Table_3_Quantitative trait loci on chromosomes 9 and 19 modulate AII amacrine cell number in the mouse retina.pdf

Sequence variants modulating gene function or expression affect various heritable traits, including the number of neurons within a population. The present study employed a forward-genetic approach to identify candidate causal genes and their sequence variants controlling the number of one type of retinal neuron, the AII amacrine cell. Data from twenty-six recombinant inbred (RI) strains of mice derived from the parental C57BL/6J (B6/J) and A/J laboratory strains were used to identify genomic loci regulating cell number. Large variation in cell number is present across the RI strains, from a low of ∼57,000 cells to a high of ∼87,000 cells. Quantitative trait locus (QTL) analysis revealed three prospective controlling genomic loci, on Chromosomes (Chrs) 9, 11, and 19, each contributing additive effects that together approach the range of variation observed. Composite interval mapping validated two of these loci, and chromosome substitution strains, in which the A/J genome for Chr 9 or 19 was introgressed on a B6/J genetic background, showed increased numbers of AII amacrine cells as predicted by those two QTL effects. Analysis of the respective genomic loci identified candidate controlling genes defined by their retinal expression, their established biological functions, and by the presence of sequence variants expected to modulate gene function or expression. Two candidate genes, Dtx4 on Chr 19, being a regulator of Notch signaling, and Dixdc1 on Chr 9, a modulator of the WNT-β-catenin signaling pathway, were explored in further detail. Postnatal overexpression of Dtx4 was found to reduce the frequency of amacrine cells, while Dixdc1 knockout retinas contained an excess of AII amacrine cells. Sequence variants in each gene were identified, being the likely sources of variation in gene expression, ultimately contributing to the final number of AII amacrine cells.

调控基因功能或表达的序列变异（sequence variants）可影响多种可遗传性状，其中包括神经元群体的数量。本研究采用正向遗传学方法（forward-genetic approach），旨在鉴定调控一类视网膜神经元——AII无长突细胞（AII amacrine cell）数量的候选因果基因及其序列变异。研究使用了由亲本品系C57BL/6J（B6/J）与A/J实验小鼠培育得到的26个重组近交系（recombinant inbred, RI）小鼠的数据，以定位调控细胞数量的基因组位点。各重组近交系间的细胞数量存在较大变异，最低约为57000个，最高约为87000个。数量性状位点（quantitative trait locus, QTL）分析共发现3个潜在的调控基因组位点，分别位于9号、11号和19号染色体（Chrs）上，每个位点均贡献加性效应，三者的总效应接近观测到的细胞数量变异范围。复合区间作图法（Composite interval mapping）验证了其中2个位点；将A/J品系9号或19号染色体基因组导入B6/J遗传背景的染色体替换品系（chromosome substitution strains）中，其AII无长突细胞数量出现了符合这两个QTL效应预期的增加。针对上述各基因组位点，研究人员结合其视网膜表达特征、已知生物学功能，以及存在的可调控基因功能或表达的序列变异，鉴定出了候选调控基因。其中位于19号染色体的Dtx4（Notch信号通路（Notch signaling）调控因子）与位于9号染色体的Dixdc1（WNT-β-连环蛋白信号通路（WNT-β-catenin signaling pathway）调节因子）得到了深入研究。研究发现，出生后过表达（Postnatal overexpression）Dtx4会降低无长突细胞的出现频率，而Dixdc1基因敲除的视网膜中则存在过量的AII无长突细胞。研究人员在两个基因中均鉴定出了序列变异，这些变异可能是基因表达差异的来源，最终影响AII无长突细胞的数量。