The gene that encodes the enzyme called tyrosine hydroxylase is known to be up r

Question

The gene that encodes the enzyme called tyrosine hydroxylase is known to be up regulated by the CREB protein. Tyrosine hydroxylase is expressed in nerve cells and is involved in the synthesis of catecholaine, a neurotransmitter. The exposure of cells to adrenaline normally up regulates the transcription of the tyrosine hydroxylase gene. A mutant cell line was identified in which the tyrosine hydroxylase gene was not up regulated when exposed to adrenaline. List all the possible mutations that could explain this defect.

Explanation / Answer

One possible mutation might lie on the gene that encodes the cell membrane receptor for adrenaline. Another possible mutation might lie in the relay of the adrenaline-mediated signal from the membrane to the nucleus. The enhancer and or promoter that binds the CREB protein and mediates its up regulation might be the site of a mutation occurring in non-transcribed DNA. The tyrosine hydroxylase gene is not likely to have a mutation on its transcribed and translated regions, because the protein is expressed though not up regulated. If the up regulation is a result of a different splicing pattern, not as likely, the site oof the mutation might be in the DNA encoding for small nuclear RNA and other kinds of RNA. The most likely culprit would be the signaling pathway, from the adrenaline receptor to the second messenger, something in the adrenaline signaling pathway. Mutations involving important molecules in adrenergic signal transduction must be considered, involving phosphatases and kinases involved in synthesis and recycling of IP3, cyclic-AMP, the GPCR's (G-protein-coupled receptors), of which adrenergic receptors are a member, these are membrane proteins, typically bearing 7 transmembrane domains. Occasionally single point mutations have caused even malignancy, as in the chemokine receptor CXCR2 due to permanent activation of the transduced signal in spite of the absence of ligand-receptor binding. In our case it might appear that there is no signal transduction even in the presence of the ligand. One possibility is iinability of the receptor to bind the ligand.

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