Sex-specific transcriptomic profiles and imprinting drive nicotine self-administration in rats and inform the genetic basis of human smoking behavior

Tobacco cigarette smoking, with nicotine (NIC) as the addictive component, is a large risk factor for human mortality. In animals, repeated NIC exposure leads to sensitization (SST) and enhances self-administration (SA) of NIC. However, the molecular basis of SST and SA and their genetic relevance to smoking behavior are poorly understood. Using F1 progeny of inbred Envigo rats (F344/BN), we carried out a transcriptional profiling of NIC SST and SA in ventral tegmental area (VTA), nucleus accumbens core (Nac) and shell (Nash). We observed male-specific NIC SST and a parental effect of NIC with SA only present in paternal F344 crosses. Gene differential expression (DE) analysis revealed sex and brain region-specific transcriptomic signatures of SST and SA, with genes downregulated in male VTA associated with both SST and SA, while genes upregulated in male Nac were associated with SA. DE genes associated with SST and SA are enriched for those related to synaptic processes, myelin sheath, and tobacco use disorder or chemdependency. Interestingly, we found that for SST the downregulated genes in the male VTA tended to be upregulated in female VTA, and the overlapping genes were strongly enriched for smoking genome-wide association study (GWAS) risk variants, which may thus explain male-specific SST. To gain mechanistic insight on the observed parental effect of SA, we analyzed the allelic imbalance of expression (AIE) in reciprocally crossed F1 rats that exhibited differential tendency of SA and found widespread region-specific AIE. For the SA-inclined rats, the genes showing AIE bias towards paternal F344 alleles in Nac were strongly enriched for genes upregulated in Nac DE—the gene set most relevant to NIC SA—and also for GWAS risk variants of smoking initiation. These findings show a prominent role for sex and region-specific gene expression and imprinting in NIC SST and SA, and suggest a mechanistic link between genes underlying these processes and human NIC addiction. This study provides a resource for understanding the biology underlying the genetic findings on human smoking phenotypes.以尼古丁（nicotine, NIC）作为成瘾性核心成分的烟草吸烟行为，是人类死亡的重要危险因素。在动物实验中，反复暴露于尼古丁可诱导敏化（sensitization, SST）现象，并增强尼古丁自身给药（self-administration, SA）行为。然而，目前学界对于敏化与自身给药的分子机制，以及二者与吸烟行为的遗传相关性，仍知之甚少。 本研究以近交系Envigo大鼠（Envigo rats）的F1子代（F344/BN）为实验模型，对腹侧被盖区（ventral tegmental area, VTA）、伏隔核核心区（nucleus accumbens core, Nac）与伏隔核壳区（shell, Nash）中尼古丁敏化与自身给药相关的转录组特征进行了分析。研究观察到雄性特异性的尼古丁敏化现象，且仅在父本为F344的杂交组中存在尼古丁自身给药的亲本效应。 差异表达（differential expression, DE）分析结果显示，尼古丁敏化与自身给药存在性别与脑区特异性的转录组特征：雄性腹侧被盖区内下调的基因同时与敏化和自身给药过程相关，而雄性伏隔核核心区内上调的基因则仅与自身给药行为相关。与敏化、自身给药相关的差异表达基因显著富集于突触过程、髓鞘结构、烟草使用障碍与药物依赖相关的生物学通路。 有趣的是，本研究发现雄性腹侧被盖区中下调的基因，在雌性腹侧被盖区内往往呈现上调趋势；二者的重叠基因显著富集于吸烟相关全基因组关联研究（genome-wide association study, GWAS）的风险变异位点，这或可解释雄性特异性敏化现象的成因。 为深入解析观察到的尼古丁自身给药亲本效应的分子机制，本研究对表现出不同自身给药倾向的正反交F1大鼠开展了表达等位基因失衡（allelic imbalance of expression, AIE）分析，发现了广泛存在的脑区特异性表达等位基因失衡现象。对于表现出自身给药倾向的大鼠，其伏隔核核心区内偏向父本F344等位基因的表达等位基因失衡基因，显著富集于伏隔核核心区差异表达基因集——该基因集与尼古丁自身给药的关联性最高——同时也富集于吸烟启动相关的全基因组关联研究风险变异位点。 本研究结果揭示了性别与脑区特异性基因表达及印记在尼古丁敏化与自身给药过程中的关键作用，并为上述过程相关基因与人类尼古丁成瘾之间的机制关联提供了理论依据。本研究可为解析人类吸烟表型相关遗传研究的生物学基础提供重要的公共数据资源。