Lim J-E, Pinsonneault J, Sadee W, Saffen D
Department of Pharmacology, The Ohio State University, Columbus, OH 43210, USA.
Mol Psychiatry. 2007 May;12(5):491-501. doi: 10.1038/sj.mp.4001923. Epub 2006 Dec 12.
Tryptophan hydroxylase isoform 2 (TPH2) is expressed in serotonergic neurons in the raphe nuclei, where it catalyzes the rate-limiting step in the synthesis of the neurotransmitter serotonin. In search for functional polymorphisms within the TPH2 gene locus, we measured allele-specific expression of TPH2 mRNA in sections of human pons containing the dorsal and median raphe nuclei. Differences in allelic mRNA expression--referred to as allelic expression imbalance (AEI)--are a measure of cis-acting regulation of gene expression and mRNA processing. Two marker SNPs, located in exons 7 and 9 of TPH2 (rs7305115 and rs4290270, respectively), served for quantitative allelic mRNA measurements in pons RNA samples from 27 individuals heterozygous for one or both SNPs. Significant AEI (ranging from 1.2- to 2.5-fold) was detected in 19 out of the 27 samples, implying the presence of cis-acting polymorphisms that differentially affect TPH2 mRNA levels in pons. For individuals heterozygous for both marker SNPs, the results correlated well (r=0.93), validating the AEI analysis. AEI is tightly associated with the exon 7 marker SNP, in 17 of 18 subjects. Remarkably, expression from the minor allele exceeded that of the major allele in each case, possibly representing a gain-of-function. Genotyping of 20 additional TPH2 SNPs identified a haplotype block of five tightly linked SNPs for which heterozygosity is highly correlated with AEI and overall expression of TPH2 mRNA. These results reveal the presence of a functional cis-acting polymorphism, with high frequency in normal human subjects, resulting in increased TPH2 expression levels. The SNPs that correlate with AEI are closely linked to TPH2 SNPs previously shown to associate with major depression and suicide.
色氨酸羟化酶同工型2（TPH2）在中缝核的5-羟色胺能神经元中表达，在该部位它催化神经递质5-羟色胺合成中的限速步骤。为了寻找TPH2基因座内的功能多态性，我们在含有背侧和中缝核的人类脑桥切片中测量了TPH2 mRNA的等位基因特异性表达。等位基因mRNA表达的差异——称为等位基因表达失衡（AEI）——是基因表达和顺式作用调控mRNA加工的一种度量。位于TPH2第7和第9外显子的两个标记单核苷酸多态性（SNP）（分别为rs7305115和rs4290270）用于对27名一个或两个SNP为杂合子个体的脑桥RNA样本进行等位基因mRNA定量测量。在27个样本中的19个检测到显著的AEI（范围为1.2至2.5倍），这意味着存在顺式作用多态性，其对脑桥中TPH2 mRNA水平有不同影响。对于两个标记SNP均为杂合子的个体，结果相关性良好（r = 0.93），验证了AEI分析。在18名受试者中的17名中，AEI与第7外显子标记SNP紧密相关。值得注意的是，在每种情况下，次要等位基因的表达均超过主要等位基因，这可能代表功能获得。对另外20个TPH2 SNP进行基因分型，确定了一个由5个紧密连锁SNP组成的单倍型块，其杂合性与AEI和TPH2 mRNA的总体表达高度相关。这些结果揭示了一种功能性顺式作用多态性的存在，在正常人类受试者中频率较高，导致TPH2表达水平升高。与AEI相关的SNP与先前显示与重度抑郁症和自杀相关的TPH2 SNP紧密连锁。
