Genetic Basis of Human Brain Evolution: a Bioinformatics and Functional Approach using Transgenic Zebrafish and Mice

FRANCHINI LF

Rosario

Conferencia; II Latin American Zebrafish Network (LAZEN) Conference; 2012

Latin American Zebrafish Network (LAZEN)

It has been hypothesized that the evolution of the unique human cognitive capacities is due to the acquisition of new temporal and spatial expression patterns of preexisting genes rather than changes in the protein-coding sequences. Using a combination of bioinformatics and functional studies including the generation of transgenic zebrafish and mice we are investigating differences in gene regulation which may have contributed to the evolution of the human brain. Evolutionary signatures of functional shifts can be detected by comparing non-coding regions that are highly conserved across mammals or primates and rapidly accumulated nucleotide substitutions only in the lineage leading to humans. Since gene loci densely populated with human accelerated elements (HAEs) are more likely to have contributed to human-specific novelties we sought to identify the transcriptional units and genomic 1 Mb intervals of the entire human genome carrying the highest number of HAEs. To this end we took advantage of four available datasets of human genomic accelerated regions obtained through different comparisons and algorithms and performed a meta-analysis of the combined data. We found that the brain developmental transcription factor NPAS3 contains the largest cluster of non-coding accelerated regions in the human genome with up to 14 elements that are highly conserved in mammals, including primates, but carry human-specific nucleotide substitutions. We then tested the ability of the 14 HAEs identified at the NPAS3 locus to act as transcriptional regulatory sequences in a reporter expression assay performed in transgenic zebrafishes. We found that 11 out of the 14 HAEs present in NPAS3 act as transcriptional enhancers during development, particularly within the nervous system. The finding that the NPAS3 gene shows a set of highly conserved regulatory regions that evolved faster in the human lineage suggests it might have acquired a new expression pattern and probably a novel function. In order to test this hypothesis, we performed a comparative expression analysis over selected NPAS3 elements in transgenic zebrafishes and mice. We found that the human and chimpanzee ortholog sequences of one of the selected HAEs shows differences in the expression during central nervous system development. Since NPAS3 is known to play a crucial role during mammalian brain development, our results indicate that the high density of HAEs present in the human NPAS3 locus could have modified the spatio-temporal expression pattern of NPAS3 in the developing human brain and, therefore, contributed to human brain evolution.