Characterization of the chicken SAA.R90S mutation

Amyloid arthropathy in chickens is a debilitating disease triggered by bacterial infection. The disease leads to substantial economic loss and is a major animal welfare concern, contributing to the widespread overuse of antibiotics. A hallmark of amyloid arthropathy is the deposition of serum amyloid A (SAA) in the large joints of affected birds. Bacterial infection initiates an acute phase response increasing SAA production in the liver, which accumulates at sites of infection. Interestingly, brown layers are most affected by amyloid arthropathy, whilst white layers are resistant to the disease. Differential disease susceptibility has an immunological basis but underlying genetic risk factors are not understood. Using a whole genome sequencing approach, we discovered a variant in the SAA gene in white layers predicted to cause an arginine to serine substitution at position 90 (SAA.R90S). When overexpressed in chicken hepatocellular carcinoma cells, SAA.R90S was found to be expressed at a higher rate, more prone to aggregation and secreted to a greater degree than the wild-type SAA protein. RNASeq analysis showed that the R90S mutant exerted a differential effect on the expression of core transcription factors linked to cell fate determination and cell differentiation. A comparative analysis to murine IL-6/SAA stimulated CD4 T-cells suggests that SAA.R90S might block an induced cell fate change towards pro-inflammatory T-helper-17-cells, which are required for immunological protection against pathogenic bacteria during an acute phase response. Our results provide first mechanistic insights into the genetic resistance of white layers to amyloid arthropathy and could be applied to commercial layer breeding programs.鸡淀粉样关节病（Amyloid arthropathy）是一种由细菌感染诱发的致残性疾病。该疾病会造成严重的经济损失，同时也是重大的动物福利议题，并加剧了抗生素的广泛滥用。鸡淀粉样关节病的典型特征是，患病禽只的大关节处会沉积血清淀粉样蛋白A（serum amyloid A, SAA）。细菌感染会启动急性期反应，促进肝脏合成SAA，随后SAA会在感染部位积累。值得注意的是，褐壳产蛋鸡（brown layers）最易感染鸡淀粉样关节病，而白壳产蛋鸡（white layers）则对该疾病具有抗性。这种疾病易感性的差异具有免疫学基础，但其背后的遗传风险因子仍未明确。本研究通过全基因组测序（whole genome sequencing）技术，在白壳产蛋鸡的SAA基因中发现了一处变异，该变异预计会导致第90位的精氨酸替换为丝氨酸（SAA.R90S）。在鸡肝癌细胞中过表达该变异蛋白时，研究发现与野生型SAA蛋白相比，SAA.R90S的表达量更高、更易发生聚集，且分泌量也更多。RNA测序（RNASeq）分析结果显示，R90S突变体对与细胞命运决定及细胞分化相关的核心转录因子的表达具有差异化调控作用。通过对小鼠白细胞介素6（IL-6）/SAA刺激的CD4阳性T细胞（CD4 T-cells）进行对比分析，本研究发现SAA.R90S可能会阻断向促炎性T辅助17细胞（T-helper-17-cells）的诱导性细胞命运转变，而这类细胞在急性期反应中对于抵御病原菌的免疫保护至关重要。本研究结果首次阐明了白壳产蛋鸡对鸡淀粉样关节病产生遗传抗性的分子机制，其成果可应用于商业化产蛋鸡的育种工作中。