5. The recessive sonic-hedgehog mutation (shh) causes a phenotype called syndact
ID: 198142 • Letter: 5
Question
5. The recessive sonic-hedgehog mutation (shh) causes a phenotype called syndactyl in which a human hand only grows 1 finger (called a digit). While another recessive mutation in gli3 causes a phenotype called polydactyl in which too many fingers form on a single hand (+5). The recessive mutation ling-ling (I) in a gene important for cilia also causes polydactyl. a) You intercross shh/+ x gli3/+ animals; your double mutants shh/shh, gli3/gli3 cause polydactyl. What is the relationship between shh and gli3? b) Shh, Gli3 and IFT (I1) are all part of a signaling pathway which is a dependent pathway. Can you further describe the relationship between shh and gli3? c) Can you perform the same experiment to determine the relationship between gli3 and ling-ling? Why or why not d) What would you conclude if instead of shh;shh, gli3/gli3 double mutants displaying polydactyl they displayed 5 fingers (normal)Explanation / Answer
(a) shh and gli3 proteins are antagonists to each other. Sonic hedgehog (Shh) is a secreted signaling protein which is synthesised in the notochord, the most ventral part of the spinal cord. The response to Shh depends on transcription factors of the Gli family.
(b) Gli3 gene is the most important gene in Sonic Hedgehog pathway. Gli3 works as a dual function transcription factor, Gli3R and Gli3A. Gli3R is a repressor form without Shh signaling. When there is no inhibitory effect of Ptch on Smo, then Smo stimulated Gli3R (Repressor) is activated to Gli3A form after Shh–Ptch interaction. Transcription of target genes then takes place.
(c) I think it is not possible because the intercross between a cilia and an animal can not be performed.
(d) Sonic hedgehog is an important protein of mammalian signaling pathway family called hedgehog. SHH acts as a ligand of the hedgehog signaling pathway. Acting as a morphogen, it plays a key role in regulating vertebrate organogenesis, such as in the growth of digits on limbs and organization of the brain. SHH also seems to promote the activation function of downstream molecule Gli3. Thus the signaling pathway takes place in a concerted pathway. So as the double mutant shh/shh and gli3/gli3 double mutants show polydactyl and these two proteins show antagonistic effect to each other thus in case of wild type genes, normal finger pattern (5 fingers) is seen.
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