaling in stomach development, and HOXA2 mediated insulin like growth factor signaling in the branchial arches. Persistent SIX2 expression in Wilms’ tumor sustains a progenitor like state that promotes oncogenesis, whereas loss of SIX2 impairs beta cell maturation and differentiation in relevant contexts. |
