• Homeobox (Hox) Genes: Master Developmental Genes
• Human HOX Genes
• Homeobox Regulation of Gene Expression
• Hox Gene Expression in the Mammalian Limb
• Hox Gene Knockouts in Mice Lead to Limb Defects
• Retinoic Acid: A Developmental Morphogen
• Hox Genes: Mouse Knockout Mutants
• HOX GENES: Mutation in Michigan Family
• Cell Death & Digit Formation
• Syndactyly in Humans

• 1st identified in Drosophila (homeotic genes)--regulate development of body segments--bizarre development when mutated
• Homologous genes are present from Arthropods to Vertebrates (homeobox genes)
• Highly conserved genes
• Crucial to development
• Functions still not well understood

• 39 Hox genes (i.e., they must be important) distributed in 4 linkage groups
• Contain homeobox domain (highly conserved)
• This domain gives them their name
• Homeobox domain encodes a small protein called homeodomain protein segment
• This protein binds DNA & regulates gene activity (i.e., transcription factor)
• Genes are expressed in sequences that correlate with development of specific regions

The HoxD gene complex is expressed in a specific pattern in the developing mouse forelimb. The pattern of gene expression correlates with the linear arrangement of the genes in the genome. The following figure shows a summary of the expression of the Hox genes relative to the final developed limb. The next figure shows the pattern of expression of the HoxD genes in the developing limb. HoxD9 is expressed earliest and continues throughout development. In contrast, HoxD13 is only expressed in the final stages of digit formation. HoxD12-10 are expressed in decreasing amounts throughout limb development.

The patterns of Hox gene expression mediate specific events in limb development. The following figure shows how knockouts of Hoxa11 or Hoxd11 lead to the loss of the radius and ulna from the mouse forelimb.

Retinoic acid (RA) is the active form of Vitamin A and has many developmental functions (Review: Kastner et al, 1985. Cell 83: 859-869). Excess RA and vitamin A deficiency each affect left-right symmetry (for example, for normal formation of the mammalian heart) through alterations in gene expression (Chauzaud et al, 1999. Development 126: 2589-2596). For an excellent review of the development of the vertebrate heart see Fishman and Chien, 1997. Development 124: 2099-2117. While retinoic acid has specific effects on limb development, the general concensus is that it is not a true morphogen in the development of the mammalian limb. For more info on retinoic acid, see the supplementary material on the chick limb.

• Targeted mutation KO'd Hox13 gene
• Abnormalities in limbs and also in genitals.

• Defects: thumbs and toes
• Women: uterine abnormalites & sterility
• Examine DNA: Mutation present in HOXA13
• Thus, HOXA13 gene important for development (Dickman, 1997. Science 245: 1568)

• Some humans are born with webbed fingers
• Webs removed just after birth
• In normal development: Cell Death (Apoptosis) Occurs between Digits
• Large amount of lysosomal enzyme activity

• Syndactyly = Webbed (fused) Digits
• Due to lack of Cell Death (apoptosis)