MOLECULAR AND FUNCTIONAL EVOLUTION OF TWO RETROPOSON-DERIVED ENHANCERS CONTROLLING NEURON-SPECIFIC EXPRESSION OF THE SAME BRAIN GENE

RUBINSTEIN M, FRANCHINI LF, DE SOUZA FSJ, LAM D, LOW MJ

Dublin

Congreso; Society for Molecular Biology and Evolution 2012; 2012

Society for Molecular Biology and Evolution

The proopiomelanocortin gene (POMC) is expressed in a group of neurons present in the arcuate nucleus of the hypothalamus. Neuron-specific POMC expression in mammals is conveyed by two conserved distal enhancers, named nPE1 and nPE2. Previous transgenic mouse studies showed that nPE1 and nPE2 independently drive reporter gene expression to POMC neurons. Here, we investigate the evolutionary mechanisms that shaped not one but two neuron-specific POMC enhancers, and tested whether nPE1 and nPE2 drive identical or complementary spatio-temporal expression patterns. Sequence comparison among representative genomes of most vertebrate classes and mammalian orders showed that nPE1 is a placental novelty. Using in silico paleogenomics we discovered that nPE1 originated from the exaptation of a Mammalian-apparent LTR Retrotransposon (MaLR) sometime between the metatherian/eutherian split (147 MYA) and the placental mammal radiation (~90 MYA). Thus, the evolutionary origin of nPE1 differs, in kind and time, from that previously demonstrated for nPE2 which was exapted from a CORE-SINE retroposon before the origin of prototherians, 166 MYA. Analysis of compound transgenic mice expressing the fluorescent markers tomato and EGFP under the transcriptional control of nPE1 or nPE2, respectively, demonstrated coexpression of both reporter genes along the entire arcuate nucleus from the onset of Pomc expression in the presumptive mouse diencephalon. Mice deficient in either nPE1, nPE2 or both enhancers simultaneously show unique differential phenotypes that are beginning to reveal the importance of functionally overlapping enhancers. Thus, the independent exaptation of two unrelated retroposons into functional analogs acting as neuron-specific enhancers of POMC constitute an authentic first example of convergent molecular evolution of cell-specific enhancers to govern gene expression throughout the entire lifespan.