Solid phase transition as a facilitator of olfactory receptor choice

Project Summary Singular and stable olfactory receptor (OR) gene choice is paramount for odor detection and identification by mammalian olfactory systems. We previously showed that a complex network of interchromosomal interactions between intergenic OR enhancers, the Greek Islands, reduces the complexity of choice to a handful OR alleles that transiently contact ~6 Greek Island hubs assembled in each olfactory neuron. Subsequently, an RNA mediated symmetry breaking process silences all but one of the co-transcribed ORs, assuring stable association of that allele with the Greek Island hub for the life of the neuron, while the other ORs became permanently silenced. Here, we show that the unusual stability and specificity of ultra-long-range Greek Island interactions are explained by solid phase transitions of Lhx2 and Ldb1 proteins in the Greek Island hub. Our in vitro data show that these two proteins promote assembly of gel-like nucleoprotein complexes when incubated with Greek Island DNA, which form in a DNA-sequence-dependent fashion. Preliminary in vivo imaging data suggest that these DNA-induced solid phase transitions may also take place in vivo, in cultured primary olfactory neurons, providing an exciting and novel mechanism that explains both the specificity and stability of ultra-long-range cis and trans genomic interactions. We therefore propose experiments that will confirm the concept of DNA-induced solid phase separation as a means of 3D genome organization, experiments that will explore the exact role of these biophysical transitions in the stability and specificity of genomic interactions, and experiments that will test whether solid phase transitions in the Greek Island hub protect the chosen OR allele from silencing by the liquid phases of the surrounding heterochromatin. Beyond providing significant mechanistic insight to the genomic choreography that culminates in singular and stable OR gene choice, our studies will provide generalizable principles for the mechanisms of long-range genomic interactions, and paths for future therapeutic interventions, as there is emerging evidence for the role of such genomic interactions in human disease.