Genomic and nuclear organization of olfactory receptors

The olfactory receptor (OR) genes are the largest mammalian gene family and are expressed in a monogenic and monoallelic fashion in olfactory neurons. To investigate stochastic and deterministic models for OR expression, we mapped transcription start sites for ~1100 of the ~1400 murine olfactory receptor genes and computationally analyzed putative promoters. OR promoters were extremely AT-rich and were homogenous with respect to transcription factor binding site representation. Other murine genes with similar AT-rich promoters resembled ORs--these were large, recently evolved families arranged in clusters in the genome that showed evidence of variegated or stochastic expression in terminally differentiated tissues.In investigating regulatory mechainsism that could restrict OR expression to one out of a large family of equivalent promoters, we found that the silent olfactory receptor genes form interchromosomal aggregates near pericentromeric heterochromatin in olfactory neurons. These aggregates are cell-type specific and differentiation-dependent and exclude the active allele. OR aggregation depends upon downregulation of Lamin b receptor in olfactory neurons and its removal from the nuclear envelope. Restoration of LBR to OSNs by overexpression disrupts OR aggregates, reduces chromatin density, and increases DNase accessibility of ORs. This results in downregulation of OR expression, co-expression of multiple OR alleles in each OSN, and mis-targeting of OSN axons to olfactory bulb glomeruli. We hypothesize that aggregation of silent ORs cloaks most OR alleles, restricting access of a limited number of transcription factors to a few OR promoters. When most ORs are revealed by LBR overexpression, transcription factor binding sites increase by several orders of magnitude while the number of transcription factors available to bind them remains the same. This results in downregulation of each OR and co-expression of multiple ORs