HP1B and H3K9me3 Regulate Olfactory Receptor Choice and 2 Transcriptional Identity [CUT&Run]

Diverse epigenetic regulatory mechanisms ensure and regulate cellular diversity. Among 18 others, the histone 3 lysine 9 me3 (H3K9me3) post translational modification participates 19 in silencing lineage-inappropriate genes. H3K9me3 restricts access of transcription factors 20 and other regulatory proteins to genes that control cell fate. Mouse olfactory sensory neu- 21 rons (OSN) select to express one olfactory receptor (OR) gene out of 2,600 possibilities. 22 This monoallelic and stochastic OR choice happens as OSNs differentiate and undergo 23 dramatic changes in nuclear architecture. OR genes from different chromosomes converge 24 into specialized nuclear bodies and chromatin compartments, as H3K9me3 and chromatin 25 binding proteins including heterochromatin protein 1 (HP1) are incorporated. In this 26 work, we have uncovered an unexpected role for HP1b in OR choice and neuronal iden- 27 tity that cannot be rescued by HP1a in vivo. With the use of a conditional knock-in mouse 28 model that replaces HP1b for HP1a, we observe changes in H3K9me3 levels and DNA 29 accessibility over OR gene clusters. These changes alter the expression paTerns that par- 30 tition the mouse olfactory epithelium into five OR expression zones, which results in a 31 reduced OR repertoire leading to a loss of olfactory sensory neuron diversity. We propose 32 that HP1b modulates the competition of OR-promoters for enhancers to promote receptor 33 diversity, by establishing repression gradients in a zonal fashion.