Crucial and Overlapping Roles of Six1 and Six2 in Craniofacial Development

Area of Science:

• Genetics
• Developmental Biology
• Craniofacial Development

Background:

• SIX1 and SIX2 are related transcription factors implicated in distinct craniofacial syndromes.
• SIX1 mutations cause branchiootic syndrome 3 (BOS3), while SIX2 deletions are linked to frontonasal dysplasia.
• Previous studies showed Six1 deficiency recapitulates BOS3 defects, but Six2 deficiency did not cause obvious frontonasal defects in mice.

Purpose of the Study:

• To investigate the overlapping functions of SIX1 and SIX2 in craniofacial development.
• To elucidate the molecular mechanisms underlying SIX1 and SIX2's roles in facial patterning.
• To determine the contribution of neural crest cells to SIX1/SIX2-mediated craniofacial morphogenesis.

Main Methods:

• Generation and analysis of Six1 and Six2 single and double knockout mouse models.
• Expression analysis of SIX1 and SIX2 in embryonic facial structures.
• In situ hybridization to detect gene expression patterns.
• Neural crest-specific gene inactivation studies.
• Analysis of apoptosis and key developmental gene expression.

Main Results:

• Six1-/- Six2-/- double mutants exhibit severe craniofacial deformities, including agenesis, midline clefts, and syngnathia.
• Six1 deficiency leads to maxillary bone transformation, exacerbated by Six2 haploinsufficiency.
• SIX1 and SIX2 regulate endothelin and BMP4 signaling pathways, with altered expression of Dlx5, Bmp4, Msx1, and Msx2.
• Neural crest-specific Six1 inactivation in Six2-/- embryos causes midline cleft and frontal bone agenesis.
• Reduced expression of Alx1 and Alx3 and increased apoptosis observed in Six1-/- Six2-/- frontonasal mesenchyme.

Conclusions:

• SIX1 and SIX2 function redundantly to control multiple craniofacial developmental processes.
• These transcription factors play a crucial, neural crest cell-autonomous role in frontonasal morphogenesis.
• SIX1 and SIX2 are essential regulators of signaling pathways that pattern facial structures.